CA1049614A - Method and apparatus for measuring the state of charge of a battery by monitoring reductions in voltage - Google Patents

Abstract

METHOD AND APPARATUS FOR MEASURING THE
STATE OF CHARGE OF A BATTERY BY
MONITORING REDUCTIONS IN VOLTAGE

ABSTRACT OF THE DISCLOSURE

A method and apparatus are disclosed for measuring the state of charge of a battery. The apparatus comprises circuitry for sensing reductions in the output terminal voltage of the battery due to varying load conditions and producing a signal in response thereto. In one embodiment the signal com-prises a series of pulses whose number is proportional to the time that the terminal voltage is below the threshold value.
In this embodiment, the apparatus further comprises an inte-grator responsive to the number of pulses for registering the number of pulses produced in response to reductions in the terminal voltage. The output of the integrator is indicative of the state of charge of the battery. In an alternative embodiment, the signal from the sensing circuitry is inte-grated directly to develop a representation of the state of charge of the battery. Techniques for synthesizing different signals from the sensing circuitry are also disclosed.

Description

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B KGROUND OE` THE INVENTIVN

This invention is directed to a system particularly useful for measuring and indicating the state of charge of a storage battery. The system may be fabricated using any of the many devices which are capable of measuring and indicat-ing the integral oE an electrical signal. Such devices include electronic devices such as counters, electromechanical devices such as stepper motors, and electrochemical devices such as coulometers and the inventive system will be described -`
in circuits employing such devices. It is, however, contem-plated that the inventive system may be used advantageously ;~
with any integrating device.

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1~96~4 1 SUMMI~Y OF T}IE INVENTION

2 The present invention is directed to a system for

3 measuring the state of charge of a battery. The invention is

4 especially use~ul for monitoring rechargeable storage batteries such as those used in battery powered vehicles which may include 6 various battery p~owered tools, such as fork lifts or the like, 7 and it will be described in detail in this context. However, 8 the inventive system may be used with any battery powered system 9 using rechargeable or non-rechargeable batteries.
Circuitry is provided for integrating a signal related 11 to the magnitude and duration of fluctuations in the battery 12 terminal voltage and for displaying the state of charge of the I3 battery in terms of percentage charge remaining in the battery~
14 The display is similar to a display showing the fuel remaining in a conventional gasoline powered vehicle and is therefore quite 16 easy for an operator familiar only with gasoline powered vehicles 17 to read and understand. The system may also be provided with a 18 deep discharge detector which, when the remaining charge in the 19 battery has been depleted below a predetermined level, disables .-the various tools on the vehicle, leaving only those systems that ; 21 are essential for the operator to be able to return to a battery 22 charging station.
?3 In tha preferred er~odiment, connection of the battery .~;
2 to the vehicle results in the actuation of a circuit which de-2 tects whether the voltage present at the terminals of the battery ' 26 is above a certain threshold value. Insofar as a newly charged ; 2 battery has an output voltage which is significantly higher than 2 the nominal terminal voltage, the threshold value is picked to , 2 be about 10 percent above the norninal terminal voltage. If this 3 threshold voltage is detected by the circuit, it furnishes a : ' . . . . .
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fairly reliable indication that the battery is freshly charged and causes the state of charge monitoring circuitry to produce an indication that the battery is fully charged.
As the battery is used, varying load conditions placed across the battery cause the voltage to be reduced. The magni-tude and duration of each of these voltage reductions is monitored by a threshold circuit which produces an output whenever the term~
inal voltage falls below a predetermined threshold. In accord-ance with one embodiment of the invention, the output of the threshold circuit is connected to circuitry which generates a train of pulses in response to reductions in voltage. The number of pulses generated is a function of the time during which the terminal voltage is below the threshold voltage. Illustrat-ively, the pulse generating circuitry takes the form of either a voltage controlled oscillator or a relaxation oscillator.
The pulse generating circuitry is in turn connected to integrating means for counting the pulses and accumulating the count, thus generating an integral which is proportional to the total time that the terminal voltage is below the threshold voltage. This counting means may take the form of an electronic counter or a stepping motor.
The output of the integrating means furnishes an indi-cation of the state of charge. This indication is more accurate than prior art devices since the integrating means accumulates ;~
the count for each time the terminal voltage falls below the ;~
threshold value. In the case of the electronic counter, the out~
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put of the integrating means may be converted to an analogue signal and used to drive a conventional electric meter; and in the case of the stepping motor, the motor may be used to position the needle of a gauge. The output of the integrating means may also activate an alarm which warns the operator that the state of charge of his 3.

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1 vehicle battery is at a predetermined low sta~e of charge and, 2 at a lower level of charge, may disable auxiliary functions cn 3 the vehicle such as the fork lift, thereby forcing the operator 4 to return to the base station for a fresh battery.
In an alternative embodiment, the magnitude and 6 duration of voltage reductions caused by varying load conditions 7 placed across the battery are monitored by a multiple-threshold 8 circuit whose output signal is related to the magnitude and 9 duration of the voltage reductions. This signal is stored by an integrator which drives a display. The output of the inte-11 grator, which may be displayed by a simple display device such 12 as a d'Arsonval electric m~oter, furnishes an indication of the 13 state of charge. The output of the integrator may also activate 14 an alarm as in the first embodiment.

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1 BRIEF DESCRIPTION OF T~IE DRAWINGS
2 Figure 1 is a schematic diagram of the power system of a battery powered vehicle incorporating an illustrative control 4 system for monitoring the state of charge of the batteryi Figure 2 is an alternative embodiment of a control 6 system constructe~d in accordance with the present invention;
7 Figure 3 is a perspective view of an indicator for use 8 in conjunction with still another embodiment of the invention;
9 Figures 4-6 are plan views of the indicato~ illustrated in Figure 3 in various positions corresponding to different 11 levels of charge; .
12 Figure 7 is a schematic diagram of a battery state ..
13 of charge monitoring system incorporating the indicator illus-14 trated in Figures 3-6; .
. Figu~e 8 is a schematic illustration in block diagram 16 form of an alternative monitoring system constructed in accord-17 ance with the present invention; and 18 Figure 9 is a schematic representation of an alterna-19 tive threshold detection circuit which may be used in the circuit 20 of Figure 8. .
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-~ - ,: ,,, 10~L~614 1 ETAILED DESCRIPTION OF Tll~ PREFI~RRED EMBODIM13NTS
2 For c~nvenience the invention will be described in a 3 number of illustrative embodiments particularly useful for measuring the state of charge of the rechargeable batteries in a ba-ttery powered vehicle. However, it is noted that the invention may be~applied to any battery powered system whether 7 the system employs rechargeable or nonrechargeable batteries.
8 Turning first to Figure l, power is supplied to the 9 system by a battery l via mating connectors 3 and 5. Connection of the battery to the system couples power to the essential ll circuits 7 in the system which include all the electrical sub-12 systems in the vehicle that are not to be disabled in response 13 to the detection of a depleted state of charge in the battery.
14 Connection of the battery to the system also results in the application of the battery terminal voltage to voltage-dividing 16 resistors 9, ll and 13 with the result that the magnitudes of 17 the voltages at points 15, 17 and l9 are functions of the 18 magnitude of the voltage present at the output terminals of .
l9 battery l. The appearance of a voltage at point 15 results in .
the application of that voltage to sequencer 21. In response, 21 the sequencer produces a logical "0" output which is coupled to 22 an AND gate 23 causing it to be disabled and producing a logical 23 "0" output. AND gate 23 is disabled in order to make it unrespon 24 sive to any transients which may pass through the state of charge -2 detecting circuitry via connectors 3 and 5. After a fixed period 2 of time which may be typically in the order of one second, or 2 as long as is necessary for all transients to subside, the output of the sequencer becomes logical "l".
2 The voltage at point 19 is coupled to a reset compara-3 tor 25 which compares it with a reference voltage source 27 ,' . ..` . ( ::

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powered by battery 1. Reference voltage source 27 may be any of a number of known circuits which provide constant voltages as outputs even though they may be powered by a source which varies within certain limits. Such circuits are well known and may typically compxise Zener diode regulated voltage sources or the like.
For batteries of the lead acid variety, the voltage sent by reference voltage source 27 to comparator 25 is selec-ted to be equal in magnitude to the voltage present at point 19 when the output terminal voltage of battery 1 is on the order of 10% higher than the nominal terminal voltage of the battery. This 10% figure is selected because, for lead acid batteries, the terminal voltage of the battery when it is fully charged is usually about 10% higher than its nominal terminal voltage. Thus, if the output terminal voltage of the battery is about 10% higher than its nominal terminal voltage, compara-tor 25 will detect this condition by comparing the voltage at point lg to the voltage coupled to the comparator by source 27 and will produce a logical 1 output. It has been empirically 2Q shown that this technique is generally quite reliable. Of course, the particular value of terminal voltage which one wishes to test for varies as a function of the nominal terminal voltage of the battery and the battery type.
The logical 1 output of comparator 25 dxives one of the inputs of AND gate 23. The other input of AND gate 23 is driven by sequencer 21. AS noted above, the sequencer changes its output from logical 0 to logical 1 after transients in the system have subsided. The presence at the input of AND
gate 23 of the logical '1 output of comparator 25, which indi-cates that the battery is fully charged, and the logical 1 - 7.
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output of se~uencer 21, which indicates that transients have subsided, causes the output of AND gate 23 to produce a logical "1" output. This logical 1' output is coupled to and drives the "clear' input of a counter 29, thereby clearing any signal that may be in the counter. Outputs 39a-g of counter 29 have two digital states 0 and 1' and display the output of counter 29 in binary code with output 39a being the least significant digit and output 39g the most significant.
When the counter is cleared, all of its outputs 39a-g are logical 0~, indicating that the battery is fully charged.
Outputs 39d and 39g are coupled to a NAND gate 41 whose output drives lockout circui-t 43. The two logical "0"s at the input of NAND gate 41 cause it to have a logical "1"
output. This logica~ "1 output causes lockout circuit 43 to close the contacts 45 of a relay 47 and couples power to the nonessential electrical circuits 49, such as the power lift of an electrical truck. Since output 39g is the most significant output, it will not change to a '1' output until half the total - capacity of counter 29 is counted. When this happens, the logical '1' will actuate an alarm 57, notifying the operator~
The output of NAND gate 41 will not change until more than half the total capacity of counter 29 has been counted when a digital value which includes a logical "1' at outputs 39d and 39g is first reached. At this time, lockout circuit 43 will open con-tacts 45, which will remain open, thereby disabling nonessential circuits 49. The operator of the vehicle is thus forced to return to the charging station because the nonessential task performing circuits such as the power lift of the vehlcle are disabled while such nonessential circuits such as the traction motor are still operable. As will be evident, the point at 8.

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which loc]cout circuit 43 is activated, can be modified simply by selectiny the appropriate output leads 39a-g to control NAND
gate 41.
- It has been found experimentally that the time that the voltage is below threshold is a measure of the state of charge of the battery. Thus, as the battery is used, a picture of its sta-te of charge can be continuously constructed by cumulatively storing the periods of time that the terminal voltage remains below a given threshold.
Reductions in the terminal voltage are detected by a tracking comparator 35 which compares the voltage present at point 17 to the voltage provided to comparator 35 by reference voltage source 27. The voltage coupled by reference voltage source 27 to comparator 35 is selected to be approximately equal to the voltage at point 17 when the output terminal voltage of the battery is at the desired threshold. The voltage at point ~ ~
17 is coupled to comparator 35 by a tracking filter 36 which ~ ;
prevents transients and other signals unrelated to depletion in the state of charge in the battery from being registered in the monitoring circuitry. Setting the response of filter 36 to eliminate transients faster than 10 milliseconds to ;~ 100 milliseconds has been found to give excellent results, as this effectively eliminates microsecond and millisecond transients which are not related to charge depletion.
AND gate 37, which is coupled to the clocking input of counter 29, is responsive to the output of tracking comparator 35, to a free-running oscillator 39 and to the output of NAND gate `
41. When counter 29 is cleared by the pulse from AND gate indicative of full charge, all of its outputs become logical "0", and outputs 39d and 39g cause NAND gate 41 to produce a logical "1"
at its output.

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If there is then a drop in battery voltage below threshold, this drop will be detected by comparator 35 which will in turn apply a logical 1 to the input of AND gate 37.
With the output of comparator 35 and NAND gate 41 logical 1 , the periodic pulsed application of a logical 1 signal to AND
gate 37 from oscillator 39 will result in the periodic appli-cation of a pulsed logical 1 signal from AND gate 37 to the clocking input of counter 29. This advances the count of the counter which accumulates the number of pulses applied from gate 37, said number being proportional to the total time that the terminal voltage has been below the threshold value. In ordinary usage, the terminal voltage will have to fall below the threshold value several times before the count in counter 29 becomes high enough to trigger alarm 57.
The output of counter 29 is converted to an analogue signal by summing resistors 51a-g. Resistors Sla-g have success-ively lower values, each resistor having a value of resistance one half that of the previous resistor. Thus, resistor 51a has a value of R ohms, resistor 51b a value of R/2 ohms, resistor 51c a value of R/4 ohms and so forth. ~he current output from resistor 51b is thus twice the current output from resistor 51a, while the current output from resistor 51c is four times the output current from re-sistor 51a, etc. The outputs of the resistors are coupled together and sent to an inverting amplifier 53 which sums them, Because amplifier 53 is an inverting amplifier it has a maximum output when outputs 39a-g are all logical 0 . This results in a full scale deflection of meter 31 which is gradually decreased to zero as pulses are stored in counter 29. Because these pulses are peri-; odic and are only coupled from the oscillator during the time that the tracking comparator senses a voltage below threshold, the num-ber of pulses stored is proportional to the total amount of time that 10.

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1 trclcking compara~or 35 has detec~cd a voltage below threshold.
2 Thus, the display on panel meter 31 reveals the state of charge 3 of the battcry. -4 ~ Depletion of the state of charge in a battery powered ~ system results in longer and deeper transient voltage reductions 6 in rcsponse to transien-t load condi-tions. It may therefore be 7 desirable to vary the threshold of comparator 35 in response to 8 the integral stored in counter 29. Specifically, because of the 9 increasing magnitude of voltage reductions with decreasing state of charge, it may be desirable to be able to lower the threshold 11 value in response to a lower level of charge in the battery.
12 This may be done by connecting a resistor 53' from the output of 13 amplifier 53 to the input of tracking comparator 35, as is 14 illustrated in phantom lines.
16 The longer and deeper transients in output terminal 16 voltage which occur in response to increasingly lower states of 17 charge in a battery may also be compensated for by making the 18 response of trac~ing filter 36 a function of the integral stored 19 in counter 29. This may be done using the feedback path, shown in phantom lines in Figure 1, extending between amplifier 53 and 21 filter 36. -22 Under various circumstances, battery 1 may be~ dis-23 connected from the system and then reconnected. In order to 2 prevent counter 29 from losing the count stored in it, it is 2 necessary to provide the system with a memory battery 79 which supplies power to counter 29 during the interval that the battery 2 1 is disconnected. Memory battery 79 is coupled to counter 29 2 by diode 81 which i5 biased into the nonconductiny region by 2 diode 83 when battery 1 is connected in the circuit.
3 ~eferring to Figure 2, an alternative embodiment of the 11 a .
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1 apparatus of the present invention is illustrated. The operation 2 of this system is largely identical to that of the system illus-3 trated in Figure 1. The primary difference is that oscillator 4 39, tracking comparator 35 and AND gate 37 have been replaced by tracking comparator 35', self-resetting integrator 39' and ~ND
6 gate 37'~ When a`voltage bclow the threshold value is detected 7 by comparator 35', it produces an output current which is fed 8 via diode 61 and resistor 63 to capacitor 65. The voltage pre-9 sent across the capacitor is applied to terminal 67 of a uni-junction transistor 69. Terminal 71 of the unijunction transisto 11 is provided with a bias voltage by a voltage divider comprising 12 resistors 73 and 75 which are connected to a source of DC power.
13 Whenever the output of tracking comparator 35' becomes active, 14 it sends current into capacitor 65, thereby raising the voltage at terminal 67. This ~oltage is stored in capacitor 65 after the 16 tracking comparator returns to its unactivated state, and it 17 resumes increasing as soon as tracking comparator 35' is again 18 actuated. .
lg When the voltage at terminal 67 becomes high enough, unijunction transistor 69 is driven into conduction, thereby 21 producing an output at its terminal 77. This signal is coupled 22 to AND gate 37' and, when NAND gate 41 is active, results in 23 the passing of a pulse to counter 29 and advancement of the 2 counter. During the course of completely discharging a battery, capacitor 65 is charged and discharged a great number of times X6 with the resultant application of a pulse from terminal 77 and 2 AND gate 37' every time the capacitor is discharged. It is thus seen that self-resetting integrator 39' produces a train of 2 pulses in response to tracking comparator 35' in place of oscilla 3 tor 39 of Fig. 1.
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~n alternative embodiment of the present invention is illustrated in Figures 3-7. Referring to Figure 3, an indicator 100 comprising a stepping motor (not shown in this ~igure) that drives a disk 103 via shaft 101 replaces the counter, memory battery and display unit in the other embodiments. The stepping motor is advanced an increment every time it receives an elec-trical pulse. I'his advances disk 103 which includes an indicator tab 105 indicating the state of charge of the battery on a cali-brated scale 107. The angular position of the disk 103 serves as an indication of the state of charge of the battery. Disk 103 includes four tracks 104, 111, 113, and 115 which contain information cutouts 117, 119, 121 and 123, respectively. Cutout 117 serves to indicate when the indicator is in the full position.
Cutout 119 serves as a warning slot indicating that the battery has been seriously discharged and that lift lockout will soon occur. Cutout 121 provides a control signal for the lift lockout `-and finally, cutout 123 provides control information for pre-venting further advancement of the disk when discharge is complete.
This particular information is especially important if the indi-cator is so constructed that the scale takes up a major portion ~
of the angle through which the disk moves and further advancement ~;
of the disk is likely to advance the tab 105 to the full position though the battery is depleted. It is also noted that, if desired, some economy may be obtained by using cutout 121 to perform the function of hole 123, thus eliminating the need for one of the tracks with its associated circuitry. In this case, cutout 121 would be shaped like hole 123.
The information-contained in the cutouts is read by ~ . .
four light emitting diodes 125, 127, 129 and 131 and correspond-ing photocells 133, 135, 137 and 139. Initially, the disk is 13.
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at the Eull position where light is allowed to pass only from light emitter 125 to its associated photocell 133, as illustra-ted in Figures 3 and ~. As the battery is depleted and the disk advances in response theeeto, the light from diode 127 begins to pass through cutout 119 onto photocell 135, activating appro-priate warning circuitry and indicating that lift lockout will soon follow. This condition is illustrated in Figure 5. The disk next passes to the position shown in Figure 6, where light also begins to pass from light emitting diode 129 to photocell 137. This causes the actuation of the lift lockout circuitry.
Finally, as the battery continues to be discharged and the disk continues to rotate, light from light emitting diode 131 begins to pass through cutout 123 and impinge upon photocell 139. This prohibits further rotation of disk 103 and thus prevents it from continuing to rotate until it again reaches the full position.
A typical control circuit for use with indicator 100 is shown in Figure 7. Power to the system is supplied by a battery 201 via mating connectors 203 and 205. Connection of the battery to the system couples power to essential circuits 207 which include all the systems in the vehicle that are not to be disabled in response to the indication of a depleted state of charge in the battery. Connection of the battery to the sys-tem also results in application of the battery terminal voltage to voltage-dividing resistors 209, 211 and 213, with the result that the magnitudes of the voltages at points 215, 217 and 219 are functions of the magnitude of the voltage present at the output terminals of battery 201.
The appearance of a voltage at point 215 results in the application of that voltage to a sequencer 221. In response ~ -.

~ L0~9614 1 the sequ~ncer produc~s a logical "0" output, which is coupled to 2 an AND gate 223, causing it to be disabled and producing a logica 3 "0" output. AND gate 223 is disabled in order to cause it to be 4 unresponsive to any transients which may pass through the system during connection o~ the battery via connectors 203 and 205.
6 After a fixed period of time which may typically be in the order 7 of one second, or as long as is necessary for all transients to 8 subside, the output of the sequencer becomes logical "1".
9 Connection of the battery to the system also results in the presence at point 219 of a voltage 11 whose magnitude is proportional to the voltage present at the 12 output terminals of battery 201. This voltage is coupled to a 13 reset comparator 225 which compares it with a reference voltage 14 supplied by a reference voltage source 227 which is powered by battery 201. If comparator 225 detects that the voltage present 16 at the output terminals of battery 201 i5 abovP a threshold which i7 may typically be in the order of 10 percent higher than the 18 nominal terminal voltage of the battery for batteries of the lead .
19 acid variety, comparator 225 will produce a logical "1" output.
This output, together with the logical "1" output of the se-21 quencer, will actuate AND gate 223 causing a logical "1" pulse 22 to appear at its output.
23 This logical "1" pulse is then coupled to a bistable reset circuit 229, thereby setting the circuit and coupliny a 2 relatively large voltage via diode ~31 and resistor 233 to a 2 voltage controlled oscillator 235. This voltage controlled 2 oscillator has a characteristic that when no signal is coupled 2 to it, it produces an extremely low frequency oscillation having 2 a frequency essentially equal to zero. When a relatively small 3 voltage siynal ., , ,15. ' ' ' ., - ~
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~V4~61~1 is coupled to it, it has a relatively low Ere~uency signal at its output; and when a relatively large voltage signal is coupled to it, it has a relatively high freguency signal at its output. Thus, the relatively large voltage signal coupled to osciallator 235 re-sults in a relatively high frequency signal which is coupled to stepping motor driver amplifier 237. This amplifier dr~ves coil 239 oE the stepping motor that drives disk 103 relatively quickly until pointing tab 105 reaches the position indicative of full charge. This condition is detected by photocell 133 when light 10 from light emitting diode 125 falls upon it in this position through cutout 117. This condition is sensed by full charge sens-ing circuit 140 which resets the output of bistable flip-flop 229, thus removing voltage from voltage controlled osciallator 235.
The terminal voltage is monitored by a comparator 241 which compares the voltage present at point 217 to a voltage which is a function of one of the outputs of reference voltage 227. The voltage present at point 217 is coupled by filter 242 whose func-tion is identical to filter 36 in Figures 1 and 2. The output oE
comparator 241 is coupled to voltage controlled oscillator 235 20 via AND gate 246, diode 245 and resistor 243.
AND gate 246 is controlled by detector 248. When there is no light impinging on photocell 139, as in the case of the full charge position, detector 248 produces a logical 1 , enabling AND
gate 246. Thus, after indicator 100 has been set to a full indi- ~ -i7 cation, the detection by comparator 241 of a drop in voltage below the predetermined threshold results in the application of a logi-cal 1 input to AND gate 246 which is enabled by the output of detector 248. The output of AND gate 246 is a relatively low voltage signal which is applied through diode 245 and resistor 243 to oscillator 235 to produce a relatively low fre~uency signal and thus advance the stepping motor at a relatively low 16.
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As the stepping motor is advanced, light Erom light emitter 127 is caused to impinge upon photocell 135 through cutout slot 119. Actuation of photocell 135 results in the detection of this condition by warning lamp circuit 247 which in turn illuminates a warning lamp advising the operator that lift lockout is about to occur.
Further advancement of the motor results in further ;~
rotation of disk 103, causing cutout slot 121 to align with light emitting diode 129 and photocell 137, thus actuating photocell 137 and coupling a signal to lift lockout lamp circuit 249 which in turn actuates a lift lockout lamp to advise the operator of the fact that the lift has been locked out. Lockout of the lift is accomplished by coupling a signal from photocell 137 to lift lock-out circuit 251 to actuate the lockout circuit, thereby opening the contacts 253 of a relay 255 and disconnecting power from nonessen-20 tial electrical circuits 257. It is noted that relay 255 may be re-placed by any well known equivalent such as an SCR or triac. As in the case of the apparatus disclosed in Figures 1 and 2 when a full charge is detected, the lockout circuit may be reset to connect power to the nonessential circuits 257. In the interest of sim- ~
plicity, this resetting apparatus is not shown in Figure 7. ~ .
Still further rotation of disk 103 results in aligning circuit 123 with light emitting diode 131 and photocell 139. This results in a logical 0 at the output of detector 248. This log-ical 0 disables further advancement of indicator 100.
Another alternative embodiment of the invention is illus-; trated in Figure 8. When lt is desired to put a vehicle into 17.

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-service, a battery 310 i5 connected via connectors 312 ancl 314into the vehicle s monitoring system. This causes actuation of a timing circuit 316 which, after a delay typically in the order of one second, actuates comparator 318 for a period also typi-cally in the order of one second. If the voltage present at the output terminals of battery 310 is unusually high in comparison to the nominal terminal voltage, comparator 318 produces a pulse at its output. This pulse serves as an indication to the remain-ing circuitry in the system that the battery is sufficiently 10 charged. The function of comparator 318 and timing circuit 316 is similar to that of sequencer 21 and reset comparator 25 of Fig. 1 and will not be discussed further.
Reference voltages are provided for comparator 318 and several other elements of the system by a reference voltage cir-cuit 315 that produces reference voltages A, B, C and D using conventional circuitry. As shown reference voltage A is applied to comparator 318.
An integrator 328 is used to store a signal that is representative of state of charge. Initially, integrator 328 in -~
the vehicle monitoring circuit has an integral stored in it which represents the state of charge of the last battery used in the vehicle. When a new battery is placed into the vehicle, it thus becomes necessary to reset the integrating device. When compara-tor 318 senses that an unusually high voltage is present across its input and hence that a new battery has been connected to the system, it sets a bistable circuit (e.g. a flip-flop) 320, whose output is used to set integrating device 328 to zero as will be explained below. Bistable 320 also actuates clamp circuit 322 which, through unity gain amplifier 324, cause the display of a ~i 30 full charge indication on meter 326. The clamp circuit thus `
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comparator 318 regardles~ of the integral stored in the inte-qrator. This is necessitat~d due to the act that the integrator may ~ake several minutes to reset, bu~ it is desired to display the fully charged condition of the battery ~unediately.
Integrator 328 may comprise any circuit which is cap-able of integrating electrical info~mation and providing an out-put signal which is propor~ional to the integral, Such an integrator circuit is shown in Eugene P. Finger and Edward M.
Marwell's Uniked States Patent No. 4,012,681 entitled "Battery 10 Control Syste3n For Battery Operated Vehicles" filed January 3, 1975 and issued March 15, 1977, Edward M. Marwell and Curtis Busman's United States Patent No. 3,255,413 ~ssued June 7, 1977 and entitled "Elec~ro-Chemical Coulometer Including Differential Capacitor Measuring Elemen~s" and Eugene P. Fingeris United States Patent Nos. 3,704,431 and 3,704,432 both issued No~ember - 28, 1972 and entitled Coulometer Controlled Variable ~requency Generator" and "Capacitive Coulcaneter Improvements".
In such integrator circuits, integration is performed ;
by an electrochemical coulometer which receives curren~ from an - 20 electrical signal source, the integral of whose output is indica-tive of the parameter which one wished to monitor. Advancement of the coulo~neter results in changing the l~ngth of the mercury columns in the coulometer and consequently the capacitive coupling between one of the mercury columns and a metallic plate disposed around the body of the coulometer. An oscillator in series with a capacitor is put in parallel with the coulometer, thereby causing an AC voltage to appear on the plate. This electrical voltage is proportional to the capacitance between the column of mercury in electrical contact with the capacitor and the plate 30 disposed around the coulometer bo~ly. This AC voltage present - on the plate is then amplified and sent to a simple amplitude .

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~496~4 detector which produces a DC output proportional to the peak-to-peak value of the AC voltage coupled to the plate. In the circuit of Figure 8, ~his DC voltage is the output of integrator 328. As will be explained below, it is representative of the state of charge of the battery and is coupled to amplifier 324.
In such a system, resetting of the integrator is accom-plished by passing a current through the coulometer. This cur-rent is in a direction opposite that of the signal source which advances the coulGmeter and has a magnitude relatively large compared to the magnitude of the current produced by that signal source. This may most conveniently be done by incorporating an SCR in bistable 320 and passing the output of bistable 320 through the coulometer.
The value of the integral stored in integrator 328 is sensed by comparator 330 and compared with a value corresponding to full charge voltage as determined by reference voltage B.
When the integrator reaches full charge, the comparator resets bistable 320 which, in turn, disables clamp circuit 322. The output of integrator circuit 328 is then free to drive amplifier 324, thereby displaying on meter 326 the integral representative of the state of charge of battery 310 which is stored in the integrator.
As the charge stored in battery 310 is depleted, the - placement of varying load conditions across the battery results in a corresponding fluctuation in the voltage present at the battery terminals. The present invention obtains a signal indica-tive of the state of charge of a battery by monitoring the magni-tude and duration of drops in terminal voltage. The magnitude and duration of the decrease in terminal voltage is detected by ~ -circuit 332 and sent to the integrator 328. Circuit 332 is a :
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1~3614 1 circuit which will produce a cuxrent at its output which is responsive to the voltage at the battery terminals. Thus, in 3 the preferred embodiment, as the voltage at the output of battery terminal 310 drops below a threshold, circuit 332 is actuated to feed a current to integrator 328, thereby advancing 6 integrator 328 so~that the voltage level displayed on meter 326 7 decreases from that indicative of full charge. The output of 8 circuit 332 is active only for the time when the voltage is 9 below its threshold value and returns to its inactive state in response to a rise in terminal voltage above that threshold value.
11 In most batteries, the terminal voltage will drop below 12 the thre~hold more frequently and for longer periods of time as 13 the battery charge is increasingly depleted, but the relationship 14 between time below threshold and state of charge is not linear.
-15 This non-linear relationship is compensated for through the non-16 linear action of circuit 332.
17 A particularly advantageous non-linear circuit 332 is 18 illustrated in Figure 8. This device comprises a threshold 1~ detector 334 which, when the voltage at the terminals of battery 20 310 drops below its threshold, produces an electrical signal 21 which advances integrator 328. ~urther reductions in terminal 22 voltage below successively lower thresholds results in actuation 23 of successive threshold detectors 336 and 338. Detectors 334, 24 336 and 338 are se~uentially and individually actuated (i.e., ; 25 non-cumulatively) in response to voltage reductions with detectors .26 334, 336 and 338 having, respectively, high, medium and low thres-27 holds and, respectively, high, medium and low outputs. This 28 results in successively reducing the effect on integrator 328 of ~ successively greater reductions in terminal voltage. Thus, as : 50 the frequency, duration and magnlt~de of voltage reductions ~ ~ 21.
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1~9~L4 increase, their increasing magnitude results in a successively decreasing effect on integrator 328.
~ lternatively, detector 334 may produce a constant output once it is actuated, and detectors 336 and 338 may be successively and cumulatively actuated to produce outputs having opposite sense to and lesser but fixed magnitude in comparison to the output of detector 334. Detectors 336 and 338 would thus have the effect of reducing the output of circuit 332 as the magnitude of voltage reductions increases. In the alternative, the use of a non-linear display device will also serve the func-tion of linearizing the display.
When the output of integrator 328 reaches a value corresponding to a first predetermined low state of charge in battery 310 as determined by reference voltage C, it triggers threshold circuit 340 which actuates a low capacity warning light in order to warn the operator of the battery's condition. Fur-ther use of the battery with corresponding further reductions in the output of integrator 328 results in the actuation of threshold circuit 342 when the output of the integrator reaches a still lower value determined by reference voltage D. Actuation of circuit 342 removes electricity from nonessential systems on a vehicle such as the lift, thereby leaving the vehicle with power applied only to such essential functions as the traction motor and forcing the operator to return to a central station for a newly charged battery.
Insofar as certain integrating devices such as electro-chemical integrators may be damaged if they are driven beyond their limits of integration, deep discharge rejector 344 and over charge rejector 346 will be responsive to the output of the integrator to prevent further integration at a point before the 22.

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1 limits o~ integration of in~ecJratOr 328 are exceeded. ~ejector 2 circuits 3~4 and 3~6 will thus protect inteyrator 328, during 3 discharge of the battery and resetting of integrator 328, respectively. In the case of a system using an electrochemical coulometer as an integrator, rejectors 344 and 346 may simply 6 take of the form of current sources which are activated by 7 threshold circuits to produce at the limits of integration a 8 current opposite in direction to the current which is advancing g the electrochemical coulometer.
~0 Referring to Figure 9, an alternate threshold detection ~ ~
11 circuit 332' is illustrated that can be used in place of the -circuit 332. ~hreshold circuit 332' comprises thxeshold detectors 13 350a-n that are triggered at a voltage level responsive not only 14 to the voltage present at the output of battery 310 but also to a feedback voltage coupled by resistors 352a-n from the output la of integrator 328. Threshold detectors 350a-n advance integrator 17 328 at a rate proportional to the value of their respective out-1~ put resistors P~a-n. Threshold detectors 350a-n may simply be 1~ comparators which change their output at different threshold 20 values which are a function of the voltage fed back from integra-21 tor 328 by resistors 352a-n and reference voltages coupled from 2~ reference voltage source 315' by resistors 356a-n. For the 23 circuit shown in Figure 9, detectors 350a-n are successively 24 and cumulatively actuated at dif~erent threshold values determined ~5 by reference voltage source 315' in a similar fashion as the alternative non-linear circuits 332 of Figure 8 discussed three 27 paragraphs above. Thus, comparator 350b couples a current through 28 resistor Rb which is oppositP in sense and lower in magnitude ~ than the output of compara~or 350a so that as the voltage at the 50 battery terminal becomes lower and comparator 350b is actuated, ~/' ,, ,. .,~

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~ 4g~:~4 1 the output of circuit 332' is reduced and causes a reduction in 2 the rate at which the int~rator advanccs. Comparators 350c-n 3 operate in similar fashion with the limitation that the cumula-tive e~fect of operation of comparators 350b-n does not exceed the effect of operation of comparator 350a to which they are ~ opposed.
7 The operation of this circuit iis such that as the out-8 put of the integrator indicates the storage of an increasingly 9 large integral (and therefore greater depletion of the battery), the output voltage that is fed back by resistor 352a serves to 11 effectively lower the threshold values of the terminal voltage 12 of battery 310 which will cause actuation of threshold detectors 13 350a-n. Thus, as the battery is depleted and the frequency and -14 duration of the battery's tendency to drop below any given fixed threshold increases, the thresholds are lowered in order to 16 require greater and greater reductions in terminal voltage to 17 actuate comparators 350a-n. This feedback arrangement thus 18 reduces or nullifies the relatively rapid advancement of the -~-19 integrator that would occur as the battery's charge is increas-ingly depleted if the threshold of the detector circuit were not 21 varied. At the same time it permits relatively rapid advancement 22 of the integrator if the battery terminal voltage should go 23 below threshold early in the operating cycle when the output 24 of integrator 328 indicates a full charge or a value close theretc .
This has the advantage of permitting the integrator to "catch 26 up" if a partially charged battery has been connected by mistake 27 to the monitoring system. If such a battery has been given a 28 short high charge, its terminal voltage initially may be high 29 enough to be accepted by comparator 318~ T~ereafter, however, the terminal voltage falls off quickly permikting relatively .
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rapid advancement of the integratorV signifying that the battery has not been properly charged.
Further variations in the feedback circuit may be ob-tained by making resistors 352a-n non linear resistive elements.
This may also result in improving the linearity of the output of integrators 328 as an indication of the state of charge.
Although the circuit has been described using a plur-ality of comparators 350a-n, it will also work well with just a single comparator 350a. It should also be noted that just as 10 the threshold of the threshold circuit may be varied in response ; ;
to the value stored in the integrator as is done in the embod-iment illustrated in Figure 9 so also may the output of the threshold circuit be varied in response to the output of the integrator. For example, this may be done by using in place of resistors Ra-n photoresistive devices whose resistance changes in response to incident light. The output of integrator 328 may then be made to drive a light source whose light would be made to fall upon the photoresistive devices and thus vary their resistance as the autput of integrator 328 is varied. Variation ; 20 of the resistance of the photoresistive devices results in vary-ing the current output of comparators 350a-n, thereby varying the output of the threshold circuit.
As will be apparent, a plurality of threshold detectors ~; may also be used in the circuits illustrated in Figures 1 - 7 to synthesize any desired response by the selection of various thresholds and various electrical outputs for each of the plur-ality of threshold detectors. For example, a synthesized re-- sponse can readily be used to charge capacitor 65 of Figure 2;
or apparatus could be provided to apply clocking pulses to the 30 digital counter of Eigure 1 at different rates depending on which threshold detector was activated. The feedback circuits shown in Figures 8 and 9 can likewise be implemented in the apparatus for 25.

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Figures 1-7 as is indicated by resistor 53~ of E`igures 1 and 2.
With respect to the embodiment of ~igures 3-7, feedback may also be provided by the use of additional light emitting diode (LED) and photocell combinations.
The embodiments in Figures 1-9 disclose a system in which a threshold circuit produces an intermediate output signal during the time in which the magnitude of the battery terminal voltage is below a threshold value. This signal is then integrated to provide a first output signal and in typical operation is accumulated over several different intervals in which the magnitude of the battery terminal voltage is reduced below the threshold value. It should be noted that the threshold value need not remain constant during the period that integration is taking place. For example, in those de-vices in which there is feedback from the integrator the threshold value may vary during the period of time the battery terminal volt-age is below it. It may also be advantageous to produce an inter-mediate output signal which continues for a fixed period of time after the voltage rises above the threshold. Finally, it should be recognized that transient excursions below the threshold value will not be integrated in those circuits that use a filter to eliminate such transients.
Since the threshold circuit is not activated until the magnitude of the terminal voltage is below a threshold value, the first output signal may also be described as a function of the magnitude of the terminal voltage. In the embodiments shown in Figs. 1-7, the magnitude of the intermediate output signal does not vary with the magnitude of the battery terminal voltage pro- `~
vided that magnitude is less than the threshold value. Where a plurality of threshold detectors are used, however, the magni-tude of the intermediate output signal does vary as a func-tion of the magnitude of the battery terminal voltage even .
26.
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,, -' ,'' ' ' ' . . ~,: '' ~ 9~4 1 I when the batt~ry terminal volta~e is below the first threshold 2 ¦ value. In the embodiments shown in Figures 8 and 9, this 3 ¦ variation is in discrete steps. ~ppropriate circuitry for 4 ¦ providing an analogue intermediate output signal will also be

5 ¦ evident to those skilled in the art.

6 ¦ The intermediate output signal from each of the thres-

7 ¦ hold detectors shown in Figure 9 will be recognized as a

8 ¦ function of the difference between the battery terminal voltage

9 ¦ and a reference voltage that is a function o~ the output o~

10¦ integrator 328 and the output of reference source 315'. As will

11¦ be evident, various transfer functions can be synthesized

12¦ depending on the particular feedback circuitry used. All these

13¦ modi~ications are contemplated to be within the scope of the 1~¦ invention. In addition, since the invention can be practiced 15¦ using either a positive voltage polarity or a negative voltage 16¦ polarity, it will be recognized that either one is fully the 17¦ equivalent of the other and both are within the intended scope 18¦ of the claims.
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Claims (44)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for monitoring the condition of a battery comprising:
(a) means for monitoring the output terminal voltage of the battery and producing an intermediate output signal when the magnitude of the terminal voltage is below a threshold value;
(b) means for integrating said intermediate output signal over a plurality of reductions in the magnitude of the output terminal voltage to less than said threshold value; and (c) means for producing a first output signal related to said integral.

2. The apparatus of claim 1 wherein said intermediate output signal is a function of the time that the magnitude of said terminal voltage is below said threshold voltage and is also a function of the difference between the output terminal voltage and a reference voltage.

3. The apparatus of claim 1 wherein said intermediate output signal is a function of the time that the magnitude of said terminal voltage is below said threshold value and is also a function of the difference between the output terminal voltage and a reference voltage that is a function of the first output signal.

4. The apparatus of claim 1 wherein said intermediate signal is a function of the time that the magnitude of said terminal voltage is below said threshold value and is also a function of the output terminal voltage, a reference voltage, and the first output signal.

5. The apparatus of claim 1 wherein said inter-mediate output signal is a function of the time that the mag-nitude of said terminal voltage is below said threshold value.

6. The apparatus of claims 1, 2, or 5 wherein said sensing and signal producing means comprises a comparator re-sponsive to the output terminal voltage and a source of reference voltage to produce an intermediate output signal when the mag-nitude of the terminal voltage is below said threshold value.

7. The apparatus of claim 1 wherein:
(a) the intermediate output signal comprises a series of pulses, the number of pulses being a function of the time that the magnitude of said terminal voltage is below said threshold value; and (b) said integrating means is responsive to said pulses to produce said first output signal, said first output signal being a function of the number of pulses produced in response to reductions in the magnitude of the terminal voltage.

8. The apparatus of claim 7 further comprising a voltage controlled oscillator for producing said series of pulses when the magnitude of the terminal voltage is below said threshold value.

9. The apparatus of claim 7 further comprising:
(a) an oscillator for continuously producing a series of pulses; and (b) gate means that passes said series of pulses when the magnitude of the terminal voltage is below said thres-hold value.

10. The apparatus of claim 7 further comprising relaxation oscillator means for producing said series of pulses when the magnitude of the terminal voltage is below said thres-hold value.

11. The apparatus of claim 1, wherein:
(a) the intermediate output signal comprises a series of pulses, the number of pulses being a function of the time that the magnitude of said terminal voltage is below said threshold value;
(b) said integrating means is a stepper motor coupled to receive and integrate said pulses; and (c) indicator means are connected to said stepper motor for indicating said first output signal.

12. The apparatus of claim 11 wherein said indicator is a disk which is rotated by the stepper motor, the angular position of the disk corresponding to the integral stored in the stepper motor, at least one hole being defined in said disk corresponding to a range of integral values, said apparatus further comprising:
at least one light source disposed on one side of said disk and at least one photodetector disposed on the other side of said disk, said light source and photodetector being positioned so that for certain angular positions of said disk the light source, hole and photodetector will be aligned to produce an electrical signal indicative of the position of the disk.

13. The apparatus of claim 1, further com-prising:
(a) means responsive to the connection of said apparatus to a battery for sensing whether the magnitude of the output terminal voltage of said battery is above an upper threshold; and (b) means responsive to an output of said sensing means for resetting said integrating means.

14. The apparatus of claim 13 wherein the integrating means is reset over a period of time, said apparatus further comprising:
(a) means responsive to said resetting means for coupling a signal to a display device indicating that there is a full charge in said battery; and (b) means responsive to said integrating means for detecting when said integrating means has been reset and for then disabling said resetting means.

15. The apparatus of claim 1, wherein:
(a) said means for monitoring terminal voltage com-prises a plurality of threshold detectors, each of said detectors having a different threshold value and being respon-sive to said terminal voltage to produce an intermediate output signal when the magnitude of the terminal voltage drops below its respective threshold value; and (b) said integrating means integrate all intermediate output signals, whereby a desired threshold detection response may be synthesized through the selection of various thresholds and various electrical outputs for each of the plurality of threshold detectors.

16. The apparatus of claim 15 wherein said plurality of threshold detectors are successively actuated and their intermediate output signals are cumulated.

17. The apparatus of claim 15 wherein said plurality of threshold detectors are sequentially actuated one at a time in a predetermined sequence corresponding to the threshold level of each of said plurality of said threshold detectors so that no more than one detector is producing an intermediate output signal at any time.

18. The apparatus of claims 1, 2, or 5 further com-prising means for disconnecting at least a portion of a load connected to said battery when the first output signal reaches a predetermined level.

19. A method for measuring the state of charge of a battery connected in a system in which it is subjected to transient reductions in voltage, comprising the steps of:
(a) monitoring the output terminal voltage of the battery;
(b) producing an intermediate output signal when the magnitude of said terminal voltage is below a threshold value;
(c) integrating said intermediate output signal over a plurality of reductions in the magnitude of the output terminal voltage to less than said threshold value; and (d) producing a first output signal related to said integral.

20. The method of claim 19 wherein the intermediate output signal is a function of the difference between the output terminal voltage and a reference voltage.

21. The method of claim 19, wherein the intermediate output signal is a function of the difference between the out-put terminal voltage and a reference voltage that is a function of the first output signal.

22. The method of claim 19 wherein the intermediate output signal is a function of the output terminal voltage, a reference voltage and the first output signal.

23. The method of claim 19 wherein:
(a) said signal producing step comprises the step of producing a series of pulses in response to reductions in the out-put terminal voltage, the number of pulses being a function of tile time that said terminal voltage is below said threshold value; and (b) the integrating step comprises the step of integrating said pulses.

24. The method of claim 23 wherein said pulses are produced by coupling the output of an oscillator to an inte-grator in response to the detection that the output terminal voltage is below said threshold value.

25, The method of claim 23 wherein said integration is performed by coupling said pulses to a stepper motor.

26. The method of claim 19 wherein the intermediate output signal is a function of the time that the magnitude of the output terminal voltage is below said threshold value.

27. The method of claims 19, 20, or 26 further comprising the steps of:
(a) sensing, in response to connection of the bat-tery to the system, whether the magnitude of the output terminal voltage of said battery is above an upper threshold; and (b) resetting said stored integral to an initial value if the magnitude of the output terminal voltage is above said upper threshold.

28. The method of claim 19 wherein a plurality of intermediate output signals are produced and all intermed-iate output signals are combined, each of said output signals being produced in response to a reduction in the magnitude of bat-tery terminal voltage below a different threshold value, whereby a desired threshold detection response may be synthesized through the selection of various thresholds and various magnitudes of electrical outputs for each of said thresholds.

29. The method of claim 28 wherein said plurality of intermediate output signals are successively produced and said signals are cumulated.

30. The method of claim 28 wherein said plurality of outputs are sequentially produced so that no more than one output is being produced at any time, the particular output produced being a function of the magnitude of the reduction in voltage.

31. The apparatus of claims 1, 2 or 5 further com-prising means for varying said threshold value, during said monitoring, as a function of said first output signal.

32. The method of claims 19, 20, or 26 further comprising the step of varying the threshold value as a function of the first output signal while measuring battery state of charge.

33. The apparatus of claims 1, 2, or 5 further compris-ing display means driven by said first output signal.

34. The method of claims 19, 20, or 26 further compris-ing the step of using said first output signal to drive a display of battery state of charge.

35. Apparatus for monitoring the condition of a battery comprising:
a) means for monitoring output terminal voltage of the battery and producing at least a single pulse for each sensed reduction in such voltage to less than a threshold value;
b) means for counting said pulses over a plurality of such pulse producing reductions in the magnitude of the output terminal voltage to less than said threshold value; and c) means for producing a first output signal related to said count.

36. The apparatus of claim 35 wherein the number of pulses produced is a function of the time that the magnitude of said terminal voltage is below said threshold value.

37. The apparatus of claim 35 further comprising means for varying said threshold value, during said monitoring, as a function of the number of pulses counted by said counting means.

38. The apparatus of claims 35, 36, or 37 further com-prising means for disconnecting at least a portion of a load connected to said battery when the count reaches a predetermined level.

39. The apparatus of claims 35, 36, or 37 further com-prising display means driven by said first output signal.

40. A method for measuring the state of charge of a battery connected in a system in which it is subjected to tran-sient reductions in voltage, said method comprising the steps of:
a) monitoring the output terminal voltage of the battery;
b) producing at least a single pulse for each sensed reduction in the magnitude of the output terminal voltage to less than a threshold value;
c) integrating said pulses over a plurality of reductions in the magnitude of the output terminal voltage to less than said threshold value; and d) producing a first output signal related to said integral.

41. The method of claim 40 wherein the number of pulses produced is a function of the time that the magnitude of said terminal voltage is below said threshold value.

42. The method of claim 40 further comprising the step of varying the threshold value as a function of the first output signal while measuring battery state of charge.

43. The method of claims 40, 41, or 42 further comprising the step of disconnecting at least a portion of a load connected to said battery when the count reaches a predetermined level.

44. The method of claims 40, 41, or 42 further comprising the step of using said first output signal to drive a display of battery state of charge.