METHOD OF OPERATING AN ELECTRIC VEHICLE AND ELECTRIC VEHICLE

Abstract

In a method for operating an electric vehicle, including a first energy storage device (e.g., a rechargeable battery storage device), a second energy storage device (e.g., a double-layer capacitor device), an energy supply unit which provides energy, for charging the first and/or second energy storage device, and a first electrical consumer connected to the second energy storage device via an intermediate circuit, the first energy storage device is connected to the energy supply unit via a bidirectional converter unit, the second energy storage device is connected to the energy supply unit, a first power flows from the first energy storage device to the second energy storage device if an intermediate circuit voltage falls below a definable voltage, and a second power flow from the second to the first energy storage device is prevented.

Claims

1-15. (canceled)

16. A method for operating an electric vehicle that includes a first energy storage device, a second energy storage device, an energy supply unit adapted to provide energy to charge the first energy storage device and/or the second energy storage device, and a first electrical consumer connected to the second energy storage device via an intermediate circuit, the first energy storage device being connected to the energy supply unit via a bidirectional converter unit, the second energy storage device being connected to the energy supply unit, comprising: delivering a first power flow from the first energy storage device to the second energy storage device in response to an intermediate circuit voltage in the intermediate circuit falling below a predefinable voltage; and preventing a second power flow from the second energy storage device to the first energy storage device.

17. The method according to claim 16, wherein the electric vehicle is arranged as a driverless, mobile assistant system for an intralogistics application, the first energy storage device includes a rechargeable battery storage device, the second energy storage device is chargeable and dischargeable, includes a double-layer capacitor device, and/or is chargeable and dischargeable faster than the first energy storage device, the energy supply unit is adapted to provide energy periodically to charge the first energy storage device and/or the second energy storage device, the first electrical consumer includes a drive device for travel movement of the vehicle, a lifting device, and/or a handling device, and the second power flow is prevented from the second energy storage device to the first energy storage device at any given time.

18. The method according to claim 16, energy is supplied to the energy supply unit with or without contact and/or energy is supplied to the energy supply unit periodically during a trip.

19. The method according to claim 16, wherein the second energy storage device includes an end-of-charge voltage and an end-of-discharge voltage, and a value of the predefinable voltage is greater than a value of the end-of-discharge voltage and less than a value of the end-of-charge voltage.

20. The method according to claim 16, wherein a value of the predefinable voltage depends on an average power required by the first electrical consumer.

21. The method according to claim 16, wherein a value of the predefinable voltage is varied dynamically during operation of the vehicle.

22. The method according to claim 16, wherein a value of the predefinable voltage is equal to a quotient of a maximum power required by the first electrical consumer and a maximum permissible current for the first electrical consumer.

23. The method according to claim 16, wherein the second power flow is prevented by a diode arranged between the second energy storage device and the converter unit.

24. The method according to claim 16, wherein an actual value of a charge-reversal current flowing between a first connection point and a second connection point, the first connection point being connected to the energy supply unit and the converter unit, the second connection point being connected to the second energy storage device and the electrical consumer, and wherein the second power flow is prevented by deactivating the converter device, at least for charging the first energy storage device, in response to a detected actual value of the charge-reversal current falls below a minimum charge-reversal current limit value.

25. The method according to claim 24, wherein the actual value of the charge-reversal current flowing between the first connection point and the second connection point is device by a current-measurement device, and wherein the minimum charge-reversal current limit value is not negative.

26. The method according to claim 16, wherein an actual value of a charge-reversal current flowing between a first connection point and a second connection point is detected, the first connection point being connected to the energy supply unit and the converter unit, the second connection point being connected to the second energy storage device and the electrical consumer, an actual value of the intermediate circuit voltage is detected, the converter includes a throttle, a DC voltage converter, and a regulator with a cascade controller, the second power flow being prevented in that the cascade controller sets or regulates a converter voltage present at the DC voltage converter as a function of detected actual values of the charge-reversal current and the intermediate circuit voltage to the extent that a negative charge-reversal current is prevented.

27. The method according to claim 26, wherein the actual value of the charge-reversal current is detected by a current-measurement device, and the actual value of the intermediate circuit voltage is detected by a voltage measurement.

28. The method according to claim 26, wherein the cascade controller includes a current regulator, a superimposed voltage regulator, and a limiter arranged between the voltage regulator and the current regulator, and the limiter limits a target value for the charge-reversal current to values greater than a minimum charge-reversal current limit value.

29. The method according to claim 28, wherein the minimum charge-reversal is not negative.

30. The method according to claim 19, wherein the first power flow occurs when the intermediate circuit voltage is less than the end-of-charge voltage and a voltage in the first energy storage device is above a predefinable minimum voltage.

31. The method according to claim 16, wherein the vehicle includes further electrical consumers, all of the electrical consumers being connected to the second energy storage device via the intermediate circuit.

32. The method according to claim 31, wherein all of the consumers are connected to the second energy storage device via the intermediate circuit in parallel.

33. An electric vehicle, comprising: a first energy storage device; a second energy storage device; an energy supply unit adapted to provide energy to charge the first energy storage device and/or the second energy storage device; a first electrical consumer connected to the second energy storage device via an intermediate circuit; wherein the first energy storage device is connected to the energy supply unit via a bidirectional converter unit; wherein the second energy storage device is connected to the energy supply unit; and wherein the vehicle is configured to deliver a first power flow from the first energy storage device to the second energy storage device in response to an intermediate circuit voltage in the intermediate circuit falling below a predefinable voltage and to prevent a second power flow from the second energy storage device to the first energy storage device.

34. The vehicle according to claim 33, wherein the vehicle is arranged as a driverless, mobile assistance system for an intralogistics application, the first energy storage device includes a rechargeable battery storage device, the second energy storage device is chargeable and dischargeable, includes a double-layer capacitor device, and/or is chargeable and dischargeable faster than the first energy storage device, the energy supply unit is adapted to provide energy periodically to charge the first energy storage device and/or the second energy storage device, and the vehicle is adapted to prevent the second power flow from the second energy storage device to the first energy storage device at any given time.

35. The vehicle according to claim 33, wherein the vehicle is adapted to perform a method that includes: delivering the first power flow from the first energy storage device to the second energy storage device in response to the intermediate circuit voltage in the intermediate circuit falling below the predefinable voltage; and preventing the second power flow from the second energy storage device to the first energy storage device.

36. The vehicle according to claim 33, wherein the energy supply unit includes a controllable power source, and/or the first energy storage device is arranged on the electric vehicle separably and replaceably.

37. The vehicle according to claim 33, wherein the first energy storage device includes an overvoltage protection, an undervoltage protection, and/or an overcurrent protection by a current measurement and/or voltage measurement.

38. The vehicle according to claim 33, wherein the first energy storage device includes an overtemperature protection by a temperature measurement.

39. The vehicle according to claim 33, wherein the second energy storage device includes an overvoltage protection and/or an overcurrent protection by a current measurement and/or voltage measurement.

40. The vehicle according to claim 33, wherein the second energy storage device includes an overtemperature protection by a temperature measurement.

41. An electric vehicle, comprising: a first energy storage device; a second energy storage device; an energy supply unit adapted to provide energy to charge the first energy storage device and/or the second energy storage device; a first electrical consumer connected to the second energy storage device via an intermediate circuit; wherein the first energy storage device is connected to the energy supply unit via a bidirectional converter unit; wherein the second energy storage device is connected to the energy supply unit; wherein the vehicle is configured to deliver a first power flow from the first energy storage device to the second energy storage device in response to an intermediate circuit voltage in the intermediate circuit falling below a predefinable voltage and to prevent a second power flow from the second energy storage device to the first energy storage device; and wherein the vehicle is adapted to perform a method that includes: delivering the first power flow from the first energy storage device to the second energy storage device in response to the intermediate circuit voltage in the intermediate circuit falling below the predefinable voltage; and preventing the second power flow from the second energy storage device to the first energy storage device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] A mobile assistance system according to an example embodiment of the present invention with a consumer is schematically illustrated in FIG. 1. The mobile assistance system is also referred to below as MAS.

[0063] FIG. 2 schematically illustrates the progression of the intermediate circuit voltage U.sub.ZK over time for the case in which a consumer is supplied with energy from the intermediate circuit.

[0064] A mobile assistance system according to an example embodiment of the present invention with a consumer is schematically illustrated in FIG. 3.

[0065] A regulator structure is schematically illustrated in FIG. 4.

DETAILED DESCRIPTION

[0066] FIG. 1 is a schematic block diagram of those components of an MAS that are assigned to the energy management of the vehicle. For the energy supply, the MAS has an energy supply unit 1, which is in the form of a controllable power source in this exemplary embodiment. For this purpose, the energy supply unit 1 has a regulator, which regulates the supply current I.sub.0 of the energy supply unit 1 and, as a result, provides a supply voltage U.sub.0. This supply voltage U.sub.0 is DC voltage and the supply voltage U.sub.0 for an MAS is, for example, in the range from 120 V to 600 V. Whenever a voltage is mentioned herein, it always means DC voltage unless it is explicitly defined as AC voltage.

[0067] The energy supply unit 1 for the MAS can be configured in different manners. For example, a simple charging device with a plug-in contact can be implemented, so that the MAS can be supplied with energy by contact at certain charging stations. Likewise, a contact-based energy supply can be implemented while the MAS is in motion, for example, by conductor lines. As an alternative to this, a non-contact power supply can be implemented, for example, an inductive power supply. This can take place through coupled primary and secondary inductors. Both a supply at stationary charging stations and a supply while the MAS is in motion are possible, for example, through primary conductors placed in or on the building's floor.

[0068] If an external power supply is present, the supply current I.sub.0, which is positive, is provided by the energy supply unit 1. If there is no external energy supply, for example, because the MAS is in motion on a section without conductor lines or an inductive supply, no supply current is provided or the supply current I.sub.0 is zero.

[0069] The charging device 1 is connected to a bidirectional converter unit 2, to which, in turn, a first energy storage device 3 is connected. In the present example, the bidirectional converter unit 2 is arranged as a bidirectional DC/DC converter and the first energy storage device 3 is arranged as a battery storage device. The bidirectional DC/DC converter 2 therefore makes it possible to supply energy to the battery storage device 3 or to draw energy from the battery storage device 3. The DC/DC converter can be arranged as a non-isolated or as an isolated DC/DC converter in terms of potential.

[0070] For example, the battery storage device 3 has voltages U.sub.1 in the range of low voltages, e.g., 12 V, 24 V, or 48 V.

[0071] The current which is supplied to the DC/DC converter 2 is referred to as the first charging current I.sub.1. The first charging current I.sub.1 is positive when the battery storage device 3 is being supplied with energy, i.e., it is being charged. The first charging current I.sub.1 is correspondingly negative when energy is being drawn from the battery storage device 3, i.e., it is being discharged.

[0072] The DC/DC converter 2 optionally converts the supply voltage U.sub.0 into the battery voltage U.sub.1 when the battery storage device 3 is being charged, or it converts the battery voltage U.sub.1 into the supply voltage U.sub.0 when the battery storage device 3 is being discharged. Charging and discharging are possible both in the case of an existing external energy supply and in the case without an external energy supply, depending on the requirements of the intralogistic application. The voltage level U.sub.0 is therefore provided by the energy supply unit 1 and/or by the DC/DC converter 2.

[0073] Furthermore, the charging device 1 is connected to a second energy storage device 4 which is arranged as a double-layer capacitor 4 in this exemplary embodiment. Instead of a double-layer capacitor, an assembly of several double-layer capacitors connected in parallel and/or in series can also be used. The following explanations for a double-layer capacitor therefore apply analogously to a double-layer capacitor device. The double-layer capacitor 4 and the DC/DC converter 2 are connected to the charger 1 in parallel, for example. In addition, an electrical consumer 5 is connected in parallel to the double-layer capacitor 4, which, in this exemplary embodiment, is arranged as a drive device for the traction of the vehicle. For example, the drive device can be implemented as a 3-phase AC motor with a 3-phase inverter connected upstream. For example, the inverter converts the DC voltage present at the inverter into a 3-phase AC voltage, with which the three-phase motor, for example, a squirrel-cage rotor, is operated. The drive device 5 can also have several motors, each of which can be operated by its own inverter. In addition, the inverter can also be arranged with feedback capability, so that, when the drive motors are operated as a generator, energy can be supplied back to charge the double-layer capacitor 4.

[0074] In addition to drive devices for traction of the MAS, other consumers are also possible, such as lifting devices for picking up a load or handling devices for moving an object, for example, a robot arm. These devices can be connected to the first electrical consumer 5 in parallel, for example.

[0075] In the exemplary embodiment illustrated in FIG. 1, a diode 6 is arranged between a first connection point 7 of the DC/DC converter 2 and a second connection point 8 of the double-layer capacitor 4; the function of the diode will be described later. This diode 6 results in two voltage levels. While voltage level U.sub.0 is present at the first connection point 7, the double-layer capacitor 4 and the drive device 5, with the common second connection point 8 thereof, are at voltage level U.sub.ZK with intermediate circuit voltage U.sub.ZK. The double-layer capacitor 4 and the drive device 5 are therefore connected by a common intermediate circuit and isolated from the DC/DC converter 2 via the diode. The DC/DC converter 2, the double-layer capacitor 4, and the drive device 5 can therefore be supplied with the supply voltage U.sub.0 if an external energy supply is present. If the voltage in the intermediate circuit U.sub.ZK is less than the supply voltage U.sub.0, it increases until it has reached the level of the supply voltage U.sub.0. Because of the voltage drop across the diode 6, this voltage will be slightly less than the supply voltage U.sub.0. However, since the supply voltage U.sub.0 is usually in the range of 120 V and more, the voltage drop across the diode 6 is negligible, for example. If there is no external power supply and the DC/DC converter 2 is not supplying any power either, the supply voltage U.sub.0 can be less than U.sub.ZK, while the intermediate circuit voltage is still kept at a positive level by the double-layer capacitor. The diode 6 permanently prevents charge reversal from the double-layer capacitor 4 to the battery storage device 3, i.e., always, that is to say at any point in time. Thus, it is not possible and also not desirable for energy to be transferred from the double-layer capacitor 4 to the battery storage device 3.

[0076] The current which is supplied to the double-layer capacitor 4 is referred to as the second charging current I.sub.2. The second charging current I.sub.2 is positive when the double-layer capacitor 4 is being supplied with energy, i.e., when it is being charged. The second charging current I.sub.2 is correspondingly negative when energy is being drawn from the double-layer capacitor 4, i.e., when it is being discharged.

[0077] The current which is supplied to the drive device 5 is referred to as the load current I.sub.3. The load current I.sub.3 is positive when the drive device 5 is being supplied with energy, i.e., the drive motors are being operated as motors. The load current I.sub.3 is correspondingly negative when the drive device 5 is supplying energy back, for example, because the drive motors are being operated as a generator during braking.

[0078] The diode 6 arranged between connection points 7 and 8 prevents energy from the intermediate circuit from reaching the DC/DC converter 2 or reaching the battery storage device 3 via the DC/DC converter 2. Thus, it is not possible to transfer energy from the double-layer capacitor 4 to the battery storage device 3 at any given time. In other words, a power flow from the double-layer capacitor to the battery storage device is prevented by the diode. A power flow is possible and desirable in the reverse direction. Energy from the battery storage device 3 can therefore be used to increase the voltage U.sub.ZK in the intermediate circuit. The double-layer capacitor 4 can therefore be charged by the charger and/or by the battery storage device, while the battery storage device can only be charged by the charger.

[0079] The current that flows between the two connection points 7, 8 and therefore through the diode 6 is referred to as the charge-reversal current I.sub.1,2. The charge-reversal current is positive if current flows from the first connection point 7, i.e., from the DC/DC converter or from the charger 1, to the second connection point 8, i.e., in the direction of the intermediate circuit or in the direction of the double-layer capacitor 5 and the drive device 5. A negative charge-reversal current I.sub.1,2 is prevented by the diode 6 in the present example embodiment. Energy that was once stored in the double-layer capacitor 4 should no longer be used to charge the battery storage device 3.

[0080] The double-layer capacitor 4 has an end-of-charge voltage U.sub.L, i.e., a maximum voltage to which it can be charged at maximum, and an end-of-discharge voltage U.sub.E, i.e., a minimum voltage which is, for example, greater than zero. These characteristic voltage values are predetermined by the configuration of the double-layer capacitor. Typical values are, for example, U.sub.L=350 V and U.sub.E=120 V. Therefore, it must be possible to supply the drive device 5 with the necessary power in this voltage range. Therefore, if only the double-layer capacitor 4 is to be used as the single energy storage device, the drive device 5 would be configured for a maximum expected power for the lowest voltage and consequently the greatest current. This would result in larger sizes having to be used for the drive motors, for example. In order to improve the configuration so that smaller sizes can be used, the DC/DC converter 2 with the battery storage device 3 ensures that the intermediate circuit voltage U.sub.ZK does not fall below a definable voltage level U.sub.S. This is schematically illustrated in FIG. 2 for the case in which the drive device 5 requires energy continuously and there is no external energy supply. The double-layer capacitor 4 is initially fully charged at voltage level U.sub.L. Over time, the voltage U.sub.ZK in the intermediate circuit drops until it reaches a defined switching voltage U.sub.S. At this point in time at the latest, the DC/DC converter 2 is operated such that energy is transferred from the battery storage device 3 into the intermediate circuit in order to keep the voltage level there at least at U.sub.S. The intermediate circuit voltage U.sub.ZK is ascertainable, for example, by a simple voltage measurement. Overall, it is possible to dimension the drives to be smaller. The voltage U.sub.S at which the switching takes place can therefore be used as the rated voltage for dimensioning the power electronics and the drives and is referred to as the switching voltage U.sub.S.

[0081] The command for activating the DC/DC converter 2 can be executed, for example, by a vehicle controller. This vehicle controller controls the energy management of the vehicle and the corresponding travel movements. Alternatively or additionally, it is also possible for the DC/DC converter itself to have control electronics, to which the value of the measured intermediate circuit voltage U.sub.ZK is provided and which initiates appropriate steps so that energy is transferred into the intermediate circuit if the voltage drops below the switching voltage U.sub.S.

[0082] The battery storage device has a large capacity and compensates for the weakness of the low energy content of the double-layer capacitor. If the power supplied from the battery storage device into the intermediate circuit corresponds to the average power of an intralogistic application, the voltage at the double-layer capacitor and thus in the intermediate circuit will remain at U.sub.S on average. This leads to favorable operating conditions for the double-layer capacitor. The double-layer capacitor buffers possible power peaks of the drive device, while the battery storage device can be configured for the continuous consumption from the drive device. If greater power reserves are available at the DC/DC converter, the double-layer capacitor can also be recharged to the end-of-charge voltage U.sub.L.

[0083] For the exemplary values of U.sub.L=350 V and U.sub.E=120 V mentioned above, the following advantage results as compared to a system with only one double-layer capacitor. Assuming that the maximum current of the power electronics of the drive device is limited to I.sub.V,max=10 A, a maximum power of P.sub.V,max=U.sub.E I.sub.V,max=1,200 W would result in the case of just one double-layer capacitor with U.sub.E=120 V. If the switching point is selected for the case with two energy storage devices, for example, at U.sub.S=180 V, the result is a maximum power of P.sub.V,max=U.sub.S I.sub.V,max=1, 800 W.

[0084] If the drive device 5 is operated as a generator, the energy generated thereby can be used to recharge the double-layer capacitor 4. For example, this provides that the voltage present at the double-layer capacitor 4 does not exceed the end-of-charge voltage U.sub.L. This can be implemented, for example, using overvoltage protection.

[0085] As described, the diode 6 has the task of preventing a charge reversal from the double-layer capacitor 4 to the battery storage device 3. The energy that the double-layer capacitor 4 has absorbed should not reach the battery storage device 3 and should only be available to the drive device 5. As an alternative to the diode, the, e.g., permanent prevention of this power flow can also be represented in terms of regulation or control technology in a further exemplary embodiment. For this purpose, the charge-reversal current I.sub.1,2 is measured using a current-measuring device. In this case, the current-measuring device replaces the diode illustrated in FIG. 1; otherwise, all previously described units and currents relating to FIG. 1 apply to the further exemplary embodiment. The current-measuring device transmits a detected actual value I.sub.1,2_actual of the charge-reversal current I.sub.1,2 to the DC/DC converter 2, which includes signal electronics, for example. These signal electronics store a defined minimum charge-reversal current limit I.sub.1,2_min, below which the DC/DC converter is deactivated, at least for charging the battery storage device. This means that this deactivation only includes the voltage conversion from voltage level U.sub.0 to voltage level U.sub.1, while the reverse voltage conversion direction is not deactivated.

[0086] Alternatively, a complete deactivation of the DC/DC converter, i.e., switch-off, can also be implemented. The charge-reversal current limit value I.sub.1,2_min is, for example, a positive value and is, e.g., so far above the value zero that the charge-reversal current I.sub.1,2 does not fall below zero due to possible time delays, i.e., it does not become negative. The control technology therefore prevents a power flow from the double-layer capacitor 4 to the battery storage device 2. The deactivation of the DC/DC converter 2, for example, includes only the voltage conversion from voltage level U.sub.0 to voltage level U.sub.1. The reverse voltage conversion direction is therefore not deactivated.

[0087] A further exemplary embodiment is described in relation to FIGS. 3 and 4. As in the foregoing exemplary embodiment, the charge-reversal current I.sub.1,2 is measured between the first connection point 7 and the second connection point 8 by a current-measuring device 12 arranged between these connection points. A power flow from the double-layer capacitor to the battery storage device is thus prevented using control technology. The current-measuring device 12 replaces the diode 6 illustrated in FIG. 1. The current-measuring device 12 transmits the detected actual value I.sub.1,2_actual of the charge-reversal current I.sub.1,2 to the converter device 2, which is described in more detail below. The converter device 2 includes, as described below, a cascade controller 9, a bidirectional DC-DC converter 11, and a throttle 10. The designation of the currents or the description of the current directions is the same as in the exemplary embodiment first described above. The DC-DC converter 11 converts the converter voltage designated as U.sub.W into the battery voltage U.sub.1 and vice-versa. Since there is no diode, the voltage level U.sub.0 is equal to the intermediate circuit level U.sub.ZK. This converter voltage U.sub.W can differ from the intermediate circuit voltage U.sub.ZK since an inductor 8 is arranged between the DC voltage converter 11 and the first connection point 7. In the present example, this inductor 10 is part of the converter device 2, so that the voltage with which the converter device 2 can be supplied corresponds to the supply voltage U.sub.0.

[0088] The charge reversal from the double-layer capacitor 4 to the battery storage device 3 is prevented with control technology by the cascade controller 9 as described below. In addition to the actual value of the charge-reversal current I.sub.1,2_actual, the cascade controller 9 is also supplied with the actual value U.sub.ZK_actual of the intermediate circuit voltage U.sub.ZK. The actual value U.sub.ZK_actual is detected using a voltage measurement.

[0089] FIG. 4 illustrates the cascade control implemented in the cascade controller 9, including a current regulator 15 with a superimposed voltage regulator 13. A setpoint limiter 14 is arranged between the two regulators 13, 15.

[0090] First, a target value for the intermediate circuit voltage U.sub.ZK_target is specified, for example, by the vehicle controller. This is compared with the detected actual value of the intermediate circuit voltage U.sub.ZK_actual and the difference is provided to the voltage regulator 13 as an input variable. The voltage regulator 13 generates a target value for the charge-reversal current I.sub.1,2_target as an output variable. In order to avoid negative charge-reversal currents I.sub.1,2, the target value for the charge-reversal current is limited by a limiter 14 to values that are greater than a minimum charge-reversal current limit value I.sub.1,2_min. In other words, values for target value I.sub.1,2_target>I.sub.1,2_min are not changed by the limiter 14, and values for target value I.sub.1,2_target<I.sub.1,2_min are set by the limiter 14 to the minimum charge-reversal current limit value I.sub.1,2_min. The minimum charge-reversal current limit value I.sub.1,2_min is a non-negative value. I.sub.1,2_min=0 is also possible for an ideal regulator. The new value for the target value of charge-reversal current I.sub.1,2_target thusly generated by the limiter 14 is compared with the actual value of charge-reversal current I.sub.1,2_actual and the difference is provided to the current regulator 15 as an input variable. Finally, the current regulator generates a control value for converter voltage U.sub.W_control as an output variable. This means that converter voltage U.sub.W_control is only set and not regulated.

[0091] In the situation described with respect to FIG. 2, i.e., when there is no external energy supply (I.sub.0=0) and the drive device 5 is continuously supplied with energy, the cascade controller could be used to set the target value of intermediate circuit voltage U.sub.ZK_target to the switching voltage U.sub.S. As long as the intermediate circuit voltage U.sub.ZK is greater than the switching voltage U.sub.S, i.e., U.sub.ZK_actual>U.sub.S=U.sub.ZK_target the voltage regulator 13 will attempt to reduce the intermediate circuit voltage U.sub.ZK by specifying a target value for the charge-reversal current I.sub.1,2_target, which is negative. Due to the limitation, however, this target value is set to the minimum charge-reversal current limit value I.sub.1,2_min, for example, I.sub.1,2_min=0.1 A.

[0092] If the charge-reversal current is still above this limit value, i.e., I.sub.1,2_actual>0.1 A, the voltage regulator 15 will vary the control variable U.sub.W_control such that the charge-reversal current is reduced, i.e., the battery storage device 3 does not support the double-layer capacitor 4. The limit value I.sub.1,2_min provides that the cascade controller can still react in a timely manner when this limit is undershot, so that the charge-reversal current I.sub.1,2 does not become negative.

[0093] If the intermediate circuit voltage U.sub.ZK falls below the switching voltage U.sub.S due to the energy consumption of the drive device 5, the current regulator 13 will specify a greater positive target value I.sub.1,2_target in order to keep the voltage level at U.sub.S.

[0094] Even in other situations in which a negative charge-reversal current I.sub.1,2 could result, for example, a sudden drop in the external energy supply or energy being supplied back by the drive device 5, the cascade controller 9 provides that, in the end, there is no power flow from the double-layer capacitor 4 to battery storage device 3. In this context, it should be pointed out that in the present description a power flow, for example, from the double-layer capacitor 4 to the battery storage device 3, is always understood to mean a charge transfer which contributes significantly to a change in the corresponding energy content of an energy storage device.

LIST OF REFERENCE CHARACTERS

[0095] 1 Energy supply unit [0096] 2 Bidirectional DC/DC converter [0097] 3 First energy storage device [0098] 4 Second energy storage device [0099] 5 Electrical consumer [0100] 6 Diode [0101] 7 First connection point [0102] 8 Second connection point [0103] 9 Cascade controller [0104] 10 Throttle [0105] 11 DC converter [0106] 12 Current-measuring device [0107] 13 Voltage regulator [0108] 14 Limiter [0109] 15 Current regulator [0110] U.sub.L End-of-charge voltage [0111] U.sub.S Switching voltage [0112] U.sub.E End-of-discharge voltage [0113] I.sub.0 Supply current [0114] I.sub.1 First charging current [0115] I.sub.2 Second charging current [0116] I.sub.1,2 Charge-reversal current [0117] I.sub.3 Load current