Fuel cell vehicle

A fuel cell vehicle may include: an electric traction motor; an inverter; a fuel cell system; a first boost converter including first low voltage terminals connected to a fuel cell and first high voltage terminals connected to the inverter, the first boost converter including a first capacitor connected between positive and negative terminals of the first high voltage terminals; a first relay connected between the first boost converter and the inverter; and a controller, wherein the controller is configured to: shut down the fuel cell system; while a voltage of the fuel cell is higher than a voltage threshold, discharge the first capacitor and maintain a voltage thereof higher than the voltage of the fuel cell; and when the voltage of the fuel cell becomes lower than the voltage threshold, stop discharging the first capacitor and disconnect the first boost converter from the inverter by opening the first relay.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2018-003666 filed on Jan. 12, 2018, the contents of which are hereby incorporated by reference into the present application.

TECHNICAL FIELD

The teaching disclosed herein relates to a fuel cell vehicle provided with a boost converter which boosts an output voltage of a fuel cell.

BACKGROUND

Japanese Patent Application Publication No. 2011-90823 (Patent Literature 1) describes fuel cells vehicle provided with a boost converter which boosts an output voltage of a fuel cell. Low voltage terminals of the boost converter are connected to the fuel cells, and high voltage terminals thereof are connected to an inverter. The inverter converts boosted DC power to AC power suitable for driving an electric traction motor. In general, the boost converter has a current smoothing capacitor connected between a positive terminal and a negative terminal of the high voltage terminals. When the fuel cells are stopped, the capacitor retains a voltage higher than the output voltage of the fuel cells. The fuel cells also retain a corresponding voltage for a while after having stopped.

In the fuel cell vehicle of Patent Literature 1, when a main switch of the vehicle is turned off, the fuel cells and the capacitor are discharged according to the following procedure. Firstly, an oxygen supplier is stopped but a pump is not stopped, and a fuel supply is continued for a while. This fuel supply enables electricity generation to continue until remaining oxygen is consumed. In the fuel cell vehicle of Patent Literature 1, the pump is stopped after the remaining oxygen has been consumed. Next, the boost converter is activated, and remaining charges in the fuel cells are discharged to a high voltage side of the boost converter. The boost converter is stopped after the remaining charges in the fuel cells have been discharged. Then, the inverter is operated to discharge the capacitor through the inverter.

SUMMARY

In the fuel cell vehicle of Patent Literature 1, the electricity are forcibly discharged from the fuel cells by operating the boost converter. Such forced discharge of the electricity may cause deterioration of catalyst and the like of the fuel cells. The fuel cell vehicle which discharges the capacitor of the boost converter and the fuel cells upon stopping power can be improved regarding this aspect.

A fuel cell vehicle disclosed herein comprises an electric traction motor, an inverter, a fuel cell system, a first boost converter, a first relay, and a controller. The inverter is configured to convert DC power to AC power for driving the electric traction motor. The fuel cell system may include a fuel cell, an air compressor, and a fuel cell pump. The fuel cell generates electricity by supplies of hydrogen fuel and oxygen. The first boost converter includes first low voltage terminals connected to the fuel cell and first high voltage terminals connected to the inverter. A first capacitor is connected between a positive terminal and a negative terminal of the first high voltage terminals. The first relay is connected between the first boost converter and the inverter. The controller stops power according to a following sequence. The controller firstly shuts down the fuel cell system. Then, while a voltage of the fuel cell is higher than a predetermined voltage threshold, the controller discharges the first capacitor with a voltage of the first capacitor maintained to be higher than the voltage of the fuel cell. When the voltage of the fuel cell becomes lower than the predetermined voltage threshold, the controller stops the discharging of the first capacitor and disconnects the first boost converter from the inverter by opening the first relay.

In the aforementioned stop sequence, the fuel cell system is shut down prior to the discharging of the first capacitor. Due to this, the voltage of the fuel cell naturally and gradually decreases. In the fuel cell vehicle disclosed herein, the controller discharges the first capacitor such that the voltage of the first capacitor is maintained to be higher than the voltage of the fuel cell. The first capacitor is discharged as the voltage of the fuel cell decreases. Since charges are not forcibly discharged from the fuel cell, damages to catalyst and the like may be suppressed.

Details and further improvements of the technique disclosed herein will be described in the following detailed description.

DETAILED DESCRIPTION

Some features of a fuel cell vehicle disclosed herein will be described. A configuration of a fuel cell vehicle of an embodiment is especially effective in a fuel cell vehicle in which a relay for electrically disconnecting a fuel cell and an inverter is arranged between a first boost converter and the inverter, not between the fuel cell and the first boost converter. The aforementioned fuel cell vehicle may avoid damaging the fuel cell upon discharging a capacitor in the first boost converter connected to the fuel cell.

A controller may discharge the capacitor, for example, by driving the inverter and allowing current to flow in a electric traction motor. Alternatively, in a case of being provided with a discharge resistance, the controller may discharge the capacitor by using the discharge resistance. A voltage threshold is set to a value at a degree by which no severe damage will be caused in peripheral components even when a current leak occurs. The voltage threshold may be, for example, 42 volts.

The fuel cell vehicle disclosed herein may further include a battery, a second boost converter, and a second relay. The second boost converter includes second low voltage terminals connected to the battery and second high voltage terminals connected to the inverter. The second relay is connected between the battery and the second boost converter. A second capacitor is connected between a positive terminal and a negative terminal of the second high voltage terminals. In a fuel cell vehicle as above, it is desirable to discharge the second capacitor as well. The controller may execute a following procedure in addition to the aforementioned stop sequence. The controller disconnects the battery from the second boost converter by opening the second relay prior to discharging the second capacitor. After having stopped the fuel cell, while a voltage of the fuel cell is higher than a predetermined voltage threshold, the controller discharges the first and second capacitors with voltages of the first and second capacitors maintained to be higher than the voltage of the fuel cell. When the voltage of the fuel cell becomes lower than the voltage threshold, the controller stops the discharging of the first capacitor and disconnects the inverter from the first boost converter by opening the first relay. As a final step, the controller discharges the second capacitor. The controller may discharge the first and second capacitors by using the inverter and the electric traction motor, or the discharge resistance.

The controller may execute a following sequence. In a case where the voltage of the fuel cell is lower than the voltage threshold prior to starting discharging of the first capacitor, the controller may discharge the first capacitor such that the first capacitor has a voltage equal to the voltage threshold and open the first relay. In a case where the voltage of the fuel cell is higher than the voltage threshold before starting the discharge, the first and second capacitors may be drastically discharged to the voltage threshold at once to complete the discharge within a short period of time.

EMBODIMENT

A fuel cell vehicle100of an embodiment will be described with reference to the drawings.FIG. 1shows a block diagram of a power system of the fuel cell vehicle100. Broken arrow lines in the drawing indicate signal lines. A reference sign37shows an air pipe, and a reference sign38shows a fuel pipe. A hold arrow line depicted along the air pipe37shows a flow of air (oxygen), and a bold arrow line depicted along the fuel pipe38shows a flow of fuel gas.

The fuel cell vehicle100includes a fuel cell2, a battery6, a first boost converter10, a second boost converter20, a first relay4, a second relay7, a first inverter5, and an electric traction motor32. The electric traction motor32may be referred to as the traction motor32or the motor32for short.

An output voltage of the fuel cell2is, for example, 0 to 200 volts, and an output voltage of the battery6is, for example, 300 volts. On the other hand, a drive voltage of the traction motor32is, for example, 300 to 600 volts. There are cases where the drive voltage is higher than the output voltages of the fuel cell2and the battery6. Due to this, the fuel cell vehicle100is provided with the first boost converter10and the second boost converter20. Low voltage terminals12of the first boost converter10are connected to the fuel cell2, and high voltage terminals13thereof are connected to the first inverter5. The first relay4is connected between the first boost converter10and the first inverter5. Low voltage terminals22of the second boost converter20are connected to the battery6, and high voltage terminals23thereof are connected to the first inverter5. The second relay7is connected between the second boost converter20and the battery6.

Although a circuit configuration of the first boost converter10will be described later with reference toFIG. 2, a capacitor14and a voltage sensor15are connected between a positive terminal13aand a negative terminal13bof the high voltage terminals of the first boost converter10. Although a circuit configuration of the second boost converter20will be described later with reference toFIG. 3, a capacitor24and a voltage sensor25are connected between a positive terminal23aand a negative terminal23bof the high voltage terminals of the second boost converter20.

The voltage sensor15is configured to measure a voltage between the positive terminal13aand the negative terminal13bof the high voltage terminals of the first boost converter10. The voltage between the positive terminal13aand the negative terminal13bis equal to a voltage between both ends of the capacitor14. The voltage sensor25is configured to measure a voltage between the positive terminal23aand the negative terminal23bof the high voltage terminals of the second boost converter20. The voltage between the positive terminal23aand the negative terminal23bis equal to a voltage between both ends of the capacitor24. Other than the voltage sensors15,25, the fuel cell vehicle100is further provided with a voltage sensor3configured to measure the output voltage of the fuel cell2. Measurement data of the voltage sensors3,15,25is sent to an controller9.

As aforementioned, the broken arrow lines inFIG. 1show the signal lines. A letter string “to Cntllr” represents “to Controller” (to the controller9), meaning that the signal line for sending a signal (data) to the controller9is connected to the controller9. A letter string “from Cntllr” represents “from Controller” (from the controller9), meaning that the signal line for the controller9to send a signal is connected to the controller9. Other than the ones shown in the drawing, the fuel cell vehicle100is further provided with various signal lines and devices, however, depiction thereof is omitted.

The first inverter5is configured to convert boosted DC power to AC power suitable for driving the motor32. The first boost converter10, the second boost converter20, and the first inverter5are controlled by the controller9. The controller9determines a target output of the motor32from a vehicle speed and an output voltage of an accelerator opening. The target output includes a target voltage and a target frequency. The controller9determines output ratios of the first boost converter10and the second boost converter20from output current and the output voltage of the fuel cell2. The controller9controls the first and second boost converters10,20such that the target voltage and the output ratios of the first and second boost converters10,20are realized, and controls the first inverter5such that the target frequency is realized.

A second inverter8is connected also to the high voltage terminals13of the first boost converter10and the high voltage terminals23of the second boost converter20. The second inverter8is configured to drive an air compressor31. Further, a third inverter33is connected between the second relay7and the second boost converter20. The third inverter33is configured to drive a fuel pump34.

A fuel cell system40includes the fuel cell2, the air compressor31, and the fuel pump34. The air compressor31is configured to supply air (oxygen) to the fuel cell2through the air pipe37. The air compressor31is an oxygen supplier configured to supply the oxygen to the fuel cell2. The fuel pump34is configured to supply hydrogen fuel in a fuel tank35to the fuel cell2through the fuel pipe38. The second inverter8and the third inverter33are also controlled by the controller9. In other words, the controller9is configured to further control the air compressor3and the fuel pump34.

The first relay4is a switch configured to connect and disconnect the inverters5,8and the first boost converter10(and the fuel cell2), and the second relay7is a switch configured to connect and disconnect the battery6and the second boost converter20. When a main switch36of the vehicle is turned on, the controller9closes the first relay4to connect the fuel cell2and the first boost converter10to the inverters5,8, and closes the second relay7to connect the battery6to the second boost converter20. When the controller9closes the second relay7, the fuel cell vehicle100is enabled to travel by using the battery6. When the controller9closes the first relay4and the second relay7and activates the fuel cell system40, the fuel cell vehicle100enters a state in which traveling by using both the battery6and the fuel cell2is enabled.

On the other hand, when the main switch36of the vehicle is switched to, off, the controller9stops the fuel cell2, opens the first relay4to disconnect the inverters5,8from the first boost converter10(and the fuel cell2), and opens the second relay7to disconnect the second boost converter20from the battery6. A power stop process by the controller9will be described later.

A circuit configuration of the first boost converter10will be described with reference toFIG. 2.FIG. 2is a circuit diagram of the first boost converter10. The first boost converter10includes a transistor16, diodes17a,17b, a reactor18, capacitors14,19, and the voltage sensor15. One end of the reactor18is connected to a positive terminal12aof the low voltage terminals12of the first boost converter10. Other end of the reactor18is connected to an anode of the diode17a. A cathode of the diode17ais connected to the positive terminal13aof the high voltage terminals13. A collector of the transistor16is connected to a connection node between the reactor18and the diode17a. An emitter of the transistor16is connected to a negative terminal12bof the low voltage terminals12. The negative terminal121) of the low voltage terminals12is directly connected to the negative terminal13bof the high voltage terminals13. The diode17bis connected in inverse parallel to the transistor16. The capacitor14and the voltage sensor15are connected between the positive terminal13aand the negative terminal13bof the high voltage terminals13, as aforementioned. The capacitor19is connected between the positive terminal12aand the negative terminal12bof the low voltage terminals12.

The transistor16is controlled by the controller9(seeFIG. 1). When the transistor16repeats to be turned on and off at a predetermined duty ratio, a voltage applied to the low voltage terminals12is boosted and the boosted voltage is outputted from the high voltage terminals13. The first boost converter10of the circuit ofFIG. 2is a chopper-type voltage converter, and as such, its output voltage pulsates. The capacitor14connected between the positive terminal13aand the negative terminal13bof the high voltage terminals13is provided to suppress pulsation of the output voltage of the first boost converter10. The capacitor14has a large capacity, and a large quantity of electric energy may be stored therein. Since the diode17ais connected between the positive terminal12aof the low voltage terminals12and the positive terminal13aof the high voltage terminals13, current does not flow from the high voltage terminals13to the low voltage terminals12. In some embodiments, as the capacitor14does not need to retain a high voltage during when the fuel cell vehicle100is not used, the capacitor14needs to be discharged upon stopping the power supply.

A circuit configuration of the second boost converter20will be described with reference toFIG. 3.FIG. 3is a circuit diagram of the second boost converter20. The second boost converter20includes transistors26a,26b, diodes27a,27b, a reactor28, capacitors24,29, and the voltage sensor25. The two transistors26a,26bare connected in series between the positive terminal23aand the negative terminal23bof the high voltage terminals23. The reactor28is connected between a midpoint of the series connection of the two transistors26a,26band a positive terminal22aof the low voltage terminals22. The capacitor29is connected between the positive terminal22aand a negative terminal22bof the low voltage terminals22. The negative terminal22bof the low voltage terminals22is directly connected to the negative terminal23bof the high voltage terminals23. The diode27ais connected in inverse parallel to the transistor26a, and the diode27bis connected in inverse parallel to the transistor26b. The capacitor24and the voltage sensor25are connected between the positive terminal23aand the negative terminal23bof the high voltage terminals23as aforementioned.

The second boost converter20shown inFIG. 3has both a boost function to boost a voltage applied to the low voltage terminals22and output the boosted voltage to the high voltage terminals23and a step-down function to step down a voltage applied to the high voltage terminals23and output the stepped-down voltage to the low voltage terminals22. The second boost converter20is a so-called bidirectional DC-DC convertor. Since the boost function is emphasized in the disclosure herein, the circuit ofFIG. 3is termed the second boost converser20.

The transistors26a,26bare controlled by the controller9(seeFIG. 1). When the transistor26brepeats to be turned on and off at a predetermined duty ratio, the voltage applied to the low voltage terminals22is boosted and the boosted voltage is outputted from the high voltage terminals23. When the transistor26arepeats to be turned on and off at a predetermined duty ratio, the voltage applied to the high voltage terminals23is stepped down and the stepped-down voltage is outputted from the low voltage terminals22. When the transistor26aand the transistor26brepeat to be turned on and off with complementary PWM signals, the boost function and the step-back function switch automatically according to a balance between the voltage of the low voltage terminals22and the voltage of the high voltage terminals23. The traction motor32frequently switches between a mode of outputting drive torque by using electric power and a mode of generating regenerative power by using deceleration energy of the vehicle. The aforementioned functions of the second boost converter20are suitable for controlling the traction motor in which the modes of power consumption and power generation frequently switch.

The second boost converter20of the circuit ofFIG. 3is also a chopper-type voltage converter, and as such, its output voltage pulsates. The capacitor24connected between the positive terminal23aand the negative terminal23bof the high voltage terminals23is provided to suppress the pulsation of the output voltage of the second boost converter20. The capacitor24has a large capacity, and a large quantity of electric energy may be stored therein. Since the diode27ais connected between the positive terminal22aof the low voltage terminals22and the positive terminal23aof the high voltage terminals23, current does not flow from the high voltage terminals23to the low voltage terminals22while the transistor26ais off. In some embodiments, as the capacitor24does not retain a high voltage during when the fuel cell vehicle100is not used, the capacitor24needs to be discharged upon stopping the power supply.

As aforementioned, upon stopping the power supply in the fuel cell vehicle100, the high voltages are retained in the capacitor14of the first boost converter10and the capacitor24of the second boost converter20, and as such, the capacitors14,24need to be discharged. Especially, since a cutoff switch is not interposed between the first boost converter10and the fuel cell2, the capacitor14of the first boost converter10, which is in constant connection with the fuel cell2, needs to be discharged. The capacitor19of the first boost converter10also needs to be discharged, however, since the capacitor19will be discharged when the capacitor14is discharged, so the discharge of the capacitor14will be emphasized herein. The same applies to the capacitor29of the second boost converter20.

FIGS. 4 and 5show flowcharts of the power stop process. Processes ofFIGS. 4 and 5are executed by the controller9. The controller9includes a memory storing programs, and a CPU configured to execute the stored programs. The controller9loads programs corresponding to the processes ofFIGS. 4 and 5from the memory and executes the same. The processes ofFIGS. 4 and 5are initiated when the main switch36of the vehicle (seeFIG. 1) is turned off.

FIG. 6shows a time chart of the power stop process. A graph G1is a time chart indicating drive/stop of the air compressor31and the fuel pump34. A graph G2shows a chronological change in the voltage of the fuel cell2. A graph G3shows the voltage of the capacitor14of the first boost converter10. A graph G4shows the voltage of the capacitor24of the second boost converter20. Until time T8, the voltage of the capacitor24is equal to the voltage of the capacitor14. A graph G5is a time chart indicating drive/stop of the first inverter5. A graph G6is a time chart indicating open/close of the first relay4. A graph G7is a time chart indicating open/close of the second relay7. The stop process will be described with reference toFIGS. 4 and 5as well asFIG. 6.

Hereinbelow, for the sake of simplicity of explanation, the voltage of the fuel cell2will be termed an FC voltage, and the voltages of the capacitors14,24will be termed capacitor voltages.

When the main switch36is turned off, the controller9firstly stops the air compressor31and stops the fuel pump34(step S2). Since the supplies of fuel and oxygen are stopped, the fuel cell2stops. At this occasion, the controller9also executes other process(es) necessary for stopping the fuel cell2. In the time chart ofFIG. 6, the main switch36is switched to off at time T1, and the controller9stops the air compressor31and the fuel pump34. In other words, the controller9stops the fuel cell2at time T1. Since the fuel cell2has been stopped, the voltage of the fuel cell2(the FC voltage) gradually drops from time T1(the graph G2).

Next, the controller9opens the second relay7to disconnect the second boost converter20from the battery6(step S3). At time T2, the second relay7is opened and the battery6is disconnected from the second boost converter20.

Next, the controller9drives the first inverter5to flow charges of the capacitors14,24to the motor32to discharge the capacitors14,24. Hereinbelow, this discharging process will be described in detail.

At time T2, the first inverter5is activated (the graph G5). At this occasion, the controller9controls current to flow in coils of the motor32such that the motor32does not rotate. Since the fuel cell2is stopped and the battery6is disconnected, the power of the capacitors14,24flows to the motor32. That is, the capacitors14,24are discharged. In discharging the capacitors14,24, the controller9sets a target discharging voltage of the capacitors14,24such that the voltages of the capacitors14,24are maintained to be higher than the voltage of the fuel cell2. Since the fuel cell2and the first boost converter10are electrically connected, if the target discharging voltage of the capacitors14,24becomes lower than the voltage of the fuel cell2, current flows from the fuel cell2to the motor32. The controller9determines the target discharging voltage such that the current does not flow out from the fuel cell2.

In the discharging process, the controller9firstly compares the voltage of the fuel cell2(the FC voltage) with a predetermined voltage threshold Vth (step S4). The voltage threshold Vth is set to a value that is low enough not to impose serious adverse effect to surrounding components even when a current leak occurs. The voltage threshold Vth is, for example, 42 volts. In a case where the FC voltage is higher than the voltage threshold Vth, the controller9sets the target discharging voltage of the capacitors14,24to the FC voltage+a margin voltage Va (step S5. The margin voltage Va is set to absorb measurement errors of the voltage sensor. That is, for example, even in a case where the voltage of the fuel cell2(the FC voltage) measured by the voltage sensor3is lower than the actual FC voltage by 5 volts, if the margin voltage Va is set to 10 volts, the target discharging voltage does not become lower than the actual FC voltage. The margin voltage Va is, for example, 10 to 20 volts. In a case where the measurement value of the voltage sensor is accurate, the margin voltage Va may be zero.

In the time chart ofFIG. 6, for example, the FC voltage at time T2is a voltage Vr. The target voltage at time T2becomes the FC voltage Vr+the margin voltage Va.

Next, the controller9drives the first inverter5and discharges the capacitors14,24until the capacitor voltages drop to the target voltage (steps S6, S7). From time T2and thereafter, the controller9drives the first inverter5until the capacitor voltages reach the target voltage Vr+Va and discharges the capacitors14,24.

In step S7, the controller9compares a voltage difference between, the target voltage and the capacitor voltages with a tolerable difference Vb. The controller9continues to drive the inverter until the voltage difference becomes smaller than the tolerable difference Vb (step S7: NO, S6). In other words, the controller9discharges the capacitors14,24until the capacitor voltages match the target voltage within a range of the tolerable difference Vb.

When the voltage difference becomes smaller than the tolerable difference Vb, the controller9compares the voltage of the fuel cell2(the FC voltage) with the voltage threshold Vth again (step S7: YES, S4). As aforementioned, since the fuel cell2has been stopped, the FC voltage drops gradually. In a case where the FC voltage is higher than the voltage threshold Vth in step S4, the controller9resets the target voltage with a new FC voltage (step S5). Then, similar to the previous step, the controller9drives the inverter5until the difference between the capacitor voltages and the target voltage becomes smaller than the tolerable difference Vb (steps S6, S7). With a loop from step S4to step S7, the capacitors14,24are gradually discharged without allowing their voltages to become lower than the voltage of the fuel cell2.

In the time chart ofFIG. 6, at time T3, the capacitor voltages (the graph G3) drop to an initial target voltage (that is, Vr+Va). After this, the loop from step S4to step S7is repeated. As a result, from time T4and thereafter, the capacitor voltages drop by following the FC voltage while maintaining their states to be higher than the FC voltage by the margin voltage Va (the graph (G3).

The capacitor19of the first boost converter10is discharged as the voltage of the fuel cell2drops. The capacitor29of the second boost converter20is discharged together with the capacitor24.

In the process of step S4, when the voltage of the fuel cell2(the FC voltage) becomes lower than the voltage threshold Vth, the process proceeds to a process of step S12ofFIG. 5. In step S12, the controller9sets the target voltage of the capacitors to the voltage threshold Vth+the margin voltage Va. Then, the controller9drives the inverter5and continues the discharge until the voltage difference between the target voltage and the capacitor voltages becomes smaller than the tolerable difference Vb (steps S13, S14). Processes of steps S12to S14are countermeasures for a case where a voltage drop of the fuel cell2progresses quickly and the FC voltage is already lower than the voltage threshold Vth when the process of step S4is executed for a first time. When step S4is executed for the first time, the voltages of the capacitors14,24are equal to the voltage of the fuel cell2that was boosted by the first boost converter10. Due to this, in the case where the FC voltage is already lower than the voltage threshold Vth when the process of step S4is executed for the first time, the capacitors14,24are discharged immediately to the voltage threshold Vth by the processes of steps S12to S14. Due to the processes of steps S12to S14, a discharge time for the case where the FC voltage is already lower than the voltage threshold Vth when the process of step S4is executed for the first time can be shortened.

On the other hand, in a case where the process of step S12is executed after the loop from step S4to step S7has been executed at least once, the capacitor voltages may be already close to the voltage threshold Vth in some cases. In such cases, a branching determination of step S14immediately becomes YES, and the process of the controller9proceeds to step S15. When the determination of YES is made in step S14, the capacitor voltages are already down to the voltage threshold Vth by which no serious adverse effect will be imposed on the surrounding components even when a current leak occurs.

In the time chart ofFIG. 6, at time T5, the FC voltage becomes lower than the voltage threshold Vth. From time T5and thereafter, the capacitor voltages are retained at the target voltage (the voltage threshold Vth+the margin voltage Va).

Next, the controller9stops the inverter5(step S15). That is, the controller9stops discharging. Then, the controller9opens the first relay4to disconnect the first boost converter10and the fuel cell2from the inverter5(step S16). In the time chart ofFIG. 6, the inverter5is stopped at time T6, and the first relay4is opened at time T7.

Next, the controller9sets the target discharging voltage of the capacitor24to 0 volts, and drives the inverter5again (steps S17, S18). The capacitors24,29of the second boost converter20are discharged by processes of steps S18, S19. The controller9stops the inverter5when a voltage difference between the target voltage (=0) and the capacitor voltage becomes smaller than the tolerable difference Vb (step S19: YES, S20). The power stop process is thereby completed. In the time chart ofFIG. 6, the inverter5is driven again at time T8, and the voltage of the capacitor24thereby drops (the graph G4). The voltage of the capacitor24reaches 0 volts at time T9. On the other hand, the FC voltage gradually drops, and reaches 0 volts at time T10.

The aforementioned power stop process has the following advantages. That is, in the aforementioned power stop process, the air compressor31that supplies the oxygen to the fuel cell2and the fuel pump34that supplies the fuel to the fuel cell2are immediately stopped when the main switch36is switched to off. As such, a pump noise stops immediately after the main switch36is switched to off. Further, in the aforementioned power stop process, the capacitors14,24are discharged while their voltages are maintained to be higher than the FC voltage. Thus, charges are not forcibly moved from the fuel cell2to the capacitors14,24during discharging of the capacitors14,24. When the charges are forcibly discharged from the fuel cell2, a catalyst and the like of the fuel cell2may be damaged. In the power step process of the embodiment, such a risk can be avoided. The capacitor19is discharged with the capacitor14, and the capacitor29is discharged with the capacitor24.