BDTTERV BOOK Effl
LEND HCID
TRHCTIOn
BATTERIES
CURTIS
BRTTERV BOOK ®
LEND RCID
TRRCTIOd
BATTERIES
Written by the staff of Curtis Instruments, Inc.
Edward M. Marwell, President
Eugene P. Finger, Vice-President/Engineering
Eugene A. Sands, Vice-President/Marketing
•
Henry M. Leuine, Writing Consultant
Ken Marsh, Editor
H. Shaw Borst, Inc., Design
CURTIS INSTRUMENTS, INC.
200 Kisco Avenue, Mt. Kisco, N.Y. 10549
1
All rights reserved. No part of this
book may be reproduced in any form
or by any means without prior written
permission of the publisher, excepting
brief quotations used in connection
with reviews or essays written
specifically for inclusion in a magazine
or newpaper.
Library of Congress Catalog Card
Number: 81-65733
ISBN: 0-939488-00-0
First printing
Copyright © 1981 Curtis Instruments, Inc
V IP
An Introductory Word
Battery Book One: Lead Acid Traction Batteries is the
first of what certainly could become a series of volumes on
"chemical systems capable of storing and supplying elec-
trical energy" — batteries.
The energy crisis has focused attention on the need for
alternative energy resources. Thus, where storage of elec-
trical energy and its supply to machines, such as fork lift
trucks, are required, the role of batteries is rapidly growing.
Industry and government are substantially investing in the
research and development of new and improved batteries for
use with a wide range of electric vehicles, solar and wind
power systems and as storage systems at electric generating
plants. The futures of energy and batteries are undeniably
intertwined.
Of the many battery types used in a variety of applica-
tions, from standby-power-supply systems to household flash-
lights, lead acid is one of the more important battery
chemistries. These so-called "wet cells" are the principal
batteries used on electric vehicles.
This book focuses on the lead acid traction battery
which is the prevailing power source used for electric fork lift
trucks widely employed in warehouses and factories for
material handling. Though the book has been designed with
the electric fork lift truck user in mind, much of the informa-
tion applies to batteries in general and will be helpful to
anyone interested in learning about them.
ENERGY AND FORK LIFT TRUCK BATTERIES
"Energy" is much in the news today, with good reason.
Until the early 70s, the cost of electrical energy was not con-
sidered to be very important, and sometimes even went
unstated in evaluating the cost of material handling opera-
tions. The "payback" of capital outlay and labor costs were
the major considerations completely overshadowing the
relatively miniscule costs of energy.
No longer, however, does capital outlay, or the cost of
borrowed money, or of labor, etc., overshadow the cost of
energy. Indeed, now, many engineering and purchasing deci-
sions begin with an analysis of energy costs.
The reasons are obvious. Just as the shortages of
gasoline and diesel fuel to operate combustion-engine
vehicles have drastically increased their per-mile operating
costs, so have the increased cost of generating and deliver-
ing electrical energy raised the cost of operating battery-
powered fork lift trucks. Every fork lift truck is a user of
energy, whether that energy is derived from gasoline or from
an electric grid fed by hydro-power, nuclear power, or fossil
fuels. With all three categories of energy displaying strong
tendencies to increasing costs in the foreseeable future, it is
entirely reasonable to expect increasing concern for improv-
ing the overall efficiency with which battery-powered fork lift
trucks are operated.
As developers and manufacturers of several proprietary
instruments for monitoring the performance of batteries, we
at Curtis have actively pursued the subject of efficiency in
battery-powered vehicles.
An early example was our design of the battery state-of-
charge indicator for NASA's Lunar Rover vehicle. The object
in that case was to warn the astronauts so that they would
not drive too far from their base station . . . there being no
means available for recharging the Rover's batteries.
A parallel program is our 933 Fuel Gage for battery-
powered fork lift trucks. Here, the instrument is used to
warn the driver when the truck's battery has reached the safe
limit of discharge. Tens of thousands of these units are now
in use on fork lift trucks throughout the world.
As we work with and listen to industry people using
electric fork lift trucks, one theme emerges over and over
again. "How can we minimize our energy costs?"
The purpose of this book is to assist those people in
minimizing their energy costs:
•By helping them to correctly select batteries for their
trucks .
• By showing them how to control the use of electrical
energy in recharging their batteries.
•By helping them to avoid damaging batteries and
trucks by over-discharging batteries.
The material in this book was compiled from numerous
standard reference sources, from data published by
manufacturers of lead acid traction batteries, from published
technical papers, and from various engineering investiga-
tions carried on by Curtis as part of our ongoing study of
batteries and their applications. There are, of course, no
direct references to particular makes or models of truck,
battery or charger. Of necessity we have generalized our ex-
amples to give them the widest possible application. Thus,
where estimates of energy use, etc., are given, they are ap-
proximations based on our experience in the field and con-
firmed by reference to published product data. Note also
that we have avoided placing specific values on capital
equipment, labor, and energy.
Whenever feasible we have used nomenclature and ab-
breviations that conform to industry standards. In case of
doubt we have relied on the standards promulgated by the
IEEE/ANSI.
The text of this book has been reviewed for us by
several well-known specialists in the field, and we would like
to thank them individually:
Dr. David P. Boden, Senior Vice-President Technology,
C & D Batteries, Plymouth Meeting, PA.
Larry E. Heisey, Battery Charging Engineer,
Hobart Brothers Co., Troy, OH.
R. T. Josey, Mark C. Pope Associates, Inc., Smyrna, GA.
Service Engineering Department, Caterpillar Tractor Co.
(Tow Motor), Mentor, OH.
Dr. William Reinmuth, Professor of Chemistry,
Columbia University, NYC.
Curtis Instruments, Inc. • Mount Kisco, N.Y.
December 1980
The staff of Curtis Instruments, Inc., assumes sole responsibility for the accuracy of the information
in this book. In no way should it be assumed that the reviewers endorse the products referred to by
company name and model in the text.
ENERGY, WORK AND THE STORAGE BATTERY
Energy and Work 10
Energy and Power 11
Accumulating Energy 12
Energy Efficiency 12
Ohm's Law 14
Current and Amperes 14
The Volt 15
The Watt 15
The Kilowatt-Hour 15
The Ampere-Hour 16
Battery Capacity 16
ABOUT LEAD ACID BATTERIES
Introduction 18
How a Lead Acid Battery is Made 18
Electrolyte 22
Producing an Open Circuit Voltage 24
Producing Current 24
State-of-Charge 26
Determining Battery Capacity 28
Capacity and Discharge Rate 30
Capacity and Temperature of Electrolyte 32
State-of-Charge Measurements 32
Specific Gravity and Open Circuit Voltage 33
Voltage Under Load 34
Ampere-Hour Measurements 36
BATTERY CHARGING
Introduction 37
How Energy is Stored in the Cell 37
Battery Chargers and Charging 38
Ferroresonant Chargers 40
Pulsed Chargers 40
Maximum Charge Rate 42
Finish Rate 43
Equalizing Charges 43
Terminating the Charge 43
Gassing 43
Energy Efficiency in the Charging Process 44
How the Charger Affects Energy Efficiency 44
How the Battery Affects Energy Efficiency 45
Overall System Efficiency 46
How Depth of Discharge Affects System Efficiency 46
The Combined Effect 48
Suction 4.
OPTIMIZING ENERGY USAGE
Beginning with Conclusions 50
Selecting the Correct Battery Capacity 50
Ways to Measure State-of-Charge
onthe Fork Lift Truck 51
Specific Gravity: A Static Measure 51
Open Circuit Voltage: Another Static Measure 54
Voltage Under Load: A Dynamic Measure 54
State-of-Charge-Based Charging Can Save Energy 56
How A Typical Fleet Can Save Energy 60
Reducing Peak Power Demand Costs 62
Capital and Labor Savings 64
WEAR AND TEAR
Overworking the Battery 67
Causes and Effects 67
Epilog 68
Index 69
Figure 1: Horsepower 11
Figure 2: Energy Efficiency from Electric
Generation Plant to Work Done by Electric
Fork Lift Truck 13
Figure 3: The Concept of Electrical Current 14
Figure 4: Battery Charge/Discharge Cycles in
Two Commercial Applications 19
Figure 5: Typical Flat and Tubular Plate Cell
Construction 20
Figure 6: Battery Cell Strapping and Numbering 21
Figure 7: Schematic Representation of Reactions
at Negative and Positive Plates 23
Figure 8: Stabilized SG for 2 Cell Types vs State-of-
Charge at the 6-Hour Rate 27
Figure 9: Time Required for SG to Stabilize During
Discharge Rest Intervals 29
Figure 10: A Conventional Test Setup for Determining
Battery Capacity 29
Figure 11: Typical End Point Voltage as Function of
Discharge Rate (Valid when manufacturer
rates battery with a current-dependent end-
point voltage [from Manufacturer's data].) 31
Figure 12: How Battery Capacity Varies with
Discharge Rate 31
Figure 13: How Battery Capacity may be Affected by
Electrolyte Temperature 33
Figure 14: How Stabilized Open Circuit Voltage
Reflects Stabilized SG 35
Figure 15: Variation of Open Circuit Voltage as Cell
Recovers After Load 35
Figure 16: Cell Voltage at Two Constant Currents 36
Figure 17: How Ampere-Hours are Returned to the
Battery During an 8-Hour Charge 39
Figure 18: Lag of SG Measured During Charging
Process Against Theoretical SG
vs State-of-Charge 39
Figure 19: Current/Voltage Relationship in a
Ferroresonant Charger 41
Figure 20: How a Pulsed Charger Operates 41
Figure 21: How the Charger Affects Efficiency 45
Figure 22: Effect of Three Depths of Discharge
(DOD) on System Efficiency 47
Figure 23: Overall Energy Efficiency in Charging a
Battery Discharged to Various Depths
(8-Hour Charge) 49
Figure 24: Relative Current Drain for Typical
Industrial Trucks 52
Figure 25: A Hypothetical Industrial Truck Operation 53
Figure 26: Stabilized SQ and Open Circuit Voltage as
Measures of State-of-Charge for Various
Discharge Rates 55
Figure 27: Voltage Under Load as the Measure
of State-of-Charge 57
Figure 28: A Practical Application of Voltage Cinder
Load as a Measure of State-of-Charge 58
Figure 29: How Energy Requirements are Affected by
Depth of Discharge 59
Figure 30: Reduction of Peak Power Demand with
State-of-Charge-Based Charging Schedule 63
Figure 31: Effect of Alternative Charging Schedules on
Peak Power Demand 64
Table 1: Effect of Battery State-of-Charge on Charger
Efficiency in an 8-Hour Charge Period 48
Table 2: Effect of Battery State-of-Charge on Battery
Efficiency in an 8-Hour Charge Period 48
Table 3: Overall System Efficiency When Charging for
an 8-Hour Charge Period 48
Table 4: Energy Usage Data for a Hypothetical
20-Truck. 2-Shift Fleet 61
Table 5: Battery Workshift Capacity 62
Table 6: Impact of Workshift-Based vs State-of-Charge-
Based Charging on Peak Demand Charges
for a 20-Truck Fleet 64
Table 7: Batteries Required Per Shift 65
Table 8: Total Number of Charges Required Annually
to Support Fleet 66
ENERGY, WORK AND THE STORAGE BATTERY
Energy and Work
"Energy" is the ability to do "work," a definition that
relates pretty well to the world of practical experience.
Everyone knows that "it takes energy to get the work done."
Energy can take many forms. In addition to human
"energy," there are mechanical energy, heat energy, chem-
ical energy, electrical energy, nuclear energy, etc.
Although each of these forms of energy is in a form
slightly different from the others, all share the basic feature:
each provides the ability to do work.
To turn the idea around, work is the process of "spend-
ing" energy, usually in a way that we humans call "useful,"
although it isn't strictly necessary that the work be useful in
our sense of the word. Since nature doesn't care about
usefulness, expending energy in any form at all is properly
called work.
Potential energy is energy accumulated in a useful form
but not yet used. A relevant example is the chemical energy
accumulated in a charged storage battery. Connecting the
battery to a charger — which, in turn, is connected via the
power lines to a distant generating plant — stores, in chem-
ical form, a small part of the energy output of the gener-
ating plant. It makes no difference whether the energy is
generated in a hydro-electric or a steam-electric plant: the
same small part of the energy output of the plant is stored,
as potential energy in chemical form in the battery.
When the battery is connected to the circuits of an elec-
tric fork lift truck, its chemical energy is converted into elec-
trical energy and released, a little at a time, to the truck,
which converts it into mechanical energy in the form of use-
ful, measurable work: moving heavy coils of wire from one
end of the plant to another, stacking loaded pallets, and so
on. Each expenditure of energy reduces the potential stored
in the battery, and so less is then available. When all of the
usable energy stored in the battery has been "used up" the
battery is discharged. To get more work out of it we must
recharge it by restoring its supply of energy.
Whatever the source of the energy at the power lines, it
takes work to generate it, accumulate it, and transmit it, and
more work to convert it into chemical form in the recharged
battery. For practical purposes, all of these processes — from
generating to charging — are part of the energy cost and are
basic components of every electric truck operator's utility
bill.
1 1
Energy and Power
Energy is the ability to do work and power is the rate at
which the work is done. Lifting 55 pounds 10 feet in the air
takes 550 foot-pounds of energy, and doing that much work
in 1 second is 550 foot-pounds per second or 1 horsepower.
The term horsepower was invented in the 18th Century to
create a practical unit for the rate of doing work. Presumably, it
represents working "as fast as a horse," a rate we define as 550
foot-pounds per second. (Figure 1.)
Another unit used to represent the rate of doing work is
the "watt." Because both the horsepower and the watt repre-
sent rates of doing work, they can be equated to one another,
and it turns out that 1 horsepower is the same rate of work as
approximately 750 watts*.
Where work and energy are concerned, only the total
amount accumulated or spent matters . . . not how quickly or
slowly. Where power is concerned, however, the time factor
enters. The faster a given amount of energy is spent, the
higher the power rating; the slower the same amount of
energy is spent, the lower the power rating.
"Here and elsewhere, we have deliberately rounded values to simplify
arithmetic.
12
Accumulating Energy
There are many ways to accumulate energy. For example,
feeding our horse so that he can work — that is, deliver
horsepower — is one way of accumulating energy, and so is
charging a storage battery, which is actually called an ac-
cumulator in some other countries.
In accumulating energy, the important considerations are:
the quantity of energy to be stored in the accumulator; the
form in which the energy is accumulated, stored, and released
for use; and the overall efficiency of the process of ac-
cumulating, storing, and releasing the energy.
In the lead acid storage battery, large quantities of energy
are accumulated by chemical activity that is produced by the
charging process.The energy is then released on demand in
the convenient form of electric current.
Energy Efficiency
With charging and discharging batteries, as with all energy
transfer processes, energy losses occur. The inequality between
what is put into a system and what is drained from it is the
system's energy efficiency. Generally, the energy efficiency of
lead acid batteries is about 76%, meaning that 76% of the
energy that was put into the battery during charging is all that
is available for release during discharge. The energy efficiency
is given as an approximate number since discharge rates and
temperature can affect it.
The battery charger, which interfaces the battery with a
source of AC power, also has an efficiency rating and, thus, is
considered when calculating the overall efficiency of a bat-
tery system. A good charger is about 85% efficient, making
for a combined charger/battery efficiency of about 65%,
meaning that 65% of the electricity from the AC line fed into
the battery is available as DC energy to a machine's com-
ponents, such as controllers, motors, drive trains, etc., which
are also not 100% efficient in their use of energy.
Another factor affecting the battery's efficiency is how and
when it is charged. For a further discussion of charging
regimens and their effect on energy efficiency and economy,
see Section 4, Optimizing Energy Usage.
As an interesting note: only about 7% of the potential
energy put into the power plant is available for actual work;
for example, the lifting and moving of pallets from one place
to another in a warehouse, when using an electric fork lift
truck.
13
Figure 2: Energy Efficiency from Electric Generation Plant to
POWER PLANT TRANSMISSION LINES
30% EFFICIENT
If Calculated from kWh Delivered from the Power Plant,
The Efficiency is about 23% <fi Work Done Point
DRIVE TRAIN 85%
CONTROLLER
85%
MOTORS 70%
7% TOTAL EFFICIENCY AT WORK DONE POINT
14
Ohm's Law
Our use of electrical energy is dependent on the "flow"
of electrical energy, the magnitude of the "force" propelling
it and the "resistance" to its flow that naturally occurs in all
materials through which it flows. The materials and the cir-
cuits they make up through which electrical energy flows, in
effect, convert the energy into other forms of energy, such as
heat and light, or work, such as turning a motor.
When a battery is used to power a fork lift truck, it
operates with a "force" that affects the "flow" of its stored
energy depending on the "load" — the power needs of the
truck's network of "resistances."
The relationship between "flow", "force" and
"resistance" is expressed mathematically in Ohm's Law,
which was discovered by George Simon Ohm in the early
1800s. The Law states that E ("force" in volts) is equal to I
("flow" in amperes) times R ("resistance" in ohms). E = IR and
its variants, 1 = E/R and R = E/I, comprise one of the basic
tools electrical engineers have for designing electric powered
machines.
Current and Amperes
Current flow is the means by which the battery releases
Figure 3: The Concept of Electrical Current
In sections of this book, current is discussed in conuentionat terms as flowing
from the positive to the negative; while, in other sections, it is discussed in the
electrochemist s terms as flowing from the negative to the positive.
15
its energy in electrical form. Current is the flow of electric
charge from the positive terminal of the battery through the
"load" (motors, pumps, controllers, etc.) of the truck, and
back to the negative terminal of the battery. (Figure 3.)
The flow of current from the battery depletes the
battery's stored charge. The rate of that depletion or current
drain is measured in amperes. An ampere is the flow of 1
coulomb* per second. It is also defined as the amount of cur-
rent that can be forced through a resistance of 1 ohm by an
electrical potential of 1 volt.
The Volt
The volt is the unit of electrical potential, or pressure,
that "forces" the current from the battery through the load
and back to the battery. Since batteries are made up of many
cells connected in series the total voltage of a battery,
naturally, is the sum of the voltages of all its cells. A typical
lead acid battery used in a fork lift truck may have 18 cells,
nominally 2 volts each; the nominal battery voltage is
therefore 36 volts.
The Watt
The watt is the electrical unit that defines the rate at
which work is done, or energy is spent. Mathematically, we
can say that Watts = Volts x Amperes.
The bigger the current (amperes) and/or the voltage
(volts), the faster the stored energy can be converted into
work.
The Kilowatt-Hour
If the watt is the rate of doing work, then the total
amount of work actually done is the product of watts and
hours: watt-hours. If a battery delivers 1000 amperes at 24
volts for 2 hours, the total amount of energy delivered is
1000 x 24 = 24,000 watts x 2 hours = 48,000 watt-
hours.
To keep the significant digits from attracting too many
zeros, we use the prefix "kilo" (k), which means 1000, and,
sometimes, "mega" (M) for millions. The energy delivered in
this example is therefore 48 kilowatt-hours.
*The coulomb is a unit of electrical energy that has been in use since
before the discovery of the electron.
It is actually made up of 6,000.000,000,000,000,000 electrons.
16
A rating of 1000 watt-hours (1 kWh) is equivalent to 1
horse working 1 Vi hours.
Where batteries are concerned, the kilowatt-hour rating
at a stated discharge current is an accurate description of ex-
actly how much energy the battery can deliver before it is
discharged. Since electric bills are rendered on the basis of
the number of kilowatt-hours of usage, it is often useful to
make calculations about energy usage and efficiency directly
in kilowatt-hours.
The Ampere-Hour
An ampere-hour is the total amount of electrical charge
transferred when a current of 1 ampere flows for 1 hour.
Therefore, the total usable charge stored in a battery can be
stated in terms of ampere-hours — how long a current of a
particular amperage can be drawn from the battery.
The ampere-hour rating accurately predicts the battery's
capacity at a specified load current; batteries are therefore
rated in ampere-hours at specified currents. A battery that
can be discharged at 125 amperes for 6 hours before reach-
ing its end-point voltage is rated at 125 amperes x 6 hours
= 750 ampere-hours. Its "capacity" is therefore stated as
"750 ampere-hours at the 6-hour discharge rate (at +25°C)."
Battery Capacity
The term battery capacity relates to the amount of
usable electrical energy stored in the battery. It is important
to keep in mind that a manufacturer's rated capacity is given
for 100% discharge of the battery. The recommended usable
capacity, however, is generally 80% of the rated capacity to
insure maximum battery life. For practical purposes, battery
capacity is usually stated in ampere-hours because a par-
ticular number of ampere-hours of capacity is equatable to
operating a given vehicle for a given length of time before its
output voltage reaches the end point. A battery rated at 1200
ampere-hours is, therefore, thought of as maximally having
960 ampere-hours of usable capacity. Capacity is also af-
fected by discharge rate and other variables that are discuss-
ed more thoroughly in the following pages.
Most manufacturers also provide capacity ratings in
terms of kilowatt-hour specification when describing their
batteries. As with ampere-hour ratings, the conditions under
which kilowatt-hour specifications are determined must be
specifically stated to be meaningful.
17
End Point Voltage (Final Voltage)
Today's lead acid traction battery has a downward-
curving discharge characteristic, meaning that the voltage of
the battery decreases gradually as it is discharged. The end
point voltage is that which determines that the battery is
discharged. Defining an end point voltage is an attempt to
provide users with a cut-off beyond which the battery should
not be used or damage to it and the equipment it is powering
may occur.
Depending on the rate of current drain and the equip-
ment, end point voltage can vary. However, when a usage
pattern of the battery and equipment are predictable, as is
pretty much the case with fork lift trucks, an end point
voltage is quite meaningful.
The end point voltage for electric fork lift truck applica-
tions is selected by general agreement among battery and
truck manufacturers. Still, for a 2 volt cell, there are differing
opinions on just where the end point should be set with a
normal range lying between 1.65 and 1.75 volts per cell (with
the battery "loaded"). In some cases, especially at high
discharge rates, this range is extended as low as 1.2 volts per
cell by some manufacturers.
Battery manufacturers may suggest that their batteries
can safely be discharged beyond the accepted range of end
point voltages. No significant loss of battery life will be
caused by such operation, they say. Further, they may even
maintain that such operation is cost-effective as far as battery
life is concerned.
Truck manufacturers, however, may take a different
position. They say that allowing the battery voltage to fall ap-
preciably below the specified end point may do irreparable
damage to electrical equipment installed on the truck. They
point out that operating at undervoltage can, for example,
overheat motors causing ultimate failure and/or burn relay
contacts.
From the users' point of view, though, one kind of
damage may be as bad as another. A damaged battery must
be replaced; a damaged pump motor or other component
must be repaired or replaced. In either case, loss of the use
of the truck — down time — is a problem that may be more
severe than the physical damage. Users, therefore, may have
more of an interest in establishing — and accurately detect-
ing — the end point voltage than either of the two suppliers.
ABOUT LEAD ACID BATTERIES
Introduction
There are two types of lead acid batteries generally used
for vehicle applications — the ordinary automotive battery
(used for starting, lighting, and ignition) and the traction bat-
tery used to supply motive power for electric vehicles.
Automotive batteries are designed for infrequent, very high
current drains of short duration, and recharging begins as
soon as the engine reaches operating speed. Traction bat-
teries, on the other hand, are designed to be discharged con-
tinuously at relatively moderate current drains because there
is no practical way to recharge the battery during operation.
The stored charge of a traction battery, therefore, runs
steadily down from its starting condition until the battery is
recharged. A reasonable service life from such a battery
might be considered as 1000 to 2000 cycles of discharge and
charge; and typical life spans for industrial batteries, properly
used and cared for in fork lift trucks, are about 5 years,
sometimes even longer.
Figure 4 shows typical charge/discharge characteristics
for batteries used in two common commercial applications
— a taxi and a fork lift truck, both used in 2-shift operations.
How a Lead Acid Battery Is Made
The lead acid battery is made up of several identical
cells, each of which contains two plates, one positive, the
other negative. Both plates are immersed in an electrolyte
that is a mixture of sulfuric acid and water.
Two types of cell construction are common: flat plate
and tubular plate. The overall functions of the two types are
identical, but their mechanical construction and performance
differ slightly.
In a flat plate cell (Figure 5), each positive plate is a
cast metallic lead frame which contains the lead dioxide ac-
tive material. The negative plates contain spongy metallic
lead active material within a similiar grid structure. Positive
and negative plate areas are usually identical.
In a tubular plate cell (Figure 5), the positive plates sur-
round lead alloy spines. The lead dioxide is in close contact
with the spine over its entire length, and is retained by a
special sleeve. Negative plates are of spongy metallic lead in
a grid form identical to those in flat plate cells.
19
20
21
In either case, the cell is filled with electrolyte, which is
slightly heavier than water. The ratio between the weight of a
given volume of electrolyte and the same volume of water is
the specific gravity of the electrolyte.
Figure 5 shows how a typical industrial cell is assembled.
In order to provide sufficient current output (amperes)
each cell consists of many plates (for example, 11 positive
and 12 negative). Because each positive plate is positioned
between two negative plates, there is always one fewer
positive than negative. The positive plates in each cell are
connected in parallel to provide a positive bus of the re-
quired current output, which is connected to the positive ter-
minal of the cell. Similarly, the negative plates are bussed
and connected to the negative terminal.
The cells are connected by external metal straps that
hook them into a series circuit ... a circuit in which the
negative plates of one cell are connected to the positive
plates of the next, so that the voltages of all cells are added
to provide the total voltage of the battery. Typically the cells
are numbered in sequence beginning with the cell containing
the positive terminal of the battery (number 1) and ending
with the cell containing the negative terminal. (Figure 6.)
There can be any number of cells in a battery, but the
numbers most commonly used are: 3, 6, 9, 12, 15, 16, 18,
20, 24, 30, 36, and 40.
Figure 6: Battery Cell Strapping and Numbering
22
Battery rating information is generally displayed in cod-
ed form, stamped into the lead of the first negative terminal
or on a nameplate on the side of the battery. As an example,
the code for a particular battery might read as follows:
12
C
85
11
Number
Manufacturer's
Ampere-hour
Total Number
of cells
Cell Type
Capacity per
of Plates
Positive Plate
Per Cell
(5 Positive, 6
Negative)
The ratings for this battery are:
Voltage: 12 cells x 2 volts each = 24 volts
Capacity: 1 1 — 1 positive plates x 85 Ah each = 425
2
Electrolyte
The electrolyte in a lead acid battery is a mixture of
sulfuric acid and water. Sulfuric acid is a very active com-
pound of hydrogen, sulfur, and oxygen. Its chemical formula
is H 2 S0 4 . In water, the sulfuric acid molecules separate into
two ions, hydrogen and "sulfate," the latter of which is made
up of sulfur and oxygen atoms. Each sulfate ion contains two
"excess" electrons and each therefore carries two negative
electrical charges. Each hydrogen ion, having been stripped
of one electron, carries one positive electrical charge.
Because sulfuric acid is highly reactive, it ionizes almost
completely and so there are very few fully assembled
molecules of sulfuric acid in the electrolyte at any instant.
Furthermore, the ions are in constant motion, attracted and
repelled by one another, by the water, and by any impurities
in the mixture. This constant random motion eventually
causes the ions to diffuse evenly throughout the electrolyte.
If any force disturbs this even distribution, the random mo-
tion eventually restores it. However, since the electrolyte
is contained in a complex structure of cells, redistribution
takes a relatively long time. This fact turns out to play a
key role in our ability to measure the exact state-of-charge
of the battery at any instant, as will be shown later.
In sections of this book, current is discussed in conuentional terms as flowing
from the positive to the negatiue; while, in other sections, it is discussed ^
in the electrochemist's terms as flowing from the negatiue to the positive. ~
23
Figure 7: Schematic Representation of Reactions at Negative
and Positive Plates
TO NEGATIVE TERMINAL
(±)
TO POSITIVE TERMINAL
□ lead
X OXYGEN i
A. SULFURIC ACID
*■
e
'LEAD SULFATE
Pb + S0 4 " 2 f +2e" +
+ PbS0 4 + 2e -»PbS
LEAD
DIOXIDE
LEAD
SULFATE
Pb0 2
PbS0 4
"SPONGY LEAD
NET REACTION: Pb + PbO 2 + 2H 2 S0 4 -* 2PbS0 4 + 2H 2
24
Producing an Open Circuit Voltage
The chemical reaction between the sulfate ions and the
spongy lead of the negative plate produces lead sulfate, a
compound that does not dissolve in water. This reaction frees
two electrons and thereby produces a net negative electrical
potential at the negative plate. (Figure 7.)
The presence of these free electrons slows down the
chemical reaction at the negative plate because their
negative charge repels other negatively charged sulfate ions.
Fewer ions can then reach the negative plate to react with
the spongy lead to form more lead sulfate. The overall reac-
tion cannot continue very long, therefore, unless the excess
electrons are permitted to leave the negative plate.
Meanwhile, at the positive plate, other sulfate ions react
with the lead of the lead dioxide to produce lead sulfate; at
the same time, the hydrogen ions of the acid react with the
oxygen of the lead dioxide to form water. This combination
of reactions produces a net positive potential at the positive
plate. (Figure 7.) Here, too, the reaction can only continue as
long as the electrical conditions are right. Within a short
time, the supply of free electrons in the metal of the positive
terminal is used up and no further chemical change can take
place unless more are supplied.
The difference between the two potentials at the plates is
the open circuit voltage or electromotive force (emf) of the
cell. This emf (about 2.1 volts) will remain unchanged as
long as no path is provided for the excess electrons to leave
the negative plate and no source of electrons is provided for
the positive plate. In this condition, there is little or no
chemical activity in the cell, which means that a charged cell
can be stored for a fairly long time without significant loss of
energy. The open circuit voltage typically will drop by less
than a millivolt (0.001V) per day, during storage, if there is
no loss of electrolyte — a process referred to as "self-
discharging."
Producing Current
The available source of electrons to make up the deficit
at the positive plate is, of course, the excess of free electrons
at the negative plate. Since these free electrons are produced
by the reaction between the acid and the lead, the total
number of free electrons available is set by the amount of
acid and lead available to react. A similar limitation exists
for the positive plate; the total number of free electrons it
25
can absorb is set by the amount of acid and lead dioxide
available to react.
Since any flow of electrons is a transfer of charge, the
total amount of charge stored in the cell is established by the
total amounts of plate material and sulfuric acid available to
react. The total amount of charge stored in the cell deter-
mines the capacity of the cell.
If a wire is connected between the two plates, the excess
electrons instantaneously rush from negative to positive. This
electron current* is very high because the wire is a short cir-
cuit between the terminals. If the wire is very thick (has no
resistance at all), the total number of electrons transferred is
determined only by the amount of electrolyte that has
reacted — and continues to react — with the two plates. The
net charge transfer is 2 electrons per molecule of acid. Since
the number of molecules of acid is inconceivably large, a
gigantic current could flow between the shorted terminals,
transferring nearly all of the cell's stored charge from one
terminal to the other in a very short time.
If electrical resistance ... a load ... is connected be-
tween the terminals, then the current is limited by the
resistance of the load, and the cell's charge is transferred
from terminal to terminal, via the load, at a slower rate, i.e.,
a smaller electron current. For a typical traction cell, the cur-
rent can be hundreds of amperes. This current will flow as
long as the load is connected and as long as there is active
material left in the cell to sustain it.
Since no physical process is perfect, the electrolyte/plate
reactions offer resistance to this internal current and
therefore lose some of the transferred energy in the form of
heat. The electrical effect of this internal resistance of the
cell appears as a loss of potential (a voltage drop) at each
plate. The cell's total voltage under load is therefore less
than its open circuit voltage. The amount of energy lost to
this internal resistance depends on the load current and on
the concentration of acid in the cell . . . especially the acid
concentration at the positive plate. The larger the load
"When discussing the electrochemical reactions in a battery, it is useful to
refer to electron flow as current.
26
current, the greater the loss of energy. Also, the lower the
acid concentration at the plates, the higher the internal
resistance of the cell.
When current is produced by the cell, acid, lead dioxide
and lead are converted to lead sulfate and water. Each acid
molecule that reacts is no longer part of the electrolyte. This
process, by reducing the concentration of acid in the water,
gradually reduces the ability of the cell and leaves less
energy in it.
In the design of batteries, the amounts of acid and plate-
active materials are balanced so that the release of energy
relates to the rate at which current is likely to be drawn. Bat-
teries designed for low-rate applications, such as for storage
in solar power systems, contain a larger amount of acid in
proportion to plate-active material. They are designed to be
plate-limited when used beyond their rated capacity. No plate
materials will be available for releasing usable energy.
Batteries designed for high-rate applications, such as
automotive ignition, etc., have a smaller amount of acid in
proportion to plate-active material. They are designed to be
acid-limited when used beyond their rated capacity.
As acid concentration becomes too low, a cell becomes
incapable of releasing usable energy at the rate for which it
was designed. Additional energy can only be drawn from it if
the current rate is reduced. As it is driven to excessively low
acid concentrations (through deep discharging), the coatings
of lead sulfate produced by the chemical reactions at the
plates will not reconvert. Upon charging, acid concentration
is restored and plate coatings will again reconvert.
The traction battery used with fork lift trucks falls be-
tween the automotive and storage battery in its proportion of
acid and plate-active material. It is generally considered to be
acid-limited for rates exceeding the 6-hour capacity.
State-of-Charge
The cell's state-of-charge is determined by the amount
of active material available to sustain a usable current flow
through a load. At the outset, all of the active material is
available and the cell is fully charged. When it can no longer
produce usable current, the cell is fully discharged. At any
27
point between these two extremes, the state-of-charge of the
cell is expressed as a percentage of the total difference in
charge between the fully charged and fully discharged states.
Since the state-of-charge is set by the availability of ac-
tive material in the cell, it is conventional (but not alone suffi-
cient) to define the cell's state-of-charge in terms of the
specific gravity of the electrolyte. As defined above, specific
gravity, a measure of density, is the ratio of the mass of the
mixture of sulfuric acid and water in the electrolyte to pure
water at a specified temperature. It is common to speak of,
for example, 1300 SG in lieu of 1.300 specific gravity: a con-
venience simply achieved by multiplying 1.300 by 1000. For
the purposes of this book, from this point on, specific gravity
measurements shall be expressed in SG form. All SG
measurements are corrected to + 25 °C.
The relationship between state-of-charge and specific
gravity is usually shown in a form similar to Figure 8. Note,
however, that this illustration does not take into account the
dynamic activity inside the cell while current is flowing. It
shows only the long-term average relationship when the load
has been disconnected and the sulfate ions have had a
chance to diffuse evenly throughout the cell.
28
The time required for this diffusion process to be com-
pleted varies according to the rate, depth and length of dis-
charge and is different in cells of different design. Figure 9
shows this effect as measured on a typical cell that has been
discharged at a moderate rate. In this test, it took more than 16
hours for specific gravity to fully stabilize.
Since the lead sulfate forms at the plates, the specific
gravity of the electrolyte is lowest near the plates and highest
farther from them. Measuring specific gravity during or short-
ly after discharge actually provides false information about
actual average specific gravity, with an error factor that
depends on the depth and duration of the cell's recent
discharges.*
Determining Battery Capacity
Battery capacity is determined through manufacturer
testing. Manufacturers have test procedures which are utiliz-
ed to establish the hour rate and ampere-hours of their bat-
teries. Prior to making a capacity measurement, the battery
is fully charged (typically 1290-1300 SG). Then it is con-
nected to a load that draws a desired current. The battery's
output current and its voltage are monitored continuously for
the specified time. A conventional test setup is shown in
Figure 10. In this case, the battery capacity was intended by
its manufacturer to be 960 ampere-hours at the 6-hour rate;
that is, the battery is designed to be capable of delivering
160 amperes for 6 hours. The final (end point) voltage is
specified as 30.6 volts (1.7 volts per cell). The resistance of
the load in our hypothetical test setup is adjustable from
0.23 ohms to 0.19 ohms.
At the start of the test, the resistance is set to 0.23 ohms
(160 amperes at 36.4 volts). As soon as the battery delivers
some of its charge, its output voltage begins to fall. To keep
the load current at 160 amperes, the load resistance must
therefore be reduced slightly. This adjustment of the load
*ln practice, the daily measure of specific gravity is made at the same point in
the battery's operating sequence (for example, at the end of each shift). In this
case, approximately the same conditions will have been reached when the
measurement is made and the results will therefore be fairly consistent. Such
measurements will, however, be offset from the true value of specific gravity by
some unknown and uncompensated amount, which can be determined by let-
ting the battery stabilize and remeasuring the specific gravity.
29
1145
Figure 9: Time Required for SG to Stabilize During Discharge
Rest Interuals
1140
1135
1130
53 1125
5 10
Time After Load Removed in Minutes
DISCHARGE RATE APPROXIMATELY 6 HOURS
TEMPERATURE CONSTANT
500 1000
5000 10.000
Figure 10: A Conventional Test Setup for Determining
Battery Capacity
AMMETER
CONSTANT 160 AMP
BATTERY 36V 960 A-H
30
resistance is continued until the battery output voltage
reaches 1.7 volts per cell (load resistance of 0.19 ohms at
160 amperes). For this battery of 18 cells the end point
voltage is 18 x 1.7 or 30.6 volt at 100% discharged.
The end point voltage signifies, by general agreement,
the practical, 100% discharge of the cell.* The length of
time it takes for this end point voltage to be reached is the
"hours" part of the "ampere-hour" rating; the constant cur-
rent, of course, is the "amperes" part.
In the U.S., traction batteries are usually specified at the
6-hour discharge rate. In other countries, a 5-hour rate is
common. The rate is the constant current drain that depletes
the battery's charge so that at the end of that many hours,
the end point voltage across the load is only 1.7 volts per
cell. For situations in which other discharge rates apply,
manufacturers may specify other end point voltages ... some
ranging as low as 1.2fvolts at very high discharge rates or as
high as 1.85 volt at very low discharge rates. A typical set of
end point voltages is shown in Figure 1 1. In the U.S., traction
battery data at various discharge rates is usually presented
using 1.7 volts per cell as the 100% discharge end point.
Capacity and Discharge Rate
If we assume that the capacity of a typical 960 ampere-
hour battery is unaffected by discharge rate, we would expect
it to discharge in 3 hours with a current of 320 amperes (960
Ah divided by 320 A = 3 Hrs). Actually, at a current drain of
320 amperes, the final voltage of 1.7 volts per cell is reached
after only about 2.5 hours. The capacity of the battery in
ampere-hours and the discharge rate are not linearly related.
For example, our typical battery delivered 160 amperes for 6
hours, which we call 100% capacity, but only 265 amperes
for 3 hours, 17% less than might be expected, and 350
amperes for 2 hours, 27% less than expected. The point to
keep in mind is that the heavier the continuous load on the
*For all practical purposes, a cell is discharged only to 80% of its capacity
because energy drawn from the cell after that point causes voltage to drop at a
steep and rapid rate. In the world of lead acid traction batteries and fork lift
trucks, a battery is considered discharged at 80%, while at 100% discharge it
is well into the area of deep discharge. It would seem prudent to simply term
the 80% level as 100%, but it is not the province of this book to alter any
such widely used convention.
t For some street electric vehicle applications, voltages as low as 1.0 have
been specified.
31
Figure 1 I: Typical End Point Voltage as a Function of
Discharge Rate: (Valid when manufacturer rates
battery with a current-dependent end point voltage,
[from Manufacturers' data].)
1.85
1.75
£ 1.55
g.
CT1.45
1
5 1.35
§1.25
i 1.15
200A
r~(5h-RATE)
mnnah
-i c
: won
L^-1.675
VPC
1 .u
T —
300A
1 .57 VPC
(3h-RATE)
qnnAti
(1
660A \
i-RATE)
560Ah
10
50 100
500 1000
5000 10,000
Figure 12: How Battery Capacity Varies with Discharge Rate
.1 .2
Current Relative to 6-Hour Rate
6.0 8.0 10.0
32
battery, the less capacity it has. Figure 12 shows the manner
in which discharge rate affects the capacities of two similarly
rated batteries from two different manufacturers.
Capacity and Temperature of Electrolyte
Another important factor that affects battery capacity is
electrolyte temperature. Generally speaking, the higher the
temperature the more rapidly any chemical action will pro-
ceed. The speed with which the acid combines with the plate
materials is much higher when the electrolyte is hot. Con-
versely, when the electrolyte is cold, the reactions move
slower.
At high temperatures, the faster chemical action at the
plates permits more material to take part in the chemical
reactions, which is roughly equivalent to having more
material available to react. Since battery capacity ultimately
depends on the amount of material available to react, in-
creasing the temperature of the cell increases its capacity.*
This effect is so pronounced that at the freezing point of
water, capacity at the 5-hour rate is only 65% of capacity at
80 °F. (See Figure 13.) For this reason, any specification of
battery capacity must state the temperature at which the
specification applies.
State-of-Charge Measurements
The constant-current method outlined earlier (Figure 10)
is the way in which batteries are evaluated at the factory to
produce the specifications by which users select the correct
battery for any application. But once the battery has been
selected and is installed on the truck, the user is interested in
"state-of-charge" at any moment, as well as rated capacity.
The standard capacity-measuring test is no help here because
*lt is generally agreed in the battery industry that continuously high
temperature can be related to grid deterioration of the plates. Considering 80 °F
as a normal temperature, for each 15°F above normal, Industry experts say
that battery life will be reduced by half. A typical battery discharged at normal
rates to 80% DOD at about 80 °F (25 °C) will show a raise of electrolyte
temperature of about 12°F. To return the battery to normal, a cooling period of
up to 12 hours may be necessary. Thus, manufacturers caution against using
batteries two cycles per day. This does not allow time for cooling and results in
reduced battery life.
33
currents are constantly changing. Four techniques are used
to measure state-of-charge:
• Specific gravity measurements
• Open circuit voltage measurements
• Measurement of battery voltage under load
• Ampere-hour measurements
Specific Gravity and Open Circuit Voltage
The open circuit voltage of a cell is a precise indicator of
specific gravity when a cell is fully stabilized. And as such,
the open circuit voltage is a precise measure of state-of-
charge. Because open circuit voltage is determined solely by
the concentrations of acid at the plates, it will not agree with
specific gravity readings unless the acid is uniform
everywhere in the cell. Then, measuring the open circuit
voltage after stabilization is equivalent to measuring the
specific gravity. This relationship is shown in Figure 14. The
34
time required for stabilization can be hours, depending on
the depth and duration of discharge and is different for cells
of different design. Cinder laboratory conditions, Figure 14 is
a valuable relationship; in practical applications, however, it
is ambiguous at best. The unstabilized open circuit voltage
will always read higher than at the equivalent point in Figure
14 if the cell has just been taken off the charger. Conversely,
the unstabilized open circuit voltage will always be lower
than at the equivalent point in Figure 14 if the cell has
recently been discharged.
Figure 15 shows open circuit voltage of a typical cell
measured at various times after disconnecting the load. In
this test, the open circuit voltage rose rapidly but did not
reach its stable value of 1.982 volts until more than 100
hours had elapsed. The peak of 1.990 volts reached after
some 6 hours was not sustained.
Voltage under Load
Under test conditions like those shown in Figure 10, we
can examine the way voltage under load is related to battery
capacity. For example, let's assume that we are testing a
traction battery with a capacity of 1050 Ah at the 6 hour rate.
At a moderate load of 200 amperes, we find that the
voltage stays constant (within about 7%) for nearly 4 hours
(actually 3.96 hours, as shown in Figure 16). Up to this point
the battery has delivered 792 Ah, or 80% of its capacity.
If we repeat the test, but draw 400 amperes, the nominal
voltage to 80% discharge holds constant to within about 8%,
but for only about 1.6 hours (1.57 hours as shown in Figure
16). Up to this point the battery would only deliver 628 Ah,
80% of its capacity.
In either case, when the battery reaches 80% discharge,
its voltage under load begins to fall rapidly, as is shown in
Figure 16, and the fall-off rate gets steeper and steeper as
the 100% discharge point is approached.
From Figure 16 you can see that the voltage under load
— when measured at a constant current — is highly predict-
able. Any change in voltage is determined by the number of
ampere-hours drawn from the battery. Thus, the change in
voltage under load is a measure of the charge withdrawn
and, therefore, of the capacity remaining.
Of course, there are other factors to be taken into ac-
count. The first among these is that any measurement of bat-
35
Figure 14: How Stabilized Open Circuit Voltage Reflects
1.90 1.95 2.00
Stabilized Open Circuit Ceil Voltage (volts)
2.05
2.10
2.15
2.00
1.99
1 98
1.97
1.96
1.95
Figure 1 5: Variation of Open Circuit Voltages as Cell Recovers
After Load
1
| 1 ' 94
S
| 1.93
o
tz
o 1.92
.1 .5 1.0
Time After Load Removal (minutes)
10
TEMPERATU
DISCHARGE
RE CO
RATE =
*JSTANTT0±2°
= 6 HRS.
C
50 100
500 1000
5000 10.000
36
tery characteristics is highly dependent on electrolyte
temperature. The higher the temperature the greater the bat-
tery capacity, as shown in Figure 13.
The second factor is that our test measurement was
made with a constant load, which does not reflect the real
world at all. In fact, we know that interrupting or reducing
the load long enough to allow some "recovery" actually in-
creases the remaining capacity. Also an increase in the load
reduces the amount of remaining capacity.
In either case, the measured voltage under load changes
as the conditions change. If electrolyte temperature in-
creases, so does voltage under load; if the load is interrupted
and the battery "recovers," the measured voltage increases,
and so on. There is no way to tell from the measured voltage
what caused the change, but the voltage under load always
decreases as capacity is withdrawn from the battery.
Ampere-hour Measurements
An ampere-hour meter integrates current in amperes
with time in hours. Displaying ampere-hours of consumption,
it can be used to indicate state-of-charge. Given the rated
capacity of a battery, the state-of-charge can be calculated by
subtracting ampere-hours consumed from rated capacity.
This can be done by the Ah instrument and displayed directly
as state-of-charge.
BATTERY CHARGING
Introduction
A lead-acid battery can be discharged and recharged
many times. In each cycle, the charging process stores
energy in the battery in the form of potentially reactive com-
pounds of sulfuric acid, lead and lead oxide. The discharge
process is another chemical reaction among those com-
ponents that release the stored charge in electrical form.
Since no chemical or physical process can ever be 100% effi-
cient, more energy is always used to charge the battery than
can be recovered from it. Thus, determining the optimum
conditions for battery charging grows in importance as the
cost of energy increases.
How Energy Is Stored in the Cell
Forcing a direct current into the cell in the reverse direc-
tion replaces energy drawn from the cell during discharge.
The effect on the electrolyte and the plates during this charg-
ing process is essentially the reverse of the discharge pro-
cess. Lead sulfate at the plates and the water in the elec-
trolyte are broken down into metallic lead, lead dioxide,
hydrogen and sulfate ions. This re-creation of plate materials
and sulfuric acid restores the original chemical conditions in-
cluding, in time, the original specific gravity.
The amount of energy it takes to re-create the original
specific gravity is, of course, at least the same as the energy
produced by the chemical reactions during discharge. This
energy is supplied by the charger in the same form that it
was removed from the battery: as volts and ampere-hours (or
kilowatt-hours). Thus, if the battery produced 36 kilowatt-
hours during discharge, it takes at least 36 kilowatt-hours to
recharge it, plus additional kilowatt-hours to make up for
losses in the energy-transfer processes.
During the first few hours that an 80% discharged bat-
tery is on the charger, the charging current is relatively high.
For example, in the first four hours of charging, about 70%
of the ampere-hours previously withdrawn from the battery
has been restored. (See Figure 17.) For the next three hours,
as battery voltage approaches the charging voltage, the
charging current through the electrolyte gradually decreases,
so that from the end of the fourth hour until the end of the
seventh, the state-of -charge increases by about 30%.
At this point, the number of ampere-hours returned to
the battery is about the same as the number withdrawn, but
the battery will still accept additional ampere-hours up to
about 105% of the number withdrawn. Beyond about 105%
38
(the nominal value for a "strong" battery) virtually all
ampere-hours supplied to the battery are consumed in elec-
trolysis and in heating the electrolyte. However, up to about
this point the added ampere-hours serve mainly to make up
for internal "coulombic" inefficiences.
For the charge cycle as for the discharge cycle, stabiliz-
ed specific gravity is a measure of the state-of-charge. Also,
as during discharge, specific gravity does not respond in-
stantly throughout the electrolyte. Instead, the specific gravi-
ty is highest at the plates, where sulfate ions are released and
the greatest number of them are concentrated. Farther from
the plates, specific gravity remains lower until the freed
sulfate ions have diffused evenly throughout the electrolyte.
Specific gravity, therefore, lags well behind the state-of-
charge of the battery, as shown in Figure 18. The maximum
specific gravity lag is considerably greater in the charging
process than in discharging. Starting at approximately 1140
SG (for a typical 80% discharged cell), after an hour on
charge, the specific gravity rises 4 "points," only 3% of the
total rise of 150 points. But nearly 20% of the ampere-hours
have been returned to the battery in that same hour.
By the end of the third hour, specific gravity has risen
only a total of 32 points, to 1 172 SG, or 21 % of the total
rise, yet the returned charge is now about 50%. During hours
4, 5 and 6, specific gravity begins to catch up and, at the end
of the sixth hour, specific gravity is 1278 SG, or 92% of its
final value, compared to a returned charge of 95%.
Battery Chargers and Charging
The basic types of battery chargers available today are
motor generator, ferroresonant and pulsed. Use of the correct
charger is an important factor in maximizing the overall
efficiency of the battery system. Used correctly, under proper
conditions, a modern battery charger will routinely provide
overall efficiencies on the order of 85% with a battery of
18-24 cells; 80% with 12 cells and 75% with a 6-cell battery.
Four methods exist to control the DC current and
voltage supplied to a battery in the charging process: two-
rate; voltage detect and time; taper; and pulsed.
In the two-rate method, charging begins at a high rate
that is dropped to a much lower rate after 80-85% of the
ampere-hours have been returned to the battery. This lower
39
Figure 17. How Ampere-Hours Are Returned to the Battery
During an 8-Hour Charge
110
01 2345678
Time (hours)
40
rate then tapers to a finish rate. The rate-change point coin-
cides with the electrolyte's gassing voltage, at which bub-
bling of hydrogen occurs. A voltage sensor/relay is commonly
used to trigger the rate change.
A variation of the two-rate method is the voltage detect
and time method in which the gassing voltage triggers a
timer which turns off the charger in a specified time after a
finishing charge period.
In the taper method, the voltage starts at a high rate and
steadily tapers downward as cell voltages rise to their
charged levels.
The pulsed method involves supplying a burst of DC un-
til a maximum voltage level is reached, at which time the
supply is cut off. As the voltage decays and hits a minimum
level, the supply is restored and so on, back and forth.
Ferroresonant Charger
Ferroresonant chargers are widely used in the U.S.A. to
charge traction batteries. The ferroresonant charger is usual-
ly a fully automatic unit that produces a charge current that
tapers steeply from a large initial value to the finish rate. A
typical ferroresonant charger produces a current-voltage pat-
tern like the one shown in Figure 19.
The internal voltage of the ferroresonant charger is
essentially constant throughout the charge period, usually 8
hours. The output current, however, is limited by the battery
voltage. At the beginning of the charge period, the battery
voltage is considerably lower than the charging voltage and
the maximum charging current flows. (This maximum current
is usually set at from 16-26 amperes per 100 Ah of rated bat-
tery capacity.) As the battery is recharged, its voltage in-
creases, gradually reducing the charging current to the finish
rate of 2 to 5 amperes (7 for a battery near the end of its life)
per 100 Ah of battery capacity.
Pulsed Chargers
Another type of charger, in wide use for traction bat-
teries in Europe, operates on a different principle: pulsating
direct current. In this case, the charger is periodically
isolated from the battery terminals and battery open circuit
voltage is automatically measured. If open circuit voltage is
above a preset limit, the charger remains isolated; when open
circuit voltage decays below that limit (as it always must),
41
42
the charger is reconnected for another period of equal dura-
tion. Figure 20 shows this procedure.
When the battery's state-of-charge is very low, charging
current is connected almost 100% of the time. This is
because the open circuit voltage is below the preset level or
rapidly decays to it. However, as the battery's state-of-charge
increases, it takes longer and longer for the open circuit
voltage to decay to the preset limit.
The open circuit voltage, charging current and the pulse
period duration are chosen so that when the battery is fully
charged, the time for the open circuit voltage to decay is ex-
actly the same as the pulse duration. When the charger con-
trols sense this conditon, the charger is automatically switch-
ed over to the finish rate current, in which short charging
pulses are delivered periodically to the battery to maintain it
at full charge.
Maximum Charge Rate
The maximum charge rate is set by the maximum
allowable temperature rise in the battery's electrolyte and the
requirement not to produce excessive gassing. A lead acid
battery that has been normally discharged can absorb elec-
trical energy very rapidly without overheating or excessive
gassing. A practical temperature limit that is widely accepted
is that the electrolyte should not rise above 46.1 °C (115°F)
with a starting electrolyte temperature of 29.4 °C (85 °F).
In the case of the battery that has been fully discharged,
the charging current can safely start out as high as 1 ampere
for every ampere-hour of battery capacity.
Studies have shown that if the charging rate in amperes
is kept below a value equal to the number of ampere-hours
lacking full charge, excessive temperatures and gassing will
not occur. This is known as the "Ampere-hour Law". For in-
stance, if 200 ampere-hours have been discharged, the charg-
ing rate may be anything less than 200 amperes, but must be
reduced progressively so that the charging current in
amperes is always less than the number of ampere-hours the
battery lacks to be at 100% charge.
As a practical matter, the discharge state of a battery is
not known; thus, chargers must utilize techniques which pro-
vide less than optimum charging rates.
The final charging voltage is limited by chemical con-
siderations and temperature. The safe limit for the lead acid
43
traction battery commonly used with fork lift trucks is
generally agreed to be between about 2.40 and 2.55 volts per
cell when charged at 25 °C ambient temperature.
Finish Rate
The most common finish rate is approximately 5
amperes per 100 Ah of rated capacity, a rate low enough to
avoid severe overcharging but high enough to complete the
charging process in the eight hours normally available.
Equalizing Charges
By maintaining the finish rate for an extended period (up
to 6 hours), a battery with cells at slightly varying voltages
and/or depths of discharge can be equalized. The continued
input of charge (overcharging) to the battery serves to "boil"
off water in those cells of higher voltage and/or depths of
discharge. Upon completion of the process, levels must be
checked and water added as required to depleted cells. New
batteries, referred to as low maintenance systems, may not
permit adding of water and therefore are not designed for
equalizing charges.
Terminating the Charge
Overcharging can materially shorten the life of a battery,
and no amount of overcharging can increase battery capacity
beyond its rated value. There are several "rules of thumb"
that are followed in deciding when to end the charge:
• When the charge is complete, the voltage levels off and
there is no further increase
• Charge current readings level off at the finish rate
• The battery gasses freely
• The specific gravity reaches a stable value.
Gassing
Hydrogen bubbles are produced at the negative plates
and oxygen at the positive plates during charging. After the
battery reaches full charge almost all added energy goes into
this gassing. The gassing process begins in the range of 2.30
to 2.38 volts per cell, depending on cell chemistry and con-
struction. After full charge, gassing releases about 1 cubic
foot of hydrogen per cell for each 63 ampere-hours supplied.
Since a 4% concentration of hydrogen in air is explosive,
ventilation of battery rooms is required for safety.
44
Energy Efficiency in the Charging Process
Nothing is free, least of all energy. Since the charging
process can never be 100% efficient, we must be careful
about how energy is used in this process.
The charging process converts energy supplied by the
local utility to kilowatt-hours stored in the battery which is
subsequently available for transfer to a load ... in our use,
an electric fork lift truck. Two major components are involv-
ed in this process: the battery itself and the charger. The
charger interfaces with the power line on one side and with
the battery on the other. The battery, in turn, interfaces with
the charger on its input side and with the truck on its output
side.
How the Charger Affects Energy Efficiency
Energy is consumed in the charging process. While most
of the charging energy goes into restoring the original
chemical conditions in the cell, some is lost in the battery,
and some is lost in the charger, mainly as heat.
We define the efficiency of the charger as the efficiency
with which power line energy is supplied to the battery in
usable form. This efficiency varies not only from type to type
and from manufacturer to manufacturer, but also may vary
from unit to unit. Let's explore this with measurements in a
"typical" case (charging a 36V, 1200 ampere-hour battery).
• During the typical 8-hour charging period, the charger
supplies energy to the battery at a rate that depends
on the battery's state-of-charge at any instant. This ac-
cumulation of energy is shown in Figure 21 as curve C.
Note that this cumulative curve rises steeply and then
gradually becomes flatter until at the end of 8 hours it
is almost completely horizontal.
• The charger draws energy from the power line to pro-
vide the battery operating energy. This energy is
shown as curve L in Figure 21. The shape of this
curve is similar to that of the curve C, but L is
always higher than C because the charger takes more
energy from the line than it delivers to the battery.
• Working from these two curves, it is possible to deter-
mine the actual charger efficiency up to any time on
charge. All we need to do is measure the heights of
the two curves at the desired time mark, divide C by L,
and multiply by 100. The overall charge efficiency E,
45
is determined by the values at the end of the 8-hour
period.
• In a typical case, the charger draws approximately 50
kilowatt-hours from the line and delivers about 42 of
them to the battery, when recharging our typical bat-
tery from 80% discharge. Thus, the charger is about
84% efficient over a standard 8-hour charge period
when recharging an 80% discharged battery.
How the Battery Affects Energy Efficiency
The battery accepts only part of the energy supplied by
the charger; furthermore, it also delivers only part of that
energy to the load. The efficiency with which the battery
releases the energy supplied it by the charger can be
demonstrated in a manner similar to that used to determine
charger efficiency.
• During the 8-hour charge period, our typical battery
accepts 42 kilowatt-hours of energy from the charger,
as shown in Figure 21.
• To understand clearly the battery's efficiency as part of
the total efficiency of the electric truck system, it must
be remembered that the amperes are being delivered
to the battery at the charger voltage which, for a
typical 36 volt battery, might be an average of 40-42
Figure 2 1 . How the Charger Affects Efficiency
Time (hours)
46
volts, while the battery discharge is at an average of
33-35 volts. Even if the total ampere-hours of charge
and discharge were the same (and normally we charge
an additional 5%) the kilowatt-hours (which is the total
energy) would vary by the difference of the average
voltage during charge and the average voltage during
discharge. Consequently the battery efficiency is much
less than may be thought if one only considers the
ampere-hours charged and discharged.
• The overall battery efficiency is determined by com-
paring the 32 kilowatt-hours delivered from the bat-
tery* with the 42 kilowatt-hours delivered to it:
32 + 42 x 100 = 76%.
Overall System Efficiency
The overall system efficiency is the efficiency with which
the power line energy (50 kilowatt-hours) is converted to
energy delivered to the load (32 kilowatt-hours): 32 -h 50 x
100 = 64%. This figure duplicates the overall system effi-
ciency calculated from the two factors, charger efficiency and
battery efficiency: 84% x 76% = 64%.
How Depth of Discharge Affects System Efficiency
Efficiency is affected by the depth of discharge of a bat-
tery when it's placed on charge. Any battery that is less than
80% discharged forces the charger to become a waster of
energy. Significant amounts of power line energy are con-
verted into small amounts of useful battery charge. In the
case of a battery that has been essentially idle during the
previous shift (less than 10% discharge), blindly placing it on
charge for eight hours will waste nearly half of the energy
delivered to the battery.
The three graphs of Figure 22 show, in an 8-hour charge
period, the hour-by-hour cumulative line energy input (L) to
the charger and the charger energy input to the battery (C)
for each of three cases: a battery placed on charge at 80%
discharged; another at 40% and a third at 20% discharged.
The dramatic effect on charger and battery efficiency is
obvious when the data from Figure 22 are presented in
Table 1 and Table 2.
* Battery output (kWh) equals average voltage times Ah delivered to 80%
DOD.
47
48
Table 1: Effect of Battery State-of -Charge on
Charger Efficiency in an 8-Hour Charge Period.
% Discharge
at start of
Charge
Total Line
Energy
(kWh)
Total Energy
to Battery
(kWh)
Charger
Efficiency
(%)
80
40
50
35
42
27
84
77
20
24
17
71
Table 2: Effect of Battery State-of-Charge on
Battery Efficiency in an 8-Hour Charge Period
%
Discharge
at Start of
Charge
Energy to Battery
kWh Ah
Energy
kWh
to Load
Ah
Efficiency
%
80
42
1030
32
960
76
40
27
652
17
480
63
20
17
424
8.6
120
50
The Combined Effect
In Table 3, the overall system efficiency, the product of
charger and battery efficiencies, is shown for each of the
above three cases.
Table 3: Ouerall System Efficiency When Charging
for an 8-Hour Charge Period
% Discharge
at Start of
Charge
Charger
Efficiency
(%)
Charge/
Discharge
Efficiency
(%)
Overall
Efficiency
(%)
80
84
76
64
40
77
63
49
20
71
50
36
49
From this examination, it becomes quite clear that if a
fixed, 8-hour charging routine is to be followed, the overall
efficiency with which energy is used is determined mostly by
the state-of -charge of the battery when it goes to the charger
as shown in Figure 23.
Figure 23. Ouerall Energy Efficiency in Charging a BaLtery
Discharged to Various Depths (8-Hour Charge)
10 20 30 40 50 60 70 80
Percent of Discharge @ Start of Charge
OPTIMIZING ENERGY OSAGE
Beginning with Conclusions
After only a brief study of Figure 23, it appears that the
optimum battery selection is one that results in 80%
discharge by the end of a pre-established work program. Said
another way, the battery should have a rated capacity of
125% of the energy it is expected to deliver during
discharge. It makes no difference whether the intention is to
charge the battery at the end of each workshift or whether
the battery is to be charged at the completion of a particular
task. The conclusion remains the same: the battery should be
placed on charge when it has been 80% discharged.
Selecting the Correct Battery Capacity
The selection of battery capacity for a given truck and
application becomes more significant with each increase in
the capital cost of batteries and charging stations as well as
with each increase in the cost of energy.
Each industrial truck application presents a separate
battery-selection problem with more involved than the size or
lifting ability of the truck. Since trucks may be used for dif-
ferent purposes, each application presents a specific work
profile that can be thought of as a series of rapidly varying
current drains.
In any application, this battery drain varies from instant
to instant during the entire time the truck is in use. As
shown in Figure 24, every task the truck performs represents
a different battery drain . . . that can, for example, range
from 5 amperes, while steering, to 1000 amperes, motor in-
rush current on the pump motor.
Figure 25 shows a hypothetical operation for a typical
truck — lifting loads and transferring them to nearby locations.
Our typical truck takes 6 separate steps to complete these
operations, and each step drains energy from the battery.
Of course, there is no simple way to calculate in advance
exactly how much energy will be needed to perform any
single step of an operation, let alone a whole day's work. The
only real solution is to measure the number of ampere-hours
used by the truck when it performs each step. This is done
with an ampere-hour meter installed on the truck.*
With the Power Prover Ampere-Hour Meter* installed
on the test truck, the driver performs the stipulated sequence
of steps and the meter readings show the total ampere-hours
The Curtis Model 1020 Power Prouer® Ampere-Hour Meter is an ac-
curate, easy-to-lnstall ampere-hour meter widely used for this purpose.
51
and the amperes used in the operation. The number used in
each step of the operation can be monitored by recording the
meter readings while the truck operates.
A procedure of this kind that includes representative
operations performed by the truck provides a simple and ac-
curate basis for selecting battery capacity ... or for verifying
assumptions about ampere-hour requirements.
Ways to Measure State-ol '-Charge on the Fork Lift Truck
The reason for measuring the state-of-charge as the bat-
tery is in use is twofold: to protect the battery from deeply
discharging and, thereby, internal damage; and to protect the
truck's electrical components from the negative effects of
low voltages, a consequence of deeply discharged batteries.
At average discharge rates of 8 hours or less, a measurement
which accounts for the remaining capacity as a function of
discharge rate can serve to protect both the battery and the
truck.
An ampere-hour meter provides useful information that
greatly simplifies specifying the correct battery and measures
ampere-hours used, but not the rate at which it is used. And
the rate at which it is used is crucial in measuring the state-
of-charge because battery capacity is different at different
rates of discharge.
Aside from ampere-hour metering, three basic measures
have been used in industry to determine battery state-of-
charge: specific gravity, open circuit voltage, voltage under
load.
Specific Gravity: A Static Measure
Not only are specific gravity measurements not conven-
ient to make during the work period, but their value is
limited because it takes time for the specific gravity to
stabilize after the battery load is disconnected. Although a
convenient measure of overall battery condition, specific
gravity measurements give no valid indication of the
discharge history that produced the reading. Any given
reading of stabilized specific gravity can either be the
result of heavy discharge for a short period or of prolonged
discharge at a very light load. This effect is shown in Figure
26, in which the specific gravity and the open circuit are plot-
ted against % of discharge for currents from 25 amperes to
800 amperes for a 1200 ampere-hour battery rated at 6
hours. Any specific gravity line, for example, 1 165 SG,
52
Figure 24: Relative Current Drain for Typical Industrial Trucks
1500
1000
500
too
10
POWER
STEERING
PUMP MOTOR
DRIVE MOTOR
LIFT PUMP
MOTOR
LIFT PUMP
MOTOR + AUX
PUMP MOTOR
(SIDE SHIF OR
SWING REACH)
SWING REACH
DRIVE, LIFT
AND TURN
INRUSH; VERY SHORT TIME
(LESS THAN 1 SECOND)
SHORT TIME INTERVAL
(1TO 15 SECONDS)
LONG TIME INTERVAL
(15 SECONDS AND GREATER)
INACTIVE REGION OF LIFT PUMP AND
DRIVE MOTOR BUT ACTIVE REGION
OF POWER STEERING PUMP
53
Figure 25: A Hypothetical Industrial Truck Operation
NO LOAD, START & RUN
10 SEC. @ 125 AMPERES (.347 AH)
■1^3 — START
.347 AH
LIFT LOAD 15',
12 SEC. @ 230 AMPERES
(.767 AH)
, 00 ,30
CURTIS I
POWER P R O V E R
1.114 AH
PUT LOAD IN PLACE.
1 SEC. @ 150 AMPERES (.042 AH)
BACK OUT,
3 SEC. m 150 AMPERES (.125 AH)
TURN, 1 SEC. to 100 AMPERES (.027 AH)
RUN. 11 SEC. 200 AMPERES(.611 AH)
CURTIS
1.919 AH
TURN, 1 SEC. @ 100 AMPERES (.027 AH)
RUN, 5 SEC. to 125 AMPERES (.174 AH)
TURN
i'Hs"'""'Z
CURTIS
2.1 AH
RUN, 30 SEC.
(1.25 AH)
LIFT LOAD V,
1 SEC. to 230 AMPERES
(.064 AH)
LIFT LOAD
eurtisuM
3.4 AH
LIFT LOAD 10',
8 SEC. <§ 230 AMPERES (.511 AH)
SIDE SHIFT
2 SEC. to 250 AMPERES (.139 AH)
FORWARD/STACKLOAD
2 SEC. to 150 AMPERES (.083 AH)
PLACE LOAD
4DO 600
° AMPERES D£
EURTISOS
4.1 AH
54
intersects a number of discharge lines. 1165 SG, in particular,
corresponds to 80% discharge at the 6-hour rate (200
amperes). However, if actual operation is at 600 amperes, the
truck motor will have been subjected to repeated, excessively
low voltages (less than 1.7 volts per cell) for a significant
amount of time.
Of course, if the truck has been used in essentially the
same manner during each workshift, then the specific gravity
(unstabilized) measured at the end of the shift will be pretty
much the same from day-to-day. If there were any sudden
change in the reading, it would be informative, but would not
reveal anything about the state-of-charge except in a general
way.
Open Circuit Voltage: Another Static Measure
Since specific gravity and open circuit voltage are direct-
ly related, a similar line of reasoning shows that unstabilized
open circuit voltage is also not a valid measure of battery
condition. In Figure 26, any of the constant voltage lines can
represent any number of battery discharge histories. Hence
the open circuit voltage is not a useful measure of state-of-
charge during operations. Note that battery manufacturers
always determine capacity by measuring voltage while the
load is connected. Disconnecting the load and immediately
measuring open circuit voltage reveals nothing about the
state-of-charge.
Voltage Under Load: A Dynamic Measure
When the truck operates, it presents varying electrical
loads to the battery. As soon as the battery is "loaded" the
open circuit voltage drops abruptly to the initial value of
voltage under load. As long as the load current stays con-
stant, the voltage under load slowly decreases as the battery
discharges. If we keep track of the average voltage under
load, we can tell how fast the battery is being discharged at
any instant. Voltage under load measurements account for
the rate at which a battery is discharged, whereas specific
gravity and open circuit voltage reveal nothing about rate.
Figures 27 and 28 illustrate aspects of voltage under
load. In Figure 27, voltage under load is shown as a measure
of state-of-charge for 5 constant currents from 0-100%
discharged. In Figure 28, varying current rates, based on the
work procedure illustrated in Figure 24, are shown as a
magnified micro-section tracked by an instrument with ap-
propriate electronic computing circuitry.
55
56
Since time always moves from left to right in Figure 27,
the net effect of many different loads is to move the
measurement point steadily toward the right, always along
one load line or another. On the fork lift truck, there are
many more load line variations. Every interval, during which
the measured voltage follows a given load line, contributes a
particular percentage of discharge.
One way to accomplish voltage averaging in an instru-
ment is to continuously compare the varying battery voltage
with a reference voltage which changes as a function of bat-
tery state-of-charge. Whenever the battery voltage is less
than the reference voltage, the time below the reference
voltage is measured and stored in the instrument's memory.
The output of the memory sets the value of the reference
voltage and represents the state-of-charge of the battery. It is
displayed on a meter ("fuel" gage) located right on the truck
in the driver's view.
Figure 28 shows how the reference moves in response
to the battery voltage and how the "fuel" gage displays the
state-of-charge.
State-of-Charge-Based Charging Can Save Energy
Some plant managers feel that trucks must be available
without interruption throughout a workshift. Thus it becomes
necessary to provide each fork lift truck with a fully charged
battery at the start of each shift and to return the battery for
charging at shift end. In any two- or three-shift operation, this
practice normally requires at least twice as many batteries as
trucks plus a charging station for each truck. For substantial
fleets this means large capital investments, considerable
floor space, significant personnel requirements, and addi-
tional energy costs.
This practice of workshift-based charging is especially
prevalent in manufacturing plants where stopping the
assembly line for any reason cannot be tolerated. At one
automotive manufacturing site, at which a stalled line would
be excessively costly, management has initiated a program in
which fully charged batteries are installed on 150 trucks in
40 minutes at the end of every workshift.
For other than such above noted critical requirements,
with the use of a reliable and economical means of monitor-
ing battery state-of-charge on the truck, another approach to
charge scheduling has emerged. It is no longer necessary to
provide each truck with a fully charged battery at the start of
58
59
each shift and to work the battery through to shift end.
Rather, if there is still significant charge left in the battery at
the end of the shift, the battery can be left on the truck and
worked until the need for charging is indicated. Then, and
only then, the truck returns for a freshly charged battery. Its
discharged battery is then placed on charge and, 8 hours
later, is ready for use again.
As shown earlier, if an 8 -hour charge period is used
(as is generally the case), the optimum discharge point is
80%. Figure 29 shows how rapidly the energy requirements
rise when batteries are charged for 8 hours after having been
discharged less than 80%. Working each battery to the 80%
discharge point before returning it for charging permits the
most effective use of energy in the system and provides
significant savings in energy costs.
Since each of the batteries reaches the 80% discharge
point at a time that depends on the way it is worked, most
will work longer than one 8 -hour shift. Since a new bat-
tery isn't required for each truck at the end of every shift, it
isn't necessary to have at least one replacement battery per
truck, nor is it necessary to have at least one charger for
each truck. Fewer batteries and chargers mean less capital
investment, less space used for charging, and fewer people to
do the work.
Figure 29: How Energy Requirements are Affected by
Depth of Discharge
1 80 i 1 1 1 1 1 , 1 1 1 1 1
5 10 15 20 25 30 35 40 45 50 55 50 65 70 75 80
Percent of Discharge (« Start of Charge
60
Further, when a battery is placed on charge it draws
maximum current from the line. Placing all of the fleet's bat-
teries on charge at one time, therefore, creates a large de-
mand, which is reflected in the cost of energy in the form of
higher utility peak demand charges. However, starting bat-
teries on charge at different times throughout the entire shift
reduces this demand and, therefore, the cost of energy.
How A "Typical" Fleet Can Save Energy
To dramatize the energy saved by charging batteries
under optimum conditions, we have prepared data for a
hypothetical 20-truck fleet operating for a 5-day week of 2
shifts per day. The data cover one month of 20 working days
and show the following:
• That it is possible to significantly reduce the amount
of energy required to operate the fleet
• That by appropriate fleet management it is possible to
greatly reduce the magnitude of peak power demand
• That this modified fleet operation holds the promise of
reducing the capital and labor costs associated with in-
dustrial trucks.
Our data and calculations are not derived from operating
a real fleet. They do, however, suggest how to reduce the
cost of operation of any real fleet of trucks.
In this fleet, all trucks are equipped with the same bat-
tery type: a 36 volt, 1200 ampere-hour unit. In the original
operating mode, each truck started each shift with a fully
charged battery. We have divided the batteries into six
classes (A-F) based on their average state-of-charge at the
end of a typical shift. The classes, the number of batteries in
each class, and the discharge data are listed in Section I of
Table 4.
Section 2 of Table 4 shows the energy used by each bat-
tery (kilowatt-hours are calculated at the average output
61
Table 4:
Energy Usage Data for a Hypothetical
20-Truck, 2 -Shift Fleet
SECTION 1
Fleet Composition
SECTION 2
Battery Data
SECTION 3
Fleet/Shift Data
Class
&
Number
of
Trucks
Average
%
D.O.D.
at Shift
End
Output to
Load
Ah kWh
Power
Line
Input to
Charger
(kWh)
Output to
Load
Ah kWh
Power
Line
Input to
Charger
(kWh)
Overall
Efficiency
(%)
A-l
80.0
960 33.6
50.2
960 33.6
50.2
67
B-2
69.3
832 29.2
46.6
1664 58.4
93.2
63
C-3
58.7
704 24.9
42.8
2112 74.7
128.4
58
D-8
48.0
576 20.5
38.5
4608 164.0
308.0
53
E-4
42.7
512 18.2
36.0
2048 72.8
144.0
51
F-2
37.3
448 16.0
33.3
896 32.0
66.6
48
Fleet Totals Per Shift
12,288 435.5
790.4
55
Per Month (40 shifts) 491,520 17,420
31,616
55
voltage). Section 3 then totals these per-battery figures by
battery class. The overall efficiency is calculated by dividing
the output energy (kWh) by the AC input energy and multiply-
ing by 100. Since there are 40 shifts in our 20-day month,
the totals are multiplied by 40 to obtain an estimate of
energy used by the entire fleet over a full month of opera-
tion. A very substantial 31.6 megawatt-hours is used . . . but
at only 55% overall efficiency to produce the required 17.4
megawatt-hours of work delivered by the batteries.
By the simple expedient of returning each truck for a
fresh battery when its "fuel" gage reads between 75% and
80% discharged, we can sharply reduce this energy waste.
Since overall system efficiency is 67% for batteries that are
charged after being 80% discharged, we can reduce the
power line energy needed for our hypothetical fleet to 25.9
megawatt-hours if each battery is 80% discharged before be-
ing recharged. This means that about 5.7 megawatt-hours
can be saved each month.
62
Reducing Peak Power Demand Costs*
If the 80% discharge point for our typical battery is 960
ampere-hours, then when that battery is discharged to 832
ampere-hours, or 69.3% (as in Class B), there is still a frac-
tion of a workshift left in the battery. In fact, a Class B bat-
tery will acually last 1.15 shifts. Each of the other classes of
battery will last correspondingly longer into the second shift,
as shown in Table 5.
Table 5: Battery Workshift Capacity
Class
Ah Used
Total Ah
Total Workshift
in One Shift
Available
Capacity per Truck
per Charge (Shifts)
A
960
960
1.0
B
832
960
1.15
C
704
960
1.36
D
576
960
1.67
E
512
960
1.88
F
448
960
2.14
Table 5 shows how battery charges can be spread out
across the two workshifts because individual batteries will
reach 80% discharge at different times, depending on in-
dividual work profiles. With time, the spread will grow even
more random, so that charge starts will be evenly spread
throughout the two shifts. As shown in Figure 30, the net ef-
fect is to reduce the peak power demand below its absolute
maximum of nearly 220 kilowatts. (The maximum peak
power demand occurs when all 20 batteries are placed on
"Peak Power Demand: The use of electricity is made up of varying periods of
high and low demand. Power companies build and provide sufficient generating
capacity to meet periods of highest demand. To help defray the large capital
outlay made to build and maintain maximum generating capacity the electric
company charges customers what is referred to as a "demand charge". The
charge is levied as an added cost of electricity. Two meters are used to monitor
the comsumption of electricity by commercial and industrial customers: one to
track kWh's and one to monitor highest average kilowatts of demand. The de-
mand meter reads 15 minute intervals of which the two highest are used to
make up an average half-hour for the monthly demand charge. Thus, it
behooves a customer to limit concentration of demand for power. In the electric
fork lift truck environment, this means not putting all batteries on charge at the
same time or charging batteries at periods of low demand (nighttime).
63
charge at once.) For example, splitting the batteries into two
groups of 10 cuts the peak demand to about 198 kilowatts, a
10% reduction. And in the optimum case, in which one bat-
tery starts on charge every 24 minutes throughout the shift,
peak demand falls to 130 kilowatts, a reduction of over 40%.
Figure 31 is a more general chart that shows the effect
of different charging schedules on peak demand for any size
fleet. To use the chart, calculate the maximum peak demand
— the peak demand when 100% of the batteries are placed
on charge at the same time. Then decide how many batteries
you will be charging in each group and the interval between
groups. (The interval is equal to 8 hours multiplied by the
percent placed on charge. For example, if 5% are to be
started at once, the interval is 5% of 8 hours, or 24 minutes.)
Figure 31 shows that when 5% are placed on charge
every 24 minutes, the peak demand is only 59% of max-
imum. If your peak demand were (for example) 300 kW, the
state-of-charge-based charging procedure would reduce the
peak to about 177 kW. Table 6 shows the dollar impact of
workshift-based vs state-of-charge-based charging.
Figure 30: Reduction of Peak Power Demand with
State-of -Charge Charging Schedule
240
220
200
180
S 160
E> 140
S 120
I 100
1 2 4 5 10
Number of Batteries Starting Charge Simultaneously (Group Size)
20
64
Table 6: Impact of Workshift-Based us State-of-Charge-Based
Charging on Peak Demand Charges for a 20 Truck-Fleet
Charging Schedule
Power Demand
Number
of Batteries
at a time
Time between
Start of Charging
(Hours)
Peak
Demand
(kW)
Average Monthly
kW Demand
Charge (based on
$13.78*/kW)
20
8
221
$3050.
10
4
177
2440.
5
2
149
2055.
2
. 8
135
1860.
1
. 4
130
1790.
* $13.78 is the Summer kW Demand Charge levied by Consolidated Edison
which semes the New York Metropolitan area (7/80).
Capital and Labor Savings
Since each battery can last for at least one shift, and
most last for part of a second, it is no longer necessary to
65
have two batteries for every truck. The total of 40 batteries
previously required for our fleet of 20 trucks is reduced to
only 35, a 12% saving in capital investment. We reach this
conclusion by evaluating the number of batteries required
per shift as shown in Table 7.
Table 7: Batteries Required per Shift
Class
Shifts
Batteries
Number
Spare
Next
Per
Per Shift
of
Batteries
Highest
Battery
Trucks
Required
Whole
Number
A
1
1
1
1 =1
1
B
1.15
0.87
2
1.74 = 2
C
1.36
0.74
3
2.22 = 3
D
1.67
0.60
8
4.80 = 5
E
1.88
0.53
4
2.12 = 3
F
2.14
0.47
2
0.94 = 1
15
+20
Total Batteries Required
35
From the reduction in the required number of batteries
there follows, naturally, a reduction in the number of
chargers required, and in the number of square feet of space
devoted to charging, changing, and maintaining of batteries.
There also follows from the reduction in the number of
batteries and chargers a 30% reduction in the total number
of battery charges required during the year's operation of the
fleet.
Under the workshift-based procedure, 200 battery
charges per week supported 20 trucks, 2 shifts per day. This
amounted to 10,400 battery charges per year as shown in
Table 8.
In the state-of-charge-based procedure, made possible by
on-board monitoring of battery capacity, there are only 35
batteries (instead of 40) to support the fleet. Thus, in 52
weeks, only 6812 charges are required for the fleet, a net
reduction of 3588 charges per year.
66
Table 8: Number of Charges Required Annually
to Support Fleet
Old Procedure
New Procedure
Number
Number
Charges
Charges
per Class
Charges
per Class
per Class
per
Class
per Shift
per Week
per Shift
per Shift
Week
A
1
10
1
1
10
B
2
20
2
1.74
18
C
3
30
3
2.22
23
D
8
80
8
4.8
48
E
4
40
4
2.12
22
F
2
20
2
0.94
10
Totals
20
200
20
12.82
131
x52
x52
10,4000
6872
Every time a battery is removed from a truck and charg-
ed a certain amount of labor is involved. In addition to the
business of lifting, emplacing, and breaking and making con-
nections, there is also the labor of checking specific gravity,
topping off electrolyte level, cleaning, etc., all of which must
be done every time a battery is charged, regardless of its
state-of-charge when it is removed from the truck.
A net reduction of 3588 charges per year (more than 1.7
per workshift for our typical double-shift, 20-truck fleet) is
certainly a labor saving worth examining on its own merits.
Further, since only a small number of trucks arrive at
the charging station at any given time, owing to the fact that
their batteries seldom reach full discharge at the same in-
stant, there is a considerable reduction in waiting time as
compared to the workshift-based procedure previously
followed. Trucks (and drivers), therefore, are more
productive on the average, since less time is spent standing
idle, waiting for fresh batteries.
WEAR AND TEAR
Overworking the Battery
Overworking the battery can have a detrimental effect on
its performance and life. For example, if the truck is worked
well beyond the normal rating of its battery . . . for example,
by repeatedly lifting very large loads very fast for a long time
. . . it is possible for the battery voltage to fall below the
manufacturer's specified end point. While it is true that let-
ting the battery recover (for a time that may extend from
minutes to hours, depending on the depth of discharge) will
bring it back to a useful state-of-charge, it is also true that
repeated heavy discharges of this kind can damage the plates
by overheating, sulfation and cell (polarity) reversal.
The makers of trucks and their electrical components offer
another set of objections to overworking the battery.
Operating at lower-than-specified voltage can do irreparable
damage to relays, SCRs (Silicon Controlled Rectifier), contac-
tors, motors, etc.
Causes and Effects
Damage to a battery and/or a truck caused by deep
discharge is the result of failure to detect the 80% discharge
point of the battery and its continued use. In the case of
component failure, inadequate maintenance is often at fault.
The use of a reliable, accurate and repeatable "fuel"
gage on the truck will always prevent both battery and truck
damage because the "fuel" gage will always detect the 80%
recommended discharge limit. A properly designed "fuel"
gage with a lift lockout will actually prevent the driver from
working the truck past this limit and will force him to return
for battery charging.
To get the most out of traction batteries, every truck
should be equipped with a reliable, accurate, repeatable
"fuel" gage and controller; operating procedures should be
arranged so that batteries are placed on charge only when
80% discharged; chargers should be maintained in good
operating condition; and a regular routine of inspection and
preventive maintenance should be followed. To do less is to
waste energy, time, and money.
Curtis Instruments, Inc. has undertaken to produce and
make available this text as a part of its effort to more com-
pletely understand batteries and their use with electric fork
lift trucks and other industrial electric vehicles, and to share
that understanding with others related to the material
handling industry.
In the future, we hope to distribute supplements to this
book in the form of "application notes" addressing such
areas as battery manufacturers' rating systems, how they
compare and concepts of standardization of graphical
displays for test data; a universal energy units conversion
table; etc.
We welcome reader comments, additions, corrections (if
any), etc. Keeping the information accessible and flowing will
have a most definite positive impact on all our related
industries.
A
Acid-limited, 26
Accumulator, see also Battery, 12
Ampere, 15
Ampere-hour, 16
measurement for,
battery selection, 50, 51
state-of-charge, 32, 33, 36
protecting battery and truck, 51
rating, 16, 28, 30
Ampere-hour Law, 42
Ampere-hour meter, 36
Curtis 1020 Power Prover® ,50, 51
use in battery selection, 50, 51
Automotive battery, 18, Fig. 4
Battery
acid-limited, 26
ampere-hour, rating, 16
assembly, 18, 21, Figs. 5-7
automotive, 18
capacity, 16
factors affecting, 24-26, 30, 32, 50,
51, Figs. 12, 13
manufacturer's test for, 28, 30,
Fig. 10
measurement of, 34, 36, 50, 51
current flow, 14, 15, Fig. 3
damage, 32n, 67
discharge characteristics, 17, 18, Fig. 4
efficiency, 12, Fig. 2
factor in overall efficiency, 44-49,
Tables 1-3, Figs. 21-23
optimizing, 56-63
electrochemical reactions,
at plates, 22, 24, 25, 32, 43, Fig. 7
during charge/discharge, 12, 24-28,
37, 38
electrolyte, see also Electrolyte, 18,
21, 22, Fig. 7
kilowatt-hour rating, 15, 16
low maintenance systems, 43
open circuit voltage, see also Open
circuit voltage, 24
optimizing performance of, 67
optimum discharge point, see also
Optimum discharge point, 16, 50
overcharging, 43
overworking, dangers of, 67
plates, 18, 21
plate-limited, 26
rating information, 16, 22
selection, 50, 51
self-discharge, 24
state-of-charge, 26-28
during charging, 37, 38, Fig. 18
factors determining, 26-28
terminals, 21, Fig. 7
traction, 18
typical voltage, 15
c
Cell, see also Battery
nominal voltage, 15, 24
numbering and strapping, 21, Fig. 6
series connection, 15, 21, Fig. 6
Charge
electrochemical reactions at plates,
22, 24, Fig. 7
producing a current in batteries, 24-26
Charge/discharge cycle
automotive battery, 18, Fig. 4
electrochemical reactions during,
24-28, 37, 38, Fig. 7
traction battery, 18, Fig. 4
Charger, 10, 38-42
efficiency,
factor in overall efficiency, 44-49,
Tables 1-3, Figs. 21-23
rating, 12
types of, 38, 40
Charging, 37, 38, 44, Figs. 17, 18
Ampere-hour Law, 42
efficiency, 44-49, Fig. 21
equalizing charges, 43
factor in energy cost, 10, 37
final voltage, 42, 43
finish rate, 38-43
gassing voltage, 40, 43
maximum charge rate, 40, 42
methods, 38, 40
optimum point (of discharge) for, 16,
50, 56, 59, 67, Fig. 29
overcharging, 43
schedules, see Charging schedules
specific gravity lag, 38, Fig. 18
temperature, 32n, 42
termination, 43
70
Charging schedules
effect on capital, labor costs, 64-66,
Tables, 7, 8
effect on energy cost, 56, 59, 60, Fig. 6
effect on peak power demand, 56-64,
Table 6, Figs. 30, 31
state-of-charge-based. 56, 59, 60
workshift-based, 56, 63, 64
Coulomb, 15
Current, 14, 15, Fig. 3
production of, in battery, 24-26
D
Discharge
constant current, method of determin-
ing capacity, 28, 30, Fig. 10
deep, 30n
effect at plates, 26
prevention of, via state-of-charge
measurement, 51. 67
depth of discharge (DOD),
effect on system efficiency. 46-49,
Tables 1-3, Fig. 22
electrochemical reactions during, 24-26
optimum point. 16, 50, 56, 59, 60,
67, Fig. 29
rate. 28, 30
effect on battery capacity, 16, 30,
32, Fig. 12
effect on end point voltage. 28, 30,
Fig. 11
self-discharging, 24
E
80% discharge point, 16, 50, 56, 59,
60, 67, Fig. 29
Electric energy
conversion of, 10
coulomb, 15
Ohm's Law, 14
relationship to battery capacity, 16
Electric fork lift truck
battery selection for, 50
end point voltages for, 17
fleet operation, 60-66, Tables 4-8,
Figs. 30, 31
measuring operations of, 50, 51, 54,
56. Figs. 24, 25
Electrolyte, 18, 21, 22, Fig. 7
characteristics during charging, 37, 38
electrochemical activity in, 22-26
gassing voltage, 40, 43
specific gravity, 21
measurement of state-of-charge, 27,
37, 38
temperature effect on capacity, 32,
Fig. 13
effect on voltage under load. 36
limits, in setting maximum charge
rate, 42, 43
emf (Electromotive force), see Open
circuit voltage
End point voltage, 17, 28, 30
for electric fork lift truck, 17
as function of discharge rate, 28,
30, Fig. 1 1
prevention of battery, truck damage,
17, 67
Energy, 10-12
loss, factors causing, 24-26, 37, 38. 44
from power plant to work. 12, Fig. 2
savings, via state-of-charge-based
charging, 56-61, Fig. 29
storage of. in lead acid battery, 22-28.
37, Fig. 17
usage, optimizing. 50-67
waste, depth of discharge and, 46-49,
Tables 1-3
prevention of, 67
reduction of, 60-63
Energy costs
factors affecting,
battery selection, 50
charging conditions, 37
charging schedules, 56-66
peak power demand, 62-64. Table 6
work processes, 10
Energy efficiency, 12, Fig. 2
factors affecting.
battery, 45, 46
charger, 44, 45, Fig. 21
charging schedules, 56-66, Tables 4-8
overall system, 46-49, Table 3,
Fig. 22
F
Ferroresonant charger, 40. Fig. 19
Final voltage, see End point voltage
Finish rate, 38-43
Flat plate cell, 18, 21, Fig. 5
Fork lift truck, see Electric fork lift truck
71
'Fuel'gage
display of state-of -charge, 56
factor in energy waste reduction, 61
prevention of deep discharge, 67
G
Gassing, 43
chemistry of, 43
factor in charge rate and termination,
42, 43
ventilation, 43
voltage, 38, 40. 43
H
Horsepower, see also Watt, 11, Fig. 1
Hydrogen, 40, 43
Hydrogen ion, 22. 24, Fig. 7
K
Kilowatt-hour (kWh), 15, 16
L
Lead, active plate material, 18, 24-26,
37, 38, Fig. 7
Lead-acid battery, see Battery
Lead dioxide, active plate material, 18,
24-26, 37, 38, Fig. 7
Lead sulfate. Fig. 7
product of discharge process, 24-28
reactions during charging, 37, 38
Lift lockout, 67
Load, 15, 25, 26
effect on discharge, 30, 32
factor in energy loss, 25. 26
M
Manufacturer's capacity rating, 16
Manufacturer's capacity test, 28, 30,
Fig. 10
Maximum charge rate. 42, 43
Motor generator charger. 38
o
Ohm, 14, 15
Ohm's Law, 14
Open circuit voltage (emf). 24
effect of electrolyte/plate reactions on,
24-26
measured during charging, 40, 42,
Fig. 20
stability during storage. 24
a state-of-charge measurement, 32-34,
51, 54, Figs. 14, 15. 26
Optimum discharge point, 16, 50, 56,
59, 67, Fig. 29
Overcharging. 43
P
Peak power demand, 62n
affected by charging schedules, 56-64,
Table 6, Figs. 30, 31
reducing costs of. 62, 63, Table 6
Plates. 18, 21
damage to, 32n, 67
electrolyte/plate reactions, 22-28, 32,
37, 38, 43, Fig. 7
Plate-limited, 26
Power, 1 1
Power Prover* ampere-hour meter, 50,
51
Pulsed charger, 38, 40, 42, Fig. 20
R
Rating, see also Battery capacity
ampere-hour, 16
battery, information. 16, 22
capacity. 16
factors affecting, 28-32
kilowatt-hour, 15, 16
s
SG (specific gravity), 21, 26-28
effect of electrolyte/plate reactions on,
26-28, 37, 38
factor in charge termination, 43
lag, 38, Fig. 18
measurement, 32-34, 38. 51. 54,
Figs. 14, 18, 26
relationship to open circuit voltage,
33, 34, Fig. 14
to state-of-charge, 26-28, Fig. 8
stabilization of, 26-28, 33, 34, 38.
Fig. 9
72
as state-of-charge measure, 32-34, 38,
51, 54, Figs. 14, 18, 26
State-of-charge, 26-28
effect on charger, battery efficiency,
46, 48, 49, Tables 1, 2
effect on overall efficiency, 46, 48,
49, Table 3
factors affecting, 22-28
on-board monitoring of, 56, Fig, 28
relationship to specific gravity, 26-28,
Fig. 8
State-of-charge-based charging schedule,
56, 59. 60
effect on energy use, 56-63,
Tables 5-8
effect on economy, 65, 66, Table 6-8
vs workshift-based charging schedule,
63, 64, Table 6
State-of-charge measurement
ampere-hour measurement, 32, 33,
36, 50, 51
open circuit voltage measurement,
32-34, 50, 51, 54, Figs. 14, 15, 26
preventing deep discharge, 51
specific gravity measurement, 32-34,
38, 50, 51, 54, Figs. 14, 18, 26
voltage under load measurement, 32,
33, 34, 36, 50, 51, 54, 56.
Figs. 16, 27, 28
Sulfate ion, 22, 24, 37, 38, Fig. 7
Sulfuric acid, see also Electrolyte, 18,
22. Fig. 7
electrochemical reactions in, 22-27,
Fig. 7
effect of charge/discharge cycle on,
26, 37
T
Taper charging, 38, 40
Temperature
effect on battery life, 32n
effect on capacity, 32, 34, 36,
Fig. 13
limits in setting maximum charge rate,
42, 43
Traction battery, see Battery
Truck damage, 67
Tubular plate cell, 18, 21, Fig. 5
Two-rate charging, 38, 40
V
Volt, 15
Voltage
cause of drop in, 24-26
final charging, 42, 43
gassing, 40, 43
nominal battery, 15
rating information, 22
Voltage detect and time charging, 38, 40
Voltage sensor relay, 38, 40
Voltage under load
constant current measurement of, 34,
36, Fig. 16
factors affecting, 24-26, 36
relationship to battery capacity, 34, 36
state-of-charge measurement, 32, 33,
34, 36, 50, 51, 54, 56, Figs. 16,
27, 28
w
Watt, 11,15
Watt-hour, 15, 16
Work, 10, 15
Workshift-based charging schedule, 56
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