Method for diagnosing and regenerating a fuel cell

Abstract

The invention relates to a method for diagnosing the health status of a fuel cell (10) in a power generation unit (1) fitted to a motor vehicle, according to which, during a maintenance phase of the motor vehicle in a workshop, the following steps are performed: - connecting a nitrogen (N2) tank to an air intake line (40) of the power generation unit, - supplying an anode of the fuel cell with hydrogen (H2) and a cathode of the fuel cell with nitrogen, - controlling the voltage generated by the fuel cell so that it varies, - measuring the current intensity generated by the fuel cell when the voltage varies, - comparing the measured current intensity with a reference, and - diagnosing said health status based on the result of said comparison. Figure for the abstract: Fig. 1

Description

Title of the invention: Method for diagnosing and regenerating a fuel cell. Technical field of the invention

[0001] The present invention relates generally to fuel cells.

[0002] It relates more particularly to a method for diagnosing the health status of a fuel cell in a current generator assembly equipping a motor vehicle, as well as a method for maintaining this fuel cell.

[0003] It also relates to a current generator assembly mounted in a vehicle and specially designed to implement one and / or the other of these two processes. State of the art

[0004] It is known to generate an electric current using a fuel cell in which, on a first electrode, an oxidation reaction of a reducing fuel occurs, and, on a second electrode, a reduction reaction of an oxidant.

[0005] As a general rule, such a fuel cell uses dihydrogen as a reducing fuel and dioxygen contained in the air as an oxidant.

[0006] These fuel cells, particularly those known as "proton exchange membrane fuel cells (PEMFCs)," are therefore very sensitive to air pollutants. They are especially sensitive to sulfur dioxide (SO2) and nitrogen dioxide (NO2). These pollutants accumulate on the surfaces of their electrodes, hindering oxidation and reduction reactions and thus reducing the overall efficiency of the fuel cell.

[0007] This progressive deterioration in fuel cell performance leads to increased maintenance costs and a reduction in the cell's lifespan. This deterioration is all the more rapid when the fuel cell is used in areas where the air is polluted (particularly in urban areas).

[0008] This deterioration is however reversible, provided that the concentration of pollutants on the electrodes is not too high.

[0009] Document US2003180586 thus proposes to inject nitrogen instead of air into the fuel cell in order to regenerate it.

[0010] This regeneration must be carried out regularly to avoid irreversible deterioration of the fuel cell, which in practice proves to be costly.

[0011] Various methods are known for determining an ECSA parameter (for "electrochemically active surface") which allows us to determine to what extent The fuel cell is damaged by pollutants and must indeed be regenerated, but these methods all require significant time to implement. Furthermore, they are completely unsuitable for regenerating fuel cells used in motor vehicles. Presentation of the invention

[0012] In order to remedy the aforementioned drawbacks of the prior art, the present invention proposes a method for determining during vehicle maintenance whether or not it is necessary to carry out a fuel cell regeneration operation.

[0013] More specifically, the invention proposes a method for diagnosing the health status of a fuel cell in a current generator assembly equipping a motor vehicle, according to which, during a maintenance phase of the motor vehicle in a workshop, the following steps are provided: - connection of a nitrogen tank to an air intake line of the current generator assembly, - supplying the anode of the fuel cell with dihydrogen and the cathode of the fuel cell with dinitrogen, - controlling the voltage applied to the fuel cell so that it varies, - measurement of the current intensity generated by the fuel cell when the voltage varies, - comparison of the measured intensity with a reference, and - diagnosis of said health condition based on the result of said comparison.

[0014] Thus, thanks to the invention, it is possible to connect a nitrogen tank to the fuel cell in the workshop in order to easily test this fuel cell to determine if it needs to be regenerated.

[0015] This solution also makes it easy to detect which is the best process for regenerating it.

[0016] Other advantageous and non-limiting features of the diagnostic method according to the invention, taken individually or in all technically possible combinations, are as follows: - the fuel cell comprising at least one cell, the voltage is controlled to vary between two terminals, including a lower terminal less than 0.1 V per cell (this value to be multiplied by the number of cells), and an upper terminal between 0.3 V and 0.9 V per cell, said lower terminal preferably being non-zero and said upper terminal preferably being equal to 0.8 V per cell; - the voltage is controlled to vary linearly between these two terminals; - the measured intensity is compared with the reference only in a window where said voltage is between 0.1 and 0.3V per cell; -if the measured intensity is equal to the said reference, within a deviation, the diagnosis indicates that no regeneration of the fuel cell is necessary, otherwise the diagnosis indicates that a regeneration of the fuel cell is necessary; -if the diagnosis indicates that fuel cell regeneration is required, it also indicates what type of regeneration process to apply, said type being different depending on whether the measured intensity is positive or negative.

[0017] The invention also proposes a method for maintaining a motor vehicle equipped with a current generator assembly including a fuel cell, in which it is planned to implement: - a diagnostic procedure as described above, then, - depending on the diagnostic result, a fuel cell regeneration step.

[0018] Other advantageous and non-limiting features of the maintenance method according to the invention, taken individually or in all technically possible combinations, are as follows: - at the comparison stage of the diagnostic process, it is determined whether the measured intensity is substantially equal to said reference and whether it is positive or negative within a predetermined voltage window; - the regeneration step is implemented only if the measured intensity is not substantially equal to said reference, according to a process which is different depending on whether the measured intensity is positive or negative; - the regeneration step comprises, if the measured intensity is positive, two separate operations and, if the measured intensity is negative, a single operation, preferably identical to one of said two separate operations; - one of the two separate operations is carried out by supplying the anode of the fuel cell with dihydrogen and the cathode of the fuel cell with dinitrogen and by controlling the voltage applied to the fuel cell so that it varies, and the other of the two separate operations is carried out by supplying the anode of the fuel cell with dihydrogen and the cathode of the fuel cell with air charged with ozone and by controlling the current delivered by the fuel cell so that it varies.

[0019] The invention also proposes a motor vehicle current generator assembly comprising: - a fuel cell, - an air intake line leading to the fuel cell, equipped with a system for connecting an external tank to the motor vehicle, - a DC-DC converter which is connected, on one side, to the fuel cell, and, on the other, to an electrically consuming accessory such as a battery, and - a control unit which is adapted to control the DC-DC converter in such a way that the fuel cell presents a voltage across its terminals equal to a voltage setpoint.

[0020] Preferably, the control unit includes a switch adapted to toggle between: - a first state in which the control unit is adapted to control the DC-DC converter in such a way that the fuel cell presents a voltage across its terminals equal to a voltage setpoint, and - a second state in which the control unit is adapted to control the DC-DC converter in such a way that the fuel cell delivers a current of equal intensity to a set intensity.

[0021] Advantageously, the control unit is controlled by an electronic or human supervisor in order to switch the switch into one state or the other.

[0022] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Detailed description of the invention

[0023] The following description with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0024] On the attached drawings:

[0025] [Fig.1] is a schematic view of a current generator assembly according to the invention, comprising a fuel cell;

[0026] [Fig.2] is an electrical diagram illustrating a filter connected between the battery and fuel of the [Fig.l] and a DC-DC converter of the current generator assembly of the [Fig.l];

[0027] [Fig.3] is a diagram illustrating the calculation of a control for the DC- DC of the [Fig.2];

[0028] [Fig.4] is a graph illustrating the voltage variations imposed on the battery at fuel of the [Fig.l] during test cycles;

[0029] [Fig.5] is a graph illustrating current variations measured at the output of the fuel cell of the [Fig.l] during test cycles, when the fuel cell is polluted by sulfur dioxide (SO2);

[0030] [Fig.6] is a graph illustrating current variations measured at the output of the fuel cell of [Fig.1] during other test cycles, when the fuel cell is polluted by nitrogen dioxide (NO2).

[0031] In [Fig.1], a current generator assembly 1 is shown on board a motor vehicle, and more specifically in a land vehicle such as, for example, a car, a van, a bus or a truck.

[0032] It is specifically designed to supply electric current to an electric motor via an inverter, directly and / or indirectly (via a battery of accumulators).

[0033] This current generator assembly 1 includes a fuel cell 10.

[0034] Here, this fuel cell 10 is of the proton exchange membrane type PEMFC.

[0035] It preferably comprises several identical cells, each of which includes: - two electrodes, namely an anode and a cathode, - two bipolar plates, one of which is to distribute a reducing fuel (typically dihydrogen H2) towards the anode, and a second plate to distribute an oxidizing fuel (for example dioxygen O2) towards the cathode, and - a proton exchange membrane acting as an insulating electrolyte (it blocks the passage of electrons while allowing H+ ions to pass through).

[0036] Finally, each cell of the fuel cell 10 has two electrical terminals connected to the two electrodes.

[0037] In practice, each cell of the fuel cell 10 is capable of generating a low voltage. This is why the fuel cell comprises a large number of identical or similar cells (several hundred), connected in series with each other by their terminals. This fuel cell 10 therefore has two main terminals between which these cells are connected.

[0038] The current generator assembly 1 here includes a DC-DC converter 20 allowing the modulation of power applied by the fuel cell 10 to the high voltage network, the voltage of which is imposed by the battery of accumulators.

[0039] This DC-DC converter 20 conventionally comprises two input terminals, connected to the main terminals of the battery here via a filter 80, and two output terminals. Its two output terminals are connected to a current-consuming device, typically a rechargeable battery.

[0040] The current generator assembly 1 also includes a dihydrogen supply circuit 30 which opens into the first bipolar plate.

[0041] This hydrogen supply circuit 30 includes herein, in particular, a reservoir of dihydrogen 31 and a valve 32 for regulating the dihydrogen flow rate.

[0042] The current generator assembly 1 further includes a residual fluid discharge line 33, which originates in the first bipolar plate and opens to the outside. It also includes a recirculation line 34 which allows a portion of the fluid circulating in the discharge line 33 to be drawn back into the first bipolar plate. This recirculation line 34 is equipped with another flow control valve 35.

[0043] The current generator assembly 1 also includes an air intake line 40 which successively comprises an air filter 41 drawing in and filtering air from the atmosphere, an air compressor 42 compressing the filtered air, an air cooler 43 which cools the compressed air and an air humidifier 44. It opens into the second bipolar plate.

[0044] The current generator assembly 1 also includes a fluid evacuation line 60, allowing the evacuation of, in particular, the water resulting from the chemical reaction taking place in the fuel cell 10. This fluid evacuation line 60 originates in the second bipolar plate, passes through the air humidifier 44 in order to supply it with water, and includes an exhaust silencer 61 or a storage element.

[0045] According to the invention, the current generator assembly 1 also includes a system for connecting a reservoir external to the vehicle to the air intake line 40, allowing the second bipolar plate to be supplied with a gas other than outside air during vehicle maintenance phases.

[0046] In practice, this connection system includes a gas injection line 50 which opens into the air intake line 40, for example between the air humidifier 44 and the second bipolar plate. This gas injection line 50 has an inlet connector through which it can be connected to a gas reservoir 52 in order to inject into the second bipolar plate a gas other than air (for example, nitrogen N2) or a mixture of air and an additional gas (for example, nitrogen N2 or ozone O3). It is also equipped with a flow control valve 51.

[0047] When the fuel cell is operating, it emits heat which must be dissipated. For this reason, the current generator assembly 1 includes a cooling circuit 70 which passes against the fuel cell cells 10 and which includes a pump 72 for circulating a heat transfer fluid in a loop in the circuit, and a heat exchanger 71 for cooling the heat transfer fluid.

[0048] A deionizer 73 is provided in this circuit, here in parallel with the heat exchanger 71, to prevent the heat transfer fluid from becoming excessively charged with ions.

[0049] Figure 2 shows in detail the filter 80 provided between the main terminals of the fuel cell 10 and the input terminals of the DC-DC converter. This filter 80 is here formed by electrical components.

[0050] In practice, this is a second-order filter, here of the LC type, which comprises: - a coil 81 connected in series between the positive terminal of the fuel cell and the corresponding input terminal of the DC-DC converter 20, and - a capacitor 82 connected in parallel with the main terminals of the fuel cell.

[0051] Means are then provided for measuring the intensity i0 of the current delivered by the fuel cell 10 and the voltage Uo across its terminals. Here, the intensity is measured at the positive terminal of the fuel cell 10.

[0052] In the context of the invention, the current generator assembly 1 can be controlled either in standard mode, to deliver an electric current, or in diagnostic mode.

[0053] The standard mode is the one used to supply the electric motor or the battery with electric current, for example during vehicle driving or battery charging phases. It can also be used to regenerate the fuel cell 10.

[0054] The diagnostic mode is intended to be used during diagnostic phases of the current generator assembly 1. It can also be used to regenerate the fuel cell 10.

[0055] Preferably, the current generator assembly 1 then includes a control unit 90 adapted to control the DC-DC converter 20 in either of these two modes.

[0056] This control unit 90 includes a processor and a memory (or a programmable logic circuit), as well as various input and output interfaces.

[0057] Thanks to its input interfaces, it is adapted to receive instructions and measurements of intensity i0 and voltage Uo.

[0058] Thanks to its output interfaces, it is suitable for controlling the DC-DC converter 20.

[0059] Thanks to its memory, it stores a computer application, consisting of computer programs comprising instructions whose execution by the processor allows the computer to implement the process described below. In the variant where it includes programmable logic, its logic gates are programmed to implement this process.

[0060] Figure 3 illustrates how the control of the DC-DC converter 20 is calculated by the control unit 90.

[0061] Initially, the control unit 90 obtains from a supervisor an indicator Ml of the mode used, the value of which indicates whether the current generator assembly 1 should operate in standard mode or diagnostic mode. The supervisor can be the operator in charge of maintaining the motor vehicle and its fuel cell 10, or a programmed electronic system.

[0062] The control unit 90 also acquires, depending on the mode, a current intensity setpoint to be delivered ic (in standard mode) from the fuel cell or a voltage setpoint to be delivered Uc (in diagnostic mode) to the fuel cell.

[0063] It also acquires measurements of intensity i0 and voltage Uo, as well as a measurement of the voltage UHt across the terminals of the accumulator battery.

[0064] A first summing component 101 calculates the difference between the setpoint voltage to be delivered Uc and the measured voltage Uo. This measured voltage can, however, be filtered beforehand. A low-pass filter 111 is provided for this purpose, allowing the high-frequency components of the measured voltage (typically switching noise) to be removed and reducing the risk of aliasing; its cutoff frequency is typically on the order of 10 kHz.

[0065] At the output of this first summing component 101, a correction block 105 allows obtaining a desired voltage at the output of the DC-DC converter 20. This correction block 105 is for example of the proportional-integral type.

[0066] A second summing component 102 calculates the difference between the set current intensity to be delivered ic and the measured current intensity i0. This measured current intensity can, however, be filtered beforehand. A low-pass filter 112 is provided for this purpose, allowing the removal of high-frequency components of the measured current intensity, typically above 3 kHz.

[0067] At the output of this second summing component 102, a correction block 106 allows a voltage difference to be obtained. This correction block 106 is, for example, of the proportional-integral type.

[0068] A third summing component 103 calculates the sum between this voltage difference and the filtered voltage Uo. Adding this measurement (commonly called 'feed forward') eliminates the need for it to be provided by the integrator of the controller 106, thereby improving the dynamic / stability trade-off of the regulation.

[0069] A switch 120 allows the choice, taking into account the indicator Ml of the mode used, of either the output of the first corrector block 105 (if the diagnostic mode is selected), or the output of this third summing component 103 (if the standard mode is selected).

[0070] This yields a voltage that the DC-DC converter 20 must apply at its output. This correction, related to the voltage UHt by a divider block 130, makes it possible to obtain the duty cycle q to be used to modulate the input voltage of the DC- DC 20 (in pulse width modulation). Thus, it is possible to obtain a setpoint in a very responsive manner.

[0071] It will be noted on [Fig.3] that for the same reasons as above, the voltage UHt can be filtered by a low-pass filter 132 before being used by the divider block 130.

[0072] At this stage, we can explain how the control unit can proceed to the diagnosis of the fuel cell pack 10 during a maintenance operation of the motor vehicle in the workshop.

[0073] A maintenance phase is defined as a set of operations performed to check, maintain, repair, or replace vehicle components in order to ensure its proper functioning and extend its lifespan. This phase is carried out in a mechanical workshop equipped for this purpose. It may include updating electronic systems. This maintenance phase ensures that the vehicle operates optimally and meets safety standards. It can be preventive (regular maintenance) or corrective (repair following a breakdown).

[0074] As a general rule, before maintenance (when the vehicle is used by its owner), the standard mode is selected.

[0075] To perform this maintenance, a workshop operator then begins by selecting the diagnostic mode.

[0076] In addition, it connects a humidified nitrogen N2 reservoir to the inlet of the gas injection line 50. The flow rates of nitrogen N2 and hydrogen H2 are then the same as those used during the regeneration phase.

[0077] Nitrogen is moistened to prevent the fuel cell 10 from drying out during diagnosis (and the first regeneration operation).

[0078] When this is done, the control unit 90 requires a voltage sweep. In other words, it controls the DC-DC converter 20 so that the voltage across the fuel cell 10 varies between two terminals (by varying the aforementioned duty cycle q).

[0079] The applied voltage setpoint Uc is illustrated in [Fig. 4]. This voltage setpoint varies over time in a sawtooth pattern between two terminals. In other words, it varies linearly from one terminal to the other, with the slope changing sign each time it reaches one of the two terminals. Here, two cycles are applied. In other words, the applied voltage setpoint Uc rises and then falls twice.

[0080] The terminals are here respectively equal to 0.08 V and 0.8 V for a fuel cell 10. Since the cells of the fuel cell 10 are connected in series here, these values ​​must therefore be multiplied by the number of cells used.

[0081] When the voltage setpoint Uc varies, for example at a rate of 20 mV / s, the control unit measures the intensity i0 of the induced current (delivered by the fuel cell 10), which will be linked to the state of health of the fuel cell 10.

[0082] To illustrate this concordance, the results of similar tests, in which the voltage is controlled to vary in sawtooth between 0.08 and 1.2 V (and not between 0.08 and 0.8 V), have been shown in [Fig. 5].

[0083] A first curve Cl illustrates the result of a standard ECSA test applied to a fuel cell in good health (for example when it is new).

[0084] The other curves illustrate the results when the fuel cell was affected by sulfur dioxide SO2 pollution.

[0085] It is observed that these different curves overlap, except in two areas where the voltage setpoint Uc is between 0.08 V and 0.15 V (where a decrease in the intensity i0 of the measured current occurs when the battery is polluted), and between 0.85 V and 1.2 V (where an increase in the intensity io of the measured current occurs when the battery is polluted).

[0086] Figure 6 also shows the results of similar tests carried out on other fuel cells 10 (during which the voltage is controlled to vary in a sawtooth pattern between 0.08 and 1.2 V).

[0087] A first curve C2 illustrates the result when the fuel cell is in good condition. Another curve C3 illustrates the result when the fuel cell has been affected by nitrogen dioxide (NO2) pollution.

[0088] Here again we observe that these different curves overlap, except in two areas for which the voltage setpoint Uc is between 0.08 V and 0.15 V and between 0.85 V and 1.2 V.

[0089] In practice, it is observed that between 0.08 V and 0.15 V, the measured current intensity i0 is positive in the case of sulfur dioxide (SO2) pollution but negative in the case of nitrogen dioxide (NO2) pollution. This observation therefore makes it possible to distinguish between the two types of pollution.

[0090] Preferably, only the results obtained in the voltage setpoint window Uc between 0.08 V and 0.15 V will then be considered, since they allow us to distinguish these two types of pollution and it is preferable to avoid applying voltages above 0.8 V to a fuel cell (to avoid carbon corrosion on the cathode side).

[0091] The diagnosis is therefore carried out by determining whether the measured current intensity i0 is different from the expected intensity in the aforementioned voltage window, and if so, whether it is negative or positive.

[0092] To carry out this comparison, it is possible to consider a value of intensity i0 (for a given voltage within the window), or several values ​​of intensity (for example the average of the intensity i0 within the window).

[0093] Thus, if the intensity is close to or equal to the expected intensity, the diagnosis indicates that the health of the fuel cell is good and that no regeneration is necessary.

[0094] Otherwise, if the measured intensity is negative, the diagnosis indicates nitrogen dioxide (NO2) pollution; otherwise, it indicates sulfur dioxide (SO2) pollution. In this case, regeneration of the fuel cell pack is necessary.

[0095] Depending on the type of pollution affecting this pack, the regeneration process differs.

[0096] In the case of pollution by nitrogen dioxide NO2, this process involves two distinct operations, whereas in the case of pollution by sulfur dioxide SO2, only the first of these two operations is necessary.

[0097] To implement these regeneration operations, it is necessary to connect the output terminals of the DC-DC converter to a current consumer, for example here to a battery that is not fully charged.

[0098] For the first regeneration operation, as before, the cathode is supplied with nitrogen N2 (as during the diagnostic) and the anode with hydrogen H2, while the control unit 90 forces the DC / DC converter to present a voltage across the cell terminals that varies between 0.08V and 0.8V per cell, for example at a rate of 20 mV / s. A cycle will then be defined as a rise followed by a fall in the voltage, between 0.08V and 0.8V.

[0099] During this first regeneration operation, the control unit 90 forces the voltage to vary over at least one cycle and, preferably, over several cycles. Thus, here at least 5 cycles are applied. In practice, about ten cycles are applied, this number resulting from a compromise between the quality of the regeneration result and the duration of the regeneration.

[0100] The flow rates of nitrogen N2 and hydrogen H2 are regulated here by valves 32, 51 so as to remain constant. These flow rates, expressed in normal liters per hour, are calculated here as follows:

[0101] Qh2 = 6* Active / 25 * Nceii

[0102] QN2 = 9 * Active / 25 * Nceii

[0103] In these two equations, NCeii is the number of cells in the fuel cell, while Sactive is the active area of ​​each cell in the cell, expressed in cm2.

[0104] For the second regeneration operation, it is planned this time to supply the cathode with air charged with ozone O3 and the anode with hydrogen H2, while the unit The 90 control unit controls the DC-DC converter not by voltage but by current (as in standard mode). To do this, the operator selects standard mode.

[0105] Then, the operator must connect an ozone O3 reservoir to the inlet of the gas injection line 50. The control unit will then be able to control the valve 51 and the compressor 42 so as to produce a mixture here less than 1% of ozone (preferably equal to 0.4% in months).

[0106] The DC-DC converter 20 is then controlled so that the fuel cell delivers a current that varies between 0.2 A / cm2 and 1 A / cm2, for example at a speed of 20 mA / s.

[0107] During this second regeneration operation, the control unit 90 forces the intensity to vary for at least one minute, and more preferably for several minutes. In practice, the duration of this second operation is approximately 15 minutes, this duration resulting from a compromise between the quality of the regeneration result and the duration of the regeneration. In practice, about ten cycles are applied, this number resulting from a compromise between the quality of the regeneration result and the duration of the regeneration.

[0108] Following regeneration, the control unit can perform a new diagnosis of the fuel cell 10, in a manner similar to that described above.

[0109] Thus, during this new diagnosis, the control unit 90 checks whether the measured current intensity i0 is equal to or different from the expected intensity in the selected voltage window.

[0110] If these intensities are equal (within a predetermined difference), the diagnosis indicates that the health of the fuel cell is good.

[0111] Otherwise, the diagnostic indicates that the fuel cell is in poor condition. In this event, the control unit informs the operator that the fuel cell has a fault, in which case the operator may consider replacing the cell.

[0112] The present invention is in no way limited to the embodiment described and represented, but a person skilled in the art will be able to make any variation in accordance with the invention.

[0113] Typically, one could perform only the diagnosis, without seeking to regenerate the fuel cell.

Claims

Demands

1. A method for diagnosing the health status of a fuel cell (10) of a power generation unit (1) equipping a motor vehicle, according to which, during a maintenance phase of the motor vehicle in a workshop, the following steps are taken: - connecting a nitrogen (N2) tank to an air intake line (40) of the power generation unit (1), - supplying an anode of the fuel cell (10) with hydrogen (H2) and a cathode of the fuel cell (10) with nitrogen (N2), - controlling the voltage (Uo) of the fuel cell (10) so that it varies, - measuring the intensity (i0) of the current generated by the fuel cell (10) when the voltage (Uo) varies, - comparing the measured intensity (i0) with a reference, and - diagnosing said health status based on the result of said comparison.

2. Diagnostic method according to claim 1, wherein, the fuel cell (10) comprising at least one cell, the voltage (Uo) is controlled to vary between two terminals, of which a lower terminal less than 0.1 V per cell, and an upper terminal between 0.3 V and 0.9 V per cell, said lower terminal preferably being non-zero and said upper terminal preferably being equal to 0.8 V per cell.

3. Diagnostic method according to claim 1 or 2, wherein the measured intensity (i0) is compared with the reference only in a window where said voltage (Uo) is between 0.1 and 0.3V per cell.

4. A method for maintaining a motor vehicle equipped with a current generator assembly (1) comprising a fuel cell (10), wherein it is planned to implement: - a diagnostic method according to any one of claims 1 to 3 and then, - depending on the result of the diagnosis, a regeneration step of the fuel cell (10).

5. A maintenance method according to claim 4, wherein: - in the comparison step of the diagnostic method, it is determined whether the measured intensity (i0) is substantially equal to said reference and if it is positive or negative within a predetermined voltage window (Uo), and - the regeneration step is implemented only if the measured intensity (i0) is not substantially equal to said reference, according to a process which is different depending on whether the measured intensity (i0) is positive or negative.

6. Maintenance method according to claim 5, wherein the regeneration step comprises, if the measured intensity (io) is positive, two separate operations and, if the measured intensity (i0) is negative, a single operation, preferably identical to one of said two separate operations.

7. Maintenance method according to claim 6, wherein: - one of the two separate operations is carried out by supplying the anode of the fuel cell (10) with dihydrogen (H2) and the cathode of the fuel cell (10) with dinitrogen (N2) and by controlling the voltage (Uo) applied to the fuel cell (10) so that it varies, and - the other of the two separate operations is carried out by supplying the anode of the fuel cell (10) with dihydrogen (H2) and the cathode of the fuel cell (10) with ozone-charged air and by controlling the current (i0) delivered by the fuel cell (10) so that it varies.

8. Motor vehicle current generator assembly (1), comprising: - a fuel cell (10), - an air intake line (40) leading into the fuel cell (10), equipped with a connection system for a tank external to the motor vehicle, - a DC-DC converter (20) which is connected, on one side, to the fuel cell (10), and, on the other, to an electrical current-consuming accessory such as a battery, and - a control unit (90) which is adapted to control the DC-DC converter (20) in such a way that the fuel cell (10) has a voltage (Uo) across its terminals equal to a voltage setpoint (Uc).

9. Current generator assembly (1) according to claim 8, wherein the control unit (90) comprises a switch (120) adapted to switch between:

10. - a first state in which the piloting unit (90) is adapted to control the DC-DC converter (20) such that the fuel cell (10) presents a voltage (Uo) across its terminals equal to a voltage setpoint (Uc), and - a second state in which the piloting unit (90) is adapted to control the DC-DC converter (20) in such a way that the fuel cell (10) delivers a current of intensity (i0) equal to a set intensity (ic). Current generator assembly (1) according to claim 9, wherein the control unit (90) is controlled by an electronic or human supervisor in order to switch the switch (120) into one state or the other.

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