Method and system for identifying leaks in a membrane of a fuel cell

Abstract

A method for identifying a leak (4') in a membrane (4) of a fuel cell (5) during operation of a motor vehicle, the method comprising the following steps: reducing the power provided by the fuel cell (5) from the output power to a minimum value; determining a measured value of the current cell voltage of the fuel cell (5) while the power provided by the fuel cell (5) is reduced to the minimum value; and evaluating the state of the membrane (4) of the fuel cell (5) based on the determined measured value to identify the leak (4'), wherein the power reduced by the fuel cell (5) during the determination of the measured value of the current cell voltage is provided by at least one other energy source with the same magnitude.

Description

Technical Field

[0001] The invention is based on a method according to the preamble of the independent method claim and on a system according to the preamble of the independent system claim. Background Art

[0002] A common source of error in fuel cells is related to leaks in the membrane used to separate the reaction gases. Since the occurrence of such leaks cannot be excluded in the corresponding manufacturing process or by applying subsequent inspection methods, methods and systems for identifying such leaks are indispensable. In particular, when using PEM fuel cells, the detection of larger leaks is indispensable for safety-related reasons, so that the relevant system can be shut down as quickly as possible when such leaks occur, and thus possible explosions caused by hydrogen-oxygen reactions can be avoided. From the prior art, in addition to methods based on detection by hydrogen sensors in the exhaust gas, detection based on cell voltage monitoring (CVM=cell voltage monitoring) is mainly known to identify leaks in the membrane of the fuel cell. Here, the last-mentioned method is based on the detection of a sudden drop in the cell voltage, which is caused by the direct transmission of hydrogen through the membrane. Here, the sudden drop in the cell voltage is caused by the fact that the hydrogen content transmitted directly through the membrane is not oxidized to protons at the anode and therefore does not contribute to the increase in the cell potential. Summary of the invention

[0003] The subject matter of the invention is a method having the features of the independent method claims and a system having the features of the independent system claims. Further features and details of the invention are derived from the corresponding dependent claims, the description and the drawings. Features and details described in conjunction with the method according to the invention are of course also applicable in conjunction with the system according to the invention and vice versa, respectively, so that reference is always made to one another with regard to the disclosure of the individual inventive aspects.

[0004] The method according to the invention according to the main claim is used in particular for detecting leaks in a membrane of a fuel cell during operation of a motor vehicle. The advantages of the method can be seen as the disclosure of a particularly effective A measuring method is provided which can be carried out under full load. In this case, effective detection of leaks under full load is particularly problematic, since the cell voltage of the fuel cell drops as the battery load increases due to activation losses and ohmic losses, and the method for detecting leaks by means of monitoring the cell voltage is based precisely on the detection of a sudden drop in the cell voltage, so that the voltage difference required for detection can no longer be recorded as the battery load increases. Therefore, effective measurements by means of the mentioned method can usually only be carried out in a state in which the relevant battery is hardly or not loaded at all and therefore almost or not at all supplied or tapped by the fuel cell. However, this is particularly disadvantageous when using fuel cells in mobile systems in which a permanent power supply of the system must be ensured.

[0005] Here, the method according to the invention for identifying a leak in a membrane of a fuel cell can be used in particular in fuel cell vehicles, such as motor vehicles, cranes or forklifts. Here, within the scope of the invention, a "leak" is understood to be a groove, in particular an opening or a hole, through which a solid, liquid or gas can enter or exit. Here, in the method according to the invention, the power provided by the fuel cell is first reduced to a minimum value starting from the output power. The minimum value of the power provided by the fuel cell is ideally a value of 0 watts, i.e. a value that can be measured when the fuel cell does not provide any power at all. However, it is also possible that the minimum value is higher, for example several kilowatts, etc. Here, the power reduction to the minimum value can be carried out step by step or continuously (stufenlos).

[0006] After the power provided by the fuel cell has been reduced to a minimum value, according to the invention, a measured value of the current cell voltage of the fuel cell is determined while the power provided by the fuel cell is reduced to a minimum value. The determination of the measured value can be performed directly or indirectly. In the case of direct determination, for example, a value that is relevant to the detection of a leak in the membrane of the fuel cell can be measured directly, while in the case of indirect determination, only the already measured value is received. The measured value is in particular a measured value of the current cell voltage of the fuel cell.

[0007] After determining the measured value of the current cell voltage of the fuel cell, within the scope of the method according to the invention, the state of the membrane of the fuel cell is evaluated based on the determined measured values ​​in order to detect a leak. Here, both the time interval for determining the measured values ​​and the rate of determining the measured values ​​can preferably be freely and variably selected until the evaluation is performed based on the measured values. For a particularly effective evaluation of the state of the membrane of the fuel cell, it is advantageous to perform the evaluation only after determining a certain number of measured values, in particular after averaging and / or weighting the measured values. Here, the state evaluated within the scope of the method according to the invention relates in particular to the current leak size in the membrane of the fuel cell.

[0008] According to a characteristic feature of the method, the power reduced by the fuel cell during the determination of the measured value of the current battery voltage is provided by at least one other energy source with the same magnitude. The at least one other energy source is preferably an electrical energy source, in particular an electrochemical energy source. The energy source can be a current source or a voltage source, such as a battery, a capacitor, a supercapacitor, etc. Similarly, the power reduced by the fuel cell can also be provided by more than one (for example, by two or three) energy sources. Within the scope of the method according to the invention, it has been recognized that by compensating for the power reduced during the determination of the measured value, a sensitive identification method can be achieved even in full operation.

[0009] In order to ensure within the scope of a reliable method that the power reduced by the fuel cell can also be provided by at least one other energy source, preferably during the entire execution of the method according to the invention, it can be advantageously provided according to the invention that a check on the current feasibility of the method is performed before the provided power is reduced. Here, such a check can in particular include a check of the current charge state of at least one other energy source. In addition, the check can preferably include a comparison of the currently available charging capacity with the predicted energy consumption during the execution of the method according to the invention. Here, the predicted energy consumption can be based on a determinable duration of the execution of the method, during which at least one other energy source compensates for the energy supply reduced by the fuel cell. However, in addition, other data can also be included in the estimation of the predicted energy consumption, such as the current road section characteristics, the current weather conditions, the current traffic conditions and user-specific preferences (such as the fastest possible transportation, etc.).

[0010] For example, a particularly simple check of the current charge capacity of the at least one other energy source can also be performed indirectly based on the last time the at least one other energy source was used. Thus, the check can, for example, include a comparison of the operating life of the fuel cell at full load with a comparison value. This can ensure in a particularly simple manner that the at least one other energy source does not provide any charge capacity during the operating life of the fuel cell at full load and that its charge capacity does not decrease during this time, but at most increases through recuperation processes or the like.

[0011] In the context of an economical and at the same time safe implementation of the method according to the invention, it can be provided that the power provided by the fuel cell is reduced during the method and is increased again to the same extent after the determination of the measured value of the current cell voltage, in particular after the evaluation performed and according to the evaluation performed, wherein the power reduced by the fuel cell is provided by at least one further energy source and the power provided by the at least one further energy source is correspondingly reduced by the same amount after the increase of the power provided by the fuel cell. It is particularly meaningful here to compensate the power reduced by the fuel cell by at least one further energy source until the evaluation is completed, so that in the event of a leak being detected, the operation of the fuel cell is not restarted until the problem is eliminated and / or in order to take other safety-related steps such as interrupting the gas supply. Conversely, in the context of a particularly economical implementation of the method according to the invention, it can also be meaningful to provide the power reduced during the method again by the fuel cell after the determination of the measured value of the current cell voltage, instead of further by at least one further energy source. In the latter case, the at least one further energy source only has to provide the power reduced by the fuel cell for a shorter duration, so that the energy source can be designed to be smaller. When the method according to the invention is used in a vehicle, this means lower weight and correspondingly lower fuel consumption of the vehicle.

[0012] As mentioned at the outset, since the cell voltage of the fuel cell decreases as the cell load increases due to activation losses and ohmic losses, and these processes lead to the voltage difference between the leaking state and the non-leaking state required for detecting leaks in the fuel cell membrane becoming smaller as the cell load increases, it is advantageously possible according to the invention within the scope of a measurement method that is as sensitive as possible to provide that the power provided by the fuel cell is reduced to a value of less than 2% of the maximum power, preferably to a value of less than 1% of the maximum power, in particular to a value of less than 0.1% of the maximum power before and / or during the determination of the measured value of the current cell voltage and in particular during and according to the evaluation performed. Thus, depending on the power of the relevant fuel cell system, the power provided by the fuel cell can be reduced, for example, to a value of less than 2 kW, preferably to a value of less than 1 kW, in particular to a value of less than 0.1 kW. The best possible measurement conditions with regard to sensitivity exist when no power is provided at all or when the highest cell voltage is present, i.e. conditions in which the detection can be performed most sensitively based on the method according to the invention. Therefore, with regard to a particularly sensitive embodiment of the method, it can also be provided that the method is carried out at least partially at the no-load voltage of the fuel cell. In particular, within the scope of a particularly effective embodiment of the method, it may be meaningful to determine the measured values ​​of the cell voltage at different power values ​​of the fuel cell and to extrapolate based on an ideal curve course. Therefore, it is not necessary to reduce the power provided by the fuel cell to the no-load voltage. Alternatively, the measured values ​​of the cell voltage can be determined at different power values ​​of the fuel cell and extrapolated to the cell voltage at the no-load voltage based on these values. Then, the cell voltage value determined in this way at the no-load voltage can be evaluated within the scope of the evaluation according to the invention with regard to the presence of a leak in the membrane of the relevant fuel cell.

[0013] In this context, within the scope of particularly efficient measurement or evaluation, it may be particularly useful to reduce the power provided by the fuel cell in steps before and / or during the ascertainment of the measured value of the current cell voltage and in particular during and as a function of the evaluation performed. The step-by-step reduction of the provided power makes it possible, in particular, to extrapolate these values ​​more precisely or to compare the curve profile more precisely with an ideal curve profile.

[0014] With regard to an implementation scheme of the method according to the invention which is as economical as possible, it may also be meaningful to determine the measuring cycle as short as possible. By means of the shortest possible measuring cycle, at least one other energy source provided for power provision can be configured as small as possible, for example only for providing 20 to 100 kW. Therefore, it is proposed according to the invention that the measured value is determined in less than 10 seconds, preferably in less than 5 seconds, and in particular in less than 2 seconds. By using a smaller and therefore lighter energy source, the relevant system has significantly lower consumption and is therefore also advantageous from an economic point of view. In addition, the corresponding smaller energy source can be arranged in a more space-saving and thus significantly more flexible manner.

[0015] As a basis for the state evaluation with respect to the detection of a leak in the membrane of a fuel cell, it can be provided according to the invention that the evaluation of such a state includes at least one comparison between a measured value and a reference value, wherein the measured values ​​preferably originate from different sensors and in particular the measured values ​​are averaged and / or weighted before the comparison with the reference value. Here, in particular the weighting can be performed with respect to the validity of the value (for example with respect to the position and / or accuracy of the respective sensor). Within the scope of the invention, a value that can be variably determined is considered as a reference value, based on which the state evaluation with respect to the detection of a leak in the membrane of a fuel cell can be performed. In the simplest case, the reference value represents the theoretically ideal expected value of the cell voltage at the correspondingly provided system power. However, the reference value is in particular system-related and / or related to the current environmental conditions, such as the ambient temperature, the ambient pressure, etc. Thus, a larger system, for example, has a larger permissible leakage rate, or the correspondingly used sensor device measures a measured value that changes according to the environmental conditions. Therefore, within the scope of a particularly effective method, it is provided according to the invention with respect to the provided comparison with the reference value: the cell voltage value that can be achieved theoretically in an ideal situation is correspondingly adapted with respect to the relevant system and / or the current environmental conditions.

[0016] Since the comparison according to the invention between the measured values ​​and the reference values ​​cannot always be used to uniquely and clearly assess the state of the leakage in the membrane of the fuel cell, it is also conceivable according to the invention to send a warning indication and / or switch the fuel cell into emergency operation according to the evaluation performed. Here, the type of warning indication and / or the type of emergency operation can depend in particular on the size of the leak or on the evaluation. In this way, during the emergency operation, the fuel cell can be operated only or at least partially by at least one auxiliary energy source. This adaptation of the operation of the fuel cell in the emergency operation can also be carried out with respect to the charge state of at least one other energy source, or optionally can also be adapted to the current concentration of harmful substances in the ambient air, in particular when air drawn from the surrounding environment is used as an oxygen source for the relevant fuel cell system.

[0017] In order to ensure reliable, permanent and continuously optimized protection against leaks in the fuel cell membrane, it is advantageously provided according to the invention that the individual steps of the method are repeated cyclically during operation of the fuel cell. Preferably, the individual steps of the method according to the invention are carried out successively at short intervals, so that even leaks that occur temporarily can be reacted to. Furthermore, the cyclic implementation of the method according to the invention also allows the use of at least one additional energy source of smaller size, if it is assumed that the energy source is recharged during the pauses, for example by recuperation.

[0018] The subject matter of the invention also includes a system for identifying a leak in a membrane of a fuel cell having the features of an independent device claim. In particular, the system comprises: at least one control unit for reducing the power provided by the fuel cell from the output power to a minimum value; at least one measuring unit for determining a measured value of the current cell voltage of the fuel cell during the power provided by the fuel cell reduced to the minimum value; at least one processing unit for evaluating the state of the membrane of the fuel cell based on the determined measured values ​​to identify a leak; and at least one further energy source for providing the reduced power during the determination of the measured value of the current cell voltage with the same magnitude. The system according to the invention thus has the same advantages as those already described in detail with reference to the method according to the invention. The system according to the invention can be integrated either into a mobile system or into the fuel cell system itself. In order to ensure flexible, simple and efficient communication between the individual system units, the individual system components can preferably communicate wirelessly based on a server or cloud and / or via the Internet. For energy-saving operation, the system can also be formed as a learning unit and change parameters and thus adapt the operation based on the collected data and experience values. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Further advantages, features and details of the present invention are apparent from the following description, in which exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. The features mentioned in the claims and in the description may be essential to the present invention individually or in any combination.

[0020] The accompanying drawings show:

[0021] Figure 1a a cross-sectional view showing a properly functioning fuel cell without leaks;

[0022] Figure 1b shows a cross-sectional view of a defective fuel cell having a leak disposed within a membrane of the fuel cell;

[0023] Figure 2shows polarization curves of a properly functioning PEM fuel cell and a defective PEM fuel cell having a leak disposed within the membrane;

[0024] Figure 3 A diagram showing the course of the cell voltage of a properly functioning and a defective fuel cell during the implementation of the method according to the invention;

[0025] Figure 4 A flow chart is shown to illustrate the sequence of the method according to the invention for detecting a leak in a membrane of a fuel cell.

[0026] In the drawings, the same reference numerals are used for the same technical features. DETAILED DESCRIPTION

[0027] Figure 1a A cross-sectional view of a properly functioning fuel cell 5 without leakage is shown. In a fuel cell system, in order to provide more energy, multiple fuel cells 5 are combined in a fuel cell stack. For simplicity, only one fuel cell 5 is shown here. The fuel cell 5 includes an anode 6 and a cathode 8, which are separated from each other by a membrane 4. The anode 6 and the cathode 8 are both electrically connected to the membrane 4. In operation, the anode 6 is circulated by an anode gas (in the present case, hydrogen 1). In addition to the hydrogen 1, the gas present at the anode also includes individual nitrogen molecules 2, which can be present in the gas, especially when hydrogen 1 is recovered from the exhaust gas. In operation, the cathode 8 is circulated by a cathode gas (in the present case, fresh air containing oxygen), which includes a nitrogen content 2 and an oxygen content 3. Here, the membrane 4 is formed as a proton exchange membrane (PEM membrane), which is permeable to protons, but for the reactants of the fuel cell reaction, hydrogen 1 and oxygen 3 are largely impermeable. During the operation of the fuel cell 5, the fuel (here, hydrogen 1) is catalytically oxidized to protons at the anode 6 with the release of electrons. Protons pass through the proton exchange membrane into the cathode chamber filled with oxygen-containing gas. Electrons are extracted from the fuel cell 5 and flow to the cathode 8 through an electrical connection not shown here. At the cathode 8, the oxidant (here oxygen 1) is reduced to anions by absorbing electrons, and these anions react directly with the protons to form water. Through the described reaction, a measurable voltage is generated between the anode and the cathode, which depends in particular on the reactants, the mass of the battery, the temperature and the load of the battery. At a temperature of 25° C., in a hydrogen / oxygen fuel cell, the theoretically achievable cell voltage is 1.23V. However, due to the lower conversion rate caused by impure reactants, the wear process of the battery and especially during the operation of the battery, a cell voltage of only about 1V is usually achieved.

[0028] Figure 1bA cross-sectional view of a defective fuel cell 5 is shown, which has a leak 4' arranged in the proton exchange membrane 5. In this case, reactants, such as here parts of the hydrogen 1 arranged in the anode chamber, can penetrate into the cathode chamber through the leak and react there directly with the oxygen 3 to form water. The hydrogen 1 converted in this way therefore does not contribute to the cell voltage, so that depending on the size of the leak 4', a lower cell voltage is measured and a lower energy is provided. In the case of larger leaks 4', such direct reactions can also be problematic due to their high reaction energy and also for safety-related reasons.

[0029] Figure 2 The polarization curves of a properly functioning and a defective PEM fuel cell 12 are shown, the defective PEM fuel cell having a leak 4' arranged in the membrane 4 of the cell 5. If the fuel cell 5 is loaded with current, the cell voltage of the loaded cell 5 decreases as the load increases due to activation losses and ohmic losses. In this case, a characteristic continuous current-voltage curve is obtained as the load changes - the so-called current-voltage curve or polarization curve of the fuel cell 5. Based on this characteristic curve change process, it becomes clear why the method according to the present invention for detecting leaks in the membrane 4 of the fuel cell 5 based on voltage drop should preferably be carried out in a completely unloaded state of the fuel cell 5 at the no-load voltage 14 (OCV: open circuit voltage) due to the measurement sensitivity. In this state, the voltage difference V between the properly functioning system 10 and the defective system 12 having a leak 4' arranged in the membrane 4 is a On the other hand, if the measurement is performed under a high load of the fuel cell 5, the measurement based on the battery voltage V b Poor detection of leak 4'.

[0030] Figure 3The course of the cell voltage of a properly functioning and defective fuel cell 5 during the implementation of the method according to the invention is shown. Here, the course of change can be divided into three sections a, b, c. Here, section a describes the course of the cell voltage before the power provided by the fuel cell 5 is reduced, at the current 30a provided by the fuel cell 5, while section b describes the course of the cell voltage after the power provided by the fuel cell 5 is reduced to a minimum value, at the provided current 30b, and shows a preferred measurement section 40 for recording measured values. Finally, section c describes the course of the cell voltage after the measured value recording is performed, in which the power provided by the fuel cell 5 is increased again to the initially provided current 30c. Here, curve 32 shows the course of the current provided by at least one further energy source. Curve 34 shows the course of the voltage of a properly functioning fuel cell 5, while curve 36 describes the course of the voltage of a defective fuel cell 5 with a leak 4' arranged in the membrane 4.

[0031] In a first section a during operation of the fuel cell 5 with the current 30a provided by the battery, the cell voltage 34a of a properly functioning fuel cell can hardly be distinguished from the cell voltage 36a of a defective fuel cell (see Figure 2 Voltage difference V b ). However, after the fuel cell current has been reduced according to the invention from the operating state 30a to the minimum value 30b, a functional system can be clearly distinguished from a defective system based on the cell voltage. The cell voltage 34b of a functional cell increases to the value that the cell reaches in the unloaded state, i.e. at the no-load voltage (see Figure 2 ). In contrast, the cell voltage 36b of a defective cell drops, since an exchange between reactants occurs due to the leak 4', which results in a direct reaction between hydrogen molecules 1 and oxygen molecules 3, so that the hydrogen 1 converted into water in this way does not contribute to the cell potential. In this phase, which represents a preferred measuring section and preferably lasts from 2 to 5 seconds, leaks 4' in the membrane 4 of the fuel cell 5 can be detected very sensitively.

[0032] In order to be able to operate the fuel cell system also during this phase 40, in which the power provided by the fuel cell 5 or the current provided is reduced to a minimum value 30b, the reduced power is compensated by increasing the current 32a of at least one other energy source to the current 32b, so that a constant power is provided to the relevant drive device during the entire change process.

[0033] Then, after recording the measured values ​​for assessing the state of the leakage in the membrane 4 of the fuel cell 5, the current provided by the fuel cell 5 is increased again to the value 30c, while the current provided by the at least one other energy source is reduced again to the value 32c to the same extent. In response to this, the detected cell voltage of the functional cell drops again, while the cell voltage of the defective cell rises again, so that, as in the first section a, functional cells can only be distinguished from defective cells very difficultly based on the cell voltage.

[0034] Figure 4 A flow chart is shown for illustrating the sequence of the method according to the invention for detecting a leak in a membrane of a fuel cell 5. The method comprises steps 20 to 28.

[0035] During operation of the fuel cell system, in an optional step 20, a check is first performed regarding the current feasibility of the method. This optional method step is first performed in order to ensure that, preferably during the entire execution of the method according to the invention, the power reduced by the fuel cell 5 can also be provided by at least one other energy source. Such a check can in particular include a check of the current state of charge of the at least one other energy source. Furthermore, the check can preferably include a comparison of the currently available charge capacity with the predicted energy consumption during the execution of the method according to the invention.

[0036] After this optional step for checking the current feasibility of the method according to the invention, in step 22, the power provided by the fuel cell 5 is reduced from the output power to a minimum value and simultaneously compensated by at least one other energy source. The minimum value of the power provided by the fuel cell 5 is ideally a value of 0 watts, i.e. a value that can be measured when the fuel cell 5 does not provide any power at all. However, it is also possible that the minimum value is higher, for example several kilowatts, etc. The reduction of the power to the minimum value can be carried out step by step or continuously. The at least one other energy source is preferably an electrical energy source, in particular an electrochemical energy source.

[0037] After the power provided by the fuel cell 5 has been reduced to a minimum value and simultaneously compensated by at least one further energy source, a measured value of the current cell voltage of the fuel cell 5 is ascertained in step 24 of the method according to the invention. The time intervals for ascertaining the measured values ​​and the rate of ascertaining the measured values ​​are preferably freely and variably selectable. The ascertainment is preferably performed in a small time window, for example within 2 to 5 seconds.

[0038] Finally, after the measured value of the current cell voltage of the fuel cell 5 has been determined, within the scope of the method according to the invention, in step 26, the state of the membrane of the fuel cell 5 is evaluated based on the determined measured value in order to detect a leak. In this case, the evaluation of such a state includes at least one comparison between the measured value and a reference value, wherein the measured values ​​preferably originate from different sensors and the measured values ​​are averaged and / or weighted, in particular before the comparison with the reference value. In this case, the reference value is preferably a variably determinable value, based on which the state can be evaluated with respect to the detection of a leak 4' in the membrane 4 of the fuel cell 5. In the simplest case, the reference value represents the theoretically ideal expected value of the cell voltage at the corresponding current provided. However, the reference value is in particular system-dependent and / or depends on the current environmental conditions, so that before an effective comparison, the reference value should preferably be adapted accordingly.

[0039] Finally, after the evaluation, in a final step 28, at least if no leak 4' is detected, the power provided by the fuel cell 5 is increased and the power provided by the at least one other energy source is correspondingly reduced. If a leak 4' in the membrane 4 of the relevant fuel cell 5 is detected within the scope of the evaluation, a warning signal can be generated alternatively or cumulatively and / or the fuel cell 5 can be switched into emergency mode or no longer switched on.

Claims

1. A method for detecting a leak (4') in a membrane (4) of a fuel cell (5) during operation of a motor vehicle, the method comprising the following steps: a) reducing the power provided by the fuel cell (5) from the output power to a minimum value; Characterized in that the method further comprises the following steps: b) determining a measured value of the current cell voltage of the fuel cell (5) during the period in which the power provided by the fuel cell (5) is reduced to the minimum value; c) evaluating the state of a membrane (4) of the fuel cell (5) based on the determined measured values ​​in order to detect a leak (4'); The power reduced by the fuel cell (5) during the ascertainment of the measured value for the current battery voltage is provided by at least one further energy source in the same amount.

2. The method according to claim 1, characterized in that Before the provided power is reduced, a check is performed as to the current feasibility of the method.

3. The method according to claim 1 or 2, characterized in that: The power provided by the fuel cell (5) is reduced during the method, and the power provided by the fuel cell (5) is increased again to the same extent after the measured value of the current battery voltage is determined, wherein the power reduced by the fuel cell (5) is provided by the at least one other energy source, and after the power provided by the fuel cell (5) is increased, the power provided by the at least one other energy source is correspondingly reduced by the same amount.

4. The method according to claim 1 or 2, characterized in that: Before and / or during the ascertainment of the measured value of the current battery voltage, the power provided by the fuel cell (5) is reduced to a value of less than 2% of the maximum power.

5. The method according to claim 1 or 2, characterized in that: Before and / or during the ascertainment of the measured value for the current battery voltage, the power provided by the fuel cell (5) is reduced in stages.

6. The method according to claim 1 or 2, characterized in that: The measured values ​​are determined in less than 10 seconds.

7. The method according to claim 1 or 2, characterized in that: Evaluating a state to identify a leak (4') in a membrane (4) of the fuel cell (5) comprises at least one comparison between a measured value and a reference value.

8. The method according to claim 1 or 2, characterized in that: Depending on the evaluation, a warning indication is issued and / or the fuel cell (5) is switched into emergency operation.

9. The method according to claim 1 or 2, characterized in that: The individual steps of the method are cyclically repeated during operation of the fuel cell (5).

10. The method according to claim 1 or 2, characterized in that: The method is used in a vehicle.

11. The method according to claim 2, characterized in that The checking includes comparing the operating life of the fuel cell (5) at full load with a comparison value.

12. The method according to claim 3, characterized in that After and as a function of the evaluation carried out, the power to be provided by the fuel cell (5) is increased again to the same extent.

13. The method according to claim 4, characterized in that During and in dependence upon the evaluation carried out, the power provided by the fuel cell (5) is reduced to a value less than 2% of the maximum power.

14. The method according to claim 4, characterized in that Before and / or during the ascertainment of the measured value of the current battery voltage, the power provided by the fuel cell (5) is reduced to a value of less than 1% of the maximum power.

15. The method according to claim 4, characterized in that Before and / or during the ascertainment of the measured value of the current battery voltage, the power provided by the fuel cell (5) is reduced to a value of less than 0.1% of the maximum power.

16. The method according to claim 5, characterized in that During and in dependence upon the evaluation carried out, the power provided by the fuel cell (5) is reduced in stages.

17. The method according to claim 6, characterized in that The measured values ​​are determined in less than 5 seconds.

18. The method according to claim 6, characterized in that The measured values ​​are determined in less than 2 seconds.

19. The method according to claim 7, characterized in that The measured values ​​originate from different sensors.

20. The method according to claim 7, characterized in that The measured values ​​are averaged and / or weighted prior to comparison with the reference value.

21. The method according to claim 10, characterized in that The method is used in a fuel cell vehicle.

22. A system for operating a motor vehicle, the system being configured to perform the method according to any one of claims 1 to 21, the system comprising at least one control unit configured to reduce the power provided by the fuel cell (5) starting from the output power to a minimum value; It is characterized in that The system further comprises: at least one measuring unit for determining a measured value of a current cell voltage of the fuel cell (5) during the period in which the power provided by the fuel cell (5) is reduced to the minimum value; at least one processing unit for evaluating the state of a membrane (4) of the fuel cell (5) based on the determined measured values ​​in order to detect a leak (4'); Therein, the system has at least one further energy source for providing the power reduced during the ascertainment of the measured value for the current battery voltage with the same magnitude.

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