METHOD FOR MEASURING THE IMPEDANCE OF A FUEL CELL STACK IN A VEHICLE

Abstract

Method for measuring the impedance of a fuel cell stack in a vehicle, comprising the method: Determine, via a control system, whether an impedance measurement of the fuel cell stack is requested during driving of the vehicle powered by the fuel cell stack; Switching off a first relay connected between the fuel cell stack and a battery charged by the fuel cell stack, by the controller, when the impedance measurement of the fuel cell stack is requested; Connecting a stack load to the fuel cell stack via a second relay by controlling and supplying air to the fuel cell stack; and The controller measures the impedance of the fuel cell stack.

Description

(a) Technical field

[0001] The present disclosure relates to a method for measuring the impedance of a fuel cell stack in a vehicle, in particular the method for accurately measuring the impedance of the fuel cell stack while the vehicle is being driven. (b) Description of the state of the art

[0002] In general, impedance measurement is one of the methods for determining the state of a fuel cell stack. Physical phenomena within the fuel cell stack can be determined by measuring an impedance for each frequency band.

[0003] It is possible to assess ohmic losses by estimating the amount of water in the fuel cell stack in a high-frequency range. Furthermore, it is possible to estimate mass transfer losses in a cathode catalyst layer and a gas diffusion layer of the fuel cell in a low-frequency range. Finally, it is possible to assess activation losses, including hydrogen ion migration and electrochemical reaction properties, in a porous catalyst layer of the fuel cell in the region between the high-frequency and low-frequency ranges.

[0004] In general, the impedance at a steady-state operating point (for example, constant current or constant voltage) of the fuel cell stack is measured by applying a small AC signal to a terminal of the fuel cell stack for each frequency band and using the amplitude and phase difference of a corresponding response signal.

[0005] However, during the driving of a vehicle, there is a problem in that it is difficult to bring the performance of the fuel cell stack into a steady state at a given operating point for an impedance measurement, and even if the vehicle reaches a steady state, it is difficult to maintain the state.

[0006] Even when the fuel cell stack's performance reaches a steady state at a predetermined operating point, a problem arises: accurately measuring the fuel cell stack's impedance is difficult because the impedance of various peripheral components connected to the fuel cell stack influences its impedance value. Consequently, the measurement accuracy of the fuel cell stack's impedance is reduced. SUMMARY

[0007] Accordingly, the present disclosure proposes a method for measuring the impedance of a fuel cell stack in a vehicle, wherein the method is able to accurately measure the impedance of the fuel cell stack while a vehicle is being driven.

[0008] To achieve the above objective of the present disclosure, a method for measuring the impedance of a fuel cell stack in a vehicle is provided, the method comprising: determining whether an impedance measurement of the fuel cell stack is requested during driving of the vehicle powered by the fuel cell stack; switching off a first relay connected between the fuel cell stack and a battery charged by the fuel cell stack when the impedance measurement of the fuel cell stack is requested; connecting a stack load to the fuel cell stack via a second relay and supplying air to the fuel cell stack; and measuring the impedance of the fuel cell stack.

[0009] According to the method of the present disclosure, if the impedance measurement of the fuel cell stack is not requested, the first relay can be switched on so that an output current of the fuel cell stack is supplied to the battery through the first relay while the vehicle is being driven.

[0010] Additionally, according to the method of the present disclosure, when it is determined that the impedance measurement of the fuel cell stack is requested, the state of charge (SoC) of the battery can be raised to a first reference value (α) by an output current of the fuel cell stack before the first relay is switched off.

[0011] A constant airflow is supplied to the fuel cell stack by connecting the stack load and supplying air to the fuel cell stack, such that the output current of the fuel cell stack supplied to the stack load can have an amplitude within a predetermined range. According to the method of the present disclosure, if the amplitude of the output current of the fuel cell stack deviates from the predetermined range, the airflow supplied to the fuel cell stack can be controlled so that the amplitude of the output current of the fuel cell stack is controlled or regulated such that it lies within the predetermined range.

[0012] When measuring the impedance of the fuel cell stack, the battery's state of charge (SoC) can be monitored while the fuel cell stack's impedance is measured for each configured frequency band. The fuel cell stack impedance measurement can be stopped when the battery's SoC is equal to or below a second reference value, which is lower than the first reference value by a predefined amount.

[0013] According to the method of the present disclosure, the second relay can be switched off and the first relay can be switched on when the impedance measurement of the fuel cell stack is complete. To prevent the relays from melting, it is preferred that the air supply to the fuel cell stack be interrupted before the second relay is switched off, and that a voltage applied to a terminal on the battery side of the first relay is maintained at or above an output voltage of the fuel cell stack before the first relay is switched on.

[0014] According to a method for measuring the impedance of a fuel cell stack in a vehicle as disclosed herein, when an impedance measurement of the fuel cell stack is requested, the output current of the fuel cell stack can exhibit an amplitude within a predetermined range, which is defined as a steady state over a specific period. Consequently, it is possible to measure the impedance of only the fuel cell stack, excluding the peripheral components of the fuel cell stack (peripheral stack devices), and it is possible to ensure sufficient time for measuring impedances across different frequency bands.

[0015] Accordingly, according to the method for measuring an impedance of the present disclosure, an impedance of the fuel cell stack can be accurately measured for any required frequency range.

[0016] Furthermore, a non-volatile, computer-readable medium is provided containing program instructions that are executed by a processor, the computer-readable medium comprising: program instructions that determine whether an impedance measurement of a fuel cell stack is requested during the operation of a vehicle powered by the fuel cell stack; program instructions that switch off a first relay connected between the fuel cell stack and a battery charged by the fuel cell stack when the impedance measurement of the fuel cell stack is requested; program instructions that connect a stack load to the fuel cell stack via a second relay and supply air to the fuel cell stack; and program instructions that measure an impedance of the fuel cell stack. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other tasks, features and other advantages of the present disclosure will become clearer from the following detailed description in conjunction with the accompanying drawings, in which: Fig. 1 represents a connection state of a fuel cell stack and peripheral stacking devices during the driving of a vehicle; Fig. 2 represents a separate state of the fuel cell stack and the peripheral stacking devices during an impedance measurement; Fig. 3 represents a state in which multiple loads are individually connected to the fuel cell stack using different relays; Fig. 4 represents a control process for measuring the impedance of the fuel cell stack; and Fig. 5 represents an operating point for accurately measuring the impedance of the fuel cell stack. DETAILED DESCRIPTION

[0018] It is understood that the term "vehicle" or "vehicle-" or other equivalent terms as used herein include motor vehicles in general, such as passenger cars including sports utility vehicles (SUVs), buses, trucks, various utility vehicles, watercraft including a variety of boats and ships, aircraft and the like, and include hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other vehicles powered by alternative fuels (for example, fuel derived from sources other than petroleum). As referenced herein, a hybrid vehicle is a vehicle that has two or more sources of propulsion, such as both gasoline-powered and electric-powered vehicles.

[0019] The terminology used herein is intended for the purpose of describing certain embodiments and is not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It is further understood that the expressions "possess" and / or "possessing," when used in this description, describe the presence of the specified features, numbers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more features, numbers, steps, operations, elements, components, and / or groups thereof. As used herein, the expression "and / or" includes any and all combinations of one or more of the associated listed elements.Unless explicitly stated otherwise, the words "include / include" and variations such as "includes / includes" or "indicating / comprehensive" are understood to mean the inclusion of the mentioned elements but not the exclusion of any other elements. Furthermore, the terms "...unit," "...-er," "...-or," and "...module" used in the description refer to units for processing at least one function and operation and can be realized / implemented by hardware components or software components and combinations thereof.

[0020] Furthermore, the control logic of the present invention can be implemented as non-volatile, computer-readable media on a computer-readable medium comprising executable program instructions that are executed by a processor, a controller / control unit, or the like. Examples of computer-readable storage media include, but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes, floppy disks, flash drives, smart cards, and optical data storage devices. The computer-readable recording medium can also be decentralized in networked computer systems, such that the computer-readable medium is stored and executed in a distributed manner, e.g., by a telematics server or a controller area network (CAN).

[0021] The present disclosure is described below with reference to the accompanying drawings, so that the disclosure can be easily carried out by a person skilled in the art in the field to which the disclosure belongs.

[0022] To measure the impedance of a vehicle's fuel cell stack, the fuel cell stack's power output must reach a steady state at a predetermined current or voltage. In other words, to measure the impedance of the fuel cell stack in the vehicle, the fuel cell stack's power output must reach a constant current state, where the power output is maintained at a predetermined current value, or a constant voltage state, where the power output is maintained at a predetermined voltage value.

[0023] As in Fig. As shown in Figure 1, when a fuel cell stack 1 is provided in a vehicle, the fuel cell stack 1 is connected to various peripheral devices (also referred to as the Balance of Plant (BOP)), such as: a battery 2, which is charged by the output current of the fuel cell stack 1; a high-voltage DC-DC converter (HDC) 4, which performs electrical power conversion during charging and discharging of the battery 2; a low-voltage DC-DC converter (LDC) 5, which performs electrical power conversion supplied to an air blower 6, a coolant pump 7, or the like; a motor 3, which propels a vehicle using the battery 2 as a power supply source; the air blower 6, which supplies air to the fuel cell stack 1; and the coolant pump 7, which supplies coolant to the fuel cell stack 1.Therefore, it is not easy to accurately measure the impedance of the fuel cell stack 1 while the vehicle is being driven.

[0024] Furthermore, even if the fuel cell stack's power output reaches a steady state at a predetermined current or voltage, and then the fuel cell stack's impedance is measured, the measured impedance values ​​include the impedance of the peripheral devices, thus reducing the accuracy of the impedance measurement.

[0025] In order to accurately measure the impedance of the fuel cell stack, it is therefore necessary to measure the impedance of the fuel cell stack only in a steady state in which a constant current is output by the fuel cell stack.

[0026] Furthermore, it is important to maintain a steady state in which a constant current is output from the fuel cell stack for a predetermined time in order to measure an impedance for each frequency band, since it takes a relatively long time to measure an impedance of the fuel cell stack in a low frequency band.

[0027] The present disclosure discloses a method for accurately measuring the impedance of the fuel cell stack 1 in such a way that the influence of the peripheral stack devices is eliminated by controlling the current of the fuel cell stack 1 so that it is in a steady state for a predetermined period by using a first relay 11, a stack load 14, a second relay 12 or the like, wherein the first relay (or a first circuit opening / closing element) 11 is arranged between the fuel cell stack 1 and the peripheral devices of the fuel cell stack 1 (hereinafter referred to as peripheral stack devices); wherein the stack load 14 is operated by the fuel cell stack 1 serving as a power supply source, and wherein the second relay 14 is arranged between the fuel cell stack 1 and the stack load 14.

[0028] A control signal (instructions / commands) generated in a control process for measuring the impedance of the fuel cell stack 1 can be generated by a vehicle-internal control system (controller), in particular a fuel cell control system, which is responsible for the overall control of the fuel cell system.

[0029] A method for measuring the impedance of a fuel cell stack according to the present disclosure comprises: determining whether an impedance measurement of the fuel cell stack 1 is requested during the operation of a vehicle powered by the fuel cell stack 1; stopping the supply of air to the fuel cell stack 1 and switching off the first relay 11, which is connected between the fuel cell stack 1 and the battery 2, when the impedance measurement of the fuel cell stack 1 is requested; connecting the stack load 14 to the fuel cell stack 1 via the second relay 12 and re-supplying air to the fuel cell stack 1; and measuring the impedance of the fuel cell stack 1 by an impedance meter 10 when a current with an amplitude within a predetermined range is output.

[0030] Each of the above steps can be controlled by the fuel cell control system.

[0031] Battery 2 is one of the peripheral stack devices. Battery 2 is charged by power generated and output by the fuel cell stack 1 and supplies motor 3 with drive current via the HDC 4, which is a first power conversion device. Motor 3 is operated by electrical power from battery 2, supplied by the first power conversion device 4, so that a vehicle is propelled by a drive force generated by motor 3.

[0032] The impedance meter 10 is connected to an output terminal of the fuel cell stack 1 and measures an impedance of the fuel cell stack 1 under a specified operating condition (see Fig. 5), in which an output current or output voltage of the fuel cell stack 1 is maintained in a constant current or constant voltage state.

[0033] Fuel cell stack 1 is supplied with fuel (hydrogen and air) by the fuel cell system to generate electricity for charging battery 2 or for other functions. The output terminal of fuel cell stack 1 can be equipped with a diode 13, which prevents current from flowing from the peripheral stack devices to fuel cell stack 1. The diode 13 prevents current flowing between fuel cell stack 1 and the first relay 11 from flowing to fuel cell stack 1.

[0034] The first relay 11 is located between the fuel cell stack 1 and the peripheral stacking devices, including the battery 2 and the motor 3. The first relay 11 serves to open and close a circuit connected between the peripheral stacking devices and the fuel cell stack 1. The fuel cell controller manages the on / off operations of the first relay 11. When the first relay 11 is switched off, the supply of current from the fuel cell stack 1 to the peripheral stacking devices is interrupted (see Fig. 2).

[0035] The second relay 12 is located between the fuel cell stack 1 and the stack load 14 and operates by opening and closing a circuit connected between the fuel cell stack 1 and the stack load 14. The fuel cell controller manages the on / off operations of the second relay 12. When the second relay 12 is switched on, the output current of the fuel cell stack 1 is supplied to the stack load 14. At this point, the output current of the fuel cell stack 1 is supplied directly to the stack load 14 without conversion.

[0036] The stack load 14 can be configured as a cathode oxygen depletion (COD) heating device, providing a function for rapidly warming or heating a coolant supplied to the fuel cell stack during a cold start of a vehicle and a function for removing residual oxygen from the fuel cell stack when starting or stopping a vehicle. However, the stack load 14 cannot be configured as a COD heating device. Any component other than the stack load 14 can be used, provided that the component can maintain a steady-state output current and voltage in the fuel cell stack 1.

[0037] Furthermore, the stack load 14 can consist of several loads connected in parallel. The multiple loads can be selectively electrically connected to the fuel cell stack via different relays (i.e., opening / closing elements of a circuit). In particular, as shown in Fig. As shown in Figure 3, the fuel cell stack 14 consists of a first load 14a and a second load 14b connected in parallel. The first load 14a can be connected to or disconnected from the fuel cell stack 1 via a second-first relay 12a, and the second load 14b can be connected to or disconnected from the fuel cell stack 1 via a second-second relay 12b. That is, the first load 14a and the second load 14b can be individually connected to the fuel cell stack 1 via the second-first relay 12a and the second-second relay 12b, respectively.

[0038] As described above, it is possible to control the multiple loads 14a and 14b connected to the fuel cell stack 1 using a device such as a relay, so that the load of the fuel cell stack 1 is controlled, and thus an impedance of the fuel cell stack 1 can be measured in different current ranges.

[0039] Additionally, when the vehicle is driven, the first relay 11 is switched on to supply the power of the fuel cell stack 1 to the battery 2 via the first relay 11 (see Fig. 1) When the first relay 11 is switched on, the circuit connected between the fuel cell stack 1 and the battery 2 is closed, so that the output current of the fuel cell stack 1 flows through the first power conversion device 4 and thus the battery 2 is charged.

[0040] The following describes a process for measuring the impedance of the fuel cell stack according to the present disclosure with reference to Fig. 4 described.

[0041] As shown in FIG., it is determined whether the impedance measurement of the fuel cell stack 1 is requested while the vehicle is driving (S10). The impedance measurement request can be generated by the impedance meter 10, and the occurrence of the request can be determined by the fuel cell controller.

[0042] When it is determined that an impedance measurement request is generated, the state of charge (SoC) of battery 2 is compared to a preset first reference value (α) (S11). If the SoC of battery 2 is below the first reference value (α), the SoC of battery 2 is monitored while battery 2 is being charged. Charging of battery 2 continues until the SoC of battery 2 reaches the first reference value (α) (S12). That is, when the impedance measurement of fuel cell stack 1 is requested, the impedance measurement is preceded by an increase in the SoC of battery 2 to the first reference value (α).

[0043] If the first relay 11 is switched off while the vehicle is in motion, the peripheral stacking devices, including the motor 3, must be operated only by the remaining charge of battery 2 (see Fig. 2) Therefore, battery 2 must be charged before the first relay 11 is switched off to ensure that the state of charge (SoC) of battery 2 is at the first reference value (α). Charging of battery 2 can be carried out by the fuel cell controller. The first reference value (α) can be set to a value required to operate the peripheral stacking devices, including motor 3, without requiring additional charging of battery 2 until the impedance measurement is complete.

[0044] After the state of charge (SoC) of battery 2 reaches the first reference value (α), it is determined again whether the impedance measurement of fuel cell stack 1 is requested. If the impedance measurement of fuel cell stack 1 is still requested, the first relay 11 is switched off by a command from the fuel cell controller (S13).

[0045] The air supply to the fuel cell stack 1 by the fuel cell control can be stopped before the first relay 11 is switched off to prevent the first relay 11 from melting (S13).

[0046] To prevent the first relay 11 from melting, it is preferred that, before the first relay is switched off, the voltage of the fuel cell stack 1 be lower than the voltage of an output terminal on the battery side of the first relay 11. In particular, to prevent a voltage difference between the terminal on the fuel cell stack side of the first relay 11 and the terminal on the battery side of the first relay 11, it is preferred that the voltage applied to the terminal on the fuel cell stack side of the first relay 11 be reduced to the level of or lower than the voltage applied to the terminal on the battery side of the first relay 11, and then the first relay 11 be switched off. By preventing the voltage difference, it is possible to prevent current from accidentally flowing to the first relay 11.After the first relay 11 is switched off, the voltage applied to the terminal on the fuel cell stack side can be increased.

[0047] After the first relay 11 is switched off, the second relay 12 electrically connects the stack load 14 to the fuel cell stack 1, and air is supplied to the fuel cell stack 1 (S14). When the second relay 12 is switched on, the current from the fuel cell stack 1 is supplied to the stack load 14, thus driving the stack load 14. The fuel cell stack 1 can be supplied with a constant airflow to generate electricity. When the constant airflow is supplied, the fuel cell stack 1 outputs a static current. That is, when the constant airflow is supplied to the fuel cell stack, the fuel cell stack 1 outputs a current that is defined as a steady state.

[0048] A constant hydrogen flow rate is accompanied by a constant air flow rate supplied to fuel cell stack 1. At this point, fuel cell stack 1 outputs a small alternating current (Idc + iac) with a small amplitude (see Fig. 5) and the small alternating current has a small amplitude within a specified range, which is defined as a steady state.

[0049] If the amplitude of the output current of the fuel cell stack deviates from the predetermined range defined as a steady state, it is difficult to accurately measure the impedance of the fuel cell stack 1.

[0050] Thus, after the air supply to fuel cell stack 1 is initiated, the output current of fuel cell stack 1 is monitored. If it is determined that the amplitude of the output current is outside the specified range, the amount of air supplied to fuel cell stack 1 is controlled so that the amplitude of the output current remains within the specified range. While the fuel cell control system manages the air supply and hydrogen supply, the amount of air supplied to fuel cell stack 1 is also controlled.

[0051] When fuel cell stack 1 begins to generate electricity using the supplied constant airflow, the impedance measurement of fuel cell stack 1 is started based on the output current of fuel cell stack 1 (S15). The impedance of fuel cell stack 1 is measured for each set frequency band, and each frequency band for which impedance measurement of fuel cell stack 1 is required can be preset and stored in the fuel cell controller.

[0052] The vehicle is even operated while the impedance of fuel cell stack 1 is measured, during which the state of charge (SoC) of battery 2 is gradually reduced. To verify the SoC of battery 2, the fuel cell controller monitors it in real time during the impedance measurement of fuel cell stack 1. Specifically, during the impedance measurement, the SoC of battery 2 is continuously and repeatedly compared to a second reference value (β) (S16). If the SoC of battery 2 falls below the second reference value (β), the operation to measure the impedance of fuel cell stack 1 (i.e., the operation of the impedance meter) is immediately stopped.

[0053] The second reference value (β) can be set to a value that is lower than the first reference value (α) by a predetermined amount or more, and stored in the fuel cell control unit. Furthermore, a predetermined value indicating a reduction in vehicle stability can be used as the second reference value (β).

[0054] When the impedance measurement of the fuel cell stack 1 is stopped or completed (S17), the operation of the stack load 14 is stopped and the first relay is switched on (S18). The second relay 12 can be switched off to stop the operation of the stack load 14.

[0055] To prevent the relays from melting when the operating state of the first relay 11 and the second relay 12 is switched, it is preferred that a voltage applied to the battery-side terminal be maintained at or above the output voltage of the fuel cell stack before the first relay 11 is switched on, and it is preferred that the air supply to the fuel cell stack 1 be stopped before the second relay 12 is switched off. The voltage applied to the battery-side terminal of the first relay 11 can be maintained at or above the output voltage of the fuel cell stack 1 before the first relay has completed switching to the ON state. The second relay 12 can be temporarily switched off before the second relay 12 has completed switching to the OFF state.

[0056] Before the first relay 11 is switched to the ON state, the voltage value of the terminal on the battery side of the first relay 11 is kept at or above the output voltage value of the fuel cell stack 1, so that it is possible to prevent any accidental current flowing to the first relay 11.

[0057] As described above, when measuring the impedance of the fuel cell stack, the vehicle is powered solely by the battery while the fuel cell stack's impedance is being measured. Furthermore, the fuel cell stack's output current is maintained at a constant amplitude under steady-state load, while the connection between the fuel cell stack and the peripheral stack devices is disconnected. By measuring the impedance while the fuel cell stack is operating, the impedance of the fuel cell stack alone can be accurately measured without being influenced by the peripheral stack devices or similar components.

[0058] Since the fuel cell stack outputs a steady current for a predetermined time, a time is also ensured for measuring an impedance in different frequency ranges.

[0059] Although the embodiment of the present disclosure has been described for illustrative purposes, the person skilled in the art will recognize that various modifications, additions and substitutions are possible without deviating from the scope and teaching of the disclosure as disclosed in the attached claims.

Claims

[1] Method for measuring the impedance of a fuel cell stack in a vehicle, comprising the method: Determine, via a control system, whether an impedance measurement of the fuel cell stack is requested during driving of the vehicle powered by the fuel cell stack; Switching off a first relay connected between the fuel cell stack and a battery charged by the fuel cell stack, by the controller, when the impedance measurement of the fuel cell stack is requested; Connecting a stack load to the fuel cell stack via a second relay by controlling and supplying air to the fuel cell stack; and The controller measures the impedance of the fuel cell stack. [2] Method according to claim 1, wherein, when the impedance measurement of the fuel cell stack is not requested, the first relay is switched on, so that an output current of the fuel cell stack is supplied to the battery through the first relay during driving of the vehicle. [3] Method according to claim 1, wherein, when the impedance measurement of the fuel cell stack is requested, a state of charge (SoC) of the battery is raised to a first reference value by an output current of the fuel cell stack before the first relay is switched off. [4] Method according to claim 1, wherein when the first relay is switched off, the air supply to the fuel cell stack is interrupted before the first relay is switched off. [5] Method according to claim 1, wherein when the first relay is switched off, a voltage of the fuel cell stack is reduced to a voltage which is lower than a voltage of a terminal on a battery side of the first relay before the first relay is switched off. [6] Method according to claim 1, wherein a constant air flow is supplied to the fuel cell stack by connecting the stack load and supplying air to the fuel cell stack, such that an output current of the fuel cell stack supplied to the stack load has an amplitude within a predetermined range. [7] Method according to claim 6, wherein, if the amplitude of the output current of the fuel cell stack deviates from the predetermined range, the air flow supplied to the fuel cell stack is controlled so that the amplitude of the output current of the fuel cell stack is controlled such that it lies within the predetermined range. [8] Method according to claim 3, wherein when measuring the impedance of the fuel cell stack the state of charge (SoC) of the battery is monitored while the impedance of the fuel cell stack is measured for each set frequency band. [9] Method according to claim 8, wherein the impedance measurement of the fuel cell stack is stopped when the SoC of the battery is equal to or less than a second reference value which is smaller than the first reference value by a predetermined value. [10] Method according to claim 1, wherein the second relay is switched off and the first relay is switched on when the impedance measurement of the fuel cell stack is completed. [11] Method according to claim 10, wherein the air supply to the fuel cell stack is interrupted before the second relay is switched off. [12] Method according to claim 10, wherein a voltage applied to a terminal on the battery side of the first relay is held at or above an output voltage of the fuel cell stack before the first relay is switched on. [13] Method according to claim 1, wherein the stack load is composed of several loads connected in parallel and the several loads are selectively connected to the fuel cell stack via different relays. [14] Method according to claim 1, wherein a diode is provided between the fuel cell stack and the first relay to prevent current from flowing to the fuel cell stack. [15] Non-volatile computer-readable medium containing program instructions that are executed by a processor, comprising the computer-readable medium: Program instructions that determine whether an impedance measurement of a fuel cell stack is requested during the driving of a vehicle powered by the fuel cell stack; Program instructions that switch off a first relay connected between the fuel cell stack and a battery charged by the fuel cell stack when the impedance measurement of the fuel cell stack is requested; Program instructions that connect a stack load to the fuel cell stack via a second relay and supply air to the fuel cell stack; and Program instructions that measure the impedance of the fuel cell stack.

Images