Battery system and process

Abstract

The present invention discloses a battery system with at least two battery modules (2a, 2b), wherein each battery module (2a, 2b) comprises: at least one battery (5a, 5b); a first controllable first switching device (3a, 3b, 3a', 3b') which is configured to couple the respective battery module (2a, 2b) into a current path (7) of the battery system (1) or to electrically bridge the respective battery module (2a, 2b); at least one controllable second switching device (6) configured to electrically connect at least two battery modules (2a, 2b) in parallel, such that a current-limited voltage equalization takes place between the at least two battery modules (2a, 2b), wherein the second switching devices (6) have at least two module terminals (12, 13) and for each pair of module terminals (12, 13) a switching element (14) configured to electrically couple the respective two module terminals (12, 13), wherein each module terminal (12, 13) can be electrically coupled to the positive terminal of the battery (5a, 5b) of a different battery module (2a, 2b) or wherein each module terminal (12, 13) can be electrically coupled to the negative terminal of the battery (5a, 5b) of a different battery module (2a, 2b);wherein the module terminals (12, 13) of one of the plurality of second switching devices (6) can be electrically coupled to the poles of the batteries of the battery modules (2a, 2b) that are not coupled to the first second switching device (6), a control device (15) which is configured to control the controllable first switching devices (3a, 3b, 3a', 3b') and the controllable second switching device (2a, 2b) depending on the requested electrical power.

Description

State of the art

[0001] It is becoming clear that in the future, both stationary applications, such as wind turbines, and vehicles, such as hybrid or electric vehicles, will increasingly utilize electronic systems that combine new energy storage technologies with electric drive technology. In conventional applications, an electric machine, such as a rotating field machine, is controlled by a converter in the form of an inverter. A characteristic feature of such systems is a so-called DC link, through which an energy storage device, usually a battery, is connected to the DC side of the inverter. To meet the power and energy requirements of a given application, several battery cells are connected in series.Since the current provided by such an energy storage device must flow through all battery cells, and a battery cell can only conduct a limited current, additional battery cells are often connected in parallel to increase the maximum current.

[0002] Connecting multiple battery cells in series, in addition to resulting in a high overall voltage, presents the problem that the entire energy storage system fails if a single cell fails, as no battery current can then flow. Such a failure of the energy storage system can lead to a failure of the entire system. In a vehicle, a failure of the traction battery can cause the vehicle to break down. In other applications, such as the rotor blade adjustment of wind turbines, adverse conditions, such as strong winds, can even lead to damage to the wind turbines. Therefore, high reliability of the energy storage system is always essential, with "reliability" referring to a system's ability to operate flawlessly for a specified period.

[0003] In a simple series connection of several battery cells, the large voltage range across the different charge states of the battery cells also leads to limitations in the design of the other system components with regard to efficiency, installation space, and cost. For example, the electric motor must be designed in such a way that the required power can be provided even at the lower voltage limit, i.e., when the battery is discharged. On the other hand, it must also be able to withstand operation at the upper voltage limit, i.e., when the battery is fully charged.

[0004] From US 2002 / 0 175 644 A1 a system for controlling a three-phase electric machine is known, which has a controllable energy storage device with switchable DC voltage sources and a downstream inverter.

[0005] Document DE 10 2011 006 761 A1 discloses a switching matrix with a plurality of supply terminals designed to be connected to a plurality of energy sources, each providing a supply voltage.

[0006] Document US 2010 / 0261048A1 discloses a dynamically reconfigurable framework for a large-scale battery system. The framework consists of multiple battery cells arranged side-by-side to form a battery cell array coupled to an application load. Each battery cell circuit includes: a battery cell with an input and an output terminal; a first switch between the load and the battery cell input; a second switch between the battery cell input and the output of an adjacent battery cell; and a third switch between the battery cell output and the output of an adjacent battery cell. The battery cell array also includes a local controller that selectively controls the switches in the battery cell circuits.

[0007] Document DE 699 22 044 T2 discloses high-voltage power sources and a high-voltage power source consisting of a multitude of metal thin-film batteries.

[0008] Document US 6,430,692 B1 discloses a battery backup system comprising multiple batteries and switches that connect the batteries to either a high-power or a low-power load. The batteries are selectively connected in series or parallel by the switches. In parallel, the batteries power the low-power load; in series, they power the high-power load. When switching from the high-power series connection to the low-power parallel connection, there is a transition period during which a backup battery powers the low-power load.

[0009] Document US 6 058 032 A discloses a multi-pulse width modulation power conversion device for the speed-controlled drive of a three-phase motor comprising three units.

[0010] Document US 2002 / 0175644A1 discloses a multi-stage DC link inverter and a method for improving torque behavior and current control in low-inductance permanent magnet motors and switched reluctance motors. Disclosure of the invention

[0011] The present invention discloses a battery system with the features of claim 1 and a method with the features of claim 6. Accordingly, the following is planned:

[0012] A battery system with at least two battery modules, wherein each battery module has at least one battery and a controllable first switching device configured to couple the respective battery module into a current path of the battery system or to electrically bridge the respective battery module, and with at least one controllable second switching device configured to electrically connect the at least two battery modules in parallel and a control device configured to control the controllable first switching devices and the controllable second switching device depending on the required electrical power.

[0013] A method for operating a battery system according to the invention comprising the steps of providing a battery system with at least two battery modules, each comprising at least one battery and a first switching device, and at least one second switching device, coupling or bridging a battery module in a current path of the battery system by the respective first switching device, and controlling the first switching devices and the second switching device depending on the electrical power drawn, such that the at least two battery modules are connected electrically in parallel or in series. Advantages of the invention

[0014] The underlying insight of the present invention is that conventional battery systems have an inflexible structure.

[0015] The underlying idea of ​​the present invention is to take this knowledge into account and to provide a battery system in which individual battery modules can be flexibly coupled together.

[0016] For this purpose, switching devices are provided in individual battery modules, each of which has at least one battery. These switching devices are designed such that the batteries can be connected to or bypassed in a current path of the battery module. If several such battery modules according to the invention are connected in series, they can be connected in series as needed. With the aid of the first switching devices according to the invention, it is therefore possible to flexibly adjust the voltage in the current path of the battery system. In particular, with the aid of the first switching devices according to the invention, the voltage in the current path can be adjusted so that a load can be operated directly with it, and, for example, no voltage converter is necessary.

[0017] The present invention further provides at least a second switching device configured to connect two battery modules in parallel. Connecting two or more battery modules in parallel significantly reduces the currents in the conductors of the individual battery modules. This, in turn, significantly reduces the ohmic conduction losses and the heat dissipation requirements in the individual battery modules and thus in the entire battery system.

[0018] Without connecting battery modules in parallel, the following power losses result: Pv=R*I2

[0019] Where R is the internal resistance of a battery module.

[0020] When two modules are connected in parallel, the resulting power loss is reduced by: Pv=2*R*(I / 2)2=0.5*R*I2 and thus a 50% reduction in power loss. When three battery modules are connected in parallel, the resulting power loss is: Pv=3*R*(I / 3)2=0.33*R*I2

[0021] As the number of battery modules connected in parallel increases, the power loss decreases.

[0022] Finally, a control device is provided which controls the switching devices.

[0023] Advantageous embodiments and further developments are described in the dependent claims and in the description with reference to the figures.

[0024] In one embodiment, the first switching device has a positive battery terminal, a negative battery terminal, a positive output terminal and a negative output terminal and is configured to electrically couple the positive battery terminal with the positive output terminal and to electrically couple the negative battery terminal with the negative output terminal, wherein the positive battery terminal can be electrically coupled to a positive terminal of the battery and the negative battery terminal can be electrically coupled to a negative terminal of the battery.

[0025] For example, if a DC electrical load is connected to the battery system, the first switching devices can be designed as so-called half-bridge switching devices, as described in Fig. Figure 4 illustrates this. Half-bridge switching devices, which have two separate switching elements, make it possible to connect multiple battery modules in the same direction (i.e., with the same polarity) into the current path of the battery system, thus varying the voltage applied to the battery system outputs. For example, at low speeds, the full battery voltage is not required at an electric motor. If a lower voltage is provided at the battery system outputs compared to the maximum voltage, switching losses in the inverter and a lower harmonic content in the phase voltage and phase current of the motor result. This leads to an increased efficiency of the overall system.

[0026] In one embodiment, the first switching device is further configured to electrically couple the positive battery terminal with the negative output terminal and to electrically couple the negative battery terminal with the positive output terminal.

[0027] If the first switching devices are designed as so-called full-bridge switching devices, as in Fig. As shown in Figure 3, individual battery modules can also be connected with reversed polarity in the current path of a battery system. This requires four separate switching elements in such a full-bridge switching device. This makes it possible to provide not only different voltage levels but also voltages of positive and negative polarity at the outputs of the battery system. The use of several battery systems according to the invention therefore makes it possible, for example, to directly control a three-phase motor. For this purpose, a battery system is provided for each phase of the motor, and the individual battery systems are controlled in such a way that the necessary voltages are established at the outputs of the respective battery systems.

[0028] In one embodiment, the second switching device has at least two module connections and a switching element for each pair of module connections, which is designed to electrically couple the respective two module connections, wherein each module connection can be electrically coupled to the positive terminal of the battery of a different battery module or wherein each module connection can be electrically coupled to the negative terminal of the battery of a different battery module.

[0029] If the second switching device is constructed with simple switching elements, it is possible to provide very simple and inexpensive second switching devices. Furthermore, if the number of module connections in a second switching device is variable, it can be adapted to a wide variety of applications. For example, if a second switching device has four module connections and correspondingly two switching elements, three battery modules can be connected in parallel. In another embodiment, the connections that are connected to the same battery terminal are electrically coupled to each other within the second switching device and connected to the respective battery terminal via a single module connection.

[0030] In one embodiment, a further second switching device is provided, and the module terminals of this second switching device can be electrically coupled to the battery terminals of the battery modules that cannot be coupled to the first second switching device. If two second switching devices are provided to couple two battery modules together, then, particularly in a battery system with full-bridge switching devices, only four semiconductor switches of the first and second switching devices are traversed by a current when two battery modules are connected in parallel. This further reduces the power loss of the battery system according to the invention.

[0031] In one embodiment, when controlling the first and second switching devices such that the at least two battery modules are electrically connected in parallel, the second switching device is controlled in such a way that a current-limited voltage equalization takes place between the at least two battery modules. This makes it possible to avoid damage to the battery modules caused by excessive currents.

[0032] The above embodiments and further developments can be combined with one another as appropriate. Further possible embodiments, further developments, and implementations of the invention also include combinations of features of the invention described previously or subsequently with regard to the exemplary embodiments, even if not explicitly mentioned. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention. Brief description of the drawings

[0033] The present invention will be explained in more detail below with reference to the exemplary embodiments shown in the schematic figures of the drawings. These show: Fig. 1 a block diagram of an exemplary embodiment of a battery system according to the invention 1; Fig. 2 a flowchart of an exemplary embodiment of a method according to the invention; Fig. 3 a block diagram of a further exemplary embodiment of a battery system 1 according to the invention; Fig. 4 a block diagram of an exemplary embodiment of a first switching device 3a, 3a', 3b, 3b', as it can be used in a battery system 1 according to the invention.

[0034] In all figures, identical or functionally equivalent elements and devices have been provided with the same reference numerals, unless otherwise specified. Embodiments of the invention

[0035] Fig. Figure 1 shows a block diagram of an exemplary embodiment of a battery system according to the invention.

[0036] Battery system 1 comprises two battery modules 2a and 2b. Each battery module 2a, 2b contains a battery 5a, 5b, which is coupled to a first switching device 3a, 3a', 3b, 3b'. Battery system 1 also comprises a second switching device 6, which is coupled to the two battery modules 2a, 2b. Furthermore, battery system 1 includes a control device 15, which is coupled to the first switching devices 3a, 3a', 3b, 3b' and the second switching device 6 in order to control them depending on a requested electrical power. Finally, battery system 1 comprises a current path 7, which connects the two first switching devices 3a, 3a', 3b, 3b' to each other and to battery system 1 to its environment.

[0037] In Fig. For example, battery modules 2a and 2b each have a voltage of 200 volts. This results in different output voltages on current path 7 of battery system 1. If the control device 15, for example, controls the first switching devices 3a, 3a' such that they connect battery 5a to current path 7, and controls the first switching device 3b, 3b' such that they do not connect battery 5a to current path 7 but bypass it, a voltage of 200 volts results on current path 7. If a very high current is required via current path 7, the control device 15 can control the second switching device 6 and the first switching device 3b, 3b' such that they connect battery 5b in parallel with battery 5a.

[0038] If, on the other hand, a higher voltage is required, the control unit 15 can control the first switching devices 3a, 3a', 3b, 3b' and the second switching device 6 such that the battery 5a and the battery 5b are connected in series in the current path 7. This results in a voltage of 400 volts in the current path 7.

[0039] The control device 15 in Fig. 1 is implemented as a microcontroller 15, which is programmed to control the first switching devices 3a, 3a', 3b, 3b' and the second switching device 6.

[0040] In further embodiments, the control device 15 is integrated into a vehicle control unit. In still further embodiments, the control device 15 is designed as a computer program module, which is stored in the memory of a vehicle control unit and executed by the processor of the vehicle control unit.

[0041] Fig. Figure 2 shows a flowchart of an exemplary embodiment of a method according to the invention.

[0042] In the method for operating a battery system according to the invention, in a first step S1, a battery system 1 is provided with at least two battery modules 2a, 2b, each comprising at least one battery 5a, 5b and each a first switching device 3a, 3b, 3a', 3b', and at least one second switching device 6. In a second step S2, a battery module 2a, 2b is coupled or bridged in a current path 7 of the battery system 1 by the respective first switching device 3a, 3b, 3a', 3b'. Finally, in a last step S3, depending on the electrical power drawn, the first switching devices 3a, 3b, 3a', 3b' and the second switching device 6 are controlled such that the at least two battery modules 2a, 2b are electrically connected in parallel or in series.

[0043] In a further embodiment of the method according to the invention, the first switching devices 3a, 3b, 3a', 3b' and the second switching device 6 are controlled in such a way that at least two battery modules 2a, 2b are electrically connected in series in the current path 7 of the battery system 1.

[0044] In a further embodiment of the method according to the invention, the first switching devices 3a, 3b, 3a', 3b' and the second switching device 6 are controlled in such a way that at least two battery modules 2a, 2b are electrically connected in parallel in the current path 7 of the battery system 1.

[0045] In a further embodiment of the method according to the invention, the first switching devices 3a, 3b, 3a', 3b' and the second switching device 6 are controlled such that at least two battery modules 2a, 2b are electrically connected in series in the current path 7 of the battery system 1 and at least two battery modules 2a, 2b are electrically connected in parallel in the current path 7 of the battery system 1.

[0046] In a further embodiment of the method according to the invention, the first switching devices 3a, 3b, 3a', 3b' and the second switching device 6 are controlled such that any number of battery modules 2a, 2b, which is greater than two, are electrically connected in series in the current path 7 of the battery system 1 and any number of battery modules 2a, 2b, which is greater than two, are electrically connected in parallel in the current path 7 of the battery system 1.

[0047] Fig. Figure 3 shows a block diagram of another exemplary embodiment of a battery system according to the invention 1.

[0048] Battery system 1 in Fig. 3 differs from battery system 1 in Fig. 1 such that batteries 5a and 5b each comprise a single battery with a positive and a negative terminal. Furthermore, the first switching devices 3a, 3b each have a positive battery terminal 8a, 8b, a negative battery terminal 9a, 9b, a positive output terminal 10a, 10b and a negative output terminal 11a, 11b.

[0049] Furthermore, the first switching devices 3a, 3b, 3a', 3b' each have four switching elements S1a, S2a, S3a, S4a and S1b, S2b, S3b, S4b. The switching elements S1a and S2a, S1b and S2b, S3a and S4a, and S3b and S4b are arranged in series. The junction between switching elements S1a and S2a is connected to the negative output terminal 11a, and the junction between switching elements S1b and S2b is connected to the negative output terminal 11b. The junction between switching elements S3a and S4a is connected to the positive output terminal 10a, and the junction between switching elements S3b and S4b is connected to the positive output terminal 10b. Finally, the terminals of the switching elements S1a and S4a or S1b and S4b that are not connected to the nodes between the switching elements S1a, S2a, S3a, S4a and S1b, S2b, S3b, S4b are connected to the positive battery terminal 8a or S1b, S2b, S3b, S4b, respectively.8b coupled and the terminals of the switching elements S2a and S3a or S2b and S3b that are not connected to the nodes between the switching elements S1a, S2a, S3a, S4a and S1b, S2b, S3b, S4b are connected to the negative battery terminal 9a or 9b respectively.

[0050] Finally, the second switching device 6 points in Fig. 3. A switching element 14 is connected between the two module terminals 12 and 13 of the second switching device 6. Module terminal 12 is connected to the positive battery terminal 8a, and module terminal 13 is connected to the positive battery terminal 8b.

[0051] In Fig. Switching elements S2a, S4a, S2b, S3b and 14 are closed. Switching elements S1a, S3a, S1b, S4b are open. Due to the in Fig. In the 3 switch positions shown, the two batteries 5a and 5b are electrically connected in parallel and switched in positive polarity into the current path 7 of the battery system.

[0052] For example, if the two batteries 5a and 5b are to be connected in series with positive polarity in the current path 7, the control unit 15 controls the switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b and 14 in such a way that the switching elements S1a, S3a, S1b and S3b and 14 are open and the switching elements S2a, S4a, S2b, S4b are closed.

[0053] If the two batteries 5a and 5b are to be connected in series with negative polarity in the current path 7, the control device 15 controls the switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b and 14 such that the switching elements S2a, S4a, S2b, S4b and 14 are open and the switching elements S1a, S3a, S1b and S3b are closed.

[0054] If the two batteries 5a and 5b are to be connected in parallel with negative polarity in the current path 7, the control device 15 controls the switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b such that the switching elements S1a, S4a, S2b and S4b are open and the switching elements S2a, S3a, S1b and S3b and 14 are closed.

[0055] The switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b and 14 are implemented as MOSFETs. In further embodiments, the switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b and 14 are configured as any semiconductor switches. In still further embodiments, the switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b and 14 are configured as semiconductor relays, relays or the like.

[0056] In another embodiment, the battery modules 2a, 2b each have at least two batteries 5a, 5b which are electrically connected in series.

[0057] Fig. Figure 4 shows a block diagram of an exemplary embodiment of a first switching device 3a', 3b', as it can be used in a battery system according to the invention.

[0058] In contrast to the switching devices 3a, 3b in Fig. 3 indicates the switching device 3a', 3b' in Fig. 4 only two switching elements S1 and S2, which together form a half-bridge switching device 3a', 3b'.

[0059] In Fig. In section 4, the two switching elements S1 and S2 are connected in series. Furthermore, a junction between the two switching elements S1 and S2 is connected to a positive output terminal 10a, 10b. Additionally, the terminal of the first switching element S1, which is not connected to the junction, is connected to the positive battery terminal 8a, 8b, and the terminal of the second switching element S2, which is not connected to the junction, is connected to the negative battery terminal 9a, 9b.

[0060] With a switching device such as those found in Fig. As shown in Figure 4, a battery module 2a, 2b can be connected in series to current path 7 or bypassed. Reversing the polarity of the battery module 2a, 2b is not possible. This allows for the provision of a battery module 2a, 2b adapted to the specific application and saves components in applications that do not require the polarity reversal function.

[0061] Although the present invention has been described above with reference to preferred embodiments, it is not limited thereto, but can be modified in many ways. In particular, the invention can be altered or modified in many ways without deviating from the core of the invention.

[0062] In an exemplary embodiment, when two or more battery modules 2a, 2b are connected in parallel, the control device 15 controls the switching devices 3a, 3b, 3a', 3b', 6 in such a way that a voltage equalization takes place between the battery modules 2a, 2b and at the same time the equalization currents between the battery modules 2a, 2b are limited.

[0063] In one embodiment, a first battery module 2a, 2b, which has a higher voltage than a second battery module 2b, 2a, is first connected to the current path 7. The second battery module 2b, 2a is then connected in parallel to the first battery module 2a, 2b by means of the second switching device 6. The switching element 14 is clocked such that the current flowing between the two battery modules 2a, 2b as balancing current does not exceed a maximum value. As soon as both battery modules 2a, 2b have the same output voltage, the switching element 14 can remain permanently closed. When the second battery module 2b, 2a has the higher output voltage, the first battery module 2a, 2b is clocked in an analogous manner and connected in parallel to the second battery module 2b, 2a.

[0064] In another embodiment, the voltage difference between two battery modules 2a, 2b is balanced by connecting the battery module 2a, 2b with the lower output voltage in reverse in the current path 7. This type of voltage balancing can be used when the voltage difference is smaller than the forward voltage of the diode junction of the semiconductor switching elements S1a, S2a, S3a, S4a, S1b, S2b, S3b, S4b. In this case, the balancing current flowing between the battery modules 2a, 2b corresponds to the string current, thus eliminating the need for a further current limiting measure and the associated avalanche losses.

[0065] In another embodiment, the switching on and off of parallel-connected battery modules 2a, 2b takes place at the zero crossing of the current in the current path 7 in order to avoid losses due to the avalanche effect.

Claims

[1] Battery system comprising at least two battery modules (2a, 2b), each battery module (2a, 2b) comprising: at least one battery (5a, 5b); a first controllable first switching device (3a, 3b, 3a', 3b') which is configured to couple the respective battery module (2a, 2b) into a current path (7) of the battery system (1) or to electrically bridge the respective battery module (2a, 2b); at least one controllable second switching device (6) configured to electrically connect at least two battery modules (2a, 2b) in parallel, such that a current-limited voltage equalization takes place between the at least two battery modules (2a, 2b), wherein the second switching devices (6) have at least two module terminals (12, 13) and for each pair of module terminals (12, 13) a switching element (14) configured to electrically couple the respective two module terminals (12, 13), wherein each module terminal (12, 13) can be electrically coupled to the positive terminal of the battery (5a, 5b) of a different battery module (2a, 2b) or wherein each module terminal (12, 13) can be electrically coupled to the negative terminal of the battery (5a, 5b) of a different battery module (2a, 2b);wherein the module connections (12, 13) of one of the plurality of second switching devices (6) can be electrically coupled to the poles of the batteries of the battery modules (2a, 2b) that are not coupled to the first second switching device (6), a control device (15) which is configured to control the controllable first switching devices (3a, 3b, 3a', 3b') and the controllable second switching device (6) depending on the requested electrical power. [2] Battery system according to claim 1, wherein the first switching device (3a, 3b, 3a', 3b') has a positive battery terminal (8a, 8b), a negative battery terminal (9a, 9b), a positive output terminal (10a, 10b) and a negative output terminal (11a, 11b) and is configured to electrically couple the positive battery terminal (8a, 8b) to the positive output terminal (10a, 10b) and to electrically couple the negative battery terminal (9a, 9b) to the negative output terminal (11a, 11b), wherein the positive battery terminal (8a, 8b) is electrically coupleable to a positive terminal of the battery (5a, 5b) and the negative battery terminal (9a, 9b) is electrically coupleable to a negative terminal of the battery (5a, 5b). [3] Battery system according to claim 2, wherein the first switching device (3a, 3b, 3a', 3b') is further configured to electrically couple the positive battery terminal (8a, 8b) with the negative output terminal (11a, 11b) and to electrically couple the negative battery terminal (9a, 9b) with the positive output terminal (10a, 10b). [4] Method for operating a battery system according to any one of claims 1 to 3, comprising the steps: Providing (S1) a battery system (1) comprising at least two battery modules (2a, 2b) comprising at least one battery (5a, 5b) and each comprising a first switching device (3a, 3b, 3a', 3b') and at least one second switching device (6); Coupling (S2) or bridging of a battery module (2a, 2b) in a current path (7) of the battery system (1) by the respective first switching device (3a, 3b, 3a', 3b'); Depending on the electrical power drawn, control (S3) the first switching devices (3a, 3b, 3a', 3b') and the second switching device (6) such that the at least two battery modules (2a, 2b) are electrically connected in parallel or in series, wherein if the at least two battery modules (2a, 2b) are electrically connected in parallel when controlling (S3) the first switching devices (3a, 3b, 3a', 3b') and the second switching device (6), the second switching device (6) is controlled such that a current-limited voltage equalization takes place between the at least two battery modules (2a, 2b). [5] Method according to claim 4, wherein a positive battery terminal of the first switching device (3a, 3b, 3a', 3b') is electrically coupled to or disconnected from a positive output terminal (10a, 10b) of the first switching device (3a, 3b, 3a', 3b') and a negative battery terminal of the first switching device (3a, 3b, 3a', 3b') is electrically coupled to or disconnected from a negative output terminal (11a, 11b) of the first switching device (3a, 3b, 3a', 3b') and wherein the positive battery terminal (8a, 8b) is electrically coupled to a positive terminal of the battery (5a, 5b) and the negative battery terminal (9a, 9b) is electrically coupled to a negative terminal of the battery (5a, 5b). [6] Method according to claim 5, wherein the positive battery terminal (8a, 8b) of the first switching device (3a, 3b, 3a', 3b') is electrically coupled to or disconnected from the negative output terminal (11a, 11b) of the first switching device (3a, 3b, 3a', 3b') and the negative battery terminal (9a, 9b) of the first switching device (3a, 3b, 3a', 3b') is electrically coupled to or disconnected from the positive output terminal (10a, 10b) of the first switching device (3a, 3b, 3a', 3b').

Images