Patent Publication Number: US-2021188103-A1

Title: Fuel cell vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application is based on Japanese Patent Application No. 2019-231443 filed with the Japan Patent Office on Dec. 23, 2019, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Field 
     The present disclosure relates to a fuel cell vehicle. 
     Description of the Background Art 
     WO2011/004493 discloses a fuel cell vehicle on which fuel cells are mounted (a fuel cell being referred to as an “FC” and a fuel cell vehicle being referred to as an “FCV” below). The FCV includes an FC stack and a battery. The battery functions as a source of storage of excessive electric power, a source of storage of regenerative energy during regenerative braking, and an energy buffer in case of variation in load with acceleration or deceleration of a vehicle. 
     What is called a plug-in FCV a power storage of which can be charged with a power supply outside the vehicle has been studied (a power supply outside a vehicle being also referred to as an “external power supply” and charging of the power storage with an external power supply being also referred to as “external charging” below). When external charging was not carried out after system stop of the FCV and a state of charge (SOC) of the power storage is low at the time of system start-up, however, system output which is the sum of output from FCs and output from the power storage is restricted after start of travel and traveling performance may be lowered. 
     SUMMARY 
     The present disclosure was made to solve such a problem, and an object of the present disclosure is to suppress lowering in traveling performance after system start-up of an FCV including FCs and a power storage. 
     An FCV according to the present disclosure includes an FC system, a power storage, a driving device that receives electric power from at least one of the FC system and the power storage and generates travel power, a charger that carries out external charging, and a controller. When an SOC is lower than a threshold value, an output upper limit of the power storage is set to decrease with lowering in SOC. When external charging is not performed under a condition that the SOC is lower than the threshold value at the time of system stop of the FCV, the controller controls the FC system to carry out FC charging for supply of electric power from the FC system to the power storage. 
     As the SOC lowers and becomes lower than the threshold value, the output upper limit of the power storage lowers, which restricts the system output. The FCV includes a charger that carries out external charging, and by carrying out external charging during system stop, the FCV can be high in SOC at the time of system start-up. External charging, however, may not be carried out during system stop because there is no charging facility at a parking spot or a user forgets external charging. 
     In this FCV, when external charging is not carried out under the condition that the SOC is lower than the threshold value at the time of system stop, FC charging is carried out. Thus, even though external charging is not carried out during system stop, the SOC can be high at the time of next system start-up and restriction of system output at the time of start of travel after system start-up can be avoided. Therefore, the FCV can suppress lowering in traveling performance after system start-up. 
     The controller may control the FC system to carry out FC charging when a prescribed operation to carry out external charging is not performed for a prescribed time period after the system stop under the condition that the SOC is lower than the threshold value at the time of system stop. 
     In the FCV, when an operation to carry out external charging is not performed for a prescribed time period after system stop, it is determined that external charging is not carried out, and when the SOC is lower than the threshold value at the time of system stop, FC charging is carried out. Therefore, according to the FCV, even though external charging is not carried out during system stop, the SOC can be high at the time of next system start-up and restriction of system output at the time of system start-up can be avoided. 
     When the operation is performed during FC charging, the controller may stop FC charging and carry out external charging. 
     According to the FCV, when external charging is requested, even during FC charging, FC charging is stopped and external charging is carried out. Therefore, decrease in remaining amount of fuel at the time of system start-up can be suppressed. 
     When the operation is performed and when an electricity fee for external charging is higher than a prescribed amount, the controller may control the FC system to carry out FC charging without carrying out external charging. 
     According to the FCV, when an electricity fee for external charging is high, FC charging is carried out even though external charging is requested. Therefore, cost for energy for suppressing lowering in traveling performance after system start-up can be suppressed. 
     The controller may control the FC system to carry out FC charging when the SOC is lower than the threshold value at the time of system start-up of the FCV. 
     In the FCV, when the SOC is lower than the threshold value at the time of system start-up, FC charging is carried out. Therefore, restriction of system output due to lowering in SOC can be avoided. Therefore, according to the FCV, lowering in traveling performance after system start-up can be suppressed. 
     An FCV according to the present disclosure includes an FC system, a power storage, a driving device that receives electric power from at least one of the FC system and the power storage and generates travel power, a charger that carries out external charging, a display operable by a user of the FCV, and a controller. When an SOC is lower than a threshold value, an output upper limit of the power storage is set to decrease with lowering in SOC. The controller controls the display to show an operation section that allows a user to indicate execution of FC charging when the SOC is lower than the threshold value at the time of system stop of the FCV. 
     In the FCV, an operation section that allows a user to indicate execution of FC charging when the SOC is lower than the threshold value at the time of system stop is shown on the display. Thus, even though external charging is not carried out during system stop, FC charging is carried out during system stop in accordance with a user&#39;s instruction, so that the SOC can be high at the time of next system start-up. Therefore, according to the FCV, restriction of system output at the time of system start-up can be avoided, and consequently lowering in traveling performance after system start-up can be suppressed. 
     The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an overall configuration of an FCV according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram showing a travel mode provided in the FCV. 
         FIG. 3  is a diagram showing relation of a remaining amount of energy in an FC system and a battery with a system output upper limit. 
         FIG. 4  is a flowchart showing an exemplary procedure in processing performed by an FDC-ECU at the time of Ready-off. 
         FIG. 5  is a flowchart showing an exemplary procedure in processing performed by the FDC-ECU at the time of Ready-off in a first modification. 
         FIG. 6  is a flowchart showing an exemplary procedure in processing performed by the FDC-ECU at the time of Ready-off in a second modification. 
         FIG. 7  is a flowchart showing an exemplary procedure in processing performed by the FDC-ECU at the time of Ready-off in a second embodiment. 
         FIG. 8  is a diagram showing an exemplary state of display on an HMI apparatus in step S 330  in  FIG. 7 . 
         FIG. 9  is a flowchart showing an exemplary procedure in processing performed by the FDC-ECU at the time of Ready-on in a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described below in detail with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. 
     First Embodiment 
       FIG. 1  is a diagram showing an overall configuration of an FCV  1  according to an embodiment of the present disclosure. Referring to  FIG. 1 , FCV  1  includes a motor generator (which is referred to as an “MG” below)  10 , an inverter  12 , an FC system  20 , a hydrogen tank  28 , a supply valve  30 , an air filter  32 , and a compressor  34 . 
     MG  10  is an alternating-current (AC) rotating electric machine, and it is, for example, a three-phase AC synchronous motor having a permanent magnet embedded in a rotor. MG  10  generates rotational driving force as it is driven by inverter  12 . Driving force generated by MG  10  is transmitted to a not-shown drive wheel. During braking of FCV  1 , MG  10  serves as a generator and generates electric power. Electric power generated by MG  10  is rectified by inverter  12  and rectified electric power can be stored in a battery  40 . 
     Inverter  12  is provided between a power line  70  and MG  10  and drives MG  10  based on a drive signal from an MG-ECU  66  (which will be described later). Inverter  12  is implemented, for example, by a bridge circuit including switching elements of three phases. 
     FC system  20  includes an FC stack  22 , a boost converter  24 , and a relay  26 . FC stack  22  is, for example, a structure in which a plurality of (for example, several ten to several hundred) cells of a solid polymer type are stacked in series. Each cell is made, for example, by joining a catalyst electrode to each of opposing surfaces of an electrolyte membrane and sandwiching the membrane between conductive separators. Each cell generates electric power as a result of electrochemical reaction between hydrogen supplied to an anode and oxygen (air) supplied to a cathode. 
     Boost converter  24  boosts electric power (for example, at several hundred V) generated by FC stack  22  based on a control signal from an FDC-ECU  60  (which will be described later) and outputs boosted electric power to power line  70 . Relay  26  is provided in an electric path between FC stack  22  and boost converter  24  and opened while a vehicle system remains stopped or while FC system  20  is not used. 
     Hydrogen tank  28  stores hydrogen as fuel supplied to FC stack  22 . Hydrogen tank  28  is a high pressure tank that is light in weight and high in strength and includes, for example, a carbon fiber reinforced plastic layer, and can store, for example, hydrogen at several ten MPa. Hydrogen is supplied from hydrogen tank  28  through supply valve  30  to FC stack  22 . 
     Compressor  34  is a device for supplying oxygen to FC stack  22 . Compressor  34  suctions oxygen (air) through air filter  32 , compresses oxygen (air), and supplies compressed oxygen (air) to FC stack  22 . 
     FCV  1  further includes battery  40 , a direct current (DC) inlet  44 , an AC inlet  48 , a charger  50 , and relays  42 ,  46 , and  52 . 
     Battery  40  is a chargeable and dischargeable power storage. Battery  40  includes a battery assembly constituted of a plurality of cells (for example, several hundred cells). Each cell is, for example, a secondary battery such as a lithium ion battery or a nickel metal hydride battery. A lithium ion secondary battery is a secondary battery containing lithium as a charge carrier, and may include not only a general lithium ion secondary battery containing a liquid electrolyte but also what is called an all-solid-state battery containing a solid electrolyte. A power storage element such as an electric double layer capacitor may be employed instead of battery  40 . 
     Battery  40  is connected to a power line  72  with relay  42  being interposed, and power line  72  is connected to power line  70 . Battery  40  stores electric power for driving MG  10  and supplies electric power to inverter  12  through power lines  72  and  70 . Battery  40  is charged with electric power generated by MG  10  during braking of FCV  1 . Battery  40  can function as an energy buffer that accommodates variation in load caused by acceleration and deceleration of FCV  1  or stores electric power generated by MG  10  during braking of FCV  1 . 
     In the present embodiment, battery  40  can be charged with electric power supplied from a power supply (not shown) outside a vehicle through DC inlet  44  or AC inlet  48  (external charging). Furthermore, battery  40  can also be charged with electric power output from FC system  20  (charging of battery  40  by FC system  20  being also referred to as “FC charging” below). 
     DC inlet  44  is connected to a power line  74  with relay  46  being interposed and power line  74  is connected to power line  72 . DC inlet  44  is constructed such that a connector of a DC charging cable that extends from a charging stand (not shown) outside the vehicle can be fitted thereto, and DC inlet  44  receives DC power at a high voltage supplied from the charging stand and outputs DC power to power line  74 . 
     AC inlet  48  is connected to charger  50  with relay  52  being interposed. AC inlet  48  is constructed such that a connector of an AC charging cable that extends from a charging stand outside the vehicle can be fitted thereto, and AC inlet  48  receives AC power (for example, system power) supplied from the charging stand and outputs AC power to charger  50 . Charger  50  is connected to power line  74  and converts AC power input from AC inlet  48  to a voltage level of battery  40  and outputs DC power to power line  74 . 
     Relay  42  is provided between battery  40  and power line  72  and closed while the system of FCV  1  is ON or while external charging is being carried out. Relay  46  is provided between DC inlet  44  and power line  74  and closed during external charging (DC charging) through DC inlet  44 . Relay  52  is provided between AC inlet  48  and charger  50  and closed during external charging (AC charging) through AC inlet  48  and charger  50 . 
     FCV  1  is thus a plug-in FCV in which battery  40  can be charged with a power supply outside the vehicle connected to DC inlet  44  or AC inlet  48 , and it can travel with electric power stored in battery  40  by external charging. 
     FCV  1  further includes FDC-electronic control unit (ECU)  60 , a mode switch (MD-SW)  62 , a battery ECU  64 , MG-ECU  66 , and a human machine interface (HMI) apparatus  68 . Each of FDC-ECU  60 , battery ECU  64 , and MG-ECU  66  includes a central processing unit (CPU), a memory (a read only memory (ROM) and a random access memory (RAM)), and an input and output buffer (none of which is shown). The CPU develops a program stored in the ROM on the RAM and executes the program. Processing to be performed by a corresponding ECU is described in a program stored in the ROM. 
     FDC-ECU  60  calculates output requested of FC system  20  (output electric power from FC system  20 ) based on travel power requested of FCV  1  and a request for charging and discharging of battery  40 , and controls boost converter  24  such that FC system  20  outputs calculated electric power. Travel power requested of FCV  1  is calculated based on an amount of operation of an accelerator pedal and a vehicle speed. Though travel power is calculated by FDC-ECU  60  in the present embodiment, it may be calculated by another ECU (for example, a vehicle ECU (not shown) that controls the entire vehicle in a centralized manner). 
     FDC-ECU  60  controls output from FC system  20  such that output from FC system  20  does not exceed an output upper limit Wfc and output from battery  40  does not exceed an output upper limit Wout either. In the present embodiment, output upper limit Wfc of FC system  20  is set by FDC-ECU  60  and output upper limit Wout of battery  40  is set by battery ECU  64  based on the SOC and the temperature of battery  40 . Output upper limit Wfc of FC system  20  and output upper limit Wout of battery  40  will be described in detail later. 
     FDC-ECU  60  switches a travel mode in accordance with setting made through mode switch  62 . FCV  1  incorporates FC system  20  and battery  40  as power supplies, and battery  40  can store electric power. In this FCV  1 , four travel modes in accordance with usage of FC system  20  and battery  40  are available, and a user can select a travel mode by operating mode switch  62 . The travel mode will be described in detail later. 
     Mode switch  62  is a switch for a user to set the travel mode. Mode switch  62  may be a dedicated switch or may be implemented on a touch panel display of HMI apparatus  68 . 
     HMI apparatus  68  provides various types of information to a user of FCV  1  or accepts an operation by the user of FCV  1 . HMI apparatus  68  includes a display including a touch panel or a speaker. 
     Battery ECU  64  monitors a voltage, a current, and a temperature of battery  40 . A voltage, a current, and a temperature of battery  40  are detected by not-shown various sensors. Battery ECU  64  calculates an SOC of battery  40  based on values of detected voltage, current, and temperature of battery  40 . The calculated SOC value is transmitted to FDC-ECU  60 . The SOC may be calculated by FDC-ECU  60  based on values of detected voltage, current, and temperature of battery  40 . 
     Battery ECU  64  sets output upper limit Wout of battery  40 . Specifically, battery ECU  64  sets output upper limit Wout to decrease with lowering in SOC when the SOC of battery  40  becomes lower than a threshold value. Then, battery ECU  64  transmits set output upper limit Wout to FDC-ECU  60 . 
     In FCV  1 , battery  40  is connected to power line  70  without a converter being interposed, and an amount of charging and discharging of battery  40  is basically determined by a difference between travel power requested by inverter  12  and MG  10  and output from FC system  20 . Therefore, charging and discharging and the SOC of battery  40  can be controlled by control of output from FC system  20  by FDC-ECU  60  based on travel power. 
     In this FCV  1 , a target SOC representing a target of the SOC is set by FDC-ECU  60  in accordance with the travel mode. A requested amount of charging and discharging of battery  40  is then calculated based on a difference between the SOC and the target SOC such that the SOC of battery  40  is closer to the target SOC, and output from FC system  20  is controlled by FDC-ECU  60  based on the calculated requested amount of charging and discharging and travel power. 
     The target SOC will be described in detail later. Various known approaches such as an approach using an open circuit voltage (OCV)-SOC curve (a map) representing relation between the OCV and the SOC and an approach using an accumulated value of currents input to and output from battery  40  can be used as the method of calculating the SOC. 
     MG-ECU  66  receives a calculated value of travel power requested of FCV  1  from FDC-ECU  60 , generates a signal for driving MG  10  with inverter  12  based on travel power, and outputs the signal to inverter  12 . 
     &lt;Description of Travel Mode&gt; 
     As described above, FCV  1  includes FC system  20  and battery  40 . In the present embodiment, four travel modes in accordance with usage of FC system  20  and battery  40  are provided. 
       FIG. 2  is a diagram showing travel modes provided in FCV  1 . Referring to  FIG. 2 , in the present embodiment, four travel modes of an “FC mode,” an “FCEV mode,” an “EV mode,” and a “CHG mode” are provided. A user of FCV  1  can select a desired travel mode from among the travel modes by using mode switch  62 . 
     The FC mode refers to a travel mode for travel basically only with output from FC system  20  until fuel (hydrogen) in FC system  20  runs out. After fuel runs out, FCV  1  travels only with output from battery  40 . 
     In the FC mode, in order to travel only with output from FC system  20 , FDC-ECU  60  controls FC system  20  (boost converter  24 ) based on travel power such that FC system  20  outputs power comparable to power required by inverter  12 , that is, travel power (a requested value). 
     Even in the FC mode, when high travel power is requested by strong pressing of the accelerator pedal and travel power exceeds output upper limit Wfc of FC system  20 , insufficiency in power is compensated for by battery  40 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
     The FCEV mode is a hybrid mode in which output from FC system  20  and output from battery  40  are used in a balanced manner. By way of example, in the FCEV mode, FCV  1  travels with both of output from FC system  20  and output from battery  40  so as to maintain the SOC at the target SOC while FCV  1  travels in accordance with requested travel power. Specifically, FDC-ECU  60  controls FC system  20  (boost converter  24 ) based on travel power and the SOC of battery  40  such that input to and output from battery  40  are adjusted to maintain the SOC at the target SOC and power required by inverter  12 , that is, travel power (a requested value), is supplied. 
     Even in the FCEV mode, when high travel power is requested by strong pressing of the accelerator pedal and travel power exceeds the output upper limit of FC system  20 , electric power equal to or higher than output in accordance with a difference between the SOC and the target SOC is compensated for by battery  40 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
     The EV mode refers to a travel mode for travel basically only with output from battery  40  without using fuel (hydrogen) in FC system  20 . 
     Even in the EV mode, when high travel power is requested by strong pressing of the accelerator pedal and travel power exceeds output upper limit Wout of battery  40 , power comparable to insufficiency in power may be output from FC system  20 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
     The CHG mode refers to a mode in which the SOC is raised to a prescribed level by positively charging battery  40  with output from FC system  20  when the SOC of battery  40  has lowered. 
     Even in the CHG mode, when travel power is requested by pressing of the accelerator pedal, electric power is supplied from FC system  20  to inverter  12 . Furthermore, when high travel power is requested by strong pressing of the accelerator pedal, electric power is supplied also from battery  40  to inverter  12 . When regeneration by MG  10  is carried out as in braking of FCV  1 , electric power generated by MG  10  is supplied from inverter  12  to battery  40 . 
     In FCV  1 , battery  40  can be charged with a power supply outside a vehicle (external charging) as described above. When external charging was not carried out after system stop (Ready-off) of FCV  1  and the SOC of battery  40  is low at the time of system start-up (Ready-on), however, system output which is the sum of output from FC system  20  and output from battery  40  is restricted after start of travel and traveling performance may be lowered. 
       FIG. 3  is a diagram showing relation of a remaining amount of energy in FC system  20  and battery  40  with a system output upper limit. In  FIG. 3 , the abscissa represents a remaining amount of energy (%) in each of FC system  20  and battery  40 , and the ordinate represents the system output upper limit (W) which is the sum of the output upper limit of FC system  20  and the output upper limit of battery  40 . The remaining amount of energy on the abscissa represents a remaining amount of hydrogen (100% representing a fully filled state) for FC system  20  and represents the SOC for battery  40 . 
     Referring to  FIG. 3 , output upper limit Wfc of FC system  20  is constant regardless of the remaining amount of hydrogen. Namely, FC system  20  can output electric power up to output upper limit Wfc regardless of the remaining amount of hydrogen until fuel runs out. On the other hand, when the SOC becomes lower than a threshold value S 1 , output upper limit Wout of battery  40  decreases with lowering in SOC. Thus, when the SOC of battery  40  becomes lower than threshold value S 1 , the system output upper limit which is the sum of output upper limit Wfc of FC system  20  and output upper limit Wout of battery  40  decreases with lowering in SOC. When the system output upper limit decreases, traveling performance lowers because of restriction of travel power during acceleration. 
     Then, in FCV  1  according to the present first embodiment, when external charging with an external power supply is not carried out under such a condition that the SOC is lower than threshold value S 1  at the time of Ready-off, the CHG mode is automatically selected. More specifically, when the charging cable is not connected to DC inlet  44  or AC inlet  48  for a prescribed time period after Ready-off under the condition that the SOC is lower than threshold value S 1  at the time of Ready-off, it is determined that external charging is not carried out and FC charging by FC system  20  is carried out. 
     Thus, even though external charging is not carried out during system stop of FCV  1 , the SOC can be high at the time of next Ready-on and restriction of system output at the time of start of travel after Ready-on can be avoided. Therefore, lowering in traveling performance after Ready-on can be suppressed. 
       FIG. 4  is a flowchart showing an exemplary procedure in processing performed by FDC-ECU  60  at the time of Ready-off. Some of processing may be allocated to battery ECU  64  or MG-ECU  66  or may be performed by another not-shown ECU (a vehicle ECU that controls the entire vehicle in a centralized manner). 
     Referring to  FIG. 4 , when Ready-off (system stop of FCV  1 ) is requested by an operation onto a start switch (not shown) (YES in step S 10 ), FDC-ECU  60  obtains the SOC of battery  40  from battery ECU  64  and determines whether or not the SOC is lower than threshold value S 1  ( FIG. 3 ) (step S 20 ). When the SOC is equal to or higher than threshold value S 1  (NO in step S 20 ), a series of processing thereafter is not performed and the process proceeds to end. 
     When the SOC is determined in step S 20  as being lower than threshold value S 1  (YES in step S 20 ), FDC-ECU  60  determines whether or not a prescribed operation to carry out external charging has been performed (step S 30 ). Examples of the prescribed operation include an operation to connect the connector of the charging cable to DC inlet  44  or AC inlet  48 . When connection between DC inlet  44  or AC inlet  48  and the connector of the charging cable is sensed, it is determined that the operation has been performed. 
     When FDC-ECU  60  determines that the operation has not been performed (NO in step S 30 ), it determines whether or not a prescribed time period has elapsed (step S 50 ). When the prescribed time period has not elapsed (NO in step S 50 ), the process returns to step S 30 . In other words, in steps S 30  and S 50 , whether or not an operation to carry out external charging has been performed within a prescribed time period after Ready-off is determined. 
     When it is determined in step S 30  that the operation to carry out external charging has been performed (within the prescribed time period after Ready-off) (YES in step S 30 ), battery  40  is externally charged with an external power supply connected to DC inlet  44  or AC inlet  48  (step S 40 ). Specifically, when the connector of the DC charging cable is connected to DC inlet  44 , relays  42  and  46  are closed and battery  40  is charged by a DC charging facility connected to DC inlet  44 . When the connector of the AC charging cable is connected to AC inlet  48 , relays  42  and  52  are closed and charger  50  is driven and battery  40  is charged by an AC charging facility connected to AC inlet  48 . 
     When it is determined in step S 50  that the prescribed time period has elapsed (Yes in step S 50 ), FDC-ECU  60  determines that external charging is not performed, selects the CHG mode, and carries out FC charging for charging battery  40  with FC system  20  (step S 60 ). Specifically, FDC-ECU  60  sets the target SOC of battery  40  to S 2  larger than threshold value S 1  and drives boost converter  24  of FC system  20  to supply electric power from FC system  20  to battery  40 . 
     During FC charging, FDC-ECU  60  determines whether or not the SOC of battery  40  has exceeded S 2  (S 2 &gt;threshold value S 1 ) (step S 70 ). When the SOC is raised by FC charging and the SOC is determined as having attained to S 2  (YES in step S 70 ), FDC-ECU  60  quits FC charging and the process proceeds to end. 
     As set forth above, in the first embodiment, when the operation to carry out external charging is not performed for a prescribed time period after Ready-off under the condition that the SOC is lower than threshold value S 1  at the time of Ready-off, FC system  20  is driven to carry out FC charging. Thus, even though external charging is not carried out during system stop, the SOC can be high at the time of next Ready-on and restriction of system output at the time of start of travel after Ready-on can be avoided. Therefore, according to the first embodiment, lowering in traveling performance after Ready-on can be suppressed. 
     [First Modification] 
     When FC charging is started because external charging is not carried out in spite of the SOC being lower than threshold value S 1  at the time of Ready-off but when an operation to carry out external charging is performed during FC charging, FC charging may be stopped and external charging may be carried out. Thus, when external charging is requested during FC charging, decrease in remaining amount of fuel can be suppressed by switching from FC charging to external charging. 
       FIG. 5  is a flowchart showing an exemplary procedure in processing performed by FDC-ECU  60  at the time of Ready-off in a first modification. This flowchart corresponds to  FIG. 4  referred to in the first embodiment. 
     Referring to  FIG. 5 , since processing in steps S 110  to S 170  is the same as the processing in steps S 10  to S 70  in  FIG. 4 , description will not be repeated. In the first modification, when FC charging is carried out in step S 160 , FDC-ECU  60  determines whether or not a prescribed operation to carry out external charging has been performed during FC charging (step S 162 ). 
     When it is determined that the operation has not been performed (NO in step S 162 ), the process proceeds to step S 170  and FDC-ECU  60  determines whether or not the SOC of battery  40  is higher than S 2  (S 2 &gt;threshold value S 1 ). 
     When it is determined in step S 162  that an operation to carry out external charging has been performed during FC charging (YES in step S 162 ), FDC-ECU  60  stops FC charging (step S 164 ). Specifically, FDC-ECU  60  stops driving boost converter  24  of FC system  20 . 
     When FC charging is stopped, the process proceeds to step S 140  and FDC-ECU  60  carries out external charging with an external power supply connected to DC inlet  44  or AC inlet  48 . 
     As set forth above, according to the first modification, even during FC charging, when external charging is requested, FC charging is stopped and external charging is carried out. Therefore, decrease in remaining amount of fuel at the time of Ready-on can be suppressed. 
     [Second Modification] 
     Even in a case that the SOC is lower than threshold value S 1  at the time of Ready-off and an operation to carry out external charging is performed, when an electricity fee for external charging is determined as being relatively high and higher than cost for fuel, FC charging may be carried out without carrying out external charging. Cost for energy (cost for charging) for suppressing lowering in traveling performance after Ready-on can thus be suppressed. 
       FIG. 6  is a flowchart showing an exemplary procedure in processing performed by FDC-ECU  60  at the time of Ready-off in a second modification. This flowchart also corresponds to  FIG. 4  referred to in the first embodiment. 
     Referring to  FIG. 6 , since processing in steps S 210  to S 270  is the same as the processing in steps S 10  to S 70  in  FIG. 4 , description will not be repeated. In the second modification, when it is determined in step S 230  that an operation to carry out external charging has been performed (YES in step S 230 ), FDC-ECU  60  obtains from a server of an electric power utility company, a unit price of an electricity fee in carrying out external charging, and determines whether or not the electricity fee for external charging is higher than a prescribed amount (step S 232 ). 
     For example, FDC-ECU  60  calculates an amount of charging power until the SOC attains to S 2 , and when the electricity fee in replenishment of the amount of power with external charging is higher than fuel expenses, the FDC-ECU determines that the electricity fee for external charging is higher than the prescribed amount. The electricity fee for external charging can be calculated by multiplying the obtained unit price of the electricity fee by the amount of charging power. The fuel expenses in replenishment by FC charging can be calculated by obtaining in advance relation between electric power generated by FC system  20  and an amount of fuel consumption as well as a unit price of fuel. Alternatively, when the unit price of the electricity fee in carrying out external charging is higher than a price level on which distinction between a relatively high unit price during daytime and a relatively low unit price during late night is based, the electricity fee for external charging may be determined as being higher than the prescribed amount. 
     When the electricity fee is determined in step S 232  as being higher than the prescribed amount (YES in step S 232 ), the process proceeds to step S 260  and FDC-ECU  60  carries out FC charging. 
     When the electricity fee is determined in step S 232  as being equal to or lower than the prescribed amount (NO in step S 232 ), the process proceeds to step S 240  and FDC-ECU  60  carries out external charging. 
     As set forth above, according to the second modification, when the electricity fee for external charging is high, FC charging is carried out in spite of a request for external charging. Therefore, cost for energy for suppressing lowering in traveling performance after Ready-on can be suppressed. 
     Second Embodiment 
     In the first embodiment, when external charging is not carried out in spite of the SOC being lower than threshold value S 1  at the time of Ready-off, FC charging is carried out. In a second embodiment, when the SOC is lower than threshold value S 1  at the time of Ready-off, an operation button that allows a user to indicate execution of FC charging is shown on HMI apparatus  68 . The user may thus permit FC charging with the button, or when the user has simply forgotten an operation to carry out external charging, the user can have external charging done by performing the operation and not permitting FC charging. 
     The FCV according to the second embodiment is identical in overall configuration to FCV  1  according to the first embodiment shown in  FIG. 1 . 
       FIG. 7  is a flowchart showing an exemplary procedure in processing performed by FDC-ECU  60  at the time of Ready-off in the second embodiment. This flowchart corresponds to  FIG. 4  referred to in the first embodiment. 
     Referring to  FIG. 7 , since processing in steps S 310  and S 320  is the same as the processing in steps S 10  and S 20  in  FIG. 4 , description will not be repeated. In the second embodiment, when the SOC is determined in step S 320  as being lower than threshold value S 1  ( FIG. 3 ) (YES in step S 320 ), FDC-ECU  60  controls HMI apparatus  68  to show a switch (an operation button) that allows a user to select whether or not to permit execution of FC charging (step S 330 ). 
       FIG. 8  is a diagram showing an exemplary state of display on HMI apparatus  68  in step S 330  in  FIG. 7 . Referring to  FIG. 8 , a pop-up screen  80  is shown on the touch panel display of HMI apparatus  68 . On pop-up screen  80 , an input section  82  for indicating permission of FC charging and an input section  84  for indicating non-permission of FC charging are shown, for example, together with a character string “REMAINING BATTERY POWER IS LOW. DO YOU PERMIT CHARGING OF BATTERY (FC charging) USING HYDROGEN?” When the user touches input section  82 , FC charging is permitted, and when the user touches input section  84 , FC charging is not permitted. 
     Referring again to  FIG. 7 , FDC-ECU  60  determines whether or not the user has permitted FC charging on HMI apparatus  68  (step S 340 ). Then, when FC charging has been permitted (YES in step S 340 ), the process proceeds to step S 350  and FDC-ECU  60  carries out FC charging. Since processing in steps S 350  and S 360  is the same as the processing in steps S 60  and S 70  in  FIG. 4 , description will not be repeated. 
     When FC charging is not permitted (NO in step S 340 ), the process proceeds to end without steps S 350  and S 360  being performed. Though not particularly shown, when an operation to carry out external charging is thereafter performed, external charging is carried out. 
     As set forth above, in the second embodiment, when the SOC is lower than threshold value S 1  at the time of Ready-off, the operation button (input sections  82  and  84 ) that allows a user to indicate execution of FC charging is shown on HMI apparatus  68 . Thus, even though external charging is not carried out during system stop, FC charging is carried out during system stop in accordance with an instruction from a user and the SOC can be high at the time of next Ready-on. Therefore, according to the second embodiment, restriction of system output at the time of Ready-on can be avoided, and consequently lowering in traveling performance after Ready-on can be suppressed. 
     Third Embodiment 
     In the first embodiment, when external charging is not carried out under the condition that the SOC of battery  40  is lower than threshold value S 1  at the time of Ready-off, FC charging is carried out. In a third embodiment, FC charging is carried out when the SOC is lower than threshold value S 1  at the time of Ready-on. 
     The FCV according to the third embodiment is also identical in overall configuration to FCV  1  according to the first embodiment shown in  FIG. 1 . 
       FIG. 9  is a flowchart showing an exemplary procedure in processing performed by FDC-ECU  60  at the time of Ready-on in the third embodiment. A series of processing shown in this flowchart is started in response to Ready-on. 
     Referring to  FIG. 9 , FDC-ECU  60  obtains the SOC of battery  40  from battery ECU  64  and determines whether or not the SOC is lower than threshold value S 1  ( FIG. 3 ) (step S 410 ). When the SOC is equal to or higher than threshold value S 1  (NO in step S 410 ), the process proceeds to end without the series of processing thereafter being performed. 
     When the SOC is determined in step S 410  as being lower than threshold value S 1  (YES in step S 410 ), FDC-ECU  60  selects the CHG mode regardless of a state of mode switch  62  ( FIG. 1 ) and carries out FC charging (step S 420 ). Specifically, FDC-ECU  60  sets the target SOC of battery  40  to S 2  larger than threshold value S 1  and drives boost converter  24  of FC system  20  such that electric power is supplied from FC system  20  to battery  40 . 
     While FC charging is being carried out, FDC-ECU  60  determines whether or not the SOC of battery  40  has exceeded S 2  (S 2 &gt;threshold value S 1 ) (step S 430 ). When the SOC is raised by FC charging and the SOC is determined as having attained to S 2  (YES in step S 430 ), FDC-ECU  60  quits FC charging and the process proceeds to end. 
     Though not particularly shown, FDC-ECU  60  thereafter switches the travel mode in accordance with setting made through mode switch  62  ( FIG. 1 ). 
     As set forth above, in the third embodiment, FC charging is carried out when the SOC is lower than threshold value S 1  at the time of Ready-on. Therefore, restriction of system output due to lowering in SOC can be avoided. Therefore, according to the third embodiment, lowering in traveling performance after Ready-on can be suppressed. 
     The third embodiment may be combined with the first embodiment and the modifications thereof as well as the second embodiment. Specifically, when the SOC is lower than threshold value S 1  at the time of Ready-off, the processing shown in the first embodiment and the modifications thereof as well as the second embodiment may be performed and processing shown in  FIG. 9  may further be performed at the time of next Ready-on. 
     Though embodiments of the present disclosure have been described above, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.