energy storage system

Abstract

The invention relates to an energy storage system for storing electrical energy, in particular in a vehicle, comprising at least one energy accumulator (20), at least one source (14) of electrical energy for charging the at least one energy accumulator (20) in a charging mode, at least one consumer (16) of electrical energy to be fed with electrical energy by the at least one energy accumulator (20) in a feeding mode, at least one first power converter (22) for converting an input charging power provided by the at least one source (14) of electrical energy into an output charging power delivered to the at least one energy accumulator (20), and a power converter control unit (26). The power converter control unit (26) is designed to set a loss power generated in the at least one power converter (22) in dependence on a state of charge of the at least one energy accumulator (20) in the charging mode.

Description

Technical Field

[0001] The present invention relates to an energy storage system, which can store electrical energy, for example in a vehicle, particularly an electrically powered vehicle, to feed different electrical energy consumers during their operation, and the energy storage system can be charged by at least one source of electrical energy. Background Technology

[0002] In these vehicles, especially electrically powered vehicles, so-called supercapacitors (SCAPs) are also used to store electrical energy. Compared to conventional battery storage devices, supercapacitors have the advantages that they can be charged very quickly and the energy stored in them can be released very quickly. Supercapacitors are therefore particularly suitable for use in conjunction with consumers that experience high energy demands for very short periods, or in conjunction with sources that can provide a large amount of energy in a short time, i.e., sources with high charging power.

[0003] Especially in vehicle applications, the energy to be stored in the accumulator is additionally provided through energy recovery during braking. For example, in electrically powered vehicles, the electric traction motor acts as a generator during braking, converting the kinetic energy lost during braking into electrical energy and providing that electrical energy to charge the accumulator. In actuators fed by such accumulators, such as steering actuators, a short-term operating state may also occur where the actuator releases electrical energy and feeds it into the vehicle's electrical grid, which in turn feeds it into the accumulator.

[0004] In order to receive energy in an energy storage system, such as during braking or via an actuator, without the potential danger of damage from overcharging, it is necessary to design the energy storage unit, or at least a portion thereof, such that its overvoltage range is not reached even at maximum charging power. This typically requires unnecessary oversize of the energy storage unit during feed operation, i.e., when the energy is fed from the energy storage unit to a consumer, resulting in additional costs and structural space requirements. Summary of the Invention

[0005] The objective of this invention is to provide an energy storage system for storing electrical energy, particularly in vehicles, and a method for operating the energy storage system, wherein the energy storage system and method can avoid over-sizing of the energy storage device without the risk of overcharging during charging operation.

[0006] According to a first aspect of the invention, this task is solved by an energy storage system for storing electrical energy, particularly in vehicles, comprising:

[0007] -At least one energy storage device

[0008] - At least one source of electrical energy, said source being used to charge said at least one energy storage device during charging operation.

[0009] - At least one energy consumer that is fed electrical energy by the at least one energy storage device during operation.

[0010] - At least one first power converter, the first power converter being configured to convert input charging power provided by the at least one source of electrical energy into output charging power delivered to the at least one energy storage device.

[0011] - A power converter control unit, wherein the power converter control unit is configured to adjust the power loss generated in the at least one power converter according to the charging state of the at least one energy storage device during the charging operation.

[0012] In the energy storage system constructed according to the invention, by adapting the power loss of the at least one power converter to the charging state of the at least one energy storage device, it is possible that if the energy is no longer available or fully available for further charging of the at least one energy storage device, the energy or charging power originally provided can be controlled by adjusting the higher power loss during charging operation, i.e., converting it into heat and dissipating it to the environment. This prevents the at least one energy storage device from being overcharged, for example, in a fully or nearly fully charged state at a relatively high charging power. It is not necessary to determine the size of the at least one energy storage device in such a way that overcharging of the energy storage device can be eliminated in principle. The at least one energy storage device can be sized such that it is suitable for providing the energy required by the consumer of the at least one electrical energy during feed operation when the at least one energy storage device feeds the consumer.

[0013] In order for the at least one electrical energy consumer to be fed by the at least one energy storage device, at least one second power converter may be provided, the second power converter being used to convert the input feed power provided by the at least one energy storage device into the output feed power delivered to the at least one electrical energy consumer.

[0014] In a structurally advantageous design, the at least one first power converter can provide the at least one second power converter, such that the first power converter operates bidirectionally and can guide energy flow in both directions between the at least one energy storage device and the at least one energy consumer.

[0015] In order to provide a suitable charging voltage for charging the at least one energy storage device during charging operation, the at least one first power converter can be a DC / DC converter. It should be noted that such a DC / DC converter can operate bidirectionally and therefore can also provide a suitable feed voltage for the at least one energy consumer during feed operation. Of course, the at least one second power converter can also be configured as a DC / DC converter if provided as a separate component.

[0016] To prevent undesirable charging or discharging of the at least one energy storage device when it should not be charged or should not be further charged and when the at least one energy consumer should not be fed by the at least one energy storage device, at least one circuit breaker can be provided, wherein the at least one first power converter is selectively coupled to or decoupled from the at least one energy storage device via the at least one circuit breaker. For example, the at least one circuit breaker can be a semiconductor switch, especially a MOSFET, or configured as a so-called Q diode.

[0017] To ensure that, during charging operation, the energy flow to the at least one energy storage device decreases as the charging of the at least one energy storage device increases, it is proposed that the power converter control unit be configured to operate the at least one power converter with a loss power that increases with the charging state of the at least one energy storage device during charging operation. This means that as the charging of the at least one energy storage device increases, a corresponding increase in the portion of energy supplied by the at least one electrical energy source is dissipated.

[0018] To ensure that the at least one energy storage device can be charged as quickly as possible, for example, when it is discharged to a large extent or almost completely, without entering an overcharged state, the power converter control unit may be configured to operate the at least one power converter with a loss power that increases with the charging state of the at least one energy storage device only when the at least one energy storage device is in a charging state above a charging state threshold during charging operation.

[0019] Furthermore, the power converter control unit can be configured to operate the at least one power converter with minimal power loss during the charging operation, when the at least one energy storage device is in a charging state below the charging state threshold. Operating the at least one power converter with minimal power loss results in utilizing the maximum portion of the energy or charging power provided by the at least one electrical energy source for charging the at least one energy storage device.

[0020] In order to efficiently utilize the energy stored in or released by the at least one energy storage device during feed operation, it is proposed that the power converter control unit be configured to operate the at least one power converter with minimal power loss during feed operation.

[0021] In a configuration where the energy storage system is designed for very rapid charging and discharging, the at least one energy storage device may have at least one supercapacitor, preferably multiple supercapacitors connected in parallel or / and in series with each other.

[0022] According to another aspect of the invention, the task is solved by a method for operating an energy storage system for storing electrical energy, particularly an energy storage system constructed according to the invention, wherein the energy storage system has at least one energy storage device to be charged during charging operation and at least one power converter for converting input charging power provided by at least one source of electrical energy into output charging power delivered to the at least one energy storage device, wherein the method includes measures:

[0023] a) Determine the charging state of the at least one energy storage device.

[0024] b) During charging operation, the at least one power converter is operated with a loss power set depending on the charging state determined in measure a).

[0025] In order to adapt the energy introduced into the at least one energy storage device during charging operation to the charging state of the energy storage device, it can be configured, in measure b), to operate the at least one power converter such that the power loss increases with the increasing charging state.

[0026] In order to achieve the fastest possible charging of the energy storage device while avoiding overcharging, using the method according to the invention, it is proposed that, in measure b), the at least one power converter be operated with increased loss power only when the charging state of the at least one energy storage device, as determined in measure a), is above the charging state threshold.

[0027] In particular, it can be further configured such that, in measure b), when the charging state of the at least one energy storage device determined in measure a) is below the charging state threshold, the at least one power converter operates with minimal power loss.

[0028] To limit the impact of power loss, the at least one power converter may have at least two semiconductor switching elements, preferably MOSFET switching elements, that alternately switch between on and off states. In measure b), the power loss of the at least one power converter can then be adjusted by changing the switching duration of the semiconductor switching elements used to switch between the on and off states and / or by introducing or / and changing the overlap duration, wherein the overlap duration is the duration during which the two semiconductor switching elements are not in their off state.

[0029] The present invention further relates to an onboard electrical grid system for a vehicle, comprising an energy storage system constructed according to the invention, and further comprising at least one energy consumer to which electrical energy is to be fed by the energy storage system, and at least one energy source for charging the at least one energy storage device. Such an onboard electrical grid system can preferably be operated using the method according to the invention. Attached Figure Description

[0030] The invention will now be described in detail with reference to the accompanying drawings. Wherein:

[0031] Figure 1 A schematic diagram of an onboard electrical system for a vehicle is shown.

[0032] Figure 2a ) shows from Figure 1 The energy flow from the energy storage device of the vehicle-mounted electrical grid system to the energy storage device with time-recorded data;

[0033] Figure 2b This shows the charging status of the accumulator with respect to time records;

[0034] Figure 2c ) shows Figure 1 The power loss of the power converter in the vehicle-mounted electrical grid system with respect to time records;

[0035] Figure 3 This shows the relationship between the charging state of the energy storage device and the power loss of the power converter configured for the energy storage device. Detailed Implementation

[0036] exist Figure 1The schematic diagram illustrates an onboard electrical network system 10 for a vehicle. The onboard electrical network system 10 includes an energy storage system 12 that can store energy supplied by at least one electrical energy source 14 and can feed energy to at least one electrical energy consumer 16. For example, in an electrically operated vehicle, the at least one electrical energy source may be a generator, such as one or more electric motors that operate as generators during vehicle braking. The electrical energy source 14 may also have a grid connection, by which the electrically operated vehicle can be connected to a voltage grid or charging station. The at least one electrical energy consumer 16 may, for example, include a traction motor of the electrically operated vehicle, an electric steering assist system, or other consumers present in the vehicle, such as an electrically operated heating device, an audio / communication system, or the like.

[0037] The energy storage system 12, as a central component, has an energy storage unit 20 comprising multiple supercapacitors 18, as shown in the design example. The supercapacitors 18 may be connected in parallel and / or in series with each other. The energy storage system 12 may, for example, have multiple such energy storage units 20 comprising one or more supercapacitors 18. Furthermore, the energy storage system 12, or another energy storage system present in the vehicle electrical grid system 10, may have one or more battery storage units so that a sufficiently large amount of energy can be stored, especially for the operation of the electric traction motor of an electrically powered vehicle.

[0038] The energy storage system 12, which includes multiple supercapacitors 18, further includes a power converter 22 configured as a DC / DC converter and a circuit breaker 24 positioned, for example, between the power converter 22 and the energy storage unit 20. The power converter 22 and the circuit breaker 24 are under the control of the control unit 26, so that their operation or state can be set as described below. For this purpose, information representing the charging state of the energy storage unit 20, such as information representing the voltage of the energy storage unit 20 and / or the current from or to the energy storage unit 20, and information representing, for example, the on-board grid voltage present in the on-board grid 28 coupled to the energy storage system 12 in the on-board grid system 10, are transmitted to the control unit 26.

[0039] In operation where at least one energy consumer 16 of the vehicle-mounted electrical network 28 is fed by the energy storage system 12, the circuit breaker 24 is in principle in its closed position, thereby establishing a conductive connection between the energy storage 20 and the power converter 22, which is configured as a DC / DC converter. If the power converter 22 is a variable power converter whose output power or output voltage is variable, the control unit 26 can operate the power converter 22 such that it converts the voltage applied to the connection region 30 on its energy storage side of the energy storage 20 into a voltage to be set or suitable for the vehicle-mounted electrical network 28 on its connection region 32 on its vehicle-mounted electrical network side. It should be considered, especially in the construction of the energy storage 20 including multiple supercapacitors 18, that the voltage provided by the energy storage 20 decreases with the discharge, thereby allowing the power converter 22 to be operated to provide a substantially constant voltage on its connection region 32 on its vehicle-mounted electrical network side, so as to provide a substantially constant output voltage of the energy storage system 12 to the vehicle-mounted electrical network 28. In principle, the power converter 22 may also be configured as an invariant power converter, thereby having a defined conversion ratio for the voltage on the connection area 30 on the energy storage side and on the connection area 32 on the vehicle power grid side.

[0040] During charging operation, i.e., when the energy storage device 20 is partially or fully discharged, the charging power provided by the at least one electrical energy source 14, i.e., during braking of an electrically operated vehicle, can be used to charge the energy storage device 20. In this state, the circuit breaker 24 is also operated in its closed position to establish a connection between the connection region 30 on the energy storage side of the power converter 32 and the energy storage device 20. The charging power provided by the at least one electrical energy source 14 is converted by the power converter 22 in such a way that a suitable energy flow for the energy storage device 20 is achieved between the connection region 30 on the energy storage side and the energy storage device 20, for example, by providing a suitable charging voltage for the energy storage device 20 on the connection region 30 on the energy storage side.

[0041] Next, referring to Figures 2 and 3, the operation for charging and discharging the energy storage device 20 will be described. Here, Figure 2a This shows the energy flow between the energy storage device 20 and the on-board electrical network 28, that is, the power P applied for discharging or charging the energy storage device, respectively. L / E Here, between times t0 and t1, the vehicle electrical system 10 is in a state in which, for example, for vehicle acceleration, for providing steering assistance, or the like, the energy storage device 20 discharges and the energy released by the energy storage device is introduced, for example, into the traction motor or actuator of the electrically operated vehicle, that is, in principle into the at least one energy consumer 16. This is achieved by means of curve K1 in Figure 2a As described in ().

[0042] At time t1, for example, when the acceleration process ends and the braking process begins, during the braking process, the traction motor of the electrically operated vehicle acts as a generator and thereby provides charging power in the on-board electrical network 28 for charging the energy storage device 20.

[0043] Curve K2 describes another operating state in which no energy is extracted from the energy storage 20 and fed into the vehicle electrical network 28 between times t0 and t1. At time t1, the energy recovery state begins, in which an energy flow occurs towards the energy storage 20 based on the charging power generated in the vehicle electrical network 28.

[0044] Figure 2b The state of charge (SOC) of the energy storage device 20 or its supercapacitor 18 is described. As illustrated by curve K1, which represents the process described by curve K3, the energy storage device 20 first discharges between times t0 and t1 and then recharges from time t1 using the charging power provided in the onboard electrical network 28.

[0045] Configured through Figure 2a The curve K2 in the diagram represents the operation, in Figure 2b Curve K4 in the figure describes the charging state of the energy storage device 20. Since there is essentially no energy extracted from the energy storage device 20 between time t0 and t1, the charging state of the energy storage device 20 remains essentially unchanged.

[0046] As the system transitions to charging operation, the charge in the energy storage device increases. Because, unlike the operation under curves K1 and K3, in the operation under curves K2 and K4, the energy storage device 20 is essentially not discharged and therefore, for example, almost fully charged, there is a problem in principle, such as... Figure 2b The branch K4' of curve K4 in the diagram describes what may be exceeded during the charging operation of the accumulator 20, specifically the overpressure limit S. O And thus may reach the overpressure range, which in turn may lead to damage to one or more of the supercapacitors 18 or the potential danger of charging to a voltage that is, in principle, permissible or intended to be set in the vehicle electrical network 28.

[0047] In order to address this problem, Figure 1The energy storage system 12 described herein is configured such that the power loss of the power converter 22 is variable by means of control via the control unit 26. This means that a portion of the energy directed through the power converter 22 can be adjusted, and this portion can be released either toward the energy storage unit 20 or toward the vehicle electrical network 28. The higher the power loss of the power converter 22 or the lower its efficiency, the greater the portion of energy dissipated into the environment, for example as heat, through the power converter 22 and thereby not introduced into the energy storage unit 20 or the vehicle electrical network 28.

[0048] Figure 2c For the two previous references Figure 2a The operation described in 2a) and 2b) illustrates how variations in the power loss or efficiency of the power converter 22 can affect the energy flow to the energy storage 20 during charging operation. Figure 2c In the example described in (), until time t1 is reached, that is, during the feed operation of the energy storage system 12, it is set to operate the power converter 22 such that the power loss P is reduced. V It has a minimum value P Vmin In this way, it is ensured that the energy stored in the energy storage device 20 can be used as efficiently as possible to operate the at least one electrical energy consumer 16.

[0049] At time t1, that is, when transitioning to charging operation, the power loss P depends on the charging state of the energy storage device 20. V Power converter 22 is operated, wherein the dependency is defined such that as the state of charge of energy storage 20 increases, the power loss of power converter 22 also increases or the efficiency of the power converter decreases. This means that the higher the state of charge of energy storage 20 or its supercapacitor 18, the lower the power loss P. V It is also relatively large, and correspondingly, during charging operation, a larger share of the energy provided by the at least one electrical energy source 14 is dissipated and a smaller share is directed toward the energy storage unit 20.

[0050] Figure 2c The power converter 22 is configured with different power loss settings on the power converter using curves K5 or K6, which are represented by curves K1, K3 or K2, K4. When the state of charge of the energy storage 20 is relatively small during the transition from discharging based on the energy storage 20 to charging operation at time t1, the power converter 22 initially operates with a smaller power loss P. V Operation. The power converter 22 only operates at higher power loss when the charging state or charging power is increased.

[0051] Curve K6 shows that when the energy storage 20 is already relatively fully charged upon entering charging operation, the power loss on the power converter 22 is significantly higher, resulting in a larger share of energy being dissipated, and as in Figure 2b As described by branch K4'' of curve K4 in the diagram, a smaller energy flow toward the accumulator 20 can prevent the overpressure limit S from being exceeded. O And thus prevents it from entering the overvoltage range or overcharge range.

[0052] For example, the power loss or efficiency of a power converter 22 configured as a DC / DC converter can be affected in different ways. Typically, such power converters are constructed with switching transistors, especially MOSFET switching elements, which are connected to each other in the form of a half-bridge with two transistors and are intended to operate such that neither transistor is conducting at any given time, in order to avoid a dead time for cross-current, during which neither transistor is conducting during the conversion process.

[0053] The power loss or efficiency in such a circuit can be altered by influencing the duration of the transition between the off-state and on-state of these transistors. The faster the transition occurs, the lower the power loss. A slower switching process prolongs the duration of poor transistor conduction during such a transition. Therefore, by affecting the duration of the transition, the power loss can be specifically influenced. In the design of such a semiconductor switch as a MOSFET switching element, the duration of the transition can be affected by the magnitude of the gate current. A decrease in gate current leads to a delay in the switching process and, correspondingly, an increase in power loss.

[0054] The power loss occurring in such a power converter 22 can also be affected by shortening or eliminating the dead time that is generally present during the switching process, resulting in a brief period during which both transistors of a half-bridge are simultaneously turned on and generate power loss through the cross current that occurs there. The degree and duration of the simultaneous conduction of the two transistors of a half-bridge also limit the degree of power loss. The shortening or elimination of the dead time can be achieved, for example, by reducing the speed of the switching process according to the previously described operating method, so that the previously non-conducting transistor begins to transition to its conducting state before the previously connected transistor is in its non-conducting state.

[0055] Figure 3 Describe the power loss P under the State of Charge (SOC) setting dependent on the energy storage 20. V Alternative operating methods at that time. Figure 3As shown by curve K7, in this configuration, the charge state threshold S, which allows the energy storage device 20 to be in a state of maximum charge, is reached, for example, 80% to 90%. V The power converter 22 operates with minimal power loss P Vmin Running. Upon reaching the charging state threshold S... V At this time, the power converter 22 operates in such a way that its power loss P V It increases with the increase of the State of Charge (SOC). This is reflected in curves K8, K9, and K... 10 As shown, the power loss P of the power converter 22 can be predetermined. V Different possibilities exist for adapting to the State of Charge (SOC) of the energy storage device 20. For example, in principle, different (curve) trends can be selected or adjusted. At the state of charge threshold S... V The power loss P in the above State of Charge (SOC) V The (curve) trend can also be selected, for example, by depending on the charging power provided in the vehicle-mounted electrical network 28 or the vehicle-mounted electrical network voltage present during charging operation, thereby, for example, being configured with a higher charging power, exceeding the state-of-charge threshold S. V Firstly, when choosing a charging power, the power loss P is also considered when the charging power is relatively small. V A stronger rise.

[0056] By adapting the power loss of the power converter 22, which is configured as a DC / DC converter, according to the invention, it is possible that the energy supplied in the vehicle electrical network 28 during charging operation is introduced into the energy storage 20 only to such an extent that it is ensured that the energy storage 20 is not overcharged, and thus damage to one or more supercapacitors 18 of the energy storage 20 is avoided due to overcharging. In particular, it is also possible to limit the charging of the energy storage 20 to such an extent that it is not fully charged, so that in the event of a subsequent short-term energy release, for example, through the energy released during the feeding operation of an actuator fed by the energy storage 20, the energy can be at least partially received in the energy storage 20.

[0057] The variation in power loss or efficiency of the power converter 22 can be adapted, for example, to the energy or charging power supplied in the vehicle electrical network 28. In principle, it may also be configured such that the power converter 22 is switchable only between two states with different power loss or different efficiencies, so that on the one hand, the power converter 22 always operates with a smaller or minimum power loss during feed operation, and on the other hand, it operates with a higher power loss or lower efficiency when the state of charge threshold is exceeded during charging operation, and similarly operates with a lower power loss or higher efficiency in other situations. In the operation according to the invention, multiple state of charge thresholds may also be set, for example, to allow a stepped transition between different power loss values ​​in the power converter 22 when the state of charge of the energy storage device 20 changes or approaches an overpressure limit.

Claims

1. An energy storage system for storing electrical energy, particularly in vehicles, said energy storage system comprising: - At least one energy storage device (20). - At least one source of electrical energy (14), said source being used to charge the at least one energy storage device (20) during charging operation. - At least one energy consumer (16) that is to be fed electrical energy by the at least one energy storage device (20) during the feeding operation. - At least one first power converter (22) for converting input charging power provided by the at least one source of electrical energy (14) into output charging power delivered to the at least one energy storage device (20). - Power converter control unit (26), wherein the power converter control unit (26) is configured to, during the charging operation, adjust the power loss generated in the at least one power converter (22) according to the state of charge (SOC) of the at least one energy storage device (28). V ).

2. The energy storage system according to claim 1, characterized in that, At least one second power converter is provided, which is used to convert the input feed power provided by the at least one energy storage device (20) into the output feed power supplied to the at least one energy consumer (16).

3. The energy storage system according to claim 2, characterized in that, The at least one first power converter (22) provides the at least one second power converter.

4. The energy storage system according to any one of claims 1-3, characterized in that, The at least one first power converter (22) is a DC / DC converter.

5. The energy storage system according to any one of claims 1-4, characterized in that, At least one circuit breaker (24) is provided, wherein the at least one first power converter (22) can be selectively coupled to or decoupled from the at least one energy storage device (20) via the at least one circuit breaker (14).

6. The energy storage system according to any one of claims 1-5, characterized in that, The power converter control unit (26) is configured to, during the charging operation, increase the power loss (P) as the state of charge (SOC) of the at least one energy storage device (20) increases. V ) to operate the at least one power converter (22).

7. The energy storage system according to claim 6, characterized in that, The power converter control unit (26) is configured to, during the charging operation, only operate at a charging state threshold (S) of the at least one energy storage unit (20). V In the above-mentioned state of charge (SOC), the power loss (P) increases with the state of charge (SOC) of the at least one energy storage device (20). V ) to operate the at least one power converter (22).

8. The energy storage system according to claim 7, characterized in that, The power converter control unit (26) is configured to, during the charging operation, determine the charging state threshold (S) of the at least one energy storage device (20). V At a state of charge (SOC) below 1000, with minimal power loss (P0). Vmin ) to operate the at least one power converter (22).

9. The energy storage system according to any one of claims 1-8, characterized in that, The power converter control unit (26) is configured to operate with minimal power loss (P) during the feed operation. Vmin ) to operate the at least one power converter (22).

10. The energy storage system according to any one of claims 1-9, characterized in that, The at least one energy storage device (20) has at least one supercapacitor (18), preferably multiple supercapacitors (18) connected in parallel or / and in series with each other.

11. A method for operating an energy storage system (12) for storing electrical energy, particularly an energy storage system (12) according to any one of claims 1-10, wherein, The energy storage system (12) has at least one energy storage unit (20) to be charged during charging operation and at least one power converter (22) for converting input charging power provided by at least one source of electrical energy (14) into output charging power delivered to the at least one energy storage unit (20), wherein the method includes measures: a) Determine the state of charge (SOC) of the at least one energy storage device (20). b) During charging operation, the power loss (P) is adjusted based on the state of charge (SOC) determined in measure a). V ) to operate the at least one power converter (22).

12. The method according to claim 11, characterized in that, In measure b), the at least one power converter (22) is operated such that the power loss (P) V It increases with increasing state of charge (SOC).

13. The method according to claim 12, characterized in that, In measure b), the state of charge (SOC) of the at least one energy storage device (20) determined in measure a) is at the state of charge threshold (S). V Only when the power loss (P) is above a certain level will the increased power loss (P) be applied. V ) to operate the at least one power converter (22).

14. The method according to claim 13, characterized in that, In measure b), when the state of charge (SOC) of the at least one energy storage device (20) determined in measure a) is at the state of charge threshold (S... V When the power loss is below ) or less, the minimum power loss (P) is achieved. Vmin ) to operate the at least one power converter (22).

15. The method according to any one of claims 11-14, characterized in that, The at least one power converter (22) has at least two semiconductor switching elements, preferably MOSFET switching elements, that alternately switch between an on state and an off state, and in measure b), the power loss of the at least one power converter (22) is adjusted by changing the switching duration of the semiconductor switching elements used to switch between the on state and the off state and / or by introducing or / and changing the overlap duration, wherein the overlap duration is the duration during which the two semiconductor switching elements are not in their off state.

16. An on-board electrical grid system for a vehicle, the on-board electrical grid system comprising an energy storage system (12) according to any one of claims 1-10, further comprising at least one consumer (16) of electrical energy to be fed by the energy storage system (12) and at least one source (14) of electrical energy for charging the at least one energy storage device (20).

17. The vehicle-mounted electrical grid system according to claim 16, characterized in that, The energy storage system (12) operates in accordance with the method according to any one of claims 11-15.

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