Skip to main content

Full text of "Batteries Delco UP Sand Standby Supplies OCR"

See other formats






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 

200 Kisco Avenue, Mt. Kisco, N.Y. 10549 


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 


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 

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" 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 

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 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 


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 


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. 


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 


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 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 

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 


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 


Figure 2: Energy Efficiency from Electric Generation Plant to 


If Calculated from kWh Delivered from the Power Plant, 
The Efficiency is about 23% <fi Work Done Point 






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 

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. 


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 

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- 

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. 


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. 


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 

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. 



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. 




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 


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: 








Total Number 

of cells 

Cell Type 

Capacity per 

of Plates 

Positive Plate 

Per Cell 

(5 Positive, 6 


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 


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. ~ 


Figure 7: Schematic Representation of Reactions at Negative 
and Positive Plates 




□ lead 





Pb + S0 4 " 2 f +2e" + 

+ PbS0 4 + 2e -»PbS 



Pb0 2 
PbS0 4 


NET REACTION: Pb + PbO 2 + 2H 2 S0 4 -* 2PbS0 4 + 2H 2 


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- 

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 


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. 


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. 


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 


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. 


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 

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. 



Figure 9: Time Required for SG to Stabilize During Discharge 
Rest Interuals 




53 1125 

5 10 

Time After Load Removed in Minutes 


500 1000 

5000 10.000 

Figure 10: A Conventional Test Setup for Determining 
Battery Capacity 


BATTERY 36V 960 A-H 


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. 


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].) 



£ 1.55 



5 1.35 


i 1.15 



-i c 

: won 



1 .u 

T — 


1 .57 VPC 



660A \ 



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 


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 

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. 


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 


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- 


Figure 14: How Stabilized Open Circuit Voltage Reflects 

1.90 1.95 2.00 

Stabilized Open Circuit Ceil Voltage (volts) 





1 98 

Figure 1 5: Variation of Open Circuit Voltages as Cell Recovers 
After Load 


| 1 ' 94 

| 1.93 

o 1.92 

.1 .5 1.0 

Time After Load Removal (minutes) 




= 6 HRS. 


50 100 

500 1000 

5000 10.000 


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. 



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% 


(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 


Figure 17. How Ampere-Hours Are Returned to the Battery 
During an 8-Hour Charge 


01 2345678 
Time (hours) 


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), 



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 


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. 


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. 


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 

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, 


is determined by the values at the end of the 8-hour 

• 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) 


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% 



Table 1: Effect of Battery State-of -Charge on 
Charger Efficiency in an 8-Hour Charge Period. 

% Discharge 
at start of 

Total Line 

Total Energy 
to Battery 













Table 2: Effect of Battery State-of-Charge on 
Battery Efficiency in an 8-Hour Charge Period 


at Start of 

Energy to Battery 
kWh Ah 


to Load 





















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 


















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 


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. 


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 

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 

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, 


Figure 24: Relative Current Drain for Typical Industrial Trucks 


















Figure 25: A Hypothetical Industrial Truck Operation 


10 SEC. @ 125 AMPERES (.347 AH) 

■1^3 — START 

.347 AH 

12 SEC. @ 230 AMPERES 
(.767 AH) 

, 00 ,30 



1.114 AH 


1 SEC. @ 150 AMPERES (.042 AH) 

3 SEC. m 150 AMPERES (.125 AH) 
TURN, 1 SEC. to 100 AMPERES (.027 AH) 
RUN. 11 SEC. 200 AMPERES(.611 AH) 


1.919 AH 

TURN, 1 SEC. @ 100 AMPERES (.027 AH) 
RUN, 5 SEC. to 125 AMPERES (.174 AH) 




2.1 AH 

RUN, 30 SEC. 
(1.25 AH) 


1 SEC. to 230 AMPERES 

(.064 AH) 



3.4 AH 


8 SEC. <§ 230 AMPERES (.511 AH) 

2 SEC. to 250 AMPERES (.139 AH) 

2 SEC. to 150 AMPERES (.083 AH) 


4DO 600 


4.1 AH 


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 

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 

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. 



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-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 



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 


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 


Table 4: 

Energy Usage Data for a Hypothetical 
20-Truck, 2 -Shift Fleet 

Fleet Composition 

Battery Data 

Fleet/Shift Data 





at Shift 

Output to 

Ah kWh 

Input to 


Output to 

Ah kWh 

Input to 






960 33.6 


960 33.6 





832 29.2 


1664 58.4 





704 24.9 


2112 74.7 





576 20.5 


4608 164.0 





512 18.2 


2048 72.8 





448 16.0 


896 32.0 



Fleet Totals Per Shift 

12,288 435.5 



Per Month (40 shifts) 491,520 17,420 



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. 


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 


Ah Used 

Total Ah 

Total Workshift 

in One Shift 


Capacity per Truck 

per Charge (Shifts) 

























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). 


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 





S 160 

E> 140 

S 120 
I 100 

1 2 4 5 10 

Number of Batteries Starting Charge Simultaneously (Group Size) 



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 

of Batteries 
at a time 

Time between 
Start of Charging 


Average Monthly 

kW Demand 
Charge (based on 














. 8 




. 4 



* $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 


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 








Per Shift 













1 =1 






1.74 = 2 





2.22 = 3 





4.80 = 5 





2.12 = 3 





0.94 = 1 



Total Batteries Required 


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 

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. 


Table 8: Number of Charges Required Annually 

to Support Fleet 

Old Procedure 

New Procedure 





per Class 


per Class 

per Class 



per Shift 

per Week 

per Shift 

per Shift 
















































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. 


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 


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 


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 


Cell, see also Battery 
nominal voltage, 15, 24 
numbering and strapping, 21, Fig. 6 
series connection, 15, 21, Fig. 6 


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 

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 


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 



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 


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 


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 


display of state-of -charge, 56 
factor in energy waste reduction, 61 
prevention of deep discharge, 67 


Gassing, 43 

chemistry of, 43 

factor in charge rate and termination, 

42, 43 
ventilation, 43 
voltage, 38, 40. 43 


Horsepower, see also Watt, 11, Fig. 1 
Hydrogen, 40, 43 
Hydrogen ion, 22. 24, Fig. 7 


Kilowatt-hour (kWh), 15, 16 


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 


Manufacturer's capacity rating, 16 
Manufacturer's capacity test, 28, 30, 
Fig. 10 

Maximum charge rate. 42, 43 
Motor generator charger. 38 


Ohm, 14, 15 
Ohm's Law, 14 

Open circuit voltage (emf). 24 

effect of electrolyte/plate reactions on, 

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 


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, 

Pulsed charger, 38, 40, 42, Fig. 20 


Rating, see also Battery capacity 
ampere-hour, 16 
battery, information. 16, 22 
capacity. 16 
factors affecting, 28-32 
kilowatt-hour, 15, 16 


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 


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 


Taper charging, 38, 40 


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 


Volt, 15 

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 


Watt, 11,15 
Watt-hour, 15, 16 
Work, 10, 15 

Workshift-based charging schedule, 56