GB2341830A

GB2341830A - Controlling battery temperature in a hybrid vehicle

Info

Publication number: GB2341830A
Application number: GB9920198A
Authority: GB
Inventors: Martin Ranson; Carl Charles Bourne; Christopher Mellors
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1998-09-02
Filing date: 1999-08-27
Publication date: 2000-03-29
Also published as: GB9818965D0; GB9920198D0

Abstract

A vehicle 10 has a heat engine 12 with a cooling circuit 16 including a radiator 18 and a traction battery 14 provided with cooling and temperature management circuit 26 including a heat exchanger 24 and a pump 28. A valve 22 selectively allows engine coolant in circuit 16 to pass through the heat exchanger 24 so that the hotter of the two circuits will transfer heat to the cooler. The battery can thus be heated to its correct operating temperature and excess heat can be transferred to the engine cooling circuit or dissipated by the heat exchanger 24. The engine cooling circuit 16 may incorporate an electrically-driven pump to allow circulation and engine heating when the engine is not running.

Description

- 1. 2341830 A Vehicle This invention relates to vehicles and in

particular to a vehicle including a heat engine and a battery used to supply energy to traction motors.

It is known to provide a vehicle with a hybrid powertrain which includes a heat engine and an electrical machine which is supplied with traction power at least in part from a battery. It is a problem with some known traction batteries that their efficiency and performance, during both charging and under load, is impaired by low ambient or environmental temperatures. Clearly, such limitation may be a significant problem where a vehicle may be operated at low temperatures or where wind chill may be a problem.

It will be understood that consistence of performance and reliability are important with regard to motor vehicles. Thus, if a vehicle response characteristics should significantly change with temperature, it is possible that the driver will be at least confused by varying response. Such variations may even lead to dangerous situations where the speed of availability and level of power for acceleration and over-taking changes from day-to-day and possibly over the course of a day between morning, mid-day and evening.

It is an object of this invention to provide an improved vehicle.

According to the invention there is provided a vehicle including a heat engine, a traction battery and control means, the heat engine having a temperature management circuit whereby liquid is controllably circulated in order to regulate the temperature of that engine and the battery having a temperature management circuit whereby a liquid is controllably circuited in order to regulate the temperature of that battery, the temperature management circuit of the heat engine being selectively coupled by the control means to the temperature management circuit of the battery to exchange heat between the respective temperature management circuits such that heat is transferred from the temperature management circuit of the heat engine to the temperature management circuit of the battery if the liquid circulated in the temperature management circuit of the engine is abovea pre-determined value.

is The temperature management circuits may share a common heat exchanger which is arranged to transfer heat from the hotter to the cooler of the temperature management circuits.

The liquid of at least one of the temperature management circuits may be selectively prevented from circulating through said heat exchanger.

When the liquid of one of the temperature management circuits has been prevented from circulating through the heat exchanger, the heat exchanger may be used to cool the liquid of the one of the temperature management circuits.

Heat may be transferred into the temperature management circuit of the battery if the temperature of the battery is below a predetermined level.

Heat may be transferred from the temperature management circuit of the battery to the temperature management circuit of the engine if the temperature of the liquid in the temperature management circuit of the battery is above a predetermined level.

Heat may be transferred from the temperature management circuit of the battery to the temperature management circuit of the engine if the temperature of the liquid in the temperature management circuit of the engine is below a predetermined level.

The temperature management circuit of the battery further may comprise a pump arranged to control the circulation of the liquid of that circuit there around.

The temperature management circuit of the engine may further comprise a valve means arranged to control the circulation of the liquid of that circuit through the common heat exchanger. The valve means may be thermostatically controlled.

The valve means may be arranged to close in the event of a fault in the regulation of heat transfer between the temperature management circuits, so that there is a reduction in the transfer of heat from the temperature management circuit of the engine into the temperature management circuit of the battery.

The temperature of the battery may be controlled to a predetermined level by said exchange of heat between the temperature management systems. The vehicle may further comprise a battery management control means arranged to regulate the temperature of the battery. The control means may be arranged to monitor the temperature of individual cells of the battery. The control means may be arranged to control the operation of the pump so as to control the rate of heat transfer in the heat exchanger and may be, in addition or in the alternative, arranged to control the operation of the valve means so as to control the rate of heat transfer in the heat exchanger.

Thermal inertia of the battery may be used to store heat. The heat may be stored using the thermal inertia of the battery during a period of high engine load.

The vehicle may furthe7r comprise a liquid circulator in the temperature management circuit of the engine which is capable of circulating liquid independently of engine operation, wherein the stored heat may be used to assist engine warming before or during a cold start by circulating the liquid in the engine temperature management system through the heat exchanger and preheating that liquid. The transfer of the stored heat may be regulated such that the temperature of the battery does not fall below a predetermined level.

The engine and traction battery may comprise a hybrid powertrain of the vehicle. The invention also provides a battery management control means for a vehicle according to the invention.

The invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a vehicle according to the invention; and Figure 2 is a graph of battery capacity versus temperature for a traction battery of the vehicle of Figure 1.

Referring to the figures, a vehicle 10 has a hybrid powertrain which includes a heat engine 12 and at least one lead-acid traction battery 14.

The engine 12 has a temperature management system in the form of a liquid cooling circuit 16 which circulates liquid coolant through the engine 12 and a radiator 18. The liquid cooling circuit 16 has a branch 20 including an electrically controlled valve 22 arranged to selectively allow 1 engine coolant to circulate around the branch 20 and thereby through a 10 transfer heat exchanger 24.

The traction battery 14 also has a temperature management system in the form of a liquid cooling circuit 26 including a coolant circulation pump 28 arranged to selectively pump traction battery coolant around the cooling circuit 26 and thereby through the heat exchanger 24.

The vehicle further comprises a battery management system (BMS) 30 which controls the operation of the valve 22 using logic which takes into account both a signal from an engine temperature sensor 32 which monitors the temperature of engine coolant and also a signal from a battery temperature sensor array 34 which monitors the temperature of the 20 individual cells of the traction battery 14.

The heat exchanger 24 is arranged to enable heat transfer between the cooling circuits 20, 26 when they are both circulating liquid through that exchanger 24. The hotter of the two circuits 20, 26 will tend to lose heat, most of which will tend to be transferred into the cooler one of circuits 20, 5 26.

If the valve 22 is closed or if the pump 28 is not operating, there will be no circulation through the portion of the heat exchanger 24 associated with that circuit 20, 26. Under these circumstances, if the other of the circuits 20, 26 is still circulating liquid through its portion of the heat exchanger 24, then the temperature of that liquid can be varied by, for example, cooling by ducting or blowing air through the heat exchanger 24 or it could be heated by including an electrical heater element in the duct or blower.

It is known that the performance and life of traction batteries is a function of the temperature of the batteries. At low temperatures electrolytic activity is low and the effective state of charge of the battery is reduced. At high temperatures battery life is reduced and thermal runaway can occur, which might lead to explosive battery venting. Temperature variations in some advanced types of battery packs suitable for use as traction batteries are subject to critical voltage limits for charging and discharging. It is also important to be able to substantially equalise the temperature gradient across the individual cells in a traction battery, in order to ensure that there will be as equal performance as possible across the traction battery pack.

In hybrid vehicles fitted with such advanced lead-acid traction batteries, a substantial amount of heat is generated during the charging and discharging cycle of the batteries. This heat generation needs to be managed to prevent premature failure of the modules forming the traction battery pack. For example, it has been estimated that for every 7 to 10 OC rise in b attery ambient temperature above about 25 OC, the expected life of the battery is reduced by 50%. To increase the power output of the traction battery, however, higher temperatures are desirable, up to a maximum of about 55 OC. This could be particularly useful for optimising the traction battery to the driving cycle of the vehicle.

It can be seen from Figure 2 in particular, that the performance of the battery 14 varies considerably in dependence on variations in its operating temperature.

For a user to fully utilise the hybrid vehicle 10, the electrical powertrain (not show fully) should be able to deliver repeatable performance under as many differing conditions as possible. This is facilitated by this invention as the stable operating environment for the traction battery 14 allows for predictions in its performance which are more accurate than they would probably be without the temperature regulation.

The BMS 30 provides the traction battery 14 with a controlled environment in which the temperature of the battery coolant is varied to keep the battery temperature as close to its optimum operating point as possible. The interaction of engine coolant with traction battery coolant provides a convenient source of heat, with the added benefit of recycling heat wh ich would otherwise be wasted by dissipation into the atmosphere from the radiator. On the other hand, the engine coolant can also be used as a heatsink if the traction battery 14 starts to overheat.

In order to try and ensure that the system fails safe, if there is a failure in any part of the thermal management system which requires a reduction in battery usage or possibly even a complete shut-down, the BMS 30 is arranged to shut the valve 22 so that no more heat can be put into the traction battery 14. This fail safe feature is preferred, although it would be possible to produce a more simple system in which the valve 22 was simply thermostatically controlled.

The thermal inertia of a traction battery 14 is very high and this means that there is a potential for storing heat within the traction battery 14. The stored heat could be used to aid engine warm-up from a cold start, though it would be necessary to ensure that the temperature of the traction battery 14 did not fall below its minimum allowable temperature. To implement this, the BMS 30 is arranged to allow transfer of heat from the traction battery 14 to the engine 12 only if the traction battery is above a pre- set level, for example 25 OC. In similar fashion, it would also be desirable to set a minimum engine coolant temperature at which the valve 22 can open, so as not to sink heat away from a cold engine, and this temperature might for example be in the order of 65 OC.

1 Another aspect to thermal inertia is that it takes a relatively long time 10 for the traction battery 14 to respond to heat input. This means that a traction battery 14 used in an arrangement according to this invention would suffer a low risk of damage from transient heat inputs and that there is also the potential for short term storage of heat at higher than normal temperatures, for example during periods of high engine load or if the vehicle 10 is equipped for regenerative braking. On the other hand, it would be better not to expose the battery coolant to the engine coolant for too long in the event that the engine coolant was running at too high a temperature, so as to avoid excessive heat being transferred to the traction battery 14.

If the vehicle 10 is designed to undergo extensive periods of charging, 20 for example overnight charging at a user's home, the temperature which builds up in the traction battery 14 might be quite substantial. During such periods of charging, the engine 12 is not likely to be running and its temperature will drop. In a modification to the invention, an independent engine coolant circulator is added, for example an electric pump, and it is controlled by the BMS 30. The BMS 30 ca n now control the engine coolant temperature to a degree dictated by the surplus heat in the traction battery cooling circuit 26 which results from the charging of the traction battery 14, and this can be used to keep the engine 12 warm or to preheat it so as to help under cold start conditions. Any heat left which is surplus could be dissipated into a passenger compartment to also warm that up.

m

Claims

-12CLAIMS

1. A vehicle (10) including a heat engine (12), a traction battery (14) and control means, the heat engine (12) having a temperature management circuit (16) whereby liquid is controllably circulated in order to regulate the temperature of that engine (12) and the battery (14) having a temperature management circuit (26) whereby a liquid is controllably circulated in order to regulate the temperature of that battery (14), the temperature management circuit (16) of the heat engine (12) being selectively coupled by the control means to the temperature management circuit (26) of the battery (14) to exchange heat between the respective temperature management circuits (16, 26) such that heat is transferred from the temperature management circuit (16) of the heat engine (12) to the temperature management circuit (26) of the battery (14) if the liquid circulated in the temperature management circuit (16) of the engine (12) is above a pre-determined value.

2. A vehicle according to Claim 1, wherein the temperature management circuits (16, 26) share a common heat exchanger (24) which is arranged to transfer heat from the hotter to the cooler of the temperature management circuits (16, 26) as determined by the control means.

3. A vehicle according to Claim 2, wherein the liquid of at least one of the temperature management circuits (16, 26) can be selectively prevented by the control means from circulating through said heat exchanger (24).

4. A vehicle according to Claim 3, wherein, when the liquid of one of the temperature management circuits (16, 26) has been prevented from circulating through the heat exchanger (24) by the control means, the heat exchanger (24) can be used to cool the liquid of the other of the temperature management circuits (16, 26).

5. A vehicle according to any preceding claim wherein, if the temperature of the battery (14) is below a predetermined value determined by a battery temperature sensor (34), the control means is configured so that heat is transferred by exchange into the temperature management circuit (26) of the battery (14) from the temperature management circuit (16) of the engine (12).

6. A vehicle according to any preceding claim wherein, if the temperature of the liquid in the temperature management circuit (26) of the battery (14) is above a predetermined value, the control means is configured so that heat is transferred by exchange from the temperature management circuit (26) of the battery (14) to the temperature management circuit (16) of the engine (12).

7. A vehicle according to any preceding claim wherein, if the temperature of the liquid in the temperature management circuit (16) of the engine (12) is below a predetermined value, the control means is configured so that heat is transferred by exchange from the temperature management circuit (26) of the battery (14) to the temperature management circuit (16) of the engine (12).

8. A vehicle according to any preceding claim, wherein the temperature management circuit (26) of the battery (14) further comprises a pump (28) arranged to control the circulation of the liquid circulated there around.

9. A vehicle (10) according to any preceding claim, wherein the control means as part of the temperature management circuit (16) of the engine (2) further comprises a valve means (22) arranged to control the circulation of the liquid of that circuit (16) through the common heat exchanger (24).

10. A vehicle (10) according to Claim 9, wherein the valve means (22) is thermostatically controlled.

11. A vehicle (10) according to Claim 9 or Claim 10, wherein the valve means (22) is arranged to close in the event of a fault in the regulation of heat transfer between the temperature management circuits (16, 26), so that there is a reduction in the transfer of heat from the temperature management circuit (16) of the engine (12) into the temperature management circuit (26) of the battery (14).

12. A vehicle (10) according to any preceding claim, wherein the temperature of the battery (14) is controlled by the control means to a predetermined value by said exchange of heat between the temperature management circuits (16, 26).

13. A vehicle (10) according to any preceding claim wherein the control means includes battery management control means (30) arranged to regulate the temperature of the battery (14) by the liquid controllably circulated in the temperature management circuit (26) of the battery (14) and by heat exchange with the liquid controllably circulated in the temperature management circuit (16) of the engine (12).

14. A vehicle (10) according to Claim 13, wherein the battery control means (30) is arranged to monitor the temperature of individual cells of the battery (14) in order to regulate the temperature of the battery (14) and/or individual cells of that battery (14).

15. A vehicle (10) according to Claim 13 or Claim 14 when dependent on Claim 8 or any other claim when dependent thereon, wherein the battery control means (30) is arranged to control the operation of the pump (28) so as to. control the rate of heat transfer in the heat exchanger (24) between the liquids circulated in the respective temperature management circuits (16, 26).

16. A vehicle according to Claim 13 or Claim 14 when dependent on Claim 9 or any other claim when dependent thereon, wherein the battery control means (30) is arranged to control the operation of the valve means (22) so as to control the rate of heat transfer in the heat exchanger (24) between the liquids circulated in the respective temperature management circuits (16, 26).

17. A vehicle (10) according to any preceding claim, wherein the temperature of the battery (14) is increased by the control means by deliberate exchange of heat to the liquid circulated in the temperature management circuit (26) of the battery (14) such that the inherent higher thermal inertia of that battery (14) at the increased temperature is used to store heat.

18. A vehicle (10) according to Claim 17, wherein heat is stored by the control means using the thermal inertia of the battery (14) during a period of high engine temperature as a result of operational load.

19. A vehicle (10) according to Claim 17 or Claim 18, further comprising a liquid circulator in the temperature management circuit (16) of the engine (12) which is capable of circulating liquid independently of engine (12) operation, wherein the stored heat in the battery (14) is used to assist engine warming before or during a cold start by circulating the liquid in the engine temperature management circuit (16) through the heat exchanger (24) and so preheating that liquid circulated through the heat exchange (24) with liquid circulated in the temperature management circuit (26) of the battery (14).

20. A vehicle according to Claim 19, wherein the transfer of the stored heat is regulated such that the temperature of the battery does not fall below a predetermined level.

21. A vehicle according to any preceding claim, the engine and the battery comprising a hybrid powertrain of the vehicle.