WO2021165654A1

WO2021165654A1 - Charging system

Info

Publication number: WO2021165654A1
Application number: PCT/GB2021/050324
Authority: WO
Inventors: Stephen Greetham
Original assignee: Dyson Technology Limited
Priority date: 2020-02-21
Filing date: 2021-02-11
Publication date: 2021-08-26

Abstract

A charging-system comprising a system and a charging station. The charging station comprises a station battery pack, and the system comprises a traction drive unit, a system battery pack and a boost converter. The system battery pack has a maximum voltage greater than a terminal voltage of the station battery pack. The station battery pack is connected to an input side of the boost converter, which steps up the terminal voltage of the station battery pack and outputs a boost voltage. The system battery pack is connected to an output side of the boost converter and is charged with the boost voltage. The traction drive unit comprises an electric motor and an inverter, and the inverter forms part of the boost converter.

Description

CHARGING SYSTEM

Field of the Invention

The present invention relates to a charging system that includes a system, such as an electric vehicle, and a charging station.

Background of the Invention

DC fast charging stations for systems, such as an electric vehicles, are typically expensive. A significant proportion of this cost resides in the power electronics required to deliver a rated power to the vehicle.

Summary of the Invention

The present invention provides a charging-system comprising a system and a charging station, wherein the charging station comprises a station battery pack, the system comprises a traction drive unit, a system battery pack and a boost converter, the system battery pack has a maximum voltage greater than a terminal voltage of the station battery pack, the station battery pack is connected to an input side of the boost converter, the boost converter steps up the terminal voltage of the station battery pack and outputs a boost voltage, the system battery pack is connected to an output side of the boost converter and is charged with the boost voltage, the traction drive unit comprises an electric motor and an inverter, and the inverter forms part of the boost converter.

With the charging system of the present invention, the charging station may comprise little more than a battery pack. The cost associated with the charging station may therefore be greatly reduced. In particular, the power electronics typically required of a DC charging station, for converting AC power into DC power, are unnecessary. The system uses the inverter of the traction drive unit as part of the boost converter. As a result, a cost effective solution is provided for boosting the terminal voltage of the station battery pack and for charging the system battery pack. The phase winding may also form part of the boost converter, thus further reducing the cost. Moreover, where the electric motor comprises a plurality of phase windings, each of the phase windings may form part of the boost converter, which then operates as, and has the advantages of, an interleaved boost converter.

The electric motor may comprise a phase winding, and the phase winding may form part of the boost converter. As a result, the cost of the boost converter is further reduced. Moreover, the electric motor may comprise a plurality of phase windings, and each of the phase windings may form part of the boost converter. This then has the benefit that the boost converter may operate as a multi-phase interleaved boost converter.

The system battery pack may comprise a plurality of modules. The modules may then be connected in parallel during charge and connected in series during discharge. By connecting the modules in parallel during charging, the system battery pack may be charged at a lower boost voltage. By connecting the modules in series during discharge, the system battery pack is able to deliver a given electrical power (e.g. to the traction drive unit) at a lower current.

The charging-station may comprise a generator, such as a solar or wind generator, for charging the station battery pack. Where the charging station is a domestic charging station, the station battery pack may be charged during the day, and the system may be charged overnight.

The present invention also provides a system comprising a traction drive unit, a battery pack, a boost converter, and charge terminals for receiving a charge voltage, wherein the battery pack has a maximum voltage greater than the charge voltage, the boost converter steps up the charge voltage and outputs a boost voltage, the battery pack is charged with the boost voltage, the traction drive unit comprises an electric motor and an inverter, and the inverter forms part of the boost converter. In another aspect the present invention provides a battery system comprising a system battery pack, a boost converter and charge terminals for receiving a charge voltage, wherein the system battery pack comprises a first sub-pack and a second sub-pack, the first sub-pack has a maximum voltage greater than the charge voltage, the second sub pack has a maximum voltage less than the charge voltage, the boost converter boosts the charge voltage and outputs a boost voltage, the first sub-pack is connected to an output side of the boost converter and is charged with the boost voltage, and the second sub-pack is connected to an input side of the boost converter and is charged with the charge voltage.

The battery system therefore includes a sub-pack, namely the first sub-pack, that has a maximum voltage (i.e. a terminal voltage when fully charged) that is greater than the charge voltage. The battery system is nevertheless able to charge the first sub-pack through the use of the boost converter. The second sub-pack is located on the input side of the boost converter and provides an energy buffer between the charging station (or other charger providing the charge voltage) and the boost converter, both of which act as current sources. This then avoids the need for any large capacitance or other storage device on the input side of the boost converter. By dividing the battery pack into at least two sub-packs and locating the sub-packs on opposite sides of a boost converter during charge, the battery system is able to charge a sub-pack having a maximum voltage greater than the charge voltage in a cost effective manner.

The second sub-pack may comprise a plurality of modules that are connected in parallel such that each module is charged with the charge voltage. Moreover, the battery system may be operable in a charge mode and a discharge mode. The modules are then connected in parallel within the second sub-pack in charge mode, and the modules are connected in series within the second sub-pack in discharge mode. This then has the advantage that, during discharge of the battery pack when the modules are connected in series, the second sub-pack is capable of outputting a voltage that is greater than the charge voltage. Nevertheless, by connecting the modules in parallel during charging, the battery system is capable of charging the second sub-pack with the charge voltage. Conceivably, the modules of the second sub-pack may be connected initially in series in charge mode so as to present a higher voltage to the charging station. This then has the advantage that a higher power may be drawn from the charging station, thereby reducing the charge time. As the terminal voltage of the second sub-pack increases and approaches the charge voltage, the modules may then be connected in parallel so that full charging of the modules can be achieved.

The battery system may be operable in a charge mode and a discharge mode. The first sub-pack is then connected to the input side of the boost converter and the second sub pack is connected to the output side of the boost converter in charge mode, and the first sub-pack and the second sub-pack are connected in series or parallel across discharge terminals in discharge mode. The two sub-packs are therefore located on opposite sides of the boost converter during charging, but are connected together, either in series or parallel, across the discharge terminals during discharge. The second sub-pack may be connected initially in parallel with the first sub-pack during discharge. However, as the battery pack discharges and the voltage drops below a threshold, the second sub-pack may be connected in series with the first sub-pack. As a result, the voltage of the battery pack increases and thus the operating voltage range of the battery pack decreases.

The first sub-pack may comprise X strings connected in series, each string comprising a plurality of cells connected in parallel. The second sub-pack may comprise N modules, and each module may comprise X/N strings connected in series. This then has the benefit that, during discharge when the first sub-pack and the second sub-pack are connected in parallel and the modules of the second sub-pack are connected in series, both sub-packs will have the same terminal voltage and thus no or very little current will flow between the two sub-packs.

The battery system may comprise a voltage sensor for sensing a magnitude of the charge voltage. When the charge voltage is greater than the maximum voltage of the first sub pack, the first sub-pack and the second sub-pack are charged with the charge voltage. For example, the first and second sub-packs may be connected in series or in parallel across the charge terminals. Conversely, when the charge voltage is less than the maximum voltage of the first sub-pack, the first sub-pack is charged with the boost voltage and the second sub-pack is charged with the charge voltage. This then has the advantage that when the charge voltage is relatively high (i.e. greater than the maximum voltage of the first sub-pack), the boost converter may be omitted and both the first sub-pack and the second sub-pack may be charged directly with the charge voltage. Whether the first sub pack and the second sub-pack are connected in series or parallel will then depend on the magnitude of the charge voltage. Conceivably, the two sub-packs may be connected initially in series so as to present a higher voltage to the charging station. As the voltage across the two sub-packs increases and approaches the charge voltage, the sub-packs may then be connected in parallel so that full charging of both sub-packs is achieved.

Both sub-packs may be charged concurrently; that is to say that both sub-packs may be charged at the same time. However, a faster charge time may be achieved by charging the sub-packs sequentially. In particular, a faster charge time may be achieved by charging the second sub-pack prior to the first sub-pack. The power drawn from the charging station is limited by the terminal voltage of the second sub-pack. By charging only the second sub-pack, the terminal voltage of the second sub-pack will rise at a faster rate. When the terminal voltage of the second sub-pack exceeds a threshold, the first sub pack may then be charged with the boost voltage. Importantly, since the second sub-pack is now at a higher terminal voltage, a higher power may be drawn from the charging station when charging the first sub-pack. As a result, an overall decrease in the charge time of the battery pack may be achieved.

The present invention also provides a product comprising an electric motor having a phase winding, an inverter coupled to the phase winding, and a battery system as described in any one of the preceding paragraphs, wherein the inverter forms part of the boost converter. By using the inverter as part of the boost converter, a cost effective solution is provided for charging the battery pack. The phase winding may also form part of the boost converter, thus further reducing the cost. Moreover, where the electric motor comprises a plurality of phase windings, each of the phase windings may form part of the boost converter, which then operates as, and has the advantages of, an interleaved boost converter.

The present invention further provides an electric vehicle comprising the battery system described in any one of the preceding paragraphs. The electric vehicle may comprise a traction drive unit having an electric motor and an inverter, and the inverter may form part of the boost converter.

There is also provided an electric vehicle comprising a switching arrangement coupled between an electric motor and a battery pack, the switching arrangement configurable between a driving mode in which the switching arrangement is controlled as an inverter to convert power from the battery pack to drive the electric motor and a charging mode in which the switching arrangement is controlled as a boost converter to convert power from an external source to charge the battery pack.

In an embodiment the switching arrangement is controlled to use a phase winding of the electric motor for the boost converter. Additionally, or alternatively, an external inductor may be coupled to the switching arrangement to form the boost converter.

In an embodiment having a three-phase motor, the switching arrangement is configured as a three-phase inverter in the drive mode and a three-phase interleaving boost converter in the charging mode. The interleaving boost converter may be arranged to operate with equal current and phase in each phase winding to avoid generating torque in the motor, however alternative arrangements may be employed.

In an embodiment, the battery pack comprises a first sub-pack having a maximum voltage greater than a charge voltage of the external source and a second sub-pack having a maximum voltage less than the charge voltage, the switching arrangement is coupled between the external source and the first sub-pack to charge this at a boost voltage in the charging mode, and the second sub-pack is coupled to the external source to charge this at the charge voltage. 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 l is a schematic diagram of a particular system, namely an electric vehicle;

Figure 2 illustrates a traction drive unit of the electric vehicle;

Figure 3 is a circuit diagram of a battery pack of the electric vehicle;

Figure 4 illustrates two different configurations of the battery pack in which (a) two sub packs are arranged in parallel, and modules within one of the sub-packs are arranged in series, and (b) the two sub-packs are arranged in series, and the modules within the sub pack are arranged in parallel;

Figure 5 illustrates circuitry within the electric vehicle that enables charging of the battery pack at two different charge voltages;

Figure 6 is a schematic diagram of a configuration within the electric vehicle in which the motor and inverter of the traction drive unit serve as a boost converter;

Figure 7 shows the time taken to charge the battery pack when (a) the sub-packs of the battery pack are charged concurrently, and (b) the sub-packs are charged sequentially; and

Figure 8 illustrates a charging system that includes a charging station and the electric vehicle.

Detailed Description of the Invention The system (electric vehicle) 1 of Figure 1 comprises a battery pack 2 and at least one traction drive unit 3 for propelling the electric vehicle 1 using power drawn from the battery pack 2. The battery pack 2 comprises a first sub-pack 11 and a second sub-pack 12. The second sub-pack 12 comprises three modules 15, 16 and 17 which, as described below, may be arranged in parallel or series.

Referring now to Figure 2, the traction drive unit 3 comprises an electric motor 5, an inverter 6, and a gearbox 7. The electric motor 5 is a three-phase motor and comprises three phase windings 9. The inverter 6 is coupled to the phase windings 9 of the motor 5 and comprises a plurality of switches SW61-SW66 and a controller (not shown) for controlling the switches. The gearbox 7 is coupled to the electric motor 5 and to a pair of drive shafts 8, and transfers the torque generated by the motor 5 to the drive shafts 8.

Figure 3 illustrates circuitry forming part of the battery pack 2. In addition to the two sub-packs 11, 12, the battery pack 1 comprises a pair of terminals 10 to which the sub packs 11,12 may be connected via a number of contactors SW1-SW12. The battery pack 2 also comprises a number of voltage sensors V1-V10 and current sensors A1-A2 for monitoring voltages and currents at various points within the circuitry of the battery pack 2. In particular, the voltage sensors may be used to determine the state-of-charge of the sub-packs 11, 12 and the modules 15-17. The battery pack 2 further includes a pre charge circuit in the form of contactor SW3 and resistor Rl.

Through suitable configuration of the contactors SW1-SW12, the two sub-packs 11, 12 may be connected in series or parallel across the terminals 10. Moreover, the modules 15-17 of the second sub-pack may be connected in series of parallel. By way of example, the two sub-packs 11,12 may be connected in parallel across the terminals 10 and the modules 15-17 may be connected in series within the second sub-pack 12 by closing contactors SW1, SW2, SW4, SW6, SW8 and SW11. This particular configuration is illustrated in Figure 4(a). In another example, the two sub-packs 11,12 may be connected in series across the terminals 10 and the modules 15-17 may be connected in parallel within the second sub-pack 12 by closing contactors closing contactors SW1, SW2, SW5, SW7, SW9, SW10 and SW12. This particular configuration is illustrated in Figure 4(b).

Each sub-pack comprises a number of strings of cells (X) which are connected in series, with each string comprising a number of cells (Y) connected in parallel. In this particular embodiment, the first sub-pack 11 comprises 216 strings of cells connected in series, and each string comprises 30 cells connected in parallel. Each module 15-17 of the second sub-pack comprises 72 strings of cells connected in series, and each string comprises 10 cells connected in parallel. Consequently, when the modules 15-17 are arranged in series within the second sub-pack 12, the second sub-pack 12 has 216 strings of cells connected in series, with each string comprising 10 cells connected in parallel. And when the modules 15-17 are arranged in parallel within the second sub-pack 12, the second sub pack 12 effectively has 72 strings of cells connected in series, with each string comprising 30 cells connected in parallel.

The ability to reconfigure the layout of the sub-packs 11,12 and modules 15-17 within the battery pack 2 has a number of advantages, as will now be described.

During discharging of the battery pack 2, the sub-packs 11,12 and modules 15-17 may initially be configured in the manner shown in Figure 4(a). If we assume a maximum voltage of 4.2V per cell then each of the two sub-packs 11,12 has a voltage of 907.2V. A voltage of 907.2V, which is relatively high for a system battery pack 2, has the advantage that a given power may be delivered to the motor 5 at lower currents, which in turn reduces power losses. As the battery pack 2 discharges and the voltage across the terminals 10 drops to a threshold or transition voltage of 680.4V (3.15V/cell), the configuration of the battery pack 2 switches to that shown in Figure 4(b). At the transition voltage, the voltage of each module 15-17 of the second sub-pack is 226.8V. Consequently, upon switching to the configuration shown in Figure 4(b), the voltage across the terminals 10 jumps from 680.4V to 907.2V. The battery pack 2 then continues to discharge until the voltage across the terminals reaches 806.4V (2.8V/cell), at which point the battery pack 2 is deemed to be fully discharged. As a result, the full operating voltage range of the battery pack 2 is 680.4V to 907.2V. By contrast, without reconfiguring the battery pack 2, the operating voltage range would be 604.8V to 907.2V. By having a narrower operating voltage range, auxiliary systems of the system that are powered by the battery pack 2 may be less complex and cheaper.

When charging the battery pack 2 with a 1000V DC charger, the sub-packs 2 and modules 15-17 are configured in the manner shown in Figure 4(a). The modules 15-17 of the second sub-pack 12 are therefore arranged in series. This then has the advantage that the second sub-pack 12 presents a higher voltage to the charger. As a result, a higher power may be drawn from the charger, which typically operates as a current source, thereby reducing the charge time.

When charging the battery pack 2 with a 500V DC charger, the sub-packs 11,12 can be reconfigured, along with the motor 5 and inverter 6, such that both sub-packs 11,12 can be charged, in spite of the fact that the charge voltage (500V) is less than the maximum voltage (907.2V) of the sub-packs 11,12.

Figure 5 illustrates part of the circuitry within the vehicle 1. The battery pack 2 is unchanged from that of Figure 3 with the exception of an intermediate terminal lOi and contactor SW13 which is used when charging with a 500V DC charger. The circuitry includes a pair of charge terminals 20 connected to the terminals 10, lOi of the battery pack 2 via a number of contactors SW20-SW22. The inverter 6 is, of course, connected to the battery terminals 10. However, the neutral point of the phase windings 9 is now connected to one of the charge terminals 20 via contactors SW21 and SW23.

When connected to a 1000V DC charger, contactors SW20 and SW22 are closed, along with contactors SW1, SW2, SW4, SW6, SW8 and SW11 of the battery pack 2. As a result, the two sub-packs 11,12 are connected in parallel across the charge terminals 20, with the modules 15-17 being connected in series within the second sub-pack 12. Again, this is the arrangement shown in Figure 4(a). When connected to a 500V DC charger, contactors SW21-SW23 are closed, along with contactors SW1, SW2, SW4, SW7, SW9, SW10, SW12 and SW13 of the battery pack 2. As a result, the second sub-pack 12 is connected directly across the charge terminals 20, with the modules 15-17 being connected in parallel within the second sub-pack 12. The first sub-pack 11, on the other hand, is connected to the charge terminals 20 via the motor windings 9 and the inverter 6. The resulting arrangement is shown in Figure 6.

As can be seen in Figure 6, the motor windings 9 and the inverter 6 collectively form a boost converter. More specifically, the windings 9 and inverter 6 collectively form a three-phase interleaved boost converter. The second sub-pack 12 is then located on the input side of the boost converter 9,6, and the first sub-pack 11 is located on the output side. The boost converter boosts the charge voltage, Vcharge, which in this case is 500V, and generates a higher boost voltage, Vboost, which in this case is 1000V, for use in charging the first sub-pack 11. The second sub-pack 12, which is located on the input side of the boost converter, provides an energy buffer between the charger and the boost converter, both of which act as current sources. This then avoids the need for any large capacitance or other storage device on the input side of the boost converter.

It is therefore possible to charge both sub-packs 11, 12 of the battery pack 2 using a 500V DC charger. Both sub-packs may be charged concurrently; that is to say that both sub packs may be charged at the same time. However, a faster charge time may be achieved by charging the sub-packs 11,12 sequentially when a full charge has been specified. In particular, a faster charge time may be achieved by charging the second sub-pack 12 prior to the first sub-pack 11. The power drawn from the charger, which typically acts as a current source, will be defined by the terminal voltage of the second sub-pack 12. By first charging only the second sub-pack 12, the terminal voltage of the second sub-pack 12 will rise at a faster rate. When the second sub-pack 12 reaches a given state of charge (i.e. when the terminal voltage of the second sub-pack reaches a threshold), the first sub pack 11 may then be charged. Since the second sub-pack 12 is now at a higher terminal voltage, a higher power may be drawn from the charger when charging the first sub-pack 11. As a result, an overall decrease in the charge time of the battery pack 2 may be achieved. This is illustrated in Figure 7, which shows the time taken to charge the battery pack 2 when (a) the sub-packs 11, 12 are charged concurrently, and (b) the sub-packs 11,12 are charged sequentially.

In the embodiment described above, the motor windings 9 and inverter 6 operate as a three-phase interleaved boost converter. This then has the advantage of reduced switching frequency and/or current ripple. Additionally, through appropriate control of the inverter 6, it is possible to avoid torque being generated by the motor 5. In spite of these advantages, one could conceivably use just a single motor winding or a number of motor windings in a non-interleaved operation, and a parking lock 29 may be provided to prevent rotation of the motor 5.

The circuitry of Figure 5 includes a boost inductor 28, which is coupled to the neutral point of the phase windings 9. This boost inductor 28 acts to increase the inductance of the windings 9 when used as a boost converter. However, it will be appreciated that, if the motor windings 9 provide sufficient inductance, the boost inductor 28 may be omitted.

In the embodiment described above, the motor windings 9 and the inverter 6 collectively form the boost converter. Conceivably, however, the vehicle may comprise a separate boost converter in order to boost the charge voltage. However, re-using the motor windings 9 and the inverter switches SW61-SW66 avoids the need for additional circuit components, thus saving cost and space. Conceivably, the boost converter may comprise the inverter 6 but not the motor windings 9. This then avoids the potential risk of the motor 5 generating torque during charging. The boost converter would then comprise a separate inductor(s) coupled to the inverter 6.

The vehicle 1 comprises a number of contactors, which are relatively simple, robust, cheap and easy to control. However, contactors have a relatively limited number of open- and-close cycles, 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 vehicle 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, are not susceptible to opening in response to vibration or a mechanical impulse, and are more compact and lighter than contactors. Accordingly, in a more general sense, the vehicle 1 may be said to comprise a number of switches, which may be contactors, power devices or a combination of the two.

Figure 8 shows a charging system 40 in which the electric vehicle 1 is connected to a charging station 30. The charging station 30 comprises a station battery pack 31 and a generator 32. The generator 32 may comprise photovoltaic cells and/or a wind turbine(s) for converting solar and/or wind energy into electrical energy for charging the station battery pack 31. Other types of generators may additionally or alternatively be used.

The station battery pack 31 has a terminal voltage that is less than the minimum voltage of the battery pack 2. For this particular application, it is not essential for the battery pack 2 to have sub-packs 11, 12 or modules that can be reconfigured. The station battery pack 31 may comprise one or more end-of-first-life battery packs that have been recovered from electric vehicles. These are battery packs that have a significantly reduced charge capacity and are therefore no longer suitable for use in a vehicle, but which are nevertheless capable of storing useful charge. The charging station 30 may be a commercial station comprising many battery packs, or a domestic station having just a single battery pack.

In a similar manner to that described above, the motor 5 and inverter 6 of the vehicle 1 are configured to implement a boost converter that boosts that charge voltage, Vcharge, of the charging station 30 to generate a higher boost voltage, Vboost, which is then used to charge the vehicle battery pack 2.

The use of a boost converter in an electric vehicle avoids the need to upgrade commercial charging stations from, for example, 500V to 1000V. Additionally, the cost associated with the charging station may be greatly reduced. In particular, the charging station may comprise a battery pack and a generator for charging the battery pack. As a consequence, the power electronics typically required of a DC charging station, for converting AC power into DC power, are avoided. Conceivably, the charging station need not have a generator. For example, the charging station may comprise a battery pack that is periodically replaced, with the depleted battery pack being transported elsewhere to be charged. The provision of a boost converter within the vehicle also enables the use of low-voltage domestic energy storage systems to charge the battery pack of the vehicle.

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

1. A charging-system comprising a system and a charging station, wherein the charging station comprises a station battery pack, the system comprises a traction drive unit, a system battery pack and a boost converter, the system battery pack has a maximum voltage greater than a terminal voltage of the station battery pack, the station battery pack is connected to an input side of the boost converter, the boost converter steps up the terminal voltage of the station battery pack and outputs a boost voltage, the system battery pack is connected to an output side of the boost converter and is charged with the boost voltage, the traction drive unit comprises an electric motor and an inverter, and the inverter forms part of the boost converter.

2. A charging-system as claimed in claim 1, wherein the electric motor comprises a phase winding, and the phase winding forms part of the boost converter.

3. A charging-system as claimed in claim 2, wherein the electric motor comprises a plurality of phase windings, and the boost converter is a multi-phase interleaved boost converter.

4. A charging-system as claimed in any one of the preceding claims, wherein the system battery pack comprises a plurality of modules, the modules are connected in parallel during charge and are connected in series during discharge.

5. A charging-system as claimed in any one of the preceding claims, wherein the charging station comprises a generator for charging the station battery pack.

6. A system comprising a traction drive unit, a battery pack, a boost converter, and charge terminals for receiving a charge voltage, wherein the battery pack has a maximum voltage greater than the charge voltage, the boost converter steps up the charge voltage and outputs a boost voltage, the battery pack is charged with the boost voltage, the traction drive unit comprises an electric motor and an inverter, and the inverter forms part of the boost converter.

7. A system as claimed in claim 6, wherein the electric motor comprises a phase winding, and the phase winding forms part of the boost converter.

8. A system as claimed in claim 7, wherein the electric motor comprises a plurality of phase windings, and the boost converter is a multi-phase interleaved boost converter.

9. A system as claimed in any one of claims 6 to 8, wherein the battery pack comprises a plurality of modules, the modules are connected in parallel during charge and are connected in series during discharge.