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Title:
ELECTRIC VEHICLE BATTERY MANAGEMENT APPARATUS AND METHOD
Document Type and Number:
WIPO Patent Application WO/2017/207997
Kind Code:
A1
Abstract:
A battery management apparatus and method for an electric vehicle is disclosed, the electric vehicle comprising an electric drivetrain comprising one or more electric motors, first and second batteries configured to supply electrical power to the electric drivetrain, a first regenerative braking system configured to generate electrical power for charging the first battery, and a second regenerative braking system configured to generate electrical power for charging the second battery. If a difference between the output voltages of the first and second batteries is less than a threshold, the first and second batteries are electrically connected in parallel to supply electrical power to the electric drivetrain, and if the difference between the output voltages is greater than the threshold, the first and second batteries are separately connected to the electric drivetrain. Additionally, in response to the difference between the output voltages being greater than the threshold, a relative amount of electrical power generated by the first and second regenerative braking systems is changed so as to supply more electrical power to the one of the first and second batteries having the lower output voltage, while keeping the total amount of electrical power generated by the first and second regenerative braking systems under braking substantially constant.

Inventors:
LESZCZYNSKI, Mirek (Banbury CrossSoutham Road, Banbury Oxfordshire OX16 2SN, OX16 2SN, GB)
Application Number:
GB2017/051565
Publication Date:
December 07, 2017
Filing Date:
June 01, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ARRIVAL LIMITED (Unit 2, Banbury CrossSoutham Road, Banbury OX16 2SN, OX5 1QU, GB)
International Classes:
B60L7/18; B60L7/16; B60L11/18
Domestic Patent References:
WO2015021998A12015-02-19
Foreign References:
US20040238244A12004-12-02
US20030122512A12003-07-03
EP0083557A21983-07-13
US5941328A1999-08-24
US20130221897A12013-08-29
Attorney, Agent or Firm:
CORK, Robert (Venner Shipley LLP, The Surrey Technology CentreThe Surrey Research - Park 40 Occam Road, Guildford Surrey GU2 7YG, GU2 7YG, GB)
Download PDF:
Claims:
Claim s

1. A battery management apparatu s for an electric vehicle, the electric vehicle comprising an electric drivetrain comprising one or more electric motors, first and second batteries configured to supply electrical power to the electric drivetrain , a first regenerative braking system configured to generate electrical power for charging the first battery, and a second regenerative braking system configured to generate electrical power for charging the second battery, the battery management apparatus comprising: power distribution means for controlling a flow of electrical power between the first and second batteries and the electric drivetrain ;

voltage monitoring means for monitoring an output voltage of the first battery and an output voltage of the second battery; and

battery control means for controlling the power distribution means to electrically connect the first and second batteries in parallel to supply electrical power to the electric drivetrain in response to a difference between the output voltages of the first and second batteries being less than a threshold, and to separately connect the first battery and the second battery to the electric drivetrain in response to the difference between the output voltages being greater than the threshold,

wherein in response to the difference between the output voltages of the first and second batteries being greater than the threshold, the battery control means is configured to change a relative amount of electrical power generated by the first and second regenerative braking systems so as to supply more electrical power to the one of the first and second batteries having the lower output voltage, while keeping the total amount of electrical power generated by the first and second regenerative braking systems under braking substantially constant.

2. The battery management apparatus of claim 1, wherein in response to the difference between the output voltages of the first and second batteries being greater than the threshold, the battery control means is configured to control the power distribution means to charge or discharge the first battery and/ or the second battery to reduce the difference between the output voltages, and to electrically connect the first and second batteries in parallel in response to the difference between the output voltages falling below the threshold.

3. The battery management apparatus of claim 1 or 2, wherein the first and second regenerative braking systems are configured to generate power from different parts of the electric drivetrain.

4. The battery management apparatus of any one of the preceding claims, wherein the electric drivetrain comprises a plurality of electric motors, and the battery control means is configured to separately connect the first battery and the second battery to the electric drivetrain so that the first and second batteries supply electrical power to different ones of the electric motors.

5. The battery management apparatus of claim 3 , wherein the electric drivetrain includes a rear electric drivetrain comprising one or more electric motors configured to drive rear wheels of the electric vehicle, and

wherein the battery control means is configured to determine which of the first and second batteries currently provides the higher output voltage out of the first and second batteries, and to control the power distribution unit to supply electrical power to the rear electric drivetrain from the one of the first and second batteries which provides the higher output voltage.

6. The battery management apparatus of any one of claims 1 to 3, wherein the battery control means is configured to separately connect the first battery and the second battery to the electric drivetrain so that only one of the first and second batteries supplies electrical power to the electric drivetrain at any time.

7. An electric vehicle comprising:

an electric drivetrain comprising one or more electric motors;

first and second batteries configured to supply electrical power to the electric drivetrain ; and

battery management apparatus according to any one of the preceding claims.

8. The electric vehicle of claim 7, further comprising:

a range extender unit comprising an internal combustion engine configured to generate electrical power,

wherein the battery control means is configured to determine which of the first and second batteries currently provides the lower output voltage out of the first and second batteries, and to control the power distribution unit to charge said one of the first and second batteries from the range extender unit to increase the output voltage of said one of the first and second batteries.

9. The electric vehicle of any one of the preceding claims, further comprising: the first and second regenerative braking systems.

10. A battery management method of an electric vehicle comprising an electric drivetrain comprising one or more electric motors, first and second batteries configured to supply electrical power to the electric drivetrain , a first regenerative braking system configured to generate electrical power for charging the first battery, and a second regenerative braking system configured to generate electrical power for charging the second battery, the method comprising:

monitoring an output voltage of the first battery and an output voltage of the second battery;

electrically connecting the first and second batteries in parallel to supply electrical power to the electric drivetrain in response to a difference between the output voltages of the first and second batteries being less than a threshold; and

separately connecting the first battery and the second battery to the electric drivetrain in response to the difference between the output voltages being greater than the threshold,

wherein in response to the difference between the output voltages of the first and second batteries being greater than the threshold, the method further comprises : changing a relative amount of electrical power generated by the first and second regenerative braking systems so as to supply more electrical power to the one of the first and second batteries having the lower output voltage, while keeping the total amount of electrical power generated by the first and second regenerative braking systems under braking substantially constant.

11. The battery management method of claim 10 , further comprising:

in response to the difference between the output voltages of the first and second batteries being greater than the threshold, charging or discharging the first battery and/ or the second battery to reduce the difference between the output voltages ; and electrically connecting the first and second batteries in parallel in response to the difference between the output voltages falling below the threshold.

12. The battery management method of claim 10 or 11, wherein the electric drivetrain comprises a plurality of electric motors, and in response to the difference between the output voltages being greater than the threshold, the first battery and the second battery are separately connected to the electric drivetrain so that the first and second batteries supply electrical power to different ones of the electric motors.

13. The battery management method of claim 10 or 11, wherein the first and second regenerative braking systems are configured to generate power from different parts of the electric drivetrain ,

wherein the electric drivetrain includes a rear electric drivetrain comprising one or more electric motors configured to drive rear wheels of the electric vehicle, and the method further comprises :

determining which of the first and second batteries currently provides the higher output voltage out of the first and second batteries ; and

controlling the power distribution unit to supply electrical power to the rear electric drivetrain from the one of the first and second batteries which provides the higher output voltage.

14. The battery management method of claim 10 or 11, wherein the first battery and the second battery are separately connected to the electric drivetrain so that only one of the first and second batteries supplies electrical power to the electric drivetrain at any time.

15. A non-volatile computer readable storage medium adapted to store computer program instructions which , when executed, perform a method according to any one of claims 10 to 14.

Description:
Electric Vehicle Battery Manage m en t Apparatus and Metho d Tech nical Fie ld

The present invention relates to a battery management apparatus and method for an electric vehicle. In particular, the present invention relates to an apparatus and method for managing two or more batteries in an electric vehicle.

Background

The drive for more fuel efficient and environmentally friendly transport solutions is seeing an increasing level of development in the field of electric vehicles. Such vehicles include not only passenger vehicles for personal transport, but also commercial vehicles such as buses and trucks. Such electric vehicles (EVs) include pure battery electric vehicles (BEVs) powered by batteries alone, and range extender electric vehicles (REEVs) which also include a small internal combustion engine (ICE) to generate electricity to supplement the battery power source. All such EVs include battery packs for supplying electrical power to the electric drive motor(s). Such battery packs typically comprise a number of battery cells connected in series.

In some electric vehicles, multiple battery packs are electrically connected in parallel to provide power to the electric drive motor(s). However, problems can arise when battery packs with different output voltages are connected in parallel, making it difficult to replace individual battery packs. For example, connecting batteries with different output voltages in parallel can result in a current surge from one battery to the other.

The invention is made in this context. Sum m ary of th e Invention

According to a first aspect of the present invention, there is provided a battery management apparatus for an electric vehicle, the electric vehicle comprising an electric drivetrain comprising one or more electric motors, first and second batteries configured to supply electrical power to the electric drivetrain, a first regenerative braking system configured to generate electrical power for charging the first battery, and a second regenerative braking system configured to generate electrical power for charging the second battery, the battery management apparatus comprising: power distribution means for controlling a flow of electrical power between the first and second batteries and the electric drivetrain ; voltage monitoring means for monitoring an output voltage of the first battery and an output voltage of the second battery; and battery control means for controlling the power distribution means to electrically connect the first and second batteries in parallel to supply electrical power to the electric drivetrain in response to a difference between the output voltages of the first and second batteries being less than a threshold, and to separately connect the first battery and the second battery to the electric drivetrain in response to the difference between the output voltages being greater than the threshold, wherein in response to the difference between the output voltages of the first and second batteries being greater than the threshold, the battery control means is configured to change a relative amount of electrical power generated by the first and second regenerative braking systems so as to supply more electrical power to the one of the first and second batteries having the lower output voltage, while keeping the total amount of electrical power generated by the first and second regenerative braking systems under braking substantially constant.

In some embodiments according to the first aspect, in response to the difference between the output voltages of the first and second batteries being greater than the threshold, the battery control means is configured to control the power distribution means to charge or discharge the first battery and/ or the second battery to reduce the difference between the output voltages, and to electrically connect the first and second batteries in parallel in response to the difference between the output voltages falling below the threshold. In some embodiments according to the first aspect, the first regenerative braking system may be configured to generate electrical power from the front wheels under braking of the electric vehicle, and the second regenerative braking system may be configured to generate electrical power from the rear wheels under braking of the electric vehicle, or vice versa.

In some embodiments according to the first aspect, the electric drivetrain comprises a plurality of electric motors, and the battery control means is configured to separately connect the first battery and the second battery to the electric drivetrain so that the first and second batteries supply electrical power to different ones of the electric motors. In some embodiments according to the first aspect, the electric drivetrain includes a rear electric drivetrain comprising one or more electric motors configured to drive rear wheels of the electric vehicle, and the battery control means is configured to determine which of the first and second batteries currently provides the higher output voltage out of the first and second batteries, and to control the power distribution unit to supply electrical power to the rear electric drivetrain from the one of the first and second batteries which provides the higher output voltage.

In some embodiments according to the first aspect, the battery control means is configured to separately connect the first battery and the second battery to the electric drivetrain so that only one of the first and second batteries supplies electrical power to the electric drivetrain at any time.

According to a second aspect of the present invention , there is provided an electric vehicle comprising: an electric drivetrain comprising one or more electric motors ; first and second batteries configured to supply electrical power to the electric drivetrain ; and battery management apparatu s according to the first aspect.

In some embodiments according to the second aspect, the electric vehicle further comprises a range extender unit comprising an internal combustion engine configured to generate electrical power, and the battery control means is configured to determine which of the first and second batteries currently provides the lower output voltage out of the first and second batteries, and to control the power distribution unit to charge said one of the first and second batteries from the range extender unit to increase the output voltage of said one of the first and second batteries.

In some embodiments according to the second aspect, the electric vehicle further comprises the first and second regenerative braking systems. In some embodiments according to the second aspect, the generating means may comprise both the range extender unit and the first and second regenerative braking systems.

According to a third aspect of the present invention , there is provided a battery management method of an electric vehicle comprising an electric drivetrain comprising one or more electric motors, first and second batteries configured to supply electrical power to the electric drivetrain , a first regenerative braking system configured to generate electrical power for charging the first battery, and a second regenerative braking system configured to generate electrical power for charging the second battery, the method comprising: monitoring an output voltage of the first battery and an output voltage of the second battery; electrically connecting the first and second batteries in parallel to supply electrical power to the electric drivetrain in response to a difference between the output voltages of the first and second batteries being less than a threshold ; and separately connecting the first battery and the second battery to the electric drivetrain in response to the difference between the output voltages being greater than the threshold, wherein in response to the difference between the output voltages of the first and second batteries being greater than the threshold, the method further comprises : changing a relative amount of electrical power generated by the first and second regenerative braking systems so as to supply more electrical power to the one of the first and second batteries having the lower output voltage, while keeping the total amount of electrical power generated by the first and second regenerative braking systems under braking substantially constant.

In some embodiments according to the third aspect, the battery management method further comprises : in response to the difference between the output voltages of the first and second batteries being greater than the threshold, charging or discharging the first battery and/ or the second battery to reduce the difference between the output voltages ; and electrically connecting the first and second batteries in parallel in response to the difference between the output voltages falling below the threshold.

In some embodiments according to the third aspect, the electric drivetrain comprises a plurality of electric motors, and in response to the difference between the output voltages being greater than the threshold, the first battery and the second battery are separately connected to the electric drivetrain so that the first and second batteries supply electrical power to different ones of the electric motors.

In some embodiments according to the third aspect, the first and second regenerative braking systems are configured to generate power from different parts of the electric drivetrain , wherein the electric drivetrain includes a rear electric drivetrain comprising one or more electric motors configured to drive rear wheels of the electric vehicle, and the method further comprises : determining which of the first and second batteries currently provides the higher output voltage out of the first and second batteries ; and controlling the power distribution unit to supply electrical power to the rear electric drivetrain from the one of the first and second batteries which provides the higher output voltage.

In some embodiments according to the third aspect, the first battery and the second battery are separately connected to the electric drivetrain so that only one of the first and second batteries supplies electrical power to the electric drivetrain at any time.

According to a fourth aspect of the present invention, there is provided a non-volatile computer readable storage medium adapted to store computer program instructions which, when executed, perform a method according to the third aspect.

Brie f De scription of th e Drawings

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

Figure 1 schematically illustrates an electric vehicle and battery management apparatus, according to an embodiment of the present invention ;

Figure 2 schematically illustrates a battery management apparatus, according to an embodiment of the present invention ;

Figure 3 is a flowchart showing a battery management method, according to an embodiment of the present invention ;

Figure 4 is a flowchart showing a method of charging or discharging a battery to reduce the voltage difference between two batteries, according to an embodiment of the present invention ; and

Figure 5 is a graph plotting the output voltage of a battery as a function of the state of charge (SOC), according to an embodiment of the present invention.

Detaile d Description

Referring now to Fig. 1, an electric vehicle 100 according to an embodiment of the present invention comprises an electric drivetrain 101, 102 for converting electrical power into mechanical power to drive the electric vehicle 100. The electric vehicle 100 further comprises a master battery 103 configured to supply electrical power to the electric drivetrain, and two alternate power sources in the form of a swappable second battery 104 and a range extender unit 105. The second battery 104 may be referred to as a slave battery. Finally, in the present embodiment the electric vehicle 100 further comprises a battery control unit 106 and power distribution unit 107. The range extender unit 105 can be any suitable generator configured to generate electrical power, for example, a petrol or diesel combustion engine arranged to drive an electric generator. The range extender 105 can extend the operating range of the electric vehicle 100 beyond the limit that would be possible when operating on battery power alone. Like the master battery 103, in the present embodiment both alternate power sources 104, 105 are also configured to supply electrical power to the electric drivetrain. In some embodiments, a range extender 105 may be mechanically coupled to the wheels of the electric vehicle 100 so as to drive the wheels directly, rather than supplying electrical power to the motors of the electric drivetrain 101, 102.

In the present embodiment, both the second battery 104 and the range extender 105 are configured to be removed and swapped with a replacement battery. That is, one or both of the second battery 104 and the range extender 105 can be removed and replaced with another battery. For example, the range extender 105 may be removed and replaced with the replacement battery when the electric vehicle 100 is to be operated in an area with strict emission controls, where use of the range extender 105 would not be permitted. The second battery 105 may be swapped with the replacement battery when the second battery 105 is discharged, or is no longer operating correctly. In general, the electric drivetrain of the electric vehicle 100 can include one or more electric motors. In the present embodiment the electric vehicle 100 is provided with separate front 102 and rear 101 electric drivetrains, each of which includes two electric motors. In this embodiment, each electric motor is arranged to drive one wheel of the electric vehicle 100. However, other arrangements are possible in other embodiments of the invention. For example, the front and/ or rear drivetrains may each only include a single electric motor, or may include more than two electric motors. Alternatively, in some embodiments the front or rear drivetrain may be omitted, with only the front wheels or the rear wheels being driven. In the present embodiment the electric vehicle 100 further comprise first and second regenerative braking systems 108 a, 108 b that are configured to generate electrical power for charging the master battery 103 and/ or the slave battery 104 under braking of the electric vehicle 100. Together, the range extender unit 105 and the first and second regenerative braking systems 108 a, 108 b can collectively be referred to as generating means, that is, means for generating electrical power that can be used to charge the master battery 103 and / or the slave battery 104. In the present embodiment, the regenerative braking system comprises a plurality of subsystems configured to generate electrical power from different parts of the electric drivetrain. In the present embodiment the regenerative braking system 108 a, 108b includes a rear regenerative braking system 108 a configured to generate electrical power for charging the master battery 103 from the rear wheels of the electric vehicle 100 , and further includes a front regenerative braking system 108b configured to generate electrical power for charging the slave battery 104 from the front wheels of the electric vehicle 100. It should be understood that the separation of regenerative braking between front and rear wheels as described here is merely exemplary, and other arrangements are possible in other embodiments. For example, in an articulated lorry comprising multiple rear axles, each battery may be charged from a different one of the rear axles via regenerative braking. The battery control unit 106 can control the power distribution unit 107 to manage the flow of electrical power between the master battery 103 , the alternate power sources 104, 105, the electric drivetrain 10 1, 102, and the first and second regenerative braking systems 108 a, 108 b. For example, the power distribution unit 107 may include an arrangement of electrical switches that can be configured to electrically connect each of the master battery 103 , second battery 104, and range extender unit 105 to one or both of the front and rear drivetrains 101, 102. This allows each of the front and rear drivetrains 101, 102 to be driven by one or more of the available power sources 103 , 104, 105, in any combination. When the same drivetrain is powered by both the master battery 103 and the swappable second 104, both batteries 103 , 104 are electrically connected in parallel. Alternatively, the electric vehicle can be operated in a mode in which the master battery 103 supplies electrical power to one of the front and rear drivetrains 101, 102 and the second battery 104 supplies electrical power to the other of the front and rear drivetrains 101, 102, so that the two batteries 103, 104 are not electrically connected to each other.

Furthermore, the power distribution unit 107 can be controlled to electrically connect one of the batteries 103, 104 to the other battery 103 , 104 and/ or the range extender unit 105, to charge said one of the batteries 103, 104 from the other battery 103 , 104 and/ or from the range extender unit 105. By controlling the power distribution unit 107, the battery control unit 106 can independently charge or discharge either of the master battery 103 and the second battery 104. An example of the power distribution unit 107 is illustrated schematically in Fig. 2, according to an embodiment of the present invention. In this embodiment, the positive and negative terminals of the master battery 103 are respectively connected to positive and negative terminals of the rear drivetrain 101, and positive and negative terminals of the slave battery 104 are respectively connected to positive and negative terminals of the front drivetrain 102. The power distribution unit 107 comprises an electrical switch 204 that is configured to be switchable to open or close an electrical connection between the positive terminals of the master battery 103 and the slave battery 104, and to simultaneously open and close an electrical connection between the negative terminals of the master battery 103 and the slave battery 104. When the switch 204 is open, the master battery 103 and the slave battery 104 are connected separately to the rear and front drivetrains 101, 102 respectively. When the switch 204 is closed, the master battery 103 and the slave battery 104 are electrically connected in parallel so that each battery 103, 104 supplies power to both the front and rear drivetrains 101, 102.

In another embodiment, independently controllable switches may be provided for the positive and negative connections between the master battery 103 and the slave battery 104. Furthermore, in some embodiments the negative terminals of both the master battery 103 and the slave battery 104 may be connected to a common ground, and only the positive connection between the batteries 103, 104 may be switched.

In yet another embodiment, the power distribution unit may be configured to provide independently switchable electrical connections between the positive terminal of either battery and each of the front and rear drivetrains, such that either battery is capable of being connected to none, one, or both of the drivetrains. Such an arrangement provides full flexibility in terms of distributing electrical power from the batteries to the front and rear drivetrains. For example, a single battery may be connected to both drivetrains to supply power simultaneously to both drivetrains whilst the other battery is electrically disconnected, so that only one of the first and second batteries supplies electrical power to the electric drivetrain at any time. This may be useful when there is a large difference between the output voltages and it is desired to rapidly discharge one battery without draining the other battery, so as to quickly reduce the difference between the output voltages. As another example, such a power distribution unit could be controlled to supply electrical power to the rear electric drivetrain from the battery which provides the higher output voltage, so that more power is provided to the rear wheels in order to drive the electric vehicle.

In an electrical circuit, power losses due to electrical resistance increase with the current in a non-linear fashion. When an electric vehicle 100 is powered by multiple batteries, as in the example shown in Fig. 1, it is more efficient to connect the batteries in parallel since this reduces the current flowing through each battery, thereby reducing the power loss. However, when two batteries in the electric vehicle 100 have different output voltages, electrically connecting the batteries in parallel can result in current flowing directly from one battery to another without passing through a load. This can result in rapid overheating which may damage the batteries or other components of the vehicle. In the worst case scenario, one or both of the batteries may ignite.

In the present embodiment, a battery management apparatus is provided to manage two or more batteries in the electric vehicle 100. For example, the battery management apparatus may be used to manage the master battery 103 , the swappable slave battery 104, and/ or a replacement battery installed in place of either the swappable slave battery 104 or the range extender unit 105. The battery management apparatus is illustrated in more detail in Fig. 2. In the present embodiment, the battery

management apparatus comprises the power distribution unit 107, a voltage monitoring unit 203, and the battery control unit 106.

The voltage monitoring unit 203 can comprise any suitable means for monitoring an output voltage of the master battery 103 and an output voltage of the slave battery 104. For example, the master battery 103 and/ or the slave battery 104 may include integrated voltage measuring equipment to measure the output voltage of the battery and report the output voltage to the voltage monitoring unit 203. Alternatively, the voltage monitoring unit 203 may be configured to directly measure the output voltage of one or both of the master and slave batteries 103 , 104. As a further alternative, in some embodiments the master battery 103 and/ or slave battery 104 may include a state of charge (SOC) monitoring unit configured to estimate the current SOC of the battery and transmit the estimated SOC to the voltage monitoring unit 203 , which can then estimate the current output voltage of the battery from a known SOC-voltage curve, for example as plotted in Fig. 5. Referring now to Fig. 3 , a battery management method is illustrated according to an embodiment of the present invention. The battery management method can be implemented by the battery management apparatus shown in Fig. 2. In the present embodiment the battery management method is implemented in software, and the battery control unit 106 comprises a processing unit 201 and non-volatile computer readable memory 202 adapted to store computer program instructions which, when executed by the processing unit 201, perform the battery management method. Any suitable type of computer memory may be used, and the processing unit 201 may include one or more processing cores. In other embodiments the battery management method may be implemented using dedicated hardware, for example an Application Specific Integrated Circuit (ASIC).

First, in step S301 the battery control unit 106 uses the voltage monitoring unit 203 to determine a difference between the output voltages of the master battery 103 and the slave battery 104. Then, in step S302 the battery control unit 106 checks whether the voltage difference is less than a preset threshold. The threshold can be set to a suitable limit below which the batteries can safely be connected in parallel. If the voltage difference is less than the threshold, then the battery control unit 106 proceeds directly to step S306 and controls the power distribution unit 107 to electrically connect the master battery 103 and the slave battery 104 in parallel to supply electrical power to the electric drivetrain 101, 102, by closing the switch 204 between the positive and negative battery terminals.

Alternatively, in response to a determination that the difference between the output voltages is greater than the threshold, in step S303 the battery control unit 106 proceeds to control the power distribution unit 107 to separately connect the master battery 103 and the slave battery 104 to the electric drivetrain 101, 102, by opening the switch 204. When a switching arrangement as shown in Fig. 2 is used, opening the switch 204 results in the master battery 103 only supplying power to the rear drivetrain 101 whilst the slave battery 104 only supplies power to the front drivetrain 102.

In this way, the battery control unit 106 can prevent the master and slave batteries 103 , 104 from being electrically connected in parallel when the output voltages differ by more than a certain threshold, to prevent damage occurring to either battery or to other components of the electric vehicle 100. On the other hand, if the output voltages are close enough for the batteries to safely be connected in parallel, the battery control unit 106 automatically connects the batteries 103 , 104 in parallel so as to minimise power losses in the electric vehicle 100.

In some embodiments, the battery management method may terminate at this point. In this way the battery control unit 106 effectively acts as a fail-safe mechanism, by preventing the batteries from being connected in parallel when it might be dangerous to do so. However, in the present embodiment the battery control unit 106 proceeds to actively manage the states of charge of the batteries if the voltage difference exceeds the threshold, so as to reduce the difference in the output voltages to a point at which the batteries can then be connected in parallel to minimise power losses.

As described above, the separate subsystems 108 a, 108b of the regenerative braking system are configured to generate power under braking from different parts of the drivetrain 101, 102, and to supply power to different batteries in the electric vehicle. For example, the rear regenerative braking system 108 a may be configured to supply electrical power to the master battery 103 under braking of the electric vehicle 100 and the front regenerative braking system 108b may be configured to supply electrical power to the slave battery 104 under braking of the electric vehicle 100 , or vice versa. Accordingly, in step S304 the battery control unit 106 controls the first and second regenerative braking system 108 a, 108b to change the relative amount of electrical power generated by the first and second regenerative braking systems 108 a, 108 b so as to supply more electrical power to the battery which has the lower output voltage, while keeping the total amount of electrical power generated by the front and rear regenerative braking systems 108 a, 108 b under braking substantially constant. In this way, battery control unit 106 controls the power distribution unit 107 to charge or discharge the master battery 103 and/ or the slave battery 104 so as to reduce the difference between the output voltages.

The regenerative braking system can influence the braking performance of the electric vehicle. By keeping the total amount of electrical power substantially constant, as described above, the battery control unit 106 can ensure that the vehicle 100 continues to provide consistent braking performance in line with the driver's expectations. If the total output of the regenerative braking system was reduced instead, the braking performance would be altered, potentially causing the driver to lose control of the vehicle. The battery control unit 106 then periodically checks the voltage difference in step S305, and continues to charge or discharge the batteries as necessary whilst continuing to operate the batteries separately. Then, in response to the difference between the output voltages falling below the threshold at step S305, the battery control unit 106 finally proceeds to step S306 and controls the power distribution unit 107 to electrically connect the master and slave batteries 103, 104 in parallel.

Referring now to Fig. 4, a method of charging or discharging a battery to reduce the voltage difference between two batteries is illustrated, according to an embodiment of the present invention. The battery management method can be implemented by the battery management apparatus shown in Fig. 2, and can be performed during step S304 of the method shown in Fig. 3.

In the present embodiment, when it has been determined that the difference in output voltage between two batteries is too large for the batteries to safely be connected in parallel, the battery control unit 106 proceeds to check in step S401 whether any generating means is available. For example, in some embodiments a range extender unit 105 may be capable of being removed and therefore may not always be present, or a fuel tank for the range extender unit 105 may be running low or empty. Similarly, in some embodiments a regenerative braking system may not be included, or may be present but may not be available due to a malfunction.

If a generator is available, then in step S402 the battery control unit 106 proceeds to check which of the master and slave batteries 103, 104 currently provides the lower output voltage. Next, in step S403 the battery control unit 106 charges the battery with the lower output voltage from the generating means so as to increase the output voltage the battery, thereby reducing the difference in output voltages between the two batteries. For example, the battery control unit 106 may activate the range extender unit 105 and/ or control the power distribution unit 107 to connect the battery to the regenerative braking system 108 a, 108b. The battery control unit 106 then proceeds to check whether the difference in output voltages has fallen below the threshold, as described above with reference to step S305 of Fig. 3.

On the other hand, if a generator is not available in step S401, then in step S404 the battery control unit 106 checks which battery currently provides the higher output voltage. Then, in step S405 the battery control unit 107 controls the power distribution unit 107 so as to preferentially discharge the battery which has the higher output voltage. That is, the battery control unit 107 ensures that the battery with the higher output voltage is discharged more rapidly than the battery with the lower output voltage, which could be disconnected entirely from the drivetrain whilst the other battery is being discharged.

Furthermore, in some embodiments the battery control unit 106 may perform both branches of the flowchart in Fig. 4 simultaneously. For example, to rapidly reduce the voltage difference between two batteries, the battery with the lower output voltage may be charged from the range extender unit 105 and/ or the regenerative braking system 108 a, 108 b , whilst the battery with the higher output voltage is connected to both front and rear drivetrains 10 1, 102 to rapidly discharge the battery.

Finally, embodiments of the invention have been described in which two batteries are electrically connected in parallel when the difference in output voltages is less than a threshold. In any given embodiment, the system may be capable of tolerating a certain finite imbalance between output voltages without damage occurring, and the threshold may be set accordingly. The threshold effectively defines an acceptable range within which the batteries can safely be electrically connected in parallel. To put it another way, the acceptable range is a range within which the output voltages of the master battery 103 and the secondary battery 104 are sufficiently close to allow the master battery 103 and the secondary battery 104 to be electrically connected in parallel, without damage occurring to either battery or to other components of the electric vehicle.

The threshold may be defined in absolute or relative terms. For example, the threshold may be defined as an absolute value, for instance 2 Volts (V). Alternatively the threshold may be defined in relative terms as a certain percentage or fraction of a voltage, for example 1%, 2% or 3 % of the lower output voltage, the higher output voltage, or an average of the output voltages of any number of batteries that are to be connected. The battery management apparatus may cease actively managing the states of charge of the batteries as long as the difference between the output voltages remains within the defined acceptable threshold. If the difference between the output voltages subsequently becomes larger than the defined acceptable threshold, the battery management apparatus may resume active management of the state of charge of one or both batteries until the output voltage difference moves back within the acceptable threshold.

Whilst certain embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the

accompanying claims.