Patent Publication Number: US-11380913-B2

Title: Fuel cell system, method of controlling fuel cell system, and fuel cell system-mounted vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 15/909,752, filed Mar. 1, 2018, which claims priority to Japanese Patent Application No. 2017-69608, filed Mar. 31, 2017, the disclosures of each of which are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a fuel cell system, a method of controlling a fuel cell system, and a fuel cell system-mounted vehicle. 
     BACKGROUND ART 
     In a fuel cell system, a high voltage electrical system including a fuel cell and a high voltage circuit driving an electromotor is isolated from surrounding components in contact with the fuel cell system or a supporting body supporting the high voltage electrical system. A cooling circuit cooling the fuel cell is not a current circuit, and thus the insulation thereof from the supporting body is not considered generally. Therefore, the increase of conductivity of cooling liquid may decrease insulation of the fuel cell system through the cooling circuit. For example, it is known that when the fuel cell system is stopped, metal ion is eluted into cooling liquid from metal parts forming a cooling circuit cooling a fuel cell, and the metal ion increases conductivity of the cooling liquid. In view of such a problem, a technology of removing metal ion in cooling liquid during operation of a fuel cell and reducing conductivity of the cooling liquid is proposed. 
     However, in the conventional technology, the process for reducing conductivity of cooling liquid is performed after the fuel cell is started without considering insulation decrease through paths other than a cooling circuit, e.g., insulation decrease through a high voltage circuit. When the insulation decrease is occurred on a path other than a cooling circuit, a current path electrically connecting the cooling circuit and the high voltage circuit in the outside of the fuel cell system may be formed. Thus, when the fuel cell is started, a high voltage may be applied on the outer current path. 
     SUMMARY 
     Therefore, it is desired to start, in a fuel cell system, a fuel cell without forming a high voltage path connecting a cooling circuit and a high voltage circuit in the outside of the fuel cell system. 
     Solution to Problem 
     The present disclosure is made to solve the above-described problems, and may be achieved by the following aspects. 
     The first aspect provides a fuel cell system. A fuel cell system according to the first aspect includes a fuel cell, a high voltage circuit configured to drive an electromotor, a relay disposed between the fuel cell and the high voltage circuit, the relay configured to electrically connect or block the fuel cell to or from the high voltage circuit, a cooling circuit provided with the fuel cell, the cooling circuit including cooling liquid to cool the fuel cell, a conductivity reduction unit disposed on the cooling circuit, the conductivity reduction unit configured to reduce conductivity of the cooling circuit, a storage unit that stores insulation decrease information specifying an insulation decrease occurred region in the fuel cell system, and a control unit that obtains the insulation decrease information upon receipt of a request for starting the fuel cell, and perform, when the specified insulation decrease occurred region is not a fuel cell region including the fuel cell and the cooling circuit, conductivity reduction process on the cooling liquid using the conductivity reduction unit before having the relay connect, and has the relay connect after completing the conductivity reduction process. 
     In the fuel cell system according to the first aspect, when the specified insulation decrease occurred region is not a fuel cell region including the fuel cell and the cooling circuit, the control unit performs conductivity reduction process on the cooling liquid using the conductivity reduction unit before having the relay connect, and has the relay connect after completing the conductivity reduction process. Therefore, it is possible to start the fuel cell without forming a high voltage path connecting the cooling circuit and the high voltage circuit in the outside of the fuel cell system. 
     In the fuel cell system according to the first aspect, the control unit may have the relay connect without performing the conductivity reduction process when the specified insulation decrease occurred region is not the fuel cell region and a stop period of the fuel cell system is shorter than a predetermined period, or may perform the conductivity reduction process and has the relay connect after completing the conductivity reduction process when the specified insulation decrease occurred region is not the fuel cell region and the stop period of the fuel cell system is equal to or longer than the predetermined period. In such a case, even when the specified insulation decrease occurred region is not a fuel cell region, whether the conductivity reduction process is performed is determined considering conductivity of cooling liquid. Thus, it is possible to achieve both the quick start of the fuel cell and the prevention of formation of the high voltage path in the outside of the fuel cell system. 
     In the fuel cell system according to the first aspect, the control unit may have the relay connect without performing the conductivity reduction process when the specified insulation decrease occurred region is the fuel cell region. In such a case, it is possible to quickly start the fuel cell without forming the high voltage path in the outside of the fuel cell system. 
     The fuel cell system according to the first aspect further includes a secondary battery connected to the high voltage circuit, in which the control unit may drive, when the conductivity reduction process is performed, the electromotor using the secondary battery before completing the conductivity reduction process, and have the relay connect after completing the conductivity reduction process to drive the electromotor using the fuel cell. In such a case, it is possible to drive the electromotor even during the conductivity reduction process and prevent formation of the high voltage circuit in the outside of the fuel cell system. 
     In the fuel cell system according to the first aspect, the cooling circuit includes a cooling liquid pump allowing circulation of the cooling liquid, and the control unit may perform the conductivity reduction process by driving the cooling liquid pump using power of the secondary battery and letting the cooling liquid flow to the conductivity reduction unit. In such a case, it is possible to perform conductivity reduction process of cooling liquid without starting the fuel cell. 
     In the fuel cell system according to the first aspect, the insulation decrease information further includes information of whether insulation decrease is occurred, and the control unit may perform, when the insulation decrease information indicates occurrence of insulation decrease, the conductivity reduction process in accordance with the insulation decrease occurred region, or has the relay connect without performing the conductivity reduction process when the insulation decrease information does not indicate occurrence of insulation decrease. In such a case, it is possible to perform activation process of the fuel cell system considering whether the insulation decrease is occurred. 
     The fuel cell system according to the first aspect further includes an insulation detection device that detects insulation decrease in the fuel cell system, in which the control unit may specify whether a detected insulation decrease occurred region is the fuel cell region or a region other than the fuel cell region, generate the insulation decrease information, and store the insulation decrease information in the storage unit. In such a case, it is possible to generate information for starting the fuel cell without forming the high voltage path in the outside of the fuel cell system in the next activation of the fuel cell system. 
     In the fuel cell system according to the first aspect, the control unit may specify the insulation decrease occurred region, generate the insulation decrease information, and store the insulation decrease information in the storage unit when the fuel cell system is stopped. In such a case, it is possible to generate information for starting the fuel cell without forming the high voltage path in the outside of the fuel cell system in the next activation of the fuel cell system. 
     The second aspect provides a vehicle. A vehicle according to the second aspect includes the fuel cell system according to the first aspect, in which the high voltage circuit, the fuel cell, and the cooling circuit are supported by the vehicle, and the insulation decrease is insulation decrease between a vehicle body of the vehicle and the fuel cell system. In the vehicle according to the second aspect, it is possible to start the fuel cell without forming the high voltage path connecting the cooling circuit and the high voltage circuit in the vehicle body of the vehicle. 
     The third aspect provides a method of controlling a fuel cell system with a fuel cell. A method of controlling a fuel cell system with a fuel cell according to the third aspect includes receiving a request for starting the fuel cell, obtaining insulation decrease information specifying an insulation decrease occurred region in the fuel cell system, and performing, when the specified insulation decrease occurred region is not a fuel cell region including the fuel cell and a cooling circuit with cooling liquid circulating therein, conductivity reduction process on the cooling liquid before having a relay connect, which is arranged between a high voltage circuit for driving an electromotor and the fuel cell, and performing start process including having connection of the relay connect after completing the conductivity reduction process. 
     In the method of controlling a fuel cell system according to the third aspect, it is possible to obtain the same action effects as the fuel cell system according to the first aspect. Moreover, the method of controlling a fuel cell system according to the third embodiment may be achieved by various aspects similarly to the fuel cell system according to the first aspect. Furthermore, the method of controlling a fuel cell system according to the third aspect may be also achieved as a computer program or a recording medium readable by a computer storing the computer program. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a schematic configuration of a fuel cell system applicable to each embodiment in common. 
         FIG. 2  is a block diagram illustrating a control unit applicable to each embodiment in common. 
         FIG. 3  is an explanatory diagram for explaining an insulation decrease occurred region in a fuel cell system according to a first embodiment. 
         FIG. 4  is a flowchart illustrating a process routine of insulation decrease detection process performed in the fuel cell system according to the first embodiment. 
         FIG. 5  is a flowchart illustrating a process routine of system activation process performed in activation of the fuel cell system according to the first embodiment. 
         FIG. 6  is a flowchart illustrating a process routine of system activation process performed in activation of the fuel cell system according to a second embodiment. 
         FIG. 7  is a flowchart illustrating a process routine of system activation process performed in activation of the fuel cell system according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following will describe a fuel cell system, a method of controlling a fuel cell system, and a fuel cell system-mounted vehicle according to the present disclosure. 
     First Embodiment 
       FIG. 1  is an explanatory diagram illustrating a schematic configuration of a fuel cell system applicable to each embodiment in common.  FIG. 2  is a block diagram illustrating a control unit applicable to each embodiment in common. 
     A fuel cell system FCS according to the first embodiment includes a fuel cell  10 , a high voltage device for fuel cell  12 , a secondary battery  20 , a power control unit  21 , an auxiliary device control unit  22 , a system switch  30 , a driving motor  42 , a high voltage auxiliary device  43 , and a control unit  50 . The fuel cell system FCS includes a fuel cell region SE 1  including the fuel cell  10  and the high voltage device for fuel cell  12 , and a fuel cell outside region SE 2  including the secondary battery  20 , the power control unit  21 , the auxiliary device control unit  22 , the driving motor  42 , and the high voltage auxiliary device  43 . The fuel cell region SE 1  and the fuel cell outside region SE 2  are electrically connected to each other through a relay  41 . The power control unit  21  and the auxiliary device control unit  22  are high voltage circuits, and at least the power control unit  21  may be provided. The following will describe an example in which the fuel cell system FCS is mounted on a vehicle. 
     The fuel cell system FCS is insulated from a vehicle body BD when mounted on a vehicle. Therefore, it is regarded that insulation resistance IR 1  is arranged virtually between the fuel cell system FCS and the vehicle body BD. Moreover, the high voltage circuit forming the fuel cell system FCS, e.g., the power control unit  21  and the auxiliary device control unit  22  are also insulated from the vehicle body BD, or insulated from a low voltage circuit configuring the conventional electrical circuit. Therefore, it is regarded that insulation resistance IR 2  is arranged virtually between the high voltage circuit and the vehicle body BD and the low voltage circuit configuring the conventional electrical circuit. Note that it is also possible to regard that the fuel cell system FCS is grounded to the vehicle body BD. In the following, a region to be insulated from the high voltage circuit is collectively referred to as the vehicle body BD. 
     The fuel cell  10  is a solid polymer type fuel cell capable of generating power by electrochemical reaction between air as oxidization gas and hydrogen as fuel gas, for example. In the specification, oxidization gas and fuel gas are collectively referred to as reaction gas. The fuel cell  10  includes a cooling circuit  11  for cooling the fuel cell  10 . 
     The cooling circuit  11  includes a cooling pipe  111 , a heat exchanger  112 , an ion exchanger  113 , a switching valve  114 , and a cooling liquid pump  40 . The ion exchanger  113  is a conductivity reduction unit including therein ion exchange resin functioning as an ion removal filter, and removes conductivity ion in passing cooling liquid to reduce conductivity of the cooling liquid. 
     The cooling liquid pump  40  is a pump controlled by the control unit  50  to allow circulation of cooling liquid in the cooling pipe  111 , and is included in the high voltage auxiliary device. The cooling liquid pump  40  feeds cooling liquid from which heat has been removed by the fuel cell  10  to the heat exchanger  112 , that is, a radiator, or to the ion exchanger  113 , and introduces the cooling liquid cooled by the heat exchanger  112  or the cooling liquid subjected to conductive ion removing process by the ion exchanger  113  again to the fuel cell  10 . The switching valve  114  is a three-way valve controlled by the control unit  50  to switch a feeding destination of cooling liquid between the heat exchanger  112  and the ion exchanger  113 . The switching valve  114  switches a feeding destination of cooling liquid to the ion exchanger  113  when conductivity reduction process of cooling liquid is performed, while it switches a feeding destination of cooling liquid to the heat exchanger  112  when conductivity reduction process is not performed, that is, in normal operation. 
     The cooling circuit  11  is fixed to the vehicle body BD by conductive fastening tools such as metal bolts, nuts, for example, through a mounting frame. As cooling liquid, generally pure water or cooling liquid with low conductivity is used, and the cooling circuit  11  is substantially insulated electrically from the vehicle body BD. That is, insulation resistance IR 1  exists between the cooling circuit  11  and the vehicle body BD. However, it is known that when the vehicle is parked and the fuel cell system FCS system is stopped, metal ion is eluted into the cooling liquid from the cooling pipe  111  and the heat exchanger  112  generally formed of metal materials, the metal ion increases the conductivity of the cooling liquid. The increase of the conductivity of cooling liquid decreases insulation between the cooling circuit  11  and the vehicle body BD, that is, insulation resistance IR 1 . Then, there is performed, if necessary, conductivity reduction process of cooling liquid using the ion exchanger  113 . 
     The high voltage device for fuel cell  12  is a high voltage circuit for increasing an output voltage of the fuel cell  10  to a given voltage required to drive the driving motor  42 , and includes therein a step-up converter. DC power boosted by the high voltage device for fuel cell  12  is supplied to the power control unit  21 , converted into AC power, and supplied to the driving motor  42 . 
     The relay  41  is a relay controlled by the control unit  50  to mechanically block the fuel cell  10  from other electrical circuits of the fuel cell system FCS, and includes therein a first relay  41   a  and a second relay  41   b . When the relay  41  is connected, it is possible to drive the driving motor  42  using power generated by the fuel cell  10 . To be more specific, the first relay  41   a  and the second relay  41   b  are able to switch, by control signals from the control unit  50 , their positions to off-positions for performing electrical blocking or on-positions for performing electrical communication. When the relay  41  is turned on, the first relay  41   a  is turned on first and then the second relay  41   b  is turned on. The second relay  41   b  is provided with a precharge resistance for preventing and reducing a rush current when the relay is turned on. 
     The secondary battery  20  is connected to the power control unit  21  through the auxiliary device control unit  22 . The secondary battery  20  is connected to an insulation detection device  45 . As the secondary battery  20 , a lithium ion battery, a nickel hydrogen battery, or a capacitor may be used, for example. 
     The power control unit  21  is a control unit for mainly controlling the operation of the driving motor  42  that is a three-phase AC motor, and allows power operation or regenerative operation of the driving motor  42 . The power control unit  21  includes, for example, a control unit, an inverter for motor and a step-up/step-down converter for secondary battery that are controlled by the control unit, and a blocking gate for mechanically blocking electrical connection with the driving motor  42 . The power control unit  21  increases an output voltage of the secondary battery  20  to a driving voltage of the driving motor  42 . The power control unit  21  converts, in power operation, DC power from the fuel cell  10  boosted by the high voltage device for fuel cell  12  or DC power from the secondary battery  20  boosted by the step-up/step-down converter into AC power, and supplies it to the driving motor  42 . The power control unit  21  converts, in regenerative operation, AC power output from the driving motor  42  by the step-up/step-down converter into DC power, reduces a voltage, and supplies it to the secondary battery  20 . The AC power in regenerative operation may be used to drive the high voltage auxiliary device  43  that is an auxiliary device of the fuel cell  10 . 
     The auxiliary device control unit  22  receives power supply from the fuel cell  10  through the high voltage device for fuel cell  12  or power supply from the secondary battery  20 . The auxiliary device control unit  22  is provided with an inverter. The inverter converts DC power supplied from the fuel cell  10  or the secondary battery  20  into AC power to control an AC motor driving the high voltage auxiliary device  43 . The auxiliary device control unit  22  further includes a blocking gate for mechanically blocking electrical connection with the high voltage auxiliary device  43 . Although in  FIG. 1 , the cooling liquid pump  40  and the high voltage auxiliary device  43  are connected to one auxiliary device control unit  22  to simplify the illustration and explanation, the auxiliary device control unit  22  is arranged for each high voltage auxiliary device  43 . 
     The driving motor  42  is a three-phase AC motor for driving a vehicle, and the output thereof is controlled by the power control unit  21  having received, from the control unit  50 , control signals in accordance with a request from a driver input through an input unit such as an acceleration pedal. Other AC motors or DC motors may be used as the driving motor  42 . 
     The high voltage auxiliary device  43  is an auxiliary device driven by a motor operating at a high voltage generally equal to or higher than 100V, and includes, for example, an air compressor feeding air to the fuel cell  10 , and a hydrogen pump for allowing circulation of hydrogen gas in an anode gas supply system. Note that the cooling liquid pump  40  for circulation of cooling liquid in the cooling circuit  11  is also included in the high voltage auxiliary device. Meanwhile, a low voltage auxiliary device generally indicates an auxiliary device driven at a low voltage of 12 to 48V. 
     The insulation detection device  45  is a device for detecting insulation resistance in the fuel cell system FCS. To be specific, the insulation detection device  45  detects reduction of the above-described insulation resistance IR 1 , IR 2 . For example, the insulation detection device  45  applies an AC voltage with a predetermined frequency on a high voltage electrical system of the fuel cell system FCS, generates a voltage value reducing in accordance with insulation decrease of the high voltage electrical system, and outputs it to the control unit  50  as detection signals. Note that the high voltage electrical system indicates an electrical system to which high voltage power is supplied in the fuel cell system FCS that is defined by the fuel cell region SE 1  and the fuel cell outside region SE 2 . 
     The system switch  30  is a switch for activating or stopping the fuel cell system FCS, and corresponds to an ignition switch in an internal combustion engine vehicle. Note that the input by the system switch  30  may include accessory positions allowing operation of electric components such as an air conditioner, a navigation device, and an audio device, and on-positions allowing driving of the vehicle by the driving motor  42 . The positions may be switched to on-positions in operation of the system switch  30  with stepping of a brake pedal, or to on-positions when the system switch  30  is held down, for example. 
     The following will describe the control unit  50 . The control unit  50  includes a central processing unit (CPU)  500 , a memory  501 , an input/output interface  502 , a counter  503 , and an inner bus  504 . The control unit  50  is operated by power supply from a low voltage secondary battery (not illustrated). The CPU  500 , the memory  501 , the input/output interface  502 , and the counter  503  are connected through the inner bus  504  so as to be able to communicate bidirectionally. The memory  501  includes a memory as a storage unit storing an insulation decrease detection program P 1  for detecting insulation decrease of the fuel cell system FCS, a system activation program P 2 , P 2   a , P 2   b  executed in activation of the fuel cell system FCS in a non-volatile manner and read-only manner, e.g., a ROM, and a memory allowing reading and writing by the CPU  500 , e.g., a RAM. The memory  501  further includes insulation decrease information specifying an insulation decrease occurred region that is stored when the insulation decrease in the fuel cell system FCS is detected by execution of the insulation decrease detection program P 1 . Note that the insulation decrease information may contain information of not only an insulation decrease occurred region but also whether the insulation decrease is detected. 
     The CPU  500  loads the insulation decrease detection program P 1  stored in the memory  501  into a memory allowing reading and writing and executes it to function as an insulation decrease detection unit. In addition, the CPU  500  executes the system activation program P 2  to function as a system activation unit. In the first embodiment, the memory  501  only needs to store at least the system activation program P 2 , and may arbitrarily store other programs. Moreover, the CPU  500  may be a multi-sled type CPU singly capable of process a plurality of execution orders, or a plurality of CPUs dedicated to execute each of the programs P 1  and P 2 . When the CPU  500  is achieved by a plurality of CPUs, each CPU and a memory storing programs form an individual control unit, and cooperative process is performed by mutual communication between the control units. 
     The system switch  30  and the insulation detection device  45  are connected to the input/output interface  502  through detection signal lines. The auxiliary device control unit  21  and the auxiliary device control unit  22  including the blocking gate, the cooling liquid pump  40 , and the switching valve  114  are connected to the input/putout interface  502  through control signal lines. 
     The counter  503  may be an oscillation circuit transmitting clock signals with given time intervals or a circuit counting a given number of clock signals and outputting detection signals. In the former case, the CPU  500  counts the number of clock signals. In the latter case, the CPU  500  obtains information indicating the elapse of a given period by input of detection signals from the counter  503 . 
     The following will describe insulation decrease detection process performed by the fuel cell system FCS according to the first embodiment.  FIG. 3  is an explanatory diagram for explaining an insulation decrease occurred region in the fuel cell system according to the first embodiment.  FIG. 4  is a flowchart illustrating a process routine of insulation decrease detection process performed in the fuel cell system according to the first embodiment. 
     In a fuel cell vehicle FCV with the fuel cell system FCS mounted, the fuel cell system FCS is generally insulated from the vehicle body BD and a low voltage circuit VA 1 . To be more specific, the cooling circuit  11  of the fuel cell  10  includes the metal heat exchanger  112  and piping, and the heat exchanger  112  is fixed to the vehicle body BD through a metal frame. Cooling liquid with low conductivity flows in the cooling pipe  111 , and the insulation resistance IR 1  exists between the cooling circuit  11  and the vehicle body BD as long as the conductivity of cooling liquid is kept low. Meanwhile, when metal ion is eluted in cooling liquid, the conductivity of the cooling liquid increases, which reduces the insulation resistance IR 1  between the vehicle body BD and the cooling circuit  11 , that is, between the vehicle body BD and the fuel cell  10 . Note that the cooling circuit  11  is electrically grounded to the vehicle body BD. 
     The high voltage circuit, e.g., the power control unit  21  is insulated from the vehicle body BD, and the insulation resistance IR 2  exists between the high voltage circuit and the vehicle body BD. The high voltage circuit may be connected to the vehicle body BD through the low voltage circuit VA 1 . In such a case, the high voltage circuit is insulated from the low voltage circuit VA 1 , and the insulation resistance IR 2  exists between the power control unit  21  and the low voltage circuit VA 1 . That is, the high voltage circuit is grounded to the vehicle body BD. For example, the decrease of insulation members between the high voltage circuit and the low voltage circuit VA 1  may damage a covering of a harness connected to the high voltage circuit and decrease the insulation resistance IR 2 . 
     The insulation decrease detection process is achieved by the CPU  500  executing the insulation decrease detection program P 1  with given time intervals during operation of the fuel cell system FCS. The CPU  500  determines whether the insulation detection device  45  has detected occurrence of insulation decrease of the fuel cell system FCS (Step S 100 ). As described above, when insulation decrease is not occurred in the fuel cell system FCS, the insulation detection device  45  outputs a high voltage value to the CPU  500  as detection signals. When insulation decrease is occurred in the fuel cell system FCS, the insulation detection device  45  outputs a lower voltage value than a voltage value in the case where no insulation decrease is occurred to the CPU  500  as detection signals. When a high voltage value is input as detection signals, the CPU  500  determines that insulation decrease is not occurred in the fuel cell system FCS (No at Step S 100 ), and finishes the process routine. 
     When a low voltage value is input as detection signals, the CPU  500  determines that insulation decrease is occurred in the fuel cell system FCS (Yes at Step S 100 ), specifies an insulation decrease occurred region (Step S 102 ), and finishes the process routine. The insulation decrease occurred region is specified by the CPU  500  before the operation of the fuel cell system FCS is stopped when the system switch  30  is turned off. 
     The CPU  500  sequentially blocks the blocking gates of the power control unit  21  and the auxiliary device control unit  22 , and the relay  41 . When detection signals output by the insulation detection device  45  indicate a low voltage value after the blocking gate of the auxiliary device control unit  22  is blocked, the CPU  500  determines that insulation decrease is occurred between the auxiliary device control unit  22  or the high voltage auxiliary device  43  and the vehicle body BD. When detection signals output by the insulation detection device  45  still indicate a low voltage value after the blocking gate of the auxiliary device control unit  22  is blocked, the CPU  500  blocks the blocking gate of the power control unit  21 . When detection signals output by the insulation detection device  45  indicate a low voltage value, the CPU  500  determines that insulation decrease is occurred between the power control unit  21  or the driving motor  42  and the vehicle body BD. When detection signals output by the insulation detection device  45  still indicate a low voltage value after the blocking gate of the power control unit  21  is blocked, the CPU  500  blocks the relay  41 . When detection signals output by the insulation detection device  45  indicate a low voltage value, the CPU  500  determines that insulation decrease is occurred between the fuel cell  10 , the cooling circuit  11 , or the high voltage device for fuel cell  12  and the vehicle body BD. 
     The CPU  500  stores information of insulation decrease and information of an insulation decrease region in the memory  501  as insulation decrease information, and finishes the fuel cell system FCS. In the embodiment, the information of an insulation decrease region may be information specifying at least one of the fuel cell region SE 1  with the fuel cell  10  and the high voltage device for fuel cell  12 , and the fuel cell system outside region SE 2  with the secondary battery  20 , the power control unit  21 , the auxiliary device control unit  22 , the driving motor  42 , and the high voltage auxiliary device  43 . 
     The insulation decrease information stored in the memory  501  is used in the next activation of the fuel cell system FCS.  FIG. 5  is a flowchart illustrating a process routine of system activation process performed in activation of the fuel cell system according to the first embodiment. The system activation process in the first embodiment is achieved by the CPU  500  executing the system activation program P 2  in response to on-input of the system switch  30 , that is, an activation request, when it is indicated that insulation decrease is detected during the previous operation of the fuel cell system FCS. Note that the relay  41  is in a blocked state, that is, a non-connected state, when the fuel cell system FCS is finished. 
     The CPU  500  obtains insulation decrease information stored in the memory  501 , and obtains a specified insulation decrease region (Step S 200 ). In the first embodiment, the specified insulation decrease region is the fuel cell region SE 1  or the fuel cell outside region SE 2 . 
     The CPU  500  determines whether the specified insulation decrease region is the fuel cell region SE 1  (Step S 202 ). When the specified insulation decrease region is the fuel cell region SE 1  (Yes at Step S 202 ), the process moves to Step S 212  so that the CPU  500  has the relay  41  connect and finishes the process routine. Therefore, the CPU  500  transmits control signals in accordance with a request output from a driver to the power control unit  21 , and drives the driving motor  42  with the fuel cell  10  as a power source to allow the vehicle FCV to travel. When the specified insulation decrease region is the fuel cell region SE 1 , the insulation decrease part in the fuel cell system FCS is one. In the outside of the fuel cell system FCS, the fuel cell region SE 1  and the fuel cell outside region SE 2  are not electrically connected through the vehicle body BD, and a current path in the outside of the fuel cell system FCS, that is, an outer current path electrically connecting the fuel cell region SE 1  and the fuel cell outside region SE 2  is not formed even with the relay  41  connected. Note that the outside of the fuel cell system FCS indicates an outer configuration insulated from the high voltage electrical system of the fuel cell system FCS, e.g., the vehicle body BD and the low voltage circuit. The outer current path is a path electrically connecting the fuel cell region SE 1  and the fuel cell outside region SE 2 , which is formed in the outer configuration of the fuel cell system FCS. Therefore, even when the fuel cell  10  is started, and allowing driving of the driving motor  42  with the fuel cell  10  as a power source, no current leak to the outside of the high voltage electrical system, e.g., the vehicle body BD from the fuel cell  10  is occurred. The start of the fuel cell  10  means the state in which power generation is possible with the fuel cell  10  connected to a load, and includes at least the start of supply of reaction gas to the fuel cell  10 . In addition, to solve insulation decrease due to increase of conductivity of cooling liquid, conductivity reduction process is performed during operation of the fuel cell  10  to suppress insulation decrease. In this manner, when the insulation decrease region is the fuel cell region SE 1 , the relay  41  is connected to connect the fuel cell  10  with the power control unit  21  at timing when the drive of the driving motor  42  is requested without adjusting timing between connection timing of the relay  41  and another process. 
     When the specified insulation decrease region is not the fuel cell region SE 1  (No at Step S 202 ), the CPU  500  performs conductivity reduction process before having the relay  41  connect (Step S 208 ) 
     In the conductivity reduction process, the CPU  500  transmits control signals to the auxiliary device control unit  22  driving the cooling liquid pump  40 , and transmits control signals for switching a feeding destination of cooling liquid to the ion exchanger  113 , to the switching valve  114 . The cooling liquid pump  40  is driven using power of the secondary battery  20 . As a result, metal ion eluted in the cooling liquid is removed by the ion exchanger  113 . The CPU  500  retains the cooling liquid path through the ion exchanger  113  until the conductivity reduction process is completed (No at Step S 210 ). Once the conductivity reduction process is completed (Yes at Step S 210 ), the CPU  500  transmits control signals for switching a feeding destination of cooling liquid to the heat exchanger  112 , to the switching valve  114 . The completion of the conductivity reduction process may be determined by waiting the elapse of conductivity reduction process time predetermined experimentally, e.g., the elapse of several minutes, or may be determined when a conductivity detected by a conductivity detection sensor disposed for the ion exchanger  113  is reduced to a conductivity reference value. The cooling liquid pump  40  is driven using power of the secondary battery  20 , and thus it is possible to perform conductivity reduction process of cooling liquid without starting the fuel cell  10 . Moreover, the relay  41  is blocked, and thus the fuel cell region SE 1  and the fuel cell outside region SE 2  are not communicated electrically, which prevents application of power of the secondary battery  20  on the outer current path. 
     Once the conductivity reduction process is completed, the CPU  500  has the relay  41  connect(Step S 212 ), starts the fuel cell  10  to allow driving of the driving motor  42  using the fuel cell  10 , and finishes the process routine. Thereafter, the CPU  500  transmits control signals in accordance with a request output from a driver to the power control unit  21 , and drives the driving motor  42  to allow the vehicle FCV to travel. 
     In the above-described fuel cell system FCS according to the first embodiment, when insulation decrease of the fuel cell system FCS is occurred in the fuel cell outside region SE 2 , the relay  41  is connected after completion of the conductivity reduction process of cooling liquid. Therefore, even when the insulation between the vehicle body BD and the fuel cell region SE 1  is decreased due to increase of conductivity of cooling liquid, and the fuel cell region SE 1  and the fuel cell outside region SE 2  are connected electrically through the vehicle body BD, the fuel cell region SE 1  and the fuel cell outside region SE 2  are not electrically connected in the fuel cell system FCS, and there is not formed an outer current path electrically connecting the vehicle body BD, the fuel cell region SE 1 , and the fuel cell outside region SE 2 . In this manner, no current leak from the high voltage circuit such as the fuel cell region SE 1  and the fuel cell outside region SE 2  to the vehicle body BD is occurred. 
     In the fuel cell system FCS according to the first embodiment, even when insulation decrease is detected when the previous operation of the fuel cell system FCS is finished, conductivity reduction process is not performed if the insulation decrease region is the fuel cell region SE 1 , which allows quick start of the fuel cell  10  in response to a start request. When the insulation decrease region is the fuel cell region SE 1 , the fuel cell region SE 1  and the fuel cell outside region SE 2  are not connected electrically through the vehicle body BD. Therefore, in the fuel cell system FCS according to the first embodiment, start timing of the fuel cell  10  is changed in accordance with a specified insulation decrease region, whereby it is possible to start the fuel cell  10  more quickly and prevent a current leak from the high voltage circuit of the fuel cell system FCS to the outer current path in the outside of the fuel cell system FCS. 
     Second Embodiment 
     The fuel cell system according to the second embodiment will be described.  FIG. 6  is a flowchart illustrating a process routine of system activation process performed in activation of the fuel cell system according to the second embodiment. The hardware configuration of the fuel cell system according to the second embodiment is the same as that of the fuel cell system FCS according to the first embodiment. Thus, the same symbols as in the first embodiment are given to omit the explanation. Moreover, the system activation program P 2   a  of the second embodiment is different from the system activation program P 2   a  of the first embodiment in the point that when the insulation decrease region is the fuel cell outside region SE 2 , whether conductivity reduction process is to be performed is determined considering vehicle leave time. Therefore, the same process step as in the activation process in the first embodiment is represented with the same symbol and the explanation thereof is omitted. The different process steps will be described in detail. 
     The system activation process in the second embodiment is achieved by the CPU  500  performing, when it is indicated that the insulation decrease is detected during the previous operation of the fuel cell system FCS, the system activation program P 2   a  in accordance with an on-input of the system switch  30 . 
     The CPU  500  performs Steps S 200  and S 202 , and when the specified insulation decrease region is the fuel cell region SE 1  (Yes at Step S 202 ), the process moves to Step S 212  so that the CPU  500  has the relay  41  connect and finishes the process routine. Therefore, the CPU  500  transmits control signals in accordance with a request output from a driver to the power control unit  21 , and drives the driving motor  42  with the fuel cell  10  as a power source to allow the vehicle FCV to travel. 
     When the specified insulation decrease region is not the fuel cell region SE 1  (No at Step S 202 ), that is, when the specified insulation decrease region is the fuel cell outside region SE 2 , the CPU  500  obtains vehicle leave time ts (s) from the counter  503  or the memory  501  (Step S 204 ). The vehicle leave time ts is elapsed time from the stop of the fuel cell system FCS to the turn-on of the system switch  30 , and may be referred to as a stop period of the fuel cell system FCS. When the counter  503  counts and retains the elapsed time from the stop of the fuel cell system FCS, the vehicle leave time ts is obtained from the counter  503 . When the CPU  500  counts and stores clock signals output from the counter  503  in the memory  501 , the vehicle leave time ts is obtained from the memory  501 . 
     The CPU  500  determines whether the obtained vehicle leave time ts is shorter than predetermined reference time t 1  (s) (Step S 206 ). When the vehicle leave time ts is shorter than the reference time t 1  and the relation ts&lt;t 1  is fulfilled (Yes at Step S 206 ), the process moves to Step S 212  so that the CPU  500  has the relay  41  connect and starts the fuel cell  10  to allow power generation for power supply to the driving motor  42 . The CPU  500  transmits control signals in accordance with a request output from a driver to the power control unit  21 , and drives the driving motor  42  with the fuel cell  10  as a power source to allow the vehicle FCV to travel. The reference time t 1  is time during which metal ion may be eluted into cooling liquid so that the conductivity of the cooling liquid is increased to cause insulation decrease. The reference time t 1  is varied depending on environmental conditions, and is time in the weekly unit, for example. When ts&lt;t 1  is fulfilled, the conductivity reduction process of cooling liquid is unnecessary, and it is considered that the insulation decrease between the fuel cell  10  and the vehicle body BD through the cooling circuit  11  is not occurred. Therefore, in the outside of the fuel cell system FCS, the fuel cell region SE 1  and the fuel cell outside region SE 2  are not connected electrically through the vehicle body BD. Even when the relay  41  is connected, there is not formed an outer current path electrically connecting the vehicle body BD, the fuel cell region SE 1 , and the fuel cell outside region SE 2 . As a result, the relay  41  is connected to connect the fuel cell  10  with the power control unit  21  at timing when the drive of the driving motor  42  is requested without adjusting timing between connection timing of the relay  41  and other process. 
     When ts&lt;t 1  is not fulfilled (No at Step S 206 ), the CPU  500  performs conductivity reduction process before having the relay  41  connect (Step S 208 ). When ts&lt;t 1  is not fulfilled, the conductivity of cooling liquid is increased, which may cause insulation decrease between the fuel cell  10  and the vehicle body BD through the cooling circuit  11 . In addition, the insulation decrease in the fuel cell outside region SE 2  is also detected and specified. Therefore, in the outside of the fuel cell system FCS, the fuel cell region SE 1  and the fuel cell outside region SE 2  may be connected electrically through the vehicle body BD. When the relay  41  is connected, there is formed an outer current path electrically connecting the vehicle body BD, the fuel cell region SE 1 , and the fuel cell outside region SE 2 . When the start of the fuel cell  10  is allowed in such a state, a current generated in the fuel cell  10  may flow in the outer current path. In addition, in the conductivity reduction process, the cooling liquid pump  40  is driven with the secondary battery  20  as a power source, and thus with the connection of the relay  41 , a current from the secondary battery  20  may flow in the outer current path. Then, in the first embodiment, the conductivity reduction process of cooling liquid is performed without having the relay  41  connect to prevent application of high voltage power of the secondary battery  20  and high voltage power caused by power generation of the fuel cell  10  on the outer current path. 
     The CPU  500  retains the cooling liquid path through the ion exchanger  113  until the conductivity reduction process is completed (No at Step S 210 ). Once the conductivity reduction process is completed (Yes at Step S 210 ), the CPU  500  transmits control signals for switching a feeding destination of cooling liquid to the heat exchanger  112 , to the switching valve  114 . The completion of the conductivity reduction process may be determined by waiting the elapse of conductivity reduction process time predetermined experimentally in accordance with the vehicle leave time ts, e.g., the elapse of several minutes, or may be determined when a conductivity detected by the conductivity detection sensor is reduced to a conductivity reference value. 
     Once the conductivity reduction process is completed, the CPU  500  has the relay  41  connect (Step S 212 ), and finishes the process routine. The CPU  500  transmits control signals in accordance with a request output from a driver to the power control unit  21 , and drives the driving motor  42  using power generated by the fuel cell  10  to allow the vehicle FCV to travel. 
     In the fuel cell system FCS according to the second embodiment, whether the conductivity reduction process is to be performed is determined considering the increase of conductivity of cooling liquid in accordance with the elapsed time from the finish of the previous operation of the fuel cell system FCS. Therefore, when the conductivity reduction process is unnecessary, it is possible to immediately allow activation of the fuel cell system FCS. By contrast, when the conductivity reduction process is required, it is possible to prevent a current leak to the outside of the fuel cell system FCS from the high voltage circuit of the fuel cell system FCS. Therefore, as compared with the case in which the conductivity reduction process is performed uniformly, it is possible to prevent an electric leak to the outside of the fuel cell system FCS from the high voltage circuit of the fuel cell system FCS while improving driver&#39;s convenience. In addition, in the fuel cell system FCS according to the second embodiment, it is also possible to obtain the advantage by the fuel cell system FCS of the first embodiment. 
     Third Embodiment 
     The following will describe a fuel cell system according to the third embodiment.  FIG. 7  is a flowchart illustrating a process routine of system activation process performed in activation of the fuel cell system according to the third embodiment. The hardware configuration of the fuel cell system according to the third embodiment is same as that of the fuel cell system FCS according to the first embodiment. Thus, the same symbols as in the first embodiment are given to omit the explanation. Moreover, the system activation program P 2   b  of the third embodiment is different from the system activation programs P 2 , P 2   a  of the first and second embodiments in the point that it is a comprehensive system activation program not supposing insulation decrease, and is able to immediately allow, using power of the secondary battery  20 , the vehicle FCV to travel after system activation. Therefore, the same process steps as in the activation process in the first and second embodiments are represented with the same symbols and the explanation thereof is omitted. The different process steps will be described in detail. 
     The system activation process in the third embodiment is achieved by the CPU  500  performing the system activation program P 2   b  in response to an on-input of the system switch  30 . The CPU  500  obtains insulation decrease information stored in the memory  501 , and determines whether the insulation decrease has been detected in the previous operation of the fuel cell system FCS (Step S 200   a ). When the obtained insulation decrease information does not indicate recording of insulation decrease (No at Step S 200   a ), the process moves to Step S 212  so that the CPU  500  has the relay  41  connect. This is because when the insulation decrease is not occurred in the fuel cell system FCS, an outer current path is not formed, which dispenses with adjustment of connection timing of the relay  41 . 
     When the obtained insulation decrease information indicates recording of insulation decrease (Yes at Step S 200   a ), the CPU  500  obtains the specified insulation decrease region (Step S 200 ). Thereafter, the CPU  500  performs Steps S 202  to S 208 . 
     Receiving a vehicle FCV driving request from a driver during the conductivity reduction process, the CPU  500  transmits control signals to the power control unit  21  to allow the vehicle FCV to travel in accordance with a request output input by the driver. To be more specific, before the conductivity reduction process is completed, the power control unit  21  drives, with the secondary battery  20  as a power source, the driving motor  42  in accordance with the request output by the driver to allow the vehicle FCV to travel (Step S 209 ). 
     The CPU  500  performs Step S 210  and Step S 212 , and finishes the process routine. 
     In addition to the advantages obtained by the fuel cell system FCS according to the first and second embodiments, it is possible, in the fuel cell system FCS according to the above-described third embodiment, to allow the vehicle FCV to travel in accordance with a request by a driver without waiting completion of the conductivity reduction process. That is, in the configuration where the relay  41  is connected after completion of the conductivity reduction process of cooling liquid, when the insulation decrease of the fuel cell system FCS is occurred in the fuel cell outside region SE 2 , it is possible to drive the driving motor  42  using power of the secondary battery  20  during the conductivity reduction process to allow the vehicle FCV to travel. Moreover, whether the insulation decrease has been detected is determined when the previous operation of the fuel cell system FCS is finished. Thus, it is possible to perform activation process when the insulation decrease is occurred and activation process when the insulation decrease is not occurred through the common activation process. 
     Modifications 
     (1) First Modification 
     In the first to the third embodiments, only the cooling liquid pump  40  is operated in the conductivity reduction process, and the supply of reaction gas to the fuel cell  10  is started after the relay  41  is connected. However, the supply of reaction gas to the fuel cell  10  may be started in the conductivity reduction process before the relay  41  is connected. In such a case, it is possible to start the fuel cell  10  more quickly. Moreover, as long as the relay  41  is disconnected, the fuel cell  10  does not start power generation, and an outer current path connecting the vehicle body BD, the fuel cell region SE 1 , and the fuel cell outside region SE 2  is formed. 
     (2) Second Modification 
     In the first to the third embodiments, the cooling liquid pump  40  is operated using the secondary battery  20  in the conductivity reduction process. However, when the auxiliary device control unit for the cooling liquid pump  40  is provided between the relay  41  and the fuel cell  10 , the cooling liquid pump  40  may be operated using power generated by the fuel cell  10 . In this case, it is possible to quickly allow the vehicle FCV to travel using power from the fuel cell  10  after the conductivity reduction process. 
     (3) Third Modification 
     The first to the third embodiment have exemplified the fuel cell system FCS mounted on the vehicle body BD. However, the fuel cell system FCS may be mounted on a mobile body such as a ship and a train, or may be a stationary fuel cell system FCS. Also in these cases, it is possible to solve the problems due to insulation decrease same as the present disclosure. 
     The above has described the present disclosure based on the embodiments and the modifications. However, the above-described embodiments of the disclosure are made to facilitate understanding of the disclosure, and do not limit the disclosure. The present disclosure may be modified and improved without departing from the scope of the claims, and includes the equivalents thereof. For example, the technical features in the embodiments and modifications corresponding to the technical features of each aspect described in the summary may be appropriately substituted or combined to solve part or all of the above-described problems or achieve part or all of the above-described effects. Moreover, as long as the technical features are not described as essential in the specification, they may be deleted appropriately.