GB2241342A

GB2241342A - Battery charge indicator

Info

Publication number: GB2241342A
Application number: GB9004065A
Authority: GB
Inventors: Albert Everett Sloan
Original assignee: SLOAN POWER ELECTRONICS Ltd
Current assignee: SLOAN POWER ELECTRONICS Ltd
Priority date: 1990-02-22
Filing date: 1990-02-22
Publication date: 1991-08-28
Also published as: GB9004065D0

Abstract

In a battery charge indicator which operates with a battery (not shown) on load, a hold circuit (5) initially stores a voltage representing the open-circuit voltage of the battery. Analogue switches (3, 4) are then controlled (11) to sample the battery voltage and to provide discharge in the circuit (5) during relatively short, but well spaced, sampling periods, the voltage in the hold circuit being discharged with a time constant chosen in accordance with the ampere hour rate of the battery and the current supplied to the load. A comparator (8) compares the stored value of voltage in the hold circuit (5) with a reference voltage (7) and energises a display (9) to indicate the charge remaining in the battery. <IMAGE>

Description

BATTERY CHARGE INDICATOR There are many applications where secondary batteries are used as the source of power in which it is useful to be able to predict the stored energy remaining before the batteries require recharging. This is particularly useful in the case of battery electric vehicles of all types which may have to be driven some distance to the nearest charging facility.

Various means have been adopted to provide such an indication, most of which measure the terminal voltage of the battery.

These depend upon the fact that the open circuit terminal voltage is dependent upon the specific gravity of the electrolyte which relates directly to the condition of charge of the battery.

However these meters are not very accurate because the relationship between the specific gravity and voltage varies with age of the battery and also on temperature.

In most circumstances compensating for these changes is not a practical proposition and some inaccuracies are normally accepted.

The biggest problem, however, is in obtaining an accurate measurement of the open circuit voltage in a practical situation. This is because this is only directly measurable when the battery is not supplying any current. The direct measurement is therefore only possible when the vehicle ,for example, is not being driven. this is a severe limitation because some vehicles such as electric cars may be driven continuously for long periods of time.

The terminal voltage of the battery falls whenever current is being drawn. Various methods of overcoming these problems have been previously adopted many of which require the measurement of current and using the measured value to compensate for voltage droop. These types however require more complicated connections to measure the current. A simply installed meter which has only two connections, connected directly across the battery, has not been produced which compensates for all the various discharge conditions. Some have been designed which measure the time it takes for the terminal voltage to rise after it has been depressed by a pulse of current but this does not cover a situation in which a constant current is drawn from the battery or in which the current pulse does not end abruptly but reduces gradually, in which case false recovery rates are obtained.

The solution proposed herein is to sample the battery terminal voltage repeatedly with very short sampling periods at relatively long intervals and change a stored value in accordance with a calculated value.

The rate of discharge of a battery depends upon the level of current being drawn. It will discharge more quickly as the current increases. It is also a fact that the terminal voltage is depressed by an amount which also increases in proportion to the level of current as it is caused by the voltage drop across the battery internal resistance.

The ampere hour capacity of a battery is the parameter usually stated to indicate the discharge characteristic and is normally quoted as the product of the current the battery can supply continuously for a given period and the stated period. For example a 600 a/H battery at the five hour rate will supply 120 amperes for 5 hours.

Measuring the average voltage of the battery using a meter or an electronic integrating circuit gives poor results due to the battery droop phenomenon.

Proposal.

If a capacitor is initially charged to a voltage which is dependent upon the open circuit battery voltage, each time the system under test is first switched on, this voltage may drive a display which obviously indicates open circuit voltage and hence the state of charge. If this capacitor could be discharged at a rate dependent upon the battery droop with a time constant equal to the period used to define the battery ampere-hour capacity, the capacitor voltage would approximate to the open circuit voltage of the battery on no current.By way of an example .In the case of a battery which is fully discharged in 5 hours by a current which caused the voltage to droop by an average of 5 volts, A capacitor of the correct value could be chosen with a suitable suitable discharge resistor which would reduce the capacitor voltage by an amount equivalent to the drop expected in the open circuit voltage of the battery, by the application of a voltage equating to the voltage droop of 5 volts for the period of 5 hours.

In practice, however this would be very difficult to achieve economically because of the size of the capacitor and resistance values required. A one microfarad capacitor would need a resistor value of 10 x 109 which is far greater than the typical capacitor leakage resistance value or the leakage expected into the electronic components used for measurement.

A solution to this problem is to discharge the capacitor by a series of pulses of short duration at relatively long time intervals. The discharge per pulse would still be dependent upon the voltage droop and the sample would be stored between pulses by a good quality sample and hold circuit. typically a sample rate of 1 in 10,000 would be sufficient to give good results in many cases..

This method alone is still insufficient to give good results in all operational conditions. The battery may be discharged in a series of pulses of various current values and duration. As the sampling pulses could coincide with periods where the battery is supplying very short duration but very high current pulses. false sensing conditions would apply and excessive discharge would occur on the storage capacitor.

This can be overcome by allowing the capacitor charge to be restored when the sampling periods coincide with situations where then actual battery voltage is equal to or higher than the stored voltage by allowing the capacitor to charge via a smaller series resistance. In general the battery voltage will never rise to a higher level than the open circuit voltage for other than very short transient periods which would be filtered out by the use of the series resistance. The only conditions in which the battery voltage could rise would be if some charging process is intermittently carried out, such as in the case of regenerative braking on a vehicle, where some of the kinetic energy is converted back into stored charge in the battery. The proposed method would of course allow or this process and automatically compensate.

One embodiment of these principals is shown in fig. 1.

The battery under test is connected to the input of a potential divider (1) which divides the battery voltage by the number of cells in the battery and provides an output voltage to the voltage follower (2) equal to that of one cell of the battery. The voltage follower (2) is merely an impedance matching device which applies an insignificant load to the divider and provides a low impedance output to the sample and hold circuit consisting of analogue switches (3) and (4) and hold circuit (5).

At the point of first switch on, the voltage stored in the hold circuit (5) will be zero. The output of the voltage follower (2) will equal the open circuit cell voltage. After the supply is connected the analogue switch (4) will be turned on for a short period, by the switch on detector (10), which will allow the hold circuit (5) to charge to a voltage equal to the cell voltage. The switch on detector (10) will then turn off and remain inoperative until the supply is interrupted.

The clock circuit (11) supplies pulses of short duration at a low frequency to both of the analogue switches (3) and (4).

Typically these pulses may be of a duration of about 100 microseconds with an interpulse period of 10 seconds.

As the battery voltage reduces, the stored voltage in the hold circuit (5) will fall by an amount proportional to the difference between the stored value and the actual cell voltage during the period of the pulse. The time constant of the discharge path will be predetermined to be approximately proportional to the ampere/hour rate of the battery, taking into account the duty cycle of the sampling, by the suitable selection of storage capacitor and discharge resistor in the hold circuit. If the battery is discharged at a constant current equal to the 5 hour rate, the voltage on the hold circuit will be reduced by the difference between the open circuit voltage of a fully charged cell and that of a fully discharged cell in a time of 5 hours, i.e. from 2.12 volts to 1.84 volts.

In practise however, although constant discharging is a possibility, the battery is more likely to be discharged unevenly by pulses of various values and time periods.

The sampling method used would then be in the nature of random sampling of the actual cell voltage. Over the total time period the hold circuit (5) would store a reasonable representation of the open circuit cell voltage. It is remotely possible, however, for the sampling rate and the discharge pulses to synchronise during significant time intervals and therefore for some drift to occur between the actual or calculated open circuit voltage and the stored value. Any error produced in this way would cause a stored value which is too low.

At intervals during the discharge cycle there would be periods during which very little or no current is flowing from the battery. This would result in the cell voltage being the open circuit voltage. Some of the sampling periods will coincide with these conditions and of course this would mean that the cell voltage would be equal to or higher than the stored voltage in the hold circuit (5). The comparator (6) is designed to switch modes in this event. The output of the comparator (6) which normally would hold analogue switch (4) in the off position would now allow it (analogue switch (4)), to turn on, in addition to the switch (3), during the clock pulse from clock (11).Switch (4) connects the hold circuit (5) to the voltage follower (2) via a lower value resistor, ie. producing a much shorter time constant, which will cause the voltage in the hold circuit (5) to rise to, or close to, the actual measured cell voltage available at the output of the voltage follower (2). This operation therefore acts as a corrective measure to compensate for any drift in the accuracy of the measurements due to the sampling procedure.

In relation to the typical specified hourly rate of a battery of 5 hours the sensing circuit is operating every ten seconds so the error in the measured value is expected to be very small and within accepted tolerances.

The output of the hold circuit may then be compared to a fixed accurate voltage reference (7) within the analogue/digital converter (8) which supplies an output to any preferred display (9). This may be a bar graph, a digital reading or an analogue meter any of which may provide a continuous indication of the state of charge of the battery.

Fig 2 shows the effect of current pulses on a battery and the consequential drop in open circuit voltage. Also shown is a typical variation expected on the said storage capacitor (1) showing the reset condition referred to.

Claims

CLAIMS:

1. A method of determining the amount of stored energy remaining in a secondary battery which is being used as a source of power; said method including the steps of: (a) initially storing in a hold circuit a voltage proportional to the open circuit voltage of the battery representing its state of charge; (b) sampling at predetermined intervals of time the instantaneous terminal voltage of the battery providing power to an external load, said sampling period being relatively short compared to the sampling intervals; (c) discharging the voltage stored in the hold circuit by an amount proportional to the magnitude of the current supplied to the external load during the short sampling period; and (d) displaying the remaining voltage stored in the hold circuit in a form which represents the amount of stored energy remaining in the secondary battery.

2. The method according to claim 1, including the additional steps in the event that a sampling period coincides with the battery being in the no load state: (e) determining at the sampling period whether the open circuit voltage of the battery is higher than the voltage stored in the hold circuit; and (f) allowing the voltage stored in the hold circuit to rise to or close to the level of the open circuit terminal voltage during the sampling period.

3. The method according to claim 1 or 2, including the additional step of comparing the voltage stored in the hold circuit with a reference voltage in order to determine the value to be displayed representing the stored energy remaining in the battery.

4. The method according to any one of the preceding claims, including the step of dividing the battery terminal voltage by the number of cells in the battery.

5. Apparatus for determining the amount of stored energy remaining in a secondary battery which is being used as a source of power; said apparatus including: (a) a hold circuit for initially storing a voltage proportional to the open circuit voltage of the battery representing its state of charge; (b) means for sampling at predetermined intervals of time the instantaneous terminal voltage of the battery providing power to an external load, said sampling period being of a relatively short duration compared to the period between sampling intervals; (c) means for discharging the voltage stored in the hold circuit by an amount proportional to the magnitude of the current supplied to the external load during the sampling period;; (d) means for displaying the remaining voltage stored in the hold circuit in a form which represents the amount of stored energy remaining in the secondary battery.

6. Apparatus according to claim 5, additionally including in the event that a sampling period coincides with the battery being in the no load state: (e) means for determining during the sampling period whether the open circuit voltage of the battery is higher than the voltage stored in the hold circuit; and (f) means to permit the voltage stored in the hold circuit to rise to or close to the level of the open circuit terminal voltage during the sampling period.

7. Apparatus according to claim 5 or 6, including means for comparing the magnitude of the voltage stored in the hold circuit with a reference voltage in order to obtain a value to be displayed representing the stored energy remaining in the battery.

8. Apparatus according to any one of the preceding claims 5 to 7, including a divider for dividing the battery terminal voltage by the number of cells in the battery.

9. Apparatus according to any one of the preceding claims 5 to 8, wherein said hold circuit comprises a capacitor and resistor.

10. Apparatus according to claim 9, wherein said means (e) comprises a comparator which on detecting that the open circuit terminal voltage of the battery is higher than the value stored in the capacitor, inserts a resistor of relatively low value in comparison with said resistor in place thereof, whereby the capacitor is permitted to charge up to or close to the level of the open circuit terminal voltage during the sampling period.

11. Apparatus according to any one of the preceding claims, wherein a clock is provided to determine the duration of the sampling periods and the intervals therebetween said clock controlling an analogue switch to permit the controlled discharge of the hold circuit in accordance with the magnitude of the current supplied by the battery to the load.

12. Apparatus for determining the amount of stored energy remaining in a secondary battery which is being used as a source of power, said apparatus being constructed substantially as herein described with reference to and as illustrated in the accompanying drawings.