Patent Publication Number: US-8527126-B2

Title: Power supply system for electrically powered vehicle and method for controlling the same

Description:
TECHNICAL FIELD 
     The present invention relates to a power supply system for an electrically powered vehicle and a method for controlling the same, and more particularly to control of a power supply system for an electrically powered vehicle equipped with a main power storage device and a plurality of sub power storage devices. 
     BACKGROUND ART 
     In recent years, electrically powered vehicles such as electric cars, hybrid cars, fuel cell cars, and the like have been developed into practical use as environmentally friendly vehicles. These electrically powered vehicles are each equipped with an electric motor generating force to drive the vehicle, and a power supply system configured to include a power storage device for supplying electric power to drive the electric motor. 
     In particular for hybrid cars, there has been proposed a configuration charging a vehicle-mounted power storage device by a power supply external to the vehicle (hereinafter also referred to as an “external power supply”). Accordingly, these electrically powered vehicles have been required to have an increased distance travelable using electric power stored in the vehicle-mounted power storage device. Hereinafter, charging of a vehicle-mounted power storage device by an external power supply will also be referred to simply as “external charging”. 
     For example, Japanese Patent Laying-Open No. 2008-109840 (Patent Document 1) and Japanese Patent Laying-Open No. 2003-209969 (Patent Document 2) describe a power supply system having a plurality of power storage devices (batteries) connected in parallel. The power supply system described in Patent Documents 1 and 2 includes a voltage converter (a converter) provided for each power storage device (battery) for serving as a charging/discharging adjustment mechanism. In contrast, Japanese Patent Laying-Open No. 2008-167620 (Patent Document 3) describes a configuration of a power supply device in a vehicle equipped with a main power storage device and a plurality of sub power storage devices, the power supply device including a converter associated with the main power storage device and a converter shared by the plurality of sub power storage devices. This configuration can achieve a reduced number of elements of the device and also an increased amount of energy that can be stored.
     Patent Document 1: Japanese Patent Laying-Open No. 2008-109840   Patent Document 2: Japanese Patent Laying-Open No. 2003-209969   Patent Document 3: Japanese Patent Laying-Open No. 2008-167620   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the power supply device described in Patent Document 3, one of the plurality of sub power storage devices is selectively connected to the converter to allow the main power storage device and the selected sub power storage device to supply electric power to drive an electric motor for driving a vehicle. In such a power supply device, when the SOC (State of Charge) of the sub power storage device in use decreases, another sub power storage device is newly connected to the converter. In such a manner, the plurality of sub power storage devices are used sequentially, thereby increasing a travelable distance (EV (Electric Vehicle) travelable distance) achieved by stored electric energy. However, at the time of switching connection of the sub power storage device, it is necessary to perform a switching process through an appropriate process procedure to avoid failure in equipment due to occurrence of a short-circuit path, and the like. 
     The present invention has been made to solve such problems, and one object of the present invention is to appropriately perform, in a power supply system configured to include a main power storage device and a plurality of sub power storage devices, and a voltage converter (a converter) shared by the plurality of sub power storage devices, a connection switching process for changing a sub power storage device to be used. 
     Means for Solving the Problems 
     According to the present invention, a power supply system for an electrically powered vehicle equipped with a motor generating power to drive the vehicle includes a main power storage device, an electric power feeding line, a first voltage converter, a plurality of sub power storage devices provided in parallel to each other, a second voltage converter, a connection unit, and a switching control device. The electric power feeding line is configured to supply electric power to an inverter that controls and drives the motor. The first voltage converter is provided between the electric power feeding line and the main power storage device, and configured to convert voltage therebetween bidirectionally. The second voltage converter is provided between the plurality of sub power storage devices and the electric power feeding line, and configured to convert voltage between one of the plurality of sub power storage devices and the electric power feeding line bidirectionally. The connection unit is provided between the plurality of sub power storage devices and the second voltage converter, and configured to selectively connect a sub power storage device selected from the plurality of sub power storage devices to the second voltage converter. The switching control device is configured to control selective connection between the plurality of sub power storage devices and the second voltage converter. The switching control device includes a switching determination unit, a first electric power limiter unit, a connection switching control unit, and a second electric power limiter unit. The switching determination unit is configured to determine whether or not the selected sub power storage device should be switched based on states of charge of the plurality of sub power storage devices. A step-up-voltage instruction unit is configured to instruct the first voltage converter to provide a voltage on the electric power feeding line to be a first voltage higher than a voltage output from the main power storage device and a voltage output from a sub power storage device to be connected to the second voltage converter after switching, when the switching determination unit determines that it is necessary to switch the selected sub power storage device. The first electric power limiter unit decreases values of upper limits on electric power input/output to/from the selected sub power storage device gradually to zero after the voltage on the electric power feeding line has reached the first voltage. The connection switching control unit is configured to switch connection between the plurality of sub power storage devices and the second voltage converter, when the first electric power limiter unit sets the values of the upper limits on electric power input/output to zero. The second electric power limiter unit is configured to increase the values of the upper limits on electric power input/output gradually to values corresponding to a state of charge of the sub power storage device newly connected to the second voltage converter after the connection switching control unit switches connection between the plurality of sub power storage devices and the second voltage converter. 
     Alternatively, in a method for controlling a power supply system for an electrically powered vehicle according to the present invention, the power supply system includes the main power storage device, the electric power feeding line, the first voltage converter, the plurality of sub power storage devices, the second voltage converter, and the connection unit described above. The method for controlling includes the steps of determining whether or not the selected sub power storage device should be switched based on states of charge of the plurality of sub power storage devices, instructing the first voltage converter to provide a voltage on the electric power feeding line to be a first voltage higher than a voltage output from the main power storage device and a voltage output from a sub power storage device to be connected to the second voltage converter after switching, when the step of determining determines that it is necessary to switch the selected sub power storage device, decreasing values of upper limits on electric power input/output to/from the selected sub power storage device gradually to zero after the voltage on the electric power feeding line has reached the first voltage, switching connection between the plurality of sub power storage devices and the second voltage converter when the step of decreasing sets the values of the upper limits on electric power input/output to zero, and increasing the values of the upper limits on electric power input/output gradually to values corresponding to a state of charge of the sub power storage device newly connected to the second voltage converter after the step of switching switches connection between the plurality of sub power storage devices and the second voltage converter. 
     Preferably, the step-up-voltage instruction unit continues to instruct the first voltage converter to provide the voltage on the electric power feeding line to be the first voltage until a process for increasing the values of the upper limits on electric power input/output by the second electric power limiter unit ends. Alternatively, the method for controlling further includes the step of continuing to instruct the first voltage converter to provide the voltage on the electric power feeding line to be the first voltage until a process for increasing the values of the upper limits on electric power input/output in the step of increasing ends. 
     Preferably, the first voltage corresponds to a value of an upper limit on the voltage on the electric power feeding line controlled by the first voltage converter. 
     According to the power supply system for an electrically powered vehicle and the method for controlling the same described above, at the time of switching connection between the second voltage converter and a sub power storage device, voltage on the electric power feeding line is stepped up to the first voltage higher than any of the voltage output from the main power storage device and the voltage output from a sub power storage device to be newly used, and thereafter the sub power storage device to be newly used can be connected to the second voltage converter. This can prevent formation of a short-circuit path from the sub power storage device to be newly used via the electric power feeding line. Further, the values of the upper limits on electric power input/output to/from the sub power storage device are decreased before switching connection of the sub power storage device, and the values of the upper limits on electric power input/output are caused to return gradually after completion of switching of connection. This can prevent the power supply system from being requested to excessively charge/discharge electric power in a period in which electric power cannot be input/output to/from the sub power storage device due to switching of connection. 
     Preferably, the switching control device further includes a third electric power limiter unit, and the third electric power limiter unit is configured to temporarily relax charging and discharging limits for the main power storage device in a period from when the first electric power limiter unit starts decreasing the values of the upper limits on electric power input/output to when the connection unit completes switching of connection between the plurality of sub power storage devices and the second voltage converter. Alternatively, the method for controlling further includes the step of temporarily relaxing charging and discharging limits for the main power storage device in a period from when the step of decreasing starts decreasing the values of the upper limits on electric power input/output to when the connection unit completes switching of connection between the plurality of sub power storage devices and the second voltage converter. 
     With this configuration, charging and discharging limits on electric power of the main power storage device are temporarily relaxed in a period in which electric power cannot be input/output to/from the sub power storage device due to switching of connection of the sub power storage device. This can ensure upper limits on electric power input/output in the entire power supply system. 
     More preferably, the electrically powered vehicle further includes an internal combustion engine configured to be capable of outputting power to drive the vehicle independently of the motor, and a traveling control unit. The traveling control unit is configured to start the internal combustion engine when total required power for the electrically powered vehicle is greater than a sum of a value of an upper limit on electric power output from the main power storage device and the value of the upper limit on electric power output from the selected sub power storage device. 
     With this configuration, by appropriately setting the values of the upper limits on electric power input/output at the time of switching connection of the sub power storage device, the power supply system can be prevented from being requested to excessively charge/discharge. In addition, by temporarily relaxing charging and discharging limits for the main power storage device, the internal combustion engine can be prevented from being restarted at the time of switching connection of the sub power storage device. 
     Effects of the Invention 
     According to the present invention, in a power supply system configured to include a main power storage device and a plurality of sub power storage devices, and a voltage converter (a converter) shared by the plurality of sub power storage devices, a connection switching process for changing a sub power storage device to be used can be performed appropriately. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a main configuration of an electrically powered vehicle equipped with a power supply system in accordance with an embodiment of the present invention. 
         FIG. 2  is a circuit diagram showing a detailed configuration of each inverter shown in  FIG. 1 . 
         FIG. 3  is a circuit diagram showing a detailed configuration of each converter shown in  FIG. 1 . 
         FIG. 4  is a functional block diagram for illustrating how traveling of the electrically powered vehicle is controlled. 
         FIG. 5  is a flowchart showing a schematic procedure of a process performed to switch connection of a selected sub power storage device in the power supply system for the electrically powered vehicle according to the embodiment of the present invention. 
         FIG. 6  is a flowchart for illustrating in detail a process performed to determine whether the sub power storage device should be switched, as shown in  FIG. 5 . 
         FIG. 7  is a flowchart for illustrating in detail a pre-switching voltage step-up process shown in  FIG. 5 . 
         FIG. 8  is a flowchart for illustrating in detail an electric power limit modification process shown in  FIG. 5 . 
         FIG. 9  is a flowchart for illustrating in detail a connection switching process shown in  FIG. 5 . 
         FIG. 10  is a flowchart for illustrating in detail a return process shown in  FIG. 5 . 
         FIG. 11  is a waveform diagram of an operation performed in the process for switching the selected sub power storage device in the power supply system for the electrically powered vehicle according to the embodiment of the present invention. 
         FIG. 12  is a functional block diagram for illustrating a functional portion for the process for switching the selected sub power storage device, in a configuration controlling the power supply system of the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE REFERENCE SIGNS 
       1 : electrically powered vehicle,  2 : wheel,  3 : power split device,  4 : engine,  6 : battery charging converter (external charging),  8 : external power supply,  9 A,  9 B 1 ,  9 B 2 : current sensor,  10 A,  10 B 1 ,  10 B 2 ,  13 ,  21 A,  21 B: voltage sensor,  11 A,  11 B 1 ,  11 B 2 : temperature sensor,  12 A: converter (dedicated to main power storage device),  12 B: converter (shared by sub power storage devices),  14 ,  22 : inverter,  15 - 17 : each phase arm (U, V, W),  24 ,  25 : current sensor,  30 : control device,  39 A: connection unit (for main power storage device),  39 B: connection unit (for sub power storage device),  100 : switching determination unit,  110 : step-up-voltage instruction unit,  120 : electric power limiter unit (for main power storage device),  130 : electric power limiter unit (for sub power storage device),  140 : connection switching control unit,  200 : converter control unit,  250 : traveling control unit,  260 : total power calculation unit,  270 ,  280 : inverter control unit, BA: battery (main power storage device), BB: selected sub power storage device, BB 1 , BB 2 : battery (sub power storage device), C 1 , C 2 , CH: smoothing capacitor, CMBT: step-up-voltage command signal, CONT 1  to CONT 7 : relay control signal, D 1  to D 8 : diode, FBT: flag (stepping up voltage completed), IA, IB 1 , IB 2 : input/output current (battery), ID: variable (status of switching process), IGON: start signal, L 1 : reactor, MCRT 1 , MCRT 2 : motor current value, MG 1 , MG 2 : motor generator, PL 1 A, PL 1 B: power supply line, PL 2 : electric power feeding line, Pttl: total required power, PWMI, PWMI 1 , PWMI 2 , PWMC, PWMC 1 , PWMC 2 : control signal (for inverter), PWU, PWUA, PWDA, PWD, PWDA, PWDB: control signal (for converter), Q 1  to Q 8 : IGBT device, R: limiting resistor, SL 1 , SL 2 : ground line, SMR 1  to SMR 3 : system main relay, SR 1 , SR 1 G, SR 2 , SR 2 G: relay, TA, TBB 1 , TBB 2 : battery temperature (battery), Tqcom 1 , Tqcom 2 : torque command value, UL, VL, WL: line (three phase), V 1 : predetermined voltage, VBA, VBB 1 , VBB 2 : voltage (battery output voltage), VLA, VLB, VH: voltage, VHref: voltage command value (VH), Win: upper limit on electric power input, Win(M): upper limit on electric power input (to main power storage device), Win(S): upper limit on electric power input (to selected sub power storage device), Wout: upper limit on electric power output, Wout(M): upper limit on electric power output (from main power storage device), Wout(S): upper limit on electric power output (from selected sub power storage device). 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter reference will be made to the drawings to more specifically describe the present invention in embodiments. In the following description, identical or equivalent components are denoted by identical reference characters and will in principle not be described repeatedly. 
       FIG. 1  is a diagram showing a main configuration of an electrically powered vehicle equipped with a power supply system in accordance with an embodiment of the present invention. 
     With reference to  FIG. 1 , an electrically powered vehicle  1  includes power storage devices implemented as batteries BA, BB 1 , BB 2 , connection units  39 A,  39 B, converters  12 A,  12 B, smoothing capacitors C 1 , C 2 , CH, voltage sensors  10 A,  10 B 1 ,  10 B 2 ,  13 ,  21 A,  21 B, temperature sensors  11 A,  11 B 1 ,  11 B 2 , current sensors  9 A,  9 B 1 ,  9 B 2 , an electric power feeding line PL 2 , inverters  14 ,  22 , motor generators MG 1 , MG 2 , a wheel  2 , a power split device  3 , an engine  4 , and a control device  30 . 
     A power supply system for the electrically powered vehicle shown in the present embodiment includes a main power storage device implemented as battery BA, electric power feeding line PL 2  supplying electric power to inverter  14  driving motor generator MG 2 , converter  12 A provided between the main power storage device (BA) and electric power feeding line PL 2  to serve as a voltage converter converting voltage bidirectionally, batteries BB 1 , BB 2  implementing a plurality of sub power storage devices provided in a manner parallel to each other, and converter  12 B provided between the plurality of sub power storage devices (BB 1 , BB 2 ) and electric power feeding line PL 2  to serve as a voltage converter converting voltage bidirectionally. The voltage converter ( 12 B) is connected selectively to one of the plurality of sub power storage devices (BB 1 , BB 2 ) to convert voltage between the connected sub power storage device and electric power feeding line PL 2  bidirectionally. 
     A sub power storage device (one of BB 1  and BB 2 ) and the main power storage device (BA) have their storable capacity set so that, for example, when they are concurrently used, they can output maximum power tolerated for an electric load ( 22  and MG 2 ) connected to the electric power feeding line. This allows the vehicle without using the engine, i.e., traveling as an EV (Electric Vehicle), to travel with maximum power. If the sub power storage device&#39;s state of power storage is decreased, the sub power storage device can be exchanged to cause the vehicle to further travel, and if the sub power storage device&#39;s electric power has completely been consumed, then, in addition to the main power storage device, the engine can be used to allow the vehicle to travel with maximum power without using the sub power storage device. 
     Furthermore, such a configuration allows converter  12 B to be shared by the plurality of sub power storage devices. This can eliminate the necessity of increasing the number of converters to be equal to that of power storage devices. For further increased EV travelable distance, an additional battery can be introduced in parallel with batteries BB 1 , BB 2 . 
     Preferably, the main power storage device and the sub power storage devices mounted in this electrically powered vehicle are externally chargeable. For this purpose, electrically powered vehicle  1  further includes a battery charging device (a charging converter)  6  for connection to an external power supply  8  which is for example a commercial power supply of AC 100V. Battery charging device  6  converts alternate current into direct current and also adjusts voltage to supply electric power charged to a battery. Note that external charging may be achieved by the above described configuration and in addition a system connecting a neutral point of a stator coil of motor generator MG 1 , MG 2  to an alternate current power supply or a system causing converters  12 A,  12 B to together function as an AC/DC conversion device. 
     Smoothing capacitor C 1  is connected between a power supply line PL 1 A and a ground line SL 2 . Voltage sensor  21 A detects a voltage VLA across ends of smoothing capacitor C 1  and outputs it to control device  30 . Converter  12 A can step up the voltage across terminals of smoothing capacitor C 1  and supply it to electric power feeding line PL 2 . 
     Smoothing capacitor C 2  is connected between a power supply line PL 1 B and ground line SL 2 . Voltage sensor  21 B detects a voltage VLB across ends of smoothing capacitor C 2  and outputs it to control device  30 . Converter  12 B can step up the voltage across terminals of smoothing capacitor C 2  and supply it to electric power feeding line PL 2 . 
     Smoothing capacitor CH smoothes the voltage stepped up by converter  12 A,  12 B. Voltage sensor  13  senses a voltage VH across terminals of smoothing capacitor CH and outputs it to control device  30 . 
     Alternatively, in an opposite direction, converters  12 A,  12 B can step down voltage VH across terminals smoothed by smoothing capacitor CH and supply it to power supply lines PL 1 A, PL 1 B. 
     Inverter  14  receives direct current voltage from converter  12 B and/or  12 A, converts it into three-phase alternate current voltage, and outputs it to motor generator MG 1 . Inverter  22  receives direct current voltage from converter  12 B and/or  12 A, converts it into three-phase alternate current voltage, and outputs it to motor generator MG 2 . 
     Power split device  3  is a mechanism coupled to engine  4  and motor generators MG 1 , MG 2  to distribute motive power therebetween. The power split device can for example be a planetary gear mechanism having three shafts of rotation of a sun gear, a planetary carrier, and a ring gear. In the planetary gear mechanism, when two of the three shafts of rotation have their rotation determined, that of the other one shaft of rotation is compulsively determined. These three shafts of rotation are connected to engine  4  and motor generators MG 1 , MG 2  at their respective shafts of rotation, respectively. Motor generator MG 2  has its shaft of rotation coupled to wheel  2  by a reduction gear, a differential gear or the like (not shown). Furthermore, power split device  3  may further have a speed reducer incorporated therein for the shaft of rotation of motor generator MG 2 . 
     Connection unit  39 A includes a system main relay SMR 2  connected between the positive electrode of battery BA and power supply line PL 1 A, a system main relay SMR 1  and a limiting resistor R connected in series and connected in parallel with system main relay SMR 2 , and a system main relay SMR 3  connected between the negative electrode of battery BA (a ground line SL 1 ) and a node N 2 . 
     System main relays SMR 1  to SMR 3  have their conduction/non-conduction states controlled (or are turned on/off) by relay control signals CONT 1  to CONT 3 , respectively, issued from control device  30 . 
     Voltage sensor  10 A measures a voltage VA across terminals of battery BA. Furthermore, temperature sensor  11 A measures a temperature TA of battery BA, and current sensor  9 A measures a current IA input/output to/from battery BA. These sensors&#39; measurements are output to control device  30 . Based on these measurements, control device  30  monitors a state of battery BA represented by the state of charge (SOC). 
     Connection unit  39 B is provided between power supply line PL 1 B and ground line SL 2 , and batteries BB 1 , BB 2 . Connection unit  39 B includes a relay SR 1  connected between the positive electrode of battery BB 1  and power supply line PL 1 B, a relay SR 1 G connected between the negative electrode of battery BB 1  and ground line SL 2 , a relay SR 2  connected between the positive electrode of battery BB 2  and power supply line PL 1 B, and a relay SR 2 G connected between the negative electrode of battery BB 2  and ground line SL 2 . 
     Relays SR 1 , SR 2  have their conduction/non-conduction states controlled (or are turned on/off) by relay control signals CONT 4 , CONT 5 , respectively, issued from control device  30 . Relays SR 1 G, SR 2 G have their conduction/non-conduction states controlled (or are turned on/off) by relay control signals CONT 6 , CONT 7 , respectively, issued from control device  30 . Ground line SL 2  extends through converters  12 A,  12 B toward inverters  14  and  22 , as will be described later. 
     Voltage sensors  10 B 1  and  10 B 2  measure voltages VBB 1  and VBB 2  across terminals of batteries BB 1  and BB 2 , respectively. Temperature sensors  11 B 1  and  11 B 2  measure temperatures TBB 1  and TBB 2  of batteries BB 1  and BB 2 , respectively. Current sensors  9 B 1  and  9 B 2  measure currents IB 1  and IB 2  input/output to/from batteries BB 1  and BB 2 , respectively. These sensors&#39; measurements are output to control device  30 . Based on these measurements, control device  30  monitors states of batteries BB 1 , BB 2  represented by the states of charge (SOC). 
     Battery BA, BB 1 , BB 2  can for example be a lead-acid battery, a nickel metal hydride battery, a lithium ion battery or a similar secondary battery, an electric double layer capacitor or a similar capacitor of large capacity, or the like. 
     Inverter  14  is connected to electric power feeding line PL 2  and ground line SL 2 . Inverter  14  receives a voltage stepped up from converter  12 A and/or converter  12 B, and drives motor generator MG 1  for example to start engine  4 . Furthermore, inverter  14  returns to converters  12 A and  12 B the electric power generated by motor generator MG 1  by motive power transmitted from engine  4 . At this time, converters  12 A and  12 B are controlled by control device  30  to operate as step-down converters. 
     Current sensor  24  detects a current that flows to motor generator MG 1  as a motor current value MCRT 1 , and outputs motor current value MCRT 1  to control device  30 . 
     Inverter  22  is connected to electric power feeding line PL 2  and ground line SL 2  in a manner parallel with inverter  14 . Inverter  22  receives direct current voltage from converters  12 A and  12 B, converts it into three-phase alternate current voltage, and outputs it to motor generator MG 2  driving wheel  2 . Furthermore, inverter  22  returns to converters  12 A and  12 B the electric power generated by motor generator MG 2  as the vehicle is regeneratively braked. At this time, converters  12 A and  12 B are controlled by control device  30  to operate as step-down converters. 
     Current sensor  25  detects a current that flows to motor generator MG 2  as a motor current value MCRT 2 , and outputs motor current value MCRT 2  to control device  30 . 
     Control device  30  is constituted of an electronic control unit (ECU) having a central processing unit (CPU) and a memory (not shown) incorporated therein, and in accordance with a map and a program stored in the memory, uses each sensor&#39;s measurement to perform operation processing. Note that control device  30  may have a portion configured to allow an electronic circuit or similar hardware to perform predetermined arithmetic and logical operation processing. 
     More specifically, control device  30  receives torque command values and rotation speeds of motor generators MG 1 , MG 2 , values of voltages VBA, VBB 1 , VBB 2 , VLA, VLB, VH, motor current values MCRT 1 , MCRT 2 , and a start signal IGON. Then, control device  30  outputs a control signal PWUB instructing converter  12 B to step up voltage, a control signal PWDB instructing converter  12 B to step down voltage, and a shutdown signal instructing converter  12 B to prohibit operation. 
     Furthermore, control device  30  outputs a control signal PWMI 1  instructing inverter  14  to convert direct current voltage output from converters  12 A,  12 B into alternate current voltage for driving motor generator MG 1 , and a control signal PWMC 1  instructing inverter  14  to convert alternate current voltage generated by motor generator MG 1  into direct current voltage and return it toward converters  12 A,  12 B for regeneration. 
     Similarly, control device  30  outputs a control signal PWMI 2  instructing inverter  22  to convert direct current voltage into alternate current voltage for driving motor generator MG 2 , and a control signal PWMC 2  instructing inverter  22  to convert alternate current voltage generated by motor generator MG 2  into direct current voltage and return it toward converters  12 A,  12 B for regeneration. 
       FIG. 2  is a circuit diagram showing a detailed configuration of inverters  14  and  22  shown in  FIG. 1 . 
     With reference to  FIG. 2 , inverter  14  includes a U phase arm  15 , a V phase arm  16 , and a W phase arm  17 . U phase arm  15 , V phase arm  16 , and W phase arm  17  are connected between electric power feeding line PL 2  and ground line SL 2  in parallel. 
     U phase arm  15  includes insulated gate bipolar transistor (IGBT) devices Q 3 , Q 4  connected in series between electric power feeding line PL 2  and ground line SL 2 , IGBT devices Q 3 , Q 4 , and their respective anti-parallel diodes D 3 , D 4 . Diode D 3  has its cathode connected to IGBT device Q 3  at the collector, and its anode connected to IGBT device Q 3  at the emitter. Diode D 4  has its cathode connected to IGBT device Q 4  at the collector, and its anode connected to IGBT device Q 4  at the emitter. 
     V phase arm  16  includes IGBT devices Q 5 , Q 6  connected in series between electric power feeding line PL 2  and ground line SL 2 , and their respective anti-parallel diodes D 5 , D 6 . IGBT devices Q 5 , Q 6  and anti-parallel diodes D 5 , D 6  are connected similarly as in U phase arm  15 . 
     W phase arm  17  includes IGBT devices Q 7 , Q 8  connected in series between electric power feeding line PL 2  and ground line SL 2 , and their respective anti-parallel diodes D 7 , D 8 . IGBT devices Q 7 , Q 8  and anti-parallel diodes D 7 , D 8  are also connected similarly as in U phase arm  15 . 
     Note that, in the present embodiment, an IGBT device is indicated as a representative example of a power semiconductor switching element controllable to be turned on/off. In other words, it is also replaceable with a bipolar transistor, a field effect transistor or a similar power semiconductor switching element. 
     Each phase arm has an intermediate point connected to motor generator MG 1  at each phase coil at each phase end. In other words, motor generator MG 1  is a three-phase permanent magnet synchronous motor, and the three U, V, W phase coils each have one end connected together to an intermediate point. The U phase coil has the other end connected to a line UL drawn from a connection node of IGBT devices Q 3 , Q 4 . The V phase coil has the other end connected to a line VL drawn from a connection node of IGBT devices Q 5 , Q 6 . The W phase coil has the other end connected to a line WL drawn from a connection node of IGBT devices Q 7 , Q 8 . 
     Inverter  22  shown in  FIG. 1  is different in that it is connected to motor generator MG 2 . However, its internal circuit configuration is similar to that of inverter  14 , and accordingly it will not be described repeatedly in detail. Furthermore,  FIG. 2  shows an inverter receiving control signals PWMI, PWMC, however, this is to avoid complexity. As shown in  FIG. 1 , different control signals PWMI 1 , PWMC 1  and control signals PWMI 2 , PWMC 2  are input to inverters  14 ,  22 , respectively. 
       FIG. 3  is a circuit diagram showing a detailed configuration of converters  12 A and  12 B shown in  FIG. 1 . 
     With reference to  FIG. 3 , converter  12 A includes a reactor L 1  having one end connected to power supply line PL 1 A, IGBT devices Q 1 , Q 2  connected in series between electric power feeding line PL 2  and ground line SL 2 , and their respective anti-parallel diodes D 1 , D 2 . 
     Reactor L 1  has the other end connected to IGBT device Q 1  at the emitter and to IGBT device Q 2  at the collector. Diode D 1  has its cathode connected to IGBT device Q 1  at the collector and its anode connected to IGBT device Q 1  at the emitter. Diode D 2  has its cathode connected to IGBT device Q 2  at the collector, and its anode connected to IGBT device Q 2  at the emitter. 
     Converter  12 B shown in  FIG. 1  is different from converter  12 A in that the former is not connected to power supply line PL 1 A and instead to power supply line PL 1 B. However, its internal circuit configuration is similar to that of converter  12 A, and accordingly it will not be described repeatedly in detail. Furthermore,  FIG. 3  shows a converter receiving control signals PWU, PWD, however, this is to avoid complexity. As shown in  FIG. 1 , different control signals PWUA, PWDA and control signals PWUB, PWDB are input to inverters  14 ,  22 , respectively. 
     In the power supply system for electrically powered vehicle  1 , battery BA (the main power storage device) and a sub power storage device selected from batteries BB 1 , BB 2  (hereinafter also referred to as a “selected sub power storage device BB”), and motor generators MG 1 , MG 2  supply and receive electric power therebetween. 
     Control device  30  receives values detected by voltage sensor  10 A, temperature sensor  11 A, and current sensor  9 A, and in accordance therewith sets an SOC(M) indicating the main power storage device&#39;s residual capacity, an upper limit on electric power input Win(M) indicating an upper limit value of electric power charged thereto, and an upper limit on electric power output Wout(M) indicating an upper limit value of electric power discharged therefrom. 
     Furthermore, control device  30  receives values detected by voltage sensors  10 B 1 ,  10 B 2 , temperature sensors  11 B 1 ,  11 B 2  and current sensors  9 B 1 ,  9 B 2 , and in accordance therewith sets an SOC(B) of selected sub power storage device BB and upper limits on electric power input and output Win(S), Wout(S) thereto and therefrom, respectively. 
     Generally, an SOC is indicated by a ratio (%) of each battery&#39;s current charged amount to its fully charged state. Furthermore, Win, Wout are indicated as such an upper limit value of electric power that, when that electric power is discharged for a predetermined period of time (e.g., for approximately 10 seconds), the battery of interest (BA, BB 1 , BB 2 ) is not overcharged/overdischarged. 
       FIG. 4  is a functional block diagram for illustrating how control device  30  controls traveling of electrically powered vehicle  1 , more specifically, a configuration of power distribution control between engine  4  and motor generators MG 1 , MG 2 .  FIG. 4  shows function blocks, which are implemented by control device  30  executing a previously stored, predetermined program and/or by processing of an operation by electronic circuitry (hardware) in control device  30 . 
     With reference to  FIG. 4 , a total power calculation unit  260  calculates total power Pttl required for the entirety of electrically powered vehicle  1  from a vehicular speed and an operation of a pedal (an accelerator pedal). Note that total required power Pttl may also include power required (i.e., the engine&#39;s output), depending on the vehicle&#39;s condition, for generating electric power by motor generator MG 1  to charge a battery. 
     A traveling control unit  250  receives upper limits on electric power input/output Win(M), Wout(M) to/from main power storage device BA, upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB, total required power Pttl from total power calculation unit  260 , and a regenerative brake request made when a brake pedal is operated. Traveling control unit  250  generates a control motor command, or torque command values Tqcom 1  and Tqcom 2 , to allow motor generators MG 1 , MG 2  in total to receive/output electric power within a charging limit (Win(M)+Win(S)) and a discharging limit (Wout(M)+Wout(S)) in total for main power storage device BA and selected sub power storage device BB. Furthermore, to ensure total required power Pttl, it is assigned between power provided by motor generator MG 2  to drive the vehicle and that provided by engine  4  to do so. In particular, externally charged battery&#39;s electric power is maximally utilized to restrict engine  4  from operation, or the power provided by engine  4  to drive the vehicle is set to correspond to a range allowing engine  4  to be highly efficiently operable, to control the vehicle to travel to achieve high fuel-efficiency. 
     An inverter control unit  270  receives torque command value Tqcom 1  and motor current value MCRT 1  of motor generator MG 1 , and therefrom generates control signals PWMI 1 , PWMC 1  for inverter  14 . Similarly, an inverter control unit  280  receives torque command value Tqcom 2  and motor current value MCRT 2  of motor generator MG 2 , and therefrom generates control signals PWMI 2 , PWMC 2  for inverter  22 . Furthermore, traveling control unit  250  generates a control engine command in response to a value requested of power provided by the engine to drive the vehicle, as set. Furthermore, a control device (an engine ECU) (not shown) controls the operation of engine  4  in accordance with the control engine command. 
     In a travel mode in which the vehicle travels actively using a battery&#39;s electric power (i.e., in an EV mode), when total required power Pttl is equal to or smaller than the batteries&#39; total upper limit on electric power output Wout(M)+Wout(S), control device  30  does not operate engine  4 , and motor generator MG 2  alone provides power to drive the vehicle to travel. When total required power Pttl exceeds Wout(M)+Wout(S), engine  4  is started. 
     In contrast, in a travel mode in which the EV mode is not selected (i.e., in an HV mode), control device  30  controls distribution of driving power between engine  4  and motor generator MG 2  to maintain the batteries&#39; SOC at a predetermined target value. In other words, traveling control under which travel with engine  4  is more actuatable than in the EV mode is carried out. 
     In the EV mode, charging and discharging are controlled to preferentially use the electric power of selected sub power storage device BB rather than that of main power storage device BA. As such, when the vehicle is traveling and currently used, selected sub power storage device BB is decreased in SOC, selected sub power storage device BB needs to be switched. For example, if battery BB 1  is set as selected sub power storage device BB in starting the vehicle, necessity will arise to subsequently disconnect battery BB 1  from converter  12 B and connect battery BB 2  as a newly selected sub power storage device BB to converter  12 B, i.e., to perform a connection switching process. 
     On this occasion, battery BB 2  newly set as selected sub power storage device BB generally has an output voltage higher than that of battery BB 1  that has been used so far. As a result, connection of a new high-voltage battery may cause occurrence of an unintended short-circuit path, which may pose a problem in protection of equipment and the like. Therefore, in the process for switching connection of the sub power storage device, full attention should be paid to prevent occurrence of a short-circuit path. Further, since supply and recovery of electric power by selected sub power storage device BB cannot be performed during a period of the connection switching process described above, it is required to limit charging and discharging to prevent occurrence of overcharging and overdischarging in the entire power supply system during that period. 
     Hereinafter, the connection switching process for the sub power storage device with attention being paid to such disadvantages will be described. 
       FIG. 5  is a flowchart showing a schematic procedure of a process performed to switch a selected sub power storage device in the power supply system for the electrically powered vehicle according to the embodiment of the present invention. Furthermore,  FIGS. 6 to 10  are flowcharts for specifically illustrating steps S 100 , S 200 , S 300 , S 400 , and S 500  in  FIG. 5 . 
     Control device  30  can execute a previously stored, predetermined program periodically as predetermined, to repeatedly perform a control process procedure in accordance with the flowcharts shown in  FIGS. 5 to 10 , periodically as predetermined. Thereby, the connection switching process for the sub power storage device in the power supply system for the electrically powered vehicle according to the embodiment of the present invention can be implemented. 
     With reference to  FIG. 5 , in step S 100 , control device  30  performs a process for determining whether a selected sub power storage device should be switched. If control device  30  determines that it is necessary to switch the selected sub power storage device, the following steps S 200  to S 500  are performed. If control device  30  determines in step S 100  that it is not necessary to switch the selected sub power storage device, steps S 200  to S 500  are substantially not performed. 
     In step S 200 , control device  30  performs a pre-switching voltage step-up process, and in step S 300 , control device  30  performs an electric power limit modification process so that a request is not generated to the power supply system to excessively charge/discharge while connection of the sub power storage device is being switched. In step S 400 , control device  30  performs a connection switching process for actually switching connection between selected sub power storage device BB and converter  12 B, and after the process is completed, control device  30  performs in step S 500  a return process to start supplying electric power by newly selected sub power storage device BB. 
       FIG. 6  is a flowchart for illustrating in detail the process performed to determine whether the selected sub power storage device should be switched (S 100 ), as shown in  FIG. 5 . 
     As will be described hereinafter, a variable ID is introduced to indicate the progress (i.e., a status) of the connection switching process. Variable ID is set to any of −1 and 0 to 4. ID=0 indicates a state in which no request for switching a sub power storage device is generated. In other words, when ID=0, currently selected sub power storage device BB supplies electric power, while whether selected sub power storage device BB should be switched or not is determined periodically as predetermined. If there is no sub power storage device that can newly be used due to failure in equipment or consumed electric power in the battery, it is assumed that ID is set to −1 (ID=−1). 
     With reference to  FIG. 6 , in step S 105 , control device  30  determines whether ID=0 or not. If ID=0 (YES in S 105 ), in step S 110 , control device  30  determines whether or not the selected sub power storage device should be switched. Determination in step S 110  is basically made based on the SOC of the currently selected sub power storage device. That is, if the SOC of the sub power storage device in use becomes lower than a prescribed reference value, it is determined that the selected sub power storage device should be switched. 
     In step S 150 , control device  30  confirms a result of determination in step S 110  as to whether switching is necessary or not. When it is determined that switching is necessary (YES in step S 150 ), control device  30  designates selected sub power storage device BB to be newly used in step S 160 . In a case where two batteries BB 1 , BB 2  are mounted as the sub power storage devices as shown in  FIG. 1 , newly selected sub power storage device BB is automatically determined without the need to perform the process in step S 160 . However, in a case where three or more sub power storage devices BB 1  to BBn (where n is an integer equal to or greater than 3) are mounted in the configuration of  FIG. 1 , a new sub power storage device to be used subsequently is designated based on the respective SOCs of the sub power storage devices that are not currently used, and the like. Then, control device  30  sets ID=1 in order to proceed with the connection switching process. Namely, ID=1 indicates a state in which a request for switching selected sub power storage device BB is generated and the switching process is started. 
     On the other hand, if control device  30  determines in step S 110  that it is not necessary to switch the selected sub power storage device (NO in S 150 ), control device  30  maintains ID=0 in step S 170 . If ID≧1 is once satisfied and the switching process has been started, or if there is no sub power storage device that can newly be used and ID=−1 is set (NO in S 105 ), the processes in steps S 110  to S 180  are skipped. 
       FIG. 7  is a flowchart for illustrating in detail the pre-switching voltage step-up process (S 200 ) shown in  FIG. 5 . 
     With reference to  FIG. 7 , in the pre-switching voltage step-up process, control device  30  confirms whether ID=1 or not in step S 205 . If ID=1, a switching request for switching selected sub power storage device BB is made and the switching process is started (YES in S 205 ), control device  30  generates in step S 210  a command to converter  12 A to step up voltage VH on electric power feeding line PL 2  to a predetermined voltage V 1 . In response to the step-up voltage command, a voltage command value VHref for electric power feeding line PL 2  is set to be equal to V 1 , and control signal PWUA for converter  12 A is generated to implement this voltage command value. 
     Note that predetermined voltage V 1  is set to be higher than any higher one of respective output voltages of main power storage device BA and selected sub power storage device BB to be newly connected (for example, BB 2 ). For example, predetermined voltage V 1  set at an upper limit control voltage VHmax that can be stepped up by converter  12 A can ensure that voltage VH when a step-up voltage command is issued is higher than both of the output voltages of main power storage device BA and selected sub power storage device BB after switching. Alternatively, in view of reducing a loss caused at converter  12 A, predetermined voltage V 1  may be determined, as occasion demands, to have a margin, depending on voltages output from main power storage device BA and selected sub power storage device BB after switching at that time. 
     If the step-up voltage command is generated in step S 210 , control device  30  determines in step S 220  whether or not voltage VH has reached predetermined voltage V 1 , based on a value detected by voltage sensor  13 . Determination as YES is made in step S 220 , for example, when VH≧V 1  continues for a predetermined period of time. 
     Once voltage VH has reached predetermined voltage V 1  (YES in S 220 ), control device  30  furthers the ID from 1 to 2. Until voltage VH reaches V 1  (NO in S 220 ), ID=1 is maintained. In other words, ID=2 indicates a state in which the pre-switching voltage step-up process ends and the switching process can be furthered. If ID≠1 (NO in S 205 ), the processes in subsequent steps S 210  to S 230  are skipped. 
     Thus, when the pre-switching voltage step-up process (step S 200 ) ends, control device  30  performs the electric power limit modification process as shown in  FIG. 8 . 
       FIG. 8  is a flowchart for illustrating in detail the electric power limit modification process (S 300 ) shown in  FIG. 5 . 
     With reference to  FIG. 8 , in the electric power limit modification process, control device  30  initially determines whether or not ID=2 in step S 305 . If ID=2 is not satisfied (NO in S 305 ), processes in subsequent steps S 310  to S 340  are skipped. 
     If ID=2 (YES in S 305 ), control device  30  starts temporary relaxation of charging and discharging limits for main power storage device BA in step S 310 . Specifically, absolute values of upper limits on electric power input/output Win(M), Wout(M) to/from main power storage device BA are temporarily increased. 
     Further, in step S 320 , control device  30  gradually decreases absolute values of upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB. For example, Wout(S), Win(S) are decreased gradually toward 0 at a predetermined fixed rate. 
     In step S 330 , control device  30  determines whether or not Wout(S), Win(S) have reached 0. Until Wout(S)=Win(S)=0, step S 320  is repeated to continuously decrease Wout(S) and Win(S). 
     Once Wout(S) and Win(S) have reached 0 (YES in S 330 ), control device  30  furthers the ID from 2 to 3 in step S 340 . In other words, ID=3 indicates a state in which the pre-switching voltage step-up process and the electric power limit modification process have ended and switching of connection between sub power storage devices BB 1 , BB 2  and converter  12 B can be started. 
     When the electric power limit modification process shown in  FIG. 8  ends, control device  30  performs the connection switching process for the sub power storage device in step S 400 . 
       FIG. 9  is a flowchart for illustrating in detail the connection switching process for the sub power storage device (S 400 ), as shown in  FIG. 5 . 
     With reference to  FIG. 9 , in the connection switching process for the sub power storage device, control device  30  initially determines whether or not ID=3 in step S 405 . If ID≠3 (NO in S 405 ), processes in subsequent steps S 410  to S 450  are skipped. 
     If ID=3 (YES in S 405 ), control device  30  stops converter  12 B to prepare for switching connection of the sub power storage device in step S 410 . More specifically, in converter  12 B, IGBT devices Q 1 , Q 2  are forced to be turned off in response to a shutdown command, and in that condition, control device  30  generates in step S 420  a relay control signal for actually switching connection of the sub power storage device. For example, in order to disconnect battery BB 1  from converter  12 B and connect battery BB 2  with converter  12 B, relay control signals CONT 4 , CONT 6  are generated to turn off relays SR 1 , SR 1 G, and relay control signals CONT 5 , CONT 7  are generated to turn on SR 2 , SR 2 G. 
     Furthermore, in step S 430 , control device  30  determines whether or not relay connection switching as instructed in step S 420  has been completed. When the connection switching has been completed (YES in S 430 ), control device  30  restarts converter  12 B to start a switching operation in step S 440 , and furthers the ID from 3 to 4 in step S 450 . 
     In other words, ID=4 indicates a state in which switching of connection between the sub power storage devices and converter  12 B by means of the relays has been completed. 
     When the connection switching process in step S 400  ends, control device  30  performs the return process in step S 500 . 
       FIG. 10  is a flowchart for illustrating in detail the return process (S 500 ) shown in  FIG. 5 . 
     With reference to  FIG. 10 , in the return process, control device  30  initially determines whether or not ID=4 in step S 505 . If ID≠4 (NO in S 505 ), processes in subsequent steps S 510  to S 570  are skipped. 
     If ID=4 (YES in S 505 ), in step S 510 , control device  30  ends the temporary relaxation of charging and discharging limits for main power storage device BA started in step S 310  ( FIG. 7 ). Thereby, Wout(M) and Win(M) basically return to values before the start of the switching process for selected power storage device BB. 
     Further, control device  30  gradually increases upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB decreased to 0 in the electric power limit modification process (step S 300 ), to values of Win, Wout to/from a newly selected sub power storage device (for example, battery BB 2 ). 
     Then, in step S 530 , control device  30  confirms whether or not upper limits on electric power input/output Win(S), Wout(S) have returned to the values of Win, Wout to/from newly selected sub power storage device BB. During a period until return is completed (NO in S 530 ), step S 520  is repeatedly performed to gradually increase upper limits on electric power input/output Win(S), Wout(S) at a fixed rate. 
     When return of upper limits on electric power input/output Win(S), Wout(S) is completed (YES in S 530 ), control device  30  returns the ID back to 0 in step S 540 . Thereby, a state in which normal supply and recovery of electric power by main power storage device BA and newly selected sub power storage device BB can be performed is reproduced in the power supply system. 
     Further, the process proceeds to step S 550  and control device  30  turns off the step-up voltage command generated in step S 210  ( FIG. 6 ). Thus, the voltage command value for electric power feeding line PL 2  is also set to an ordinary value set in accordance with the states of motor generators MG 1 , MG 2 . 
     After completion of a series of switching processes, control device  30  may further determine whether or not there is a possibility that further switching of the selected sub power storage device is performed during traveling of the vehicle in step S 560 . If there is no possibility of further switching, control device  30  sets ID=−1 in step S 570 . If ID=−1 is set, steps S 100  to S 500  in  FIG. 4  are substantially not performed, and thus the switching process for the selected sub power storage device is not started until the vehicle stops operation. 
     On the other hand, if there is a possibility of further switching, control device  30  skips step S 570  and maintains ID=0. As a result, the switching determination process in step S 100  is performed periodically as predetermined, and thereby the switching process for the selected sub power storage device is restarted as necessary. 
     Note that, in the exemplary configuration of  FIG. 1  in which only two sub power storage devices are mounted, it is possible to omit the process in step S 560 , that is, always set ID=−1 once the switching process for the selected sub power storage device is completed, thereby limiting the number of the switching process for the selected sub power storage device performed during driving of the vehicle to only one. 
     Alternatively, in a power supply system equipped with three or more sub power storage devices or a power supply system having a configuration such that a sub power storage device not in use can be charged during driving of a vehicle, the power supply system can be configured such that a second or later switching process for a selected sub power storage device can be performed by maintaining ID=0 depending on a situation. 
       FIG. 11  shows an operation waveform in the process for switching the selected sub power storage device in the power supply system for the electrically powered vehicle according to the embodiment of the present invention described with reference to  FIGS. 5 to 10 . 
     With reference to  FIG. 11 , during a period until time t 1  when ID=0, the switching determination process is performed periodically as predetermined, based on the SOC of the currently selected sub power storage device (e.g., battery BB 1 ). 
     At time t 1 , in response to a decrease in the SOC of battery BB 1 , the switching determination process (step S 100 ) is performed to issue a switching request to switch selected sub power storage device BB, and ID=1 is set to start the switching process. 
     Thus, the pre-switching voltage step-up process (step S 200 ) is performed and converter  12 A increases voltage VH on electric power feeding line PL 2  toward predetermined voltage V 1 . The process for stepping up voltage on electric power feeding line PL 2  is completed at time t 2 , and accordingly, the ID is changed from 1 to 2. 
     When ID=2 is set, the electric power limit modification process (S 300 ) is performed to temporarily relax charging and discharging for main power storage device BA. Specifically, temporary increase in the absolute values of upper limits on electric power input/output Win(M), Wout(M) is started. Further, upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB are decreased toward 0 gradually at a fixed rate. Note that, during this period, converter  12 B is controlled to stop charging/discharging of the currently selected sub power storage device (battery BB 1 ). Alternatively, converter  12 B may be shut down from time t 1 . 
     At time t 3 , upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB are decreased to 0, and in response, the ID is changed from 2 to 3. Once ID=3 is set, the connection switching process for the sub power storage device is started. More specifically, with converter  12 A being shut down, relays SR 1 , SR 1 G are turned off, and thereafter relays SR 2 , SR 2 G are turned on. Then, when the relay connection switching process is completed and battery BB 2  as a newly selected sub power storage device is connected to converter  12 B, converter  12 B is restarted. By completing this connection switching process, the ID is changed from 3 to 4 at time t 4 . 
     When ID=4 is set, upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB are gradually increased at a fixed rate, and thereby battery BB 2  is started to be used as a newly selected sub power storage device. Accordingly, the temporary relaxation of charging and discharging limits for main power storage device BA ends, and Wout(M), Win(M) are basically caused to return to values before time t 2 . 
     When Win(S), Wout(S) to/from selected sub power storage device BB return to original values corresponding to Wout, Win from/to battery BB 2  at time t 5 , the ID returns to 0. Then, the process for stepping up voltage on electric power feeding line PL 2  is also stopped. 
     Thereby, a series of switching processes for the selected sub power storage device are completed, and a state in which normal supply and recovery of electric power using selected sub power storage device BB (battery BB 2 ) can be performed is reproduced. 
     At time t 5 , if it is determined whether there is a possibility that a further process for switching the sub power storage device is performed during driving of the vehicle, and ID=−1 is set when there is no possibility of occurrence of the switching process as described in  FIG. 10 , load subsequently imposed on control device  30  can be alleviated. 
     Next, a configuration of a functional portion for the process for switching the selected sub power storage device described in  FIGS. 5 to 10 , which a part of a configuration controlling the power supply system of the embodiment of the present invention will be described with reference to  FIG. 12 .  FIG. 12  shows function blocks, which are implemented as control device  30  executing a predetermined program to provide software processing, or by dedicated electronic circuitry (or hardware processing). 
     With reference to  FIG. 12 , a switching determination unit  100  receives SOC(BB 1 ), SOC(BB 2 ) indicating the states of charge of batteries BB 1 , BB 2 , respectively, and determines whether or not the SOC of selected sub power storage device BB currently in use is lower than a predetermined reference value. When variable ID shared by the function blocks is 0, switching determination unit  100  performs the determination process described above periodically as predetermined, and if it is necessary to switch the selected sub power storage device, switching determination unit  100  changes the ID from 0 to 1. Thus, a request for switching the selected sub power storage device is generated. In other words, switching determination unit  100  has a function corresponding to the process in step S 100  in  FIG. 5 . 
     When the request for switching the selected sub power storage device is generated and ID=1 is set, a step-up-voltage instruction unit  110  outputs a step-up voltage command signal CMBT to a converter control unit  200  controlling converter  12 A. 
     Converter control unit  200  generates control signals PWUA, PWDA for converter  12 A, based on voltages VH, VLA and voltage command value VHref, so that voltage VH on electric power feeding line PL 2  reaches voltage command value VHref. 
     Furthermore, when step-up-voltage instruction unit  110  generates step-up voltage command signal CMBT, converter control unit  200  sets voltage command value VHref=V 1  and generates control signal PWUA. If voltage sensor  13  detects voltage VH having reached predetermined voltage V 1  continuously for at least a predetermined period of time, converter control unit  200  sets a flag FBT indicating that stepping up voltage is completed, to ON. 
     In response to flag FBT set to ON, step-up-voltage instruction unit  110  changes the ID to 2, and continues to output step-up voltage command signal CMBT until a connection switching control unit  140 , which will be described later, completes relay connection switching and accordingly ID=4 is set. In other words, step-up-voltage instruction unit  110  has a function corresponding to step S 200  in  FIG. 5  and step S 550  in  FIG. 10 . 
     An electric power limiter unit  120  sets upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB. Normally, upper limits on electric power input/output Win(S), Wout(S) are set based on selected sub power storage device BB or battery&#39;s SOC (SOC(BB 1 ) or SOC(BB 2 )), temperature (TBB 1  or TBB 2 ), and output voltage (VB 1  or VB 2 ). 
     In the process for switching the selected sub power storage device, in contrast, when ID=2 is set, electric power limiter unit  120  decreases upper limits on electric power input/output Win(S), Wout(S) gradually at a fixed rate toward 0, and when Win(S), Wout(S) have reached 0, electric power limiter unit  120  changes the ID from 2 to 3. Further, when connection switching control unit  140  sets ID=4, electric power limiter unit  120  increases upper limits on electric power input/output Win(S), Wout(S) to values corresponding to Win, Win of newly selected sub power storage device BB after switching. Then, when the increasing process is completed, electric power limiter unit  120  changes the ID from 4 to 0. 
     In other words, electric power limiter unit  120  implements the processes in steps S 320  to S 340  in  FIG. 8  and the processes in steps S 520  to S 540  in  FIG. 10 , and functions of the “first electric power limiter unit” and the “second electric power limiter unit” of the present invention. 
     An electric power limiter unit  130  sets upper limits on electric power input/output Win(M) and Wout(M) to/from main power storage device BA. Normally, upper limits on electric power input/output Win(M), Wout(M) are set based on main power storage device BA&#39;s SOC (BA), temperature TA, and output voltage VA. 
     In the switching process for the selected sub power storage device, in contrast, when ID=2 is set, electric power limiter unit  130  temporarily increases the absolute values of upper limits on electric power input/output Win(M) and Wout(M), and thereby temporarily relaxes charging and discharging limits for main power storage device BA. Then, when connection switching control unit  140  sets ID=4, electric power limiter unit  130  causes upper limits on electric power input/output Win(M) and Wout(M) to return to normal values. 
     In other words, electric power limiter unit  130  implements the processes in step S 310  in  FIG. 8  and in step S 510  in  FIG. 10 , and a function of the “third electric power limiter unit” of the present invention. 
     When electric power limiter unit  120  sets ID=3, connection switching control unit  140  generates a command to shut down converter  12 B, and also generates relay control signals CONT 4  to CONT 7  to switch connection between converter  12 B and sub power storage devices BB 1 , BB 2 . For example, when selected sub power storage device BB is switched from battery BB 1  to battery BB 2 , relay control signals CONT 4  to CONT 7  are generated to turn off relays SR 1 , SR 1 G and turn on relays SR 2 , SR 2 G. Once this relay connection switching process is completed, connection switching control unit  140  stops the shutdown command described above to restart converter  12 B, and changes the ID from 3 to 4. 
     Connection switching control unit  140  corresponds to the process in step S 400  in  FIG. 5  (the processes in S 405  to S 450  in  FIG. 9 ). 
     As described above, according to the power supply system for the electrically power vehicle in accordance with the present embodiment, in the process for switching a sub power storage device that is used, voltage on electric power feeding line PL 2  is increased and thereafter connection of the sub power storage device is switched. This ensures that connection switching is performed without forming a short-circuit path originating from a newly used sub power storage device in the system. Furthermore, during the switching process for the selected sub power storage device, upper limits on electric power input/output Win(S), Wout(S) to/from selected sub power storage device BB are appropriately limited, which can prevent the power supply system from being requested to excessively charge/discharge. As a result, in a power supply system configured such that a plurality of sub power storage devices share a single voltage converter (a converter), a connection switching process for a sub power storage device performed to switch the selected sub power storage device can be performed appropriately and smoothly. 
     It should be understood that the embodiment disclosed herein is illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.