Fuel cell system and secondary battery diagnostic procedures

Abstract

Fuel cell system, comprising: a fuel cell (10); Accessory devices (26, 31) used to supply gas to the fuel cell; an accessory device controller (72) configured to control operation of the accessory devices; a secondary battery (50); a current sensor (62) configured to measure an amount of current supplied by the secondary battery; a voltage sensor (61) configured to measure a discharge capacity of the secondary battery; and a diagnostic control device (71) adapted to check the secondary battery, wherein: the accessory device control means (72) is configured to perform, in an output stop state where the fuel cell does not output electric power, a residual water purging process in which water remaining in the fuel cell is purged from the fuel cell system by driving the accessory devices using electric power supplied from the secondary battery and supplying the gas to the fuel cell; and the diagnosis control means (71) is configured to perform diagnosis of the secondary battery using an integrated value obtained by integrating current amounts supplied from the secondary battery in a predetermined voltage range of the discharge voltage that changes as a result of discharge when electric power is supplied to the accessory devices, by performing the residual water purging process.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to a fuel cell system including a secondary battery and a secondary battery diagnostic method.2. DESCRIPTION OF THE RELATED ARTIn a fuel cell system, a secondary battery may be provided to supply electric power to accessories used to supply reaction gas or a pump for circulating hydrogen gas. The discharge capacity of a secondary battery gradually deteriorates with the lapse of time. Therefore, the secondary battery must be periodically subjected to diagnosis. As a secondary battery diagnostic method, a diagnostic method is proposed (see Japanese Patent Application JP 2013-150 417 A, the diagnostic method including: acquiring an integrated value of discharge currents in a predetermined voltage range when the discharge voltage of the secondary battery decreases while the secondary battery supplies electric power to accessory devices and the like; and diagnosing the degree of capacity deterioration based on the integrated value of discharge currents. DE 11 2008 003 022 T5 discloses a fuel cell system which comprises a fuel cell which is supplied with fuel gas and oxidizing gas and which generates electrical power by an electrochemical reaction between these gases. The fuel cell system further includes: a power storage unit that can be charged with the electric power generated by the fuel cell; a purge routine execution means that executes a purge routine for removing residual water in the fuel cell using the electric power stored in the power storage unit after the power generation of the fuel cell is stopped; and a judgment unit that judges whether or not the gas supply in the fuel cell system is exhausted, wherein the purge routine execution means restricts the execution of the purge routine from being executed when the judgment unit determines that the gas supply is exhausted. A residual water purging method for a fuel cell system is the subject matter of DE 10 2015 117 485 A1. The method includes predicting whether the outside temperature becomes equal to or lower than a first predetermined temperature while the fuel cell system is operating; performing residual water purging processing on only an oxidizing gas supply / discharge mechanism when it is predicted that the outside temperature becomes equal to or lower than the first predetermined temperature and then stopping the operation of the fuel cell system; predicting whether the temperature of a predetermined component included in the fuel cell system becomes equal to or lower than a second predetermined temperature after stopping the operation of the fuel cell system; and performing residual water purging processing on the fuel gas supply / discharge mechanism when it is predicted that the temperature of the predetermined component becomes equal to or lower than the second predetermined temperature.SUMMARY OF THE INVENTIONHowever, it may take a considerable time, for example, about one hour, to perform a secondary battery diagnostic method of the related art. Therefore, for example, in a configuration in which a fuel cell system is installed on a vehicle to provide electric power for running the vehicle, diagnosis can be performed for about one hour after the vehicle is brought into a shop, which involves considerable effort for the user. In addition, even in a fuel cell system that is located on the ground or a building and used, the system may fail for diagnosis for about one hour, which is not user-friendly. Therefore, a technique capable of suppressing deterioration of user-friendliness related to diagnosis of a secondary battery included in a fuel cell system is desired.The invention can realize the following aspects.(1) A first aspect of the invention relates to a fuel cell system including: a fuel cell; accessory devices used to supply gas to the fuel cell; an accessory device controller configured to control an operation of the accessory devices; a secondary battery; a current sensor configured to measure an amount of current supplied from the secondary battery; a voltage sensor configured to measure a discharge voltage of the secondary battery; and a diagnosis controller configured to perform diagnosis on the secondary battery. The accessory device controller is configured to, in an output stop state where the fuel cell does not output electric power, perform a residual water purging process in which water remaining in the fuel cell is purged from the fuel cell system by driving the accessory devices using electric power supplied from the secondary battery and supplying the gas to the fuel cell. The diagnosis control means is configured to perform diagnosis of the secondary battery using an integrated current value obtained by integrating amounts of current supplied from the secondary battery in a predetermined voltage range of a discharge voltage of the secondary battery that changes as a result of discharge when electric power is supplied to the accessory devices by performing the residual water scavenging process.In the fuel cell system according to the first aspect of the invention, the diagnosis control device performs diagnosis on the secondary battery using the integrated current value obtained by integrating the amounts of current supplied from the secondary battery in a predetermined voltage range of the discharge voltage that changes as a result of the discharge when electric power is supplied to the accessory devices, by performing the residual water purging process in the output stop state. Therefore, the discharge of the secondary battery can be prevented only for diagnosis on the secondary battery. Accordingly, for example, it is not necessary to put the fuel cell system in a shop or intentionally put the fuel cell in the output stop state to perform diagnosis on the secondary battery, and deterioration of user-friendliness can be suppressed.(2) The fuel cell system according to the first aspect of the invention may further include a storage unit configured to store an integrated value of amounts of current supplied from the secondary battery. The diagnosis controller may be configured to determine the integrated current value by repeating the following processes (i) and (ii) until the discharge from the secondary battery in the voltage range is completed, when the residual water scavenging process is completed before the discharge from the secondary battery in the voltage range is completed: (i) a process in which the storage unit is caused to store an integrated value of current amounts supplied from the secondary battery from the start of the residual water scavenging process and until the residual water scavenging process is completed; and (ii) a process in which an amount of current supplied from the secondary battery is integrated with the integrated value stored in the storage unit when the next residual water scavenging process is performed.In the fuel cell system according to the first aspect of the invention, when the residual water scavenging process is completed before the discharge in the predetermined voltage range is completed, the storage unit stores the integrated values of the amounts of current supplied from the secondary battery from the start of the residual water scavenging process until the residual water scavenging process is completed. Then, when the next residual water purging process is performed, the amount of current supplied from the secondary battery is integrated with the integrated value stored in the storage unit. The above processes are repeated. Thus, even if the residual water scavenging process is completed before the discharge in the predetermined voltage range is completed, the integration of the amounts of current supplied from the secondary battery in the predetermined voltage range can be completed by performing the residual water scavenging process once or more times. Therefore, an integrated current value required for the diagnosis of the secondary battery can be obtained, and accurate diagnosis of the secondary battery is possible.(3) In the fuel cell system according to the first aspect of the invention, the gas may include air as the oxidizing gas, and the accessories may include an air compressor. With the fuel cell system according to the first aspect of the invention, the integrated current value is generally obtained during the supply (discharge) of electric power to the air compressor having high power consumption. Therefore, an integrated current value required for the diagnosis of the secondary battery can be obtained within a short time. Further, diagnosis of the secondary battery may be performed while the residual water purging process is performed on the cathode side of the fuel cell.(4) In the fuel cell system according to the first aspect of the invention, the gas may include hydrogen gas as the fuel gas. The fuel cell system may further include: a tank in which hydrogen gas is stored; an anode gas supply path through which the hydrogen gas is supplied from the tank into the fuel cell; an anode off-gas discharge path through which off-gas including the hydrogen gas from the fuel cell is discharged; a bypass flow path through which the anode off-gas discharge path and the anode gas supply path are connected; and a pump disposed in the bypass flow path and supplying off-gas discharged from the anode off-gas discharge path to the anode gas supply path. The accessory devices may include the pump. With the fuel cell system according to the first aspect of the invention, diagnosis of the secondary battery can be performed while the residual water purging process is performed on the anode side of the fuel cell.(5) In the fuel cell system according to the first aspect of the invention, the diagnosis controller may be configured to compare the integrated current value with an integrated threshold value that is an integrated current value at which deterioration of the secondary battery is at a lower limit of an allowable range, and configured to determine that the secondary battery deteriorates when the integrated current value is lower than the integrated threshold value. With the fuel cell system according to the first aspect of the invention, the diagnosis controller compares the integrated current value with the integrated threshold value which is an integrated current value at which the deterioration of the secondary battery is at the lower limit of the allowable range, and determines that the secondary battery deteriorates when the integrated current value is lower than the integrated threshold value. Therefore, the diagnostic control means can accurately determine whether or not the secondary battery deteriorates.(6) In the fuel cell system according to the first aspect of the invention, the diagnosis controller may be configured to compare a gradient of the integrated current value with a threshold gradient of an integrated current value at which deterioration of the secondary battery is at a lower limit of an allowable range during a predetermined time lapse, and configured to determine that the secondary battery deteriorates when the gradient of the integrated current value is lower than the threshold gradient.(7) In the fuel cell system according to the first aspect of the invention, the diagnosis control means may determine whether or not a discharge voltage of the secondary battery at the start of the residual water purging process is lower than an upper limit value of the voltage range, do not determine the integrated current value and do not perform diagnosis of the secondary battery when the diagnosis control means determines that the discharge voltage of the secondary battery is lower than the upper limit value of the voltage range, and determine the integrated current value and perform diagnosis of the secondary battery when the diagnosis control means determines that the discharge voltage of the secondary battery is not lower than the upper limit value of the voltage range. With the fuel cell system according to the first aspect of the invention, the diagnosis control means does not acquire the integrated current value and does not perform diagnosis of the secondary battery when the amounts of current supplied from the secondary battery in the predetermined voltage range cannot be integrated. When the amounts of current supplied from the secondary battery in the predetermined voltage range can be integrated, the diagnosis controller determines the integrated current value and performs diagnosis of the secondary battery. Therefore, the execution of an unnecessary process can be suppressed, and accurate diagnosis of the secondary battery is possible.(8) A second aspect of the invention relates to a secondary battery diagnosis method for performing diagnosis on a secondary battery that supplies electric power to accessory devices for supplying gas to a fuel cell, the secondary battery diagnosis method including: (a) performing a residual water scavenging process in which water remaining in the fuel cell is purged from the fuel cell system by driving the accessory devices using electric power supplied from the secondary battery and supplying the gas to the fuel cell, in an output stop state where the fuel cell does not output electric power; (b) obtaining an integrated current value by integrating amounts of current supplied from the secondary battery in a predetermined voltage range of a discharge voltage of the secondary battery that changes as a result of discharge by performing the residual water scavenging process; and (c) performing a secondary battery diagnosis using the integrated current value.The aspects of the invention may be embodied in various forms. For example, the aspects of the invention may be realized in various forms, for example, as a secondary battery diagnostic system, a secondary battery diagnostic method, and a method of operating a fuel cell system.BRIEF DESCRIPTION OF THE DRAWINGSFeatures, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like reference numerals designate like elements: FIG. 1 is a block diagram showing a schematic configuration of a fuel cell system according to an embodiment of the invention; FIG. 2 is a flowchart showing the flow of a residual water purging process when the vehicle is at a standstill; FIG. 3 is a flowchart showing the flow of a process for controlling the start of accessory devices; FIG. 4 is a flowchart showing the flow of a secondary battery diagnostic process; and FIG. 5 is a diagram showing a change of an integrated current value after starting integrating current values.DETAILED DESCRIPTION OF EMBODIMENTSA. EmbodimentA1. System DesignFIG. 1 is a block diagram showing a schematic configuration of a fuel cell system 100 according to an embodiment of the invention. The fuel cell system 100 according to the embodiment is mounted on a vehicle to supply electric power to a vehicle drive motor (not shown). The fuel cell system 100 includes a fuel cell 10, an anode-side reactant gas supply / discharge mechanism 20, a cathode-side reactant gas supply / discharge mechanism 30, a secondary battery 50, a DC-DC converter 41, a first boost converter 42, an inverter 43, a second boost converter 44, a voltage sensor 61, a current sensor 62, a diagnostic controller 73, an accessory device controller 72, a start controller 73, a storage unit 80, a first DC power line wire 91, a second DC power line wire 92, a third DC power line wire 93, a fourth DC power line wire 94, a fifth DC power line wire 95, and a sixth DC power line wire 96.The fuel cell 10 is a solid polymer fuel cell that generates electric power by supplying hydrogen gas and air as reaction gas, and has a configuration in which a plurality of unit cells are stacked. The fuel cell is not limited to a solid polymer fuel cell, and any fuel cell such as a phosphoric acid fuel cell or a solid oxide fuel cell may be used. The fuel cell 10 is connected to an input terminal of the first boost converter 42 via the first DC conductor wire 91.The anode-side reaction gas supply / discharge mechanism 20 supplies hydrogen gas as fuel gas to the fuel cell 10 and discharges anode off gas from the fuel cell 10. The anode-side reaction gas supply / discharge mechanism 20 includes a tank 21, an anode gas supply path 22, an anode off-gas discharge path 23, a bypass flow path 24, a gas-liquid separator 25, and a circulation pump 26.Hydrogen gas is stored in the tank 21. The tank 21 and the fuel cell 10 are connected via the anode gas supply path 22, so that the hydrogen gas is supplied from the tank 21 to the fuel cell 10. In the anode gas supply path 22, a shut-off valve (not shown) and an injection nozzle (not shown) are provided. The anode off-gas discharge path 23 is connected to an anode off-gas branch pipe (not shown) provided in the fuel cell 10, and feeds the anode off-gas discharged from the fuel cell 10 to the anode off-gas discharge path 23. A first end of the bypass flow path 24 is connected to the gas-liquid separator 25, a second end of the bypass flow path 24 is connected to the anode gas supply path 22, and the hydrogen gas discharged from the gas-liquid separator 25 is supplied to the anode gas supply path 22 via the bypass flow path 24. The gas-liquid separator 25 separates water and the hydrogen gas contained in the anode off-gas supplied via the anode off-gas discharge path 23 from each other, and discharges the hydrogen gas to the bypass flow path 24. The gas-liquid separator 25 is also connected to a cathode off-gas discharge path described below, and supplies the separated water to the cathode off-gas discharge path. The circulation pump 26 is provided in the bypass flow path 24 and supplies the hydrogen gas discharged from the gas-liquid separator 25 to the anode gas supply path 22, and as a result, a part of the hydrogen gas supplied to the fuel cell 10 but not consumed in the fuel cell 10 is supplied to the fuel cell 10 again.The cathode-side reaction gas supply / discharge mechanism 30 supplies air as oxidizing gas (cathode gas) to the fuel cell 10 and discharges cathode off gas from the fuel cell 10. The cathode-side reaction gas supply / discharge mechanism 30 includes an air compressor 31, a cathode gas supply path 32, and a cathode off-gas discharge path 33. The air compressor 31 takes in air from the atmosphere, compresses the air, and supplies the compressed air to the cathode gas supply path 32, the cathode gas supply path 32 is connected to a cathode branch pipe (not shown) provided in the fuel cell 10, and supplies the air to the fuel cell 10, the cathode off-gas discharge path 33 is connected to a cathode off-gas branch pipe (not shown) provided in the fuel cell 10, and the cathode off-gas discharged from the fuel cell 10 is discharged from the fuel cell system through the cathode off-gas discharge path 33. As described above, the cathode off-gas discharge path 33 is also connected to the gas-liquid separator 25, and the water discharged from the gas-liquid separator 25 to the cathode off-gas discharge path 33 is discharged from the fuel cell system due to the influence of the cathode off-gas through the cathode off-gas discharge path 33. In addition, a small amount of the hydrogen gas discharged from the gas-liquid separator 25 to the cathode off-gas discharge path 33 is diluted with the cathode off-gas, and then is discharged from the fuel cell system through the cathode off-gas discharge path 33.In the embodiment, the secondary battery 50 is configured using a lithium ion battery and supplies electric power to accessory devices such as the air compressor 31, the circulation pump 26, and a radiator fan (not shown). The discharge capacity of the secondary battery 50 deteriorates with the lapse of time. Accordingly, in a case where a fixed amount of electric power is supplied, a period of time in which the electric power can be supplied by charging the secondary battery 50 once is decreased with the lapse of time. In the embodiment, the deterioration of the discharge capacity of the secondary battery 50 is also referred to as "deterioration of the secondary battery 50".The DC-DC converter 41 is electrically connected to the secondary battery 50 via the fifth DC conductor wire 95 and is also electrically connected to the circulation pump 26 via the sixth DC conductor wire 96. The DC-DC converter 41 converts a voltage input from the secondary battery 50 into a predetermined voltage suitable for the circulation pump 26, and outputs the converted voltage.The first boost converter 42 is electrically connected to the fuel cell 10 via the first DC conductor wire 91, and is also electrically connected to the inverter 43 via the second DC conductor wire 92. The first boost converter 42 boosts a voltage input from the fuel cell 10 to a target voltage and outputs the target voltage to the inverter 43. The inverter 43 converts DC voltages output from the first boost converter 42 and the second boost converter 44 into three-phase AC voltages, and supplies the three-phase AC voltages to the air compressor 31 and the vehicle drive motor (not shown). In a state (hereinafter also referred to as an "output stop state") where the fuel cell 10 does not output electric power, electric power is supplied from the secondary battery 50 to the air compressor 31. Thus, in this case, the inverter 43 converts a DC voltage output from the second boost converter 44 into a three-phase AC voltage and supplies the three-phase AC voltage to the air compressor 31. In a case where the output electric power of the fuel cell 10 is insufficient and lower than a target output power of the vehicle drive motor, the inverter 43 discharges the secondary battery 50 and compensates the insufficient amount of electric power. The inverter 43 converts regenerative electric power generated by the vehicle drive motor into DC power, and outputs the DC power to the second boost converter 44 via the third DC conductor wire 93. The second boost converter 44 is electrically connected to the secondary battery 50 via the fourth DC conductor wire 94, and is also electrically connected to the inverter 43 via the third DC conductor wire 93. The second boost converter 44 boosts a voltage input from the secondary battery 50 to a target voltage and outputs the target voltage to the inverter 43. The second boost converter 44 decreases a voltage of regenerative electric power supplied from the inverter 43 via the third DC conductor wire 93, and outputs the decreased voltage to the secondary battery 50 via the fourth DC conductor wire 94.The voltage sensor 61 is provided in the fourth DC conductor wire 94 and measures the discharge voltage of the secondary battery 50. likewise, the current sensor 62 is provided in the fourth DC conductor wire 94 and measures the amount of current supplied from the secondary battery 50. The voltage sensor 61 and the current sensor 62 are electrically connected to the diagnosis controller 71, and report respective measurement values (voltage value and current value) to the diagnosis controller 71.The diagnosis controller 71 performs diagnosis of the secondary battery by 50. more specifically, the diagnosis controller 71 performs a secondary battery diagnosis process described below, and performs diagnosis as to whether or not the discharge capacity of the secondary battery 50 deteriorates. The accessory device controller 72 is electrically connected to the circulation pump 26 and the air compressor 31, and controls driving and stopping of the circulation pump 26 and the air compressor 31. The accessory device controller 72 performs a residual water purging process while the vehicle is at a standstill. The start controller 73 includes a timer 74 and performs a process for controlling the start of accessory devices. A predetermined time is set in the timer 74, and the time is measured until the predetermined time has elapsed after the timer 74 is started. The time set in the timer 74 may be any time period. As described below, residual water on the anode side and the cathode side is rinsed out respectively in the set time. For example, assuming that the residual water purging process is performed once a night, eight hours may be set. The diagnostic controller 71, the accessory controller 72, and the start controller 73 are configured using an electronic control unit (ECU) including a microprocessor and a memory, and are driven by electric power supplied from the secondary battery 50. The start controller 73 controls supply of electric power to the diagnostic controller 71 and the accessory device controller 72, and the storage unit 80 is electrically connected to the diagnostic controller 71 and can store an integrated current value in a secondary battery diagnostic process described below. The storage unit 80 may be configured as a memory included in the diagnosis controller 71. In the embodiment, the timer 74 is configured using a software timer, but may be configured using a hardware timer instead of the software timer.The fuel cell system 100 may include a cooling medium supply / discharge mechanism (not shown) in addition to the anode side reaction gas supply / discharge mechanism 20 and the cathode side reaction gas supply / discharge mechanism 30. The cooling medium supply / discharge mechanism is a mechanism for adjusting the temperature of the fuel cell 10, supplies a cooling medium such as a coolant to the fuel cell 10, discharges the cooling medium from the fuel cell 10, and supplies the cooling medium that has undergone heat exchange with the discharged cooling medium again to the fuel cell 10.In the fuel cell system 100 configured as described above, the fuel cell 10 enters the output stop state when a vehicle is at a standstill, when a shift lever is at parking (P) and a start switch is turned off. In the discharge stop state, the below-described residual water purging process is performed while the vehicle is at a standstill. As a result, water remaining in the fuel cell 10, the flow path of the reaction gas and the exhaust gas on the anode side, the flow path of the reaction gas and the exhaust gas on the cathode side, and the like is regularly discharged from the fuel cell system. This configuration is intended to suppress freezing of water remaining in the fuel cell 10, the respective flow paths, and the like, so that gas diffusivity does not deteriorate compared to the normal state, and so that opening and closing of various valves (not shown) can be controlled. In the fuel cell system 100, when the residual water purging process is performed while the vehicle is at a standstill, the secondary battery diagnostic process described below is performed. As a result, the deterioration of user-friendliness related to the diagnosis of the secondary battery 50 is suppressed.Here, a relationship of the vehicle standstill residual water scavenging process and the secondary battery diagnostic process described below with the accessory device start control process described below will be briefly described. In the residual water purging process when the vehicle is at a standstill, the accessory device controller 72 drives the accessory device controllers such as the circulation pump 26 and the air compressor 31 so that the reaction gas is supplied to the fuel cell 10. As a result, the residual water purging process is performed in which water remaining in the fuel cell 10 is discharged from the fuel cell system, when a vehicle is at a standstill, a shift lever is at parking (P), and a start switch is turned off, but supply of electric power to the accessory device controller 72 is interrupted. On the other hand, even in a state where supply of electric power to the accessory device controller 72 is interrupted, electric power is supplied to the start controller 73. By performing the accessory device start control process, the start controller 73 supplies electric power to the accessory device controller 72, and as a result, the accessory device controller 72 can periodically perform the residual water purging process while the vehicle is at a standstill. When a vehicle is stationary, a shift lever is in parking (P), and a start switch is turned off, supply of electric power to the diagnostic controller 71 is interrupted as in the case of supplying electric power to the accessory controller 72. however, as in the case of the accessory controller 72, electric power is regularly supplied to the diagnostic controller 71 by the accessory start control process of the start controller 73. As a result, the diagnosis controller 71 can periodically perform the secondary battery diagnosis process.A2. Residual water flushing-out process with the vehicle at standstillFIG. 2 is a flowchart showing the flow of the residual water purging process when the vehicle is at a standstill. When the supply of electric power to the accessory device controller 72 is started as a result of the accessory device start control process described below, the residual water purging process is started with the vehicle at a standstill.The accessory device controller 72 notifies the start of the residual water purging process to the diagnostic controller 71 (step S 105). The accessory device controller 72 performs an anode-side residual water purging process (step S 110). More specifically, the accessory device controller 72 opens a purge valve (not shown) provided in the anode off-gas discharge path 23 so that a predetermined amount of hydrogen gas is supplied to the fuel cell 10 per predetermined period of time by the circulation pump 26 and the injector (not shown). As a result, the anode-side water purging process is performed. At this time, electric power is supplied from the secondary battery 50 to the circulation pump 26. In this way, the hydrogen gas is supplied to the fuel cell 10, and water remaining on the anode side of the fuel cell system 100 is discharged from the fuel cell system. "Water remaining on the anode side of the fuel cell system 100" may include: water remaining in pores formed in an anode side catalyst layer and an anode side gas diffusion layer of each individual cell; water remaining in an anode gas supply branch pipe and the anode off-gas discharge branch pipe in the fuel cell 10; water remaining in the anode gas supply path 22; water remaining in the anode off-gas discharge path 23; water remaining in the gas-liquid separator 25; water remaining in the bypass flow path 24; and water remaining in the circulation pump 26. The water may also include: water (back diffusion water) that permeates from the cathode side of each individual cell through an electrolyte membrane; and liquid water generated by condensation of water vapor contained in an atmosphere.The accessory device controller 72 performs a cathode-side residual water purging process (step S 115). More specifically, the accessory device controller 72 drives the air compressor 31 to supply a predetermined amount of air to the fuel cell 10 per predetermined period of time, and adjusts the opening degree of a backpressure valve (not shown) provided in the cathode-side exhaust gas discharge path 33 to discharge the cathode exhaust gas from the fuel cell system 100. At this time, electric power is supplied from the secondary battery 50 to the air compressor 31. In this way, the air is supplied to the fuel cell 10, and water remaining on the cathode side of the fuel cell system 100 is discharged from the fuel cell system. "water remaining on the cathode side of the fuel cell system 100" may include: water remaining in pores formed in a cathode side catalyst layer and a cathode side gas diffusion layer of each individual cell; water remaining in a cathode gas supply branch pipe and the cathode off gas discharge branch pipe in the fuel cell 10; water remaining in the cathode gas supply path 32; and water remaining in the cathode off gas discharge path 33. The water may further include: water generated by an electrochemical reaction on the cathode side of each individual cell; and liquid water generated by condensation of water vapor contained in a cathode side atmosphere.When the cathode-side water purging process (step S 115) ends, the accessory device controller 72 sends a notification that the water purging is ended to the diagnostic controller 71 and the start controller 73 (step S 120), and the remaining water purging process ends while the vehicle is at a standstill.A3. Process for Controlling Start of Accessory DevicesFIG. 3 is a flowchart showing the flow of the process for controlling the start of accessory devices; when a vehicle is at a standstill, a shift lever is at parking (P), and a start switch is turned off, the start controller 73 performs a process for controlling the start of accessory devices to periodically perform the remaining water scavenging process with the vehicle at a standstill and the secondary battery diagnostic process described below. When a vehicle is stationary, a shift lever is at parking (P), and a start switch is turned off, the timer 74 is started.The start controller 73 waits until the timer 74 has expired (step S 205). When the timer 74 has expired (step S 205: YES), the start controller 73 starts supplying electric power to the diagnostic controller 71 and the accessory controller 72 (step S 210). Then, the start controller 73 waits until the notification that the water purging is ended is received from the accessory device controller 72 (step S 215). When the notification that the water scavenging is ended is received (step S 215): YES), the start controller 73 is restarted (step S 220), and the process returns to step S 205. Due to the accessory device start control process, electric power is supplied to the accessory device controller 72 over the preset time of the timer 74, so that the residual water purging process is performed with the vehicle at standstill. In addition, electric power is supplied to the diagnosis controller 71 over the preset time of the timer 74, so that the above-described secondary battery diagnosis process is performed.A4. Secondary Battery Diagnostic ProcessThe diagnosis controller 71 performs the secondary battery diagnosis process in response to the supply of electric power to the diagnosis controller 71 when the vehicle is at a standstill (in an output stop state). First, a method for performing diagnosis of the secondary battery 50 will be briefly described. When the secondary battery 50 is discharged, the discharge voltage gradually decreases. At this time, current values supplied from the secondary battery 50 in a predetermined discharge voltage range (hereinafter, simply referred to as "predetermined voltage range") are integrated, and the diagnosis controller 71 performs a diagnosis as to whether or not the discharge capacity of the secondary battery deteriorates using the integrated value of the current values (hereinafter, also referred to as "integrated current value"). In a case where the discharge capacity deteriorates with the lapse of time, the integrated current value becomes smaller in the predetermined voltage range. Therefore, in a case where the obtained integrated current value is lower than an integrated threshold value, the diagnosis controller 71 determines that the secondary battery 50 deteriorates. Here, in this embodiment, the upper limit value of the predetermined voltage range is set as a voltage value in a state of so-called full charge. The lower limit value of the predetermined voltage range is set as a voltage value, in which even in a case where a start switch of a vehicle is turned on in a state where the voltage value has decreased to the lower limit value, required electric power can be supplied to the accessory devices and the vehicle can travel.FIG. 4 is a flowchart showing the flow of the secondary battery diagnostic process. When the secondary battery diagnostic process is started, the diagnostic controller 71 waits until it receives a notification that the water scavenging is started from the accessory device controller 72 (step S 305). When the notification that the water scavenging is started is received (step S 305: YES), the diagnostic controller 71 performs calibration of the current sensor 62 (step S 310). More specifically, the diagnosis controller 71 measures a current value a plurality of times in a state where no current flows through the fourth DC power line wire 94, and performs a process (so-called zero point adjustment) in which the zero point is shifted over the average value of the measured current values. The calibration of the current sensor 62 is performed between step S 105 and step S 110 shown in FIG. 2.The diagnosis controller 71 determines whether or not the diagnosis can be performed (step S 315). As described above, for diagnosis of the secondary battery 50, the integrated current value in the predetermined voltage range is obtained. Therefore, in a case where the discharge voltage of the secondary battery 50 is lower than the upper limit value of the predetermined voltage range from the beginning, the integrated current value of the predetermined voltage range cannot be obtained. Thus, in a case where the measurement value of the voltage sensor 61, that is, the discharge voltage of the secondary battery 50, is lower than the upper limit value of the predetermined voltage range, the diagnosis controller 71 determines that the diagnosis cannot be performed (step S 315: NO), and the secondary battery diagnosis process ends. At this time, the secondary battery diagnostic process may end after information regarding "the state where the diagnosis cannot be performed) has been stored as history data in the storage unit 80. On the other hand, in a case where the discharge voltage of the secondary battery 50 is the upper limit value of the predetermined voltage range or higher, the diagnosis controller 71 determines that the diagnosis can be performed (step S 315: YES), and then determines whether or not the previous diagnosis has been interrupted (step S 320).In the residual water purging process when the vehicle is at a standstill, the discharge voltage of the secondary battery 50 becomes lower along with the supply of electric power to the air compressor 31, the circulation pump 26, and the like. However, in a case where the residual water purging process is completed while the vehicle is at a standstill until the discharge voltage reaches the lower limit of the predetermined voltage range, the supply of electric power to the air compressor 31 and the circulation pump 26 is stopped, and thus the integrated current value does not reach the lower limit of the predetermined voltage range. In this case, the secondary battery diagnostic process is interrupted as described below, and data indicating the interruption is stored in the storage unit 80. Therefore, in step S 320, the diagnosis controller 71 determines whether or not the previous diagnosis has been interrupted by checking whether or not the data indicating the interruption is stored in the storage unit 80.In a case where the diagnosis controller 71 determines that the previous diagnosis has not been interrupted (step S 320: NO), the diagnosis controller 71 starts integrating current values measured by the current sensor 62 (step S 330). The diagnosis controller 71 determines whether or not the discharge voltage of the secondary battery 50 reaches the lower limit value of the predetermined voltage range (step S 335). In a case where the diagnosis controller 71 determines that the discharge voltage of the secondary battery 50 reaches the lower limit value of the predetermined voltage range (step S 335: YES), the diagnosis controller 71 ends the integration of the current values (step S 340), and stores the integrated current value in the storage unit 80 (step S 345). In the residual water flushing process when the vehicle is stationary, predefined accessory devices are activated over a predefined time. In step S 340, the discharge voltage of the secondary battery 50 reaches the lower limit value of the predetermined voltage range before the accessory devices are stopped. The diagnosis controller 71 determines whether or not the discharge capacity of the secondary battery 50 deteriorates using the integrated current value stored in the storage unit 80 (step S 350), and the secondary battery diagnosis process ends. The details of the determination regarding the deterioration in step S 350 will be described with reference to FIG. 5.FIG. 5 is a diagram showing a change of an integrated current value after the start of integrating the current values. In FIG. 5, the horizontal axis represents the time elapsed since the start of current value integration, and the vertical axis represents the integrated current value. In addition, in FIG. 5, a straight line L 1 represents a change in the integrated current value in a case where deterioration of the discharge capacity of the secondary battery 50 is within an allowable range, and a straight line L 2 represents a change in the integrated current value in a case where deterioration of the discharge capacity of the secondary battery 50 is lower than the allowable range. FIG. 5 shows a change in the integrated current value starting at a time T 0 and ending at a time T 1. The power consumed by the accessory devices in the residual water flushing process when the vehicle is at a standstill is constant in each case. Therefore, the rate of decrease of the discharge voltage is substantially constant. Therefore, the change (decrease) in the discharge voltage and a lapse of time are substantially proportional to each other. The discharge capacity deterioration allowable range of the secondary battery 50 represents a discharge capacity deterioration range that does not pose a problem for the operation of the fuel cell vehicle 100 and that can be determined through experimentation.Regardless of whether or not the discharge voltage of the secondary battery 50 deteriorates, the integrated current value increases with time. However, in a case where the deterioration is lower than the allowable range, the integrated current value shown as straight line L 2 is lower than that shown simultaneously as straight line L 1. Accordingly, an integrated current value SI 1 at which the deterioration at the time T 1 is lower than the allowable range is lower than an integrated current value SI 1 at which the deterioration at the time T 1 is in the allowable range. In this embodiment, an integrated current value corresponding to the lower limit of the allowable range of deterioration of the secondary battery 50 is determined in advance from an experiment or the like and set as the integrated threshold value SIt. In step S 350, in a case where the integrated current value obtained in steps S 330 to S 340 is lower than the integrated threshold SIt, the diagnostic controller 71 determines that the discharge capacity of the secondary battery 50 deteriorates. In a case where the integrated current value obtained in steps S 330 to S 340 is the integrated threshold value SIt or higher, the diagnosis controller 71 determines that the discharge capacity of the secondary battery 50 does not deteriorate.In step S 3535, in a case where the diagnosis controller 71 determines that the discharge voltage of the secondary battery 50 does not reach the lower limit value of the predetermined voltage range (step S 3535: NO), as shown in FIG. 4, the diagnosis controller 71 determines whether or not the notification that the water scavenging is ended is received from the accessory device controller 72 (step S 355). In a case where the diagnosis controller 71 determines that the notification that the water scavenging has been completed has not been received (step S 555: NO), the process returns to step S 3535.On the other hand, in a case where the diagnosis controller 71 determines that the notification that the water scavenging has been ended has been received (step S 55: YES), the diagnosis controller 71 ends the integration of the current values (step S 360) and stores the integrated current value and data indicating that the diagnosis has been interrupted in the storage unit 80 (step S 365), and the secondary battery diagnosis process ends. The integrated current value stored in the storage unit 80 in step S 365 is an integrated value of the discharge currents in a case where the discharge voltage changes in a voltage range narrower than the predetermined voltage range. In the embodiment, the integrated current value stored in the storage unit 80 in step S 365 is referred to as an interruption integrated current value.In a case where the secondary battery diagnostic process ends after step S 365 is performed, it is not determined whether or not the discharge capacity of the secondary battery 50 is deteriorated. The reason for this is as follows. The remaining water purging process when the vehicle is at a standstill ends before the discharge capacity of the secondary battery 50 reaches the lower limit of the predetermined voltage range, and the secondary battery 50 is not discharged any further. Therefore, a value required for diagnosis cannot be obtained as an integrated current value. However, in a case where the next remaining water purging process is started while the vehicle is at a standstill and the next secondary battery diagnostic process is also started accordingly, the diagnostic controller 71 determines in step S 320 that the previous diagnosis has been interrupted (step S 320: YES). In this case, the diagnostic controller 71 acquires the integrated interrupt value from the storage unit 80 (step S 325), and then the process starts from step S 330. In a case where step S 325 is performed, the diagnosis controller 71 integrates the measured current value with the integrated interruption value. For example, it is assumed that: in the discharge stop state, the first remaining water purging process is performed while the vehicle is at standstill and ends before the discharge voltage reaches the lower limit value of the predetermined voltage range; and then, the second remaining water purging process is performed while the vehicle is at standstill after a predetermined time has passed in the discharge stop state. In this case, the secondary battery 50 is not charged after the end of the first residual water purging process while the vehicle is at a standstill. Therefore, the discharge capacity of the secondary battery 50 at the start of the second remaining water purging process when the vehicle is at standstill is substantially equal to the discharge capacity of the secondary battery 50 at the end of the first remaining water purging process when the vehicle is at standstill. Accordingly, in a case where the current value is integrated with the interruption integrated value so that the discharge capacity of the secondary battery 50 reaches the lower limit value of the predetermined voltage range, the integrated value of the discharge current is obtained while the discharge capacity of the secondary battery 50 decreases from the upper limit value to the lower limit value of the predetermined voltage range. Accordingly, the diagnosis controller 71 can accurately determine whether or not the discharge capacity of the secondary battery 50 deteriorates using the integrated value of the discharge currents.In step S 315 in which the next secondary battery diagnostic process is performed after the interruption of the diagnosis, the diagnosis controller 71 checks that the data indicating interruption of the diagnosis is stored in the storage unit 80, and then, even in a case where the discharge capacity of the secondary battery 50 is lower than the upper limit value of the predetermined voltage range, the diagnosis controller 71 determines that the diagnosis can be performed.The result of the secondary battery diagnostic process, that is, the secondary battery 50 deteriorates or does not deteriorate, can be used, for example, as follows. By lighting or flashing a predetermined LED lamp or the like included in a dashboard of the vehicle, the diagnosis controller 71 can notify a user of the deterioration of the secondary battery 50. In addition, the diagnosis controller 71 notifies a user of the deterioration of the secondary battery 50 using sound of a speaker. In addition, the content that is notified is not limited to the data that notifies deterioration of the secondary battery 50, and may include data that urgently prompts a user to bring a vehicle to a shop or data that urgently prompts a user to stop a vehicle.In the fuel cell system 100 according to the first embodiment, the diagnosis controller 71 performs diagnosis of the secondary battery 50 using the integrated current value obtained by integrating the amounts of current supplied from the secondary battery in a predetermined voltage range of the discharge voltage of the secondary battery 50 that changes as a result of discharge when electric power is supplied to the accessory devices (for example, the circulation pump 26 and the air compressor 31) by performing the residual water purging process in the discharge stop state. Therefore, the discharge of the secondary battery 50 can be avoided only for performing diagnosis on the secondary battery 50. Accordingly, for example, it is not necessary to put the vehicle (the fuel cell system 100) in a shop or intentionally put the fuel cell 10 in the output stop state in order to perform diagnosis on the secondary battery, and deterioration of user-friendliness can be suppressed.In a case where the remaining water scavenging process is completed with the vehicle at standstill before the discharge is completed in the predetermined voltage range, the integrated interruption value from the start to the end of the remaining water scavenging process with the vehicle at standstill is stored in the storage unit 80. Then, when the next residual water purging process is performed while the vehicle is at a standstill, the amount of current supplied from the secondary battery 50 is integrated with the integrated interruption value stored in the storage unit 80. Accordingly, even in a case where the remaining water scavenging process is completed with the vehicle at standstill before the discharge is completed in the predetermined voltage range, the integration of the amounts of current supplied from the secondary battery 50 in the predetermined voltage range can be completed by performing the remaining water scavenging process once or more times. Therefore, an integrated current value required for the diagnosis of the secondary battery 50 can be obtained, and accurate diagnosis of the secondary battery is possible.In general, the integrated current value during supply (discharge) of electric power to the air compressor 31 having high power consumption is obtained. Therefore, an integrated current value required for the diagnosis of the secondary battery 50 can be obtained within a short time. The diagnosis controller 71 compares the integrated current value with the integrated threshold value SIt that is an integrated current value at which the deterioration of the secondary battery 50 is at the lower limit of the allowable range, and determines that the secondary battery 50 deteriorates when the integrated current value SIt is lower than the integrated threshold value. Therefore, the diagnosis controller 71 can accurately determine whether or not the secondary battery 50 deteriorates.In addition, the diagnosis controller 71 does not determine the integrated current value and does not perform diagnosis of the secondary battery 50 if the discharge voltage of the secondary battery 50 at the start of integration of the current values is lower than the upper limit value of the predetermined voltage range, that is, in a case where the amounts of current supplied from the secondary battery 50 in the predetermined voltage range cannot be integrated. In a case where the amounts of current supplied from the secondary battery 50 in the predetermined voltage range can be integrated, the diagnosis controller 71 acquires the integrated current value and performs diagnosis of the secondary battery by 50.B. Modification ExamplesB1. Modification Example 1In the embodiment, the cathode-side water purging process is performed after the anode-side water purging process while the vehicle is at a standstill. However, the order of the water purging processes may be interchanged. In the residual water purging process when the vehicle is at a standstill, the residual water purging process is performed on both the anode side and the cathode side. However, the residual water purging process may be performed on either the anode side or the cathode side. For example, in a configuration where the residual water purging process is performed only on the cathode side, the secondary battery 50 may be checked while the residual water purging process is performed on the cathode side of the fuel cell 10. In a configuration where the residual water scavenging process is performed only on the anode side, the secondary battery 50 may be checked while the residual water scavenging process is performed on the anode side of the fuel cell 10. Accordingly, the same effects as those of the embodiment can be exhibited. For example, in a case where the residual water scavenging process is performed when a start switch is turned off, the residual water scavenging process may be performed only on the anode side with the vehicle at standstill. In a case where the water scavenging process is not performed when a start switch is turned off, the remaining water scavenging process may be performed on either the anode side or the cathode side with the vehicle at standstill.B2. Modification Example 2In the embodiment, the remaining water purging process is regularly performed in the discharge stop state with the vehicle at a standstill, but the invention is not limited thereto. For example, the temperature of the fuel cell 10 or the outside air temperature may be measured regularly, so that in a case where the measured temperature is lower than a predetermined temperature in a predetermined temperature range whose center is 0° C., the residual water purging process is performed with the vehicle at standstill. In addition, for example, in a case where the average lowest temperature is 0° C. or lower in a predetermined period of time (for example, three days) immediately before the residual water purging process, the residual water purging process may be performed with the vehicle at standstill.B3. Modification Example 3However, in the embodiment, in a case where the remaining water purging process ends when the vehicle is at a standstill before the discharge capacity of the secondary battery 50 reaches the lower limit value of the predetermined voltage range, the current value is integrated with the previous integrated interruption value in the next secondary battery diagnostic process. However, the invention is not limited to this configuration. For example, in a case where the diagnosis is interrupted, the integrated interruption value may not be stored in the storage unit 80, and in a case where the second or subsequent secondary battery diagnosis process is performed, the diagnosis (integration of the current values) may not be performed until the discharge capacity of the secondary battery 50 becomes the upper limit value of the voltage range or higher. Also in the configuration, in a case where the fuel cell system 100 is subsequently started to charge the secondary battery 50, the discharge voltage may be higher than the upper limit value of the predetermined voltage range. Accordingly, in a case where the fuel cell 10 subsequently enters the output stop state, the integrated current value in the secondary battery diagnostic process can be accurately obtained.B4. Modification Example 4In the embodiment, the diagnosis controller 71 compares the integrated current value with the integrated threshold value SIt, and determines that the secondary battery 50 deteriorates in a case where the integrated current value is lower than the integrated threshold value SIt. However, the invention is not limited to this configuration. For example, the diagnostic controller 71 may determine a slope of the straight line shown in FIG. 5, and in a case where the slope is less than a predetermined value, may determine that the secondary battery 50 deteriorates. For example, an integrated value of current values at two times during the integration of the current values may be stored, so that the slope of the straight line may be determined based on the integrated current value. In addition, the degree of deterioration may be specified instead of determining whether or not the secondary battery 50 deteriorates. In the configuration, the specification of the degree of deterioration corresponds to the diagnosis of the secondary battery 50.B5. Modification Example 5In the embodiment, in a case where the discharge voltage of the secondary battery 50 is lower than the upper limit value of the predetermined voltage range, the diagnosis controller 71 determines that the diagnosis cannot be performed (step S 315: NO), and the secondary battery diagnosis process ends. However, the invention is not limited to this configuration. For example, the current values may be constantly integrated without performing step S 315. By setting a voltage range in which the upper limit value is relatively low as the predetermined voltage range, the discharge capacity of the secondary battery 50 becomes likely to become higher than the upper limit value of the predetermined voltage range at the start of integration of the current values. Thus, in this case, step S 315 may not be performed. In the configuration of the embodiment, the predetermined voltage range may be set as a wide voltage range, and an integrated current value having a higher reliability may be obtained to perform diagnosis as to whether or not the secondary battery 50 deteriorates.B6. Modification Example 6In step S 315 of the embodiment, in a case where the discharge voltage of the secondary battery 50 is lower than the upper limit value of the predetermined voltage range, the diagnosis controller 71 determines that the diagnosis cannot be performed. However, the invention is not limited to this configuration. For example, the diagnosis controller 71 may determine that the diagnosis cannot be performed not only in a case where the discharge capacity of the secondary battery 50 is lower than the upper limit value of the predetermined voltage range but also in a case where a shift lever of a vehicle is in a mode other than parking, for example, driving (D) or reverse (R), that is, in a mode corresponding to a state where the fuel cell system 100 is turned on.B7. Modification Example 7In the embodiment, the gas used in the residual water purging process when the vehicle is at a standstill is a reaction gas (hydrogen gas and air). However, another gas may be used instead of or in addition to the reaction gas. For example, the residual water purging process using a specific gas for the residual water purging process may be enabled by supplying the specific gas to the fuel cell 10 and providing a function for executing exhaust gas. As the specific gas, for example, nitrogen gas may be used.B8. Modification Example 8In the embodiment, some configurations realized by hardware may be realized by software. Conversely, some configurations realized by software may be realized by hardware. In addition, in a case where some or all of the functions of the invention are realized by software, the software (a computer program) may be realized in the form of a computer readable recording medium storing the software. "Computer readable recording medium" includes not only portable recording media such as a flexible disk and a CDROM, but also internal storage devices within computers such as a RAM and a ROM, and external storage devices fixedly attached to computers, for example, a hard disk. That is, "computer readable medium" has a broad meaning, including any recording medium that can permanently retain data.

Claims

A fuel cell system comprising: a fuel cell (10); accessory devices (26, 31) used to supply gas to the fuel cell; an accessory device controller (72) configured to control an operation of the accessory devices; a secondary battery (50); a current sensor (62) configured to measure an amount of current supplied from the secondary battery; a voltage sensor (61) configured to measure a discharge capacity of the secondary battery; A diagnostic controller (71) configured to check the secondary battery, wherein: the accessory device controller (72) is configured to, in an output stop state where the fuel cell does not output electric power, perform a residual water purging process in which, by driving the accessory devices using electric power supplied from the secondary battery and supplying the gas to the fuel cell, water remaining in the fuel cell is purged from the fuel cell system; The diagnosis controller ( 71) is configured to perform diagnosis of the secondary battery using an integrated value obtained by integrating amounts of current supplied from the secondary battery in a predetermined voltage range of the discharge voltage that changes as a result of the discharge when electric power is supplied to the accessory devices, by performing the residual water purging process.The fuel cell system according to claim 1, further comprising a storage unit (80) configured to store an integrated value of amounts of current supplied from the secondary battery, wherein the diagnosis controller (71) is configured to, when the residual water purging process is completed before the discharge by the secondary battery is completed in the voltage range, repeat the integrated current value by repeating the following processes (i) and (ii) until the discharge by the secondary battery is completed in the voltage range: (i) a process in which the storage unit (80) is caused to store the integrated value of amounts of current supplied from the secondary battery, the current being supplied from the start to the end of the residual water purging process; (ii) a process in which the amount of current supplied from the secondary battery is integrated with the integrated value stored in the storage unit (80) when the residual water scavenging process is performed.The fuel cell system according to claim 1 or 2, wherein: the gas includes air as the oxidizing gas; and the auxiliary devices include an air compressor (31).The fuel cell system according to any one of claims 1 to 3, wherein: the gas includes hydrogen gas as a fuel gas; the fuel cell system further includes: a tank (21) in which hydrogen gas is stored, an anode gas supply path (22) through which the hydrogen gas is supplied from the tank to the fuel cell, an anode off-gas discharge path (23) through which off-gas including hydrogen gas is discharged from the fuel cell, a bypass flow path (24) through which the anode off-gas discharge path and the anode gas supply path are connected, and a pump (26) disposed in the bypass flow path (24) and supplying the off-gas discharged from the anode off-gas discharge path to the anode gas supply path; and wherein the accessories include the pump.The fuel cell system according to any one of claims 1 to 4, wherein the diagnosis controller (71) is configured to compare the integrated current value with an integrated threshold value (SIt) corresponding to a lower limit of an integrated current value at which the deterioration of the secondary battery is at a lower limit of an allowable range, and is configured to determine that the secondary battery deteriorates when the integrated current value is lower than the integrated threshold value (SIt).The fuel cell system according to any one of claims 1 to 4, wherein the diagnosis controller (71) is configured to compare a gradient of the integrated current value during a predetermined time course with a threshold gradient of a lower limit for an integrated current value at which deterioration of the secondary battery is at a lower limit of an allowable range during the predetermined time course, and is configured to determine that the secondary battery deteriorates when the gradient of the integrated current value is lower than the threshold gradient.The fuel cell system according to any one of claims 1 to 5, wherein the diagnosis control means (71) determines whether or not the discharge voltage of the secondary battery at the start of the residual water purging process is lower than an upper limit value of the voltage range, does not determine the integrated current value and does not perform diagnosis of the secondary battery when the diagnosis control means (71) determines that the discharge voltage of the secondary battery is lower than the upper limit value of the voltage range, and determines the integrated current value and performs diagnosis of the secondary battery when the diagnosis control means (71) determines that the discharge voltage of the secondary battery is not lower than the upper limit value of the voltage range.A secondary battery diagnostic method for performing a diagnosis on a secondary battery that supplies electric power to accessory devices for supplying gas to a fuel cell, the secondary battery diagnostic method comprising: (a) performing a residual water purging process in an output stop state where the fuel cell does not output electric power, wherein by driving the accessory devices using electric power supplied from the secondary battery and supplying the gas to the fuel cell, water remaining in the fuel cell is purged from the fuel cell system. (b) determining an integrated value by integrating amounts of current supplied from the secondary battery in a predetermined voltage range of a discharge voltage of the secondary battery that changes due to a discharge when electric power is supplied to the accessory devices by performing the residual water purging process; (c) performing a secondary battery diagnosis using the integrated current value.

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