GB2592241A

GB2592241A - A System

Info

Publication number: GB2592241A
Application number: GB2002434.5A
Authority: GB
Inventors: Greetham Stephen
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2020-02-21
Filing date: 2020-02-21
Publication date: 2021-08-25
Anticipated expiration: 2040-02-21
Also published as: GB202002434D0; GB2592241B

Abstract

A system, particularly for an electric vehicle, comprises a battery pack and an electric drive unit. The battery pack comprises a first voltage bus, a second voltage bus, a first sub-pack 210, a second sub-pack 220, a plurality of switches configurable to connect the first sub-pack to the first voltage bus and to connect the second sub-pack to the second voltage bus, and a battery controller for configuring the switches. The electric drive unit comprises a motor 310 and an inverter module, the inverter module comprising first and second inverters 360, 370 coupled to windings of the motor, and an inverter controller 380 for controlling switches of the inverters. The first sub-pack supplies a first voltage to the first inverter via the first voltage bus, and the second sub-pack supplies a second voltage to the second inverter via the second voltage bus. At least one of the sub-packs comprises a plurality of battery modules, and the battery controller configures the switches of the battery pack such that the battery modules are arranged in one of series and parallel during discharging of the battery pack and in the other of series and parallel during charging of the battery pack.

Description

A SYSTEM

Field of the Invention

The present invention relates to a system, and in particular to a system having an electric motor coupled to a dual inverter.

Background of the Invention

Improvements in the efficiency of systems such as an electric vehicle are clearly desirable and lead to improvements, for instance, increases in the driving range. Furthermore, it is desirable that the performance of components such as the powertrain are maintained as the battery pack discharges.

Summary of the Invention

The present invention provides a system comprising a battery pack and an electric drive unit, wherein: the battery pack comprises a first voltage bus, a second voltage bus, a first sub-pack, a second sub-pack, a plurality of switches configurable to connect the first sub-pack to the first voltage bus and to connect the second sub-pack to the second voltage bus, and a battery controller for configuring the switches; the electric drive unit comprises a motor and an inverter module, the inverter module comprising first and second inverters coupled to windings of the motor, and an inverter controller for controlling switches of the inverters; the first sub-pack supplies a first voltage to the first inverter via the first voltage bus; the second sub-pack supplies a second voltage to the second inverter via the second voltage bus; the switches of the inverters are configurable between a first configuration in which the first voltage is applied to one or more of the windings, and a second configuration in which a second voltage is applied to one or more of the windings; at least one of the sub-packs comprises a plurality of battery modules; and the battery controller configures the switches of the battery pack such that the battery modules are arranged in one of series and parallel during discharging of the battery pack and are arranged in the other of series and parallel during charging of the battery pack.

The inverters may be controlled such that the voltage applied to the windings is selected in response to a desired or commanded speed and/or torque. For example, when the system is an electric vehicle, the first voltage may be applied to the motor windings when driving at relatively constant speed, and the second voltage may be applied at during acceleration. As a result, the efficiency of the motor may be improved.

At least one of the sub-packs comprises a plurality of modules that can be arranged in one of series and parallel during discharging of the battery pack and in the other of series and parallel during charging of the battery pack. This then has the advantage that the modules may be arranged in parallel during discharging so that one of the inverters supplies a relatively low voltage, such as 450V. The modules may then be arranged in series during charging so that they can be charged more quickly using a high voltage charger, such as a 1000V DC charger. Alternatively, the modules may be arranged in series during discharging so that one of the inverters supplies a relatively high voltage, such as 900V.

However, the modules may then be arranged in parallel during charging so that the sub-pack can be charged using a low voltage charger, such as a 500V DC charger.

Conceivably both sub-packs may comprise a plurality of battery modules. Moreover, the battery controller may configure the switches of the battery pack such that the modules of the first sub-pack are arranged in parallel during discharging of the battery pack and in series during charging of the battery pack, and the modules of the second sub-pack are arranged in series during discharging of the battery pack and in parallel during charging of the battery pack.

The first inverter may comprise first switches and the second inverter may comprise second switches, such that the first switches are of a different type to the second switches. In particular, the first switches may be silicon carbide switches, and the second switches may be gallium nitride switches. The silicon carbide switches are better suited to the higher voltages whereas the gallium nitride switches are generally less expensive and compatible with the lower voltages. This combination of silicon carbide and gallium nitride thus enables higher switching speeds within the inverters in a cost-efficient manner.

The switches may be configurable into a configuration in which a charge current flows from the first sub-pack to the second sub-pack. This allows the first sub-pack to maintain the state-of-charge of the second sub-pack, so that the second voltage is better-regulated. This provides a number of advantages. For example, auxiliary components of the system operating at lower voltage (e.g. 12V) may be powered from the second sub-pack and the improved regulation of the second voltage means that a simpler and cheaper power supply unit may be used to step down the voltage. Additionally or alternatively, the first sub-pack may be used during low load, for example in an electric vehicle, normal driving (i.e. driving requiring a relatively low torque), and the second sub-pack may be used during high load, for example acceleration (i.e. driving requiring a relatively high torque). By regulating the second voltage, the ability to provide power from the second sub-pack, and thus the ability to accelerate, is maintained. When the first voltage is less than the second voltage, the switches can be configured so that the motor windings can be used to boost the voltage such that the second sub-pack can be charged.

The first sub-pack may comprise battery cells having a first maximum discharge current, the second sub-pack may comprise battery cells having a second maximum discharge current, and the second maximum discharge current may be higher than the first maximum discharge current. Cells having a higher maximum discharge current (generally referred to as power cells) are advantageous in that they are capable of delivering higher power, albeit for a shorter period of time, for a given charge capacity. However, power cells are generally more expensive than cells having a lower maximum discharge current (generally referred to as energy cells). Having sub-packs of different discharge current allows the battery pack to deliver bursts of higher power in a cost-effective manner. In particular, the second sub-pack may comprise power cells for delivering bursts of high power during periods of high load (acceleration), whereas the first sub-pack may comprise cheaper energy cells for delivering lower power during periods of low load (normal driving).

The second voltage may be higher than the first voltage. By having a higher terminal voltage and a higher discharge current, the second sub-pack is capable of outputting higher power to the motor. The second voltage may be at least double the first voltage, and the second maximum discharge current may be at least double the first discharge current. Such an arrangement results in a second sub-pack which can provide at least four times the power as the first sub-pack.

Brief Description of the Drawings

In order that the invention may be more readily understood, reference will now be made by way of example only to the accompanying drawings in which: Figure 1 shows a schematic depiction of a particular system, namely an electric vehicle; Figure 2 shows a schematic view of the underside of the electric vehicle of Figure 1, Figure 3 shows a schematic depiction of an electric drive unit of the electric vehicle; Figure 4 shows a schematic depiction of an electrical circuit of the electric vehicle; Figure 5 details the configuration of switches of an inverter of the electric drive unit, Figure 6 illustrates the arrangement of the windings of the motor of the electric drive unit when the switches of the inverter are configured as detailed in Figure 5; Figure 7 shows a schematic depiction of a battery pack of the electric vehicle; Figure 8 illustrates a first configuration of the switches in which a positive voltage is applied to the windings of the motor by a first battery sub-pack; Figure 9 iflustrates a second configuration of the switches in which a charge current flows from the first battery sub-pack to a second battery sub-pack, Figure 10 illustrates a third configuration of the switches in which a negative voltage is applied to the windings of the motor by the first battery sub-pack, and Figure 11 illustrates a fourth configuration of the switches in which a charge current flows from the first battery sub-pack to a second battery sub-pack

Detailed Description of the Invention

The system (an electric vehicle) 100 of Figures 1 and 2 comprises two pairs of wheels 110, each of which is connected to an electric drive unit (EDU) 300. The EDUs 300 are connected to a battery pack 200 which comprises a first battery sub-pack 210 and a second battery sub-pack 220. In use the EDUs 300 provide a motive force to the wheels 110 in order to propel the electric vehicle 100.

As shown in Figure 3, each EDU 300 comprises an electric motor 310, a gearbox 320 and an inverter module 330. Each EDU 300 is connected to a pair of wheels 110 via drive shafts 400. In use, the inverter module 330 converts the DC voltage supplied by the battery pack 200 into an AC voltage which is applied to the motor 310. The motor, in response, drives the gearbox 320, which in turn causes the drive shafts 400 and the wheels 110 to rotate.

Referring now to Figure 4, each inverter module 330 comprises a first inverter 360 and a second inverter 370. The electric motor 310 is a three-phase motor and comprises three phase windings 312. The first inverter 360 and the second inverter 370 are connected to opposing ends of the phase windings 312 in what is generally referred to as a dual-inverter open winding arrangement. The first inverter 360 is additionally connected to the first battery sub-pack 210 such that current can flow between the first battery sub-pack 210 and the electric motor 310 via the first inverter 360. Similarly, the second inverter 370 is additionally connected to the second battery sub-pack 220 such that current can flow between the second battery sub-pack 220 and the electric motor 310 via the second inverter 370. The first inverter 360 comprises a plurality of first inverter switches 362 (labelled as SW1 to SW6), the second inverter 370 comprises a plurality of second inverter switches 372 (labelled as SW7 to SW12), and the inverter module 330 comprises an inverter controller 380 which is operable to configure the first inverter switches 362 and the second inverter switches 372 so as to control the current supplied to the phase windings 312 of the electric motor 310.

The first battery sub-pack 210 supplies a first voltage, V1, to the first inverter 360 and the second battery sub-pack 220 supplies a second voltage, V2, which is different to the first voltage, to the second inverter 370. By configuring the first inverter switches 362 and the second inverter switches 372, it is possible to apply a number of different voltages to the windings 312 of the motor 310. In particular, the switches 362,372 may be configured such that any one of the following voltages may be applied to the windings: +V1, -V1, +V2, -V2, +V1+V2, +V1-V2, -V1+V2 and -V-V2. More generally, it may be said that the switches 362,372 are configurable into a first configuration in which the first voltage is applied the windings 312, a second configuration in which the second voltage is applied to the windings 312, and a third configuration in which a third voltage, corresponding to the sum of the first and second voltages, is applied to the windings 312. In each of the three configurations, the polarity of the first voltage and the second voltage may be positive or negative.

Figure 5 details the configuration of the switches 362,372 in order to apply each of the voltages to the windings 312 of the motor 310. The configurations detailed in Figure 5 lead to an arrangement in which the upper winding of Figure 4 is connected in series with the middle and lower windings, which are connected in parallel. This particular arrangement is shown in Figure 6, with the upper winding being labelled 312a, the middle winding 312b, and the lower winding 312c. It will be appreciated that a different set of switch configurations will be required in order to apply each of voltages to the windings 312 for the arrangements in which (i) the middle winding 312b is connected in series with the upper and lower windings 312a,312c, which are connected in parallel, and (ii) the lower winding 312c is connected in series with the upper and middle windings 312a,312b, which are connected in parallel. A positive voltage is assumed when the potential on the left side of the upper winding (as it appears in Figure 4) is greater than that on the right side. A voltage of zero may be achieved by opening all of the switches. However, a voltage of zero may also be achieved by closing all of the high-side switches or all of the low-side switches, as detailed in Figure 5. This particular configuration then permits current in the windings 312 to continue to flow around the inverters 360,370 in a process generally referred to as freewheeling.

By way of example, in order to apply the voltage +Vt to the windings 312, the inverter controller 380 closes switches SW1, SW4, SWG, SW8, SW10 and SW12, and opens switches SW2, SW3, SW5, SW7, SW9 and SW11. Current then flows from the first battery sub-pack via SW I, through the upper winding of the motor 310 shown in Figure 4, through SW8 and then returns to the motor via SW10 and SW12. The current then flows through the middle and lower windings of the motor 310 and returns to the first battery sub-pack 210 via SW4 and SW6.

For a given applied voltage, there is typically a single point in the torque-speed characteristic of a motor where the efficiency of the motor is greatest. With the system 100 of the present invention, a number of different voltages may be applied to the motor 110. As a result, it is possible to achieve multiple peak efficiency points within the torque-speed characteristic of the motor 110. By controlling the voltage that is applied to the windings 312 of the motor 110 according to a desired speed and/or torque of the motor 110, it is possible to increase the efficiency of the system 100. For example, the first voltage VI may be applied to the motor 110 when operating at low torque conditions (e.g. when coasting), the second voltage V2 may be applied to the motor 110 when operating at high torque conditions (e.g. during normal acceleration), and the sum of V1 and V2 may be applied to the motor 110 when operating at maximum torque (e.g. during hard acceleration).

Assume a scenario in which the first voltage, VI, provided by the first battery sub-pack 210 is 450V and the second voltage, V2, provided by the second battery sub-pack 220 is 900V.

The inverter controller 380 executes a voltage vector control algorithm to generate a voltage vector. The voltage vector is determined in accordance with a desired speed and/or torque for the motor 110. In an alternative, the inverter controller 380 may receive the voltage vector from another component or module within the vehicle 100. The inverter controller 380 uses the amplitude of the voltage vector to determine the configuration of the inverter switches 362,372. If the amplitude of the voltage vector is less than a first threshold value, the inverter switches 362,372 are configured in a first configuration such that the first voltage, V1, is applied to the windings 312. If the amplitude of the voltage vector is greater than the first threshold value but less than a second threshold value, the inverter switches 362,372 are configured in a second configuration such that the second voltage, V2, is applied to the windings 312. If the amplitude of the voltage vector is greater than the second threshold value then the inverter switches 362,372 are configured in a third configuration such that the sum of the first and second voltages, V I+V2, is applied to the windings 312. As a consequence, the voltage applied to the motor depends on the magnitude of the voltage vector. More particularly, one of three different voltages, namely 450V, 900V and 1350V, is applied to the windings of the motor according to the desired speed and/or torque of the motor.

In the example given above, the voltage provided by the first battery sub-pack is 450V and the voltage provided by the second battery sub-pack is 900V. Consequently, it is possible only to apply three different voltages to the winding 312 of the motor 110 This is because the difference between the two voltages (i.e. V2-V1) is also 450V However, if the voltage provided by the first battery sub-pack was, say, 400V and the voltage provided by the second battery sub-pack was, say, 1000V then it would be possible to apply four different voltages to the motor, namely 400V (V1), 600V (V2-V1), 1000V (V2) and 1400V (V1+V2). This would then enable the three-level control scheme described in the preceding paragraph to be extended to a four-level control scheme, in which the switches 362,372 are configurable into a fourth configuration such that the difference in the first and second voltages, V1-V2, is applied to the windings 312.

Figure 7 shows a schematic depiction of the battery pack 200, which comprises the first battery sub-pack 210 and the second battery sub-pack 220. The battery pack 200 further comprises a first bus 240 which comprises terminals 242 and 244, a second bus 250 which comprises terminals 252 and 254, a plurality of contactors 230 (labelled as SW20 to SW30) and a contactor controller 280. In operation, the contactor controller 280 controls the configuration of the contactors 230 (that is, to select whether a contactor is an open or a closed state) so as to configure the battery pack 200 and thus effect the operation of the system 100.

The first battery sub-pack 210 comprises a first battery module 212 and a second battery module 214. The contactors 230 may be configured such that the first battery module 212 and the second battery module 214 are connected in parallel or in series. When the system 100 is operating in a motoring mode, the first and second battery modules 212, 214 are connected in parallel, as shown in Figure 4. In this case, contactors SW23 and SW25 are closed, and SW24 is open. By reversing this arrangement, that is by opening contactors SW23 and SW25 and closing contactor SW24, the first and second battery modules 212, 214 are connected in series. As described below, the first and second battery modules 212, 214 may be connected in series during charging. The first battery sub-pack 210 is connected to the first bus 240 and can be isolated from the terminals 242, 244 of the first bus by contactors SW20 and SW22. The first battery sub-pack 210 is isolated from the second battery sub-pack 220 using contactors SW26 and SW27. The second battery sub-pack 220 is connected to the second bus 250 and can be isolated from the terminals 252, 254 of the second bus by contactors SW28, and SW30. Contactor SW21 and resistor R1 form a pre-charge circuit for the first bus 240, whilst contactor SW29 and resistor R2 form a pre-charge circuit for the second bus 250.

DC chargers for electric vehicles typically act as a current source. Accordingly, when the system 100 is connected to a 1000V DC charger, the modules 212, 214 of the first battery sub-pack 210 are advantageously connected in series so as to present a higher voltage to the charger. This then has the advantage that a higher power may be drawn from the charger, thereby reducing the charge time. Accordingly, when the system 100 is connected to a 1000V DC charger, the contactors 230 are configured such that the first and second battery modules 212, 214 are connected in series. More specifically, the 1000V DC charger is connected to the terminals 252, 254 of the second bus 250. The controller 280 then determines which of the two battery sub-packs 210, 220 has the lower voltage; in this instance, the voltage of first battery sub-pack 210 is that provided when the modules 212, 214 are connected in series. The controller 280 then closes those contactors necessary to connect the battery sub-pack with the lower voltage to the terminals 252, 254. The battery sub-pack with the lower voltage then begins to charge. Subsequently, when the voltages of the two sub-packs are the same, the controller 280 closes those contactors necessary to connect the other battery sub-pack to the terminals 252, 254. At this stage, both battery sub-packs 210, 220 are connected to the terminals 252, 254 and are simultaneously charged. So, for example, if the first battery sub-pack 210 initially has the lower voltage, the controller 280 closes contactors 5W24, 5W26, SW27 and 5W30 so as to charge the first battery sub-pack 210 with the modules 212, 214 connected in series. Once the voltage of the first battery sub-pack 210 reaches that of the second battery sub-pack 220, the controller 280 closes contactor 5W28 such that both the first battery sub-pack 210 and the second battery sub-pack 220 are charged simultaneously.

The system 100 may also be charged using a 500V DC charger. In this instance, only the first battery sub-pack 210 is charged and the modules 212, 214 are connected in parallel during charging. The controller 280 therefore closes contactors SW23, SW25, SW26, 5W27 and 5W30 such that the two battery modules 212, 214 are connected in parallel across the terminals 252, 254. Although only the first battery sub-pack 210 is charged by the 500V DC charger, the first battery sub-pack 210 may be used to charge the second battery sub-pack 220 during motoring, as will now be described.

Through appropriate control of the inverter switches 362,372, current can be transferred from the first battery sub-pack 210 to the second battery sub-pack 220, in spite of the first battery sub-pack 210 having a lower voltage. In order to achieve this, the windings 312 of the motor 310 are used to boost the voltage of the first battery sub-pack 210. For example, let us say that vehicle 100 is motoring at low torque and the switches 362,372 of the inverter module 330 are configured to apply the first voltage, VI, to the windings 312. Moreover, let us say that voltage is positive and that, in common with the table of Figure 5, the upper winding is connected in series with the middle and lower windings, which are connected in parallel. This may be achieved with the configuration of switches shown in Figure 8. When in this particular configuration, current flows through the upper winding in a direction from left to right. In order to charge the second battery sub-pack 220 using the first battery sub-pack 210, switch SW7 is closed and switch SW8 is opened, as shown in Figure 9. Current in the upper winding will want to continue flowing from left to right. As a result, the voltage across the upper winding increases until such time as current is driven into the second battery sub-pack 220. Consequently, although the voltage of the first battery sub-pack 210 is less than the voltage of the second battery sub-pack 220, the voltage of the first battery sub-pack is boosted by the winding, which is nothing more than an inductor, upon closing SW7 and opening SW8. Current in the winding collapses fairly quickly at which point switch SW8 is closed and switch SW7 is opened. This process of switching between the configurations shown in Figures 8 and 9 (using, for example, PWIVI) may continue until such time as a desired voltage or state-ofcharge is achieved for the second battery sub-pack. Part of the power drawn from the first battery sub-pack 210 is therefore used to power the motor 310 in order to propel the vehicle 100, and part of the power drawn from the first battery sub-pack is used to charge the second battery sub-pack 220.

Figures 8 and 9 illustrate switch configurations when a positive voltage, +V1, is applied to the windings 312. For completeness, Figures 10 and 11 illustrate the switch configurations when a negative voltage, +VI, is applied to the windings 312. Again, the upper winding is connected in series with the middle and lower windings, which are connected in parallel. As can be seen, by closing switches SW9 and SW11 and opening switches SW10 and SW12, current in the middle and lower windings is driven into the second battery sub-pack.

In Figures 8 to 11, the upper winding is connected in series with the middle and lower windings, which are connected in parallel. As noted above in connection with Figure 5, it will be appreciated that a different set of switch configurations will be required for the arrangements in which (i) the middle winding is connected in series with the upper and lower windings, which are connected in parallel, and (ii) the lower winding is connected in series with the upper and middle windings, which are connected in parallel.

It is therefore possible to charge both battery sub-packs 210, 220 even if only a 500V DC charger is available. With intelligent charge management, a control unit within the vehicle could determine that the vehicle is approaching a 500V DC charger and, in response, the inverter controller 380 may configure the switches 362,372 such that first battery sub-pack 210 charges the second battery sub-pack 220 prior to arriving at the charger. As a result, a greater overall charge may be achieved for the battery pack.

The capability to charge the second battery sub-pack 220 from the first battery sub-pack 210, regardless of the relative state-of-charge of the first and second battery sub-packs, provides a number of further advantages. For example, the second battery sub-pack 220 may be used only in response to a request for acceleration. Once the request for acceleration has been met, it is then possible for the state-of-charge of the second battery sub-pack 220 to be restored to 100% (or substantially 100%) by transferring charge from the first battery sub-pack 210 to the second battery sub-pack 220.

The first inverter 360 may be a lower power inverter than the second inverter 370, with the first battery sub-pack 210 being used to provide power for the system 100 when the motor 310 is operating at low torque and/or low speed. For example, the first inverter 360 may have a power rating of say, 100kW, and the second inverter may have a power rating of, say, 200kW. The same switches may be used in both the first inverter 360 and the second inverter 370. Alternatively, the switches 362 of the first inverter 360 may be different to the switches 372 of the second inverter 370. Using different switches for the two inverters 360,370 enables high switching frequencies to be achieved in a cost-effective way. In particular, gallium nitride (GaN) switches may be used for the switches of the first inverter 360 as their breakdown voltage (typically no greater than 650V DC) is compatible with the voltages supplied by the first battery sub-pack 210. For the second inverter, GaN switches would not be appropriate due to the higher voltages involved and thus the switches 372 of the second inverter 370 may comprise silicon carbide (SiC) devices, which have breakdown voltages in excess of 1200V DC and are significantly more expensive than GaN devices. The combination of GaN devices in the first inverter 360 and SiC devices in the second inverter 370 provides high switching frequencies in a cost-effective manner.

The first battery sub-pack 210 may have a higher power capacity than the second battery sub-pack 220. This enables the first battery sub-pack to be used to provide motive power for the vehicle in steady state conditions and to maintain the state-of-charge of the second battery sub-pack. By maintaining the state-of-charge of the second battery sub-pack at 100%, the system 100 will retain the ability to respond to the driver's request for acceleration. The first battery sub-pack 210 may comprise energy cells, which have a relatively high power capacity but have a lower maximum discharge rate. The second battery sub-220 pack may comprise power cells, which have a reduced impedance compared to an energy cell such that they can have a greater maximum discharge rate. Such an arrangement enables the second battery sub-pack to provide high currents to the second inverter such that the engine can provide significant acceleration when required. The combination of energy cells in the first battery sub-pack and power cells in the second battery sub-pack provide a cost effective balance between power and performance.

The higher capacity of the first battery sub-pack allows the second battery sub-pack to be recharged through the transfer of power between the first and the second battery sub-pack. By appropriate power transfer the state-of-charge of the second battery sub-pack can be maintained at 100% (or substantially 100%) across a significant range of state-of-charge for the first battery sub-pack. For example, the state-of-charge of the second battery sub-pack can be maintained at 100% until the state-of-charge of the first battery sub-pack reaches a predetermined threshold value, for example 12.5%. By maintaining the state-of-charge of the second battery sub-pack at 100% the performance of the system 100 is maintained.

The second battery sub-pack 220 may also be used to generate power for the auxiliary components of the system] 00. These are components which require a low voltage power supply, typically 12V, rather than the hundreds of volts which are supplied from the battery pack. Conventionally, a system comprises an auxiliary power supply unit (PSU) which steps down the voltage of the battery pack and outputs a 12V supply voltage. If the state-of-charge of the battery pack were to vary from 100% to 12.5%, the resulting operating voltage range will be relatively wide. The PSU, which is required to operate over the full voltage range of the battery pack, is therefore often complex and expensive. By using the methods described above, the state-of-charge of the second battery sub-pack may be held close to 100% and thus a narrow operating voltage range may be achieved.

By using the second battery sub-pack 220 to power the auxiliary components, a simpler and cheaper PSU may be used to step down the voltage.

The above embodiments are to be understood as illustrative examples of the invention only, and that further embodiments are envisaged. For example, the system may comprise only a single EDU, which is connected to either the front or the rear wheels. Additionally, in the embodiment described above, the battery pack 200 comprises contactors 230 for connecting the sub-packs 210, 220 to the voltage buses 240, 250 and for connecting the modules 212, 214 within the first battery sub-pack 210 in series. Contactors have the advantage that they are simple, robust, cheap and easy to control. However, contactors have a relatively limited number of open-and-close cycles. Additionally, contactors have a relatively slow response time and may open inadvertently in response to vibration or a mechanical impulse (such as that which may arise when the vehicle hits a pothole). As an alternative to contactors, the battery pack may comprise power devices (i.e. semiconductor switches), either alone or in combination with contactors. Although more expensive and their control is more complex, power devices have a much higher number of open-and-close cycles. Additionally, power devices are not susceptible to opening in response to vibration or a mechanical impulse, and they are more compact and lighter than contactors. In a more general sense, therefore, the battery pack may be said to comprise switches and that these switches may be contactors, power devices or a combination of the two.

In the embodiment described above, the first battery sub-pack comprises two battery modules. Conceivably, the first battery sub-pack may comprise a greater or smaller number of modules. For example, the first battery sub-pack may comprise a single 450V module, or three 300V modules. As noted above, having multiple battery modules has the advantage that modules may be arranged in parallel during discharging so that first inverter can supply a lower voltage (e.g. 450V) to the motor. The modules can then be arranged in series during charging so that they can be charged more quickly using a higher voltage charger (e.g. a 1000V DC charger). The second sub-pack may equally comprise one or more modules. Moreover, the modules of either sub-pack may be arranged in series (rather than in parallel) during discharging and in parallel (rather than in series) during charging. For example, the second battery sub-pack may comprise two modules of 450V, which are arranged in series during discharging to supply a voltage of 900V to the motor, but can then be arranged in parallel during charging so that the second battery sub-pack can be charged using a 500V DC charger. Accordingly, in a more general sense, the battery pack may be said to comprise sub-packs that may have a plurality of battery modules. The battery modules can then be arranged in one of series and parallel during discharging and in the other of series and parallel during charging.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMSA system comprising a battery pack and an electric drive unit, wherein: the battery pack comprises a first voltage bus, a second voltage bus, a first sub-pack, a second sub-pack, a plurality of switches configurable to connect the first sub-pack to the first voltage bus and to connect the second sub-pack to the second voltage bus, and a battery controller for configuring the switches; the electric drive unit comprises a motor and an inverter module, the inverter module comprising first and second inverters coupled to windings of the motor, and an inverter controller for controlling switches of the inverters; the first sub-pack supplies a first voltage to the first inverter via the first voltage bus; the second sub-pack supplies a second voltage to the second inverter via the second voltage bus; the switches of the inverters are configurable between a first configuration in which the first voltage is applied to one or more of the windings, and a second configuration in which a second voltage is applied to one or more of the windings; at least one of the sub-packs comprises a plurality of battery modules; and the battery controller configures the switches of the battery pack such that the battery modules are arranged in one of series and parallel during discharging of the battery pack and are arranged in the other of series and parallel during charging of the battery pack.
2. A system as claimed in claim 1, wherein the first sub-pack comprises a plurality of battery modules, and the battery controller configures the switches of the battery pack such that the battery modules of the first sub-pack are arranged in parallel during discharging of the battery pack and in series during charging of the battery pack.
3. A system as claimed in claim 1 or 2, wherein the second sub-pack comprises a plurality of battery modules, and the battery controller configures the switches of the battery pack such that the battery modules of the second sub-pack are arranged in series during discharging of the battery pack and in parallel during charging of the battery pack.
4. A system as claimed in any one of the preceding claims, wherein the first inverter comprises first switches, the second inverter comprises second switches, and the first switches are of a different type to the second switches.
5. A system as claimed in claim 4, wherein the first switches are silicon carbide switches, and the second switches are gallium nitride switches
6. A system as claimed in any one of the preceding claims, wherein the switches of the inverters are configurable into a configuration in which a charge current flows from the first sub-pack to the second sub-pack
7. A system as claimed in any one of the preceding claims, wherein the first sub-pack comprises battery cells having a first maximum discharge current, the second sub-pack comprises battery cells having a second maximum discharge current, and the second maximum discharge current is higher than the first maximum discharge current.
8. A system as claimed in claim 7, wherein the second voltage is higher than the first voltage.
9. A system as claimed in claim 8, wherein the second voltage is at least double the first voltage, and the second maximum discharge current is at least double the first discharge current