Redox flow battery system and method for operating same

Abstract

The invention relates to a method for operating a redox flow battery system, in which method an intervention is carried out in a battery module (1), which intervention comprises the following steps: - stopping the supply of electrolytic fluid to at least some of the cell assemblies (2) of the corresponding battery module (1); - short-circuiting the cell assemblies (2) of the corresponding battery module (1) to which the supply of electrolytic fluid was stopped when a potential difference between the negative electrolyte and the positive electrolyte in a cell assembly (2) of the corresponding battery module (1) has fallen below a predefined value; - carrying out measures; - supplying electrolytic fluid to the cell assemblies of the corresponding battery module (1) to which the supply of electrolytic fluid was stopped; - opening the short circuits of the cell assemblies (2) of the corresponding battery module (1).

Description

[0001] Redox flow battery system and method of operation

[0002] The invention relates to a redox flow battery system and a method for operating a redox flow battery system, wherein the redox flow battery system comprises a plurality of battery modules connected in series, and wherein the method relates to the reduction or elimination of imbalances occurring between series-connected battery modules during charging and discharging of the battery system, the maintenance of a battery module, or the decoupling of one or more battery modules to optimize partial load operation. The invention particularly relates to a high-performance redox flow battery system.

[0003] Redox flow battery systems and methods for reducing or eliminating imbalances that occur between series-connected battery modules during charging and discharging are known from the prior art. For example, DE 102020 108053 A1 discloses such a system and method. Individual battery modules are decoupled from the series circuit. In high-performance battery systems, switching operations must be performed at high currents. This places high stress on the switches.

[0004] Measures to reduce or eliminate these imbalances are usually referred to as 'balancing'.

[0005] The object of the invention is to provide an alternative redox flow battery system and a method for operating such a system.

[0006] This object is achieved according to the invention by a redox flow battery system and by methods according to the independent claims. Further advantageous embodiments of the present invention can be found in the subclaims.

[0007] The invention is explained below with reference to the figures. The figures show in detail: Fig. 1: Battery module according to the prior art;

[0008] Fig.2 Redox flow battery system according to the state of the art;

[0009] Fig.3 Battery module according to the invention;

[0010] Fig.4 Redox flow battery system according to the invention in a first embodiment;

[0011] Fig.5 Charge / discharge cycles without balancing;

[0012] Fig.6 Charge / discharge cycles with balancing according to the state of the art;

[0013] Fig.7 Charge / discharge cycles with balancing according to the invention;

[0014] Fig.8 Charge / discharge cycles with balancing according to the invention;

[0015] Fig.9 Charge / discharge cycles with balancing according to the invention;

[0016] Fig.10 Course of the terminal voltage of a cell arrangement during a balancing intervention according to the invention;

[0017] Fig.11 Redox flow battery system according to the invention in a further embodiment;

[0018] Fig.12 Redox flow battery system according to the invention in a further embodiment;

[0019] Figure 1 shows, on the left-hand side, a schematic representation of a battery module according to the prior art. The battery module is designated 1. The battery module comprises a cell arrangement, designated 2, and a tank device, designated 3. The cell arrangement 2 is an arrangement of a plurality of redox flow cells, which can be arranged in any desired manner. For example, it could be a single cell stack, a series connection of several stacks, a parallel connection of several stacks, or a combination of series and parallel connections of several stacks. The tank device 3 serves to store the electrolyte fluid and to supply the cell arrangement 2 with electrolyte fluid. For this purpose, the tank device 3, with a few exceptions, comprises at least two tanks, a pipe system for connecting the tanks to the cell arrangement 2, and pumps for conveying the electrolyte fluid.Figure 1 shows two separate pumps. The electrolyte fluid could just as easily be pumped with a double-head pump, i.e., with two pumps driven by a common motor. The tank device 3 is designed to supply all cells of the cell array 2 with electrolyte fluid. Thus, if the pumps 4 pump electrolyte fluid, the fluid flows through all cells of the cell array 2.

[0020] The battery module 1 shown in Figure 1 comprises a measuring device for providing a measured value that represents the state of charge (SoC) of the associated battery module and is designated by 4. Measuring device 4 is a measuring device for providing the so-called open circuit voltage (OCV). The OCV value is a measure of the battery module's state of charge (SoC).

[0021] The battery module 1 shown in Figure 1 further comprises a measuring device for providing a potential difference, designated by 5. The measuring device 5 measures a potential difference that is formed between a first potential of the negative electrolyte and a second potential of the positive electrolyte. In the measuring device 5 for providing a potential difference, the electrodes for tapping the aforementioned potentials are located in corresponding cells of the cell arrangement 2. The potential difference formed depends on the number of cells connected in series between the electrodes for tapping the potentials.Since redox flow battery modules typically include a measuring device that can detect and provide a potential difference between the two outer electrodes of the cell array 2, it is easiest to use this measuring device to provide the described potential difference. The potential difference between the two outer electrodes is referred to as the terminal voltage. The provided potential difference is always a measure of the SoC of the electrolyte fluid contained in the cell array 2.

[0022] On the right-hand side of Figure 1, a symbolic representation of the battery module 1 is shown. This symbolic representation will be used below. Figure 2 shows a schematic representation of a battery system according to the prior art. The battery system comprises at least two battery modules, one of which is designated 1, a bidirectional power conversion system (PCS), designated 6, and a control device, designated 7. The battery modules 1 are connected in series and to the converter 6. Figure 2 shows four battery modules, with the dashed lines in the diagram indicating any number of additional modules. The converter 6 connects the battery system to the grid or to a higher-level electrical system.The battery system further comprises, for each battery module 1, a first switch, one of which is designated by 8, and a second switch, one of which is designated by 9. The first switch 8 is arranged in series with the battery modules 1, with one switch being arranged in front of or behind each battery module 1. This means that with each of the first switches 8, the series connection of the battery modules can be interrupted. The second switches 9 are each arranged in a bypass line around a battery module 1 and the associated first switch 8. In Figure 2, all switches 8 and 9 are shown in the open state. In reality, the switches are controlled by the control device 7 such that, of each pair of first and second switches, exactly one switch is closed and one switch is open (alternately open and closed). This meansA switch pair has exactly two switching positions, whereby in the first switching position (first switch 8 closed and second switch 9 open) the associated battery module 1 is in the series connection of the battery system, and in the second switching position (first switch 8 open and second switch 9 closed) the associated battery module 1 is separated from the series connection of the battery system by the bypass line. Opening the first switch 8 with the second switch 9 closed prevents the module from discharging via the bypass line. Since the bypass line represents a short circuit, such a discharge of the battery module via the bypass line would also lead to very high currents that could damage or even destroy the battery module.

[0023] In a battery system as shown in Figure 2 with completely identical battery modules 1, no harmful imbalance could arise. However, real battery modules 1 differ due to manufacturing variations and aging processes. Furthermore, different operating conditions, e.g. temperature differences between the individual modules, can cause them to behave differently. For these reasons, real battery modules have different efficiency values ​​and different internal resistances. For a given charging or discharging current, higher efficiency leads to the battery module in question reaching its final state more quickly. As the same current flows through all of the battery modules 1 in the series circuit as shown in Figure 2, the high-efficiency modules reach their final state more quickly than the low-efficiency modules. To avoid damage, the charging or discharging current must beThe discharge process must be aborted as soon as a module reaches its respective final state. If this effect is not compensated for, the usable storage capacity of such a battery system decreases with each cycle (“capacity fading”). The different internal resistance of the modules has a similar effect. There are upper and lower limits for the terminal voltage which must not be exceeded or undercut. Even with identical efficiency, a module with a higher internal resistance will reach the respective limit of the terminal voltage more quickly during charging or discharging than a module with a lower internal resistance. If the first module reaches this limit, the respective process must be aborted, which also leads to a reduction in the usable capacity of the battery system. Alternatively, the system's output could also be reduced.In any case, these effects lead to a degradation of the system. Balancing is intended to reduce or completely eliminate these effects in order to maintain the usable capacity of the battery system at a consistently high level or to eliminate the degradation described. On the other hand, successful balancing enables the use of cells with a comparatively high variation in efficiency and / or internal resistance, which naturally translates into reduced production costs.

[0024] Figure 3 shows a schematic representation of a battery module 1 according to the invention. In contrast to the battery module shown in Figure 1, the battery module 1 according to the invention comprises at least two cell arrangements 2. The battery module 1 shown in Figure 3 comprises, by way of example, three cell arrangements 2. The cell arrangements 2 are each constructed as described above for Figure 1, i.e. each cell arrangement 2 is an arrangement of a plurality of redox flow cells, which can be arranged in any desired manner. For example, it could be a single cell stack, a series connection of several stacks, a parallel connection of several stacks, or a combination of series and parallel connections of several stacks. The at least two cell arrangements 2 are electrically separated from one another within the battery module 1, i.e. there are no electrical lines within the battery module 1 which connect the cell arrangements 2 to one another.Rather, the terminals of each cell arrangement 2 are led out separately. Like conventional battery modules, the battery module according to the invention comprises a tank device 3. The tank device 3 serves to store the electrolyte fluid and to supply the at least two cell arrangements 2 with electrolyte fluid. For this purpose, the tank device 3 comprises at least two tanks, a pipe system for connecting the tanks to the at least two cell arrangements 2, and pumps for conveying electrolyte fluid. For the sake of clarity, the pipes and pumps are not shown in Figure 3. As with conventional battery modules, the tank device 3 is designed such that it can supply all cells of the at least two cell arrangements with electrolyte fluid. The hydraulic system for supplying the at least two cell arrangements 2 with electrolyte fluid represents a hydraulic parallel connection of the at least two cell arrangements 2.

[0025] The pumps can be arranged in different ways. In a first embodiment, two pumps are arranged so that they can supply all cell assemblies 2 of the battery module with electrolyte fluid simultaneously. If the pumps pump electrolyte fluid, it flows through all cells of the at least two cell assemblies 2. Optionally, means can be provided for each cell assembly 2 to hydraulically isolate it. This makes it possible to selectively prevent the flow of electrolyte fluid through one or more of the cell assemblies of the battery module. The pumps then only supply the remaining cell assemblies with electrolyte fluid. The means for hydraulically isolating the cell assemblies can be, for example, shut-off valves in the supply lines to the relevant cell assemblies.In another embodiment, separate pumps are provided for each cell arrangement of the battery module to supply the associated cell arrangement with electrolyte fluid. If the pumps associated with a cell arrangement deliver electrolyte fluid, the fluid flows through all cells of the respective cell arrangement. By switching off specific pumps, the flow of electrolyte fluid through one or more of the cell arrangements of the battery module can be selectively prevented.

[0026] The battery module 1 shown in Figure 3 optionally comprises at least one measuring device for providing a potential difference, which is not shown in Figure 3. As described in relation to Figure 1, the measuring device 5 measures a potential difference which is formed between a first potential of the negative electrolyte and a second potential of the positive electrolyte. If the battery module 1 according to the invention comprises only one such measuring device, the electrodes for tapping the aforementioned potentials are located in corresponding cells of any cell arrangement 2 of the battery module. A single such measuring device 5 per battery module 1 can be used if the at least two cell arrangements 2 in the battery module differ only insignificantly from one another. As will be explained in more detail below, measuring devices 5 are not absolutely necessary for carrying out the method according to the invention.However, the operational reliability of the method according to the invention can be improved if such measuring devices 5 are used. It is particularly advantageous if a measuring device 5 for providing a potential difference is provided for each cell arrangement 2 in the battery module 1. As described above, the potential difference provided by the measuring devices 5 can advantageously be the terminal voltage of the associated cell arrangement 2.

[0027] On the right-hand side of Figure 3, two symbolic representations of the battery module 1 according to the invention are shown. The representation shown on the far right differs from the other in that the battery module 1 comprises means for each cell arrangement 2 with which the associated cell arrangement 2 can be short-circuited. The simplest embodiment for such a means is a short-circuit line and a switch arranged in the short-circuit line. Such a switch is designated 10 in Figure 3. The switch 10 can be a relay or a semiconductor arrangement for switching. More complex short-circuiting means are also conceivable. For example, a short-circuit bar could be provided parallel to the battery string, to which individual battery modules can be connected with the aid of two switches. The short-circuiting means 10 are required for the method according to the invention.They can be part of the battery module (as shown on the far right), or alternatively they can be located outside the battery modules 1.

[0028] Figure 4 shows a battery system according to the invention in a first embodiment. The battery system comprises at least two battery modules, one of which is designated 1, a bidirectional power conversion system (PCS), designated 6, and a control device, designated 7. Each battery module 1 comprises at least two cell arrangements, the number of cell arrangements being the same in all battery modules. The battery modules 1 shown in Figure 4 each comprise three cell arrangements. The cell arrangements of the battery modules 1 are connected in series and to the converter 6 such that parallel strings are formed, and the number of strings corresponds to the number of cell arrangements per battery module. The battery modules 1 connected together in this way form a string.To distinguish between the strings, we will refer to “cell arrangement strings” and a “battery string” below. Figure 4 shows five battery modules 1, with the dashed lines in the series circuit indicating any number of additional modules. The converter 6 connects the battery system to the grid or to a higher-level electrical system. For each cell arrangement, the battery system includes a means for short-circuiting the associated cell arrangement. In Figure 4, the short-circuiting means are parts of the associated battery modules. The short-circuiting means could also be arranged outside the battery modules. The control device 7 is designed such that it can control the pumps, if necessary the means for hydraulically isolating the cell arrangements, and the means 10 for short-circuiting the cell arrangements. This control can be direct or indirect.In the latter case, each battery module could include a separate controller connected to the control device 7. Such controllers are commonly referred to as BMS (battery management system). The pumps, the hydraulic isolation means, and the short-circuiting means 10 are then controlled by the BMS associated with the respective battery module. The BMS, in turn, is triggered by the control device 7.

[0029] By dividing the battery string into several parallel cell arrays, the charging and discharging current is distributed among the individual cell arrays. The current in the cell arrays is reduced accordingly. This means that the switching processes when short-circuiting the cell arrays (see below) place less strain on the short-circuiting devices than would be the case if the battery string were not subdivided.

[0030] In the various embodiments of the method according to the invention, one or more battery modules 1 are decoupled from the battery string, or one or more cell assemblies are decoupled from one or more cell assembly strings. If an entire battery module 1 is decoupled from the battery string, this means that all cell assemblies belonging to the respective battery module are decoupled from the cell assembly strings.

[0031] In the various embodiments of the method according to the invention, the decoupling occurs for different purposes or to perform different measures. In a first embodiment, the measure is a balancing intervention. In a second embodiment, maintenance measures are performed on the decoupled battery module. In a further embodiment, the measure is operating the battery system in partial load mode.

[0032] The method according to the invention is first explained using balancing interventions, with the fundamentals of balancing being discussed in more detail later. Furthermore, only the case in which entire battery modules are decoupled is initially discussed. The other embodiments follow thereafter. Figure 5 shows two charge / discharge cycles of two battery modules connected in series with different efficiency values. For clarity, the efficiency difference has been chosen to be very high. In real battery systems, the efficiency differences are much smaller. In Figure 5, the SoC curve of the battery module with the higher efficiency is shown as a solid line, and the SoC curve of the battery module with the lower efficiency is shown as a dashed line. In Figure 5, the minimum state of charge is marked as 0% and the maximum state of charge as 100%.

[0033] The more efficient battery module reaches the 100% SoC value when the less efficient battery module is not yet fully charged. Since both battery modules are subjected to the same current, the charging process must now be stopped. If you then want to discharge the battery system, the process begins when the less efficient battery module is not yet fully charged. Due to this unequal starting point for discharging and the lower efficiency, the less efficient battery module reaches the 0% SoC value when the more efficient battery module is not yet fully discharged. Since the described effects act cumulatively, the SoC curves of the two battery modules diverge further and further with the increasing number of cycles, and the usable capacity of the battery system continues to decrease.

[0034] The negative effect described can be avoided by taking balancing measures.

[0035] Figure 6 shows two diagrams, each with a charging cycle of two battery modules connected in series with different efficiencies. In each case, a balancing intervention is carried out in one half-cycle. The SoC curves shown can be generated by decoupling one or more battery modules from the series connection for a certain period of time, so that they no longer participate in the charging or discharging of the remaining battery modules during this time. Ideally, the SoC curve for the decoupled modules is horizontal (self-discharge can be neglected). For this purpose, the first and second switches 8 and 9 are used in a battery system according to Figure 2. During the charging half-cycle, the more efficient battery module is decoupled, and during the discharging half-cycle, the less efficient battery module is decoupled. The difference between the two diagrams lies in the different decoupling times.

[0036] In the left diagram of Figure 6, the battery module in question is only decoupled until the two curves are equal. For example, during charging, the efficient battery module is decoupled until the less efficient battery module reaches the same state of charge as the more efficient battery module.

[0037] In the right-hand diagram of Figure 6, the battery module in question is decoupled for a longer period, causing the two battery module curves to intersect. For example, during the charging half-cycle, the more efficient battery module is decoupled until the less efficient battery module has gained a sufficiently large "head start" in charging, so that the more efficient battery module catches up with the less efficient battery module just at 100% SoC. Such a balancing intervention could be described as a type of temporary overcompensation, since equalization only occurs some time after the intervention.

[0038] The inventors have recognized that a generic battery system as shown in Figure 2 can be constructed more simply by omitting the switches 8 for interrupting the series connection, with the balancing interventions being implemented differently than those known from the prior art. This will be explained in more detail below. The inventors have further recognized that a battery system constructed according to the invention enables the decoupling of individual or multiple battery modules (or individual or multiple cell arrangements) for maintenance purposes or for optimizing partial load operation. This will be explained in more detail following the description of the balancing interventions.

[0039] The balancing interventions according to the invention are characterized by the fact that they are only triggered during the discharging of the battery system. This means that, in a cyclically operated battery system, either all balancing interventions take place only during the discharging half-cycles or, if the application of the battery system permits, the battery system is briefly switched to discharging during the charging half-cycle if a balancing intervention becomes necessary during the charging half-cycle. In the second case, the battery system is switched back to charging after or during the balancing intervention.

[0040] Figure 7 shows charge / discharge cycles with balancing interventions according to the invention. In the discharge half-cycle, the SoC curves do not differ from the SoC curves shown in Figure 6. In the left-hand diagram, the interventions continue until the two SoC curves have equalized. In the charge half-cycle, the system switches to discharge during the intervention, whereby only the more effective battery module actually experiences a significant discharge. In the right-hand diagram, a balancing intervention only occurs in the discharge half-cycle. The intervention is of the "overcompensation" type. Figure 8 shows another charge / discharge cycle with balancing interventions according to the invention. An intervention occurs in each half-cycle. Both interventions are of the "overcompensation" type. The "overcompensation" type balancing intervention in the charge half-cycle significantly extends the charge / discharge cycle.

[0041] Figure 9 shows another charge-discharge cycle with balancing interventions according to the invention. In contrast to Figure 8, in the charging half-cycle, it is not the less efficient but the more efficient battery module that is short-circuited. As in Figure 8, the battery system is switched to discharging during the intervention, but only for a short time. Afterward, it is switched back to charging. The period for which the system switches to discharging can be very short. It only needs to be long enough to perform the first two steps of the balancing intervention (see next section). Both interventions are of the "overcompensation" type. The embodiment according to Figure 9 is characterized by a shorter cycle time compared to the embodiment according to Figure 8.

[0042] The sequence of a balancing procedure according to the invention is described in more detail below. It is assumed that the battery system is in discharge mode and that a balancing procedure is to be performed on at least one battery module. The balancing procedure comprises the following steps: - Preventing the supply of electrolyte fluid to the cell arrangements of the respective battery module

[0043] - Short-circuiting the cell arrangements of the battery module in question if the terminal voltage in the cell arrangement of the battery module in question has fallen below a predefined value

[0044] - Wait until the final balancing state has been reached

[0045] - Supply of electrolyte fluid into the cell arrangements of the battery module in question

[0046] - Opening the short circuits of the cell arrangements of the battery module in question

[0047] The easiest way to stop the supply of electrolyte is to turn off the pumps. Similarly, the subsequent re-supply of electrolyte is most easily achieved by turning the pumps on.

[0048] During the "Wait until the final balancing state is reached" step, the battery system is either discharged—as shown in Figures 7 and 8—with the discharge current flowing through all battery modules except for the battery module undergoing balancing intervention, or—as shown in Figure 9 in the charging half-cycle—the battery system is switched to charging after a short discharge phase. In this case, too, during the "Wait until the final balancing state is reached" step, the discharge or charging current flows through all battery modules except for the battery module undergoing balancing intervention. The last two steps can be performed both during charging and discharging of the battery system.

[0049] The final balancing state can be either an adjustment or an overcompensation.

[0050] Since the internal resistance of the cell arrays is much greater than the resistance of the corresponding short-circuit lines, the discharge current flows (almost entirely) past the short-circuited cell arrays through the closed short-circuit line, which, over time, leads to the desired final state of the balancing intervention. Balancing intervention according to the invention can also be performed on several battery modules simultaneously.

[0051] The processes in the battery module on which a balancing intervention according to the invention is carried out are explained in more detail with reference to Figure 10. Figure 10 shows the curve of the terminal voltage VK of a cell arrangement of the respective battery module as a function of time t. Before the balancing intervention, the cell arrangement has the terminal voltage Vo. Vo depends, among other things, on the state of charge of the respective battery module. At time t, the supply of electrolyte fluid to the cell arrangement is stopped. As a result, the electrolyte fluid in the cell arrangement discharges very quickly, since no "fresh" electrolyte fluid is supplied. This leads to a very rapid reduction in the terminal voltage. If the terminal voltage falls below the predefined threshold voltage V shas fallen, then the cell arrangement is short-circuited. In the illustration in Figure 10, this occurs at time t1. As a result, the terminal voltage collapses completely. This state is maintained until the desired final state of the balancing intervention has been reached. During this entire time, no significant amount of "fresh" electrolyte fluid may be supplied to the cell arrangement. This can be ensured, for example, by locking the pumps or closing a valve arranged in the supply line. The measures mentioned are then included, if applicable, in the step "Preventing the supply of electrolyte fluid to the cell arrangements of the battery module in question".

[0052] During a very short time interval immediately after short-circuiting, the entire remaining energy content of the cell array of the battery module in question is converted into heat. To prevent damage to the cell array, the energy content must be sufficiently small. This is ensured by ensuring that the threshold voltage V s is chosen sufficiently small, since the energy content of the cell arrangement scales with the terminal voltage. The determination of the threshold voltage V scan therefore be based on energetic considerations, taking into account at least the following parameters: electrolyte volume in the cell array, cell array structure (including electrode shape, electrode material, thermal coupling of the electrodes to the environment), discharge current, and state of charge. Due to the complexity of the possible influencing factors, an experimental verification of the effectiveness of the specified threshold voltage V is recommended. s .

[0053] Any other conceivable potential difference between the negative and positive electrolyte in the affected cell arrangement exhibits essentially the same curve as the terminal voltage VK shown in Figure 10. The curve only needs to be scaled along the y-axis. The remaining energy content in the cell arrangement also scales in the same way. Therefore, a threshold voltage corresponding to Vs can be set for any other conceivable potential difference, so that short-circuiting the cell arrangement cannot cause damage to it once this threshold voltage is exceeded.Since the terminal voltage represents only a specific potential difference, the criterion “when the terminal voltage in the relevant cell arrangement of the battery module in question has fallen below a predefined value” can be formulated more generally as follows: “when a potential difference between negative and positive electrolyte in a cell arrangement of the battery module in question has fallen below a predefined value”.

[0054] The criterion “when a potential difference between negative and positive electrolyte in the relevant cell arrangement of the relevant battery module has fallen below a predefined value” can also be implemented by allowing a sufficiently long time to elapse between stopping the supply of electrolyte fluid and short-circuiting, i.e. by selecting a sufficiently long time interval Δt = t i - t i. The value Δt to be selected depends, among other things, on the state of charge of the relevant battery module at time t i and the discharge current flowing through the cell arrangement strings in the time interval between t i and t i. In the battery systems investigated by the inventors, Δt was in a range from a few seconds to several minutes. This means that the method according to the invention can also be carried out without measuring and recording a potential difference in the cell arrangement of the relevant battery module.As already mentioned above, it can be advantageous for safety reasons if at least one such potential difference in the battery modules is detected and used to control the process. Such balancing interventions can be carried out not only by decoupling all cell arrangements of a battery module from the associated cell arrangement strings, but also by decoupling only a portion of the cell arrangements of a battery module from the associated cell arrangement strings. The phrase "a portion of the cell arrangements of a battery module" is to be understood that not all cell arrangements of the battery module in question are decoupled, but only one or some of them. It is clear that the battery module in question is then not completely decoupled from the battery string. Therefore, it continues to participate in the discharging or charging process of the battery system via the non-decoupled cell arrangements.Therefore, the relevant sections in the SoC diagrams do not run horizontally during a balancing intervention, but are simply flatter than before the decoupling. Decoupling only a portion of a battery module's cell arrays for balancing purposes enables very sensitive balancing. It is particularly suitable for balancing with a large number of short-term balancing interventions. On the other hand, the duration of the balancing interventions increases compared to balancing in which all sub-arrays of a battery module are decoupled. If only a portion of the cell arrays are decoupled, the method comprises the following steps:

[0055] - Preventing the supply of electrolyte fluid to part of the cell arrangements of the battery module in question

[0056] - Short-circuiting of the cell arrangements of the battery module in question, to which the supply of electrolyte fluid has been stopped, if a potential difference between negative and positive electrolyte in a cell arrangement of the cell arrangement in question of the battery module in question has fallen below a predefined value

[0057] - Wait until the final balancing state has been reached

[0058] - Supply of electrolyte fluid into the cell arrangements of the battery module in question, where the supply of electrolyte fluid has been prevented

[0059] - Opening the short circuits of the cell arrangements of the battery module in question The phrase “at least some of the cell arrangements of a battery module” also includes the case where all cell arrangements of the battery module in question are decoupled.

[0060] The described balancing interventions according to the invention can advantageously be combined with other known types of balancing interventions, e.g., with balancing interventions in which a load is connected in parallel with the battery module in question. In this case, the balancing interventions during discharging could be implemented using the interventions according to the invention, and the balancing interventions during charging by the aforementioned parallel connection of a load. This can prevent switching to discharge mode during the charging half-cycle. Auxiliary systems of the battery modules, such as pumps, can be considered as loads. However, this can also be an electrical resistor, so that the energy dissipated by the balancing is converted into heat.

[0061] A battery system according to the invention is also suitable for performing maintenance interventions on at least one battery module. In this case, the step "waiting until a final balancing state has been reached" is simply replaced by the step "performing maintenance measures on the relevant battery module." The maintenance measures can be any measures that repair the relevant battery module after a fault or prevent future faults. They can also be maintenance measures that only affect the decoupled cell arrangements. This can also involve replacing the decoupled cell arrangements or completely replacing the relevant battery module. The new battery module then replaces the relevant battery module (and is referred to as such in the subsequent method steps).

[0062] Furthermore, the battery system according to the invention is suitable for decoupling one or more cell arrays from the cell array strings or one or more battery modules from the battery string in order to make operation more efficient in partial load mode. Decoupling reduces the internal resistance of the battery system, thereby reducing losses. In this case, the step of "waiting until a final balancing state has been reached" is simply replaced by the step of "operating the battery system in partial load mode."

[0063] To encompass all these possibilities, the step "waiting until a balancing end state has been reached" is referred to in the independent claim as "performing measures." In the dependent claims, the measures are then specified as "waiting until a balancing end state has been reached," or as "performing maintenance measures on the battery module in question," or as "operating the battery system in partial load mode."

[0064] It should be noted that the two subsequent steps of "supplying electrolyte fluid to the cell assemblies of the battery module in question, where the supply of electrolyte fluid has been prevented," and "opening the short circuits of the cell assemblies of the battery module in question," can be initiated at the same time or performed consecutively. The order of these steps is irrelevant as long as the time interval between the two steps is not too long.

[0065] Through these two steps, the cell arrangements previously decoupled in the first two steps are recoupled into the series connections of the cell arrangement strings. If the intervention was a balancing intervention, this naturally ensures that the recoupled battery module has a suitable SoC. With the other two types of intervention described, care must be taken to ensure that coupling only takes place when the SoC of the battery module in question roughly corresponds to the SoC of the other battery modules. This means that the SoC of the battery module whose cell arrangements are to be coupled in should not deviate by more than 10% - preferably not more than 5% - from the SoC of the other battery modules during coupling. Further adjustment of the SoC can be achieved through a subsequent balancing intervention.

[0066] A further embodiment of the method according to the invention comprises the following steps in the given order: - Preventing the supply of electrolyte fluid to a part of the cell arrangements of the battery module in question

[0067] - Short-circuiting of the cell arrangements of the battery module in question, to which the supply of electrolyte fluid has been stopped, if a potential difference between negative and positive electrolyte in a cell arrangement of the cell arrangement in question of the battery module in question has fallen below a predefined value

[0068] - Implementation of measures

[0069] - Supply of electrolyte fluid into the cell arrangements of the battery module in question, where the supply of electrolyte fluid has been prevented

[0070] - Measuring the current flowing through a cell arrangement of the battery module in question

[0071] - Opening the short circuit of the cell arrangement in question as soon as the measured current has exceeded a predefined threshold

[0072] The advantage of this design is that a jump in the voltage of the battery system when opening the short circuit can be minimized.

[0073] If only some of the cell arrangements in a battery module are decoupled, this means that the individual cell arrangement strings can comprise a different number of cell arrangements. In a battery system as shown in Figure 4, this situation means that the current strength in the individual cell arrangement strings can vary. This means that the voltage drop across the individual cell arrangements can also vary. If this situation persists over a longer period of time, it can lead to problems. There are several ways to prevent this. Firstly, the decoupling processes can be designed so that the number of cell arrangements in the cell arrangement strings (i.e. the cell arrangements not decoupled) is always the same.This is the case when all cell arrays of a battery module are decoupled simultaneously (decoupling of the entire battery module), or when the number of cell arrays decoupled per cell array string at a time is the same. In the latter case, the decoupled cell arrays can also be located in different battery modules. These problems can also be avoided by designing the battery system in such a way that it is capable of controlling the current through the individual cell array strings independently of one another. Figures 11 and 12 show two different battery systems capable of this.

[0074] Figure 11 shows a further embodiment of a battery system according to the invention. Unlike the battery system of Figure 4, the battery system shown in Figure 11 includes a bidirectional converter 6 for each cell array string. Each cell array string is connected to the grid or a higher-level electrical system via the converters 6. The converters 6 enable the currents in the individual cell array strings to be controlled independently of one another.

[0075] Figure 12 shows a battery system according to the invention in a further embodiment. In contrast to the battery system in Figure 4, the battery system shown in Figure 12 comprises an additional converter for each cell arrangement string, one of which is designated 11. The converters 11 make it possible to control the current intensities in the individual cell arrangement strings independently of one another. The converters 11 are so-called DC-DC converters. These converters must also be bidirectional. Since the converters 11 are only required when the number of non-decoupled cell arrangements in the individual cell arrangement strings is different, it is advantageous if the converters 11 can be switched out of the battery system. This can be done, for example, using the short-circuit lines shown in Figure 12.

[0076] In order for the method according to the invention to be carried out, the control device 7 must be designed accordingly. The control device controls at least the pumps or the means for preventing the electrolyte supply to the cell arrangements and the means 10 for short-circuiting the cell arrangements of the individual battery modules and, if necessary, also the means for switching off the converters 11. If necessary, the control device 7 also records the potential differences in the cell arrangements of the battery modules, i.e., it is connected to the associated measuring devices 5. Furthermore, a computer program installed in the control device is provided for carrying out the steps of the method according to the invention. The computer program according to the invention can be stored on a data carrier.

[0077] List of reference symbols

[0078] 1 battery module

[0079] 2 Cell arrangement

[0080] 3 Tank facility

[0081] 4 Measuring device for determining the OCV

[0082] 5 Measuring device for determining a potential difference

[0083] 6 Bidirectional converter (PCS)

[0084] 7 Control device

[0085] 8 switches for interrupting the series circuit

[0086] 9 Means for short-circuiting a battery module

[0087] 10 Means for short-circuiting a cell arrangement

[0088] 11 DC-DC converters

Claims

Patent claims 1. A method for operating a redox flow battery system comprising at least two battery modules (1), at least one bidirectional converter (6), and a control device (7), wherein each battery module (1) comprises at least two cell assemblies (2), each having a plurality of redox flow cells, wherein the cell assemblies (2) of a battery module (1) within said battery module (1) are electrically separated from one another, and wherein the number of cell assemblies (2) in all battery modules (1) is the same, and wherein the cell assemblies (2) of the battery modules (1) are connected in series and to the at least one converter 6 such that parallel cell assembly strings are formed, and wherein the number of cell assembly strings corresponds to the number of cell assemblies (2) per battery module (1), and wherein means (9) for short-circuiting the respective cell assembly (2) are provided for each cell assembly (2).and wherein each battery module (1) comprises a tank device (3) for storing electrolyte fluid and for supplying the cell arrangements (2) with electrolyte fluid and at least one pump for conveying the electrolyte fluid, characterized in that an intervention is carried out on at least one battery module (1), which comprises the following steps: - preventing the supply of electrolyte fluid to at least some of the cell arrangements (2) of the battery module (1) in question; - short-circuiting the cell arrangements (2) of the respective battery module (1), in which the supply of electrolyte fluid has been stopped, when a potential difference between negative and positive electrolyte in a cell arrangement (2) of the respective battery module (1) has fallen below a predefined value; - Implementation of measures; - supply of electrolyte fluid into the cell arrangements of the battery module (1) in question, in which the supply of electrolyte fluid has been prevented; - Opening the short circuits of the cell arrangements (2) of the battery module (1) in question.

2. The method according to claim 1, wherein the intervention comprises the following steps in the specified order: - preventing the supply of electrolyte fluid to at least some of the cell arrangements (2) of the battery module (1) in question; - short-circuiting the cell arrangements (2) of the respective battery module (1) in which the supply of electrolyte fluid has been stopped, when a potential difference between negative and positive electrolyte in a cell arrangement (2) of the respective battery module (1) has fallen below a predefined value; - Implementation of measures; - supply of electrolyte fluid into the cell arrangements of the battery module (1) in question, in which the supply of electrolyte fluid has been prevented; - measuring the current flowing through the short-circuited cell arrangements (2) of the battery module (1) in question; - Opening the short circuits of the cell arrangements concerned (2) as soon as the measured current has exceeded a predefined threshold.

3. Method according to claim 1 or 2, wherein the measures consist of waiting until a balancing end state has been reached.

4. Method according to claim 1 or 2, wherein the measures involve carrying out maintenance measures on the battery module (1) in question.

5. The method according to claim 1 or 2, wherein the measures involve operating the redox flow battery system in partial load mode.

6. The method according to any one of claims 1 to 5, wherein the battery system is in a discharging mode prior to the intervention.

7. The method according to any one of claims 1 to 5, wherein the battery system is in a charging mode prior to the intervention and the intervention comprises as a first step: - Switching the battery system to a discharge mode; includes.

8. Redox flow battery system designed to carry out the method according to one of claims 1 to 7.

9. Computer program for carrying out the steps of the method according to one of claims 1 to 7.

10. A data carrier on which a computer program according to claim 9 is stored.

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