WO1993010590A1

WO1993010590A1 - Battery management system

Info

Publication number: WO1993010590A1
Application number: PCT/GB1992/002144
Authority: WO
Inventors: John Molyneux
Original assignee: Silent Power Gmbh Für Energiespeichertechnik
Priority date: 1991-11-20
Filing date: 1992-11-20
Publication date: 1993-05-27
Also published as: JPH07501199A; GB2261735A; EP0613593A1; GB9124671D0

Abstract

A battery management system is disclosed for use with a battery (1) formed of serially connected sub-units. The charge output from the battery (1) is measured by a current measuring circuit (3) and the open circuit voltage across each of the sub-units (1a and 1b) of the battery (1) are measured using the voltage measuring circuit (2). The digital processor (4) is provided with memory (7) and deduces a value for the charge capacity of the battery using the measured values for the open circuit voltage and total output charge, together with information on values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic of the battery (1). Alternatively, the open circuit voltage of each of the sub-units (1a and 1b) of the battery (1) can be measured using the voltage measuring circuit (2) during a discharge cycle of the battery. A power controller (8) under the control of the digital processor (4) operates to reduce the power available from the battery when the open circuit voltage falls below a predetermined threshold, thus preventing over discharge of the battery.

Description

BATTERY MANAGEMENT SYSTEM

The present invention relates to battery management apparatus, and a method of monitoring and controlling the discharge of the battery.

The control of the flow of charge into and out of a battery is vital in order to ensure safe and efficient operation. When batteries are used in such vehicles, it is desirable to have a measure of the capacity available. In order for this to be accurate, the charge capacity of the batteries needs to be known. This can deteriorate with age significantly reducing for instance the range of an electric vehicle. It is thus desirable to have an accurate up-to-date measure of battery capacity. It is also desirable to provide protection for a battery to prevent its over-discharge during use.

It is thus an object of the present invention to provide efficient management apparatus for use with a battery.

The present invention provides battery management apparatus for use with a battery formed of one or more serially connected sub-units, said apparatus comprising voltage measuring means operative to determine a value of open circuit voltage for the or each sub-unit of said battery; charge measuring means operative to measure, when said voltage value is determined, the total charge delivered by said battery since said battery was last fully charged; memory means containing information for deriving values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic of said sub-unit; and processing means operative to deduce from said information together with at least one said determined open circuit voltage value and the related measured total charge delivered, a value for the charge capacity of said battery.

The present invention also provides battery management apparatus for use with a battery formed of one or more serially connected sub-units, said apparatus comprising voltage measuring means operative repeatedly to determine a value for the open circuit voltage of the or each subunit of said battery during a discharge cycle of said battery? and protection means operative to reduce the power available from said battery when the determined open circuit voltage value for a sub-unit of said battery falls below a predetermined threshold voltage.

The present invention also provides a method of monitoring a battery formed of one or more serially connected sub-units; said method comprising the steps of providing information for deriving values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic'of a said sub-unit of said battery; determining a value for the open circuit voltage of the or each sub-unit of said battery; measuring, when said open circuit voltage value is determined, the total charge delivered by said battery since said battery was last fully charged; and determining a value for the charge capacity of said battery from said information together with at least one said determined open circuit voltage value and the related measured total charge delivered.

The present invention further provides a method of controlling the discharging of a battery formed of one or more serially connected sub-units; said method comprising the steps of repeatedly determining a value for the open circuit voltage of the or each sub-unit of said battery during a discharge cycle of said battery; and reducing the power available from said battery when a determined open circuit voltage value for a sub-unit of said battery falls below a predetermined threshold voltage.

The present invention is applicable to the management of batteries which have a fairly well defined voltage/capacity curve characteristic. One such type of battery is the sodium sulphur battery.

Examples of the present invention will now be described with reference to the drawings, in which :-

Figure 1 is a diagrammatic illustration of a battery management system according to one aspect of the present invention;

Figure 2 is a diagram of one example of the digital processor of Figure 1;

Figure 3a is a diagram of one example of the temperature sensing circuits of Figure 1;

Figure 3b is a diagram of one example of the voltage measuring circuit of Figure 1;

Figure 4 illustrates the theoretical curve for the open circuit voltage versus the charge status for the sodium sulphur battery;

Figure 5 illustrates the on-load cell voltage for various loads versus a discharge for a sodium sulphur battery;

Figure 6a and b is a flow diagram of the charge loop of the battery management system;

Figure 7 is a flow diagram of the discharge loop of the battery management system.

Referring now to Figure 1, the battery management system is designed to maintain the battery at an appropriate operating condition and to protect the battery against accidental and deliberate abuse. The battery management system can interface with an electric vehicle controller, a battery charger and a mains supply, to cut back the current demand, both in charging and discharging operation, in response to the battery status. The battery management system can also monitor and control battery temperature.

In Figure 1, a battery 1 is formed of a number, in this case two, of sub-units la and lb. These sub-units la and lb are monoblocs formed of an array of individual cells. In the example shown four sodium sulphur cells each giving a voltage of 2.076 volts are series connected in a string and there are five strings arranged in parallel. Therefore, each monobloc provides a voltage of 8.304 volts when fully charged. The serially connected strings of a monobloc are arranged in parallel to ensure that if any cell in a string fails, only one string is lost leading to, in the case shown, a 20% reduction in capacity. There is no limit to the number of parallel connected strings that can be used and in practice many more than five are used. The present invention is applicable to any member of serially connected sub-units of a battery, whether each sub-unit is an array or not.

In order to monitor the battery 1, a voltage measuring circuit 2 is provided to measure the voltage across each monobloc. Also, a current measuring circuit 3 is provided to measure the current flowing through the battery. The outputs of the voltage and current measuring circuits 2 and 3 are input into a digital processor 4. The digital processor 4 is therefore able to monitor the state of the individual monoblocs la and lb forming the battery 1 using the voltage measuring circuit 2. The total charge flowing into or out of the battery can be measured by integrating the current measured by the circuit 3 over time. In use the battery 1 will be connected across a load 5 by a switch 6. During the discharge of the battery 1 through the load 5 the charge output from the battery 1 can be measured using the circuit 3. The battery current can be derived from a suitably isolated sensor such as a shunt with associated isolation amplifier or a Hall-Effect device. The processor 4 continuously monitors the voltage across each monobloc la and lb and compares the voltage across each monobloc la and lb with a threshold voltage stored in a memory 7. The processor 4 also measures the total charge output from the battery 1 using the circuit 3 and compares with a value for the predicted charge capacity of the battery 1 which is stored in the memory 7. If the charge taken out of the battery is a predetermined proportion of the predicted charge capacity, or if a measured voltage across one of the monoblocs la and lb is less than a predetermined threshold, then the digital processor 4 will send a signal to a power controller 8 which reduces the power available to the load 5.

Temperature sensing circuits 9 are provided for each of the monoblocs. la and lb in order that the digital processor 4 can monitor the temperature of the monoblocs. If the measured temperature rises above a threshold temperature value which is stored in the memory 7, then the digital processor 4 outputs a control signal to the power controller 8 to reduce the power available from the battery 1.

In order to recharge the battery 1, a battery charging circuit 10 is switchably connected across the battery 1. The battery charging circuit 10 is under the control of the digital processor 4 in order to ensure that the battery is not overcharged.

Figure 2 illustrates a modular form for the digital processor 4. In this arrangement the digital processor 4 is divided into a central micro-controller 20. The central micro-controller 20 is interfaced to four battery section monitoring modules 21a to 21d using ulticore cables with eight twisted pairs. The micro-controller 20 can conveniently control up to nine battery section monitoring modules depending on the number of monoblocs, this number being dependent on the capabilities of the micro¬ controller. Each battery section monitoring module 21a to 2Id can measure the power from the monobloc (section) , the voltage across the monobloc, and monobloc temperature. The central microprocessor 20 receives signals (a voltage) representing the battery current from the current measuring circuit 3, and is powered by the battery voltage or alternative supply such as the 12V supply in a vehicle. The central micro-controller 20 also has an RS232 port which may be used e.g. for test or diagnostic purposes. Output from the central micro-controller 20 are signals indicating the state of charge of the battery and power available (e.g. mark/space waveform) . These signals are transmitted to the vehicle controller (not shown) via a ulticore cable with two twisted pairs.

A suitable microprocessor for use as the micro¬ controller is a Motorola MC 68 HC 11 operating at 8 MHz in an external memory mode. External memory (not shown) utilised by the micro-controller 20 can typically be an 8K EPROM with an 8K battery-backed RAM, for non-volatile memory in case of power failure. The EPROM is used for program storage whilst the RAM is used for storage of determined values in case of power failure.

All inputs/outputs to and from the digital processor 4 are electrically isolated from the battery for safety purposes.

Figure 3a illustrates an example of the temperature sensing circuits 9 of Figure 1. Type K thermocouples 30 are coupled to each monobloc and the output is filtered using passive filters 31.

Figure 3b illustrates an example of the voltage measuring circuit 2 of Figure 1. The monobloc voltage to be measured is filtered by passive filter 40 before being converted into a frequency signal by a voltage to frequency converter 41. The frequency signal is then transmitted to the micro-controller 20 via an opto-isolator 42. The opto-isolator provides for safe measurement of the frequencysignal.

Figure 4 illustrates a theoretical curve for the open circuit voltage versus charge held for a sodium sulphur battery. The open circuit voltage for a single cell of a sodium sulphur battery is 2.076 volts. For a new battery the end of discharge voltage which defines the bottom of the normal desirable discharge region is 1.9 volts for a single cell. This defines the 100% capacity. During the discharge of a sodium sulphur cell up until 80% of capacity theoretically the cell voltage remains unchanged. At 80% of discharge capacity, the voltage then falls to 1.9 volts at 100%. For an older battery, the charge capacity is reduced due to cell deterioration. However, the characteristics of the cell are still the same. At 80% of discharge capacity the voltage falls off to 1.9 volts at 100% of this reduced capacity. Clearly therefore for applications where it is necessary to know the capacity of the battery, the capacity must be monitored in order to keep a check on cell deterioration.

In practice the curve is not the theoretical shape of Figure 4 but more as is shown in Figure 5. Figure 5 illustrates the voltage across a cell under various loads, versus the discharge. It can be seen that under increased load the voltage measured is much reduced due to the internal resistance of the cell. However, the shape of the curve is unaltered. In order to monitor the conditions of the cell accurately, the open circuit voltage must be measured. However, it is not always practical and in such circumstances the load voltage can be measured and the internal resistance of the cell can be compensated for. Also, the profile illustrated in Figure 4 is only achieved if the battery is allowed to stand for a long time or if the battery has been charged for a few minutes before being open circuited.

Although Figures 4 and 5 illustrate the characteristics of a single cell, in practice typically four cells in series are utilised in each monobloc and these serially arranged strings are arranged in parallel. Thus for a monobloc la or lb the voltage curve being monitored would normally have an open circuit voltage of 8.304 volts and an end of discharge voltage of 7.6 volts.

The ampere hour efficiency of the sodium sulphur battery is exactly 100%, so determination of the state of charge for a battery is carried out by measuring the charge input and output from the battery. Thus, by measuring accurately the charge output from the battery since the battery was last fully charged, the position on the curve in Figure 4 is known if the capacity of the battery is known. This is fine for a new battery, but when the capacity of the battery starts to deteriorate due to age, the capacity prediction can become out-of-date, possibly resulting in an over-discharge of the battery. It is thus important that the predicted capacity of a battery be updated as the battery ages.

In one embodiment of the present invention the capacity of the battery 1 formed of monoblocs la and lb can be predicted by measuring the amount of charge output from the battery since the last full charge-up and by measuring the open circuit voltage. As long as the open circuit voltage is less than 8.304 volts for a monobloc (i.e. the monobloc is more than about 80% discharge) then the position on the curve characteristic of the battery can be determined. The charge capacity of the battery can be determined by either having a look-up table of open circuit voltage values and related values for charge delivered for a range of total charge capacities characteristic of the battery monobloc, or by fitting the values to a curve which is a known relationship between the open circuit voltage and delivered charge, and is characteristic of a charge capacity. The curve to which the value fits determines the charge capacity.

As mentioned above the most accurate method of measuring the open circuit voltage for a monobloc is to make a measurement after having applied a small amount of charge to the battery. Thus the most accurate method of predicting the charge capacity of a battery is performed during the recharging operation of the battery. Figures 6a and 6b are a flow diagram of the charge loop performed by the battery management system.

In the recharging operation, it is confirmed that it is acceptable to start recharging. If so the battery charger is turned on to charge the battery for an initial predetermined period. This initial period is about five minutes after which time the current is interrupted and a measurement taken of the open circuit voltage of each monobloc la and lb. During the small recharging period the charge input into the battery is measured and deducted from the charge output from the battery. Thus after measuring the open circuit voltage and deducting the input charge, it is possible to make a first charge capacity prediction by fitting the open circuit voltage measurement and the measured output charge of the battery to a curve characteristic of the monoblocs. The battery charger is then switched on for a second predetermined period. This predetermined period is typically about twenty minutes at which time the charger is switched off to allow a second measurement of the open circuit voltage. Once again the charge input to the battery is measured and deducted from the charge output from the battery. The measurements of the charge and open circuit voltage can then be compared to the curve characteristic of the monobloc or to the values stored in the look-up table. The use of two points allows for a far more accurate fitting of the points to the curve or conversion to the look-up table values and therefore a far more accurate estimation of the charge capacity of the battery. Having made the second and more accurate charge capacity prediction, the battery charger is then switched on for a third period recharging the battery up to a point which is termed the good charge limit. Checks are made throughout the charge to see whether there have been any unexpected excursions in parameters, such as temperature, current and voltage. If no faults are detected then the batter charger is switched on for a fourth charge period to bring the battery to full charge at which time the battery charger is switched off.

This charge loop is performed by the battery management system for each and every recharging operation. However, if the battery is not discharged beyond about 80% of its capacity, then the open circuit voltage will not change and it is not possible to accurately predict a capacity. If the battery has not been discharged beyond about 80% of its capacity for some time, then the battery capacity may well have changed significantly during that time, meaning that the capacity prediction has become outdated. If the capacity prediction is not updated, the actual capacity of the battery could be exceeded resulting in possible battery damage.

Therefore for situations wherein the battery is not discharged beyond about 80% of its capacity for some time, it is necessary to provide for protection against a damaging over-discharge during heavy use.

A back-up prediction, which is less accurate than the main prediction performed during the recharging operation, is based preferably on the opportunistic measurement of the open circuit voltage of the battery. Thus the battery management system measures the open circuit voltage for the monoblocs la and lb whenever possible, i.e. whenever the battery is under no load. The voltage thus measured is not an accurate open circuit voltage since the battery has not been allowed to stand for some time or had a small recharge applied thereto. Thus this approximate measurement of the open circuit voltage is utilised in a less accurate prediction for the charge capacity of the battery. This charge capacity prediction can once again only be made if a significant proportion of the battery capacity has been utilised. If this capacity prediction indicates that the capacity is reduced compared to the capacity prediction last made, (i.e. the battery voltage has dropped earlier than expected) then this capacity prediction is input into the memory of the digital processor. This inaccurate prediction can then be updated at the next recharging operation.

The battery management system also allows for the battery to be recharged l)y regenerative braking. This greatly increases the efficiency and range of an electric vehicle utilising the battery management system.

A flow diagram of the discharge loop of the battery management system is shown in Figure 7. Initially the power level is set and the output charge measured. The remaining charge is compared with the predicted capacity to determine whether the remaining charge is below a predetermined capacity threshold. If the remaining charge is below the threshold then the output power available from the battery is reduced by the power controller 8. If during this discharge loop no current is being drawn from the battery, then conditions are met for a capacity prediction and the open circuit voltage of the battery is measured to allow the charge capacity prediction. If from this new prediction the charge capacity is below the threshold, then the output power from the battery is reduced.

Even when there are no opportunities to measure the open circuit voltage (i.e. when there is a current output from the battery) a prediction is made of the open circuit voltage by measuring the load voltage and compensating for the internal resistance of the cells. The algorithm required to do this will be discussed hereinafter. If from this prediction the charge output from the battery has exceeded the threshold, then the power output from the battery is reduced.

During the discharge loop the temperature of the monoblocs la and lb is also monitored. If the temperature exceeds a predetermined threshold then this can also lead to a reduction in the power output from the battery.

In addition to making a charge capacity prediction by measuring the open circuit voltage or determining the open circuit voltage from measuring the load voltage, the measurements of the open circuit voltage or the calculation of the open circuit voltage can be used directly to protect the battery by comparison of this measured or calculated open circuit voltage with a threshold voltage. If the measured or calculated open circuit voltage is less than the predetermined threshold, then the power output from the battery is reduced. For the monoblocs la and lb shown in Figure 1, this threshold voltage would typically be about 7.65 volts, which should correspond to about 97% of usable discharge. When it is not possible to obtain a measure of the open circuit voltage of the cell during operation, to enable a calculation of the predicted charge capacity for the cell, as mentioned hereinabove, the load voltage can be measured and the open circuit voltage calculated by compensating for the internal resistance of the battery. When a battery is being discharged, the load voltage differs from the open circuit voltage according to the equation

^VOc ^{= V}Load ^{+ I}Load ^RInt ^(A)

where V_Q equals open circuit voltage;

V_ _ad equals load voltage;

I- , equals load current;

R₁ . equals internal resistance to the battery.

For a cell formed of sub-units or monoblocs, then the measured voltage is measured across the monobloc. Therefore, for a single monobloc the internal resistance can be calculated by loading the battery with two different currents in quick succession. The load voltages and load currents can then be measured and it can be assumed that the open circuit voltage remains the same. The internal resistance can then be calculated from

^VLoad l^+ILoad l^RInt^=VLoad 2^+ILoad 2^RInt ^(B*

The calculated internal resistance value can then be used to calculate the equivalent open circuit voltage during discharge by applying Equation (A) .

However, the use of the batteries in mobile applications means that during discharge, the current is at varying levels and the above method with just two know current loads is not feasible.

An alternative method of calculating the internal resistance is to measure the current and voltage applied during the charge period. Near the full charge of the battery, it can be assumed that the open circuit voltage cell will be 2.076 volts and therefore the measured current and voltage applied to the battery will allow calculation of the internal resistance using Equation (A) . During the discharge of the battery, during operation of the discharge loop by the battery management system. Equation (A) can once more be applied to calculate the open circuit voltage when it is not possible to measure the open circuit voltage directly.

Thus the digital processor 4 operates to monitor the voltage across the battery 1 and the current input or output in order to predict the charge capacity of the battery either by making accurate measurement of the open circuit voltage during recharge operation, by measuring the less accurate open circuit voltage during discharge, or by calculating the open circuit voltage when the battery is under load. The open circuit voltage value can then be used to make a capacity prediction. Also the voltage across each monobloc is measured for comparison with a threshold voltage to ensure that the battery does not become over-discharged. The most accurate charge capacity prediction is made during the recharging cycle and is stored in the memory 7. This value is only updated by a less accurate charge capacity prediction if it is less than this value.

If a predetermined threshold of the charge capacity is utilised or if the open circuit voltage of a monobloc is detected as falling below the threshold voltage, then the power available from the battery is reduced according to equation % P = 100 - INT ( (V_T - V_Min) x C_B) (C)

where VM„m. is the lowest value for an open circuit voltage for a monobloc of the battery; V_τ is the threshold voltage; INT denotes the integer of the term in the brackets; and C_, is a constant dependant on the electrical properties of the battery. Here C_ is 1800 for the described battery.

For a monobloc formed of sodium sulphur cells the selected threshold voltage V = 7.65V. Thus, using this equation the power output of the battery is cut back to 10% when V_Mi = 7.6 volts. The digital processor 4 only considers utilising the equation when the open circuit voltage is less than 7.65 volts.

As mentioned above, the temperature of each of the monoblocs la and lb is also monitored. If the temperature of a monobloc exceeds 370°C then the power output of the battery is reduced according to the equation

% P = 100 + INT((T_T- T_Maχ) x C_τ) (D)

where T„ is the temperature of the highest temperature monobloc. T,-. is the threshold temperature; INT denotes the integer of the term in the brackets; and C_ is a constant dependant on the temperature control of the battery. Here C_ is 7 for the described temperature controller. For the sodium sulphur cell the selected threshold temperature T = 370°. Thus, using this algorithm the power is cut back to 50% for a battery monobloc temperature of 377°C. The power is cut back 100% for a temperature of 385°C. If the temperature exceeds 400°C then a fault is indicated and the battery should be isolated from the load.

The battery management system of the present invention is also able to detect and compensate for the failure of cells in a monobloc. In Figure 6b, at the end of the charging cycle, a charge capacity calculation is made. During the discharge of the battery a value is obtained for the total charge output from the battery since the previous full charge-up. During the recharging operation a value is obtained for the charge input to the battery. If the two charge values are not equal, this indicates a failure of a cell. For instance, the charge input to the battery may be greater than the charge output. The digital processor 4 is thus able to compare the two different charges and calculate a new value for the predicted charge capacity.

Thus the present invention provides a method of accurately updating the predicted charge capacity of a battery. The battery management system allows for determination of discharge by monitoring ampere hours or by monitoring the open circuit voltage. One of the advantages of terminating the discharge normally on ampere hours rather than calculating the nominal open circuit voltage, is that the voltage measurement to update a state of charge when the vehicle system is non-operational can be very accurate. In addition, since no current is flowing, the voltage measurement is independent of the monobloc resistance. The problem of relying on the calculation of the open circuit voltage using the internal resistance of the battery is that the monobloc resistance varies with battery age and battery temperature and must be updated periodically.

The predicted charge capacity and the measured charge available from the battery can be displayed in many ways. One possibility is to display 100% whenever the battery is fully charged, whatever the battery capacity. Another method is to display the maximum capacity of the battery as a ratio of the nominal or new battery capacity. The second method will give a direct measurement of the deterioration and, more importantly, will enable the driver to assess the operational range more accurately when utilised in an electric vehicle. A fully charged battery with a reading of 100% will not convey to the driver any information about the battery deterioration with age.

Claims

1. Battery management apparatus for use with a battery formed of one or more serially connected sub-units, said apparatus comprising voltage measuring means operative to determine a value of open circuit voltage for the or each sub-unit of said battery; charge measuring means operative to measure, when said voltage value is determined, the total charge delivered by said battery since said battery was last fully charged; memory means containing information for deriving values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic of said sub-unit; and processing means operative to deduce from said information together with at least one said determined open circuit voltage value and the related measured total charge delivered, a value for the charge capacity of said battery.

2. Battery management apparatus as claimed in Claim 1, wherein said memory means contains a look-up table of stored values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic of said sub-unit; and said processing means is operative to compare at least one said determined open circuit voltage value and the related measured total charge delivered with said stored values, to determine a value for the charge capacity of said battery.

3. Battery management apparatus as claimed in Claim 1, wherein said memory means contains information on the relationship between values of open circuit voltage and charge delivered for a range of total charge capacity values characteristic of said sub-unit and said processing means is operative to calculate a value for the charge capacity of said battery using said relationship, together with at least one said determined open circuit voltage value and the related measured charge delivered.

4. Battery management apparatus as claimed in any preceding claim, wherein said battery comprises a plurality of serially connected sub-units, said processing means being operative to determine a value for the charge capacity of said battery by utilising the lowest determined open circuit voltage value for said sub-units of said battery.

5. Battery management apparatus as claimed in any preceding including charge capacity memory means for storing a determined value for the charge capacity of said battery; and comparison means to compare said measured total charge delivered with said stored value for the charge capacity, in order to provide an indication of the remaining charge in said battery.

6. Battery management apparatus as claimed in any preceding claim including recharging means to supply charge to said battery; wherein charge measuring means is further operative to measure the charge supplied to said battery during a recharging operation.

7. Battery management apparatus as claimed in Claim 6, wherein said voltage measuring means is operative to measure the open circuit voltage for the or each sub-unit after said recharging means has supplied a predetermined amount of charge to said battery.

8. Battery management apparatus as claimed in Claim 6 or Claim 7, wherein said processing means is operative to determine a value for the charge capacity of said battery using at least two measurements of the open circuit voltage for the or each sub-unit of said battery, taken after said recharging means has supplied two sequential predetermined amounts of charge to said battery.

9. Battery management apparatus as claimed in any of Claims 6 to 8, wherein said processing means is operative to compare the total charge delivered by said battery since a previous full recharge with the total charge supplied to said battery during a subsequent recharge, and to recalculate the charge capacity of said battery if said charges are not equal.

10. Battery management apparatus as claimed in any preceding claim including discharge warning means operative to indicate when a predetermined proportion of the charge capacity of said battery has been utilised.

11. Battery management apparatus as claimed in any preceding claim including discharge protection means operative to reduce the power available from said battery when a predetermined proportion of the charge capacity of said battery has been utilised.

12. Battery management apparatus as claimed in any preceding claim, wherein said voltage measuring means is operative to measure the open circuit voltage for the or each sub-unit of said battery during a discharge cycle of said battery when said battery is under no load.

13. Battery management apparatus as claimed in any preceding claims, wherein said voltage measuring means is operative to measure the voltage across the or each sub-unit of said battery during a discharge cycle of said battery when said battery is under load, and to calculate a value for the or each open circuit voltage by compensating for the voltage drop across the internal resistance of the or each sub-unit of said battery.

14. Battery management apparatus as claimed in Claim 7, wherein said voltage measuring means is operative to measure the open circuit voltage for the or each sub-unit of said battery during a discharge cycle of said battery, when said battery is under no load; and is operative to measure the voltage across the or each sub-unit of said battery during a discharge cycle of said battery, when said battery is under load, and to calculate a value for the open circuit voltage by compensating for the voltage drop across the internal resistance of the or each sub-unit of said battery; said memory means being operative to store the determined value for the charge capacity of said battery whenever said open circuit voltage is measured after said recharging means has supplied a predetermined amount of charge to said battery, and operative to only store the charge capacity otherwise determined when the charge capacity otherwise determined is smaller.

15. Battery management apparatus as claimed in Claims 13 or 14, wherein said voltage measuring means is operative to calculate the internal resistance of the or each sub-unit of said battery.

16. Battery management apparatus as claimed in Claim 15, wherein said voltage measuring means is operative to measure two or more different load voltages for the or each sub-unit of said battery within a short time period; and said charge measuring means is operative to measure associated load currents; said voltage measuring means being further operative to calculate the internal resistance of the or each sub-unit of said battery using said measured load voltages and load currents.

17. Battery management apparatus as claimed in Claim 16, wherein said voltage measuring means is operative to calculate the internal resistance R_Int of the or each sub-unit of said battery using the equation

^VLoad l^+ILoad 2 ^RInt ^{= V}Load 2^+ILoad 2 ^RInt

18. Battery management apparatus as claimed in Claim 15, wherein said voltage measuring means is operative to calculate the internal resistance of the or each sub-unit of each battery using measurements of a load voltage and associated load current applied to said battery, at a time during the charge cycle of said battery when the load voltage is substantially constant with time.

19. Battery management apparatus as claimed in Claim 18, wherein said voltage measuring means is operative to correct for the difference in internal resistance of the or each sub-unit of said battery during charging and discharging of said battery.

20. Battery management apparatus as claimed in any preceding claim including discharge warning means operative to indicate when a value for the open circuit voltage for a sub-unit falls below a predetermined threshold.

21. Battery management apparatus as claimed in any preceding claim including discharge protection means operative to reduce the power available from said battery when a value for the open circuit voltage for a sub-unit of said battery falls below a predetermined threshold.

22. Battery management apparatus as claimed in Claim 21, wherein said discharge protection means is operative to reduce the power P available from said battery according to the equation

% P = 100 - INT((V_T - V_Min) x C_β)

where V„Mi.n .is the lowest value for an open circuit voltage for a sub-unit of said battery; V„ is the predetermined threshold voltage; INT denotes the integer of the term in the brackets; and C„ is a constant dependent on the electrical arrangement of the battery.

23. Battery management apparatus as claimed in any preceding claim including battery temperature measuring means operative to measure the temperature of the or each sub-unit of said battery; and temperature protection means operative to reduce the power available from said battery when the temperature measured by said temperature measuring means is greater than a predetermined threshold temperature.

24. Battery management apparatus as claimed in Claim 23, wherein said temperature protection means is operative to reduce the power P available from said battery according to the equation

% P = 100 + INT((T_T TMax') x c_τ) where T„ is the temperature of the highest temperature sub-unit of said battery; T_ψ is the predetermined threshold temperature; INT denotes the integer of the term in the brackets; and C_ is a constant dependant on the temperature control of the battery

25. Battery management apparatus for use with a battery formed of one or more serially connected sub-units, said apparatus comprising voltage measuring means operative repeatedly to determine values for the open circuit voltage of the or each sub-unit of said battery during a discharge cycle of said battery; and protection means operative to reduce the power available from said battery when the determined open circuit voltage value for a sub-unit of said battery falls below a predetermined threshold voltage.

26. Battery management apparatus as claimed in Claim 25, wherein said voltage measuring means is operative to measure the open circuit voltage of the or each sub-unit of said battery when said battery is under no load.

27. Battery management apparatus as claimed in Claim 25 or Claim 26, wherein said voltage measuring means is operative to measure the voltage across the or each sub-unit of said battery when said battery is under load, and to calculate a value for the open circuit voltage by compensating for the voltage drop across the internal resistance of the or each sub-unit of said battery.

28. Battery management apparatus as claimed in Claim 27, wherein said voltage measuring means is operative to calculate the internal resistance of the or each sub-unit of said battery.

29. Battery management apparatus as claimed in Claim 28, wherein said voltage measuring means is operative to measure two or more different load voltages for the or each sub-unit of said battery within a short time period; and said charge measuring means is operative to measure associated load currents; said voltage measuring means being further operative to calculate the internal resistance of the or each sub-unit of said battery using said measured load voltages and load currents.

30. Battery management apparatus as claimed in Claim 29, wherein said voltage measuring means is operative to calculate the internal resistance R_τmt. of the or each sub-unit of said battery using the equation

^VLoad l^+ILoad 2^RInt ^{= V}Load 2^+ILoad 2^RInt

31. Battery management apparatus as claimed in Claim 28, wherein said voltage measuring means is operative to calculate the internal resistance of the or each sub-unit of each battery using measurements of a load voltage and associated load current applied to said battery, at a time during the charge cycle of said battery, when the load voltage is substantially constant with time.

32. Battery management apparatus as claimed in Claim 31, wherein said voltage measuring means is operative to conrect for the difference in internal resistance of the or each sub-unit of said battery during charging and discharging of said battery. O 93/10590

- 26 -

33. Battery management apparatus as claimed in any of Claims 25 to 32, including discharge warning means operative to indicate when a value for the open circuit voltage for a sub-unit falls below a predetermined threshold.

34. Battery management apparatus as claimed in any of Claims 5 to 33, including discharge protection means operative to reduce the power available from said battery when a value for the open circuit voltage for a sub-unit of said battery falls below a predetermined threshold.

35. Battery management apparatus as claimed in Claim 34, wherein said discharge protection means is operative to reduce the power P available from said battery according to the equation

% P = 100 - INT((V_T - V_M±n) x C_β)

where V„Mm. is the lowest value for an open circuit voltage for a sub-unit of said battery; V_ is the predetermined threshold voltage; INT denotes the integer of the term in the brackets; and

C_ is a constant dependant on the electrical arrangement of the battery.

36. Battery management apparatus as claimed in any of Claims 25 to 35 including battery temperature measuring means operative to measure the temperature of the or each sub-unit of said battery; and temperature protection means operative to reduce the power available from said battery when the temperature measured by said temperature measuring means is greater than a predetermined threshold temperature.

37. Battery management apparatus as claimed in Claim 36, wherein said temperature protection means is operative to reduce the power P available from said battery according to the equation

% P = 100 + INT((T_T - T_Maχ) X C_τ)

where T_M is the temperature of the highest temperature sub-unit of said battery; T_τ is the predetermined threshold temperature; INT denotes the integer of the term in the brackets; and

C_ is a constant dependant on the temperature control of the battery.

38. A method of monitoring a battery formed of one or more serially connected sub-units; said method comprising the steps of providing information for deriving values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic of a said sub-unit of said battery; determining a value for the open circuit voltage of the or each sub-unit of said battery; measuring, when said open circuit voltage value is determined, the total charge delivered by said battery since said battery was last fully charged; and determining a value for the charge capacity of said battery from said information together with at least one said determined open circuit voltage value and the related measured total charge delivered.

39. A method as claimed in Claim 38, wherein said information is a look-up table of stored values of open circuit voltage related to charge delivered for a range of total charge capacity values characteristic of said sub- unit; and a value for the charge capacity of said battery is determined by comparing at least one said determined open circuit voltage value and the related measured total charge delivered with said stored values.

40. A method as claimed in Claim 38, wherein said information gives the relationship between values of open circuit voltage and charge delivered for a range of total charge capacity values characteristic of said sub-unit; and a value for the charge capacity of said battery is calculated using said relationship together with at least one said determined open circuit voltage value and the related measured charge delivered.

41. A method as claimed in Claim 38, wherein said battery comprises a plurality of serially connected sub-units, and said step of determining the charge capacity of said battery utilises the lowest determined open circuit voltage value for said sub-uriits of said battery.

42. A method as claimed in any of Claims 38 to 41 including the step of storing a determined value for the charge capacity of said battery, and comparing said measured charge to the stored value for the charge capacity, in order to provide an indication of the remaining charge in said battery.

43. A method as claimed in any of Claims 38 to 43 including a recharging cycle wherein charge is supplied to said battery and the charge supplied to said battery is measured and controlled to ensure that said battery is not ove charged.

44. A method as claimed in Claim 43, wherein the open circuit voltage for the or each sub-unit of said battery is measured after a predetermined amount of charge has been supplied to said battery.

45. A method as claimed in Claim 43 or Claim 44, wherein the charge capacity of said battery is determined using at least two measurements of the open circuit voltage for the or each sub-unit of said battery, taken after said recharging means has supplied two sequential predetermined amounts of charge to said battery.

46. A method as claimed in any of Claims 43 to 45 including the steps of comparing the measured charge delivered by said battery since a previous full recharge, to the measured charge supplied to said battery during a subsequent recharge, and recalculating the charge capacity of said battery if said charges are not equal.

47. A method as claimed in any of Claims 38 to 46 including the step of indicating when a predetermined proportion of the charge capacity of said battery has been utilised.

48. A method as claimed in any of Claims 38 to 47 including the step of reducing the power available from said battery when a predetermined proportion of the charge capacity of said battery has been utilised.

49. A method as claimed in any of Claims 38 to 48 including the step of measuring the open circuit voltage of the or each sub-unit of said battery during a discharge cycle of said battery, when said battery is under no load.

50. A method as claimed in any of Claims 38 to 49 including the steps of measuring the voltage across the or each sub-unit of said battery during a discharge cycle of said battery, when said battery is under load; and calculating a value for the or each open circuit voltage by compensating for the voltage drop across the internal resistance of the or each sub-unit of said battery.

51. A method as claimed in Claim 44 including the steps of measuring the open circuit voltage for the or each sub-unit of said battery during a discharge cycle of said battery, when said battery is under no load; measuring the voltage across the or each sub-unit of said battery during a discharge cycle of said battery, when said battery is under load, and calculating a value for the open circuit voltage by compensating for the voltage drop across the internal resistance of the or each sub-unit of said battery; storing the determined value for the charge capacity of said battery whenever said open circuit voltage is measured after a predetermined amount of charge has been supplied to said battery; and only storing the otherwise determined charge capacity when the otherwise determined charge capacity is smaller.

52. A method as claimed in Claims 50 or 51 including the step of calculating the internal resistance of the or each sub-unit of said battery.

53. A method as claimed in Claim 52 including the steps of measuring two or more different load voltages for the or each sub-unit of said battery, within a short time period; measuring the associated load currents; and calculating the internal resistance of the or each sub-unit of said battery, using said measured load voltages and load currents.

54. A method as claimed in Claim 53, wherein the iinntteerrnnaall rreessiissttaannccee RR_jm___tt. ooff tthhee or each sub-unit of said battery is calculated according to

^VLoad l^+ILoad 2 ^RInt ^VLoad 2^+ILoad 2 ^RInt

55. A method as claimed in Claim 52, wherein the internal resistance of the or each sub-unit of said battery is calculated using measurements of a load voltage and associated load current applied to said battery at a time during the recharge cycle of said battery, when the load voltage is substantially constant with time.

56. A method as claimed in Claim 55 including the step of correcting for the difference in internal resistance of the or each sub-unit of said battery during charging and discharging of said battery.

57. A method as claimed in any of Claims 38 to 56 including the step of indicating when a value for the open circuit voltage for a sub-unit of said battery falls below a predetermined threshold.

58. A method as claimed in any of Claims 38 to 57 including the step of reducing the power available from said battery when a value for the open circuit voltage for a sub-unit of said battery falls below a predetermined threshold.

59. A method as claimed in Claim 58, wherein the power P available from said battery is reduced according to the equation

% P = 100 - INT((V_T - VMm. )' x ^CB> where vr_wim. is the lowest value for an open circuit voltage for a sub-unit of said battery;

V_ is the predetermined threshold voltage;

INT denotes the integer of the term in the brackets; and

C_ is a constant dependant on the electrical B arrangement of the battery.

60. A method as claimed in any of Claims 38 to 59 including the steps of measuring the temperature of the or each sub-unit of said battery, and reducing the power available from said battery when the measured temperature exceeds a predetermined threshold.

61. A method as claimed in Claim 60, wherein the power P available from said battery is reduced according to the equation

% P = 100 + INT((T_T - -T_Maχ) X C_τ)

where T_M is the temperature of the highest temperature sub-unit of said battery; T_ is the predetermined threshold temperature; INT denotes the integer of the term in the brackets; and

C_ is a constant dependant on the temperature control of the battery.

62. A method of controlling the discharging of a battery formed of one or more serially connected sub-units; said method comprising the steps of repeatedly determining a value for the open circuit voltage of the or each sub-unit of said battery during a discharge cycle of said battery; and reducing the power available from said battery when the determined open circuit voltage value for a sub-unit of said battery falls below a predetermined threshold voltage.

63. A method as claimed in Claim 62 including the step of measuring the open circuit voltage for the or each sub-unit of said battery when said battery is under no load.

64. A method as claimed in Claim 62 or Claim 63 including the steps of measuring the voltage across the or each sub-unit of said battery when said battery is under load, and calculating a value for the open circuit voltage by compensating for the voltage drop across the internal resistance of the or each sub-unit of said battery.

65. A method as claimed in Claim 64 including the step of calculating the internal resistance of the or each sub-unit of said battery.

66. A method as claimed in Claim 65 including the steps of measuring two or more different load voltages for the or each sub-unit of said battery, within a short time period; measuring the associated load currents; and calculating the internal resistance of the or each sub-unit of said battery using said measured load voltage and load currents.

67. A method as claimed in Claim 66, wherein the internal resistance R_τm_t. of the or each sub-unit of said battery is calculated according to

^VLoad l^+ILoad 2^RInt ^{= V}Load 2^+ILoad 2^RInt

68. A method as claimed in Claim 65, wherein the internal resistance of the or each sub-unit of said battery is calculated using measurements of a load voltage and associated load current applied to said battery at a time during the recharging operation of said battery when the load voltage is substantially constant with time.

69. A method as claimed in Claim 68 including the step of correcting for the difference in internal resistance of the or ech sub-unit of said battery during charging and discharging of said battery.

70. A method as claimed in any of Claims 62 to 69 including the step of indicating when a value for the open circuit voltage for a sub-unit of said battery falls below a predetermined threshold.

71. A method as claimed in any of Claims 62 to 70 including the step of reducing the power available from said battery when a value for the open circuit voltage for a sub-unit of said battery falls below a predetermined threshold.

72. A method as claimed in Claim 71, wherein the power P available from said battery is reduced according to the equation

% P = 100 - INT((V_T - V_Min) x C_B)

where „. is the lowest value for an open circuit voltage for a sub-unit of said battery; V_ is the predetermined threshold voltage; INT denotes the integer of the term in the brackets; and

C_β is a constant dependant on the electrical arrangement of the battery.

73. A method as claimed in any of Claims 62 to 72 including the steps of measuring the temperature of the or each sub-unit of said battery, and reducing the power available from said battery when the measured temperature exceeds a predetermined threshold.

74. A method as claimed in Claim 73, wherein the power P available from said battery is reduced according to the equation

% P = 100 + INT((T_T - T_Maχ) x C_τ)

C_ is a constant dependant on the temperature control of the battery.

75. Apparatus as hereinbefore described with reference to the drawings.

76. A method as hereinbefore described with reference to the drawings.