Electrical charge storage system management

Abstract

A charge storage system 14 of a vehicle comprises a low-voltage battery 22 and a high-voltage battery 18 and is operable in a charging mode in which charge is transferred from the low-voltage battery

Description

The present disclosure relates to electrical charge storage system management. Aspects of the invention relate to a system, to a method, to computer readable instructions and to a vehicle fitted with the system. BACKGROUND Some automotive vehicles, such as battery electric vehicles (BEVs) or plug-in hybrid electric vehicles (PHEVs), include a high-voltage electrical system and a low-voltage electrical system. Typically, the high-voltage system is used for propulsion of the vehicle, while the low-voltage system is used to power all other electrical systems in the vehicle, for example ancillary devices and systems such as lighting, infotainment and climate control. Each electrical system may include a respective battery, so that the vehicle includes a high-voltage battery and a low-voltage battery. The high-voltage battery may receive charge from an external source such as a vehicle charging point, for example, and the low-voltage battery may be charged from the high-voltage battery, via a step-down DC-DC converter. It is known to provide photovoltaic (PV) systems in BEVs or PHEVs. Figure 1 shows one known implementation, in which a low-voltage battery 1 of a low-voltage system is connected directly to a PV system 2, and also to a high-voltage battery 3 of a high-voltage system through a step-down DC-DC converter 4. In this arrangement, the low-voltage battery 1 can be charged using power generated by the PV system 2 when available and can be charged by the high-voltage battery 3 at other times, under the control of a charge controller 5. The low-voltage battery 1 typically has a relatively low capacity, which may limit the proportion of the power that is generated by the PV system 2 that can be stored. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a vehicle, a method and computer readable instructions as claimed in the appended claims. According to an aspect of the invention there is provided a control system. The charge storage system comprises a low-voltage battery and a high-voltage battery and is operable in a charging mode in which charge is transferred between the low-voltage battery and the high-voltage battery, for example, from the low-voltage battery to the high-voltage battery. The control system comprises one or more processors collectively configured to: receive signals indicative of a state of charge of a battery, for example, the low-voltage battery; determine whether the indicated state of charge of the battery exceeds a first threshold value; and output one or more control signals configured to activate the charging mode for the charge storage system if the indicated state of charge of the battery exceeds the first threshold value. The control system may comprise one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein. The at least one electronic processor may be configured to access the at least one memory device and execute the instructions thereon so as to control the charge storage system. Transferring charge from the low-voltage battery to the high-voltage battery directly contributes to increased range of the vehicle. The power of charge transfer can be tuned to enhance efficiency to minimise power losses. The control system may be configured to deactivate the charging mode when the indicated state of charge of the low-voltage battery drops below a second threshold value that is lower than the first threshold value. This may avoid deep cycling of the low-voltage battery and thereby extend its life. The control system may be configured to receive signals indicative of a state of charge of the high-voltage battery, and to activate the charging mode only if the indicated state of charge of the high-voltage battery is below a third threshold value. By introducing a threshold for the state of charge of the high-voltage battery before the charging mode is enabled, the control system can act to transfer charge according to demand. The control system may be configured to activate the charging mode in dependence on an expected level of charge to be supplied to the low-voltage battery. Beneficially, this helps to avoid a scenario in which charge is transferred from the low-voltage battery and is subsequently not restored, leaving the low-voltage battery in a depleted state. This scenario could entail transferring charge back to the low-voltage battery from the high-voltage battery, increasing transfer losses. Further to this, the control system may be configured to determine the expected level of charge to be supplied to the low-voltage battery, and activating the charging mode only if the expected level exceeds a fourth threshold value. Additionally, the control system may be configured to determine the expected level of charge in accordance with one or more of the following: a time of day; a location of the vehicle; a user input to the vehicle; a present level of charge being supplied to the low-voltage battery; and a date. The vehicle in which the charge storage system resides may comprise a photovoltaic (PV) system that is configured to supply charge to the low-voltage battery. Correspondingly, the charge storage system, and in particular the low-voltage battery, may be configured to receive charge from a PV system of the vehicle. Combining a PV system with the control system allows the low-voltage battery and, in turn, the high-voltage battery, to be charged using electricity derived from solar energy. The PV system may therefore be low-voltage, which reduces the undesirable effects of partial shadowing of cells of the PV system. The control system may be configured to operate a converter of the charge storage system to provide a voltage step-up between the low-voltage battery and the high-voltage battery. The control system may be configured to activate the charging mode only if the vehicle is inactive. While a vehicle may seldom be entirely inactive, in that certain vehicle systems continue to execute while a vehicle is not in use, in this context a vehicle may be regarded as ‘inactive’ when it is not being driven or actively used by a user, for example when the vehicle is unoccupied, in a park mode, disengaged, switched off or otherwise powered down. The control system may, when activating the charging mode, close at least one electrical connector between the high-voltage battery and the low-voltage battery to connect the high and low voltage batteries electrically, in which case deactivating the charging mode may comprise opening the at least one electrical connector to disconnect the high and low voltage batteries electrically. Doing so may ensure that the high-voltage battery is electrically isolated when not in use. Furthermore, charge losses may be reduced by avoiding keeping the contacts closed unnecessarily. The control system may be configured to deactivate the charging mode once a predetermined period has elapsed following activating the charging mode. Placing a time limit on the charging mode may provide a simple way to determine when to deactivate the charging mode and, if the control system is also configured to deactivate the charging mode based on a threshold value, may help to limit operational charge loss and thermal build-up in the charge storage system. The control system may be configured to transfer charge from the low-voltage battery to the high-voltage battery, when in the charging mode, at a power level at which the charge transferred into the high-voltage battery exceeds operational losses of the charge transfer. This may help to ensure that the majority of charge transferred from the low-voltage battery reaches the high-voltage battery. The control system may be configured to transfer charge from the low-voltage battery to the high-voltage battery, when in the charging mode, at a power level that exceeds a power level at which the low-voltage battery is charged. The control system may be configured to transfer charge from the low-voltage battery to the high-voltage battery, when in the charging mode, at a substantially maximised power level. In another aspect of the invention, a vehicle charge storage system comprising the control system of the above aspect is provided. The invention also extends to a vehicle comprising the charge storage system or the control system of the above aspects. Another aspect of the invention provides a method for controlling a charge storage system of a vehicle. The charge storage system comprises a low-voltage battery and a high-voltage battery and is operable in a charging mode in which charge is transferred from the low-voltage battery to the high-voltage battery. The method comprises: receiving signals indicative of a state of charge of the low-voltage battery; determining whether the indicated state of charge of the low-voltage battery exceeds a first threshold value; and outputting one or more control signals to activate the charging mode for the charge storage system if the indicated state of charge of the low-voltage battery exceeds the first threshold value. A further aspect of the invention provides computer readable instructions which, when executed by a computer, are arranged to perform the above method. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples, and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of a known vehicle system; Figure 2 shows a vehicle in which embodiments of the disclosure may be implemented; Figure 3 shows a block diagram of a charge storage system of the vehicle of Figure 2; Figure 4 shows a block diagram of a controller of the charge storage system of Figure 3; Figure 5 shows a process flow chart representing operation of the system of Figure 3; and Figure 6 is a graph demonstrating an example of functionality of the system of Figure 3. DETAILED DESCRIPTION In general terms, embodiments of this disclosure relate to charge storage systems and associated control systems for vehicles, in which charge can be transferred from a low-voltage battery to a high-voltage battery. This configuration enables the full combined capacity of both batteries to be exploited. It follows that if the low-voltage battery is charged by a PV system, for example, that charge can then be passed on to the high-voltage battery, for example by implementing a burst-charging arrangement. This provides a convenient way to charge a high-voltage battery using power generated by a PV system in a vehicle, which may otherwise be challenging. Figure 2 shows an example of an automotive vehicle 10 in which such embodiments may be implemented. In this example, the vehicle 10 takes the form of a passenger vehicle, specifically a BEV in this example, and comprises a roof-mounted PV system 12. Embodiments of charge storage systems and associated control systems according to this disclosure may be implemented in other types of vehicles, however, and in general terms are applicable to any vehicle having a low-voltage and a high-voltage system, for example a PHEV. Figure 3 shows, in simplified schematic form, an overall electrical system 14 of the vehicle 10. The electrical system 14 includes a high-voltage system 16 having a high-voltage battery 18, and a low-voltage system 20 having a low-voltage battery 22. The electrical system 14 further includes the PV system 12, which comprises an array of photovoltaic cells 24 that are configured to generate electrical power when exposed to sunlight. Although not shown in the Figures, the low-voltage system 20 further comprises low-voltage components representing ancillary or secondary function devices of the vehicle 10, for example, HVAC, lighting, and infotainment systems. The low-voltage system 20 operates at 12V in this example, although this may vary in other implementations. In general terms, the low-voltage system 20 may comprise all electrical systems of the vehicle except the propulsion system. The components of the low-voltage system are powered by the low-voltage battery 22. The PV system 12 is connected to the low-voltage battery 22 through a maximum power point tracker 26 (MPPT), which controls the transfer of charge generated by the PV system 12 into the low-voltage battery 22. In this respect, the output of the PV system 12 may vary in dependence on the ambient temperature and the strength of the incident solar energy, and in particular may have a variable voltage. The MPPT 26 is configured to process the output of the PV system 12, provided that the output is above a cut-off voltage, into a steady input to be delivered to the low-voltage battery 22. The power delivered to the low-voltage battery 22 may be at 13-15V, for example. The low-voltage battery 22 is a lithium-ion battery in this example, which supports the expected charge cycling. The low-voltage battery 22 typically has a lower charge capacity than the high-voltage battery 18. For example, the charge capacity of an automotive low-voltage battery 22 may be approximately 50Ah to meet the demands of the components of the low-voltage system 20. The high-voltage system 16 propels the vehicle 10 and so acts as a traction system. The high-voltage battery 18 acts as a source of electrical power that is converted into motion of the vehicle 10 using one or more motors (not shown) of the high-voltage system 16. In this example, the high-voltage system 16 operates at 400V, although this may vary in other examples. For example, systems operating at 800V or 1200V are also known. In general terms, the high-voltage system may operate at above 60V, whereas the low-voltage system 20 may operate at below 60V, for example. The high-voltage battery 18 is primarily rechargeable by a source 28 that is external to the vehicle 10 such as a charging station, at a voltage compatible with the components of the high-voltage system 16. For this purpose, the high-voltage system 16 includes a terminal for connection to an external charger, and a DC-DC converter 30 that manages any voltage change that may be required between the external charger 28 and the high-voltage battery 16. The system 33 may include an AC-DC converter (not shown) where the source 28 is an AC source. The low-voltage battery 22 is also connected to the DC-DC converter 30, so that transfer of electrical charge between the high and low-voltage batteries 18, 22 is possible. So, for example, the high-voltage system 16 supports the low-voltage system 20 through the DC-DC converter 30 whenever the vehicle 10 is in an active state. When the vehicle 10 is in an inactive state charge can be transferred from the high-voltage battery 18 to the low-voltage battery 22 when the state-of-charge (SOC) of the low-voltage battery 22 falls below a threshold, to ensure sufficient power remains available for the components of the low-voltage system 20 and to prevent deep cycling of the low-voltage battery 22. The transfer of charge from the high-voltage battery 18 to the low-voltage battery 22 may be referred to as ‘buck’ charging. The DC-DC converter 30 is bi-directional in this example, in that it can provide either a step-up in voltage or a step-down. Such a converter may be referred to as a ‘buck-boost converter’. Accordingly, it is also possible to transfer charge from the low-voltage battery 22 to the high-voltage battery 18, which may be referred to as ‘boost’ charging. Since the low-voltage battery 22 receives charge from the PV system 12, this beneficially allows electrical power produced by the PV system 12 to be passed on to the high-voltage battery 18. In this respect, it is noted that charging the high-voltage battery 18 directly from the PV system 12, while desirable in view of the greater capacity of the high-voltage battery 18, is practically challenging. In this respect, generating the required voltage may entail either a photovoltaic array of a greater number of smaller cells, which may be vulnerable to shadowing, or dedicated power conversion hardware, which may have relatively high losses. By charging the low-voltage battery and then transferring that charge on to the high-voltage battery, these difficulties can be avoided. A charge controller 32 is provided to manage the transfer of electrical charge between the low and high-voltage batteries 22, 18, and also between an external charger 28 and the high-voltage battery 18. The charge controller 32 is therefore configured to operate the DC-DC converter 30 to provide the required voltage conversion for each type of energy transfer. It follows from the above that the low-voltage battery 22, the high-voltage battery 18, the DC-DC converter 30 and the charge controller 32 collectively represent a charge storage system 33 that is configured to manage and distribute electrical charge entering the vehicle electrical system 14 through either the PV system 12 or the external charger 28, according to the operational requirements of the vehicle 10. The charge controller 32 forms part of the charge management system in this embodiment and controls operation of the charge storage system. In other examples, the charge storage system may be operated by a separate charge controller. The charge controller 32, shown in Figure 4, represents a control system for the charge storage system 33 that is embodied as a single controller, although it will be appreciated that this is merely illustrative and in other examples the functionality of the charge controller 32 may be implemented using multiple controllers, for example. The charge controller 32 comprises processing means 38 and memory means 40. The processing means 38 may be one or more electronic processing device 38 which operably executes computer-readable instructions. The memory means 40 may be one or more memory device 40. The memory means 40 is electrically coupled to the processing means 38. The memory means 40 is configured to store instructions, and the processing means 38 is configured to access the memory means 40 and execute the instructions stored thereon. The charge controller 32 comprises an input means 34 and an output means 36. The input means 34 may comprise an electrical input of the controller 32. The output means 36 may comprise an electrical output of the controller 32. The input 34 is arranged to receive signals indicative of a SOC of the low-voltage battery 22. The output 36 is arranged to output control signals for implementing commands generated by the processing means 38, for example to control the DC-DC converter 30 to implement a charging mode of the charge storage system 33. In this respect, the charge storage system 33 is operable in a charging mode, in which electrical charge is transferred from the low-voltage battery 22 to the high-voltage battery 18 through appropriate control of the DC-DC converter 30. The charging mode is activated and deactivated by the charge controller 32. The charging mode may be activated and deactivated in accordance with the respective states of charge of the high and low-voltage batteries 18, 22. The charge controller 32 may also take other factors into account to determine when to activate and / or deactivate the charging mode, for example the likelihood that charge transferred from the low-voltage battery 22 will be replenished in the shortterm. For example, the charge controller 32 is configured to receive data indicative of a SOC of the low-voltage battery 22 and determine whether the indicated SOC of the low-voltage battery 22 exceeds a first threshold value. The first threshold generally defines a sufficient SOC of the low-voltage battery 22 to support a transfer of charge to the high-voltage battery 18 without compromising operation of the low-voltage system. The first threshold value may be a level of charge that is within 5%, 10% or 15% of the maximum capacity of the low-voltage battery 22, for example. The charge controller 32 may then output a control signal to activate the charging mode for the charge storage system 33 if the indicated SOC of the low-voltage battery 22 exceeds the first threshold value. Subsequently, the charge controller 32 may output a control signal to deactivate the charging mode if the SOC of the LV battery 22 drops below a second threshold that is lower than the first threshold. The second threshold defines an SOC to preserve sufficient charge reserves in the low-voltage battery 22 and to prevent the low-voltage battery 22 from exhibiting deep cycling. In this example, when the charging mode is activated a substantially fixed quantity of charge is transferred from the low-voltage battery 22 to the high-voltage battery 18, this quantity of charge being defined by the difference between the first and second thresholds, ignoring losses. In each instance of charging, the charge is transferred at a substantially constant, high power and over a substantially fixed time period. The charging mode is activated in dependence on whether the high-voltage battery 18 has sufficient spare capacity to receive the fixed quantity of charge to be delivered. In this respect, the SOC of the high-voltage battery 18 is also monitored, and a third threshold is defined representing a value for the SOC of the high-voltage 18 battery below which the charge transfer can take place. Put another way, the charging mode may be prevented from being activated if the SOC of the high-voltage battery 18 exceeds the third threshold. The value of the third threshold may be determined to account for the tendency of the high-voltage battery 18 to charge at a lower rate as its SOC approaches its maximum capacity due to the reduced ability to accept energy at this SOC. This ensures that activating the charging mode results in a reasonable increase in the SOC of the high-voltage battery 18, accounting also for the operational power losses of the charge transfer. It is noted that other charging regimes are possible. For example, a charging mode may be activated in part in response to a demand for charge from the high-voltage battery. In such an arrangement, the charging mode may be inhibited if the SOC of the high-voltage battery is above a threshold level. It is also possible for charging to be effected in a dynamic manner instead of transferri ng a fixed quantity of charge each time as in the present example, in which case suitable thresholds may be implemented to avoid hysteresis. Figure 5 illustrates a corresponding process 42 for controlling the charge storage system 33 of the vehicle 10. In particular, the process 42 is a method of transferring charge from the low-voltage battery 22 to the high-voltage battery 18 of the vehicle 10. In particular, the memory 40 of the charge controller 32 may comprise computer-readable instructions which, when executed by the processor 38, perform the process 42. In this way, the charge controller 32 is configured to perform the process 42. The process 42 comprises receiving, at step 44, signals indicative of a state of charge of the low-voltage battery 22 and the high-voltage battery 18. Next, it is determined, at step 46, whether the indicated state of charge of the low-voltage battery 22 exceeds the first threshold value, and then at step 48 if the indicated state of charge of the high-voltage battery 18 is below the third threshold. Additionally, the temperatures of each battery 18, 22 may be monitored to ensure that they are within acceptable temperature ranges, for example, between 5°C and 50°C. Next, provided the criteria of both steps 46 and 48 are met, one or more control signals are output at step 50 to activate the charging mode for the charge storage system 33. If the criteria of either of steps 46 and 48 are not met, the process 42 reverts back to step 44. Once the charging mode is enabled, the SOC for the low-voltage battery 22 is monitored in step 52. This information is used to determine, at step 54, whether the SOC of the low-voltage battery 22 is below the second threshold. If the SOC of the low-voltage battery 22 is below the second threshold, then the charge transfer is complete and the method proceeds to step 56 to deactivate the charging mode, and the process 42 returns to step 44 and continues to iterate. If the SOC of the low-voltage battery 22 is above the second threshold, then the process 42 reverts back to the monitoring step 52 and charging continues. To facilitate the charge transfer to the high-voltage battery 18, the electrical system 14 further includes auxiliary systems (not shown) such as cooling pumps and fans and disconnectable high-voltage contactors that each consume power as the charge transfer is taking place. The high-voltage contactors enable the high-voltage battery 18 to be electrically isolated when not in use, for example when the vehicle 10 is parked. These contactors are closed temporarily while the charging mode is active and holding them in a closed state consumes electrical power. This power consumption of these components may be factored into process 42 to ensure a positive net power transfer to the high-voltage battery 18. For example, the net total charge transfer rate may be determined to ensure that power losses associated with the transfer do not exceed the amount of power being transferred, so that the majority of the charge discharged from the low-voltage battery 22 reaches the high-voltage battery 18, or at least sufficient to justify the charging mode. To address this, in this example, in the charging mode, charge is transferred at high power over a short period, which is referred to as a ‘burst’ charge. The minimum rate of charge transfer during a burst charge defines a fourth threshold value, for example -1500W. The power of the burst charge at least exceeds the total power consumed to effect the transfer for the system to function efficiently, for example, which may be in the range of SOW to 160W depending on the system and the respective battery temperatures, and is configured to minimise the proportion of the transferred charge that is lost. The burst charge may occur at a maximum power of the system. By transferring charge from the low-voltage battery 22 to the high-voltage battery 18 using a series of burst charges, the high-voltage battery 18 is intermittently charged from the low-voltage battery 22. Additionally, the intermittent ‘boost’ charges will increase the temperature of the high-voltage battery 18 over time. Minimising the time spent charging at this relatively high power will serve to maintain the high-voltage battery 18 within acceptable ranges so as not to diminish the lifespan of the battery. Figure 6 shows a chart 60 which plots an SOC percentage of the low-voltage battery 22 operated by the charge controller 32 over time in hours. It can be noted that the SOC of the low-voltage battery 22 cycles over time. This is because as the low-voltage battery 22 receives charge and its SOC reaches the first threshold 62, which is shown as approximately 95% in Figure 6 but may be lower in other examples, for example 90% or less, some of the charge stored in the low-voltage battery 22 is then discharged into the high-voltage battery 18. The discharge continues until the second threshold 64 is reached. The second threshold 64 may be in the range of 20%-50%, for example, and may vary in dependence on the temperature of the low-voltage battery 22. The low-voltage battery 22 is then recharged, for example, by the PV system 12, before cycling again in the same manner. In this example, in which the PV system 12 is used to recharge the low-voltage battery 22, there are periods where the charge cycle is paused, for example where the PV system 12 is no longer exposed to the sun and therefore not generating any electricity. In general terms, the charging mode may be inhibited or interrupted at times when it is unlikely that the PV system 12 will generate power to replenish the low-voltage battery 22, for example when the sun is about to set or if the vehicle 10 is parked in a garage, to avoid a situation in which charge may need to be transferred back to the low-voltage battery 22 from the high-voltage battery 18. To account for this and similar factors, the charge controller 32 may be configured to determine an expected level of charge to be supplied to the low-voltage battery 22 in the short term, which is related to the power that the PV system 12 may be expected to generate. This, in turn, is dictated by the level of solar energy that the PV system 12 will receive, which can be predicted based on any one or more of: a time of day; a location of the vehicle; a user input to the vehicle; a present level of charge being supplied to the low-voltage battery; and a date. Time of day and a date may be received from a built-in clock (not shown) of the vehicle 10. Location of the vehicle may be provided by a GPS system (not shown) of the vehicle 10. A user input to the vehicle 10 may be received through a human-machine interface (not shown) or remotely, for example by a key fob or a mobile device application. A present level of charge being supplied to the low-voltage battery 22 may be supplied by the charge controller 32. The expected level of charge to be supplied to the low-voltage battery 22 derived in this manner can then be used to decide whether to activate the charging mode. In this respect, the charge controller 32 may inhibit the charging mode at times when the expected level of charge to be supplied to the low-voltage battery 22 is low, for example at sunset or if the vehicle 10 is parked in a location known to receive low levels of solar energy. 5 The rate of discharge or power transfer is represented as substantially constant in Figure 6, and corresponds to the fourth threshold value, which therefore defines the transfer rate for a ‘burst’ charge. The charge controller 32 may be configured to activate the charging mode only if vehicle 10 is not in use, inactive, in park mode, disengaged, switched offer otherwise powered down. It is appreciated that modern 10 vehicles are very rarely completely off, even when not in use, so the state described is not necessarily without power supply. By effecting the charge transfer during times when the vehicle 10 is not in use, there is minimal risk of affecting the drivability of the vehicle whilst the charge transfer is occurring. It will be appreciated that various changes and modifications can be made to the present invention without 15 departing from the scope of the present application.

Claims

1. A control system for controlling a charge storage system of a vehicle, the charge storage system comprising a low-voltage battery and a high-voltage battery and being operable in a charging mode in which charge is transferred from the low-voltage battery to the high-voltage battery, wherein the control system comprises one or more processors collectively configured to:receive signals indicative of a state of charge of the low-voltage battery;determine whether the indicated state of charge of the low-voltage battery exceeds a first threshold value; andoutput one or more control signals configured to activate the charging mode for the charge storage system if the indicated state of charge of the low-voltage battery exceeds the first threshold value.

2. The control system of Claim 1, configured to deactivate the charging mode when the indicated state of charge of the low-voltage battery drops below a second threshold value that is lower than the first threshold value.

3. The control system of any preceding claim, configured to receive signals indicative of a state of charge of the high-voltage battery, and to activate the charging mode only if the indicated state of charge of the high-voltage is below a third threshold value.

4. The control system of any preceding claim, configured to activate the charging mode in dependence on an expected level of charge to be supplied to the low-voltage battery.

5. The control system of Claim 4, configured to determine the expected level of charge to be supplied to the low-voltage battery, and activating the charging mode only if the expected level exceeds a fourth threshold value.

6. The control system of Claim 5, configured to determine the expected level of charge in accordance with one or more of the following: a time of day; a location of the vehicle; a user input to the vehicle; a present level of charge being supplied to the low-voltage battery; and a date.

7. The control system of any preceding claim, wherein the vehicle comprises a photovoltaic system that is configured to supply charge to the low-voltage battery.

8. The control system of any preceding claim, configured to activate the charging mode only if the vehicle is inactive.

9. The control system of any preceding claim, configured to transfer charge from the low-voltage battery to the high-voltage battery, when in the charging mode, at a power level at which the charge transferred into the high-voltage battery exceeds operational losses of the charge transfer.10, The control system of any preceding claim, configured to transfer charge from the low-voltage battery to the high-voltage battery, when in the charging mode, at a power level that exceeds a power level at which the low-voltage battery is charged.

11. The control system of any preceding claim, configured to transfer charge from the low-voltage battery to the high-voltage battery, when in the charging mode, at a substantially maximised power level.12, A vehicle charge storage system comprising the control system of any preceding claim.

13. A vehicle comprising the charge storage system of Claim 12 or the control system of any of Claims 1 to11.

14. A method for controlling a charge storage system of a vehicle, the charge storage system comprising a low-voltage battery and a high-voltage battery and being operable in a charging mode in which charge is transferred from the low-voltage battery to the high-voltage battery, the method comprising:receiving signals indicative of a state of charge of the low-voltage battery;determining whether the indicated state of charge of the low-voltage battery exceeds a first threshold value; andoutputting one or more control signals to activate the charging mode for the charge storage system if the indicated state of charge of the low-voltage battery exceeds the first threshold value.

15. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to Claim 14.

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