Provision of additional charging power from a fuel cell of a vehicle

Abstract

A method of controlling a fuel cell (204-Fig.2) and an electrical energy storage means (e.g., traction battery (206-Fig.2)) of a vehicle comprises: receiving a power demand 602; receiving an energy pa

Description

TECHNICAL FIELD The present disclosure relates to the provision of additional charging power from a fuel cell of a vehicle. The additional charging power is for charging an electrical energy storage means of the vehicle. Aspects of the invention relate to a control system, to a system, to a vehicle, to a method, and to computer readable instructions. BACKGROUND A fuel cell converts the chemical energy of a fuel, such as hydrogen, and oxygen to electrical energy. In a fuel cell powered vehicle, the generated electrical energy is stored by an electrical energy storage means (e.g., traction battery), and consumed by various consumers including an electric motor. The electric motor is configured to produce torque (drive torque) for driving the vehicle. If a torque increase is requested via an accelerator pedal or other requestor, the fuel cell’s electrical power output may be increased in response to ensure that sufficient electrical power is available. If a torque decrease is requested, the fuel cell’s electrical power output may be decreased in response. It is an aim of the present invention to address one or more disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a system, a vehicle, a method, and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for controlling a fuel cell and an electrical energy storage means (electrical energy storage device) of a vehicle, the control system comprising one or more processors collectively configured to: receive a power demand; receive an energy parameter dependent on stored electrical energy in the electrical energy storage means; determine whether the energy parameter satisfies a charging condition associated with the stored electrical energy in the electrical energy storage means relative to a target; and output a control signal configured to, in dependence on the energy parameter satisfying the charging condition, control the fuel cell to generate the power demand and additional charging power in addition to the power demand, to charge the electrical energy storage means. An advantage is improved energy management because the fuel cell is configured to generate electrical energy to sustain a target state of charge in the electrical energy storage means, while concurrently generating electrical energy to satisfy the power demand. The power demand may be a tractive power demand. The control system comprises 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; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive the power demand; receive the energy parameter; determine whether the energy parameter satisfies the charging condition; and output the control signal. Optionally, the electrical energy storage means has a peak electrical energy storage capacity no less than 5% of a peak electrical energy that the fuel cell is capable of producing in one hour. For example, the peak electrical energy storage capacity may be greater than 5 kilowatt-hours, or greater than 10 kilowatt-hours. Optionally, the peak electrical energy of the fuel cell may be greater than 50 kilowatt hours (peak power >50kW) or greater than 80 kilowatt hours (peak power >80kW). An advantage is that the electrical energy storage means has a relatively high capacity compared to the peak electrical energy of the fuel cell, and can therefore provide assistive electrical power over a long or challenging drive cycle. Assistive electrical power may be required, for example when the power demand exceeds the peak electrical energy of the fuel cell. Optionally, the target comprises a value selected from the range 60% to 95% of a peak electrical energy storage capacity of the electrical energy storage means. Optionally, the target comprises a value selected from the range 80% to 90% of a peak electrical energy storage capacity of the electrical energy storage means. An advantage is that the target maintains a high performance reserve of energy stored in the electrical energy storage means. The high performance reserve ensures that assistive electrical power from the electrical energy storage means is available throughout a long or challenging drive cycle. Optionally, the additional charging power of the control signal is dependent on a magnitude of the power demand. For example, the additional charging power may be reduced or removed in dependence on the magnitude of the power demand increasing. An advantage is that vehicle acceleration is prioritised when the power demand is high, for example when power demand exceeds a threshold. Optionally, satisfaction of the charging condition depends on an error (difference) between the stored electrical energy and the target. Optionally, satisfaction of the charging condition depends on both the error and the power demand. Optionally, the energy parameter is dependent on the error. Optionally, the energy parameter is also dependent on the power demand. An advantage is prioritising high performance driving over a long or challenging drive cycle. High performance driving refers to extra power being available when the additional charging power is not being requested from the fuel cell. Charging may only be required if the error indicates the stored electrical energy being below the target. Furthermore, the error may be allowed to increase further before additional charging power is requested, when the power demand is high compared to when the power demand is low. Optionally, a magnitude of the control signal has a nonlinear relationship with a magnitude of the error between the stored electrical energy and the target. An advantage is improved energy management because it allows the fuel cell power to be modified at different rates depending on whether the stored electrical energy is above or below the target, for example. Optionally, the additional charging power is inhibited in dependence on the error having a first, non-zero magnitude, and is implemented in dependence on the error having a second magnitude greater than the first magnitude. Therefore, charging of the electrical energy storage means does not start immediately upon the error crossing zero, and may be deferred until the energy parameter falls below a threshold associated with the stored electrical energy being below the target. The term ‘fall’ refers to an increasing distance of the stored electrical energy from the target, while below the target. An advantage is extending the period over which the electrical energy storage means can provide assistive electrical power, before it is charged by the fuel cell. The stored electrical energy is allowed to pass below the target before additional charging power is requested. When this strategy is used in conjunction with a high capacity electrical energy storage means, this allows high performance driving over a long or challenging drive cycle. Optionally, the charging condition comprises a threshold allowing the stored electrical energy to be below the target before the charging condition is satisfied, and wherein determining whether the charging condition is satisfied comprises determining whether the energy parameter is past the threshold. Optionally, satisfaction of the charging condition is dependent on the stored electrical energy being at least a predetermined amount less than the target. Optionally, the threshold corresponds to the stored electrical energy being X% below the target, and wherein X is selected from the range 1 % to 15%, or 1%to 10%. Optionally, the charging condition is an entry condition for supplying the additional charging power, and wherein the control system is further configured to determine whether the energy parameter satisfies an exit condition for ceasing to supply the additional charging power, and wherein the exit condition is different than the entry condition. Optionally, the exit condition comprises a threshold different than a threshold of the entry condition. The thresholds may be separated by a hysteresis gap. Optionally, the threshold of the exit condition is associated with the stored electrical energy being equal to or greater than the target, or less than the target but closer to the target than when the entry condition is satisfied. Optionally, the entry condition is not satisfied in dependence on the energy parameter being above a threshold, such as the threshold mentioned above. Optionally, in dependence on non-satisfaction of the entry condition, the control signal is configured to control the fuel cell to generate a different electrical power different than when the entry condition is satisfied, and different than the power demand, and wherein the electrical energy storage means is configured to supply assistive electrical power to reduce a difference between the different electrical power of the fuel cell and the power demand. Optionally, if the different electrical power of the fuel cell is less than the power demand, the electrical energy storage means is configured to supply assistive electrical power such that the sum of the different electrical power of the fuel cell and the assistive electrical power of the electrical energy storage means substantially add up to the power demand. Optionally, the different electrical power of the fuel cell is a fixed value or fixed range associated with a peak efficiency of the fuel cell. The different electrical power can therefore be referred to as a ‘peak efficiency power’. Therefore, in dependence on non-satisfaction of the entry condition, the control signal may be configured to control the fuel cell to generate a peak efficiency power different than the power demand, and wherein the electrical energy storage means is configured to reduce a difference between the peak efficiency power and the power demand. An advantage is that fuel efficiency is improved because the fuel cell is selectively operated at its peak efficiency. A fuel cell has a power curve, such that it is more efficient to run at certain power levels such as 4-15kW. The electrical energy storage means supplies or stores the difference between the peak efficiency power and the power demand. The ability to operate the fuel cell at the peak efficiency power level is acceptable provided the stored electrical energy is sufficient, as indicated by the non-satisfaction of the charging condition. When the stored electrical energy falls such that the charging condition is satisfied, the control signal may be configured to switch the fuel cell from outputting the peak efficiency power to outputting the power demand plus the additional charging power. Therefore, the fuel cell and electrical energy storage means are switchable between ‘charging’ and ‘assisting’ modes using the charging condition. In charging mode, the fuel cell provides the power demand and charges the electrical energy storage means. In assisting mode, the fuel cell provides the peak efficiency power or less, while the electrical energy storage means outputs assistive electrical power to provide the rest of the power demand. Optionally, the peak efficiency power is selected from the range 4kW to 15kW, or 4kW to 12kW. Optionally, in dependence on non-satisfaction of the entry condition, the control signal is configured to control the fuel cell to hold a static value of the peak efficiency power despite an increase of the power demand, and wherein the electrical energy storage means is configured to supply increased electrical power in dependence on the power demand increasing. The peak efficiency power of the fuel cell may remain fixed or relatively fixed for a range of values of the power demand. However, when the power demand reaches a very high level, the fuel cell power may be allowed to increase. Optionally, the assistive electrical power from the electrical energy storage means is determined via power request arbitration. Optionally, the arbitrated assistive electrical power comprises a selected one of a first assistive power or a second assistive power. Optionally, according to the first 3 assistive power, the proportion of power output by the fuel cell and by the electrical energy storage means varies in dependence on the stored electrical energy relative to the target. Optionally, according to the second assistive power, the power output of the fuel cell is fixed at the peak efficiency power. An advantage is that the arbitration determines which one of the fuel cell or the electrical energy storage means has its power output determined in dependence on an energy management priority. When the first assistive power is used, the priority is for the electrical energy storage means to increase its assistance to deplete some excess stored electrical energy. When the second assistive power is used, the priority is for the fuel cell to minimise its fuel consumption by operating substantially at its most efficient output. The electrical energy storage means provides enough assistance that the fuel cell can lower its output to the peak efficiency power. Optionally, the additional charging power from the fuel cell is determined via power request arbitration. Optionally, the control signal is dependent on a selected one of the additional charging power, a peak efficiency power, or optionally an equipment power demand. Optionally, in dependence on non-satisfaction of the entry condition, and in dependence on the assistive electrical power of the electrical energy storage means exceeding a power limit, the control signal is configured to increase the power of the fuel cell above the peak efficiency power to supply additional power corresponding to the amount of exceedance of the power limit. The power limit may define a peak electrical power that the electrical energy storage means is allowed to output. An advantage is allowing high performance driving, because if the power demand reaches such a high magnitude that the electrical energy storage means has reached its power limit, the fuel cell is allowed to increase its electrical power output. Only a slight increase may be required, so the fuel cell may still be operating close to its peak efficiency power. In other words, while the charging condition is not satisfied, the control system is optionally configured to increase the power of the fuel cell above the peak efficiency power in dependence on the power demand being greater than the power limit of the electrical energy storage means plus the peak efficiency power of the fuel cell. Optionally, the power limit is static or variable. If the latter, the power limit may depend on any one or more of: state of charge; state of health; or temperature, among other things. Optionally, in dependence on non-satisfaction of the entry condition, and in dependence on the stored electrical energy being above the target, the control signal is configured to reduce the electrical power generated by the fuel cell, by an amount which may optionally depend on how far the stored electrical energy is above the target. The assistive electrical power from the electrical energy storage means may increase in dependence on the stored electrical energy increasing relative to the target. An advantage is that when the error indicates too much stored electrical energy relative to the target, the reduction of the stored electrical energy is prioritised. This ensures that the electrical energy storage means is not overcharged and also reduces fuel consumption by the fuel cell. Optionally, the power demand may comprise a tractive demand power for one or more front wheel electric motors of the vehicle. Alternatively, the power demand may be for one or more rear wheel electric motors of the vehicle. Optionally, the power demand may be for a single electric motor of the vehicle. Optionally, the fuel cell and the electrical energy storage means are electrically connected to a same electrical bus of the vehicle. According to a further aspect of the present invention there is provided a system comprising the control system, and the fuel cell and the electrical energy storage means. According to a further aspect of the present invention there is provided a vehicle comprising the system, or the control system. According to a further aspect of the present invention there is provided a method of controlling a fuel cell and an electrical energy storage means of a vehicle, the method comprising: receiving a power demand; receiving an energy parameter dependent on stored electrical energy in the electrical energy storage means; determining whether the energy parameter satisfies a charging condition associated with the stored electrical energy in the electrical energy storage means relative to a target; and outputting a control signal configured to, in dependence on the energy parameter satisfying the charging condition, control the fuel cell to generate the power demand and additional charging power in addition to the power demand, to charge the electrical energy storage means. According to a further aspect of the invention there is provided computer readable instructions which, when executed by a computer, are arranged to perform any one or more of the methods described herein. According to a further aspect of the invention there is provided a non-transitory computer readable medium comprising computer readable instructions that, when executed by one or more electronic processors, causes the one or more electronic processors to carry out any one or more of the methods described herein. According to a further aspect of the present invention there is provided a control system for controlling a fuel cell and an electrical energy storage means (electrical energy storage device) of a vehicle, the control system comprising one or more processors collectively configured to: receive a power demand; receive an energy parameter dependent on stored electrical energy in the electrical energy storage means; and output a control signal configured to, in dependence on the energy parameter and the power demand, control the fuel cell to generate the power demand and additional charging power in addition to the power demand, to charge the electrical energy storage means. 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 falls within the scope of the appended claims. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination that falls within the scope of the appended claims, 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: FIG. 1 illustrates a perspective view illustrating an example of a vehicle; FIG. 2 illustrates a schematic view illustrating an example of a fuel cell powertrain; FIG. 3 illustrates a graph illustrating an example of a fuel cell efficiency curve; FIG. 4 illustrates a schematic view illustrating an example of a control system; FIG. 5 illustrates a schematic view illustrating an example of a non-transitory computer-readable storage medium; FIG. 6 illustrates a flowchart illustrating an example of a method; FIG. 7A illustrates a first example lookup table, FIG. 7B illustrates a second example lookup table, and FIG. 7C illustrates a graph graphically illustrating example values of an energy parameter in dependence on a state of charge error; and FIG. 8 illustrates a graph illustrating example relationships between state of charge error and a power request. DETAILED DESCRIPTION A vehicle 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying FIG. 1. In some, but not necessarily all examples, the vehicle 1 is a passenger vehicle, also referred to as a passenger car or as an automobile. In other examples, embodiments of the invention can be implemented for other applications, such as commercial vehicles. 5 FIG. 2 illustrates a powertrain 200 (system) of the vehicle 1. The powertrain 200 comprises a fuel cell 204, and an electrical energy storage means in the form of a traction battery 206. In a non-limiting example, the fuel cell 204 is a hydrogen fuel cell. The fuel cell 204 is connected to a fuel tank 202 storing a chemical fuel such as gaseous hydrogen. The fuel cell 204 is configured to convert the chemical energy of the fuel into electrical energy through an electrochemical reaction. The fuel cell 204 is also connected to an electrical bus 201 of the vehicle 1. The fuel cell 204 is configured to generate electrical energy and output the electrical energy to the electrical bus 201. The peak electrical energy of the fuel cell 204 may be greater than 50 kilowatt hours (peak power >50kW) or greater than 80 kilowatt hours (peak power >80kW). The traction battery 206 is also connected to the electrical bus 201. When discharging, the traction battery 206 is configured to output electrical energy to the electrical bus 201. When charging, the traction battery 206 is configured to receive and store electrical energy from the electrical bus 201. The traction battery 206 may have a peak electrical energy storage capacity of greater than 5 kilowatt-hours, or greater than 10 kilowatt-hours. An advantage is that the traction battery 206 has a high enough capacity to provide assistive electrical power over a long or challenging drive cycle, when its stored electrical energy is managed in accordance with the present invention. Assistive electrical power may be required, for example when power demand exceeds the peak electrical energy of the fuel cell 204. One or more electrical inverters 208 are also connected to the electrical bus 201. Each inverter 208 is connected to a corresponding electric machine 210, such as an electric motor. One of the electric machines 210 is mechanically connected to a front wheel of the vehicle 1, and the other is mechanically connected another front wheel. Alternatively, One of the electric machines 210 is mechanically connected to a rear wheel of the vehicle 1, and the other is mechanically connected to another rear wheel. Alternatively, a single electric machine 210 drives both front wheels and / or both rear wheels, for example. When positive torque is requested, the electric machine 210 and inverter 208 are collectively configured to convert the electrical energy on the electrical bus 201 into mechanical energy of the vehicle 1. When negative torque is requested for charging the traction battery 206, the opposite energy transformation takes place. FIG. 3 is a graph schematically illustrating a curve of efficiency E of the fuel cell 204 compared to an electrical power output P of the fuel cell 204. The line is not based on any particular model of fuel cell 204. The efficiency curve illustrates a peak efficiency Emax, and falling efficiency to either side of the peak efficiency Emax. The peak efficiency Emax corresponds to a peak efficiency power P_Emax, meaning the fuel cell power P at which the fuel cell 204 is operating at its peak efficiency Emax. FIG. 3 shows that the peak efficiency power P_Emax is quite low relative to a peak power of the fuel cell 204. For example, the peak efficiency power P_Emax may be from the range 5% to 15% of the rated peak power of the fuel cell 204. For an 80kW fuel cell 204, the peak efficiency power P_Emax may be in the order of 4kW to 15kW or 4kW to 12kW depending on the implementation. With reference to FIG. 4, there is illustrated a control system 400 for a vehicle 1. The control system 400 comprises one or more controllers 401. The control system 400 is configured to receive power demand data directly or indirectly from a power requestor 414 such as an accelerator pedal sensing system, or an automated driving control system. The control system 400 is also configured to receive a state of charge error from a battery monitoring system 415. The control system 400 may then determine and output a control signal to control the fuel cell 204, in dependence on the power demand data and the SoC error. Separately, the traction battery 206 automatically charges or discharges based on the load demand and bus voltage. When the fuel cell 204 is not providing all of the demanded power, the traction battery 206 will automatically supply the rest. The control system 400 as illustrated in FIG. 4 comprises one controller 401, although it will be appreciated that this is merely illustrative. The controller 401 comprises processing means 404 and memory means 406. The processing means 404 may be one or more electronic processing device 404 which operably execute computer-readable instructions. The memory means 406 may be one or more memory device 406. The memory means 406 is electrically coupled to the processing means 404. The memory means 406 is configured to store instructions, and the processing means 404 is configured to access the memory means 406 and execute the instructions stored thereon. The controller 401 comprises an input means 410 and an output means 412. The input means 410 may comprise an electrical input 410 of the controller 401. The output means 412 may comprise an electrical output 412 of the controller 401. The controller 401 may have an interface 402 comprising an electrical input / output I / O 410, 412, or an electrical input 410, or an electrical output 412, for receiving information and interacting with external components. The input 410 is arranged to receive a power demand signal from a power requestor 414. The power demand signal is an electrical signal which is indicative of a requested electrical instant power available on the electrical bus 201. The output 412 is arranged to output a fuel cell control signal, indicative of requested fuel cell power for controlling a power level of the fuel cell 204. FIG. 5 illustrates a non-transitory computer-readable storage medium 500 comprising the instructions (computer software). FIG. 6 illustrates a method 600 according to an embodiment of the invention. Example tables and graphs are provided in FIGS. 7A-8. The method 600 is a method of controlling the fuel cell 204 and the electrical energy storage means of the vehicle 1. The method 600 may be performed by the control system 400 illustrated in FIG. 4. In particular, the memory 406 may comprise computer-readable instructions 408 which, when executed by the processor 404, perform the method 600. In summary, the method 600 comprises: at block 602, receiving a power demand P_D; at block 606, receiving an energy parameter EP dependent on stored electrical energy SoC in the electrical energy storage means 206; at block 608, determining whether the energy parameter EP satisfies a charging condition associated with the stored electrical energy SoC in the electrical energy storage means 206 relative to a target; and at block 644, outputting a control signal 646 configured to, in dependence on the energy parameter EP satisfying the charging condition, control the fuel cell 204 to generate the power demand P_D and an additional charging power in addition to the power demand P_D, to charge the electrical energy storage means 206. FIG. 6 also illustrates further blocks demonstrating optional further features of the method 600. The blocks 602 to 642 calculate a required battery power. Block 644 then derives the required fuel cell power from the required battery power. It would be appreciated that the method 600 could instead be performed from the frame of reference of the fuel cell 204, in another implementation. Blocks 602-606 relate to input data for a lookup block 606. Block 602 comprises receiving a power demand P_D. Block 602 is a data source block indicating a current power demand P_D of the fuel cell 204. The power demand P_D refers to an electrical power demand, indicating an electrical power that is requested on the electrical bus 201. The power demand P_D may be determined by a controller 401 inside or outside the control system 400. The power demand P_D may be determined in dependence on a tractive power demand. The tractive power demand relates to a value of electrical bus power which is demanded for satisfying a propulsive torque request. The propulsive torque request may be determined in dependence on accelerator pedal depression. If the vehicle 1 is under automated control, the propulsive torque request may be determined at least partially automatically in dependence on machine vision sensing and an automated driving algorithm. The propulsive torque request may be converted from torque units to electrical power units. A power demand P_D may also be determined in dependence on an equipment power demand. An equipment power demand comprises the sum of an electrical power request from a voltage converter (not shown), and an electrical power request from a heating, ventilation and cooling (HVAC) system. The load of the voltage converter represents the cumulative loads of various vehicle power consuming equipment that are connected to a step-down voltage side of the voltage converter. Therefore, the equipment power demand represents a minimum amount of electrical power required on the electrical bus to power the equipment for purposes other than tractive torque or battery charging. The power demand P_D may therefore be dependent on the sum of the tractive power demand and the equipment power demand. The power demand P_D may be determined reactively in response to the current tractive power demand and current equipment power demand. Block 604 comprises receiving a current state of charge (SoC) error ‘SoC_e’ indicative of a current difference (error) between the stored electrical energy in the traction battery 206 and a target. The stored electrical energy represents the SoC of the battery, for example in percent. The target is a setpoint value of the SoC. The SoC error indicates the difference between the SoC and the setpoint. In an implementation, the target comprises a value selected from the range 60% to 95%, or 80% to 90%, of the peak electrical energy storage capacity of the traction battery 206. This is a high target within the context of vehicle battery management. An advantage is that the high target maintains a high performance reserve of energy stored in the traction battery 206. The high performance reserve ensures that electrical power from the traction battery 206 remains available even if the drive cycle becomes long or challenging. Therefore, maximum vehicle performance is maintained over long drive cycles, and seeks to avoid a situation in which the vehicle 1 is driven on fuel cell power alone. When the SoC is above the target, the SoC error is negative. When the SoC is below the target, the SoC error is positive. The sign (positive or negative) is expressed for example purposes and is non-limiting. Alternatively, the SoC may be determined predictively, based on an algorithm that calculates predicted power demand for a journey, segment, or route ahead, and an algorithm that models the future SoC based on the predicted power demand. As an example, determining the predicted SoC error may first comprise predicting power demand P_D. Predicting the power demand P_D may start with receiving electronic horizon data. The electronic horizon data is based on a predictive model of a road ahead and / or traffic ahead. The predictive model may represent a programmed journey, segment, or route. The predictive model is updated in dependence on various data sources such as one or more of location data, digital map data, route data, traffic data etc. The predicted SoC error is determined by predicting the amount of SoC required to satisfy the predicted power demand P_D. The prediction may take into account charging opportunities such as negative road gradients, or accelerator pedal lift-off opportunities such as approaches to junctions. The SoC error, whether current or predicted, may be expressed as a single value or as a variable SoC window, for example. A variable window means the control system 400 could vary the SoC target in dependence on the predicted power demand P_D. For example, if higher power demand is predicted, the control system 400 can raise the SoC target (e.g., SoC window 60-80%). If lower power demand is predicted (e.g., city driving and hence less dependency on battery energy), the control system 400 can lower the SoC target (e.g., SoC window 20% to 40%). The lookup block 606 comprises looking up an energy parameter EP in dependence on the power demand P_D and SoC error received from the input blocks 602 and 604. The energy parameter EP refers to any parameter dependent, at least in part, on stored electrical energy in the traction battery 206. The looked up energy parameter EP is dependent on the SoC error and therefore is dependent on the stored electrical energy (SoC). The energy parameter EP is any control parameter for a subsequent decision block 608, to control whether the decision block 608 selects a ‘charging’ mode (blocks 626-640) or an ‘assisting’ mode (blocks 610-624). The decision block 608 selects either a charging mode or an assisting mode in dependence on the energy parameter EP, which in turn is dependent on the SoC error and the power demand P_D. In the charging mode, the fuel cell 204 generates additional charging power in addition to the power demand P_D. In the assisting mode, the traction battery 206 provides assistive electrical power to assist the fuel cell 204 in providing the power demand P_D. The energy parameter EP may comprise a lookup function such as a lookup value in a lookup table, or the like. The values of the energy parameter EP depend on calibration. The values of the energy parameter EP in the lookup table can have any units. In a non-limiting example, the energy parameter EP has units of power. FIG. 7A illustrates an example two-dimensional lookup table. The energy parameter EP depends on both the power demand P_D (block 602) and the SoC error ‘SoC_e’ (block 604). The energy parameter EP is selected from the lookup table in dependence on the values of the power demand P_D and of the SoC error. In FIG. 7A, the values of the power demand P_D, the SoC error, and the energy parameter EP are pseudonymised because the actual values are implementation details. Furthermore, the 2x5 size of the array is simplified and may be more complex in practice. The power demand P_D is either low (L) or high (H), in FIG. 7A. A high value ‘H’ refers to any value higher than L, as determined by an appropriate power threshold of the power demand P_D. The SoC error is between -1 and 1, where 0 represents the SoC being equal to the target, -1 represents the SoC being above the target, and 1 represents the SoC being below the target. The range -1 to 1 of the SoC error represents an optimum region for the SoC. This may correspond to an actual range of 70-90% relative to a target of 80%, for example. This range may be within an upper half of the whole charging range of the traction battery 206. If a variable SoC window is used, the range could drop below the upper half as well. The energy parameter EP in FIG. 7A comprises values which are not shown, and are instead represented as two classes: charging values ‘C’ for selecting the charging mode; or assisting values ‘A’ for selecting the assisting mode. These labels relate to whether the value is above or below a threshold 704 (FIG. 7C) associated with decision block 608. The values of the energy parameter EP maybe either binary values, or may have further different values. FIG. 7C graphically illustrates example values of the energy parameter EP relative to the SoC error, where the energy parameter EP is a variable. The x-axis represents the SoC error, with an arbitrary scale of -1 to 1, where 0 represents the SoC being on target. The y-axis represents the magnitude of the energy parameter EP, in any appropriate units or scale. As shown, the energy parameter EP can optionally vary nonlinearly in dependence on the SoC error. This may be determined through calibration. Their values relative to the threshold 704 are suggestive of the right column of the lookup table of FIG. 7A, or the lookup table of FIG. 7B. FIG. 7C is described in more detail later. As shown in FIG. 7A, when the power demand P_D has a low value L, the energy parameter EP has a first relationship with the SoC error. When the power demand P_D has a higher value H, the energy parameter EP has a second, different relationship with the SoC error. Specifically, this shows 9 that in dependence on the power demand P_D increasing, the SoC is allowed to fall further relative to the target before a charging value C of the energy parameter EP is selected. FIG. 7A shows that when power demand P_D is high (H), a charging value C of the energy parameter EP may be selected in dependence on the SoC falling below a predetermined amount, shown in FIG. 7C with the label 702. The SoC error may be allowed to pass below the target until the SoC is below the target by the predetermined amount 702. The predetermined amount 702 is between 0.5 and 1 of SoC error in FIG. 7A. This corresponds to X% of SoC below the target, wherein X is selected from the range 1 % to 15%, or 1 % to 10%. When the SoC is below the predetermined amount 702, a charging value C of the energy parameter EP is selected. When it is above the predetermined amount 702, optionally by a hysteresis gap, an assisting value A of the energy parameter EP may be selected instead. The values of the target and the predetermined amount 702 may be selected such that the target minus the predetermined amount 702 is a value greater than 50% or greater than 70% of a peak SoC of the traction battery 206. This leaves a large amount of SoC in reserve. This does not necessarily mean that the target always has to be greater than 50% of the peak SoC. In some situations, the target may be lowered below 50% of the peak SoC, for example when allowed by the predicted SoC or SoC window. By contrast, when the power demand P_D is low (L), FIG. 7A shows a different relationship between the SoC error and the energy parameter EP. A charging value C of the energy parameter EP is selected even though the SoC is above the target. This prioritises charging even when the SoC is above-target. Since the fuel cell 204 is energy efficient at low power demands, this represents an energy efficient opportunity for the fuel cell 204 to charge the traction battery 206 to create ‘headroom’ above the SoC target. FIG. 7B presents a one-dimensional lookup table according to an alternative example, in which only the SoC error and not the power demand P_D is taken into account. Therefore, block 602 can be omitted. In this example, the energy parameter EP depends on the SoC error but not the power demand P_D. FIG. 7B shows that a charging value C of the energy parameter EP may be selected in dependence on the SoC falling, while the SoC error indicates that the SoC is below the target. The SoC may be allowed to pass below the target until it is below the target by the predetermined amount 702. At this point, a charging value C of the energy parameter EP is selected. At higher values of the SoC, an assisting value A of the energy parameter EP is selected. In other implementations, an equation or formula may be used to determine the energy parameter EP, instead of a lookup. In further alternative examples, the SoC error itself is the energy parameter EP so the lookup block 606 can be omitted. H In a still further alternative example, the SoC is used as the energy parameter EP, rather than the SoC error. In FIG. 6, decision block 608 receives the selected energy parameter EP, and determines whether the energy parameter EP satisfies a charging condition. The energy parameter EP is dependent on the SoC error and optionally the power demand P_D. Therefore, the charging condition is associated with the SoC error and optionally the power demand P_D. In some, but not necessarily all examples, the decision block 608 comprises determining whether the value of the energy parameter EP is less than a threshold 704 (FIG. 7C). If it is below the threshold 704, the value is a charging value C. If it is above the threshold 704, the value is an assisting value A. The charging condition can be summarised as an entry condition for entering the charging mode, the entry condition comprising the threshold 704. When the energy parameter EP is less than the threshold 704, the entry condition is satisfied and the charging mode is selected. Selection of the 10 charging mode comprises proceeding towards block 626. However, when the energy parameter EP is greater than the threshold 704, the entry condition is not satisfied and the assisting mode is selected instead. Selection of the assisting mode comprises proceeding towards block 618 instead. FIG. 7C graphically illustrates example values of the energy parameter EP of the lookup table of FIG. 7B, in combination with an example position of the threshold 704 of the decision block 608. FIG. 7C illustrates two thresholds 704, 706. This is because the decision block 608 optionally comprises different entry and exit conditions. For example, satisfaction of the exit condition may comprise determining whether the energy parameter EP is less than a second threshold 706, which is the first threshold 704 modified by a hysteresis. The hysteresis gap between the thresholds 704, 706 of the entry and exit conditions delays a return to the assisting mode after the charging mode has been executed, for example. In other words, the second threshold 706 is associated with a lower SoC error magnitude. This advantageously provides more SoC to delay the need for additional charging. When the energy parameter EP falls below the first threshold 704, corresponding to the SoC being less than the target by the predetermined amount 702, the charging condition 608 is satisfied and the charging mode is selected. When the SoC subsequently rises, the control system 400 is configured to compare the energy parameter EP with the second threshold 706, the second threshold 706 corresponding to the SoC being closer to the target than the predetermined amount 702. Then, the charging condition 608 is no longer satisfied and the assisting mode is selected instead. The positions of the thresholds 704, 706 may be determined by calibration. For example, the first threshold 704 could be closer to or further from the target. The second threshold 706 could be at or above the target. The assisting mode is now described, with reference to blocks 610 to 624. These are shown on the left side of the flowchart. The assisting mode is selected when the energy parameter EP has a value A greater than the threshold 704 of decision block 608. For example, the power demand P_D may be high, and the SoC error may indicate that the SoC is at or above the target, or is below the target by less than the predetermined amount 702. Blocks 610 to 624 comprise, among other things, a block 616 which determines a battery assist power. The battery assist power represents how much assistive electrical power the traction battery 206 can provide. This is looked up from a calibratable lookup function by block 616, in dependence on inputs such as the current / predicted SoC error (input block 612) and optionally the power demand P_D (input block 613). These may be the same parameters as received earlier at blocks 602 and 604. The SoC error may be used to determine an SoC error-derived battery assist power. In an implementation, block 616 outputs the minimum of two inputs: 1) the power demand P_D, or 2) an SoC error-derived battery assist power. FIG. 8 graphically illustrates three example calibration shapes (solid line, long dash line, short dash line) for the calibratable lookup function. The x-axis represents SoC error (SoC_e), and the y-axis represents electrical power P. Positive y-values represent assistive electrical power from the traction battery 206. Negative y-values represent battery charging power in which the traction battery 206 is charged by the fuel cell 204. A zero y-value represents a zero battery power request. Positive x-values represent SoC errors where the SoC is below the target. Negative x-values represent SoC errors where the SoC is above the target. In FIG. 8, all three calibrations are variables rather than fixed values. While the SoC is above the target (left side, negative-x), they increase in magnitude proportionally to how far the SoC is above the target. In other words, higher battery assist power is requested if the SoC increases relative to the target while being above the target. Whether they are linear (as shown) or not depends on the implementation. When the SoC is above the target, all three calibrations of FIG. 8 request battery assist power. Furthermore, the amount of battery assist power requested may depend on the magnitude of the above-target SoC error, in other words how far the SoC is above the target. In consequence, the traction battery 206 will dispose of excessive SoC and therefore reduce the amount of electrical power required from the fuel cell 204. This prevents overcharging or insufficient use of the traction battery 206, and reduces fuel consumption by the fuel cell 204. As shown in FIG. 8, the additional charging power may have a nonlinear relationship with the magnitude of the SoC error. When the SoC is below the target (right side, positive-x), the long-dash line function of FIG. 8 requests a fixed amount of battery assist power when the SoC is below the target by less than the predetermined amount 702. The solid line function requests zero power while the SoC is below the target and within a range between the target and the predetermined amount 702. The short dash line function requests battery assist power only when the SoC is above the target. Once the battery assist power has been determined, a sign check is performed at optional decision block 610. Decision block 610 determines whether the battery assist power has a value greater than a threshold of zero. This prevents negative values of the battery assist power, shown as negative y-values in FIG. 8. If decision block 610 determines that the battery assist power is greater than the threshold, the method 600 proceeds to an optional arbitration block 618. The arbitration block 618 receives at least two power requests. The first power request is the battery assist power from block 616. The second power request is an FC-fixed battery power that enables the fuel cell 204 to run at its fixed peak efficiency power P_Emax, determined by blocks 620 and 624. Block 620 determines the FC-fixed battery power, by calculating the power demand P_D minus the peak efficiency power P_Emax of the fuel cell 204 (FIG. 3). For example, if the power demand P_D is 20kW and the peak efficiency power P_Emax of the fuel cell 204 is 5kW, the result would be 15kW. This 15kW value represents how much electrical power the traction battery 206 would need to supply for the power demand P_D to be satisfied, if the fuel cell 204 outputs a fixed power corresponding to the peak efficiency power P_Emax. This power request for the traction battery 206 can be described as fuel-minimising because it allows the fuel cell 204 to operate more efficiently by supplying the difference between the peak efficiency power P_Emax of the fuel cell 204 and the power demand P_D. Block 624 saturates the FC-fixed battery power of block 620 in dependence on a power limit (battery power limit herein). This ensures that if the output of block 620 is high, the battery power limit of the traction battery 206 is respected. Optionally, the power limit is static or variable. If the latter, the power limit may depend on any one or more of: state of charge; state of health; or temperature, of the traction battery 206. The power limit may be reported by a battery management system internal to or external from the control system 400. In summary, the battery assist power and the FC-fixed battery power are different selectable requests dependent on different variables. The battery assist power from block 616 is dependent on the SoC error and optionally on the power demand P_D, but not on the peak efficiency power P_Emax of the fuel cell 204. By contrast, the FC-fixed battery power is dependent on the power demand P_D and on the peak efficiency power P_Emax of the fuel cell 204, but not on the SoC error. The arbitration block 618 compares the power requests, to determine which one is highest. For example, if the SoC error is 0, then some of the calibration curves in FIG. 8 suggests that the battery assist power may be zero. If the FC-fixed battery power is 15kW as set out above, then the arbitration block 618 will select the FC-fixed battery power of 15kW. However, if the SoC error indicates an SoC significantly higher than the target, the battery assist power may have a higher value than 15kW and may be selected instead. In other words, the more ‘full’ the traction battery 206 is, the more likely it is that the battery assist power will be selected. By contrast, the higher the power demand P_D is, the more likely it is that the FC-fixed battery power will be selected. The arbitration block 618 outputs the selected power request to an optional final saturation block 642, which applies a battery power limit as per block 624. The final saturation block 642 outputs a signal 644 indicative of an arbitrated battery power request. The arbitrated battery power 644 is sent to a fuel cell controller 644 for controlling the power output of the fuel cell 204. This may be a function within a controller of the control system 400, for example. To determine a fuel cell power request, the fuel cell controller 644 subtracts the arbitrated battery power from the power demand P_D. Therefore, any power that is not supplied by the traction battery 206 will be supplied by the fuel cell 204. This ensures that the sum of the fuel cell power request and the arbitrated battery power add up to the power demand P_D. The fuel cell controller 644 outputs a control signal 646 to the fuel cell 204 indicative of the fuel cell power request. If the FC-fixed battery power was selected at block 618, then the fuel cell power request may be substantially equal to the peak efficiency power P_Emax of the fuel cell 204. It is a fixed value and will not vary in dependence on the power demand P_D in many conditions. Since the peak efficiency power P_Emax has a low value relative to the rated peak power of the fuel cell 204, it is likely that the fuel cell 204 will often be generating less electrical power than the power demand P_D. Therefore, the traction battery 206 is configured to supply assistive electrical power equal to the FC-fixed battery power, such that the sum of the peak efficiency power P_Emax of the fuel cell 204 and the assistive FC-fixed battery power of the traction battery 206 substantially adds up to the power demand P_D. If the battery assist power was selected at block 618, then the traction battery 206 may supply an amount of power dependent on the SoC error, as shown in FIG. 8 on the positive y-axis. The fuel cell 204 provides the rest of the power. If the FC-fixed battery power from block 620 exceeds the battery power limit of block 624, the FC-fixed battery power may be saturated at the battery power limit. If the saturated FC-fixed battery power is nonetheless selected by arbitration block 618, a power shortfall is avoided because the fuel cell power request is calculated by subtracting the post-saturated FC-fixed battery power from the power demand P_D. Therefore, the fuel cell power request will supply additional power corresponding to the amount of saturation (amount of exceedance of the battery power limit). An advantage is allowing high performance driving, because if the power demand P_D reaches such a high magnitude that the traction battery 206 has reached its power limit, the fuel cell 204 is allowed to increase its electrical power output. Only a slight increase may be required, so the fuel cell 204 may still be operating close to its peak efficiency. In summary, in the assisting mode, the control system 400 outputs a control signal 646 to control the fuel cell 204 to generate the power demand P_D minus an assistive electrical power to be supplied by the traction battery 206. As described above, the assistive electrical power may be arbitrated. As described, the arbitrated assistive electrical power may comprise a selected one of a first assistive power (block 616) in which the proportion of power output by the fuel cell 204 and by the traction battery 206 varies in dependence on the SoC error, or a second assistive power (block 620) in which the power of the fuel cell 204 is fixed (peak efficiency power P_Emax). An advantage of the arbitration (block 618) is that it determines which one of the fuel cell 204 or the traction battery 206 has its power output determined in dependence on an energy management priority. When the battery assist power (block 616) is used, the priority is for the traction battery 206 to increase its assistance to deplete some excess SoC. When the FC-fixed battery power (block 620) is used, the priority is for the fuel cell 204 to minimise its fuel consumption by operating substantially at its most efficient output. The traction battery 206 provides enough assistance that the fuel cell 204 can lower its output to the peak efficiency power. Turning back to decision block 610, if the battery assist power from block 616 has a value of zero or less, the decision block 610 does not proceed to the arbitration block 618. Instead, the decision block 610 outputs a value equal to the FC-fixed battery power of block 620, to the final saturation block 642. This amounts to a rejection of the calculated battery assist power. The charging mode is now described, with reference to blocks 626 to 640. These are shown on the right side of the flowchart. The charging mode is selected when the energy parameter EP has a value C less than the threshold 704 of decision block 608. For example, the SoC error may indicate that the SoC is below the target by at least the predetermined amount 702 (FIG. 7C, 8). For example, the SoC may have fallen below 70% against a target of 75%. The SoC error is 5%. The energy parameter EP falls below the threshold 704 which corresponds to an SoC error of 5%. Blocks 626 to 640 comprise, among other things, a block 634 which determines a battery charging power. The battery charging power represents how much electrical charging power the traction battery 206 requires. This is looked up from a calibratable lookup function by block 634, in dependence on inputs such as the current / predicted SoC error (input block 632) and optionally the power demand P_D (input block 633). These may be the same parameters as received earlier at blocks 602 / 612 and 604 / 613. In FIG. 8, the negative-y values of the three example calibration shapes represent battery charging power. When the SoC is below the target by at least the predetermined amount 702, the three functions of FIG. 8 behave similarly. The amount of battery charging power selected may be fixed or variable. Once the charging mode has been selected at decision block 608, the method 600 proceeds to an optional arbitration block 626. The arbitration block 626 receives at least two battery power requests and selects the minimum of the requests. Three requests are shown. The first power request is the battery charging power from block 634. The second power request is an equipment power demand, determined by block 636. This is described earlier. The third power request is an FC-fixed battery power that enables the fuel cell 204 to run at its fixed peak efficiency power P_Emax, determined by blocks 638 and 640. This may be determined in the same manner as blocks 620 and 624. The earlier description of the FC-fixed battery power applies here. In summary, the battery charging power and the FC-fixed battery power are different requests dependent on different variables. The battery charging power from block 634 is dependent on the SoC error and optionally on the power demand P_D, but not on the peak efficiency power P_Emax of the fuel cell 204. By contrast, the FC-fixed battery power is dependent on the power demand P_D and on the peak efficiency power P_Emax of the fuel cell 204, but not on the SoC error. The arbitration block 626 compares the power requests, to determine which one is the minimum. For example, if the SoC error is to the right of the predetermined amount 702, then the calibration curves in FIG. 8 suggest that the battery charging power will have a negative value and will therefore be selected because it is less than the positive the second and third power requests. However, if the SoC error is closer to the target than the predetermined amount 702 and the charging mode is still selected, then scenarios may occur in which the battery charging power becomes higher than the second or third power request. In such a situation, the arbitration block 626 will select the second or third power request instead, whichever is lowest. The arbitration block 626 outputs the selected power request to a final saturation block 642, which applies a battery power limit as per block 634. The final saturation block 642 outputs a signal 644 indicative of an arbitrated battery power request, to the fuel cell controller 644. To determine the fuel cell power request, the fuel cell controller 644 subtracts the arbitrated battery power from the power demand P_D. If the arbitration block 626 selected the battery charging power of block 634 as the arbitrated battery power, then the fuel cell power request will correspond to the power demand P_D minus the battery charging power. If for example the power demand P_D is positive and the battery charging power is negative, the subtraction of the negative battery charging power will result in a fuel cell power request greater than the power demand P_D. Specifically, the fuel cell power request will comprise the power demand P_D and an additional charging power equal in magnitude to the battery charging power. In consequence, the fuel cell 204 will generate extra electrical power, which will simultaneously provide the demanded power and will charge the traction battery 206 at the required power level. Once the SoC of the traction battery 206 has increased above the threshold 704, or the second threshold 706 of FIG. 7C, the next run time of the method 600 will switch from the charging mode to the assisting mode, at decision block 608. 14 If the FC-fixed battery power is instead selected as the arbitrated battery power at arbitration block 626, the result will be similar to that described earlier in relation to blocks 618, 620, and 624. If the equipment power demand is selected, the battery will supply the equipment power demand while the fuel cell 204 supplies the tractive power demand. It is to be understood that the or each controller 401 can comprise a control unit or computational device having one or more electronic processors (e.g., a microprocessor, a microcontroller, an application specific integrated circuit (ASIC), etc.), and may comprise a single control unit or computational device, or alternatively different functions of the or each controller 401 may be embodied in, or hosted in, different control units or computational devices. As used herein, the term “controller,” “control unit,” or “computational device” will be understood to include a single controller, control unit, or computational device, and a plurality of controllers, control units, or computational devices collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause the controller 401 to implement the control techniques described herein (including some or all of the functionality required for the method(s) described herein). The set of instructions 408 could be embedded in said one or more electronic processors 404 of the controller 401; or alternatively, the set of instructions 408 could be provided as software to be executed in the controller 401. A first controller or control unit may be implemented in software run on one or more processors. One or more other controllers or control units may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller or control unit. Other arrangements are also useful. The, or each, electronic processor 404 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 408. The, or each, electronic memory device 406 may comprise any suitable memory device and may store a variety of data, information, threshold value(s), lookup tables or other data structures, and / or instructions therein or thereon. In an embodiment, the memory device 406 has information and instructions for software, firmware, programs, algorithms, scripts, applications, etc. stored therein or thereon that may govern all or part of the methodology described herein. The processor, or each, electronic processor 404 may access the memory device 406 and execute and / or use that or those instructions and information to carry out or perform some or all of the functionality and methodology described herein. The at least one memory device 406 may comprise a computer-readable storage medium (e.g. a non-transitory or non-transient storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors / computational devices. Examples of the form include, without limitation: a magnetic storage medium (e.g. floppy diskette); optical storage medium (e.g. CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g. EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information / instructions. It will be appreciated that embodiments of the present invention can be realised in any suitable form of hardware, software or a combination of hardware and software. For example, it is contemplated that the present invention is not limited to being implemented by way of programmable processing devices, and that at least some of, and in some embodiments all of, the functionality and or method steps of the present invention may equally be implemented by way of non-programmable hardware, such as by way of non-programmable ASIC, Boolean logic circuitry, etc. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. The blocks illustrated in FIG. 6 may represent steps in a method and / or sections of code in the computer program 408. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Claims

1. A control system for controlling a fuel cell and an electrical energy storage means of a vehicle, the control system comprising one or more processors collectively configured to:receive a power demand;receive an energy parameter dependent on stored electrical energy in the electrical energy storage means;determine whether the energy parameter satisfies a charging condition associated with the stored electrical energy SoC in the electrical energy storage means relative to a target; andoutput a control signal configured to, in dependence on the energy parameter satisfying the charging condition, control the fuel cell to generate the power demand and an additional charging power in addition to the power demand, to charge the electrical energy storage means.

2. The control system of claim 1, wherein the target comprises a value selected from the range 60% to 95% of a peak electrical energystorage capacity of the electrical energy storage means.

3. The control system of claim 2, wherein the target comprises a value selected from the range 80% to 90% of a peak electrical energystorage capacity of the electrical energy storage means.

4. The control system of claim 1, 2, or 3, wherein the additional charging power of the control signal is dependent on a magnitude of thepower demand.

5. The control system of any preceding claim, wherein a magnitude of the control signal has a nonlinear relationship with a magnitude ofan error between the stored electrical energy and the target.

6. The control system of any preceding claim, wherein the charging condition comprises a threshold 704 allowing the stored electricalenergy to be below the target before the charging condition is satisfied, and wherein determining whether the charging condition is satisfied comprises determining whether the energy parameter is past the threshold.

7. The control system of claim 6, wherein the threshold corresponds to the stored electrical energy being X% below the target, and whereinX is selected from the range 1 % to 15%.

8. The control system of any preceding claim, wherein the charging condition is an entry condition for supplying the additional chargingpower, and wherein the control system is further configured to determine whether the energy parameter satisfies an exit condition for ceasing to supply the additional charging power, and wherein the exit condition is different than the entry condition.

9. The control system of claim 8, wherein the exit condition comprises a threshold different than a threshold of the entry condition.

10. The control system of claim 9, wherein the threshold of the exit condition is associated with the stored electrical energy being equal toor greater than the target, or less than the target but closer to the target than when the entry condition is satisfied.

11. The control system of claim 8, 9, or 10, wherein in dependence on non-satisfaction of the entry condition, the control signal is configured to control the fuel cell to generate a peak efficiency power different than the power demand, and wherein the electrical energy storage means is configured to reduce a difference between the peak efficiency power and the power demand.

12. The control system of any preceding claim, wherein the electrical energy storage means has a peak electrical energy storage capacity no less than 5% of a peak electrical energy that the fuel cell is capable of producing in one hour.

13. A system comprising the control system of any preceding claim, and the fuel cell and the electrical energy storage means.1614.A vehicle 1 comprising the system of claim 13 or the control system of any one of claims 1 to 12.

15. A method of controlling a fuel cell and an electrical energy storage means of a vehicle 1, the method comprising:receiving a power demand;receiving an energy parameter dependent on stored electrical energy in the electrical energy storage means;determining whether the energy parameter satisfies a charging condition associated with the stored electrical energy in the electrical energy storage means relative to a target; andoutputting a control signal configured to, in dependence on the energy parameter satisfying the charging condition, control the fuel cell to generate the power demand and an additional charging power in addition to the power demand, to charge the electrical energy storage means.

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

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