Method for monitoring the state of a redox flow battery system

Abstract

Method for monitoring the state of a redox flow battery system, wherein the battery system comprises at least two serially-connected battery modules (1), and wherein the method comprises the following steps: S1: Disconnect at least one battery module (1) from the series circuit; S2: Discharge at least a partial volume of the electrolyte of at least one battery module (1) disconnected from the series circuit in step S1, wherein the potential difference is repeatedly detected, and wherein a potential difference is formed between a first potential of the negative electrolyte and a second potential of the positive electrolyte of at least one disconnected battery module (1); S3: Determine the SoH of at least one battery module (1) disconnected from the series circuit in step S1 from the potential difference values detected in step S3.

Description

Field of the Invention

[0001] The present invention relates to a method for operating an all-vanadium redox flow battery system, in particular. The present invention particularly relates to an all-vanadium redox flow battery system having a high output voltage. The operating method relates to a state monitoring of the state of health (SoH) of the battery system. Background Art

[0002] In order to obtain a high output voltage in an all-vanadium redox flow battery system, a plurality of single cells are usually electrically connected in series. This arrangement is called a stack. However, this cannot be continued arbitrarily because otherwise the shunt current caused by the electrolyte will become intolerably high. However, when a plurality of stacks are connected in series, the output voltage will further increase, where each stack has a separate tank unit. Such a unit composed of a stack and the associated separate tank unit is called a battery module. A series circuit of a plurality of battery modules is usually called a string. Therefore, the present invention relates to a battery system including a plurality of battery modules, wherein the battery system is configured such that during charging and discharging of the system, the battery modules are connected in series, i.e., a string is formed.

[0003] The SoH of an all-vanadium redox flow battery system can be negatively affected by different effects. The imbalance of the electrolyte (including the imbalance of ion concentrations in the negative electrolyte and the positive electrolyte) will have a negative impact on the SoH. This imbalance is usually described by the so-called average oxidation state (AOS). An AOS deviating from +3.5 indicates this imbalance. This imbalance may already exist from the start of operation or may increase during operation. The latter is caused by vanadium oxidation, further chemical side reactions, and "crossing" on the membranes of the stack. This imbalance of the electrolyte is usually also referred to as the offset of the electrolyte. Another possibility of electrolyte imbalance is that the volume of the negative electrolyte may be different from the volume of the positive electrolyte. Here, the total electrolyte volume of the battery module and / or the electrolyte volume located in the single cell will have this deviation. In the latter case, for example, bubbles in one or more single cells may be the cause. The imbalance caused by different electrolyte volumes can also increase over time.

[0004] It is known from the prior art that the offset of the electrolyte can be detected by means of an OCV unit and a reference unit (or three half-cells). Here, OCV represents the so-called "open circuit voltage" (see below). However, there are problems caused by the possible limited long-term stability of the reference unit (see WO2018 / 237181A1). In addition, the imbalance caused by different electrolyte volumes cannot be detected by means of the reference unit.

[0005] A method is also described in which the electrolyte imbalance is determined when the redox flow battery is charged for the first time (“Electrolyte Imbalance Determination of a Vanadium Redox Flow Battery by Potential-Step Analysis of the Initial Charging”, Kirstin Beyer, Jan grosse Austing, Barbara Satola, Timo Di Nardo, Marco Zobel, Carsten Agert, in European Chemical Societies Publishing ChemSusChem 2020, 13, 2066-2071). In this case, in the case of an electrolyte drift, two potential jumps separated from each other can be detected during the first charge, whereas an ideal electrolyte has only one potential jump. The time interval between the two potential jumps is a measure for the SoH (see first Figures 1 to 3 ). Summary of the invention

[0006] The object of the present invention is to provide a method for state monitoring of a redox flow battery system which allows monitoring of the state of health of the battery system during normal operation and in doing so allows detection of all described types of electrolyte imbalances.

[0007] This object is achieved according to the invention by the embodiments according to the independent claims. Further advantageous embodiments of the invention are described in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The solution according to the invention is explained below with the aid of the accompanying drawings. In the accompanying drawings:

[0009] Figure 1 shows a battery module,

[0010] Figure 2 A battery system in a first embodiment is shown,

[0011] Figure 3Shows a battery system in another embodiment,

[0012] Figure 4 Shows a battery system in yet another embodiment,

[0013] Figure 5 Shows the time variation curve of the potential difference,

[0014] Figure 6 Shows the time variation curve of the potential difference,

[0015] Figure 7 Shows the time variation curve of the potential difference. Detailed implementation

[0016] Figure 1 The battery module is schematically shown on the left. The battery module is denoted by 1. The battery module includes a single-cell facility denoted by 2 and a tank device denoted by 3. The single-cell facility 2 is a mechanism having a plurality of redox flow single cells that can be arbitrarily arranged. For example, it can be a single single-cell stack, a series circuit of multiple stacks, a parallel circuit of multiple stacks, or a combination of a series circuit and a parallel circuit of multiple stacks. The tank device 3 is used to store the electrolyte and supply the electrolyte to the single-cell facility 2. To this end, the tank device 3 includes at least two tanks for the negative electrolyte and the positive electrolyte, a pipe system for connecting the tanks to the single-cell facility 2, and a pump for transporting the electrolyte. Figure 1 Two independent pumps are shown here. The electrolyte can be transported equally well using a double-headed pump, that is, two pumps driven by a common motor. The tank device 3 is configured here such that it can supply the electrolyte to all the single cells of the single-cell facility 2. Therefore, if the pump transports the electrolyte, all the single cells of the single-cell facility 2 are flowed through by this electrolyte.

[0017] Figure 1The battery module 1 shown includes two measuring devices denoted by 4 and 5 respectively. Here, the measuring device denoted by 4 is a measuring device for providing the so-called open circuit voltage (OCV). The OCV value is a measure of the state of charge (SoC) of the battery module. The measuring device denoted by 5 is a measuring device for providing the terminal voltage of the single-cell facility 2 and thus of the battery module 1. When the battery module 1 is charging or discharging, the terminal voltage differs from the open circuit voltage by the voltage dropped across the internal resistance of the single-cell facility 2. The potential difference is measured using the two measuring devices 4 or 5, which is formed between the first potential of the negative electrolyte and the second potential of the positive electrolyte accordingly. In the OCV measuring device 4, the electrodes for intercepting the mentioned potential are located in the measuring unit, where the respective chambers for the negative electrolyte and the positive electrolyte are separated by a membrane or a separator. In the measuring device 5 for providing the terminal voltage, the electrodes for intercepting the mentioned potential are located in the respective single cells of the single-cell facility 2. Here, the formed potential difference of course depends on the number of single cells connected in series between the electrodes for intercepting the potential. When no charging current or discharging current is flowing, the terminal voltage is a multiple of the potential intercepted on the OCV measuring device, where the respective coefficient is given by the number of single cells connected in series in the single-cell facility 2.

[0018] On the Figure 1 right side of is shown a symbolic representation of the battery module 1. This symbolic representation is subsequently used.

[0019] Figure 2 The battery system in the first embodiment is shown schematically. The battery system includes at least two battery modules (one of the battery modules is denoted by 1), a bidirectional power conversion system (PCS) denoted by 6, and a control device denoted by 8. The battery modules 1 are connected in series and are connected to the converter 6. Figure 2 Four battery modules are shown, where the dashed line in the series circuit should represent any number of additional modules. The converter 6 takes on the connection of the battery system to the power grid or a higher-level electrical system. The battery system also includes a first switch and a second switch for each battery module 1, where one of the first switches is denoted by 9 and one of the second switches is denoted by 10. The first switch 9 is arranged in series with the battery module 1 respectively, where of course it is not important on which side of the respective battery module the respective switch 9 is arranged. The second switch 10 is arranged in a bypass (bypass line) bypassing each one of the battery modules 1 and the respective first switch 9. Figure 2All switches 9 and 10 are shown in the open state. However, the switches are actuated by the control device 8 such that exactly one switch in each pair of switches consisting of a first and a second switch is closed and one switch is open (selectively open and closed). This means that the switch pairs have exactly two switch positions here, where in the first switch position (the first switch 9 is closed and the second switch 10 is open), the associated battery module 1 is located in the series circuit of the battery system, and in the second switch position (the first switch 9 is open and the second switch 10 is closed), the associated battery module 1 is separated from the series circuit of the battery system by a bypass. When switch 10 is closed, the opening of the first switch 9 here prevents the module from discharging through the bypass. The control device 8 is connected to each battery module such that the control device can detect the measured values of the measuring devices 4 or 5. In addition, the control device 8 is connected to each of the switches 9 and 10 such that the control device can determine the respective switch positions in order to connect the battery module 1 to the series circuit or disconnect it from the series circuit. These connections can also be realized wirelessly.

[0020] Figure 2 The arrangement shown represents the minimum configuration for implementing the operating method according to the invention. It should be mentioned here that in most embodiments of the operating method according to the invention, only the terminal voltage of the battery module 1 is measured. That is, in these embodiments, it is not necessary for the battery module 1 to further include a measuring device for detecting the OCV.

[0021] Figure 3 The battery system according to the invention in another embodiment is shown. The battery system additionally includes another switch, namely a third switch, for each battery module 1, where one third switch is denoted by 11. In addition, the battery system includes a resistor for each battery module 1, where one resistor is denoted by 14. The third switch 11 and the resistor 14 are here respectively arranged in another bypass line bypassing each one battery module 1 such that when the associated third switch 11 is closed, each one battery module 1 is short-circuited through the resistor 14. The third switch 11 is also actuated by the control device 8. With Figure 3 this arrangement, each battery module can be selectively discharged via the associated resistor 14 by closing the associated third switch 11.

[0022] Figure 4 The battery system according to the invention in another embodiment is shown. The battery system includes another converter denoted by 7. In addition, the battery system additionally includes fourth and fifth switches (denoted by 12 and 13) and lines for each battery module, where the additional switches and lines are connected to each other and to the battery module such that each battery module 1 can be individually connected to the other converter 7. Here, the additional switches 12 and 13 are also actuated by the control device. Due to space reasons, inFigure 4 The control device is not shown. Figure 4 Also shown are additional switches by which the converters 6 and 7 can be electrically separated from the remaining arrangement, respectively. This can be advantageous. If necessary, each converter uses only one disconnecting switch. Such switches can also be used in all other embodiments. It should also be mentioned that the names "fourth" and "fifth" switches are only for clarity and do not imply that a "third" switch must also be present in embodiments having these switches. According to Figure 4 the embodiment of [], it is possible to discharge the individual battery modules 1 via an additional converter 7. For this purpose, it is sufficient if the additional converter 7 is constructed unidirectionally. However, the additional converter can also be constructed bidirectionally, so that it can also be used to charge the individual battery modules 1, which can be useful, for example, for the SoC balancing of the string or the pre-charging of the battery modules 1.

[0023] The inventors' concept is that the SoH of the battery system depends on the SoH of the individual battery modules. In addition, the inventors have recognized that during discharge, the SoH of the battery modules can also be monitored by means of the potential jumps described in the above-cited documents. The method according to the invention comprises the following steps:

[0024] S1: Actuate the first and second switches 9, 10 such that at least one battery module 1 is cut out of the series circuit;

[0025] S2: Discharge at least a partial volume of the electrolyte of at least one battery module 1 cut out of the series circuit in step S1, wherein the potential difference is repeatedly detected, and wherein a potential difference is formed between a first potential of the negative electrolyte and a second potential of the positive electrolyte of at least one cut-out battery module 1;

[0026] S3: Determine the SoH of at least one battery module cut out of the series circuit in step S1 from the potential difference values detected in step S2.

[0027] Here, with regard to step S2, it should be noted that the sign of the formed potential difference makes no difference, i.e., the potential difference can equally well be formed between a first potential of the positive electrolyte and a second potential of the negative electrolyte of at least one cut-out battery module 1. Or in other words: It only depends on the absolute value of the potential difference.

[0028] Depending on the construction of the battery system, step S2 can be carried out differently here. If according to Figure 2A battery system is formed, and the discharging process is realized in the form of self-discharge of at least one battery module 1 cut out in step S1. Since this self-discharge proceeds relatively slowly, it is advantageous that one or more of the battery modules involved are already about to be in a state of charge in which a potential mutation to be detected occurs when cut out in step 1. This is the case if the OCV value of the battery module involved is 1.4 volts or lower. The corresponding criterion can be derived from the terminal voltage.

[0029] If according to Figure 3 A battery system is formed, and the discharging process of at least one battery module 1 cut out in step S1 is realized in such a way that the associated third switch 11 is closed, so that the battery module involved discharges through the associated resistor 14. Thus, the discharging process can proceed correspondingly faster, so that the determination of SoH requires less time.

[0030] In the cases mentioned in the previous two paragraphs, the energy converted during discharging is ultimately converted into heat, that is, lost. Therefore, it is also advantageous that one or more of the battery modules involved are already about to be in a state of charge in which a potential mutation to be detected occurs when cut out in step 1. Another feature of the cases mentioned is that, in principle, more than one battery module 1 can be cut out from the series circuit of the battery system in step S1, so that the SoH of these modules can be determined simultaneously in steps S2 to S4. Therefore, the upper limit of the number of modules monitored simultaneously is limited in such a way that the battery system must be able to ensure normal operation during the time required for SoH determination. Common high-voltage battery systems can compensate for the cut-out of battery modules in each operating state. In some operating states, for example, if it can be foreseen that no power consumption or power output of the battery system is required during the time required for SoH determination, then multiple battery modules can also be easily cut out from the series circuit for SoH determination.

[0031] If according to Figure 4To form a battery system, the discharging process of at least one battery module 1 cut out in step S1 is implemented in the following manner, i.e., the fourth and fifth switches 12 and 13 belonging thereto are closed, and discharging is completed through another converter 7. In this configuration, always only exactly one battery module can be cut out and connected to another converter in order to determine the SoH belonging thereto in this way. The advantage of this configuration is that the energy converted during discharging is not lost because it is fed into a higher-level power grid through another converter 7. Therefore, the start of SoH determination can be largely independent of the corresponding existing SoC of the battery module involved. If the SoC is still far above the range of interest for SoH determination, the discharging of the battery module involved can be achieved through another converter 7 with a high discharging current until the OCV value has reached the above-mentioned 1.4 V mark. Subsequently, discharging can be carried out with a lower current to ensure a sufficiently high resolution of the potential jump. Thus, SoH determination can be carried out faster and with fewer side reactions.

[0032] In principle, any combination of the Figures 2 to 4 embodiments is conceivable. Thus, for example, a module can be discharged through another converter 7 and other modules can be discharged simultaneously through the belonging resistor 14, wherein the SoH of the module involved is determined.

[0033] According to which partial volume of the electrolyte of the battery module involved in step S2 is discharged, further variants of the method according to the invention are obtained. These variants are closely related to the corresponding target settings of SoH determination.

[0034] If the volume deviation of the entire electrolyte is to be determined, i.e., the deviation between the volume of the negative electrolyte and the volume of the positive electrolyte, then the entire electrolyte of the battery module involved must also be discharged in step S2. This is achieved in the following manner, i.e., during the implementation of step S2, an electrolyte flow through the single-cell facility 2 is set by means of a pumping power, thereby generating a super-stoichiometric flow. In this variant, the detection of the potential difference in step S2 can be selectively achieved either by using the OCV unit 4 or by using a measuring device 5 for detecting the terminal voltage. It is clear that this variant is time-consuming because a correspondingly large amount of energy must be converted during discharging. Therefore, in combination with the variants according to Figure 2 and Figure 3 a large amount of energy is also lost, so that these combinations are significantly less favorable compared to the combinations with the variant according to Figure 4

[0035] If it is desired to detect an offset of the electrolyte, it is sufficient to discharge a partial volume of the electrolyte contained in the single cell facility 2 in step S2 and thus use it for SoH determination. This is achieved in such a way that during the implementation of step S2, the electrolyte flow through the single cell facility 2 is either completely blocked (by switching off the pump) or adjusted by the pumping power, thereby generating a sub-stoichiometric flow. In this variant, the detection of the potential difference in step S2 must be achieved using the measuring device for detecting the terminal voltage 5. This variant is also suitable for detecting bubbles in the single cell facility 2. A combination with the variant according to Figures 2 to 4 is advantageous.

[0036] In principle, it is also possible to discharge only the partial volume contained in the OCV unit 4 in step S2 and thus use it for SoH determination. In this variant, the potential difference is detected in step S2 using the OCV unit 4 itself. However, the following disadvantage arises, namely that the SoH determination in this way takes a long time because the discharge is very slow due to the very small short-circuit current.

[0037] In order to determine the SoH in step S3, it must be said that the SoH can be determined absolutely or relatively here.

[0038] The absolute determination of the SoH is achieved by determining the potential step according to the above-mentioned document "Electrolyte Imbalance Determination of a Vanadium Redox Flow Battery by Potential-Step Analysis of the Initial Charging". The potential step is obtained when the potential difference detected in step S2 is considered as a function of time. Here, in order to determine the exact time position of the step, the curve can be differentiated. For the absolute determination of the SoH, it is also necessary to quantify the energy transferred between the two potential steps during discharge. For this purpose, it is necessary to measure the magnitude of the discharge current. The charge transferred is obtained by integrating the discharge current curve over time. In the case of a constant discharge current, the charge is proportional to the time elapsed between the potential steps. The energy transferred is in turn obtained by integrating the potential difference as a function of the transferred charge. For the absolute determination of the SoH, the battery system thus includes a measuring device for determining the discharge current intensity present in step S2. Depending on the configuration of the battery system, this measuring device can be arranged differently. Figure 4 An exemplary measuring device for determining the discharge current intensity is shown, which is designated by 15. In other embodiments, for example according toFigure 3 In an embodiment of the invention, the battery system can include a separate measuring device for each battery module for determining the discharge current intensity. Figure 3 In the embodiment of FIG. 1 , the measuring device is arranged in series with the resistor 14 .

[0039] For the relative determination of the SoH, the currently recorded potential difference curve can be compared with a previously recorded reference curve (see below). Another possibility is to consider only the time elapsed between two sudden changes in the potential difference curve as a SoH parameter. This case can also be referred to as a qualitative determination of the SoH, since only one parameter is determined, which is related to the actual (i.e. quantitatively determined) SoH. The SoH parameter determined in this way also allows a relative SoH determination.

[0040] Figures 5 to 7 The time curve of the potential difference value detected in step S2 is shown as an example in order to clarify the determination of SoH. This means that the x-axis of these diagrams is time and the y-axis is the detected potential difference value. Figure 5 In the curve shown, essentially only one potential jump can be observed. Therefore, the corresponding electrolyte is well balanced (AOS=3.5). Figure 6 In , the curve shows two potential jumps, wherein the first potential jump in time is steeper than the subsequent potential jump in time. The associated electrolyte is not well balanced and the AOS is less than 3.5. Figure 7 The curve shown also has two potential jumps, where Figure 6 In comparison, the temporal order is reversed. The associated electrolyte is not well balanced and the AOS is greater than 3.5. With knowledge of the discharge current intensity, the relevant AOS value can be calculated from the time interval of the two potential changes, so that the SoH can be determined absolutely. It is usually also sufficient to determine the SoH relatively, for example in order to determine when the electrolyte must be regenerated in the battery module. Many suitable indicators can be used for this purpose. For example, the detected potential difference curve can be compared with a corresponding reference curve, which is recorded during commissioning or after a previous regeneration of the electrolyte. Then, a suitable measure for the deviation of the two curves is, for example, the sum of the squares of the differences. For this purpose, a person skilled in the art can easily find other suitable indicators.

[0041] In the method according to the invention described above, at least a partial volume of the electrolyte of the battery module is discharged respectively. Thereby, the state of charge of the battery module involved naturally also changes (i.e., decreases). If this is to be prevented, then before or after implementing the described method, corresponding energy can be supplied to the battery module involved, so that the state of charge of the battery module will ultimately not change due to the implementation of the method. Here, the energy supply can be advantageously achieved by means of a further converter 7 in the configuration according to Figure 4 is advantageously implemented by means of a further converter 7.

[0042] Finally, it should be mentioned that when enough potential differences have been recorded to be able to determine the SoH in step S3, the discharge can be interrupted in step S2. In order to establish an interruption criterion for the discharge in step S2, it is thus advantageous for steps S2 and S3 to be carried out at least partially in parallel, i.e., the evaluation of the recorded potential differences has already started, and at the same time further potential differences are being recorded. If step S3 provides any of the following results, step S2 can be interrupted:

[0043] - A potential mutation is detected whose mutation height exceeds a pre-defined threshold;

[0044] - Two potential mutations are detected;

[0045] Here, all values within the OCV equivalent range between 0.7 volts and 1.2 volts are suitable as the threshold for the first criterion. Particularly advantageous are thresholds from the OCV equivalent range of 0.8 volts to 1.1 volts. Using an interruption criterion for step S2 has the advantage that the method according to the invention can be implemented more quickly. The use of modern computing systems enables the determination of the SoH in step S3 in a negligible amount of time.

[0046] It is clear here that the thresholds mentioned in the previous paragraph apply to vanadium-based battery systems. Other thresholds generally apply to other battery systems. The corresponding applies to the above-mentioned AOS values.

[0047] List of reference numerals

[0048] 1 Battery module

[0049] 2 Single-cell facility

[0050] 3 Tank device

[0051] 4 Measuring device for determining the OCV (OCV unit)

[0052] 5 Measuring device for determining the terminal voltage

[0053] 6 Bidirectional converter (PCS)

[0054] 7 Further converter

[0055] 8 Control device

[0056] 9 First switch

[0057] 10 Second switch

[0058] 11 Third switch

[0059] 12 Fourth switch

[0060] 13 Fifth switch

[0061] 14 Resistor

[0062] 15 Measuring device for determining the discharge current intensity

Claims

1. A method for monitoring the state of a redox flow battery system, wherein, The battery system includes at least two battery modules (1), a bidirectional converter (6), and a control device (8). Among them, the battery modules (1) are connected in series and connected to the bidirectional converter (6). And each battery module (1) includes a single-cell facility (2) having a plurality of redox flow single cells and a tank device (3) for storing negative electrolyte and positive electrolyte and supplying electrolyte to the single-cell facility (2). And the battery system includes a first switch (9) and a second switch (10) for each battery module (1). The first switch (9) is arranged in series with the respective battery module (1), and the second switch (10) is arranged in a bypass that bypasses the respective battery module (1) and the respective first switch (9). And the control device (8) is connected to each of the switches (9, 10) so that the control device can determine the respective switch positions to connect the battery module (1) to the series circuit or cut it out of the series circuit. It is characterized in that the method includes the following steps: S1: Drive and control the first and second switches (9, 10) to cut at least one battery module (1) out of the series circuit; S2: Discharge at least a partial volume of the electrolyte of at least one battery module (1) cut out of the series circuit in step S1. Among them, the potential difference is repeatedly detected, and a potential difference is formed between the first potential of the negative electrolyte and the second potential of the positive electrolyte of at least one cut-out battery module (1); S3: Determine the SoH of at least one battery module (1) cut out of the series circuit in step S1 from the potential difference values detected in step S2; And steps S2 and S3 are implemented at least partially in parallel. And when step S3 provides one of the following results, step S2 is interrupted: - Detect a potential mutation with a mutation height exceeding a predefined threshold; - Detect two potential mutations; Among them, the predefined threshold is within the OCV equivalent range between 0.7 volts and 1.2 volts.

2. The method according to claim 1, wherein, The battery system includes a third switch (11) and a resistor (14) for each battery module (1). The third switch (11) and the resistor (14) are respectively arranged in a bypass line that bypasses one battery module (1) so that when the respective third switch (14) is closed, one battery module (1) is short-circuited through the resistor (14). And the discharge in step S2 is carried out through the resistor (14).

3. The method according to claim 1, wherein, The battery system includes an additional converter (7), and includes a fourth and a fifth switch (12, 13) and a line for each battery module (1). The fourth and fifth switches (12, 13) are connected to each other and to the battery module (1) so that each battery module (1) can be individually connected to the additional converter (7). And the discharge in step S2 is carried out through the additional converter (7).

4. The method according to any one of the preceding claims, wherein, The battery modules (1) each include a measuring device (4) for determining the OCV, and wherein, in step S2, the potential difference is detected by means of the measuring device (4) for determining the OCV.

5. The method according to any one of claims 1 to 3, wherein, The battery modules (1) each include a measuring device (5) for determining the terminal voltage, and wherein, in step S2, the potential difference is detected by means of the measuring device (5) for determining the terminal voltage.

6. The method according to claim 4 or 5, wherein In step S2, the entire electrolyte volume of at least one battery module (1) cut out from the series circuit in step S1 is discharged.

7. The method according to claim 5, wherein, In step S2, the electrolyte volume of at least one battery module (1) cut out from the series circuit in step S1 and contained in the single cell facility (2) is discharged.

8. The method according to any one of the preceding claims, wherein, The battery system includes a measuring device (15) for detecting the current intensity of the discharge current present in step S2.

9. The method according to claim 1, wherein The predefined threshold is in the OCV equivalent range of 0.8 volts to 1.1 volts.

10. The method according to any one of the preceding claims, wherein, In step S3, the SoH is determined relatively by comparison with a reference potential difference curve.

11. The method according to any one of the preceding claims, wherein, In step S3, the SoH is determined by knowing the time elapsed between two mutations in the detected potential difference curve.

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