Patent Publication Number: US-2009226769-A1

Title: Measurement of insulation resistance of fuel cell in fuel cell system

Description:
TECHNICAL FIELD 
     The present invention relates to technology for measuring insulation resistance of a fuel cell in a fuel cell system 
     BACKGROUND ART 
     In a water-cooled fuel cell system in which the fuel cells are cooled by a circulating coolant, the electrical conductivity of the coolant rise over time due to ions eluting into the coolant. As the electrical conductivity of the coolant reaches a certain level, there is a risk that the electrical current generated by the fuel cells flows through the coolant and that the electrical power generated by the fuel cells cannot be drawn effectively. Moreover, if the coolant is decomposed by the electric current flowing through the coolant, there is a risk that heat transfer to the coolant is impaired by bubbles formed within the coolant flow passages and that cooling of the fuel cell becomes insufficient. For this reason, a conventional practice for preventing the various problems associated with increased conductivity of the coolant has been to monitor the rise in conductivity of the coolant in terms of the insulation resistance of the fuel cell, and to replace either the ion filter that removes the ions, or the coolant itself, when needed. 
     However, if the output voltage of a fuel cell fluctuates during measurement of the insulation resistance of the fuel cell, errors in insulation resistance measurements may occur and false detection of rising in conductivity may result, for example, in instances where rise in conductivity of the coolant is undetected or where a rise in conductivity of the coolant is detected despite no actual rise in conductivity having occurred. Such problems are particularly notable in a water-cooled fuel cell system in which rise in conductivity is detected via insulation resistance, but are nevertheless problems common to all manner of fuel cell systems that detect problems in the fuel cell system, such as electrical leakage, by measuring insulation resistance. 
     DISCLOSURE OF THE INVENTION 
     To achieve at least part of the above mentioned object, a fuel cell system of the present invention is provided. The fuel cell system has a fuel cell; an insulation resistance measuring unit configured to measure insulation resistance between the fuel cell and an external conductor; and a control unit configured to control power generation status of the fuel cell, wherein the insulation resistance measuring unit performs measurement of the insulation resistance under condition in which the control unit maintains the fuel cell in a stable state such that fluctuation of output voltage of the fuel cell is within a predetermined permissible range. 
     With this arrangement, measurement of insulation resistance is carried out in a steady state in which fluctuation of output voltage that may cause insulation resistance measurement error lies within a predetermined permissible range. As a result, the accuracy of measurement of insulation resistance can be improved further. 
     The present invention may be reduced to practice in various modes, for example, an insulation resistance measurement device and measurement method in a fuel cell system; a control device and control method for such a measurement device; a fuel cell employing such devices and methods; or an electric car having on-board a generator device utilizing such a fuel cell system and the fuel cell thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an electric car  10  as an embodiment of the present invention. 
         FIG. 2  is an illustration depicting measurement of insulation resistance of the fuel cell  100  by the insulation resistance measuring unit  340 . 
         FIG. 3  is a flowchart of the fuel cell  100  insulation resistance measuring routine in the First Embodiment. 
         FIG. 4(   a ) through  FIG. 4(   e ) are illustrations showing change over time in operating status of the fuel cell  100  in the First Embodiment. 
         FIG. 5  is a flowchart of the fuel cell  100  insulation resistance measurement routine in the Second Embodiment. 
         FIG. 6(   a ) through  FIG. 6(   e ) are illustrations showing operation in the output suspending mode of a fuel cell that is experiencing cross leakage. 
         FIG. 7  is an illustration showing the relationship of output current I FC  and output voltage V FC  of the fuel cell  100  before and after initiation of charging control. 
         FIG. 8  is a flowchart of the fuel cell  100  insulation resistance measurement routine in the Third Embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The best mode for carrying out the present invention is described in the following order. 
     A. First Embodiment: 
     B. Second Embodiment: 
     C. Third Embodiment: 
     D. Variations: 
     A. First Embodiment 
       FIG. 1  is a schematic diagram of an electric car  10  as an embodiment of the present invention. The electric car  10  has a fuel cell  100 , a fluid unit  200 , a power unit  300 , and a control unit  400 . The fuel cell  100  is composed of a plurality of stacked cells  102 . The fuel cell  100 , the fluid unit  200 , the power unit  300 , and the control unit  400  are installed on the car body  12  of the electric car  10 , which serves as an external conductor. 
     The fluid unit  200  includes an oxidant gas supply unit  210 , a cathode off-gas discharge unit  220 , a fuel gas supply unit  230 , a circulating pump  240 , an anode off-gas discharge unit  250 , and a coolant circulating unit  260 . 
     The oxidant gas supply unit  210  has an air pump  212 . This air pump  212  produces compressed air from the outside air. The compressed air so produced is supplied, via an oxidant gas supply line  214 , to the fuel cell  100  as the oxidant gas containing oxygen for use by the fuel cell  100 . The oxidant gas supplied to the fuel cell  100  is delivered to the cathodes of in cells  102  that make up the fuel cell  100 . At the cathode, the oxygen contained in the oxidant gas is consumed by the fuel cell reaction. The oxidant gas of reduced concentration of oxygen due to the fuel cell reaction (in general such a gas is called as “cathode off-gas”) is discharged from the fuel cell  100  to the cathode off-gas discharge unit  220 , via a cathode off-gas discharge line  222 . The cathode off-gas discharge unit  220  releases the cathode off-gas from the fuel cell  100  into the atmosphere. 
     The fuel gas supply unit  230  has a fuel gas tank  232 . This fuel gas tank  232  is filled with hydrogen gas used as the fuel gas. The hydrogen gas filling the fuel gas tank  232  is pressure-regulated by a pressure reduction device (not shown) provided to the fuel gas supply unit  230 . The pressure-regulated hydrogen gas is then supplied via a first fuel gas supply line  234  to a second fuel gas supply line  236 . The second fuel gas supply line  236  is also supplied with anode off-gas as discussed later, with the hydrogen gas being mixed with the anode off-gas and supplied to the fuel cell  100 . 
     The fuel gas supplied to the fuel cell  100  is delivered to the anodes in the cells  102 . At the anode, the hydrogen contained in the fuel gas is consumed by the fuel cell reaction. The fuel gas of reduced concentration of hydrogen due to the fuel cell reaction (in general such a gas is called as “anode off-gas”) is supplied to the circulating pump  240  via a first anode off-gas discharge line  242  and a first return line  244 . The circulating pump  240  then returns the anode off-gas to the second fuel gas supply line  236  via a second return line  246 . By returning the anode off-gas using the circulating pump  240 , the fuel gas is circulated through the second fuel gas supply line  236 , the fuel cell  100 , the first anode off-gas discharge line  242 , the first return line  244 , the circulating pump  240 , and the second return line  246 . 
     The anode off-gas discharge unit  250  is connected to the first anode off-gas discharge line  242  via a second anode off-gas discharge line  252 . When the impurity concentration of the circulating fuel gas has reached a certain level, the anode off-gas discharge unit  250  will release the anode off-gas into the atmosphere as needed. At this time, the anode off-gas discharge unit  250  carries out a deactivation process by combusting the hydrogen contained in the anode off-gas. 
     The coolant circulating unit  260  has a radiator  262  and a coolant pump  264 . The coolant pump  264  supplies coolant to the fuel cell  100 . As the coolant supplied to the fuel cell  100  flows through the coolant flow passages provided within the fuel cell  100 , the coolant absorbs the heat produced by the fuel cell reaction from the cells  102 . The coolant, which is at elevated temperature having absorbed the heat, is then supplied to the radiator  262 . The coolant supplied to the radiator  262  drops in temperature through radiation of heat into the atmosphere. The coolant whose heat has been radiated by the radiator  262  is supplied to the coolant pump  264 , whereby the coolant is circulated between the coolant circulating unit  260  and the fuel cell  100 . 
     Ions elute into the circulating coolant from the walls of the coolant flow passages. Thus, the ion concentration of the coolant will increase over time, and the electrical conductivity of the coolant rises. During this time that the coolant flows through the coolant flow passages within the fuel cell  100 , it comes into contact with the cells  102  that make up the fuel cell  100 . As the conductivity of the coolant contacting the cells  102  rises, the electrical current generated by the cells  102  flows through the coolant, so that the power generated by the cells  102  can no longer be drawn effectively. Moreover, if the coolant is decomposed by the electrical current flowing through the coolant, there is a risk that transfer of the heat produced by the cells to the coolant is impaired by bubbles formed within the coolant flow passages and that cooling of the fuel cell  100  becomes insufficient. 
     The coolant contacts both the cells  102  of the fuel cell  100  and the radiator  262 . Since the radiator  262  is typically connected electrically to the car body  12 , as the conductivity of the coolant rises, the insulation resistance between the fuel cell  100  and the car body  12  falls. Accordingly, the First Embodiment is configured so as to detect a decline in insulation resistance between the fuel cell  100  and the car body  12  (hereinafter termed simply “insulation resistance”), and detect a rise in conductivity of the coolant. 
     The power unit  300  includes a DC voltmeter  312 , an output switch  314 , a secondary cell  320 , a high voltage load  330 , and an insulation resistance measuring unit  340 . The high voltage load  330  has a converter  332 , a high voltage component  334 , and an inverter  336 . 
     The fuel cell  100  is connected to two lines  20 ,  22  provided to the power unit  300 . The DC voltmeter  312 , which measures the output voltage of the fuel cell  100 , is connected between the two lines  20 ,  22 . The line  22  connected to the fuel cell  100  is connected to a line  24  via the output switch  314 . Between the line  20  and the line  24  are parallel-connected the converter  332 , to which the converter  332  is connected; the high voltage component  334 ; and the inverter  336 . 
     A remaining capacity monitor  322  for detecting the remaining capacity of the secondary cell  320  is provided to the secondary cell  320 . As the remaining capacity monitor  322 , it is possible to use, for example, a voltage sensor or an SOC meter that cumulates charge/discharge current in the secondary cell  320  over time. 
     The converter  332  converts the voltage of the secondary cell  320  and set a target voltage Vt across the line  22  and the line  24 . With the output switch  314  connected (the ON state), the output current of the fuel cell  100  is regulated to the set voltage Vt across the line  22  and the line  24  set by the converter  332 . The connected state of the output switch  314  and control of the output current of the fuel cell  100  will be discussed later. 
     The high voltage component  334  uses the power supplied via the two lines  22 ,  24  without voltage conversion. The high voltage component  334  includes, for example, motors (not shown) for respectively driving the air pump  212 , the circulating pump  240 , and the coolant pump  246 , as well as the air conditioning unit provided to the electric car  10 . 
     The inverter  336  converts the DC power supplied to the inverter  336  via the two lines  22 ,  24  to three-phase AC power, and supplies the converted power to the motor (not shown). The motor generates driving power for the electric car  10  using the power supplied by the inverter  336 . 
     The high voltage component  334  and the inverter  336  constitute the load of the fuel cell system composed of the fuel cell  100 , the fluid unit  200 , the power unit  300 , and the control unit  400 . 
     The insulation resistance measuring unit  340  is connected on the line  20  of the power unit  300 . The insulation resistance measuring unit  340  measures insulation resistance between the fuel cell  100  and the car body  12 . Measurement of insulation resistance by the insulation resistance measuring unit  340  will be discussed later. 
     The control unit  400  is configured as a microcomputer equipped with a CPU, ROM, RAM, a timer, and so on. The control unit  400  acquires various types of signals such as output signals from the DC voltmeter  312  and the remaining capacity monitor  322 ; ON/OFF signals from the start switch of the electric car  10 ; and control signals for shift position and accelerator opening of the electric car. Various control processes are executed on the basis of these signals, and drive signals are output to the various components that make up the fluid unit  200  and the power unit  300 . 
     The control unit  400  acquires the insulation resistance measurement output by the insulation resistance measuring unit  340 . In the event that the acquired insulation resistance measurement is smaller than a predetermined minimum value for insulation resistance, it will be determined that coolant conductivity has risen. In the event of a determination that coolant conductivity has risen, the control unit  400  displays an alert prompting replacement of the coolant on the display panel (not shown) of the electric car  10 . 
       FIG. 2  is an illustration depicting measurement of insulation resistance of the fuel cell  100  by the insulation resistance measuring unit  340 . The circuit shown in  FIG. 2  is equivalent to a circuit composed of the fuel cell  100  and the power unit  300  shown in  FIG. 1 . In  FIG. 2 , insulation resistance between the fuel cell  100  and the electric car  10  ( FIG. 1 ) is shown as simple insulation resistance Rx. 
     The insulation resistance measuring unit  340  includes an AC power supply  342 , a sensing resistor Rs, a capacitor Cs, a band-pass filter (BPF)  344 , and an AC voltmeter  346 . The band-pass filter  344  is a band-pass filter having a center frequency equal to the oscillation frequency of the AC power supply  342 . Noise reaching the AC voltmeter  346  is reduced by this band-pass filter  344 . 
     As is apparent from  FIG. 2 , in the event that impedance of the capacitor Cs is sufficiently low at the oscillation frequency fs of the AC power supply  342  and the output voltage of the fuel cell  100  does not fluctuate, the resistance value Rx of insulation resistance can be derived from the measured signal voltage Vs of the AC power supply  342 , the sensed voltage Vm by the AC voltmeter  346 , and the resistance value Rs of sensing resistor, using the following expression. 
         Rx=Rs×Vm /( Vs−Vm )  (1) 
     The resistance value Rs of sensing resistor and the measured signal voltage Vs of the AC power supply  342  are pre-established values. Thus, the resistance value Rx of insulation resistance can be computed from the sensed voltage Vm by the AC voltmeter  346 . 
     When the output voltage of the fuel cell  100  fluctuates, the voltage on the line  20  fluctuates in response to the fluctuation in output voltage. Where the voltage fluctuation on the line  20  includes an AC component of frequency close to the oscillation frequency fs of the AC power supply  342  (hereinafter simply called the “AC component”), the AC component of the voltage on the line  20  passes through the band-pass filter  344  and reaches the AC voltmeter  346 . If the AC component of the voltage on the line  20  is applied to the AC voltmeter  346  in this way, the sensed voltage Vm fluctuates, and the resistance value of calculated insulation resistance differs from the actual resistance value Rx. Accordingly, in the First Embodiment, measurement of insulation resistance is carried out in condition with the fuel cell  100  maintained in a stable state wherein fluctuation of the output voltage V FC  of the fuel cell  100  remains within a predetermined permissible range. The predetermined permissible range for fluctuation of the output voltage V FC  can be calculated with reference to the configuration of the insulation resistance measuring unit  340  and the value of sensed insulation resistance, in such a way as to inhibit insulation resistance measurement error caused by the AC component of the output voltage V FC . 
       FIG. 3  is a flowchart of the fuel cell  100  insulation resistance measuring routine in the First Embodiment. This insulation resistance measuring routine is executed at predetermined intervals during operation of the electric car  10 , for example. 
       FIG. 4(   a ) through  FIG. 4(   e ) are illustrations showing change over time in operating status of the fuel cell  100  in the First Embodiment. The horizontal axis of each of the graphs shown in  FIG. 4  represents time. The vertical axis of the graph in  FIG. 4  ( a ) represents the operating mode of the fuel cell  100 . The vertical axis of the graph in  FIG. 4  ( b ) represents supply status of oxidant gas and fuel gas (hereinafter collectively referred to as “reactant gases”) to the fuel cell  100 . The vertical axis of the graph in  FIG. 4  ( c ) represents the connection status of the output switch  314  ( FIG. 1) . The solid line in the graph of  FIG. 4  ( d ) indicates temporal change in the output voltage V FC  of the fuel cell  100 , and the broken line indicates the set voltage Vt between the two lines  22 ,  24  ( FIG. 1 ) set by the converter  332  ( FIG. 1 ). The vertical axis of the graph in  FIG. 4  ( e ) represents the output current I FC  of the fuel cell  100 . 
     In Step S 100  of  FIG. 3 , the control unit  400  determines whether the fuel cell  100  is operating in an output suspending mode (described later) associated with stable output voltage. If the controller determines that operating mode of the fuel cell  100  is not the output suspending mode, control is returned to Step S 100 . Step S 100  is subsequently repeated until the operating mode of the fuel cell  100  goes to the output suspending mode. 
     In the example of  FIG. 4(   a ) through  FIG. 4(   e ), the fuel cell  100  is operating in normal operating mode prior to time to. As shown in  FIG. 4  ( b ), in normal operating mode, the reactant gases are supplied to the fuel cell  100 . As shown in  FIG. 4  ( c ), the output switch  314  at this time is maintained in the ON state, supplying the power generated by the fuel cell  100  to the high voltage load  330  ( FIG. 1) . Since the output switch  314  is in the ON state, the output voltage V FC  of the fuel cell  100  is equivalent to the set voltage Vt set by the converter  332 . This set voltage Vt is regulated according to the power requirements of the high voltage load  330 . As shown in  FIGS. 4  ( d ) and ( e ), the output current I FC  of the fuel cell  100  declines in association with higher output voltage V FC , and rises in association with lower output voltage V FC . 
     With the fuel cell  100  in normal operating mode in this way, there is a risk of error in measurement of insulation resistance, caused by fluctuation of the output voltage V FC  of the fuel cell  100 . Accordingly, in the First Embodiment, since Step S 100  of  FIG. 3  is executed repeatedly, measurement of insulation resistance of the fuel cell  100  is not executed until the fuel cell  100  goes into the output suspending mode. 
     In the example of  FIG. 4(   a ) through  FIG. 4(   e ), next, the operating status of the fuel cell  100  switches from normal operating mode to the output suspending mode at time to. Then, from time to time t 1 , the operating status of the fuel cell  100  is maintained in the output suspending mode. Operation of the fuel cell  100  in the output suspending mode takes place, for example, in the event that the secondary cell  320  ( FIG. 1)  has high remaining capacity and the power requirement of the high voltage load  330  is low. 
     The output suspending mode is an operating mode of the fuel cell  100  wherein, with the fuel cell system running, power generation by the fuel cell  100  is suspended temporarily. During operation in the output suspending mode, the control unit  400  and the high voltage load  330  are kept running through power supplied by the secondary cell  320 . Operation of the fuel cell  100  in this output suspending mode is also typically referred to as intermittent operation. 
     As shown in  FIG. 4  ( b ), in the output suspending mode, the supply of reactant gases to the fuel cell  100  is suspended. Specifically, the control unit  400  suspends driving of the air pump  212  ( FIG. 1 ) and the circulating pump  240  ( FIG. 1 ), as well as suspending the supply of hydrogen gas from the fuel gas supply unit  230 , and discharge of anode off-gas to the outside from the anode off-gas discharge unit  250 . In addition to suspending the supply of reactant gases, the control unit  400  turns the output switch  314  to OFF. With output switch  314  in the OFF state, the output current I FC  of the fuel cell  100  goes to zero, so the output voltage V FC  of the fuel cell  100  goes to open circuit voltage OCV. In the event that the fuel cell  100  is not in operation during the output suspending mode, the converter set the set voltage Vt, for example, to the voltage at the terminals of the secondary cell  320 , so as to avoid loss in the power unit  300 . 
     As shown by the flowchart in  FIG. 3 , when the fuel cell  100  is in the output suspending mode, control passes from Step S 100  to Step S 110 . In Step S 110 , the control unit  400  issues an instruction to the insulation resistance measuring unit  340 , to begin measuring insulation resistance. Once measurement of insulation resistance has been completed, the insulation resistance measurement routine shown in  FIG. 3  terminates. 
     In the example of  FIG. 4(   a ) through  FIG. 4(   e ), measurement of insulation resistance begins at time t S . Measurement of insulation resistance continues for a predetermined time interval T M  (e.g. 30 seconds) in order to suppress error caused by noise or other factors. During the period from time t S  to time t E  (t S +T M ) the output switch  314  is in the OFF state, so the output voltage V FC  of the fuel cell  100  is maintained substantially at open circuit voltage OCV. Thus, error in measurements of insulation resistance caused by fluctuation in the output voltage V FC  of the fuel cell  100  can be suppressed. 
     In the example of  FIG. 4  ( a ) through  FIG. 4(   e ), the operating status of the fuel cell  100  is switched from the output suspending mode to the normal operating mode at time t 1 . As shown in  FIG. 4  ( b ), at this time, the control unit  400  resumes supply of the reactant gases to the fuel cell  100 . Together with resumption of supply of the reactant gases, the control unit  400  places the output switch  314  in the ON state. When the output switch  314  assumes the ON state, the output voltage V FC  of the fuel cell  100  rises to the set voltage Vt set by the converter  332 . Beginning at time t 1 , the output current I FC  of the fuel cell  100  changes in response to changes in the output voltage V FC , in the same way as prior to time t 0 . 
     In this way, in the First Embodiment, measurement of insulation resistance of the fuel cell  100  takes place during an interval in which the operating status of the fuel cell  100  is the output suspending mode. For the interval of the output suspending mode, the output voltage V FC  of the fuel cell  100  is substantially at open circuit voltage OCV. Thus, error in insulation resistance measurements caused by fluctuation of the output voltage V FC  of the fuel cell  100  can be suppressed. 
     B. Second Embodiment 
       FIG. 5  is a flowchart of the fuel cell  100  insulation resistance measurement routine in the Second Embodiment. The insulation resistance measurement routine of the Second Embodiment shown in  FIG. 5  differs from the insulation resistance measurement routine of the First Embodiment shown in  FIG. 3 , in that there is an additional Step S 200  for determining whether the fuel cell  100  is operable in the output suspending mode and Steps S 210  to S 250  for measuring insulation resistance in an operating status other than the output suspending mode. 
     In Step S 200 , the control unit  400  determines whether the fuel cell  100  is operable in the output suspending mode. In the event of a determination that the fuel cell  100  is operable in the output suspending mode, control passes to Step S 100 . Insulation resistance is then measured in the output suspending mode, in the same manner as in the First Embodiment. On the other hand, if it is determined that the fuel cell  100  is not operable in the output suspending mode, control passes to Step S 210 . 
     The determination as to whether the fuel cell  100  is operable in the output suspending mode is made by determining whether the drop in the output voltage V FC  of the fuel cell  100  observed during the fuel cell  100  is operated for a predetermined length of time under conditions identical to those during execution of the output suspending mode exceeds a certain predetermined limit. In the event that the drop in output voltage V FC  exceeds the predetermined limit, there is risk of damage to the fuel cell  100 , the fluid unit  200 , and/or the power unit during switching from the output suspending mode to normal operating mode. Therefore it is determined that the cell should not be operated in the output suspending mode. An example of a fuel cell that should not be operated in the output suspending mode is a fuel cell  100  whose electrolyte membrane has deteriorated to the point that there is leakage of hydrogen from the anode to the cathode (cross leakage). 
       FIG. 6(   a ) through  FIG. 6(   e ) are illustrations showing operation in the output suspending mode of a fuel cell that is experiencing cross leakage.  FIG. 6(   a ) through  FIG. 6(   e ) differ from  FIG. 4(   a ) through  FIG. 4(   e ) in that the temporal change in the output voltage V FC  shown by the solid line in  FIG. 6  ( d ) differs from the temporal change in the output voltage V FC  shown by the solid line in  FIG. 4  ( d ). In other respects it is the same as  FIG. 4(   a ) through  FIG. 4(   e ). 
     As mentioned above, in the output suspending mode, the air pump  212  ( FIG. 1 ) is stopped, suspending the supply of oxidant gas to the fuel cell  100 . When the supply of oxidant gas is suspended, hydrogen leaking from the anode to the cathode due to cross leakage collects on the cathode side of the electrolyte membrane. As hydrogen collects on the cathode side of the electrolyte membrane, the oxygen concentration declines on the cathode side of the electrolyte membrane and the output voltage V FC  of the fuel cell drops below the open circuit voltage OCV. 
     In the example of  FIG. 6(   a ) through  FIG. 6(   e ), the output voltage V FC  of the fuel cell drops gradually beginning at the time to of switching from the normal operating mode to the output suspending mode. Then, at the time t 1  of switching from the output suspending mode to the normal operating mode, the output voltage V FC  goes to lower voltage than the set voltage Vt set by the converter  332 . When the output switch  314  is switched ON with the output voltage V FC  lower than the set voltage Vt in this way, reverse current flows into the fuel cell, and there is a possibility that the fuel cell is damaged by the reverse current. 
     Accordingly, in the Second Embodiment, there is executed a inspecting mode in which the supply of reactant gases is suspended and the output switch  314  is turned OFF, in the same manner as when the output suspending mode is executed. Then, using the DC voltmeter  312  ( FIG. 1 ), the output voltage V FC  is measured at a point in time after a predetermined time interval T has elapsed from the start of the inspecting mode. In the event that the difference between the output voltage V FC  and the open circuit voltage OCV, this difference being equivalent to the drop in the output voltage V FC  from the start of the inspecting mode, is greater than a certain predetermined limit δV, it is determined that the fuel cell should not be operated in the output suspending mode. Then, after the predetermined time interval T has elapsed since start of the inspecting mode, the fuel cell is switched from the inspecting mode back to the normal operating mode. The predetermined time interval T and the predetermined limit δV may be established appropriately from experimentally-derived values so as to enable determination as to operability in the output suspending mode, and to avoid any damage to the fuel cell etc. when determining the operability in the output suspending mode. 
     In Step S 210  of  FIG. 5 , the control unit  400  acquires the remaining capacity of the secondary cell  320  ( FIG. 1 ) and the power requirement of the high voltage load  330  ( FIG. 1 ), respectively. The remaining capacity of the secondary cell  320  is acquired by reading the output signal of the remaining capacity monitor  322 . The power requirement is calculated from the control signals for shift position, accelerator opening, etc. of the electric car  10 . 
     In Step S 220 , on the basis of the acquired secondary cell  320  remaining capacity and the power requirement of the high voltage load  330 , the control unit  400  determines whether charging of the secondary cell  320  is possible given the current status of the electric car  10 . Specifically, in the event that the remaining capacity of the secondary cell  320  is smaller than a predetermined threshold for remaining capacity and the power requirement of the high voltage load  330  is greater than a predetermined threshold for power, it is determined that charging is not possible. In the event of a determination that charging of the secondary cell  320  is not possible, control returns to Step S 210 , and Steps S 210  and S 220  are repeated until charging of the secondary cell  320  becomes possible. On the other hand, in the event of a determination that charging of the secondary cell  320  is possible, control passes to Step S 230 . 
     In Step S 230 , the control unit  400  initiates control for charging the secondary cell  320  (charging control). Specifically, the output current I FC  of the fuel cell  100  is increased by setting the set voltage Vt set by the converter  332  ( FIG. 1 ) to a level lower than a target voltage set with reference to the power requirement of the high voltage load  330 . By reducing the set voltage Vt in this way, the fuel cell  100  outputs power in excess of the level of power required by the high voltage load  330 , and this extra power can be used to charge the secondary cell  320 . 
       FIG. 7  is an illustration showing the relationship of output current I FC  and output voltage V FC  of the fuel cell  100  before and after initiation of charging control. A chargeable state is a state in which the power requirement of the high voltage load  330  is smaller than the predetermined power threshold, and thus in the state prior to initiating charging control, the output current I FC  will be low current I 1 . At this time, if the power requirement of the high voltage load  330  fluctuates and the output current I FC  fluctuates by ΔI, the output voltage V FC  fluctuates by ΔV 1 . 
     When charging control is executed and electrical current for charging the secondary cell  320  is drawn, the output current I FC  rises and reaches a current value I 2 . In this state, if the output current I FC  fluctuates by ΔI, the output voltage V FC  fluctuates by ΔV 2  which is smaller than the ΔV 1  observed prior to charging control. In this way, as the output current I FC  increases through execution of charging control, fluctuation of output voltage with respect to a given fluctuation ΔI of the output current declines from ΔV 1  to ΔV 2 . 
     In Step S 240  of  FIG. 5 , the control unit  400  begins to measure insulation resistance. As mentioned above, variation of the output voltage V FC  with respect to variation of the output current I FC  becomes smaller. Thus, error in the insulation resistance measurements taken in Step S 230  is smaller than errors in the absence of charging control. Moreover, in Step S 240 , since charging control continues until measurement of insulation resistance has been completed, it is preferable for the upper limit of remaining capacity for halting charging of the secondary cell  320  to be higher than that in the normal state. Furthermore, of components of the high voltage component  334  ( FIG. 1 ), it is preferable to halt operation of those components which it is possible to do so, in order to reduce the fluctuation ΔI of the output current. 
     In Step S 250 , the control unit  400  terminates execution of charging control. Execution of charging control is terminated by setting the set voltage Vt set by the converter  332  to a value established with reference to the power requirement of the high voltage load  330 . Then, after Step S 250 , the insulation resistance measurement routine terminates. 
     In this way, in the Second Embodiment as well, fluctuation of the output voltage V FC  in association with fluctuation of the output current I FC  are suppressed. Thus, error in insulation resistance measurements caused by fluctuation of the output voltage V FC  can be suppressed. 
     The Second Embodiment is preferable to the First Embodiment in that error in insulation resistance measurements can be reduced even in instances where it would be undesirable to operate the fuel cell  100  in the output suspending mode. On the other hand, the First Embodiment is preferable to the Second Embodiment in that control for the purpose of measuring insulation resistance is easier. 
     In the Second Embodiment, the determination as to possibility of charging control is made on the basis of both the remaining capacity of the secondary cell  320  and the power requirements of the high voltage load  330 . It is also possible to determine whether charging control is possible on the basis of the remaining capacity of the secondary cell  320  only, for example. In this case as well, it is possible, by carrying out charging control, to reduce fluctuation of the output voltage V FC  in association with fluctuation of the output current I FC , and thus error in insulation resistance measurements can be suppressed. 
     Moreover, whereas in the Second Embodiment, insulation resistance is measured with the fuel cell  100  in operation in the output suspending mode in instances where the fuel cell is operable in the output suspending mode. It is also possible to measure insulation resistance while carrying out charging control by default, without making a determination as to whether the fuel cell  100  is operable in the output suspending mode. With this approach as well, fluctuation of the output voltage V FC  in association with fluctuation of the output current I FC  will be suppressed so that error in insulation resistance measurements caused by fluctuation of the output voltage V FC  can be suppressed. 
     In the Second Embodiment, the output current I FC  of the fuel cell  100  is increased by executing charging control, in order to set the output current I FC  in an electrical current range such that the level of change in output voltage relative to the level of change in output current is small. Other methods to increase the output current I FC  may also be used. For example, it is acceptable to increase the output current I FC  by running all of the components included in the high voltage component  334  ( FIG. 1 ) in order to maximize power consumption by the high voltage component  334 . In this manner as well it is possible to increase the output current I FC  and set the output current I FC  to within an electrical current range such that the level of change in output voltage relative to the level of change in output current is small. 
     Moreover, in the Second Embodiment, a inspecting mode is executed for determining whether the output suspending mode is executable. However, the inspecting mode may be omitted. In this case, the output voltage V FC  is measured during execution of the output suspending mode, and in the event that the difference between the output voltage V FC  and the open circuit voltage OCV becomes greater than a predetermined limit, execution of the output suspending mode is interrupted. After interrupting execution of the output suspending mode, measurement of insulation resistance is carried out while executing charging control. 
     C. Third Embodiment 
       FIG. 8  is a flowchart of the fuel cell  100  insulation resistance measurement routine in the Third Embodiment. The insulation resistance measurement routine of the Third Embodiment shown in  FIG. 8  differs from the insulation resistance measurement routine of the Second Embodiment shown in  FIG. 5  in that there is an additional Step S 300  preceding Step S 200 . In other respects, it is the same as the insulation resistance measurement routine of the Second Embodiment. 
     In Step S 300 , the control unit  400  determines whether insulation resistance has already been measured subsequent to startup of the fuel cell  100 . In the event that insulation resistance has not been measured yet, control passes to Step S 200  and insulation resistance is measured in the same manner as in the insulation resistance measurement routine of the Second Embodiment. If on the other hand insulation resistance has been measured already, the insulation resistance measurement routine shown in  FIG. 8  terminates. 
     Specifically, when the power switch of the electric car  10  is turned from OFF to ON, the control unit  400  resets a flag indicating that insulation resistance measurement has taken place. Then, when insulation resistance is measured, the insulation resistance measurement flag is set. In Step S 300 , if the insulation resistance measurement flag is set, it is determined that insulation resistance measurement has taken place and the insulation resistance routine terminates. 
     In the Third Embodiment, insulation resistance is measured only once during the period from startup to shutdown of the electric car  10  (such a duration is called “one trip”). Since the conductivity of the coolant typically rises only gradually over time, measuring insulation resistance a single time during one trip may be sufficient to prevent damage caused by a rise in coolant conductivity. 
     Moreover, whereas in the Third Embodiment the insulation resistance measurement flag is reset when the power switch of the electric car  10  is turned from OFF to ON, the insulation resistance measurement flag may be reset at predetermined time intervals, at predetermined travel distance intervals, or at intervals after generation of a predetermined amount of power, for example. It will be possible to prevent damage caused by a rise in coolant conductivity in this way as well. 
     D. Variations 
     The foregoing description of the present invention based on certain preferred embodiments is provided for illustration only and not for the purpose of limiting the invention, and various modifications such as the following can be made herein without departing from the scope of the invention. 
     D1. Variation 1: 
     In the embodiments hereinabove, measurement of insulation resistance is carried out with the fuel cell maintained in a stable state by executing either the output suspending mode or charging control. In general, measurement of insulation resistance can be carried out in any stable state in which fluctuation of output voltage V FC  is kept within a predetermined permissible range. For example, such a stable state could be achieved by using power from the secondary cell  320  to compensate for fluctuation in power requirements and suppress fluctuation in output current I FC  of the fuel cell  100 . As is apparent from the preceding description, stable states in which fluctuation of output voltage V FC  is within a predetermined permissible range refer to those including states in which output voltage V FC  does not fluctuate, such as a state where the fuel cell  100  is operating in the output suspending mode. 
     D2. Variation 2: 
     In the embodiments hereinabove, a secondary cell  320  is used as the secondary power supply employed together with the fuel cell  100 . Any rechargeable electrical storage device could be used as the secondary power supply. It is possible to use a capacitor as the electrical storage device. 
     D3. Variation 3: 
     In the embodiments hereinabove, insulation resistance is measured between the fuel cell  100  and the car body  12  of the electric car  10  using the insulation resistance measurement technology of the present invention. The invention is also applicable generally to measurement of insulation resistance between the fuel cell  100  and a conductor disposed to the outside of the fuel cell  100  (external conductor). It is possible to employ the present invention for measuring insulation resistance between a metal unit of the radiator  262  ( FIG. 1 ) and the fuel cell  100 , for example. 
     D4. Variation 4: 
     In the embodiments hereinabove, the insulation resistance measurement technology of the present invention is applied to a water-cooled fuel cell system. The insulation resistance measurement technology of the present invention can also be implemented in fuel cell systems that do not use coolant. In this case, electrical leakage from the fuel cell can be detected by detecting a drop in insulation resistance of the fuel cell. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to measurement of insulation resistance in fuel cell systems employing fuel cells of various kinds.