Battery state-of-charge indicator

A circuit for indicating the charge state of a battery supplying discharge current to its load circuit under actual load operation during which battery discharge current is subject to magnitude variations and interruptions, having means for storing a stored signal whose value is representative of battery state-of-charge, means for rapidly increasing the value of the stored signal to the scaled value of the battery terminal voltage when its scaled value is below that of the battery terminal voltage, first means for decreasing the value of the stored signal at a slow rate preestablished such that the stored signal decreases from a first value representative of a fully charged battery to a second value representative of a discharged battery in a time period approximating the average time required during normal operation of the load circuit for the fully charged battery to become discharged, and second means, operative only during intervals when the battery supplies no discharge current to the load, for decreasing the value of the stored signal at a rate intermediate said slow and fast rates and related to the difference between the scaled values of the battery terminal voltage and the stored signal.

BACKGROUND OF THE INVENTION 
The present invention relates to battery monitoring apparatus and more 
particularly to a battery state-of-charge indication system. 
Storage batteries are used in numerous applications where it is important 
to know the amount of available energy remaining in the battery. For 
example, the battery state-of-charge is a critical parameter in the 
operation of battery energized electrically propelled traction vehicles, 
such as electric cars and forklift trucks. Such vehicles must rely upon 
the energy stored in the on-board batteries for propulsion, and the 
replenishing of stored energy requires special equipment which is only 
available at a charging station. Thus, means for indicating the energy 
state of the remaining battery charge can be advantageously used by the 
vehicle operator to ensure that the vehicle is returned to a charging 
station before the battery has been completely discharged. The vehicle 
batteries represent a substantial investment and the amortization of 
battery costs depends upon the available number of charge/discharge cycles 
and upon the average depth of discharge of a lead acid battery. It is well 
known that the life of a battery is reduced significantly when it is 
repeatedly discharged such that the specific gravity of the electrolyte 
falls below a specified quantity. Hence, it is desirable to provide some 
means for recognizing that the battery state-of-charge is approaching this 
level. Further, it is important that such a state-of-charge indicator 
system provide a continuous and an accurate state-of-charge output while 
the battery is connected to its normal load circuit and is operating under 
normal load conditions. For example, in the case of a battery energized 
vehicle, this permits the operator to perform his mission until the 
batteries have been discharged to a desired level. 
Various types of prior art systems have been proposed for indicating the 
energy remaining in a battery or detecting a low battery condition under 
normal load operation including the following. 
Specific gravity metering devices can detect a change in the index of 
refraction with variations in the specific gravity of the electrolyte. 
However, loads are commonly energized by plural batteries, each battery 
comprises a large number of cells and the electrolyte condition varies 
from cell to cell. This requires multiple sensors to obtain an average 
specific gravity. This results in unduly complex sensing circuits and 
means for interconnecting the sensors to the indicating system so as to 
permit normal battery exchanges. Further, the specific gravity of a cell 
may vary throughout the cell so that no one location is ideal as a sensing 
point. 
Ampere hour and watt hour meter devices operate on the assumption that the 
remaining available energy corresponds to the energy input during charging 
minus the electrical energy which has been extracted. However, such 
systems fail to account for loss in the remaining available energy 
resulting from increased current discharge rates. For example, ampere hour 
meters commonly utilize a reversible electrochemical plating cell. During 
battery charging, this cell is plated with a material at a rate 
corresponding to the magnitude of the charging current so that total 
plating is the product of current and time. During discharge, the plating 
process is reversed in a similar fashion. However, the ampere hours (AH) 
available from a lead-acid battery depend upon the rate of discharge. 
Thus, a battery rated at 300 AH at a current drain which would deplete the 
charge in 6 hours may only provide 220 AH at a current corresponding to a 
one hour discharge rate so that the meter may indicate that one-third of 
the energy is available when, in fact, the battery is completed 
discharged. In addition, the instrument must remain with the batteries 
when batteries are exchanged for an alternate set at the charging station, 
since the meter cannot be reset for the unknown charging history of the 
new set. Further, such devices are subject to additional inaccuracies 
resulting from variations of recoverable AH based on battery age and 
temperature and variable battery charging efficiencies. 
Arrangements have been proposed for indicating state-of-charge based on 
sensing the terminal voltage of batteries while energizing their load 
circuits. Commonly, a heavily dampled voltmeter is connected directly 
across the battery to provide an indication of battery charge. The battery 
voltage varies substantially with the changes of discharge current 
encountered during operation of loads such as the electric motors of 
traction vehicles. Thus, the meter produces variable and erroneous 
indications of battery charge. An electric vehicle operator may judge the 
battery charge condition by the magnitude of the voltage drop during a 
specific maneuver such as acceleration, i.e., a specific load. This 
requires a high level of skill and close observation by an operator who is 
likely to be preoccupied with vehicular operation. 
Battery condition monitors utilizing a similar voltage detection prinicple 
have been employed in some battery powered industrial trucks. These 
devices have a voltage level switch activated when the battery terminal 
voltage drops below a preset level, e.g., 80-85% of nominal voltage. If 
the voltage remains below this level for a preset time interval, e.g. 
15-30 seconds, an indicating lamp is energized and a second timer may be 
started. Upon the presence of the undervoltage condition over this second 
time interval, a specified work function of the vehile, such as the 
forklift, can thus be disabled so as to force the operator to return to 
the charging station. However, the sensing and detection means of such 
monitors are inexact and dependent upon may variables, including the 
changes in battery voltage with variations in load. In addition, no 
continuous indicating means is available to continuously advise the 
operator of the present state-of-charge and the low charge indication will 
often catch the operator by surprise. 
Battery state-of-charge indicating systems relying on detection of battery 
terminal voltage during variations of load, i.e. discharge current, thus 
have tended to be inaccurate are unreliable. Improved results have 
resulted from a battery monitoring system providing an indication of the 
state-of-charge based on the differences between nominal battery voltage 
of a fully charged battery at a predetermined discharge current and the 
actual battery voltage occuring at the same predetermined discharge 
current level. A measure of battery voltage is stored when the 
predetermined value of discharge current occurs such that the stored value 
is updated solely during the occurrence of such predetermined value. Thus, 
the stored value provides a continuous indication of battery 
state-of-charge. This battery monitoring apparatus which is disclosed in 
my U.S. Pat. No. 4,021,718, assigned to the assignee of the subject 
application, utilizes discharge current measuring means to produce a 
signal proportional to load, or discharge current. However, if such a 
current measuring device is not otherwise required for control of the 
battery load circuit, its requirement solely for use in battery 
state-of-charge monitoring increases the cost of the battery monitoring 
system which may limit its use in a highly competitive market. 
OBJECT OF THE INVENTION 
It is, therefore, an object of the invention to provide an improved method 
and apparatus for reliably providing the state-of-charge of a battery 
while energizing its load, including during intervals of varying discharge 
current. 
It is a further object of the invention to provide such an improved method 
and apparatus which is operable without requiring sensing of discharge 
current magnitude. 
It is another object of the invention to provide such apparatus for 
automatically providing a continuous state-of-charge indication 
substantially free of erratic indications or fluctuations caused by 
variations of discharge current. 
SUMMARY OF THE INVENTION 
These and other objects are attained in accordance with the invention by 
storing a value representative of battery terminal voltage, decreasing the 
stored value at a predetermined discharge rate which is independent of 
actual battery discharge current and when the scaled value of actual 
battery voltage exceeds the stored value increasing the stored value to 
the scaled value of such actual battery voltage at a rate substantially 
faster than said predetermined rate, such that the stored value is 
indicative of the battery voltage uner conditions of no-load and 
represents the battery state-of-charge. Preferably, the stored value is 
normally discharged at a predetermined low rate approximating the actual 
anticipated discharge rate of the battery, but during intervals of no 
battery discharge current, the value is discharged at a faster rate. This 
faster rate preferably decreases as a function of the decreasing 
difference between the value of actual battery voltage and of the stored 
value.

DETAILED DESCRIPTION 
It is known that the terminal voltage of a lead acid storage battery which 
has been in a quiescent state for an extended period of time is a good 
indicator of the state-of-charge, for example, ranging from 2.12 volts per 
cell when fully charged to 1.97 volts per cell when substantially 
discharged. Thus, the state-of-charge of a battery which is disconnected 
from its load may be detected such as by a suppressed zero volt meter. 
However, the battery terminal voltage drops substantially under load, i.e. 
during the existence of discharged current. Further, after the discharge 
current terminates, the battery voltage rises only gradually to its true 
open circuit voltage. Thus, measuring the voltage of batteries under open 
circuit conditions does not provide an obvious solution to provide a 
state-of-charge indication of batteries while operating under varying load 
conditions. FIG. 1 illustrates the relationship between battery terminal 
voltage and battery current for different levels of battery charge. Lines 
2 and 3 represent, respectively, fully charged and discharged load lines. 
Battery voltages and currents were measured under conditions of no-load, 
100 amp discharge current, and 200 amperes of discharge current. These 
measurements were taken of one type of battery when (1) the battery was 
fully charged and (2) when it was discharged. The measured voltages for 
the fully charged battery are 12.55 volts at zero current, 11.73 volts at 
100 amperes, and 11.15 volts at 200 amperes. The voltages for the 
discharged battery are 11.75 volts at zero current, 10.73 volts at 100 
amperes, and 9.85 volts at 200 amperes. These voltage-ampere measurements 
are shown in FIG. 1 with a voltage-ampere characteristic line 2 for the 
fully charged battery and a voltage-ampere characteristic line 3 for the 
discharged battery. This demonstrates the inverse voltage to current 
relationship existing at a specified level of charge, e.g. fully charged 
and discharged. In the specific example the load lines for the charged and 
discharged battery are approximately linear and have relatively similar 
slopes. The fully charged battery has a significantly higher no-load 
voltage than the discharged battery. Accordingly the state-of-charge of 
the battery should be determinable from the battery terminal voltage 
existing at a specified load current level within the range normally 
encountered during operation including at zero current. 
Referring now to FIG. 2, there is shown a block diagram of a battery 
state-of-charge indicating circuit according to the present invention. A 
battery 2 having terminals 3 and 4 is connected in series circuit with its 
normal load 5. Terminals 3 and 4 are further connected to voltage sensor 
and comparator 10 whose output is connected via line 7 to storage device 
15. The storage value output of this device is supplied by line 11 to 
display, or utilization, means 35 which provides an indication of the 
state-of-charge of the battery. Line 11 is additionally connected to an 
additional input of voltage sensor and comparator 10. Storage device 15 
includes storage means 9 for storing a value representative of voltage, 
and "up" portion 8 which permits the stored value to be increased when the 
signal on line 7 is representative of a voltage value greater than the 
value stored, but does not permit the stored value to be decreased when 
the signal on line 7 has a value below the value stored. Device 15 further 
has a "down" portion 12 utilized solely for decreasing the stored value, 
as described subsequently. The arrangement desired above thus provides for 
storing a value which is representative of the battery voltage upon 
initial turn-on of the system, and for rapidly increasing this stored 
value whenever the stored value diminishes below a value representative of 
actual battery voltage, such that the stored value is increased to 
represent the actual value of the battery voltage. 
As the battery is discharged, its terminal voltage decreases. It is 
therefore necessary to reduce the stored value. This is accomplished by 
fixed rate discharge means 20 whose output is connected by line 13 to the 
"down" portion 12 of discharge means 15. The discharge means 20 thus 
causes the value stored in storage means 9 to be reduced at a 
predetermined rate. Since the state-of-charge indicating system does not 
utilize a current sensor and the true discharge rate is thus not detected, 
the stored value must be reduced at an approximated rate. The discharge 
rate of discharge means 20 is thus selected to approximately track the 
reduction of no-load battery terminal voltage resulting from discharge 
currents. Discharge means 20 causes the stored value to be reduced at a 
predetermined rate which corresponds to the average of the anticipated 
discharge rate occurring during normal operation of the load. This 
anticipated discharge rate is inversely proportional to the time required 
for the battery to discharge from its fully charged to its discharged 
state. This depends upon the batteries utilized, the type of load system, 
and the normal type of operation of the load. For example, an industrial 
utility vechile such as a forklift truck, may have a battery discharge 
time of 3 hours, i.e. a fully charged battery will be discharged after 3 
hours of vehicle operation. This time constant varies with operating 
parameters. A heavier on the road vehicle operating at maximum speed, e.g. 
45 miles per hour, may have a battery discharge time of about one hour. 
The time constant of normal types of electric vehicle systems therefore is 
believed to at least exceed 30 minutes and may range upward to many hours. 
The above referenced predetermined discharge rate thus is a function of the 
difference of the stored value representative of a fully charged battery 
and the value representative of a discharged battery divided by the above 
referenced time constant. Further discussion of this topic follows in 
connection with the description of FIG. 4. The stored value is prevented 
from decreasing below the value representative of actual battery voltage, 
by the above-described arrangement for rapidly increasing the stored value 
to value representative of actual battery voltage. 
However, if the actual battery discharge rate exceeds the predetermined 
rate of discharge, the battery can be discharged while the stored value 
still indicates that substantial energy remains in the battery. One 
solution to this problem is to increase the predetermined discharge rate 
of means 20, such that the stored value is decreased more rapidly than the 
maximum battery discharge rate. During operation of a typical load 
circuit, such as for example a forklift truck, there typically are long 
intervals during which the battery is discharged followed by briefer time 
intervals when the battery is unloaded. During these no-load intervals, 
the battery voltage increases from its load voltage to its true no-load 
voltage. Under conditions where the predetermined discharge rate exceeds 
the actual discharge rate, the actual battery voltage will exceed the 
stored value when the battery is unloaded. This causes the stored value to 
increase when the load is removed for any substantial time. This is likely 
to lead to considerable wandering of the stored value output and thus to 
wandering of the output of indicating means 35. Accordingly, in a 
preferred embodiment of the invention, a more stable indication of charge 
is attained by adjusting the fixed rate to the typical discharge rate and 
by adding a second discharge means for reducing the stored value during 
intervals when the battery does not provide a discharge current. This is 
accomplished by no-load rate discharge means 25 whose output is connected 
by line 14 to the "down" portion 12 of storage means 15. Discharge means 
25 is activated solely during conditions of battery open circuit by means 
of open circuit sensor 30 which is shown connected by line 16 to discharge 
means 25. Sensor 30 may be a device responding to circuit contactors which 
open the battery circuit. Alternatively, other types of open circuit 
sensor means may be utilized. For example, it has been found that electric 
motors upon being energized by the battery produce a commutator ripple 
which can be detected by a squelch circuit which circuit can be utilized 
to inhibit the discharge means 25 during periods when the motor is 
energized. In summary, the value of battery voltage is initially stored in 
storage device 15. As the battery is discharged, its state-of-charge and 
its true no-load battery voltage is reduced. Thus, the value 
representative of actual battery voltage decreases below the stored value. 
Accordingly, during periods when the battery-load system is energized, the 
stored value is continuously discharged at a predetermined rate, which 
preferably corresponds to a typical average discharge rate. This is 
accomplished by discharge means 20 acting on storage device 15. If the 
value stored in storage device 15 is reduced below the value of the actual 
battery terminal voltage, the stored value is rapidly increased, at a rate 
substantially faster than the discharge rate of means 20, to a value 
corresponding to actual battery voltage. This is accomplished by 
comparison of actual battery voltage and stored value by comparator 10 
which supplies to the "up" portion 8 of storage device 15 an input causing 
the stored value to be increased to the value representative of the 
battery voltage. 
FIG. 3 illustrates operation of the system, including the no-load rate 
discharge means 25. FIG. 3 illustrates the relationship between the actual 
battery voltage, as represented by line 105, and the stored value, as 
represented by line 110, as a function of time. It is assumed that the 
value initially stored in storage means 15 was representative of a no-load 
voltage of the battery. Subsequently, the battery is discharged during a 
time interval, represented extending to 115, when there is discharge, or 
load, current 100 of unknown magnitude. During this interval, fixed rate 
discharge means 20 causes the stored value to be decreased at a slow rate, 
as indicated by the dashed line segment 110 extending to the time 
identified at 115. During this interval, the battery voltage is below the 
value of the stored value, since battery voltage varies inversely with the 
magnitude of discharge current. Upon termination of discharge current at 
time 115, the battery voltage gradually rises to its true open circuit 
voltage. For one type of lead acid battery, the time constant of this 
recovery appeared to be approximately 5 minutes. Upon termination of load 
current at time 115, open circuit sensor 30 activates no-load rate 
discharge means 25. The net effect of discharge means 25 and 20 is to 
increase the rate at which the stored value is reduced, as indicated by 
the segment of line 110 subsequent to time 115. This net discharge rate is 
approximately matched to the recuperation time constant of the battery. 
The most accurate indication of state-of-charge is achieved by adjusting 
this net rate of stored value reduction in proportion to the difference 
between battery terminal voltage and the stored value, i.e. an exponential 
rate. This is illustrated by the portion of stored value line segment 110 
subsequent to time 115. At time 120, the stored value and the actual 
battery voltage value intersect. At this point, the stored value is 
updated to the actual battery voltage to prevent the stored value from 
declining below the actual battery voltage value. The preferred embodiment 
of the system thus provides a state-of-charge indication essentially based 
on the magnitude of actual battery voltage occurring when there is no-load 
current. Thus, the stored value is rapidly increased to a value 
representative of actual battery voltage at a first, rapid, rate. It is 
decreased slowly at a second rate approximating average battery discharge 
during the occurrence of discharge currents, and it is decreased at a 
somewhat higher, third rate, intermediate the first and second rates, 
subsequent to the termination of discharge current until the value 
representative of battery voltage corresponds to the stored value. 
Referring now to FIG. 4, there is shown a schematic of a preferred 
embodiment of the invention. Negative terminal 3 of battery 2 is connected 
to a common bus 41 and positive terminal 4 is connected in series circuit 
through resistor 42 and potentiometer 44 to the common bus. The arm of the 
potentiometer is connected to the non-inverting input of operational 
amplifier 48. The arm of potentiometer 44 is adjusted to have a first 
predetermined voltage, e.g. 9 volts, when battery 2 is fully charged and a 
second predetermined voltage, e.g. 8.33 volts, when the battery is 
substantially discharged. Capacitor 70, which constitutes the storage 
means, is connected between the common bus 41 and via serially connected 
diode 66 and resistor 58 to the output of amplifier 48. The junction of 
diode 66 and of capacitor 70 is connected to the non-inverting input of 
operational amplifier 76 whose output is connected to its inverting input. 
Amplifier 76 thus is connected as a voltage follower to provide at its 
output an indication representative of the charge on capacitor 70 without 
materially affecting the stored value. The output of amplifier 76 is also 
connected via resistor 56 to the inverting input of operational amplifier 
48. This inverting input is additionally connected through resistor 46 to 
a first positive bus 43 having a first predetermined positive potential, 
e.g. +9 volts. Operational amplifier 48 thus operates as a comparator, 
comparing the scaled voltage of capacitor 70 with the voltage on the arm 
of potentiometer 44 which represents a scaled down level of battery 
terminal voltage. This comparison circuit provides for increasing the 
stored value in the manner previously described. Specifically, if the 
capacitor voltage is below the voltage on the potentiometer arm, the 
output of comparator-amplifier 48 is switched to a high level so as to 
charge capacitor 70 through the series circuit comprising resistor 58 and 
diode 66. Once the voltage of capacitor 70 exceeds the potentiometer arm 
value by an incremental amount, the scaled voltage output of amplifier 76, 
applied to the inverting input of comparator 48, will exceed the potential 
on potentiometer arm 44, causing the output of comparator 48 to switch to 
a low level. However, capacitor 70 is prevented from discharging into the 
low level output of comparator 48 by diode 66, whose cathode is connected 
to the positively charged side of the capacitor. The described arrangement 
thus constitutes a peak detector circuit for charging storage capacitor 70 
whenever a scaled value of actual battery voltage exceeds the value stored 
on the capacitor, such that the capacitor is charged to a value 
representing the actual battery voltage. 
The following is a description of the fixed rate discharge system. 
Discharge of capacitor 70 at the fixed rated is accomplished by the 
network comprising diode 74 and resistors 72 and 78. Diode 74 and resistor 
78 are serially connected between the output of amplifier 76 and common 
bus 41 with the anode of doide 74 being connected to the amplifier output. 
Capacitor 70, which is connected to the non-inverting input of amplifier 
48 has a residual charge. Since amplifier 76 is connected as a voltage 
follower, its output will be at the same potential as its non-inverting 
input. The output potential is sufficiently positive, e.g. +3 volts, in 
respect to the common bus to cause conduction through diode 74 and 
resistor 78. During conduction, the diode has a predetermined and 
reasonably constant small voltage drop, e.g. 0.5 volts. Resistor 72 is 
connected between the cathode of diode 74 and the junction of capacitor 70 
and the non-inverting input of amplifier 76. As previously stated, the 
potentials at the non-inverting input and the output are identical, and 
there is a predetermined constant voltage drop across diode 74. Therefore, 
resistor 72 has a constant potential across its terminals which equals the 
drop across diode 74. For example, with a diode potential of 0.5 volts, 
the potential drop across resistor 72 is also 0.5 volts. Thus there is a 
constant discharge current flow through resistor 72 whose fixed magnitude 
is equal to the potential drop, e.g. 0.5 volts, divided by the resistance 
of resistor 72. The magnitude of discharge current, I.sub.72, is: I.sub.72 
=(Capacitance of 72) (.DELTA.V)/.DELTA.T where .DELTA.V equals the 
difference between the voltage of capacitor 72 when fully charged and when 
discharged, and .DELTA.T equals the battery discharge time constant in 
seconds, i.e. the anticipated time for the battery to discharge from full 
load under the assumed operating conditions. In one case, for example, the 
circuit parameters were selected such that the capacitor has a capacitor 
of 10 .mu.fd, the capacitor voltage, i.e. the voltage at the non-inverting 
input of amplifier 76, was 9 volts for a fully charged battery and 3 volts 
for a discharged battery. According, .DELTA.V equals 9-3=6 volts. The 
battery discharge time constant for a vehicle having particularly high 
discharge requirements because of its anticipated continuous high speed 
operation, was 44 minutes, i.e. 2643 seconds. Thus, the discharge current 
was (10.times.10.sup.-6) (6)/2643=2.27.times.10.sup.-8 amps. The 
resistance of resistor 72 equals the voltage of diode 74, e.g. 0.5 volts, 
divided by the discharge current, and for the above example was 
22.times.10.sup.6 ohms. 
It should be noted that the storage capacitor being interposed between 
operational amplifiers is not subject to random and undesired discharge 
circuits which otherwise could substantially reduce the predetermined rate 
of discharge. The input, i.e. offset, current of operational amplifier 76 
was limited to a very small percentage of the current through resistors 
72. Of course, other circuits could be utilized to assure that the 
capacitor is discharged at the predetermined rate. 
The no-load rate discharge system is described below. Diode 60 and resistor 
64 are serially connected from the output of comparator 48 to the common 
bus 41. Resistor 68 and diode 62 are connected in series circuit from the 
junction of storage capacitor 70 and the non-inverting input of amplifier 
76 to the junction of diode 60 and resistor 64. The series of combination 
of resistor 68 and diode 62 provides the discharge path of the no-load 
rate discharge circuit. The junction of diodes 60 and 62 is connected to 
the collector of transistor 54 whose emitter is connected to a second 
positive bus 45, e.g. +12 volts. The base of transistor 54 is connected 
through resistor 50 to bus 45 and through resistor 52 to terminal 51. As 
further described, terminal 51 is connected to an open circuit sensor 
whose output is indicative of the presence or absence of battery discharge 
current. In the described embodiment, an open circuit detector, for 
example of the type illustrated in FIG. 5, clamps the base of transistor 
54 to zero volts during intervals when the battery supplies a discharge 
current. Transistor 54 having its emitter connected to positive bus 45 
thus conducts and applies a positive voltage to the cathodes of diodes 60 
and 62. This application of reverse bias to the diodes blocks conduction 
through resistor 68 and diode 62 and thus disables operation of the 
no-load rate discharge circuit during intervals of battery discharge 
current. 
During intervals when there is no battery discharge current, the open 
circuit sensor output coupled to resistor 52 is not clamped to zero volts. 
Accordingly, resistor 50 applies a positive potential to the base of 
transistor 54 causing the transistor to be cut off. This removes the 
positive back bias signal otherwise supplied from the emitter of 
transistor 54 to the junction of diodes 60 and 62. This permits the 
no-load rate discharge circuit to operate while the scaled voltage on 
storage capacitor 70 exceeds the voltage on the arm of potentiometer 44, 
i.e. the scaled value representative of battery terminal voltage. The 
circuit additionally includes transistor 80 whose emitter is connected to 
the junction of resistor 58 and diode 66, whose collector is connected to 
the inverting input of comparator 48, and whose base is connected to the 
output of operational amplifier 76. When there is no battery discharge 
current, and when the stored value, i.e. scaled voltage, on capacitor 70 
exceeds the voltage on the potentiometer arm, the output of comparator 48 
becomes less positive, i.e. moves in a negative direction, so as to 
forward bias the emitter base junction of transistor 80. This causes 
transistor 80 to conduct, effectively connecting resistor 58 between the 
output and inverting input of comparator 48. Thus, operational amplifier 
48 operates essentially as a voltage follower and provides at its output a 
signal representative of the voltage on the arm of potentiometer 44. 
Accordingly, voltage across resistor 68 is proportional to the difference 
between the stored value on capacitor 70 and the value representative of 
actual battery voltage. Hence, the "no-load" discharge current through 
resistor 68 is proportional to the difference between this stored value 
and actual value, and accordingly decreases as the difference between the 
stored value and the actual value decreases. In a preferred embodiment, 
the resistance of resistor 68 was selected to be 10 M. With the potential 
drops encountered, this provided a substantially faster discharge rate 
than the discharge rate produced by "fixed rate" resistor 72. The net 
discharge rate under no-load conditions was, in this case, selected to be 
about 5 minutes, so as to approximate the time required for a lead acid 
storage battery to recover to its true no-load value after the cessation 
of discharge current. 
The output voltage of amplifier 76 is representative of the charge on 
storage capacitor 70 and thus is indicative of the state-of-charge of the 
battery. The voltage ranges from a first predetermined voltage, e.g. 9 
volts, for a fully charged battery to a lower second predetermined 
voltage, e.g. 3 volts, for a discharged battery. Various known types of 
display or utilization circuits can thus be employed. The preferred 
embodiment merely provides an analog meter representation. A voltage 
divider comprising resistors 84 and 86 is connected from the positive bus 
43 to common bus 41. An ammeter 82 is connected between the output of 
amplifier 76 and the junction of resistors 84 and 86. For the specific 
arrangement where the bus 43 voltage is +9 volts, and the amplifier 76 
output ranged from 9 volts for a fully charged battery to 3 volts for a 
discharged battery, the resistance of resistor 84 is twice that of 
resistor 86 providing 3 volts at their junction, and the resistance of 
resistor 86 was further chosen such that the ratio of 9 volts, i.e. the 
amplifier 76 output for a fully charged battery, to the resistance of 
resistor 86 provides full scale deflection of the ammeter. 
The following components were utilized in a preferred embodiment of the 
invention: 
Amplifier 48--Type TL 082, Texas Instruments 
Amplifier 76--Type LM 239, National Semiconductor 
Resistors: 46--10K; 56--80K; 58--56K; 64--10K; 68--10M; 72--22M; 78--10K; 
84--20K; 86--10K. 
Capacitor 70--10 .mu.fd 
Transistors: 54--Type 2N4249; 80--Type 2N2714 
Diode: 60--Type 1N457A; 62--Type 1N457A; 74--Type 1N457A 
Various alternative circuit embodiments can be utilized in lieu of that 
illustrated in FIG. 4. These can be analog or digital circuits. For 
example, a digital system of the type illustrated in FIG. 2 can utilize a 
counter in lieu of a storage capacitor for storage element 15. In such an 
arrangement, the output of the comparator 10 would operate into the count 
up terminal of the counter. The counter output can, for example, be 
converted to an analog output. The current magnitude of the discharge 
means can be converted to digital pulses applied to a countdown terminal 
of the counter. Such arrangements are known in the prior art. For example, 
the referenced U.S. Pat. No. 4,021,718 discloses arrangements for 
decreasing the count of a counter and for converting the digital value 
stored in the counter to an analog value. Similarly, the digital system 
can be constructed utilizing a digital processor system. Various 
arrangements can also be employed to provide alarm indications or to 
modify operation of the electric load system when the battery voltage is 
discharged to predetermined levels. 
The preferred embodiment utilizes no-load discharge means providing a 
modified discharge rate during intervals when there is no battery 
discharge current. Since the state-of-charge indicator incorporates no 
means for direct detection of discharge current, there is a need for an 
alternate means for detecting no-load, i.e. no discharge current, 
conditions. In some applications, this may be provided by monitoring the 
state of line contactors or other elements which interrupt the connection 
between the battery and the load circuit. However, in motor applications, 
such as electric traction motors, the no-load indication can be obtained 
from a simple self-contained unit which requires no external connection 
other than connection to the battery circuit, e.g. the terminals of the 
battery being monitored. It has been found that a low voltage ripple of 
relatively high frequency is present at the battery terminals during 
operation of the motor due to commutation in the motor. A no-load monitor 
can thus be incorporated into the state-of-charge indicating system to 
provide the necessary no-load indication. 
FIG. 5 illustrates such a no-load detector which monitors the presence or 
absence of commutator voltage ripple and thus provides an output 
indicative of the presence or absence of battery discharge current. 
Terminals 99 and 97 are respectively connected to the battery, such that, 
for example, the positive battery terminal is connected to 99 and the 
negative terminal is connected to terminal 97 and common bus 141. Resistor 
101 and coupling capacitor 103 are connected serially between terminal 99 
and the base of transistor 111. The emitter of the transistor is connected 
to the common bus and the collector is connected through resistor 113 to 
positive bus 145, e.g. +12 volts. Feedback biasing resistor 109 is 
connected from the collector to the base. Resistor 105 and negative 
clamping diode 107 are connected in parallel between the base and the 
emitter. The transistor circuit thus operates as an AC amplifier producing 
an amplified ripple signal at the collector. 
Capacitor 115 and diode 121 are connected serially between the collector of 
transistor 111 to the base of transistor 127, whose emitter is connected 
to the common bus. Resistor 117 and diode 119 are connected in parallel 
from the junction of capacitor 115 and diode 121 to the common bus. The 
collector of transistor 127 is connected via resistor 129 to positive bus 
145, and its base is connected via biasing resistor 123 to bus 145. The 
collector device 127 is connected to the base of transistor 131, whose 
emitter is connected to the common bus and whose collector is connected to 
terminal 133, which constitutes the output of the no-load detector. 
The amplified ripple signal appearing at the collector of transistor 111 is 
coupled by capacitor 115 to load resistor 117 and a peak detector circuit 
comprising diode 121 and capacitor 125. Diode 119 isolates the peak 
circuit from positive voltage excursions such that the upper plate of 
capacitor 125 is driven negative in respect to the common bus when ripple 
voltage is present at input terminal 99. 
Transistor 127 is normally biased in the on state by bias resistor 123 so 
as to have a low collector voltage. Transistor 131 is therefore normally 
cut off. Thus, when there is no ripple, i.e. no discharge current, 
transistor 131 being cut off has a high impedance. Accordingly, terminal 
133 presents a high impedance. 
During the presence of ripple, the negative potential on the top plate of 
capacitor 125, i.e. the capacitor terminal connected to the base of 
transistor 127, provides sufficient reverse bias to overcome the positive 
bias supplied to the base by resistor 123. Accordingly, transistor 127 is 
cut off during intervals of ripple. When transistor 127 is cut off, its 
collector voltage rises sufficiently to turn on transistor 131. Thus, 
during intervals when there is ripple current, i.e. battery discharge 
current, conduction of transistor 131 causes terminal 133 to have a low 
impedance path to the common bus 141. Terminal 133 thus constitutes a high 
impedance when there is no battery discharge current and a low, i.e. zero, 
impedance to common bus 141 when there is discharge current. Terminal 133 
can thus be directly connected to terminal 51 of the circuit illustrated 
in FIG. 4 in order to control operation of the no-load discharge means. 
While preferred forms of the invention have been herein shown and described 
by way of illustration, modifications and variations thereof will probably 
occur to persons skilled in the art. It is therefore intended by the 
concluding claims to cover all such changes and modifications as fall 
withint he true spirit and scope of this invention.