DEVICE AND METHOD FOR DETERMINING THE HEALTH STATUS OF A FUEL CELL

Description

TECHNICAL FIELD

[0001] The present invention relates to the determination of the impedance of the stages of a fuel cell, and more particularly of a hydrogen cell. It makes it possible to determine the state of health of such a cell. STATE OF THE ART

[0002] A fuel cell has a stacked structure of metal plates, called bipolar, intercalated with membranes. Each pair of plates, with the membrane in the middle, is a layer or stage, comprising a positive and a negative electrode (anode and cathode) presenting an electrical potential during operation of the cell. The most delicate and crucial part for the performance of the fuel cell and its longevity is in particular the membrane, also called PEM membrane (Proton Exchange Membrane in English), that is to say the proton exchange membrane of a fuel cell. For a hydrogen cell, the membrane constitutes the interface which allows the electrochemical reaction between oxygen and hydrogen producing an electric current, in particular thanks to the presence of a catalyst.If the membrane is too dry (accumulation of inert gas within it), or on the contrary if it is too humid (accumulation of water within it), its performance, as well as the performance of the battery, will be degraded, with a possible reduction in its lifespan.

[0003] Several representative models of a fuel cell exist, often with discrete elements and capable of providing an estimate of the state of health of the cell, and in particular of its membranes. These models are very often based on a measurement of the electrical impedance of the cell and each of its stages, using filtering algorithms that rely on the total current and voltage measurements of the stages of the cell obtained during its normal operation. In order to be able to use these models, it is necessary to know the impedance of the cell, or of a stage of the cell, and more particularly to know the resistance of the membrane at a precise frequency, often equal to 1 kHz. Indeed, the impedance is the input data for the models representative of the state of health of the membrane and therefore of the cell, and it is therefore necessary to measure the impedance, in particular at 1 kHz, of each stage of the cell.It should be noted that an impedance measurement at 1 kHz, or at another frequency required by the state algorithms, allows a relevant state of health of the battery to be determined.

[0004] There are electronic components for measuring voltage, but generally their sampling frequency is not sufficient to measure an impedance at 1 kHz.

[0005] Other devices can measure the impedance of a battery and its stages during assembly. However, these are dedicated laboratory instruments used only after the battery has been gassed, i.e., before the battery is loaded into a system, such as a mobile vehicle. These instruments are not suitable for transport or integration into a portable diagnostic device.

[0006] For example, Chinese patent application CN 109 916 964 discloses a method in which several gases with different humidities are selected to purge a fuel cell and obtain different internal resistances of the cell. However, this method is not suitable for measuring the impedance of a cell in operation, particularly a cell embedded in a system, for example a mobile vehicle.

[0007] Another example is US patent application US 2019 049522, which discloses an estimation of a fuel cell's state of health (SOH) by applying an excitation current or voltage to the entire cell to determine a harmonic distortion factor of a signal resulting in voltage or current. In addition, the method can use a model to determine the cell's state of health. However, the method does not allow for the measurement of an impedance of a particular stage. Nor does it overcome measurement errors.

[0008] We can cite the American patent application US 2010 323260, which discloses a method in which an impedance value previously measured on a fuel cell is stored in memory when it is not in operation. Then, the impedance of the cell is measured and the measured value is compared with the reference impedance stored in memory in order to estimate a quantity of water present in the system. The method also does not make it possible to know the state of the membrane of a stage of the cell. It also does not overcome measurement errors, in particular measurement errors on the impedance of the cell.

[0009] Also worthy of mention is Chinese patent application CN 111077467A, which discloses an impedance measurement system for proton exchange membrane fuel cells.

[0010] An object of the present invention is therefore to propose means for improving the estimation of the state of health of a battery.

[0011] Another object is to provide means for estimating the state of health of the battery when the latter is in operation to produce electrical energy.

[0012] Another object is to provide means that are simple to produce and implement.

[0013] Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0014] To achieve this objective, according to one embodiment, a device is provided for determining the impedance of a plurality of stages of a fuel cell, each stage of the plurality of stages comprising an electrolyte placed between two electrodes, the device comprising: several connection circuits respectively associated with the stages of the plurality of stages, each connection circuit being electrically coupled to the electrodes of the associated stage, and a measurement system comprising an excitation system common to the plurality of stages, and selection means.

[0015] The measurement system comprises a circuit, called an equivalent circuit, having a reference impedance corresponding to that of a stage of the plurality of stages, and the selection means are capable of selecting a stage of the plurality of stages by electrically coupling the associated connection circuit with the excitation system, the selection means being further capable of selecting the equivalent circuit by electrically coupling the equivalent circuit with the excitation system, and the measurement system is configured to: applying to the selected stage an excitation signal generated by the excitation system; measuring a response signal of the selected stage to the application of the excitation signal; applying to the equivalent circuit an excitation signal generated by the excitation system; measuring a response signal of the equivalent circuit to the application of the excitation signal; determining an impedance of the equivalent circuit from the response signal of the equivalent circuit; calculating an impedance error from the reference impedance and the determined impedance of the equivalent circuit, and determining an impedance of the selected stage from the response signal of the selected stage and the impedance error.

[0016] Thus, the device makes it possible to determine an impedance of each of the stages of the plurality of stages and preferably of all the stages of the stack.

[0017] Determining the impedance of a stage makes it possible to determine information relating to the state of health of the stage, and therefore of the battery, such as for example a variation in the resistance of the electrolyte.

[0018] Such a device is particularly space-saving and can be easily integrated into a system where the battery is embedded, particularly a mobile system.

[0019] According to another aspect, there is provided an assembly, comprising: a fuel cell comprising a plurality of stages, each stage of the plurality of stages comprising an electrolyte placed between two electrodes; and a device for determining the impedance of the plurality of stages of the fuel cell as defined above.

[0020] According to another aspect, there is provided a method of determining impedance of a plurality of stages of a fuel cell, each stage of the plurality of stages comprising an electrolyte placed between two electrodes.

[0021] The process includes: a first measurement comprising: ∘ a selection of a stage by electrically coupling the electrodes of the stage with an excitation system common to the plurality of stages; ∘ an application to each selected stage of an excitation signal generated by the excitation system; and ∘ a measurement of a response signal of the selected stage to the application of the excitation signal; a second measurement carried out before or after the first measurement, the second measurement comprising: ∘ a selection of a circuit, called an equivalent circuit, having a reference impedance corresponding to that of a stage of the plurality of stages, by electrically coupling the equivalent circuit with the excitation system; ∘ an application to the equivalent circuit of an excitation signal generated by the excitation system; and ∘ a measurement of a response signal of the equivalent circuit to the application of the excitation signal; the method further comprising: a determination of an impedance of the equivalent circuit from the response signal of the equivalent circuit; a calculation of an impedance error from the reference impedance and the determined impedance of the equivalent circuit, and a determination of an impedance of the selected stage from the response signal of the selected stage and the impedance error. BRIEF DESCRIPTION OF THE FIGURES

[0022] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which: There figure 1 schematically represents an embodiment of an impedance determination device. The figure 2 schematically represents an embodiment of an excitation system. The figure 3 schematically represents an embodiment of an equivalent circuit.

[0023] The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. DETAILED DESCRIPTION

[0024] Before commencing a detailed review of embodiments of the invention, optional features which may optionally be used in combination or alternatively are set out below: According to one example, the measurement system comprises an electronic control unit and the selection means comprise at least one multiplexer controlled by the electronic control unit. According to one example, the equivalent electrical circuit comprises a set of components having the reference impedance as an impedance, and a voltage source for powering the set of components, the voltage source providing a voltage equivalent to that provided by a stage. According to one example, the measurement system is configured to determine the impedance of a selected stage periodically according to a first period and to determine the impedance of the equivalent circuit periodically according to a second period less than or equal to the first period.According to one example, the measurement system is located at a location such that a temperature of the location differs by less than 30°C and preferably by less than 10°C, from a temperature of the fuel cell during its operation. According to one example, the measurement system comprises a temperature sensor capable of measuring the temperature of the fuel cell, in particular by measuring the temperature in the vicinity or that of the coolant at the outlet of the cell. According to one example, the two electrically conductive elements at least partially use an area where the two electrically conductive wires of at least one connection circuit are located. According to one example, the measurement system is printed on a printed circuit board. According to one example, the measurement system is powered by a power source independent of the fuel cell.According to one example, the excitation signal has a frequency between 0.1 Hz and 10000 Hz, preferably between 100 Hz and 5000 Hz, preferably equal to 1000 Hz. According to another example, each connection circuit comprises two electrically conductive wires coupled to the selection means and the equivalent electrical circuit comprises two electrically conductive elements coupled to the selection means, the impedance of each of the two elements being between 0.9 times and 1.1 times that of each of the two wires of the connection circuits. According to another example, the section and the length of each of the two elements is between 0.9 times and 1.1 times, preferably identical, to those of each of the two wires of the connection circuits. According to another example, the plurality of stages corresponds to all of the stages of the fuel cell. Thus, the device makes it possible to measure the impedance of each stage of the fuel cell.According to another example, the device comprises a single excitation system for the plurality of stages, preferably a single excitation circuit for all of the stages of the fuel cell. According to another example, the measurement system is further configured to determine the impedance of the selected stage when the fuel cell provides electrical energy. According to one example, the electrolyte is a proton exchange membrane. According to another example, the method comprises powering the equivalent circuit with a power source independent of the fuel cell. According to another example, the method comprises periodically determining, according to a first period, the impedance of a selected stage and periodically determining, according to a second period less than or equal to the first period, the impedance of the equivalent circuit. According to another example, the fuel cell produces electrical energy.

[0025] It is specified that in the context of the present invention, the expression "A electrically coupled to B" or "A coupled to B" or "A connected to B" is synonymous with "A is in electrical connection with B" and does not necessarily mean that there is no member between A and B. Thus these expressions are understood to mean an electrical connection between two elements, this connection being able or not to be direct, this means that it is possible that between a first device A and a second device B which are electrically connected, a current flows in A, in B, and on the path connecting A to B, this path being able or not to include other electrical equipment.

[0026] Conversely, in the context of the present invention, the term "electrically directly connected" means a direct electrical connection between two elements. This means that between a first device A and a second device B which are electrically directly connected no other equipment is present, other than one or more electrical connections.

[0027] On the figure 1, a device 1 is shown for determining the impedance of a plurality of stages 2 of a fuel cell 3. A fuel cell 3 is a system for generating an electrical voltage by means of oxidation on one electrode of a reducing fuel coupled with reduction on the other electrode of an oxidant. In particular, the device 1 is aimed at hydrogen cells, for which the reducing fuel is dihydrogen and the oxidant is oxygen. Each stage 2 of the cell 3 comprises an electrolyte 4 placed between two electrodes 5, 6. The electrolyte 4, also called a membrane or PEM membrane, is a conductive substance due to the presence of mobile ions. In the case of a hydrogen cell, the mobile ions are hydrogen ions. The gases (hydrogen, oxygen, water) circulate in channels present at each stage 2 of the cell 3, not shown here for the sake of simplification.

[0028] The electrical potential of a stage 2 is between 0.6 V and 1.2 V for the stage of a battery in normal operation, that is to say a battery which produces electrical energy. The electrodes 5, 6 of a stage 2 form an anode, cathode pair and make it possible to produce a voltage, at each stage 2 of the battery 3. In other words, each stage 2 comprises a first electrode 5 of a first polarity and a second electrode 6 of a second polarity. In the example illustrated on the figure 1, the first polarity is greater than the second polarity. In general, the electrodes 5, 6 are partially metallic. Furthermore, the electrodes 5, 6 of two adjacent stages 2 are in contact with electrical collectors typically called bipolar plates 100 because they are in contact on one side of an anode, and on the other side of the cathode of an adjacent stage 2. More particularly, a first electrical collector, also called first monopolar plate 101, located at a first end of the cell 3 is in contact with a single first electrode 5. A second electrical collector, or second monopolar plate 102, located at a second end of the cell 3 is in contact with a single second electrode 6. The first and second monopolar plates 101, 102 are also called terminal plates. These bipolar plates 100 and monopolar plates 101, 102 are typically connection points for carrying out voltage measurements in the battery 3.As an example, a fuel cell 3 is shown comprising ten stages 2 stacked in series.

[0029] The device 1 comprises connection circuits 10 and a measuring system 11. The connection circuits 10 are intended to electrically couple the stages 2 with the measuring system 11. Each connection circuit 10 is therefore associated with a distinct stage 2 of the stack 3. For example, the device 1 comprises several connection circuits 10 associated respectively with certain stages 2 of the stack 3. Advantageously, the device comprises a connection circuit 10 for each of all the stages 2 of the stack 3. More particularly, each connection circuit 10 is electrically coupled to the electrodes 5, 6 of the stage 2 with which it is associated. In particular, each connection circuit 10 is electrically connected directly to the bipolar 100 and monopolar 101, 102 plates of the stage 2 with which it is associated.For example, each connection circuit 10 comprises two electrical connections 12, 13, denoted first and second electrical connections 12, 13, such as electrically conductive cables, wires or strips, respectively electrically coupled to the electrodes 5, 6 of an associated stage 2. Preferably, the measurement system 11 is electrically coupled to ten stages 2 of the fuel cell.

[0030] The measuring system 11 comprises an excitation system C1 common to the plurality of stages 2. For example, the device 1 comprises a single excitation system C1 for the plurality of stages 2, advantageously a single excitation circuit C1 for all of the stages 2 of the stack 3. The excitation system C1 is capable of generating an excitation signal intended for the stages 2 of the stack 3. The excitation signal C1 may be a variation in current or voltage. Preferably, the excitation signal is a variation in current. Preferably, the excitation signal is a sinusoidal signal. The excitation signal makes it possible to determine the impedance Zcell of a stage 2 of the stack 3. The device 1 is furthermore particularly suitable for determining the impedance Zcell of each of the stages 2 of the plurality of stages of the stack 3.

[0031] The excitation system C1 has the function of subjecting a selected stage 2 to variations in current, or voltage, by applying a signal, for example sinusoidal. The measurement system 11 makes it possible to determine the impedance Zcell of the selected stage 2 at a particular frequency of interest. In particular, the excitation signal has a frequency between 0.1 Hz and 10000 Hz, preferably between 100 Hz and 5000 Hz, preferably equal to 1000 Hz.

[0032] To determine the impedance Zcell of a stage 2, an excitation signal is applied to stage 2 at a frequency f. Stage 2 thus undergoes a current variation at the frequency f resulting in a voltage variation at the terminals of stage 2, i.e. between the electrodes 5, 6 of stage 2. A signal is measured in response to the application of the excitation signal, by measuring the variation in current and / or voltage, and an impedance value Zcell is determined from the measured variation in current and / or voltage.

[0033] In order to be able to determine the impedance Zcell of each stage 2 of the plurality of stages, the measurement system 11 comprises selection means M+, M- capable of selecting a stage 2 by electrically coupling the connection circuit 10 associated with the stage 2 with the excitation system C1. In particular, the connection circuits 10 are coupled to the selection means M+, M-, via the electrical connections 12, 13, and each selection means M+, M- is coupled to the excitation system C1 by a connection 14, 15. For example, the connection means M+, M- can be multiplexers. Each multiplexer comprises several inputs connected to the electrical connections 12, 13 of the connection circuits 10 and an output coupled to a terminal J8, J5 of the excitation system C1.More particularly, each bipolar plate 100 is electrically connected directly to the two selection means M+, M-, i.e. two electrodes 5, 6 of two adjacent stages 2 are electrically connected to the two selection means M+, M-. Furthermore, a first monopolar plate 101 is electrically connected to the first selection means M+ and a last monopolar plate 102 is electrically connected directly to the second selection means M-. In other words, a single first electrode 5 located at the first end of the stack 3 is electrically coupled to the first selection means M+, and a single second electrode 6, located at the second end of the stack 3, is electrically coupled to the second selection means M-.

[0034] Furthermore, a first selection means M+ may be coupled to a first terminal J8 of the excitation system C1 via a resistor R5, also referred to as a shunt resistor. In other words, the shunt resistor R5 has its terminals coupled respectively to the first terminal J8 and to a node J9 coupled to the output of the first selection means M+. According to one embodiment, a first selection means M+ has its inputs connected to the first electrodes 5 of each of the stages 2 of the plurality of stacks 3, via the first electrical connections 12, and the second selection means M- has its inputs connected to the second electrodes 6 of each of the stages 2 of the plurality of stages of the stack 3, via the second electrical connections 13.

[0035] Advantageously, the selection means M+ M- can be controlled, and the measurement system 11 comprises an electronic control unit MCU, for example a microcontroller, a processor or a microprocessor, configured to issue commands CD1, CD2 to the selection means M+, M- in order to select a connection circuit 10. When a connection circuit 10 is selected, it is also said that the associated stage 2 is selected. In other words, a stage 2 is selected when the stage 2 forms, with the associated connection circuit 10 and the excitation circuit C1, a closed circuit. In particular, the selection means M+, M- are controlled to select only one stage at a time before determining the impedance Zcell.

[0036] Furthermore, the measuring system 11 is configured to apply to a selected stage 2 an excitation signal generated by the excitation system C1. Furthermore, the measuring system 11 is configured to measure a response signal to the application of the excitation signal. The response signal may be a response current or a response voltage. The measurement of the response signal may be carried out by the electronic control unit MCU. In particular, the electronic control unit MCU may be connected to the terminals of the shunt resistor R5, via connections 16, 17 connected to the node J9, and to the terminal J8. Furthermore, the electronic control unit MCU may be connected downstream of the selection means M+, M-, by connections 18, 19 connected to a node J6 and to the terminal J5, the node J6 being coupled to the output of the first selection means M+.Thus, the electronic control unit MCU can measure the response voltage of a selected stage 2 by measuring the voltage between node J6 and terminal J5. The electronic control unit MCU can also measure the response current of a selected stage 2 from the voltage measured between node J9 and terminal J8. More particularly, the excitation signal is a sinusoidal signal and the response signal is also sinusoidal. The electronic control unit MCU may further comprise measurement circuits capable of measuring the response signal, i.e. the response current or voltage. The measurement circuits comprise, for example, an amplifier, a filter and an analog / digital converter for carrying out the measurements.

[0037] Furthermore, the variations of the response signal depend on the impedance of the selected stage 2, and more particularly on the state of the membrane 4 of the stage 2. Thus, by measuring the response current and / or voltage, the measurement system 11 can determine the impedance Zcell of the selected stage 2. In particular, the measurement system 11 selects one stage 2 at a time to determine the impedance Zcell of the stage 2. The measurement system 11 therefore makes it possible to estimate the state of the membrane 4 of a stage 2 from the impedance Zcell thus determined. Furthermore, the measurement system 11 can more precisely estimate the general state of the stack 3 by determining the impedance Zcell of all the stages 2 of the stack 3.

[0038] On the figure 2 , an embodiment of an excitation system C1 is shown.

[0039] In this embodiment, the excitation system C1 is an electrical circuit. The excitation system C1 comprises a discharge branch 20 comprising in series a power transistor Q2, for example a MOS transistor or a bipolar transistor, and a discharge resistor R6. In this example, the transistor Q2 is an NPN type bipolar transistor whose collector is connected to the first terminal J8 and whose emitter is connected to the discharge resistor R6. Alternatively, a PNP transistor or a MOS transistor may be used. The excitation system C1 further comprises an operational amplifier U1, a high supply node J1 of which is intended to receive a first supply potential VCC and a low supply node J3 of which is intended to receive a second supply potential lower than the first potential. The power supply for the amplifier U1 may come from an external source not shown.Preferably, the supply potential VCC is provided by a power source independent of the fuel cell 3. In the example shown, the low supply node J3 is connected to the low potential of the stage 2 whose impedance Zcell is to be measured, i.e. by the potential of the node J5. The amplifier U1 comprises an inverting input terminal (-) connected to the emitter of the transistor Q2, and an output terminal connected to the base of the transistor Q2 via a resistor R4. The circuit C1 further comprises a node J2 coupled to the microcontroller MCU and to the non-inverting input terminal (+) of the operational amplifier U1 so as to transmit a control voltage CD3, from the microcontroller MCU, to the operational amplifier U1. The control voltage CD3 applied to the amplifier U1 makes it possible to close or open the transistor Q2, to deliver or not the excitation signal to the selected stage 2.In particular, the control voltage CD3 determines the current drawn into the transistor Q2 by the selected stage 2. The excitation system C1 further comprises a power source V2 coupled between the node J2 and the terminal J5.

[0040] The discharge branch 20 is connected in parallel to the terminals J8, J5 of the excitation system C1. The terminals J8, J5 of the excitation system C1 are coupled to the terminals of the selected stage 2, via the selection means M+, M-, whose impedance Zcell is to be measured.

[0041] In order to improve the estimation of the state of the membrane 4 of the stages of the cell 3, the measurement system 11 can be configured to improve the precision of the determination of the impedance Zcell of the stages 2. The precision on the determination of the impedance Zcell is all the more relevant as the values ​​measured in voltage and / or in current are low for a fuel cell 3, and in particular a hydrogen cell. Indeed, it appears that the selection means M+, M- and the connection circuits 10 can introduce errors into the measurements of the response signals of the stages 2. More particularly, the impedance, especially resistive, non-zero of the selection means M+, M- in conduction, which can be a few ohms, can degrade the measurement of the response signals and therefore the determination of the impedance of the membrane 4. Furthermore, the length of the electrical connections 12, 13 between the stages 2 and the selection means M+, M- introduces a non-negligible error.The temperature can affect the resistance of the electrical connections 12, 13. Furthermore, the impedance of the electrical connections 12, 13 and that of the selection means M+, M- add to that of the selected stage 2 and therefore disturb the determination of the impedance Zcell of the selected stage 2. In order to improve the accuracy of the impedance Zcell, the measurement system 11 can be configured to compensate for the errors introduced by the connection circuits 10 and the selection means M+, M-.

[0042] The measuring system 11 further comprises a circuit C2, called an equivalent circuit. The equivalent circuit C2 is configured to have a reference impedance Zref. The reference impedance Zref corresponds to an impedance of a stage 2 of the battery 3. The equivalent circuit C2 can be produced from theoretical models. For example, the impedance Zcell of a stage 2 can be determined when the battery 3 is in operation, that is to say when it produces electrical energy, before its use. This determination can be carried out in the factory. Then, the equivalent circuit C2 is produced to obtain a circuit having an impedance Zref as close as possible to that previously determined Zcell.

[0043] The equivalent circuit C2 is electrically coupled to the selection means M+, M-, via two electrically conductive elements 22, 21 respectively. The electrically conductive elements 21, 22 may be electrically conductive cables, wires or strips, or tracks on a printed circuit board, for example on a board 30 where the equivalent circuit C2 is produced. The selection means M+, M- are further capable of selecting the equivalent circuit C2 by electrically coupling the equivalent circuit C2 with the excitation system C1. The measurement system 11 is further configured to: apply to the equivalent circuit C2 an excitation signal generated by the excitation system C1; measure a response signal of the equivalent circuit C2 to the application of the excitation signal; determine an impedance of the equivalent circuit Zref_meas from the response signal of the equivalent circuit C2; calculate an impedance error ez from the reference impedance Zref and the determined impedance of the equivalent circuit Zref_meas.

[0044] The measurement system 11 is further configured to determine the impedance of the selected stage Zcell from the response signal and the impedance error ez.

[0045] The electrical connections 12, 13 of the connection circuits 10 and the selection means M+, M- have non-zero resistances and introduce an error ez on the determination of the impedance Zcell of the selected stage 2. We can note ez = efils + ems, where ez corresponds to the impedance error on the determined impedance Zcell of a selected stage 2 (in ohms), efils corresponds to the impedance error induced by the electrical connections 12, 13, (in ohms), and ems the impedance error induced by the selection means M+, M- (in ohms).

[0046] The determination of the impedance Zcell of a selected stage 2 can be calculated according to equation (1): Zcell_meas = Zcell + ez (1), where Zcell_meas corresponds to the impedance of a selected stage 2 with a measurement error (in ohms) and Zcell corresponds to the impedance of a selected stage 2 without error.

[0047] Furthermore, the determination of the impedance of the equivalent circuit Zref_meas from the measurement of the response signal of the equivalent circuit is impacted by the same disturbances as the determination of the impedance of stage 2 selects Zcell_meas from the measurement of the response signal of stage 2. In addition, the reference impedance Zref of the equivalent circuit C2 is known, and we can therefore estimate the impedance of the equivalent circuit C2 by calculating the following equation (2): Zref_meas = Zref + ez (2), where Zref is the impedance of the equivalent circuit C2 (in ohms) and Zref_meas is the determined impedance of the equivalent circuit with a measurement error (in ohms). The impedance error ez on the determined impedance Zcell of a selected stage 2 is considered to be the same as the impedance error on the determined impedance Zref_meas of the equivalent circuit C2.Indeed, the measurement system 11 uses the same selection means M+, M- and the same excitation system C1 to determine both Zcell and Zref_meas.

[0048] Thus, we can deduce from equation (2), the following equation (3): ez = Zref_meas − Zref

[0049] From equation (1), we can deduce the following equation (4): Zcell = Zcell_meas − ez

[0050] We can therefore deduce from equations (3) and (4), the following equation (5): Zcell = Zcell_meas − Zref_meas + Zref

[0051] The determination of the impedance Zref makes it possible to subtract the error due to the electrical connections 12, 13 and the impedance of the selection means M+, M- from the determination of the impedance Zcell of the selected stage 2.

[0052] Thus, by knowing the value of Zref, then determining the impedance of a selected stage 2 with an impedance error Zcell_meas and determining the impedance of the equivalent circuit C2 with an impedance error Zref_meas, we determine the impedance of the selected stage 2 without Zcell error.

[0053] To obtain the most accurate determination of the impedance Zcell, it is preferable to carry out the determination of the impedance Zref_meas under conditions as close as possible to those during the determination of the impedance Zcell. Thus, the impedance of each of the two elements 21, 22 can be between 0.9 times and 1.1 times that of each of the two electrical connections 12, 13 of the connection circuits 10. Advantageously, the elements 21, 22 are made with the same material as that of the electrical connections 12, 13. Preferably, the section and the length of each of the two elements 21, 22 is between 0.9 times and 1.1 times, preferably identical, to those of each of the electrical connections 12, 13.It is not necessary to minimize the length of the electrically conductive elements 21, 22 to limit the resistance effects between the equivalent circuit C2 and the selection means M+, M- in order to determine the most accurate impedance Zcell possible.

[0054] On the figure 3, an embodiment of the equivalent circuit C2 is shown. The equivalent circuit C2 comprises a set of components, R8, L1, C4, R9, C5, R10 having as impedance, the reference impedance Zref. In particular, the equivalent circuit C2 comprises in series a voltage source V1, a resistor R8, an inductance L1, a first circuit P1 having a capacitor C4 connected in parallel with a resistor R9 and a second circuit P2 having a capacitor C5 connected in parallel with a resistor R10. Furthermore, the equivalent circuit C2 comprises a first terminal J21 coupled to the second circuit P2 and to a first electrically conductive element 21 and a second terminal J22 coupled to the voltage source V1 and to the second electrically conductive element J22.The voltage source V2 may be a linear regulator LDO (Low-dropout regulators) with a low impedance output, powered by the same voltage that powers the other components of the measurement system 11. Thus, disturbing the measurement of the response signal of the equivalent circuit C2 is avoided when determining the impedance Zref_meas. Advantageously, the voltage source V1 provides a voltage equivalent to that provided by a stage of the stack 3. For example, the source V1 provides a voltage of 1V. Thus, the impedance error ez is similar when determining the impedance Zcell of a selected stage 2 and when determining the impedance of the equivalent circuit Zref_meas. In other words, the measurement errors are similar when measuring the response signals of the selected stages 2 and when measuring the response signal of the equivalent circuit C2.

[0055] The measurement system 11 is further configured to determine the impedance of a selected stage Zcell periodically according to a first period and to determine the impedance of the equivalent circuit Zcell_meas periodically according to a second period less than or equal to the first period.

[0056] The electrically conductive elements 21, 22 of the measuring system 11 are located at a position which allows them to be subjected to a temperature very close to that of the electrical connections 12, 13 of the impedance determination device 1. For example, considering ΔT°C the temperature difference between the electrical connections 12, 13 of each connection circuit 10 and the electrically conductive elements 21, 22 coupled to the selection means (M+, M-) of the equivalent electrical circuit C2, ΔT°C < 30°C, preferably ΔT°C < 10°C.

[0057] Preferably, the measuring system 11 is located at a location such that a temperature Tmeas of the location differs by less than 30°C and preferably by less than 10°C, from a temperature Tcell of the fuel cell 3 during its operation.

[0058] For example, the two electrically conductive elements 21, 22 at least partially borrow an area where the electrical connections 12, 13 of at least one connection circuit 10 are located, so that the electrically conductive elements 21, 22 and the electrical connections 12, 13 are located in the same thermal environment.

[0059] Thus, the temperature variations which impact the impedance of the measuring system 11, in particular that of the electrically conductive elements 21, 22, impact in the same way the impedance Zref of the equivalent circuit C2. The determination of the impedance of each stage 2 is therefore free from the variations in impedance of the measuring circuit as a function of the temperature.

[0060] For this, the measuring system 11 is located so that the heat from the cell 3 is, by conduction and / or convection, transmitted in the same way to the electrical connections 12, 13 of each connection circuit 10 and to the electrically conductive elements 21, 22 of the equivalent electrical circuit C2.

[0061] For example, the measurement system may be printed on a printed circuit board 30. The board 30 may be part of a battery control system 3, also called an FCMS board (“Fuel Cell Monitoring System” in English, i.e. fuel cell control system) or a CVM board (“Cell Voltage Monitor”), i.e. measurement of the voltage of a stage of the battery or cell. Thus, the board 30 supporting the measurement system may be housed in a housing.

[0062] In particular, the control system measures the voltages of the cell 3. The card 30 can be permanently coupled to the cell 3. The measurement system 11 can be powered by a voltage source, not shown for the sake of simplification, independent of the fuel cell 3. Thus, a higher supply voltage can be used than that provided by the cell 3. The various components and circuits of the measurement system 11 can therefore be easily powered. Furthermore, the measurement system 11 does not disturb the operation of the cell 3 when determining the impedance Zcell of a stage 2.

[0063] The measuring system 11 is preferably powered by a power source, not shown for simplification purposes, independent of the fuel cell 3.

[0064] The measurement system is further configured to determine the impedance of the selected stage Zcell when the fuel cell is providing electrical energy. Advantageously, the impedance measurement can be performed during normal operation of the cell, without the need to shut down or disassemble the cell.

[0065] A method for determining the impedance of a plurality of stages 2 of a fuel cell 3 can be implemented by the device defined above. In particular, the method can alternate a series of determinations of the impedance Zcell of several stages 2 with one or more determinations of the impedance Zref_meas of the equivalent circuit C2 in order to calculate an impedance error ez and to take into account the impedance error calculated for the determination of the impedance Zcell of the stages 2. In other words, the method comprises a first measurement in which a response signal of a selected stage 2 is measured upon application of an excitation signal and a second measurement in which a response signal of the equivalent circuit C2 is measured upon application of the excitation signal. The second measurement can be carried out before or after the first measurement.

[0066] From the above description, it is clear that the proposed solution allows the health status of a battery to be determined in significantly more reliable ways than existing solutions.

[0067] The invention is not limited to the embodiments described in the preceding description.

Claims

1. Device for determining the impedance of a plurality of stages (2) of a fuel cell (3), each stage of the plurality of stages (2) comprising an electrolyte (4) placed between two electrodes (5, 6), the device comprising: - several connecting circuits (10) associated respectively with the stages of the plurality of stages (2), each connecting circuit (10) being electrically coupled to the electrodes (5, 6) of the associated stage (2), and - a measuring system (11) comprising an excitation system (C1) common to the plurality of stages (2), and selection means (M+, M-); characterised in that the measuring system (11) comprises a circuit (C2), called equivalent circuit, having a reference impedance (Zref) corresponding to that of a stage of the plurality of stages (2), in that the selection means (M+, M-) are capable of selecting a stage from the plurality of stages (2), by electrically coupling the connecting circuit (10) associated with the excitation system (C1), the selection means (M+, M-) further being capable of selecting the equivalent circuit (C2) by electrically coupling the equivalent circuit (C2) with the excitation system (C1); and in that the measuring system (11) is configured to: - apply to the selected stage, an excitation signal generated by the excitation system (C1); - measure a response signal from the selected stage to the application of the excitation signal; - apply to the equivalent circuit (C2), an excitation signal generated by the excitation system (C1); - measure a response signal from the equivalent circuit (C2) to the application of the excitation signal; - determine an impedance of the equivalent circuit (Zref_meas) from the response signal of the equivalent circuit (C2); - calculate an impedance error (ez) from the reference impedance (Zref) and from the impedance determined from the equivalent circuit (Zref_meas), and - determine an impedance of the selected stage (Zcell) from the response signal of the selected stage and of the impedance error (ez).

2. Device according to the preceding claim, wherein the measuring system (11) comprises an electronic control unit (MCU) and the selection means (M+, M-) comprise at least one multiplexer controlled by the electronic control unit (MCU).

3. Device according to one of claims 1 or 2, wherein the equivalent electric circuit (C2) comprises a set of components having as impedance, the reference impedance (Zref), and a voltage source (V1) to supply the set of components, the voltage source (V1) supplying a voltage equivalent to that supplied by a stage.

4. Device according to one of claims 1 to 3, wherein the measuring system (11) is configured to determine the impedance of a selected stage (Zcell) periodically according to a first period and to determine the impedance of the equivalent circuit (Zref_meas) periodically according to a second period less than or equal to the first period.

5. Device according to one of the preceding claims, wherein each connecting circuit (10) comprises two electrically conductive wires (12, 13) coupled to the selection means (M+, M-) and the equivalent electric circuit (C2) comprises two electrically conductive elements (21, 22) coupled to the selection means (M+, M-), the temperature of the electrically conductive wires (12, 13) coupled to the selection means (M+, M-) have a temperature difference ΔT°C with the electrically conductive elements (21, 22), such that ΔT°C < 30°C, preferably ΔT°C < 10°C.

6. Device according to the preceding claim, wherein the two electrically conductive elements (21, 22) at least partially use a zone where the two electrically conductive wires (12, 13) of at least one connecting circuit (10) are located.

7. Device according to one of the preceding claims, wherein the measuring system (11) is printed on a printed circuit board (30).

8. Device according to one of the preceding claims, wherein each connecting circuit (10) comprises two electrically conductive wires (12, 13) coupled to the selection means (M+, M-) and the equivalent electric circuit (C2) comprises two electrically conductive elements (21, 22) coupled to the selection means (M+, M-), the impedance of each of the two elements (21, 22) being between 0.9 times and 1.1 times that of each of the two wires (12, 13) of the connecting circuits (10).

9. Device according to the preceding claim, wherein the cross-section and the length of each of the two elements (21, 22) is between 0.9 times and 1.1 times, preferably identical, to those of each of the two wires (12, 13) of the connecting circuit.

10. Device according to one of the preceding claims, wherein the measuring system (11) is supplied by a power source independent from the fuel cell (3).

11. Assembly, comprising: - a fuel cell (3) comprising a plurality of stages, each stage of the plurality of stages (2) comprising an electrolyte placed between two electrodes; and - a device for determining the impedance of the plurality of stages (2) of the fuel cell (3) according to one of the preceding claims.

12. Method for determining the impedance of a plurality of stages (2) of a fuel cell (3), each stage of the plurality of stages (2) comprising an electrolyte placed between two electrodes, characterised in that the method comprises: - a first measurement comprising: ∘ a selection of a stage by electrically coupling the electrodes of the stage with an excitation system (C1) common to the plurality of stages (2); ∘ an application to each selected stage of an excitation signal generated by the excitation system (C1); and ∘ a measurement of a response signal of the selected stage to the application of the excitation signal; - a second measurement taken before or after the first measurement, the second measurement comprising: ∘ a selection of a circuit, called equivalent circuit (C2), having a reference impedance (Zref) corresponding to that of a stage of the plurality of stages, by electrically coupling the equivalent circuit (C2) with the excitation system (C1); ∘ an application to the equivalent circuit (C2) of an excitation signal generated by the excitation system (C1); and ∘ a measurement of a response signal from the equivalent circuit (C2) to the application of the excitation signal; the method further comprising: - a determination of an impedance of the equivalent circuit (Zref_meas) from the response signal of the equivalent circuit (C2); - a calculation of an impedance error (ez) from the reference impedance (Zref) and of the impedance determined from the equivalent circuit (Zref_meas), and - a determination of an impedance of the selected stage (Zcell) from the response signal of the selected stage and of the impedance error (ez).

13. Method according to the preceding claim, comprising a supply of the equivalent circuit (C2) by a supply source (V1) independent from the fuel cell (3).

14. Method according to one of claims 12 to 13, comprising a periodic determination, according to a first period, of the impedance of a selected stage (Zcell) and a periodic determination, according to a second period less than or equal to the first period of the impedance of the equivalent circuit (Zref_meas).

15. Method according to one of claims 12 to 14, wherein the fuel cell (3) produces electric energy.