Field of the Invention

The present invention relates to a battery pack having sub-packs that can be configured differently during charge and/or discharge.

Background of the Invention

The operating voltage range of a battery pack can be relatively wide. By way of example, the battery pack of an electric vehicle may have an operating voltage of around 900 V when fully charged and 500 V when fully discharged. As a consequence of the wide operating voltage range, electrical systems powered by the battery pack are often complex, large, inefficient and/or expensive.

Summary of the Invention

The present invention provides a battery pack comprising a voltage bus, a first sub-pack and a second sub-pack, wherein: the second sub-pack comprises a plurality of modules; the battery pack is configurable into a first configuration in which the first sub-pack and the second sub-pack are connected in parallel across the voltage bus and the modules are connected in series within the second sub-pack, and a second configuration in which the first sub-pack and the second sub-pack are connected in series across the voltage bus and the modules are connected in parallel within the second sub-pack; and the battery pack switches between the first configuration and the second configuration when a voltage of the battery pack transits a first threshold.

During discharging, the battery pack may switch from the first configuration to the second configuration when the voltage of the battery pack falls below the first threshold. Conversely, during charging, the battery pack may switch from the second configuration to the first configuration when the voltage of the battery pack rises above the first threshold. The voltage of the battery may be any one of the voltage of the voltage bus, the voltage of one or more of the sub-packs or a voltage of one or more of the modules. By reconfiguring the battery pack during discharging, the operating voltage range of the battery pack may be reduced. In particular, when the voltage of the battery pack drops below the first threshold, the sub-packs may be reconfigured from a parallel arrangement to a series arrangement. As a result, the bus voltage is increased and thus the operating voltage range may be reduced. Moreover, by ensuring that the modules of the second sub-pack are reconfigured from series to parallel, an excessive increase in the bus voltage may be avoided, which again helps reduce the operating voltage range. By providing a battery pack with a narrower operating voltage range, electrical systems powered by the battery pack may be less complex, smaller and/or cheaper. Reconfiguration also prevents excessively high voltages which might damage electrical components.

When charging, as the voltage of the battery pack transits the first threshold, the battery pack may switch from the second configuration to the first configuration. The reconfiguration allows the battery pack to present the highest voltage possible to the charging station, which increases the power that can be drawn from the charging station, which in turn reduces the charging time. Charging stations (and indeed most battery chargers) typically operate in constant-current mode followed by constant-voltage mode. When operating in either mode, the power drawn from the charging station is the product of the current and voltage (i.e. P = IV). By ensuring that the voltage presented by the battery pack to the charging station is as high as possible, a higher power draw is achieved during constant-current mode, and thus the battery pack may be charged at a quicker rate.

The battery pack may be configurable into a third configuration in which the first sub pack and the second sub-pack are connected in series across the voltage bus and the modules are connected in series within the second sub-pack, and the battery pack switches between the second configuration and the third configuration when the voltage transits a second threshold.

This configuration may further reduce the operating voltage range. In particular, as the battery pack continues to discharge within the second configuration and the voltage of the battery pack drops below a second threshold, the modules of the second sub-pack are reconfigured from parallel to series. As a result, the bus voltage is again increased and thus the operating voltage range may be further reduced. The third configuration also has advantages for charging by presenting a higher voltage to the charging station. The second threshold may be greater than, lower than or the same as the first threshold.

The battery pack may comprise a third sub-pack connected in series with the first sub pack and the second sub-pack when the battery pack is in first, second or third configurations.

The use of a third sub-pack may further reduce the operating voltage range as it is possible to achieve a given bus voltage using first and second sub-packs of lower voltage. As a result, the increase in the bus voltage when switching from the first configuration to the second configuration may be smaller, and thus the operating voltage range may further be reduced. For example, let us say that we require a starting bus voltage of 900 V. Without the third pack, the voltage of each of the first and second sub-packs is 900 V. Let us further say that the battery pack switches to the second configuration when the bus voltage drops to 700 V. At this point, the voltage of the first sub-pack is 700 V and the voltage of the reconfigured second sub-pack (which has just two modules) is 350 V. The bus voltage jumps from 700 V to 1050 V on switching to the second configuration. However, by including a third sub-pack with a voltage of, say, 450 V, the first and second sub-packs then have voltages of 450 V, in order to achieve our starting voltage of 900 V. The battery pack again switches to the second configuration when the voltage of the voltage bus drops to 700 V. At this point, the voltage of the third sub-pack is 350 V, the voltage of the first sub-pack is 350 V and the voltage of the reconfigured second sub-pack is 175 V. The voltage of the voltage bus therefore jumps from 700 V to 875 V on switching to the second configuration. Therefore, the operating voltage range is reduced.

The battery pack may comprise a third sub-pack connected in parallel with the first sub pack and in series with the second sub-pack across the voltage bus when the battery pack is in the first configuration, and the third sub-pack is disconnected from the voltage bus when the battery pack is in the second configuration. This configuration may further reduce the operating voltage range. As noted in an earlier arrangement above, upon switching from the first configuration to the second configuration, the bus voltage increases. This increase in voltage may be excessive. So, for example, the starting voltage of the voltage bus may be 900 V, but may rise to 1000 V following reconfiguration to the second configuration. By introducing a third sub-pack which is disconnected upon switching to the second configuration, the voltage increase upon reconfiguration may be reduced. Rather than increasing to 1000 V, the voltage increase may increase to, say, 900 V. Again, for charging, we wish to present the highest possible voltage to the charging station without exceeding the maximum charge voltage of the station. By disconnecting the third sub-pack when in the second configuration, the battery pack is able to present a high voltage to the charging station without exceeding the maximum charge voltage of the station.

The battery pack may be configurable into a third configuration in which the first sub pack, the second sub-pack and the third sub-pack are connected in series across the voltage bus and the modules are connected in parallel within the second sub-pack, and the battery pack switches between the second configuration and the third configuration when the voltage transits a second threshold.

This configuration may further reduce the operating voltage range. As the battery pack continues to discharge within the second configuration and the voltage drops below the second threshold, the third sub-pack is reintroduced. As a result, the bus voltage is again increased and thus the operating voltage range may be further reduced.

The battery pack may be configurable into a fourth configuration in which the first sub pack, the second sub-pack and the third sub-pack are connected in series across the voltage bus and the modules are connected in series within the second sub-pack, and the battery pack switches between the third configuration and the fourth configuration when the bus voltage transits a third threshold. This configuration may further reduces the operating voltage range. In particular, as the battery pack continues to discharge within the second configuration and the voltage of the battery pack drops below the third threshold, the modules of the second sub-pack are reconfigured from parallel to series. As a result, the voltage of the voltage bus is again increased and thus the operating voltage range may be further reduced. The configuration also has advantages for charging by presenting a higher voltage to the charging station. The third threshold may be greater than, lower than or the same as the first threshold.

The first sub-pack may comprise X strings connected in series and each string comprises Y cells connected in parallel, the second sub-pack comprises N modules, and each module comprises X/N strings connected in series.

When fully charged, the first and second sub-packs will have the same terminal voltage when the sub-packs are in the first configuration. This is because (i) the modules are arranged in series and (ii) each module has X/N strings. Since both sub-packs have the same terminal voltage in the first configuration, no (or very little) current flows between the two packs. By contrast, if the two sub-packs had different terminal voltages, very high current would flow from the pack of higher voltage to the pack of lower voltage until such time as the two packs are balanced.

Each string of each module comprises Y/N cells may be connected in parallel. This allows all cells within the first and second sub-packs to discharge at the same rate within each configuration. This then has the advantage that aging of the cells due to discharging is balanced throughout the battery pack. In other words, whilst aging of cells may arise due to other factors (such as temperature which may vary across the battery pack), any aging due to charging and discharging may be better balanced by connecting the cells of the strings in this way. By contrast, if the cells were to discharge at different rates, then those cells discharging at the higher rate will age faster.

By selecting an appropriate threshold(s) at which to reconfigure the sub-packs and modules, full discharge for all sub-packs may be achieved at the same time. This then avoids the situation in which one sub-pack has reached full discharge, but another sub pack still has useful charge that cannot now be extracted. The choice of threshold(s) will naturally depend upon many factors, such as the number of series and parallel connected cells within each sub-pack, as well as the number of modules.

Each module may comprise the same number of series-connected strings, with each string comprising the same number of parallel-connected cells. As a result, all cells within the second sub-pack charge and discharge at the same rate within each configuration. This then has the advantage that aging of the cells, due to charging and discharging, is better balanced.

The battery pack may comprise a plurality of switches for connecting the sub-packs to the voltage bus and for connecting the modules within the second sub-pack, a voltage sensor for sensing the voltage of the battery and a control circuit for controlling the switches. The control circuit then controls the switches such that battery pack is configured into the first configuration or the second configuration, and the control circuit switches between the first configuration and the second configuration when the voltage of the voltage sensor transits the first threshold

The present invention also provides an electric vehicle comprising a battery pack as described in any one of the preceding paragraphs.

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 an electric vehicle;

Figure 2 is a circuit diagram of a battery pack of the electric vehicle; Figure 3 illustrates a sequence of different configurations of the battery pack in accordance with a first embodiment;

Figure 4 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the first embodiment vary during discharging;

Figure 5 illustrates different configurations of the battery pack in accordance with a second embodiment;

Figure 6 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the second embodiment vary during discharging;

Figure 7 illustrates different configurations of the battery pack in accordance with a third embodiment;

Figure 8 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the third embodiment vary during discharging;

Figure 9 illustrates different configurations of the battery pack in accordance with a fourth embodiment;

Figure 10 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the fourth embodiment vary during discharging;

Figure 11 illustrates different configurations of the battery pack in accordance with a fifth embodiment;

Figure 12 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the fifth embodiment vary during discharging; Figure 13 illustrates different configurations of the battery pack in accordance with a fifth embodiment;

Figure 14 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the fifth embodiment vary during charging;

Figure 15 illustrates different configurations of the battery pack in accordance with a sixth embodiment; and

Figure 16 illustrates how the bus voltage, and the state-of-charge of each of the sub-packs of the battery pack of the sixth embodiment vary during charging.

Figure 1 illustrates an electric vehicle 1 according to an embodiment and which comprises a battery pack 2 for delivering power to one or more traction drive units for propelling the electric vehicle 1, as well as delivering power to auxiliary systems of the vehicle.

Figure 2 illustrates circuitry forming part of the battery pack 2, which is used to configure the battery pack into different configurations. The battery pack 2 includes a first sub pack 11 and a second sub-pack 12. The second sub-pack comprises a plurality of modules 20-22. The battery pack 2 further comprises a voltage bus 10 having a pair of terminals 14,15, and a plurality of switches SW1-SW12, which are used to connect the sub-packs 11,12 to the voltage bus 10. 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 voltage across each of the sub-packs 11, 12 and the modules 20-22. The battery pack 2 further includes a pre-charge circuit in the form of switch SW3 and resistor Rl.

With the circuitry of the battery pack 2, the sub-packs 11,12 and the modules may be connected to the voltage bus 10 in many different configurations. In particular, the first and second sub-packs 11,12 can be connected in series or parallel across the voltage bus 10, and the modules 20-22 can be connected in series or parallel with the second sub-pack 12. In an example, the two sub-packs 11,12 may be connected in parallel, and the modules within the second sub-pack 12 may be connected in series, by closing switches SW1, SW2, SW4, SW6, SW8 and SW11. Alternatively, the two sub-packs 11,12 may be connected in series, and the modules within the second sub-pack 12 may be connected in parallel, by closing switches SW1, SW2, SW5, SW7, SW9, SW10 and SW12. As will now be described, the battery pack 2 may be reconfigured in many different ways during discharging such that the operating voltage range of the battery pack 2 is reduced. By providing a battery pack with a narrower operating voltage range, electrical systems powered by the battery pack may be less complex, smaller, efficient and/or cheaper. Additionally, the battery pack 2 may be reconfigured during charging such that charge times are reduced. Moreover, different configurations are described for charging the battery pack according to the magnitude of the charge voltage.

Each sub-pack and module comprises X strings of cells connected in series, with each string comprising Y cells connected in parallel. Where a sub-pack or module is described below or illustrated in the Figures as 100S30P, this should be understood to mean that the sub-pack or module has 100 strings connected in series, with each string having 30 cells connected in parallel. For the purposes of the following discussion, it will be assumed the each cell has a maximum voltage of 4.2 V (100% state-of-charge) and a minimum voltage of 3.0 V (0% state-of-charge).

Figure 3 shows battery pack configurations for a reconfiguration sequence according to a first embodiment. By appropriately reconfiguring a battery pack 2, the operating voltage range of the battery pack may be reduced. The parallel and series configurations of Figure 3 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

In Figure 3a, the first sub-pack 11 is connected in parallel with the second sub-pack 12, with the modules 20, 21, 22 of the second sub-pack connected in series as shown. The first sub-pack 11 has 216 strings each with 30 cells connected in parallel (216S30P). The second sub-pack 12 has three modules 20, 21, 22 each having 72 strings each having 10 cells connected in parallel (72S10P).

If the battery pack remained in the first configuration of Figure 3a, the battery pack 2 would have a maximum or starting voltage of 907.2 V (4.2 V/cell) and a minimum or termination voltage of 648.0 (3.0 V/cell). The size or width (AV) of the operating voltage range would then be 259.2 V (i.e. 907.2 - 648.0). However, the operating voltage may be reduced by switching from the first configuration of Figure 3a to the second configuration of Figure 3b when the voltage of the battery pack falls below or transitions a threshold.

In Figure 3b, the first sub-pack 11 is connected in series with the second sub-pack 12. The modules 20, 21, 22 of the second sub-pack 12 are connected in parallel within the second sub-pack as shown. When the bus voltage falls below a threshold of 680.4 V (3.15 V/cell for both sub-packs), the battery pack switches from the first configuration of Figure 3a to the second configuration of Figure 3b. At this point, the voltage of the first sub-pack is 680.4 V and the voltage of each module is 226.8 V. Accordingly, upon switching to the second configuration, the bus voltage jumps to 907.2 V (i.e. 680.4 V + 226.8 V). The battery pack then continues to discharge in the second configuration until a termination voltage of 864.0 V (3.0 V per cell for both sub-packs) is reached.

By switching from the first configuration to the second configuration at the threshold voltage of 680.4 V, the width (AV) of the operating voltage range of the battery pack is 226.8 V, a reduction of 32.4 V compared to the first configuration alone. Moreover, by reconfiguring the modules 20, 21, 22 from series to parallel, an excessive increase in the bus voltage upon switching to the second configuration is avoided, which helps achieve a narrow voltage range.

Figure 4 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub-packs for the reconfiguration sequence of Figure 3. The first sub-pack 11 comprises 216 series-connected strings, each string having 30 parallel-connected cells. The second sub-pack 12 has three modules, and each module has 72 series-connected strings and 10 parallel-connected cells. Each module therefore has a third of the strings of the first sub-pack, and each string has a third of the cells. As a consequence of this arrangement, both sub-packs 11, 12 discharge at the same rate in each configuration. This is evident in Figure 4, in which the two dotted lines overlie one another. This then has the advantage that aging of the cells due to discharging is balanced throughout the battery pack. Whilst aging of cells may arise due to other factors (such as temperature), any aging due to discharging is better balanced. By contrast, if the cells were to discharge at different rates, then those cells discharging at the higher rate will age faster. Of course, the first sub-pack may have a different number of cells, and the second sub-pack may have a different number of modules from that described above. Accordingly, in a more general sense, the first sub-pack may be said to have X series- connected strings, with each string having Y parallel-connected cells. The second sub pack may have N modules. In order for both sub-packs to discharge at the same rate, each module would then have X/N series-connected strings, with each string having Y/N parallel connected cells.

Charging is essentially the reverse of discharging. Consequently, when charging from fully discharged, the battery pack begins in the second configuration of Figure 3b, in order to present the highest possible voltage to the charging station. The termination voltage of the configuration of Figure 3b is 864V (3.0 V per cell) compared with the lower termination voltage of the configuration of Figure 3a which is 648 V. As the bus voltage transitions or rises above a threshold voltage, the battery pack 2 is reconfigured from the second configuration of Figure 3b to the first configuration of Figure 3a. The threshold or transition voltage used may be the same as that used for the discharging process. In this case the voltages described above for the discharging process correspond to those of the charging process but in reverse and the voltage characteristic shown in Figure 4 is reversed. However, a different threshold voltage may be used for charging compared with that used for discharging. Figure 5 shows battery pack configurations for a reconfiguration sequence according to a second embodiment. By appropriately switching between these three configurations, the operating voltage range of a battery pack 2 may be further reduced. The battery pack configurations of Figure 5 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

In Figure 5a, a first sub-pack 11 is connected in parallel with a second sub-pack 12, with the modules 20, 21 of the second sub-pack connected in series as shown. A third sub pack 13 is connected in series with the first and second sub-packs 11, 12. In this example, the first sub-pack 11 has 58 strings of 30 parallel connected cells (58S30P). The second sub-pack 12 has two modules 20, 21, each having 29 strings having 20 cells connected in parallel (29S20P). The third sub-pack 13 has 158 strings of 40 parallel connected cells (158S40P).

In Figure 5b, the first sub-pack 11 and the second sub-pack 12 are connected in series, with the modules 20 and 21 of the second sub-pack being connected in parallel. The third sub-pack 13 is also connected in series with the first and second sub-packs 11, 12. The battery pack switches from the first configuration of Figure 5a to the second configuration of Figure 5b when the bus falls below a threshold during discharging. This reconfiguration is similar to that of Figure 3a and 3b, but the addition of the third sub pack 13 in series allows for the use of lower voltages for the first and second sub-packs 11, 12 to achieve the same starting bus voltage across the terminals. As a result, the increase in the bus voltage during reconfiguration can be lower. This may enable further reducing the operating voltage range compared with the embodiment of Figures 3a and 3b. Conceivably, the reconfiguration scheme may comprise just the first configuration of Figure 5a and the second configuration of Figure 5b. However, a further reduction in the width of the operating voltage range may be achieved by including the third configuration of Figure 5c. Figure 5c shows a third configuration in which the modules 20, 21 of the second sub-pack 12 are connected in series, with the second sub-pack being connected in series with the first and third sub-packs 11, 13. When battery pack is in the second configuration of Figure 5b and the bus voltage falls below a second threshold or transition voltage, the battery pack 2 is reconfigured to the third configuration shown in Figure 5c.

When fully charged and in the first configuration of Figure 5a, the battery pack has a maximum of starting voltage of 907.2 V. As the battery pack discharges and the bus voltage falls below a first threshold or transition voltage of 789.0 V, the battery pack switches from the first configuration of Figure 5a to the second configuration of Figure 5b. At this point, the voltage of the first sub-pack 12 is 216.5 V (3.73 V/cell), the voltage of each module 20, 21 of the second sub-pack 12 is 108.25 V (3.73 V/cell), and the voltage of the third sub-pack 13 is 572.5 V (3.62 V/cell). Consequently, when the battery pack switches to the second configuration, the bus voltage jumps from 789.0 V to 897.25 V. As the battery pack continues to discharge and the bus voltage falls below a second threshold or transition voltage of 815.9 V, the battery pack switches from the second configuration of Figure 5b to the third configuration of Figure 5c. At this point, the voltage of the first sub-pack 12 is 419.5 V (3.35 V/cell), the voltage of each module 20, 21 of the second sub-pack 12 is 100.7 V (3.47 V/cell), and the voltage of the third sub pack 13 is 520.7 V (3.30 V/cell). Consequently, when the battery pack switches to the third configuration, the bus voltage jumps from 815.9 V to 916.6 V. The battery pack then continues to discharge in the third configuration until a termination voltage of 822.0 V (3.0 V per cell for all sub-packs) is reached.

The operating voltage range of the battery pack is therefore 789.0 V to 916.6 V, and the width (AV) of the operating range is 127.6 V (916.6 - 789.0). This represents a significant reduction over the width of the operating voltage range when no reconfiguration is employed, which is 259.2 V (907.2 V - 648.0 V).

Figure 6 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub-packs for the reconfiguration sequence of Figure 5. As illustrated in Figure 6, the reconfiguration sequence switches from the first configuration of Figure 5a to the second configuration of Figure 5b when the voltage of the voltage bus falls below a first threshold and then switches from the second configuration of 5b to the third configuration of Figure 5c when the voltage of the voltage bus falls below a second threshold. However other switching strategies are possible including only switching from the first configuration of Figure 5a to the second configuration of Figure 5b when a threshold voltage at the voltage bus is transitioned, without involving the third configuration of Figure 5c. Similarly, in another switching strategy, the battery pack may switch from the second configuration of Figure 5b to the third configuration of Figure 5c when another threshold voltage at the voltage bus 10b is transitioned, without involving the first configuration of Figure 5a.

The various configurations of Figure 5 may be switched in reverse in order to achieve more efficient and faster charging. The configuration of Figure 5c will present the highest voltage to a charging source and therefore will provide the maximum power transfer rate when the battery pack is fully discharged. Once a first charging threshold or transition voltage is reached, for example the maximum available voltage of the charging station, the battery pack may be switched to the configuration of Figure 5b to allow further charging as the voltage across the battery pack will have fallen due to the modules 20, 21 being switched from series to parallel connected. Then once a further or second threshold or charging transition voltage is reached, again for example the maximum voltage of the charging station, the battery pack may be switched to the configuration of Figure 5a to allow further charging as the voltage across the battery pack will have fallen due to the first and second sub-packs 11 and 12 being switched from series to parallel connected. Alternatively, rather than use the maximum voltage of the charging station as the transition voltage, the first and second threshold or discharge transition voltages used during discharging of the battery pack may be employed in reverse in order to reduce the operating voltage range of the electrical components of the electric vehicle. Figure 7 shows battery pack configurations for a reconfiguration sequence according to third embodiment. Again, the battery pack configurations of Figure 7 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

In Figure 7a, a first sub-pack 11 is connected in parallel with a second sub-pack 12, with the modules 20, 21 of the second sub-pack also connected in parallel as shown. A third sub-pack 13 is connected in series with the first and second sub-packs 11, 12. In this example, the first sub-pack 11 has 33 strings of 21 cells in parallel (33S21P). The second sub-pack 12 has two modules each with 33 strings of 13 cells in parallel (33S13P). The third sub-pack 13 has 183 strings of 35 cells in parallel (183S35P).

In Figure 7b, the first sub-pack 11 and the second sub-pack 12 are connected in series, with the modules 20 and 21 of the second sub-pack again connected in parallel. The third sub-pack 13 is also connected in series with the first and second sub-packs 11, 12. The reconfiguration from Figure 7a to Figure 7b is similar to that of Figure 5a to Figure 5b, but the third sub-pack 13 is comparatively larger than the first and second sub-packs, and the modules 21, 22 remain parallel connected. This further reduces the jump in the bus voltage when the threshold voltage is passed or transited, thereby enabling a further reduction in operating voltage range. Conceivably, the reconfiguration scheme may comprise just the first configuration of Figure 7a and the second configuration of Figure 7b. However, a further reduction in the width of the operating voltage range may be achieved by including the third configuration of Figure 5c.

Figure 7c shows a third configuration in which the modules 20, 21 of the second sub-pack 12 are connected in series, with the second sub-pack 12 being connected in series with the first and third sub-packs 11, 13. When the bus voltage falls below a second discharge transition voltage or threshold in the second configuration of Figure 7b, the battery pack 2 is switched to the third configuration of Figure 7c. As the battery pack discharges in the first discharging configuration of Figure 7a, the bus voltage falls below a first discharging transition or threshold voltage of 807.8 V. This corresponds to 130.0 V across the first sub-pack 11 (3.94 V/cell), 130.0 V across the second sub-pack 12 (3.94 V/cell, with 130.0 V across each module 20, 21) and 677.8 V across the third sub-pack 13 (3.70 V/cell). At this point the battery pack is reconfigured to a second configuration shown in Figure 7b.

When fully charged and in the first configuration of Figure 7a, the battery pack has a maximum of starting voltage of 907.2 V. As the battery pack discharges and the bus voltage falls below a first threshold or transition voltage of 807.8 V, the battery pack switches from the first configuration of Figure 7a to the second configuration of Figure 7b. At this point, the voltage of the first sub-pack 12 is 130.0 V (3.94 V/cell), the voltage of each module 20, 21 of the second sub-pack 12 is 130.0 V (3.94 V/cell), and the voltage of the third sub-pack 13 is 677.8 V (3.70 V/cell). Consequently, when the battery pack switches to the second configuration, the bus voltage jumps from 807.8 V to 937.8 V. As the battery pack continues to discharge and the bus voltage falls below a second threshold or transition voltage of 825.9 V, the battery pack switches from the second configuration of Figure 7b to the third configuration of Figure 7c. At this point, the voltage of the first sub-pack 12 is 111.1 V (3.37 V/cell), the voltage of each module 20, 21 of the second sub-pack 12 is 116.2 V (3.52 V/cell), and the voltage of the third sub-pack 13 is 598.6 V (3.27 V/cell). Consequently, when the battery pack switches to the third configuration, the bus voltage jumps from 825.9 V to 942.1 V. The battery pack then continues to discharge in the third configuration until a termination voltage of 846.0 V (3.0 V per cell for all sub-packs) is reached.

The operating voltage range of the battery pack is therefore 807.8 V to 942.1 V, and the width (AV) of the operating range is 134.3 V (942.1 - 807.8). This again represents a significant reduction over the width 259.2 V when no reconfiguration is employed.

Figure 8 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub-packs for the reconfiguration sequence of Figure 7. As illustrated in Figure 8, the reconfiguration sequence switches from the first configuration of Figure 7a to the second configuration of Figure 7b when the voltage of the voltage bus falls below a first threshold and then switches from the second configuration of 7b to the third configuration of Figure 7c when the voltage of the voltage bus falls below a second threshold. The dashed lines show the SoC of the sub-packs as the battery pack discharges and is reconfigured. Other switching strategies are possible including only switching from the first configuration of Figure 7a to the second configuration of Figure 7b when a threshold voltage at the voltage bus is transitioned, without involving the third configuration of Figure 7c. Similarly, in another switching strategy, the battery pack may switch from the second configuration of Figure 7b to the third configuration of Figure 7c when another threshold voltage at the voltage bus 10b is transitioned, without involving the first configuration of Figure 7a.

The various configurations of Figures 7 may be switched in reverse in order to achieve more efficient and faster charging. The configuration of Figure 7c will present the highest voltage to a charging source and therefore will provide the maximum power transfer rate. Once a first threshold or charging transition voltage is reached, for example the maximum available voltage of the charging station, the battery pack may be switched to the configuration of Figure 7b to allow further charging as the voltage across the battery pack will have fallen due to the modules 20, 21 being switched from series to parallel connected. Then once a further threshold or charging transition voltage is reached, again for example the maximum voltage of the charging station, the battery pack may be switched to the configuration of Figure 7a to allow further charging as the voltage across the battery pack will have fallen due to the first and second sub-packs 11 and 12 being switched from series to parallel connected. Alternatively, rather than use the maximum voltage of the charging station as the threshold or transition voltage, the first and second threshold voltages used during the discharge switching strategy may be employed in reverse in order to reduce the operating voltage range of the electrical components of the electric vehicle. Figure 9 shows battery pack configurations for a reconfiguration sequence according to fourth embodiment. The battery pack configurations of Figure 9 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

This fourth embodiment differs from the second and third embodiments in that the third sub-pack is not always connected. By using a third sub-pack which is disconnected when transitioning from one configuration to the next, the jump in bus voltage can be reduced. For charging, we wish to present the highest possible voltage to the charging station without exceeding the maximum charging voltage of the station. By disconnecting the third sub-pack, it is possible to present a high voltage to the charging station that does not exceed the maximum charge voltage.

In Figure 9a, a first sub-pack 11 is connected in parallel with a second sub-pack 12 and a third sub-pack 13 which are connected in series as shown. Modules 20, 21 of the second sub-pack 12 are also connected in series. A third sub-pack 13 is connected in parallel with the first and second sub-packs 11, 12. In this example, the first sub-pack 11 has 216 strings of 135 cells in parallel (216S135P). The second sub-pack 12 has two modules each with 35 strings of 18 cells in parallel (35S18P). The third sub-pack 13 has 146 strings of 5 cells in parallel (146S5P) and a starting voltage of 546.0 V.

In Figure 9b, the first sub-pack 11 and the second sub-pack 12 are connected in series, with the modules 20 and 21 of the second sub-pack again connected in parallel. The third sub-pack 13 is now disconnected from the voltage bus 10.

In Figure 9c, the first sub-pack 11 and the second sub-pack 12 are again connected in series, but the modules 20 and 21 of the second sub-pack are now connected in series rather than parallel. The third sub-pack 13 is still disconnected from the voltage bus 10.

When fully charged and in the first configuration of Figure 9a, the battery pack has a starting voltage of 907.2 V. As the battery pack discharges in the first configuration of Figure 9a, the bus voltage falls below a first transition or threshold voltage of 707.9 V. This corresponds to 707.9 V across the first sub-pack 11 (3.28 V/cell), 270.5 V across the second sub-pack 12 (3.86 V/cell, with 135.2 V across each module 20, 21) and 437.48 V across the third sub-pack 13 (3.00 V/cell). This means that the third sub-pack 13 is fully discharged. At this point the battery pack is reconfigured to a second configuration shown in Figure 9b, in which the third sub-pack 13 is disconnected from the battery pack.

Upon reconfiguration to Figure 9b, the voltage at the voltage bus 10 jumps from 707.9 V to 843.1 V. The battery pack 2 continues to discharge in the second configuration of Figure 9b until a second threshold or transition voltage of 777.3 V is reached, whereupon the battery pack is further reconfigured to the third configuration of Figure 9c. At the point, the voltage across the first sub-pack 11 is 658.3 V (3.05 V/cell), and the voltage across each of the modules of the second sub-pack 12 is 119.0 V (3.40 V/cell). Again, the third sub-pack is disconnected.

Upon reconfiguration to Figure 9c, the modules 20, 21 of sub-pack 12 are connected in series instead of in parallel, with the first sub-pack 11 still connected to the second sub pack in series and the third sub-pack still disconnected. The bus voltage therefore jumps from 777.3 V to 896.2 V. The battery pack continues to discharge to a termination voltage of 858.0 V (3.0 V/cell for all sub-packs).

The operating voltage range of the battery pack is therefore 707.9 V to 907.2 V, and the width (AV) of the operating range is 199.2 V (907.2 - 707.9). Whilst the width of the operating voltage range is greater than that for the reconfiguration sequences of Figures 5 and 7, it nevertheless represents a sizeable reduction over the width of 259.2 V when no reconfiguration is employed.

As described, the sub-packs of this embodiment may be disconnected as the battery pack discharges in order to reduce operating voltage. The embodiment can also be used to ensure that all cells are fully discharged at the end of the discharge cycle, even though this may occur at different times for different sub-packs. Figure 10 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub packs for the reconfiguration sequence of Figure 9.

The various configurations of Figure 9 may be switched in reverse to charge the sub packs, however a different order of reconfiguring may be used and/or different reconfigurations. For example, a configuration having all sub-packs and modules connected in series (not shown) will present the highest voltage to a charging source and therefore may provide the maximum power transfer rate. Once a threshold or charging transition voltage is reached, for example the maximum available voltage of the charging station, one of the sub-packs 13 may be disconnected to allow further charging as the voltage across the battery pack will have fallen, for example the configuration of Figure 9c. Then once a further threshold or charging transition voltage is reached, again for example the maximum voltage of the charging station, the battery pack may be switched to the configuration of Figure 9b to allow further charging as the voltage across the battery pack will have fallen due to the first and second modules 20, 21 being switched from series to parallel connected. A further threshold or charging transition voltage may result in reconfiguration to Figure 9a.

Figure 11 shows battery pack configurations for a reconfiguration sequence according to fifth embodiment. Again, the battery pack configurations of Figure 11 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

This fifth embodiment differs from the earlier embodiments in that additional sub-packs are switched in as the battery pack discharges.

In Figure 11a, only a first sub-pack 11 is connected to the voltage bus. The first sub-pack has first and second modules 18, 19 connected in series. A second sub-pack 12 and a third sub-pack 13 are initially disconnected. In this example, the two modules 18, 19 of the first sub-pack 11 each have 108 strings of 34 cells in parallel (108S34P). The second sub-pack 12 has 15 strings of 26 cells in parallel (15S26P), and the third sub-pack 13 has 15 strings of 15 cells in parallel (15S15P).

In Figure l ib, the first sub-pack 11 and the second sub-pack 12 are connected in series, with the modules 18 and 19 of the first sub-pack again connected in series. The third sub pack continues to be disconnected from the voltage bus 10.

In Figure 11c, the first sub-pack 11, the second sub-pack 12 and the third sub-pack are connected in series, with the modules 20 and 21 of the frist sub-pack again connected in series. The third sub-pack continues to be disconnected from the voltage bus.

When fully charged and in the first configuration of Figure 11a, the battery pack has a maximum of starting voltage of 907.2 V. As the battery pack 2 discharges in the first discharging configuration of Figure 11a, the bus voltage falls below a first transition or threshold voltage of 850.0 V (3.94/cell for the first sub-pack). At this point the battery pack is reconfigured to a second configuration shown in Figure l ib. Upon reconfiguration to Figure l ib, the bus voltage jumps to 913.0 V, i.e. 850.0 V across the first sub-pack 11 plus 63.0 V across the second sub-pack 12. The battery pack 2 continues to discharge in the second configuration of Figure 1 lb until a second transition or threshold voltage of 851.7 V is reached, whereupon the battery pack is further reconfigured to the third configuration of Figure 1 lc. At this point, the voltage of the first sub-pack 11 is 794.9 V (3.68 V/cell), and voltage of the second sub-pack is 56.8 V (3.79 V/cell). Upon reconfiguration to Figure 11c, the bus voltage jumps to 914.7 V, i.e. 794.9 V across the first sub-pack 11, 56.8 V across the second sub-pack, and 63.0 V across the third sub-pack. The battery pack 2 continues to discharge in the third configuration of Figure 11c until the termination voltage of 738.0 V (3.0V/cell for all sub-packs).

The operating voltage range of the battery pack is therefore 738.0 V to 914.7 V, and the width (AV) of the operating range is 176.7 V (914.7 - 738.0). This again represents a significant reduction over the width of 259.2 V when no reconfiguration is employed. Figure 12 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub packs for the reconfiguration sequence of Figure 11.

The various configurations of Figure 11 may be switched in reverse to charge the sub packs, however a different order of reconfiguring may be used and/or different reconfigurations. The configuration of Figure 11c will present the highest voltage to a charging source and therefore will provide the maximum power transfer rate. Once a threshold or transition voltage is reached, for example the maximum available voltage of the charging station, the battery pack may be switched to the configuration of Figure 1 lb to allow further charging as the voltage across the battery pack will have fallen due to the third sub-pack 13 being disconnected. Then once a further transition voltage is reached, again for example the maximum voltage of the charging station, the battery pack may be switched to the configuration of Figure 1 la to allow further charging as the voltage across the battery pack will have fallen due to the second sub-pack 12 being disconnected. Alternatively, rather than use the maximum voltage of the charging station as the transition voltage, the first and second threshold voltages used during the discharge switching strategy may be employed in reverse in order to reduce the operating voltage range of the electrical components of the electric vehicle. A further alternative reconfiguration sequence for charging the battery pack of Figure 11 is illustrated in Figures 13 and Figure 15.

Figure 13 shows battery pack configurations for a reconfiguration sequence according to a sixth embodiment. These reconfigurations may be used to efficiently charge battery pack of Figure 11 using a high voltage (i.e. 1000 V) charger. Again, the configurations of Figure 13 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

This sixth embodiment is largely the reverse sequence of the reconfiguration sequence of the fifth embodiment shown in Figure 11, but with the additional charging configuration of Figure 13a. The configuration of Figure 13b corresponds with that of Figure 11c, Figure 13c with that of Figure 1 lb, and Figure 13d with that of Figure 11a.

As explained previously, when charging we want to draw the maximum available power from the charging station in order to minimise the charge time. The power drawn is both a function of the voltage and the current. All else being equal, we would therefore want to configure the battery pack such that it presents the highest possible terminal voltage to the charging station (without, of course, exceeding the maximum charge voltage of the station). However, there is also another factor to consider, namely the current drawn by the battery pack. There is a limit in the magnitude of current that each cell can draw, which is where the employment of Figure 13a provides an advantage. What that means in practice is that the maximum current that the battery pack can draw is limited by the sub-pack having the smallest number of parallel cells, or more accurately the sub-pack having a series string with the fewest number of parallel cells.

With the battery pack fully discharged, the configuration of Figure 13b presents the highest voltage to the charging station. However, the third sub-pack 13 has 15 series strings with just 15 parallel cells, the lowest number of parallel cells in the battery pack. This is the limiting sub-pack in terms of the magnitude of current, and thus power, that can be drawn from the charging station. By starting instead with the configuration of Figure 13a, with the second and third sub-packs 12 and 13 connected in parallel, there are now 41 cells in parallel which means that the first sub-pack 11 now has the lowest number of parallel cells at 34 which is more than double the lowest number of parallel cells in the configuration of Figure 13b. Therefore, by initially employing the configuration of Figure 13a, the magnitude of current that can be drawn is more than doubled. Whilst the voltage presented by the battery pack configured according to Figure 13a (693 V at 3.0 V/cell) is lower than that of Figure 13b (738 V at 3.0 V/cell), this may be more than offset by the increase in current. As a result, a higher power may be drawn from the charging station.

Whilst the configuration of Figure 13a does initially provide for drawing more power for charging, if charging remained in this configuration, the first sub-pack 11 would reach full charge before the second and third sub-packs 12 and 13. This is because each series string within sub-pack 11 has 34 parallel cells, whereas each series string within the combination of sub-packs 12 and 13 has 41 parallel cells. However, we cannot then simply disconnect the first sub-pack 11 as sub-packs 12 and 13 will not present a high enough terminal voltage for the charging station. A 1000 V charging station will typically present a charge voltage of between 200 V and 1000 V, and the terminal voltage of the battery pack needs to be somewhere in this range, otherwise the charging station won’t deliver any current. If we arrange sub-packs 12 and 13 in series, we have 30 series connected strings (i.e. 15S +15S). Therefore, the maximum possible voltage that these two sub-packs can present is 30 * 4.2 V = 126 V (and that is when the cells are fully charged).

Switching to the configuration of Figure 13b overcomes this problem by presenting the battery pack 2 with a voltage within the required charging station range and therefore allowing charging to continue. The length of time spent charging in the configuration of Figure 13a before reconfiguring to the configuration of Figure 13b is determined by the following. At the end of charging in the configuration of Figure 13b, we want the third sub-pack 13 to be fully charged but we do not want the terminal voltage of the battery pack to exceed the maximum charge voltage of the charging station. Working backwards from that enables the length of time that is spent charging using the configuration of Figure 13ato be determined. As can be seen in Figure 14, this is a relatively short duration compared with the other charging configurations used, and will ultimately depend on the number of series and parallel cells for each sub-pack.

Having noted that charging using the configuration of Figure 13a provides some advantages, it is nevertheless not essential, and charging could start with the configuration of Figure 13b. Moreover, a battery pack having sub-packs with different numbers of parallel and series cell arrangements may not present the problems noted above and therefore the configuration of Figure 13a may not provide the described advantages in some of these cases. Similarly, the configuration of Figure 13b may not be required for some battery pack arrangements. For example, charging could start using the configuration of Figure 13a until such time as sub-packs 12 and 13 are fully charged, and then jump to a configuration other than that of Figure 13b, for example to that of Figure 13d in which case the configurations of both Figure 13b and 13c are omitted.

By using second and third sub-packs 12 and 13 with a relatively low terminal voltage, when each sub-pack is introduced during discharge (see Figures 11 and 12), the rise in the terminal voltage of the battery pack is not excessive (i.e. the jumps in bus voltage in Figure 12 are not excessive) and thus a relatively narrow operating voltage range can be achieved. The difficulty or challenge in having sub-packs of low terminal voltage is that they cannot be charged on their own because the terminal voltage may be less than the minimum charge voltage of the station (e.g. 200 V). Instead, we need to charge the sub packs along with another sub-pack of higher voltage which benefits from an intelligent charging scheme such as that shown in Figure 13.

Charging of the battery pack may proceed as follows. When fully discharged, the battery pack starts in the configuration of Figure 13a. In Figure 13a, the first sub-pack 11 is connected in series with the second and third sub-packs 12, 13 which are themselves connected in parallel with each other. The modules 18, 19 of sub-pack 11 are connected in series as shown. As a result, starting voltage of the battery pack is 693 V (i.e. [108+108+15] * 3.0 V). As the battery pack 2 charges in the first configuration of Figure 13a, the bus voltage rises above a first transition or threshold voltage of 733.0 V. This corresponds to 685.6 V across the first sub-pack (3.17 V/cell) and 47.4 V across each of the second and third sub-packs 12, 13 (3.16 V/cell). At this point the battery pack is reconfigured to a second configuration shown in Figure 13b, in which the second and third sub-packs 12, 13 are now connected in series with each other and with the first sub pack 11, the modules 18, 19 of which are still connected in series. Upon reconfiguration to Figure 13b, the voltage at the voltage bus 10 jumps to 780.4 V, and the bus voltage continues to rise as the battery pack charges in the configuration of Figure 13b. The battery pack 2 continues to charge in the second configuration of Figure 13b until a second transition or threshold voltage of 920.0 V is reached, whereupon the battery pack is further reconfigured to the third configuration of Figure 13c. At this stage, the voltage across the first sub-pack 11 is 799.4 V (3.70 V/cell), the voltage across the second sub pack 12 is 56.9 V (3.79 V/cell), and the voltage across the third sub-pack 13 is 62.0 V (4.13 V/cell). The third sub-pack is essentially fully charged and is disconnected from the voltage bus 10.

Upon reconfiguration to Figure 13c, the voltage at the voltage bus 10b falls to 856.3 V, following disconnection of the third sub-pack 13. The battery pack 2 then continues to discharge in the third configuration of Figure 13c until a third threshold or transition voltage of 913.0 V is reached, which corresponds to a voltage of 850.6 V across the first sub-pack 11 (3.94 V/cell) and 62.5 V across the second sub-pack 12 (4.20 V/cell). The second sub-pack is therefore fully charged.

When the bus voltage reaches the third transition or threshold voltage, the battery pack is reconfigured to the configuration of Figure 13d in which the second sub-pack 12 is also disconnected leaving only the first sub-pack 11 to continue charging. The modules 18, 19 remain series connected, and the bus voltage falls to 850.6 V. The battery pack 2 then continues to charge until a terminal voltage of 907.2 V is reached (4.20 V/cell), at which point the first sub-pack is fully charged.

Figure 14 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub packs for the reconfiguration sequence of Figure 13.

Figure 15 shows battery pack configurations for a reconfiguration sequence according to a seventh embodiment. These reconfigurations may be used to efficiently charge battery pack of Figure 11 using a low voltage (i.e. 500 V) charger. Again, the configurations of Figure 15 may be implemented using the reconfiguration circuitry of Figure 2 as described above, although other implementations are possible and may alternatively be used.

In contrast to the embodiment of Figure 13, this seventh embodiment employs parallel connection of the modules 18, 19 of the first sub-pack to reduce the voltage presented to the charging station. As a result, the battery pack may be charged with a low voltage (500 V) charging station.

When fully discharged, the second and third sub-packs 12, 13 are not capable of presenting a voltage that exceeds the minimum charge voltage of the charging station. This is true even when the two sub-packs 12,13 are connected in series, which in this instance would be 90 V (i.e. [15+15] * 3.0 V). It is therefore necessary to charge the second and third sub-packs in combination with the first sub-pack. When fully charged, connecting the first sub-pack 11 in series with one or both of the second and third sub-packs will result in a bus voltage that exceeds the maximum charge voltage of the station (i.e. 500 V). It is therefore also necessary to complete the charging of the second and third sub-packs 12,13 before completing the charging of the first sub-pack 1.

Since the strings of the second and third sub-packs 12,13 have a relatively low number of parallel-connected cells, we ideally want to charge the second and third sub-packs 12,13 in parallel. This then has the benefit that a higher current, and thus a higher power, can be drawn from the charging station. However, in order to connect the two sub-packs in parallel, it is necessary for the sub-packs 12,13 to be at the same voltage. If the two sub packs are at different voltages, an excessively high current will flow between the sub packs when they are connected in parallel. When the battery pack is fully charged and fully discharged, the two sub-packs 12,13 will have the same terminal voltages of 63 V (100% SoC) and 45 V (0% SoC). However, owing to the sequence in which the battery pack discharges (Figure 11), the voltages of the two sub-packs will be different at all other states of partial discharge. This can be seen in Figure 12, in which the SoC of the two sub-packs are the same only at full charge and full discharge. Accordingly, with the exception of full discharge, it will be necessary to initially charge the second sub-pack before connecting the two sub-packs in parallel. The second sub-pack is initially charged because, with the exception of full charge and full discharge, the voltage of the second sub-pack is always less than that of the third sub-pack during discharge. With all of the considerations and constraints outlined above, we arrive at the sequence illustrated Figure 15. For the moment, let us assume that the battery pack is partially discharged rather than fully discharged. Moreover, let us say that the voltage of the first sub-pack is 366.0 V (3.39 V/cell), the voltage of the second sub-pack is 52.1 V (3.47 V/cell), and the voltage of the third sub-pack is 55.2 V (3.68 V/cell).

The battery pack 2 is initially configured according to Figure 15a with only the first sub pack 11 connected to the voltage bus, and with the modules 18, 19 connected in parallel. This presents a starting voltage to the charging station of 366.0 V. Conceivably, the battery pack could start in the configuration of Figure 15b. This would then have the benefit of presenting a higher voltage of 418.1 V to the charging station. However, the second sub-pack has strings of only 26 parallel cells. By contrast, the first sub-pack (with the two modules arranged in parallel) has strings of 68 parallel cells. So a much higher current and thus power may be drawn from the charging station by starting in the configuration of Figure 15a. When the bus voltage reaches a first threshold or transition voltage of 396.1 V, the battery pack switches from the configuration of Figure 15a to the configuration of Figure 15b.

In the configuration of Figure 15b, the first sub-pack 11 and the second sub-pack 12 are connected in series to the voltage bus, with the modules 18, 19 of the first sub-pack being connected in parallel. Upon switching to the configuration of Figure 15b, the bus voltage increases to 448.2 V. When the bus voltage reaches a second threshold or transition voltage of 458.0 V, the battery pack switches from the configuration of Figure 15b to the configuration of Figure 15c. At this point, the voltage of the first sub-pack is 402.8 V, and the voltage of the second sub-pack is 55.2 V. The second sub-pack is therefore at the same voltage as that of the third sub-pack, and thus the transition to the configuration of Figure 15c can be safely made.

In the configuration of Figure 15c, the first sub-pack 11 is connected in series with a group comprising the second sub-pack 12 and the third sub-pack 13, which are connected in parallel. Moreover, the modules 18, 19 of the first sub-pack continue to be connected in parallel. Upon switching to the configuration of Figure 15c, there is no change in the bus voltage, which is 458.0 V. When the bus voltage subsequently reaches a third threshold or transition voltage of 495.0 V, the battery pack switches from the configuration of Figure 15c to the configuration of Figure 15d. At this point, the voltage of the first sub-pack is 432.0 V, the voltage of the second sub-pack is 63.0 V (4.2 V/cell), and the voltage of the third sub-pack is also (4.2 V/cell). The second sub-pack and the third sub-pack are therefore fully charged.

In the configuration of Figure 15d, only the first sub-pack 11 is connected to the voltage bus, with the modules 18,19 again connected in parallel. Since both the second sub-pack and the third sub-pack are fully charged, they are disconnected from the voltage bus. The voltage bus therefore drops from 495.0 V to 432 V. The first sub-pack then continues to be charged until the bus voltage reaches a termination voltage of 453.6 V (4.2V/cell). The first sub-pack is therefore fully charged.

Charging of the battery pack begins with the configuration of Figure 15a. As noted above, the configuration of Figure 15a may be omitted and charging may start with the configuration of Figure 15b. However, a higher current and thus power may be drawn from the charging station by starting in the configuration of Figure 15a. As a result, the overall charge time may be reduced. Nevertheless, the configuration of Figure 15a could be omitted.

At the end of the configuration of Figure 15c, we need to ensure that both the second sub pack 12 and the third sub-pack 13 are fully charged without the bus voltage exceeding the charge voltage of the station. In the example provided above, the bus voltage at the end of the configuration of Figure 15c was 495 V, just 5 V less than the maximum charge voltage. This criteria ultimately defines the length of time that can be spent in the configuration of Figure 15a. If too long a time is spent in the configuration of Figure 15a then, when during the configuration of Figure 15c, the bus voltage may rise to the maximum charge voltage of the station before the second and third sub-packs are fully charged. If the second and third sub-packs are fully discharged, or for some other reason have the same voltage, then the configuration of Figure 15b may be omitted. As already noted, one might also omit the configuration of Figure 15a. Charging would then start from the configuration of Figure 15c. However, the combined number of parallel cells (26 + 15 = 41), is still significantly less than that of the first sub-pack (34 + 34 = 68). Accordingly, a faster charge time may be achieved by initially charging in the configuration of Figure 15a, and then switching to the configuration of Figure 15c.

Figure 16 illustrates the bus voltage and the state-of-charge (SoC) of each of the sub packs for the reconfiguration sequence of Figure 15, using the particular example described above.

In the embodiments described above, the voltage thresholds or transition voltages are chosen such that, during discharge, each of the sub-packs reach a state of full discharge (i.e. 0% SoC) at the same time. This then avoids an undesirable situation in which one sub-pack is fully discharged, but another sub-pack still retains useful charge that cannot now be extracted. The voltage thresholds are additionally chosen so as to reduce the operating voltage range and to avoid excessively high bus voltages. When charging, the voltage thresholds or transition voltages are chosen such that all sub-packs can be fully charged. The voltage thresholds are additionally chosen so as to increase the power that is drawn from the charging station so as to reduce charge times. It will be appreciated that the particular thresholds will depend on many factors, such as the particular reconfiguration sequence, the number of sub-packs and modules, and the number of series and parallel connected cells within each sub-pack and module.

In the embodiments described above, reference is made to switching between configurations when the voltage of the bus 10 transits a threshold, i.e. when the bus voltage falls below the threshold during discharging, or rises above the threshold during charging. It will be appreciated, however, that an alternative voltage measurement may be used to trigger the switching of configurations. For example, in the embodiment illustrated in Figure 3, the battery pack 2 may switch from the configuration of Figure 3a to the configuration of Figure 3b when the voltage of the first sub-pack 11 falls below 680.4 V, or when the voltage of any one of the modules 20-22 of the second sub-pack 12 falls below 226.8 V. Similarly, in the embodiment illustrated in Figure 15, the battery pack 2 may switch from the configuration of Figure 15b to the configuration of Figure 15c when the voltages of the second sub-pack 12 and the third sub-pack 13 are the same. Accordingly, in a more general sense, the battery pack may be said to switch between two configurations when a voltage of the battery pack transits a threshold. The voltage of the battery pack may then be any voltage within the battery pack, such as the voltage of the voltage bus, the voltage of one or more of the sub-packs or a voltage of one or more of the modules.

In each of the embodiments described above, the battery pack 2 comprises four battery units (i.e. sub-packs and modules); each battery unit is then represented by a box in Figures 3, 5, 7, 9, 11, 13 and 15. It has been found that four battery units is a good compromise between not making the switching circuitry of the battery pack too complex and reducing the operating voltage range of the battery pack. However, more battery units could alternatively be used at the expense of increasing the number of switches and the complexity of the switching controls and strategy. This may increase the flexibility of the battery pack and further reduce the operating voltage range. Similarly, fewer battery pack units could alternatively be used, which would reduce costs and complexity but may increase the operating voltage range.

Embodiments of the invention are described above, by way of example. The skilled person will appreciate that the invention is not limited to these embodiments and that other embodiments falling within the scope of the claims may be developed that lack and/or have alternative features to the embodiments described above.