METHOD FOR OPERATING A BATTERY SYSTEM

Description

[0001] The invention relates to a method for operating a battery system. The battery system comprises a plurality of strings connected in parallel, each of which has at least one battery module. In the at least one battery module, a plurality of battery cells are connected in series and / or in parallel. The individual strings can be connected and disconnected independently of one another. Individual battery cells or individual battery cell packs, each comprising a plurality of battery cells connected in parallel, can be connected and disconnected independently of one another.

[0002] The invention further relates to a battery system which is configured to carry out the method proposed according to the invention.

[0003] The invention also relates to a vehicle which is configured to carry out the method proposed according to the invention and / or comprises the battery system proposed according to the invention. State of the art

[0004] In today's electrically powered vehicles, multiple battery cells are connected not only in series, but also in parallel. This not only achieves a sufficiently high battery capacity and thus a long vehicle range, but also ensures the required power is available. Such battery cells are usually connected in parallel within a battery module, with multiple battery modules connected in series to achieve the required battery voltage. Each battery module has a battery cell monitoring unit (Cell Supervising Circuit, CSC), which measures the individual battery cell voltages and the temperature(s) within the battery module and transmits them to a higher-level battery control unit (BCU) for further analysis.

[0005] Autonomously operated electric vehicles (EVs) are subject to special requirements, particularly with regard to their drivability, as such vehicles are not permitted to break down. Depending on their degree of automation, these vehicles must meet a certain safety level (Safe Stop Level, SSL). To this end, they are classified according to different risk levels (Automotive Safety Integrity Level, ASIL), which places increased demands on battery design. Less error-prone systems or even fault-tolerant systems can involve redundancy, i.e., duplication, of individual subcomponents or even duplication of the entire battery system.

[0006] The disadvantages of redundancy, such as installation space and, in particular, costs, primarily apply to the most expensive component of the powertrain, the battery. Initial approaches are aimed at constructing batteries not only as single-stranded batteries, i.e., individual battery cells or battery cells connected in parallel are all connected in series, but also as two or even more battery modules connected in parallel. Multiple battery modules connected in parallel only provide a limited increase in installation space, since the battery cells must be halved or even quartered according to their battery capacity.

[0007] If a battery cell fails due to a fault, the battery module containing the faulty battery cell is shut down, or if multiple modules in a string are faulty, the entire string can be shut down. In battery modules with coupling devices on the individual battery cells of the battery module, the defective battery cell can be bypassed in the event of a fault. However, in such a battery system, this means that one intact battery cell from each of the other strings in the battery system must also be bypassed to maintain the same voltage across the strings. The same applies to battery systems with modules connected in parallel; intact battery cells are thus lost from the battery system.

[0008] The document US 2012 / 0091964 A1 discloses a bypass circuit for a battery and in particular a serial bypass circuit for a vehicle battery system which switches off and bypasses one or more battery cells or modules in the battery system in response to a cell or module fault or a potential cell or module fault.

[0009] The document US 2016 / 0240894 A1 describes a battery management system for monitoring and regulating the operation of a rechargeable battery, which has a plurality of battery modules which are electrically interconnected and each comprise at least one battery cell, wherein the battery management system comprises at least one control unit and at least one cell monitoring unit, and wherein the at least one cell monitoring unit is designed to receive data relating to at least one operating parameter of at least one battery cell, to record the received data and to transmit the recorded data to the at least one control unit.

[0010] The document DE 10 2012 210 910 A1 discloses a method for operating an electric traction drive system with a battery direct inverter and associated control device. Disclosure of the invention

[0011] A method for operating a battery system for a vehicle, in particular an electric vehicle, is proposed according to claim 1. The battery system comprises a plurality of strings connected in parallel, each of which has at least one battery module. In the at least one battery module, a plurality of battery cells are connected in series and / or in parallel. The individual strings can be connected and disconnected independently of one another.

[0012] Preferably, the individual battery cells can be connected and disconnected independently of one another. The battery system comprises first coupling devices within the individual battery modules, with which individual battery cells within the battery module can be disconnected and bridged via a bypass line in which second coupling devices are accommodated.

[0013] Advantageously, the battery cells can be combined into several battery cell packs, each comprising several battery cells connected in parallel. The battery system comprises first coupling devices within the individual battery modules, with which individual battery cell packs within the battery module can be disconnected and bridged via a bypass line in which second coupling devices are accommodated.

[0014] The battery system can have a battery control unit for monitoring the at least one battery module and for controlling the main switch and / or the first and second coupling devices. The at least one battery module can have a battery cell monitoring unit with sensors for detecting measured values ​​of the battery cells and of the at least one battery module. The battery cell monitoring unit communicates with the battery control unit. The battery control unit can have evaluation electronics for evaluating the measured values ​​of the battery cells and of the at least one battery module detected by the sensors.

[0015] The measured values ​​recorded by the sensors include, for example, a temperature and voltage of each individual battery cell as well as a temperature and voltage of the entire battery module.

[0016] Furthermore, the battery system can be designed such that it has first coupling devices with which battery modules can be disconnected from the string and bridged via bypass lines comprising second coupling devices.

[0017] According to the invention, a battery cell fault is first detected. This can be done by evaluating the measured values. A battery cell fault is understood to be a cell fault or an electronic fault that occurs in the electronic components associated with the battery cell, such as a battery monitoring unit or a sensor. Subsequently, the faulty battery cell or the faulty battery pack in which the faulty battery cell is located is disconnected and bypassed. The faulty battery cell or the faulty battery cell pack remains permanently bypassed.

[0018] At the same time, the faulty string containing the faulty battery cell or battery pack is switched off.

[0019] The strand voltage of the faulty strand is then compared with the strand voltage of the respective intact strands in which no faults were detected.

[0020] The intact strings are then discharged if voltage differences between the strings exceed a threshold. The faulty string remains switched off only until the string voltages of the intact strings have almost equalized to the level of the faulty string. During this time, the intact strings provide energy for the electric vehicle and deliver correspondingly higher string currents. The faulty string remains switched on when recuperation is taking place. The intact battery cells of the faulty string are charged by the recuperation current until the voltages of the strings have equalized. After this, the intact battery cells of all strings are charged.

[0021] Preferably, the faulty string is switched on when voltage differences between the strings fall below the voltage threshold.

[0022] At the same voltage level, all strings are available again and can thus deliver their maximum power until a final discharge voltage is reached. The energy loss, which ultimately affects the range, results only from the duration of a voltage adjustment phase and a discharge current. This advantageously increases the availability and reliability of the traction supply. This allows any safe-stop level to be achieved.

[0023] Preferably, the voltage threshold is in a range of 1 V to 2 V. This can prevent battery-damaging equalizing currents.

[0024] The journey can thus be continued without restrictions; repairs can be carried out afterwards. If no battery cell or battery module is replaced and the electric vehicle's battery is being charged, all strings can be charged preferentially when the battery system is being charged. The charging process is terminated when a string voltage that is reduced by one or more cell voltages is reached.

[0025] Alternatively, the intact strings can be fully charged first when charging the battery system with the faulty string disconnected. After complete charging, the intact strings are discharged until the string voltages of all strings are at the same level.

[0026] Preferably, when electronic faults are detected, the faulty string is only switched off when the string current of the faulty string falls below a current threshold. The current threshold is preferably in a range of 15 A to 25 A.

[0027] Furthermore, the method proposed according to the invention can be carried out in such a way that a battery module is switched off when battery faults and / or battery module faults are detected.

[0028] Furthermore, a battery system is proposed which is configured to carry out the method proposed according to the invention.

[0029] A vehicle is also proposed which is configured to carry out the method proposed according to the invention and / or which comprises the battery system proposed according to the invention. Advantages of the invention

[0030] With the method proposed according to the invention, a battery system can be operated cost-effectively and with minimal installation space while simultaneously achieving maximum possible capacity in the event of a fault. The implementation of the method proposed according to the invention is based purely on software, and therefore no additional hardware, such as electronics for the battery system, is required.

[0031] No redundant battery module is required for the battery system. The method proposed by the invention adjusts the voltage of the intact battery modules or strings to that of the faulty battery module or string. This minimizes range reduction and also increases the availability and reliability of the traction supply for achieving any desired safe stop level. Furthermore, the method proposed by the invention is only used in the event of a fault, thereby minimizing the load on the battery system's battery control unit.

[0032] By means of the method proposed according to the invention, multiple faults within a battery module or string or multiple faults across battery modules or strings can also be mapped.

[0033] Furthermore, the method proposed according to the invention is fault-independent and thus applicable to cell faults or electronic faults. Furthermore, the method proposed according to the invention can also be used for battery systems with coupling devices not only for individual battery cells, but also for battery modules. Short description of the drawings

[0034] Embodiments of the invention are explained in more detail with reference to the drawings and the following description.

[0035] They show: Figure 1 shows a structure of a four-strand battery system with battery cells connected in parallel and in series, Figure 2 shows a structure of a three-strand battery system in a 1p arrangement, Figure 3 shows an electric drive system with a three-strand battery system in a 1p arrangement, with battery cell coupling devices, Figure 4 shows a battery module with battery cells connected in series in a 3p arrangement with a battery cell coupling device, Figure 5 shows a battery module in a 1p arrangement with coupling devices assigned to it in the main and bypass lines, Figure 6 shows a battery module in a 3p arrangement with coupling devices assigned to it in the main and bypass lines and Figure 7 shows a schematic process flow of the method proposed according to the invention for operating a fault-tolerant battery system.

[0036] Figure 1shows a structure of a battery system 10 with four strings 12, each comprising a battery module 18, 20, 22, 23, a first string contactor 14 and a second string contactor 16. From the illustration according to Figure 1 It can be seen that a battery cell monitoring unit 36 ​​is assigned to each of the battery modules 18, 20, 22, 23. The battery modules 18, 20, 22, 23 are constructed in such a way that the battery cells 24, 26, 28, 30 are interconnected in a 12s3p circuit 32. The battery system 10 as shown in Figure 1 comprises four battery control units 40, each of which is connected to both the first string contactor 14 and the second string contactor 16 and also communicates with the battery cell monitoring unit 36 ​​of the individual battery modules 18, 20, 22, 23 via a communication line 42, such as a CAN bus. The battery system 10 as shown in Figure 1further comprises a first main contactor 60 and a second main contactor 62 for switching the battery system 10 on and off.

[0037] In the Figure 1 In the battery system 10 shown, ten to twelve battery cells 24, 26, 28, 30 usually form a battery module 18, 20, 22, 23. If several such battery modules 18, 20, 22, 23, for example eight to ten battery modules, are connected in series with one another, the required battery voltage of, for example, 400 V is achieved.

[0038] From the representation according to Figure 2 It can be seen that the battery system 10 comprises three strings 12. Within each of the strings 12, which is protected by a first string contactor 14 and a second string contactor 16, there are a first battery module 18, a second battery module 20 and a third battery module 22, wherein the battery modules 18, 20, 22 are connected in series with one another. As can be seen from the illustration according to Figure 2As can be seen furthermore, within the individual battery modules 18, 20, 22, the battery cells 24, 26, 28, 30 are connected in a series circuit 34. Figure 2 The illustration shown is also referred to as a 1p arrangement 48. Each of the battery modules 18, 20, 22 as shown in Figure 2 A battery cell monitoring unit 36 ​​with sensors not shown here is assigned to the battery system 10. The battery system 10 further comprises a battery control unit 40, which communicates with the battery cell monitoring units 36 via a communication line 42.

[0039] Instead of the Figure 2 In addition to the 1p arrangement 48 shown, the individual battery modules 18, 20, 22 can also be arranged in a 2p arrangement or a 3p arrangement 50, see illustration according to Figure 1 , be constructed. All battery modules 18, 20, 22 as shown in Figure 2 have an identical structure.

[0040] The Figure 1 and2 The battery systems 10 shown are only fault-tolerant insofar as, in the event of a fault, a battery module 18, 20, 22, 23 or string 12 is switched off, which is accompanied by a reduction in capacity and thus a reduction in power. Embodiments of the invention

[0041] In the following description of the embodiments of the invention, identical or similar elements are designated by the same reference numerals, whereby a repeated description of these elements is omitted in individual cases. The figures only schematically illustrate the subject matter of the invention.

[0042] Figure 3 shows an electric drive system 100. The electric drive system 100 comprises a three-strand battery system 10, an electric motor 80. The battery system 10 and an inverter 70 are connected to the electric motor 80.

[0043] From the representation according to Figure 3It can be seen that the strings 12 each comprise a battery module 18, 20, 22. The first string 12 comprises the first battery module 18, the second string 12 the second battery module 20 and the third string 12 the third battery module 22. The three battery modules 18, 20, 22 of the battery system 10 each comprise a battery cell monitoring unit 36. In each battery module 18, 20, 22, the battery cells 24, 26, 28, 30 are connected in series circuit 34. In contrast to the design of the battery modules 18, 20, 22, which are also connected in series circuit 34, according to Figure 2 , are in the Figure 3 First coupling devices 44 are provided in the battery module 18, 20, 22 shown in the battery system 10 of a main line 52 of the string 12. Each of the first coupling devices 44 provided in the battery modules 18, 20, 22 according to the embodiment in Figure 3A bypass line 54 is assigned to each of the battery cells 24, 26, 28, 30 shown, in each of which a second coupling device 46 is located.

[0044] If one of the battery cells 24, 26, 28, 30 of the battery modules 18, 20, 22 fails according to Figure 3 off, the first coupling device 44 assigned to this battery cell 24, 26, 28, 30 is opened, so that the respective battery cell 24, 26, 28, 30 can be switched off. Bypassing occurs by closing the second coupling device 46 in the bypass line 54 assigned to the battery cell 24, 26, 28, 30 to be switched off.

[0045] For the sake of completeness, it should be mentioned that in the battery modules 18, 20, 22 of the battery system 10 according to Figure 3 the battery cells 24, 26, 28, 30 are connected in 1p arrangement 48.

[0046] A common first string contactor 14 is assigned to the strings 12 of the battery system 10. To switch off the strings 12 independently of one another, the strings 12 each comprise a second string contactor 16.

[0047] Instead of bridging the defective battery cell 24, 26, 28, 30 from the battery module 18, 20, 22 and bridging other intact battery cells 24, 26, 28, 30 from the remaining battery modules 18, 20, 22 or strings 12 for the purpose of voltage equalization, which is accompanied by a voltage reduction and ultimately a range reduction, a voltage equalization is carried out here by actuating the second string contactor 16 of the battery module 18, 20, 22 or string 12 with the defective battery cell 24, 26, 28, 30.

[0048] To illustrate the method proposed by the invention, it is assumed that a fault exists in the first battery cell 24 of the first battery module 18. Of course, faults may exist in another battery cell or in multiple battery cells 24, 26, 28, 30 of a battery module 18, 20, 22 or in different battery modules 18, 20, 22.

[0049] If the battery control unit 40, in which a battery system management system is implemented, detects a fault in the first battery cell 24 of the first battery module 18, be it a cell fault or an electronic fault, the first and second coupling devices 44, 46 of the first battery cell 24 of the first battery module 18 are actuated to permanently bridge them. At the same time, the second string contactor 16 of the first string 12 switches over and disconnects the first string 12 from the other two strings 12, since a voltage difference exists between the battery modules 18, 20, 22 and the strings 12, which would lead to high equalizing currents, damaging the battery cells 24, 26, 28, 30 and thus causing them to age more quickly.

[0050] Since the string contactors 14, 16 should not be operated under high load, care must be taken to ensure that the current at the time of shutdown is not too high, for example, less than 20 A. This can easily be taken into account in the charging strategy during the charging process. The situation is different while driving, i.e., during the discharging process; here, an abrupt drop in power should not occur. Fault type detection of a safety-critical cell fault or a less critical electronic fault means that, in the latter case, shutdown can be delayed until the current condition is met, such as when stationary at a traffic light or during coasting when there is no torque demand. However, a safety-critical cell fault must lead to shutdown immediately after its detection. The driver can be informed of a possible imminent drop in power by a message in the cockpit.The battery management system will largely counteract the power reduction by distributing the required current between the two intact strings 12, here the second and third strings 12 or battery modules 20, 22. Thus, the battery cells 24, 26, 28, 30 of the second and third strings 12 or battery modules 20, 22 are subjected to higher string currents within the permissible limits. Thus, the driver only notices a drop in power during full-load operation, i.e., at maximum torque demand.

[0051] The second string contactor 16 of the first string 12 remains open only until the voltages of the first and second strings 12 have almost equalized to the level of the first string 12, for example, to within approximately 1 V to 2 V. During this time, the second and third strings 12 provide energy for the electric vehicle and, where possible, supply correspondingly higher string currents. If a recuperation process takes place during this phase, the second string contactor 16 of the first string 12 closes, and the intact battery cells 26, 28, 30 of the first battery module 18 are charged until the voltages have equalized, while the second string contactors 16 of the second and third strings 12 are open. After that, the battery cells 24, 26, 28, 30 of all three strings 12 are charged.

[0052] At the same voltage level, all three strings 12 are available again and can thus deliver their maximum power until a final discharge voltage is reached. The energy loss, which ultimately affects the range of the electric vehicle, results solely from the duration of the voltage adjustment phase and the discharge current. This advantageously increases the availability and reliability of the traction supply. This allows any desired safe-stop level to be achieved.

[0053] The journey can therefore be continued without restrictions; repairs can be carried out afterwards. If no battery cells or battery modules are being replaced and the electric vehicle's battery is being charged, the charging process can be terminated when the string voltage, reduced by one cell voltage, is reached. Alternatively, the two intact strings 12 can also be fully charged by opening the second string contactor 16 of the first string 12. In this case, the discharging process takes place as described above with the second string contactor 16 of the first string 12 open until the voltages of the three strings 12 are at the same level.

[0054] An advantage of this design is that multiple faults in a battery module 18, 20, 22 or a string 12, so-called double faults or multiple faults, can be handled. Faults in different strings 12 can also be mapped using the method proposed according to the invention. The method proposed according to the invention offers the advantage that no further battery cells 24, 26, 28, 30, apart from the faulty battery cell 24, 26, 28, 30, need to be switched off, which is associated with a smaller range reduction. Thus, the operation of the battery system 10 in the event of a fault is ensured without additional hardware while simultaneously maintaining the maximum possible capacity.

[0055] Figure 4 shows the first battery module 18 of the fault-tolerant battery system 10 with first and second coupling devices 44, 46, which are assigned to battery cells 24, 26, 28, 30 connected in a 3p arrangement 50.

[0056] In Figure 4 the first battery module 18 is constructed in such a way that individual battery cells 24, 26, 28, 30 connected to 3p battery cell packs 38 are connected in series circuit 34, for example twelve battery cell packs 38. The first battery module 18 according to Figure 4 is constructed in such a way that the first coupling device 44 is located in the main line 52 upstream of each 3p arrangement 50 or each battery cell pack 38 of the individual battery cells 24, 26, 28, 30. The bypass line 54 branches off upstream of this, in which the second coupling device 46 is accommodated.

[0057] By the design variant of the first battery module 18 as shown in Figure 4continued operation of the first battery module 18 is also possible if individual battery cells 24, 26, 28, 30 connected in a 3p arrangement 50 or battery cell pack 38 should fail, so that continued operation of the first battery module 18 is possible by opening or closing the first and second coupling devices 44, 46, ie by bypassing the defective battery cell 24, 26, 28, 30.

[0058] Figure 5 shows the first battery module 18 in 1p arrangement 48 with coupling devices 44, 46 assigned to it in the main and bypass lines 52, 54.

[0059] From the representation according to Figure 5A fault-tolerant battery system 10 emerges, the first battery module 18 of which, as an example, comprises battery cells 24, 26, 28, 30 connected to one another in series circuit 34. The first battery module 18 shows the master and slave battery cell monitoring units 36, which are each connected to the battery cells 24, 26, 28, 30 connected to one another in series circuit 34.

[0060] As from Figure 5 As can be seen further, the first coupling device 44 is located in the main line 52, in front of which, analogously to the embodiment according to Figure 4, the bypass line 54 branches off. The second coupling device 46 is arranged in this. To disconnect the first battery module 18 from the main line 52, the first coupling device 44 is opened and the second coupling device 46 is closed, so that a first battery module 18 that has turned out to be defective, for example, can be bypassed via the bypass line 54 within the string 12 of the battery system 10, and continued operation of the fault-tolerant battery system 10 proposed according to the invention is possible.

[0061] Figure 6 shows the first battery module 18 in 3p arrangement 50, to which a first and a second coupling device 44, 46 are assigned.

[0062] Out of Figure 6 It can be seen that the first battery module 18 shown there has a number of battery cells 24, 26, 28, 30, which are each connected in parallel in a 3p arrangement 50 or battery cell pack 38. The Figure 6The first battery module 18 of the battery system 10, which has been singled out, has a first coupling device 44 in the main line 52. The bypass line 54, in which the second coupling device 46 is accommodated, branches off upstream of the first coupling device 44. If the battery system 10, whose battery cells 24, 26, 28, 30 are connected in a 3p arrangement 50 and are connected in series 34, fails, the first coupling device 44 is opened, the second coupling device 46 located in the bypass line 54 is closed, and thus the defective battery module is bridged, so that in the battery system 10 only the defective one of the battery modules 18, 20, 22 is switched off and the battery system 10 can continue to operate, albeit with reduced power.This ensures that the fault-tolerant battery system 10 according to the present invention ensures continued operation of the autonomously driving electric vehicle, and that the journey does not need to be interrupted but can be continued, albeit with reduced power and increased duration.

[0063] Figure 7 shows a schematic process flow 200 of the method proposed according to the invention for operating a fault-tolerant battery system 10.

[0064] Out of Figure 7 It can be seen that, starting from a start in a method step 201, the method proposed according to the invention begins to run.

[0065] First, a fault in a battery cell 24, 26, 28, 30 is detected in a method step 202. This can be done by evaluating the measured values ​​of the battery cells 24, 26, 28, 30.

[0066] Subsequently, in a method step 203, the defective battery cell 24, 26, 28, 30 or the defective battery cell pack 38 in which the defective battery cell 24, 26, 28, 30 is located is disconnected and bridged. The defective battery cell 24, 26, 28, 30 or the defective battery cell pack 38 remains permanently bridged.

[0067] At the same time, in a method step 204, the faulty string 12 which has the faulty battery cell 24, 26, 28, 30 or the faulty battery cell pack 38 is switched off.

[0068] Subsequently, in a method step 205, the strand voltage of the faulty strand 12 is compared with the strand voltage of the respective intact strands 12 in which no faults were detected.

[0069] If voltage differences between the strings 12 exceed a voltage threshold, the intact strings 12 are discharged in a method step 206. The faulty string 12 remains switched off only until the string voltages of the intact strings 12 have almost equalized to the level of the faulty string 12. In this case, the intact strings 12 provide the energy for the electric vehicle and supply correspondingly higher string currents. If a recuperation process takes place during the discharging of the intact strings 12, the faulty string 12 is switched on. The intact battery cells 24, 26, 28, 30 of the faulty string 12 are charged by the recuperation current until the voltages of the strings 12 have equalized. After that, the intact battery cells 24, 26, 28, 30 of all strings 12 are charged.

[0070] If voltage differences between strings 12 fall below the voltage threshold, the faulty string 12 is switched on in a method step 207. The battery cells 24, 26, 28, 30 of all strings 12 are then discharged in a method step 208. If a recuperation process occurs during this process, the battery cells 24, 26, 28, 30 of all strings 12 are also charged.

[0071] At the same voltage level, all strings 12 are available again in a method step 209 and can thus deliver their maximum power until a final discharge voltage is reached, and the method proposed according to the invention is terminated in a method step 210 when the battery system 10 is switched off.

[0072] The invention is not limited to the embodiments described here and the aspects highlighted therein. Rather, numerous modifications are possible within the scope of the claims, which are within the scope of expert practice.

Claims

1. Method for operating a battery system (10), which comprises a plurality of parallel connected strings (12), each having at least one battery module (18, 20, 22, 23), in which a plurality of battery cells (24, 26, 28, 30) are connected in a series circuit (34) and / or in a parallel circuit, wherein the strings (12) can be switched on and off from one another, and wherein individual battery cells (24, 26, 28, 30) and / or individual battery cell packs (38), each comprising a plurality of battery cells (24, 26, 28, 30) connected in parallel, can be switched on and off from one another and can be bypassed, comprising at least the following method steps: - detecting a fault in a battery cell (24, 26, 28, 30); - switching off and bypassing the faulty battery cell (24, 26, 28, 30) and / or the faulty battery cell pack (38) in which the faulty battery cell (24, 26, 28, 30) is located; - switching off the faulty string (12) which has the faulty battery cell (24, 26, 28, 30) and / or the faulty battery cell packet (38); - comparing the string voltage of the faulty string (12) with the string voltage of the respective intact strings (12) in which no faults have been detected; - discharging the intact strings (12) if voltage differences between the intact strings (12) exceed a voltage threshold value, - switching on the faulty string (12), if the string voltages of the respective intact strings (12) have almost matched the level of the string voltage of the faulty string (12), or if a recuperation process takes place during the discharging of the intact strings (12).

2. Method according to Claim 1, characterized in that the faulty string (12) is switched on if voltage differences between the strings (12) fall below the voltage threshold value.

3. Method according to Claim 1 or 2, characterized in that the voltage threshold value lies within a range of 1 V to 2 V.

4. Method according to one of Claims 1 to 3, characterized in that while the battery system (10) is being charged, all of the strings (12) are charged, with a charging process being terminated when a string voltage reduced by one or more cell voltages is reached.

5. Method according to one of Claims 1 to 3, characterized in that while the battery system (10) is being charged, the intact strings (12) are fully charged, the charging being followed by a discharging process of the intact strings (12) until the string voltages of all of the strings (12) are at the same level.

6. Method according to one of Claims 1 to 5, characterized in that when electronic faults are detected, the faulty string (12) is switched off only when a string current falls below a current threshold value.

7. Method according to Claim 6, characterized in that the current threshold value lies in the range of 15 A to 25 A.

8. Battery system (10) which is configured to carry out a method according to one of Claims 1 to 7.

9. Vehicle which is designed to carry out a method according to one of Claims 1 to 7, and / or which comprises a battery system (10) according to Claim 8.