GB2086060A

GB2086060A - Battery monitoring system

Info

Publication number: GB2086060A
Application number: GB8128229A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1980-10-11
Filing date: 1981-09-18
Publication date: 1982-05-06
Also published as: GB2086060B

Abstract

A battery monitoring system comprises a traction battery pack having eighteen individual 12V sub- packs (10, 11, 12) and an individual voltage detector (14, 15, 16) for each sub-pack. Each voltage detector (14, 15, 16) continuously monitors the on- load voltage of its associated sub- pack and when, and only when, the voltage of one of the sub-packs falls below 9V a warning lamp 24 is energised. The monitoring system is also combined with an electric vehicle motor control system which includes an accelerator potentiometer (117) which provides an armature current demand signal to a chopper circuit. When the voltage of one of the sub-packs falls below 9V, the armature current demand signal is removed thereby reducing the armature current. <IMAGE>

Description

SPECIFICATION Battery monitoring system This invention relates to a monitoring system for a traction battery pack and further relates to such a system combined with an electric vehicle motor control system.

Towards the end of discharge of a traction battery pack there is a danger that the current drawn may cause some of the cells to start charging with reverse polarity. This effect which is known as cell reversal is very damaging to a cell and so discharge should be terminated before such cell reversal occurs.

It is accordingly an object of this invention to provide a monitoring system for a traction battery pack which is capable of detecting the onset of cell reversal, and it is a further object of this invention to combine such a system with an electric vehicle motor control system.

According to one aspect of this invention, there is provided a traction battery pack monitoring system including a traction battery pack having a plurality of cells connected in series and divided into a plurality of sub-packs each of which comprises one or more cells and an individual voltage detector for each sub-pack for continuously monitoring the on-load voltage of its associated subpack, each individual voltage detector providing an output signal when, and only when, the on-load voltage of its associated sub-pack falls below a predetermined value, said predetermined value being equal to a substantial fraction of the nominal voltage of its associated sub-pack.

The monitoring system may be combined with an electric vehicle control system comprising a main electric traction motor powered by the traction battery pack and means for controlling the current supplied to the motor, said control means being responsive to the voltage detectors and restricting the motor current when, and only when, one of the voltage detectors produces its output signal.

According to another aspect of this invention, there is provided a method of discharging a traction battery pack supplying current to the main traction motor of an electric vehicle, said battery pack comprising a plurality of cells connected in series and divided into a plurality of sub-packs each of which comprises one or more cells, said method comprising continuously monitoring the on-load voltage of each sub-pack and restricting the current supplied to the traction motor whilst the voltage of one of the sub-packs is below a predetermined voltage, said predetermined voltage being equal to a substantial fraction of the nominal voltage of the sub-pack.

This invention will now be described in more detail by way of example with reference to the accompanying drawing which is a circuit diagram of a battery monitoring system embodying this invention.

Referring now to the drawing there is shown a traction battery which comprises eighteen individual sub-packs connected in series. Only the first, second and last sub-packs are shown and these are denoted respectively by numerals 1 0, 11 and 12. Each sub-pack comprises two nominal 6V batteries connected in series and each battery comprises three 2V cells connected in series. The on-load voltage of each of the sub-packs is monitored by an associated voltage detector and the detectors associated with the sub-packs 10, 11 and 12 are shown and are denoted respectively by numerals 14, 15 and 16. As each of the voltage detectors is identical, only the detector 14 will be described.The detector 14 comprises a resistor R1, one end of which is connected to the positive terminal of the sub-pack 10 and the other end of which is connected to the cathode of a zener diode ZD1 . The anode of zener diode ZD1 is connected to the anode of a photodiode Dl, the cathode of which is connected to the negative terminal of the subpack 10.

Each of the voltage detectors is associated with an NPN photo-transistor and the photo-transistors associated with the voltage detectors 1 4, 1 5 and 1 6 are shown and denoted respectively by numerals 18, 19 and 20. The collector-emitter paths of the photo-transistor are connected in series and each photo-transistor is provided with a high value resistor connected between its base and emitter to shunt leakage currents and avoid false operation. Each of the photo-transistors forms an opto-isolator with its associated light emitting diode.

The collector of transistor 1 8 is connected through a resistor R2 to a 1 2V rail and also to the cathode of a zener diode ZD2, the anode of which is connected through a further resistor R3 to a ground rail which is also connected to the emitter of transistor 20. The junction of zener diode ZD2 and resistor R3 is connected to the base of an NPN transistor 21 , the collector of which is connected through a resistor R4 to a terminal 25 and the emitter of which is connected through a resistor R5 to the ground rail. The collector of transistor 21 is also connected to the anode of a diode D2, the cathode of which is connected to a terminal 22.The emitter of transistor 21 is also connected to the base of an NPN transistor 23, the emitter of which is connected to the ground rail and the collector of which is connected through a resistor R6 and a warning lamp 24, connected in series, to the 1 2V rail.

The monitoring system is combined with the electric vehicle DC motor control system described in our recently filed co-pending Application entitled "Battery-Powered Electric Vehicle DC Motor Control" and based on United Kingdom Patent Application No.8031860. In order to achieve this terminals 25 and 22 of the present system are connected respectively to the 8V rail and the junction of the accelerator potentiometer 17 and diode D1005 in place of resistor R1017 in the circuit shown in Figure 2 of the said co-pending Application. The accelerator potentiometer is shown in the drawing of the present Application and denoted by reference numeral 117. Thus, the monitoring system affects the voltage supply to the accelerator potentiometer. As explained in the said co pending Application and also in published PCT Application No.PCT/GB78/0046, the current supply from the battery to the electric motor is controlled by a chopper circuit. The signal on the slider of the accelerator potentiometer is an armature current demand signal and is supplied to the chopper circuit as a control signal.

The operation of the monitoring system as combined with the electric vehicle traction motor control system will now be described.

Each of the voltage detectors is arranged so that its zener diode conducts when the voltage of the associated sub-pack exceeds 9 volts.

Consequently, each of the zener diodes is normally conducting thereby energising its associated light emitting diode and rendering the associated photo-transistor conductive. In this condition, the zener diode ZD2 is non-conductive with the result that transistors 21 and 23 are off, and so the warning lamp 24 is not energised and the voltage supply to the accelerator potentiometer is not affected.

Towards the end of discharge, one or more of the cells for the traction battery will tend towards reversal at high battery discharge currents thereby causing the voltage of the associated sub-pack to fail below 9 volts. When this happens, the associated zener diode will be rendered nonconductive thereby de-energising the associated light emitting diode. The de-energisation of the light emitting diode thus provides an output signal which is indicative of the low voltage of the associated subpack.

When one of the light emitting diodes is deenergised, the associated photo-transistor will be rendered non-conductive thereby switching on transistors 21 and 23. As a result of switching on transistor 23, the warning lamp 24 will be energised. As a result of switching on transistor 21, the voltage supply to the accelerator potentiometer will be removed thereby providing a current restricting signal to the motor control system. Each time the voltage supply to the accelerator potentiometer is removed, the current demand signal is also removed thereby causing the chopper circuit to reduce the armature current.

When a vehicle is hill climbing, the armature current and consequently the battery current increases. Also, as vehicle speed increases, the mark space ratio of the current supply from the battery to the chopper circuit increases.

Consequently, as a battery approaches a low state of discharge, an output signal will be produced by one of the voltage detectors only at high speeds or when hill climbing. Then, as discharge proceeds further, an output signal from one or more of the voltage detectors will be provided progressively more frequently until a stage is reached when the warning lamp 24 is energised almost continuously and the vehicle is almost impossible to drive. Thus, the driver is given a progressive warning of the low state of charge of the battery by progressive energisation of the warning lamp and by progressive restriction of the armature current.It is to be noted that at low states of charge the ability of the vehicle to climb hills is retained at low speeds as at such speeds a high armature current can be obtained at a low mark space ratio of the chopper circuit and consequeritly at a low average battery current.

In the present example, each voltage detector provides its output signal when the voltage of the associated sub-pack falls below 9V, i.e. three quarters of the nominal sub-pack voltage.

Consequently, the first warning is given well before there is any danger of cell reversal.

Although, it is not essential to set each voltage detector to three quarters of the nominal voltage of the associated sub-pack, it is desirable to set each voltage detector to a substantial fraction of the nominal voltage, and preferably each voltage detector should be set to a value which is at least half of the nominal voltage of the associated subpack.

Claims

1. A traction battery pack monitoring system including a traction battery pack having a plurality of cells connected in series and divided into a plurality of sub-packs each of which comprises one or more cells and an individual voltage detector for each sub-pack for continuously monitoring the on-load voltage of its associated sub-pack, each individual voltage detector providing an output signal when, and only when, the on-load voltage of its associated sub-pack falls below a predetermined value, said predetermined value being equal to a substantial fraction of the nominal voltage of its associated sub-pack.

2. A system as claimed in Claim 1 in which for each individual voltage detector said predetermined value is equal to at least half of the nominal voltage of the associated sub-pack.

3. A system as claimed in Claim 2 in which for each individual voltage detector said predetermined value is equal to approximately three quarters of the nominal voltage of the associated sub-pack.

4. A system as claimed in any one of the preceding claims further including warning means responsive to the voltage detectors for providing a warning when, and only when, one of the voltage detectors produces its output signal.

5. A monitoring system as claimed in any one of the preceding claims combined with an electric vehicle control system comprising a main electric traction motor powered by the traction battery pack and means for controlling the current supplied to the motor, said control means being responsive to the voltage detectors and restricting the motor current when, and only when, one of the voltage detectors produces its output signal.

6. A combined monitoring system and electric vehicle control system as claimed in Claim 5 further including means connected between the voltage detectors and the control means for generating a current restricting signal to the control means when, and only when, one of the voltage detectors provides its output signal.

7. A combined monitoring system and electric vehicle control system as claimed in Claim 6 in which the control means includes a chopper circuit connected between the battery pack and the armature of the traction motor, the chopper circuit restricting the current supply from the battery to the armature of the traction motor when the signal generating means provides the current restricting signal.

8. A method of discharging a traction battery pack supplying current to the main traction motor of an electric vehicle, said battery pack comprising a plurality of cells connected in series and divided into a plurality of sub-packs each of which comprises one or more cells, said method comprising continuously monitoring the on-load voltage of each sub-pack and restricting the current supplied to the traction motor whilst the voltage of one of the sub-packs is below a predetermined voltage, said predetermined voltage being equal to a substantial fraction of the nominal voltage of the sub-pack.

9. A battery monitoring system substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

10. A combined battery monitoring system and electrical vehicle control system substantially as hereinbefore described with reference to and as shown in the accompanying drawing.