Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
ELECTRIC VEHICLE BATTERY MANAGEMENT APPARATUS AND METHOD
Document Type and Number:
WIPO Patent Application WO/2017/207996
Kind Code:
A1
Abstract:
A battery management apparatus for an electric vehicle is disclosed, the electric vehicle comprising an electric drivetrain comprising one or more electric motors, a first battery configured to supply electrical power to the electric drivetrain, and an alternate power source configured to supply electrical power to the first battery and/or the electric drivetrain, and the alternate power source configured to be swappable with a replacement battery. The battery management apparatus comprises battery control means for monitoring a state of charge of the first battery, and controlling the state of charge of the first battery and/or a state of charge of the replacement battery before the replacement battery is installed in the electric vehicle, so that an output voltage of the first battery and an output voltage of the replacement battery are within an acceptable range. In some embodiments, the battery management apparatus further comprises a power distribution unit configured to control a flow of electrical power between the first battery, the alternate power source and the electric drivetrain, and the battery control means can control the power distribution unit so as to charge or discharge the first battery. Battery management methods are also disclosed.

Inventors:
LESZCZYNSKI, Mirek (Banbury CrossSoutham Roa, Banbury Oxfordshire OX16 2SN, OX16 2SN, GB)
Application Number:
GB2017/051564
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:
B60L11/18; B60L15/20; G01C21/34
Foreign References:
US20130057182A12013-03-07
US20130229152A12013-09-05
US20120207620A12012-08-16
US20130282472A12013-10-24
Attorney, Agent or Firm:
CORK, Robert (Venner Shipley LLP, The Surrey Technology CentreThe Surrey Research Park, 40 Occam Roa, 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, a first battery configured to supply electrical power to the electric drivetrain , and an alternate power source configured to supply electrical power to the first battery and/ or the electric drivetrain , the alternate power source configured to be swappable with a replacement battery, the battery management apparatu s comprising:

battery control means for monitoring a state of charge of the first battery, and controlling the state of charge of the first battery and/ or a state of charge of the replacement battery before the replacement battery is installed in the electric vehicle, so that an output voltage of the first battery and an output voltage of the replacement battery are within an acceptable range. 2. The battery management apparatus of claim 1, further comprising:

a power distribution unit configured to control a flow of electrical power between the first battery, the alternate power source and the electric drivetrain ,

wherein the battery control means is configured to control the power distribution unit to transfer electrical power from the first battery to the electric drivetrain and/ or the alternate power source to decrease the state of charge of the first battery, and to control the power distribution unit to transfer electrical power from the alternate power source to the first battery to increase the state of charge of the first battery. 3. The battery management apparatus of claim 1 or 2, wherein the battery control means is physically separate from the electric vehicle, and the battery management apparatus further comprises :

a vehicle communication unit configured to communicate remotely with the electric vehicle,

wherein the battery control means is configured to receive state of charge information via the vehicle communication unit, the state of charge information comprising information indicative of the state of charge of the first battery, and the battery control means comprises a charging unit configured to control the state of charge of the replacement battery according to the received state of charge information .

4. The battery management apparatus of claim 1, 2 or 3 , wherein the battery control means is configured to predict a state of charge of the first battery at a time when the replacement battery is due to be swapped with the alternate power source, and is further configured to control the state of charge of the replacement battery so that an output voltage of the first battery in the predicted state of charge and an output voltage of the replacement battery are within the acceptable range.

5. The battery management apparatus of claim 4, further comprising:

route planning means for obtaining information relating to a planned route to be followed by the electric vehicle during a journey,

wherein the battery control means is configured to predict the state of charge of the first battery at the end of the journey based on the obtained information relating to the planned route. 6. The battery management apparatus of claim 5, wherein the information relating to the planned route includes one or more of:

a distance to be travelled on the planned route;

an expected speed of the electric vehicle during the journey;

traffic conditions on the planned route;

weather conditions on the planned route;

an elevation change over the planned route; and

a predicted time required to complete the journey.

7. The battery management apparatus of any one of the preceding claims, wherein the battery control means is configured to control the state of charge of the first battery and/ or a state of charge of a replacement battery so that the difference between an output voltage of the first battery and an output voltage of the replacement battery is within a defined acceptable limit. 8. The battery management apparatus of any one of the preceding claims, wherein the battery control means is configured to control the replacement battery to be at a predefined state of charge within a range for which the output voltage is substantially constant, at a time when the replacement battery is due to be swapped with the alternate power source.

9. The battery management apparatus of any one of the preceding claims, wherein the battery control means is configured to receive state of charge information from the first battery, the state of charge information comprising information indicative of the state of charge of the first battery.

10. An electric vehicle comprising:

an electric drivetrain comprising one or more electric motors ;

a first battery configured to supply electrical power to the electric drivetrain ; an alternate power source configured to supply electrical power to the first battery and/ or the electric drivetrain ; and

battery management apparatu s according to any one of the preceding claims.

11. The electric vehicle of claim 10 , wherein the alternate power source is a removable range extender unit configured to generate electrical power.

12. The electric vehicle of claim 11, further comprising:

a second battery electrically connected in parallel with the first battery, wherein the battery control means is configured to control a state of charge of the second battery so that an output voltage of the second battery and an output voltage of the

replacement battery are within the acceptable range.

13. The electric vehicle of claim 10 , wherein the alternate power source comprises a second battery and the electric vehicle further comprises :

a regenerative braking system configured to generate electrical power under braking of the electric vehicle, the regenerative braking system being configured to generate electrical power for charging the first battery from a first pair of wheels of the electric vehicle, and to generate electrical power for charging the second battery from a second pair of wheels of the electric vehicle,

wherein the battery control means is configured to increase the state of charge of the first battery relative to the state of charge of the second battery by changing a relative amount of electrical power generated from the first and second pairs of wheels by the regenerative braking system so as to supply more electrical power to the first battery than to the second battery, while keeping the total amount of electrical power generated under braking substantially constant.

14. A battery management method of an electric vehicle an electric drivetrain comprising one or more electric motors, a first battery configured to supply electrical power to the electric drivetrain , and an alternate power source configured to supply electrical power to the first battery and/ or the electric drivetrain , the alternate power source configured to be swappable with a replacement battery, the method comprising: monitoring a state of charge of the first battery; and

controlling the state of charge of the first battery and/ or a state of charge of the replacement battery before the replacement battery is installed in the electric vehicle, so that an output voltage of the first battery and an output voltage of the replacement battery are within an acceptable range.

15. The battery management method of claim 14, further comprising:

electrically connecting the replacement battery in parallel with the first battery, after the alternate power source has been replaced with the replacement b attery.

16. A non-volatile computer readable storage medium adapted to store computer program instructions which , when executed, perform a method according to claim 14 or 15.

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 the state of charge of an onboard battery and/ or a replacement battery.

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, a first battery configured to supply electrical power to the electric drivetrain, and an alternate power source configured to supply electrical power to the first battery and/ or the electric drivetrain, the alternate power source configured to be swappable with a replacement battery, the battery management apparatus comprising: battery control means for monitoring a state of charge of the first battery, and controlling the state of charge of the first battery and/ or a state of charge of the replacement battery before the replacement battery is installed in the electric vehicle, so that an output voltage of the first battery and an output voltage of the replacement battery are within an acceptable range. In some embodiments according to the first aspect, the battery management apparatus further comprises a power distribution unit configured to control a flow of electrical power between the first battery, the alternate power source and the electric drivetrain , wherein the battery control means is configured to control the power distribution unit to transfer electrical power from the first battery to the electric drivetrain and/ or the alternate power source to decrease the state of charge of the first battery, and to control the power distribution unit to transfer electrical power from the alternate power source to the first battery to increase the state of charge of the first battery.

In some embodiments according to the first aspect, the battery control means is physically separate from the electric vehicle, and the battery management apparatus further comprises a vehicle communication unit configured to communicate remotely with the electric vehicle, wherein the battery control means is configured to receive state of charge information about the state of charge of the first battery via the vehicle communication unit, the state of charge information comprising information indicative of the state of charge of the first battery, and the battery control means comprises a charging unit configured to control the state of charge of the replacement battery according to the received state of charge information .

In some embodiments according to the first aspect, the battery control means is configured to predict a state of charge of the first battery at a time when the

replacement battery is due to be swapped with the alternate power source, and is further configured to control the state of charge of the replacement battery so that an output voltage of the first battery in the predicted state of charge and an output voltage of the replacement battery are within the acceptable range. For example, the battery management apparatus may further comprise route planning means for obtaining information relating to a planned route to be followed by the electric vehicle during a journey, and the battery control means may be configured to predict the state of charge of the first battery at the end of the journey based on the obtained information relating to the planned route. Examples of types of information relating to the planned route that can be obtained include, but are not limited to: a distance to be travelled on the planned route; an expected speed of the electric vehicle during the journey; traffic conditions on the planned route; weather conditions on the planned route; an elevation change over the planned route; and a predicted time required to complete the journey.

In some embodiments according to the first aspect, the battery control means is configured to control the state of charge of the first battery and/ or a state of charge of a replacement battery so that the difference between an output voltage of the first battery and an output voltage of the replacement battery is within a defined acceptable limit.

In some embodiments according to the first aspect, the battery control means is configured to control the replacement battery to be at a predefined state of charge within a range for which the output voltage is substantially constant, at a time when the replacement battery is due to be swapped with the alternate power source.

In some embodiments according to the first aspect, the battery control means is configured to monitor the state of charge of the first battery by receiving state of charge information from the first battery, the state of charge information comprising information indicative of the state of charge of the first battery.

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 ; a first battery configured to supply electrical power to the electric drivetrain ; an alternate power source configured to supply electrical power to the first battery and/ or the electric drivetrain ; and battery management apparatu s according to the first aspect. In some embodiments according to the second aspect, the alternate power source is a removable range extender unit configured to generate electrical power.

In some embodiments according to the second aspect, the electric vehicle further comprises a second battery electrically connected in parallel with the first battery, and the battery control means can be configured to control a state of charge of the second battery so that an output voltage of the second battery and an output voltage of the replacement battery are within the acceptable range.

In some embodiments according to the second aspect, the alternate power source comprises a second battery and the electric vehicle further comprises a regenerative braking system configured to generate electrical power under braking of the electric vehicle, the regenerative braking system being configured to generate electrical power for charging the first battery from a first pair of wheels of the electric vehicle, and to generate electrical power for charging the second battery from a second pair of wheels of the electric vehicle, wherein the battery control means is configured to increase the state of charge of the first battery relative to the state of charge of the second battery by changing a relative amount of electrical power generated from the first and second pairs of wheels by the regenerative braking system so as to supply more electrical power to the first battery than to the second battery, while keeping the total amount of electrical power generated under braking substantially constant.

According to a third aspect of the present invention , there is provided a battery management method of an electric vehicle an electric drivetrain comprising one or more electric motors, a first battery configured to supply electrical power to the electric drivetrain , and an alternate power source configured to supply electrical power to the first battery and/ or the electric drivetrain , the alternate power source configured to be swappable with a replacement battery, the method comprising: determining a state of charge of the first battery; and controlling the state of charge of the first battery and/ or a state of charge of the replacement battery before the replacement battery is installed in the electric vehicle, so that an output voltage of the first battery and an output voltage of the replacement battery are within an acceptable range.

In some embodiments according to the third aspect, the battery management method further comprises a step of electrically connecting the replacement battery in parallel with the first battery, after the alternate power source has been replaced with the replacement battery.

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 illu strates 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 ; and

Figure 4 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. Finally, in the present embodiment the electric vehicle 100 further comprises a 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 112. 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 112 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 112 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.

The 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, and the electric drivetrain 101, 102. In the present embodiment the power distribution unit 107 includes 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 and the second battery 104 supplies electrical power to the other of the front and rear drivetrains, so that the two batteries 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 control unit 106 can independent charge or discharge either of the master battery 103 and the second battery 104.

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 one of the alternate power sources 104, 105 is swapped out for the replacement battery 112, the output voltage of the replacement battery 112 may differ to the output voltage of the master battery 103. When the output voltages are different, 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 the state of charge of the master battery 103 and the replacement battery 112, ahead of one of the alternate power sources 104, 105 being swapped with the replacement battery 112. The battery management apparatus is configured to control the state of charge of the master battery 103 and the state of charge of the replacement battery 112 so that the master battery 103 and the replacement battery 112 provide substantially the same output voltage. Additionally, if the replacement battery 112 is due to be swapped with the range extender unit 105, such that master battery 103 , the second battery 104, and the replacement battery 112 will all be u sed to power the electric vehicle 100 , the battery management apparatu s may control the state of charge of the second battery 104 so that all three batteries provide substantially the same output voltage.

Furthermore, in some embodiments the battery management apparatus may manage the states of charge of a plurality of replacement batteries, for example, when both the second battery 104 and the range extender unit 105 are to be replaced by separate replacement batteries. The battery management apparatus is schematically illustrated in Fig. 2.

The battery management apparatus of the present embodiment includes the control unit 106 in the electric vehicle 100 , and a remote charging control unit 111 connected to the replacement battery 112. The remote charging control unit 111 is physically separate to the electric vehicle 100 and may be situated at a separate location to the electric vehicle 100. For example, the remote charging control unit 111 and the replacement battery 112 can remain at a service depot whilst the electric vehicle 100 is out on a journey. The onboard control unit 106 and the remote charging control unit 111 can communicate with one another via any suitable interface. As shown in Fig. 2, in the present embodiment the onboard control unit 106 comprises computer-readable memory 201 and a processing unit 202 for executing computer program instructions stored in the memory 202. The memory 201 can be any suitable storage medium, and the processing unit 202 can include one or more processing cores. The control unit 106 further comprises a communication unit 203 for communicating wirelessly with the remote charging control unit 111. For example, the communication unit 203 may be configured to connect to a mobile telecommunication network so that the control unit 106 and remote charging control unit 111 can communicate over the Internet.

The control unit 106 is configured to monitor the state of charge of the master battery 103. In the present embodiment, the battery control means is configured to monitor the state of charge of the master battery 103 by receiving state of charge information from a state of charge (SOC) monitoring unit 204 included in the master battery 103. The state of charge information comprises information indicative of the state of charge of the master battery 103. In the present embodiment the master battery 103 includes equipment for measuring the output voltage and/ or current of the first battery, and the SOC monitoring unit 204 is configured to estimate the current state of charge based on the measured voltage and/ or current and a known relationship between the voltage, current, and the state of charge. The SOC monitoring unit 204 can then transmit state of charge information indicative of the estimated state of charge to the control unit 106.

In some embodiments, the secondary battery 104 may also include a SOC monitoring unit for transmitting state of charge information of the secondary battery 104 to the control unit 106, allowing the control unit 106 to also monitor the state of charge of the secondary battery 104.

In other embodiments, the battery management apparatus itself may include a voltage and/ or current measuring unit configured to measure the output voltage and/ or current of the master battery 103. The control unit 106 could then estimate the state of charge of the master battery 103 from the measured output voltage and/ or current. Such embodiments can enable the control unit 106 to monitor the state of charge of the master battery 103 even when the master battery 103 is not capable of reporting state of charge information, for example, if the master battery 103 does not include the necessary hardware or is otherwise unable to report the state of charge due to an internal fault. The remote charging control unit 111 of the present embodiment comprises a charging unit 211 which can be controlled to electrically charge or discharge the replacement battery 112. For example, the replacement battery 112 could be discharged by feeding electrical power back to the mains power grid. The charging unit 211 allows the remote charging control unit 111 to actively control the state of charge of the replacement battery 112. Additionally, as with the onboard control unit 106 , the remote charging control unit 111 further comprises memory 212, a processing unit 213 , and a

communication unit 214.

Although in the present embodiment certain functions of the battery management apparatus are implemented in software, in other embodiments one or both of the onboard control unit 106 and remote charging control unit 111 may be implemented u sing dedicated hardware rather than software instructions, for example, a field- programmable gate array (FPGA) or application-specific integrated circuit (ASIC) .

The method performed by the battery management apparatu s of the present embodiment is shown in Fig. 3. First , in step S30 1 the control unit 106 determines the estimated SOC of the master battery 103 by receiving SOC information from the SOC monitoring unit 204 in the master battery 103. The control unit 106 may monitor the SOC of the master battery 103 while the electric vehicle 100 is being driven , that is, during u se of the electric vehicle 100 , or while the electric vehicle 100 is stationary and powered down . Once the estimated SOC of the master battery 103 has been determined, in step S302 the SOC of one or both of the master battery 103 and the replacement battery 112 is controlled so that the master battery 103 and the replacement battery 112 provide the same output voltage. By 'controlled ', it is meant that the SOC of a battery is changed by either discharging or charging the battery as required. This process can be referred to as active SOC management. Depending on the embodiment, active SOC management may only be performed at either the electric vehicle 100 or the remote charging station , or may be performed at both locations.

In the present embodiment, in step S302 the onboard control unit 106 and the remote charging control unit 111 can share information about their available resources to determine which of the master battery 103 and the replacement battery 112 should be charged or discharged. For example, if the SOC of the master battery 103 needs to be increased to provide the same output voltage as the replacement battery 112, but there is not sufficient power available at the electric vehicle to raise the SOC of the master battery 103 by the required amount, the remote charging control unit 111 may use the charging unit 211 to discharge the replacement battery 112, with or without

simultaneous charging of the master battery 103 via the power distribution unit 107, until the output voltages are the same. Alternatively, in another embodiment the onboard control unit 106 may simply communicate the SOC of the master battery 103 to the remote charging unit 111 without performing active SOC management at the electric vehicle. In this case only the remote charging unit 111 performs active SOC management by controlling the SOC of the replacement battery 112 according to the information received from the onboard control unit 106.

Next, after the master battery 103 and the replacement battery 112 have been brought to the same output voltage, one of the alternate power sources 104, 105 is swapped out for the replacement battery 112. For example, in the present embodiment both the second battery 104 and the range extender unit 105 are configured to be capable of being removed and replaced by the replacement battery 112. It will be understood that the step of physically replacing the battery in step S303 may not be performed by the battery management apparatu s, but in practice may be performed by a human operator.

Then , once the replacement battery 112 has been installed in the electric vehicle 100 , in step S304 the onboard control unit 106 controls the power distribution unit 107 to electrically connect the replacement battery 112 in parallel with the master battery 103. The replacement battery 112 and the master battery 103 can then supply electrical power to the electric drivetrain in an efficient manner, since power losses are minimised by connecting the batteries in parallel. In some embodiments, the battery management apparatu s can be further configured to predict a state of charge of the master battery 103 at a time when the replacement battery 112 is due to be swapped with the alternate power source. For example, this may be a fixed time of day, or may be predicted based on the remaining journey time. The remote charging control unit 111 can be configured to control the state of charge of the replacement battery 112 to provide substantially the same output voltage as the master battery 103 in the predicted state of charge. As shown in Fig. 2, in the present embodiment the onboard control unit 106 is further configured to communicate with a route planning unit 205 in order to obtain information relating to a planned route to be followed by the electric vehicle during a journey. The route planning unit 205 can be any suitable device capable of providing information about the planned route, for example, a satellite navigation ('Satnav') unit. The obtained route information can be used in predicting the state of charge of the master battery 103 at the end of the journey. The information may include, for example, a distance to be travelled on the planned route, the expected speed of the electric vehicle during the journey, traffic conditions on the planned route, weather conditions on the planned route, an elevation change over the planned route, and the predicted time required to complete the journey.

Referring now to Fig. 4, a graph plotting the output voltage of a battery as a function of the state of charge (SOC) is illustrated, according to an embodiment of the present invention. In the present embodiment, the output voltage remains substantially constant for a range of SOC values. As shown in Fig. 4, the SOC curve includes a central region with a relatively shallow gradient. Within this region, a relatively large change in the SOC of the battery only produces a relatively small change in the output voltage. Outside of this range, the output voltage decreases at low SOC values, and increases at high SOC values. The battery management apparatus can be configured to control the SOC of the first battery and/ or the replacement battery to be within a SOC range for which the output voltage is substantially constant. This SOC range may be referred to as an acceptable SOC range. It may be possible for two batteries with output voltages within this acceptable SOC range to be safely connected in parallel, provided that the variation in output voltage is sufficiently small within this SOC range. The acceptable SOC range can be defined in terms of upper and lower limits that are programmed in advance into the battery control means. In such cases, the onboard control unit 106 and remote charging control unit 111 could independently manage the SOC of the master battery 103 and the replacement battery 112, respectively, without communicating with one another, by controlling the batteries to have SOCs in this range.

Referring back to Fig. 1, in the present embodiment the electric vehicle 100 further comprise a regenerative braking system 108 configured to generate electrical power for charging the master battery 103 and/ or the slave battery 104 under braking of the electric vehicle 100. In the present embodiment the regenerative braking system 108 is configured to generate electrical power for charging the master battery 103 from one pair of wheels of the electric vehicle 100 , and to generate electrical power for charging the slave battery 104 from the other pair of wheels of the electric vehicle 100. For example, the master battery 103 may be charged using power generated from the front wheels and the slave battery 104 may be charged using power generated from the rear wheels, or vice versa. It will be understood that embodiments of the invention are not limited to charging individual batteries from a single pair of wheels via regenerative braking, and in some embodiments a given battery may be charged using energy recovered from multiple pairs of wheels, for example when a vehicle comprises more axles than there are batteries.

In some scenarios, the control unit 106 may determine that it is necessary to increase the SOC of the master battery 103 relative to the SOC of the second battery 104. For example, this may occur when the SOC of the master battery 103 needs to be increased to ensure that the output voltage matches that of the replacement battery 112, and when the secondary battery 104 does not need to be charged as it is about to be replaced. In response to a determination that it is necessary to increase the SOC of the master battery 103 relative to the SOC of the second battery 104, the control unit 106 is configured to control the regenerative braking system 108 to change a relative amount of electrical power generated from the first and second pairs of wheels so as to supply more electrical power to the master battery 103 than to the slave battery 104, while keeping the total amount of electrical power generated under braking substantially constant. 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 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.

In the above-described embodiments, the state of charge of both the master battery 103 and the replacement battery 112 are actively controlled by a distributed battery management apparatus. The battery management apparatus of the present embodiment includes the onboard control unit 106 in the electric vehicle 100 , and the charging control unit 111 which is situated at a remote charging station , for example, a service depot. However, in other embodiments only one of the master battery 103 and the replacement battery 112 may be actively managed. In such embodiments, the battery management apparatu s may be solely located in the electric vehicle 100 or solely located at the remote charging station , as appropriate.

For example, in one embodiment the replacement battery 112 may always be charged to a specific predefined output voltage, knowledge of which is pre-programmed into the onboard control unit 106. The control unit 106 can then actively manage the state of charge of the master battery 103 so that the output voltage of the master battery 103 matches that of the replacement battery 112 at a time when the replacement battery is due to be installed in the electric vehicle 100.

Alternatively, in another embodiment the state of charge of the master battery 103 may not be actively managed, but instead may be passively monitored by a SOC monitoring unit 20 1 which reports back to the remote charging control unit 111. The remote charging control unit 111 can then discharge or charge the replacement battery 112 as required, in a dynamic fashion , so that the output voltage of the replacement battery 112 matches that of the master battery 103.

Finally, embodiments of the invention have been described in which the states of charge of the master battery 103 and the replacement battery 112 are controlled so that both batteries have similar output voltages. It should be understood that in this context, any references to the same' output voltage do not imply that the output voltages must be absolutely identical. In any given embodiment, the system may be capable of tolerating a certain finite imbalance between output voltages without damage occurring. An acceptable range can be defined 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 replacement battery 112 are sufficiently close to allow the master battery 103 and the replacement battery 112 to be electrically connected in parallel, without damage occurring to either battery or to other components of the electric vehicle.

In some embodiments, the acceptable range may be defined in terms of an acceptable voltage difference between the output voltage of the master battery 103 and the replacement battery 112, which may be defined in absolute or relative terms. For example, an acceptable voltage difference may be defined as 2 V, or as a certain percentage of the output voltage, for example 1%, 2% or 3 %. Alternatively, the acceptable range may be defined as a finite range of voltages by setting upper and lower voltage thresholds, and the states of charge of the master battery 103 and the replacement battery 112 can be controlled so that both batteries have output voltages within the range of acceptable voltages. 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 range. If the difference between the output voltages subsequently becomes larger than the defined acceptable range, 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 range.

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.