Patent Publication Number: US-2011068740-A1

Title: Power supply system for vehicle, electric vehicle having the same, and method of controlling power supply system for vehicle

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2009-218685 filed on Sep. 24, 2009 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a power supply system for a vehicle, an electric vehicle equipped with the power supply system, and a method of controlling a power supply system for a vehicle and, more particularly, to a power supply system for a vehicle, which is configured to be chargeable by a power supply outside the vehicle, an electric vehicle equipped with the power supply system, and a method of controlling a power supply system for a vehicle. 
     2. Description of the Related Art 
     An electric automobile, a hybrid automobile, a fuel cell automobile, and the like, are known as electric vehicles that is able to drive an electric motor for propelling the vehicle using electric power stored in an in-vehicle electrical storage device, typically, a secondary battery. Then, for these electric vehicles, there is proposed a configuration that the in-vehicle electrical storage device is charged by an external power supply outside the vehicle (hereinafter, charging of the in-vehicle electrical storage device by the external power supply is referred to as “external charging”). 
     Japanese Patent Application Publication No. 2009-27774 (JP-A-2009-27774) describes a vehicle of which the charging efficiency during external charging is improved. The vehicle includes a battery, a DC/DC converter, an auxiliary battery and a controller. The battery is chargeable by an external power supply. The DC/DC converter steps down the voltage of the battery and then outputs the voltage. The auxiliary battery is charged with the voltage output from the DC/DC converter, and supplies electric power to an auxiliary load. The controller continuously operates the DC/DC converter during operation of the vehicle, and intermittently operates the DC/DC converter during external charging. 
     The above vehicle intermittently operates the DC/DC converter during external charging, so the electrical storage device may be charged while a loss during external charging is being suppressed (see JP-A-2009-27774). 
     The technique described in JP-A-2009-27774 is useful in that the DC/DC converter that generates an auxiliary voltage is intermittently operated during external charging to suppress a loss during external charging to thereby make it possible to improve the charging efficiency. However, an unnecessarily high auxiliary voltage may possibly be supplied during operation of the DC/DC converter, and electric power consumed by the auxiliary load increases when the auxiliary voltage is high, so the charging efficiency during external charging may possibly deteriorate. 
     SUMMARY OF THE INVENTION 
     The invention provides a power supply system for a vehicle, which is able to improve the charging efficiency during external charging, an electric vehicle equipped with the power supply system, and a method of controlling a power supply system for a vehicle. 
     A first aspect of the invention relates to a power supply system for a vehicle, which is configured to be chargeable by a power supply outside the vehicle. The power supply system includes: a rechargeable electrical storage device; a charger that is configured to charge the electrical storage device with electric power supplied from the power supply outside the vehicle; a voltage converter that is configured to convert voltage of electric power output from the electrical storage device and to supply the converted electric power to an auxiliary load; and a controller that controls the voltage converter. The controller includes a) a remaining time estimation unit that estimates a remaining time up to completion of charging of the electrical storage device by the charger and b) a control unit that, when the remaining time estimated by the remaining time estimation unit is longer than a predetermined period of time, controls the voltage converter so that a voltage output from the voltage converter is lower than a voltage output from the voltage converter during system operation in which the vehicle can travel. 
     A second aspect of the invention relates to a power supply system for a vehicle, which is configured to be chargeable by a power supply outside the vehicle. The power supply system includes: a rechargeable electrical storage device; a charger that is configured to charge the electrical storage device with electric power supplied from the power supply outside the vehicle; a voltage converter that is configured to convert voltage of electric power output from the electrical storage device and to supply the converted electric power to an auxiliary load; and a controller that controls the voltage converter. The controller includes a) an SOC estimation unit that estimates a residual capacity of the electrical storage device and b) a control unit that, during charging of the electrical storage device by the charger, controls the voltage converter so that a voltage output from the voltage converter is lower than a voltage output from the voltage converter during system operation in which the vehicle can travel until the residual capacity estimated by the SOC estimation unit reaches a predetermined amount. 
     A third aspect of the invention relates to an electric vehicle. The electric vehicle includes: the power supply system according to the first aspect; and an electric motor that uses electric power stored in the electrical storage device to generate driving torque. 
     A fourth aspect of the invention relates to an electric vehicle. The electric vehicle includes: the power supply system according to the second aspect; and an electric motor that uses electric power stored in the electrical storage device to generate driving torque. 
     A fifth aspect of the invention relates to a method of controlling a power supply system for a vehicle, which is configured to be chargeable by a power supply outside the vehicle, wherein the power supply system includes a rechargeable electrical storage device; a charger that is configured to charge the electrical storage device with electric power supplied from the power supply outside the vehicle; a voltage converter that is configured to convert voltage of electric power output from the electrical storage device and to supply the converted electric power to an auxiliary load; and a controller that controls the voltage converter. The method includes: estimating a remaining time up to completion of charging of the electrical storage device by the charger; and, when the estimated remaining time is longer than a predetermined period of time, controlling the voltage converter so that a voltage output from the voltage converter is lower than a voltage output from the voltage converter during system operation in which the vehicle can travel. 
     A sixth aspect of the invention relates to a method of controlling a power supply system for a vehicle, which is configured to be chargeable by a power supply outside the vehicle, wherein the power supply system includes a rechargeable electrical storage device; a charger that is configured to charge the electrical storage device with electric power supplied from the power supply outside the vehicle; a voltage converter that is configured to convert voltage of electric power output from the electrical storage device and to supply the converted electric power to an auxiliary load; and a controller that controls the voltage converter. The method includes: estimating a residual capacity of the electrical storage device; and, during charging of the electrical storage device by the charger, controlling the voltage converter so that a voltage output from the voltage converter is lower than a voltage output from the voltage converter during system operation in which the vehicle can travel until the estimated residual capacity reaches a predetermined amount. 
     According to the above aspects, the remaining time up to completion of external charging is estimated, and, when the estimated remaining time is longer than the predetermined period of time, the voltage converter is controlled so that the voltage output from the voltage converter is lower than the voltage output from the voltage converter during system operation in which the vehicle can travel. In addition, during external charging, until the residual capacity of the electrical storage device reaches the predetermined amount, the voltage converter is controlled so that the voltage output from the voltage converter is lower than the voltage output from the voltage converter during system operation in which the vehicle can travel. With the above configuration, the voltage output from the voltage converter is controlled to be low up to a predetermined time point immediately before completion of external charging, so electric power consumed by the auxiliary load during external charging is reduced. Thus, according to the aspects of the invention, it is possible to further improve the charging efficiency during external charging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is an overall block diagram of a hybrid vehicle illustrated as an example of an electric vehicle according to a first embodiment of the invention; 
         FIG. 2  is a graph that shows a variation in auxiliary voltage during external charging; 
         FIG. 3  is a functional block diagram of portions of a controller shown in  FIG. 1 , related to control over a DC/DC converter during external charging; 
         FIG. 4  is a flowchart for illustrating control, executed by the controller shown in  FIG. 1 , over the DC/DC converter during external charging; 
         FIG. 5  is a graph that shows a variation in auxiliary voltage during external charging according to a second embodiment; 
         FIG. 6  is a functional block diagram of portions of a controller according to the second embodiment, related to control over a DC/DC converter during external charging; and 
         FIG. 7  is a flowchart for illustrating control, executed by the controller according to the second embodiment, over the DC/DC converter during external charging. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. Note that like reference numerals denote the same or corresponding portions in the drawings, and the description thereof will not be repeated. 
       FIG. 1  is an overall block diagram of a hybrid vehicle illustrated as an example of an electric vehicle according to a first embodiment of the invention. As shown in  FIG. 1 , the hybrid vehicle  1  includes an electrical storage device B 1 , a step-up converter  12 , a smoothing capacitor CH, inverters  14  and  22 , motor generators MG 1  and MG 2 , an engine  4 , a power split device  3 , and a drive wheel  2 . In addition, the hybrid vehicle  1  further includes a charger  6 , a DC/DC converter  33 , an auxiliary battery B 2 , an auxiliary load  35 , voltage sensors  10 ,  13  and  36 , a current sensor  11  and a controller  30 . 
     The electrical storage device B 1  is connected between a positive electrode line PL 1  and a negative electrode line NL. The step-up converter  12  is provided between the electrical storage device B 1  and the inverters  14  and  22 . The inverters  14  and  22  are connected to a positive electrode line PL 2  and the negative electrode line NL. The DC/DC converter  33  is connected to the positive electrode line PL 1  and the negative electrode line NL. The auxiliary battery B 2  and the auxiliary load  35  are connected to the DC/DC converter  33 . 
     The electrical storage device B 1  is a rechargeable direct-current power supply, and is, for example, formed of a nickel metal hydride secondary battery or a lithium ion secondary battery. The electrical storage device B 1  supplies electric power to the step-up converter  12  and the DC/DC converter  33 . In addition, the electrical storage device B 1  is charged by the step-up converter  12  with electric power generated by the motor generator MG 1  and/or the motor generator MG 2 . Furthermore, the electrical storage device B 1  is charged by the charger  6  when the hybrid vehicle  1  is charged by an external power supply  8  (for example, a commercial system power supply) (during external charging). Note that a large-capacitance capacitor may be used as the electrical storage device B 1 , and, as long as an electric power buffer is able to temporarily store electric power generated by the motor generators MG 1  and/or MG 2  or electric power supplied from the external power supply  8  and to supply the stored electric power to the step-up converter  12  and the DC/DC converter  33 , any electric power buffer is applicable. 
     The step-up converter  12  steps up the voltage between the positive electrode line PL 2  and the negative electrode line NL to the voltage between the positive electrode line PL 1  and the negative electrode line NL (voltage of the electrical storage device B 1 ) or above on the basis of a control signal received from the controller  30 . The step-up converter  12  is, for example, formed of a current reversible direct-current chopper circuit that has a reactor for storing energy. The smoothing capacitor CH smoothes the voltage between the positive electrode line PL 2  and the negative electrode line NL. The positive electrode line PL 2  and the negative electrode line NL are arranged between the step-up converter  12  and the inverters  14  and  22 . 
     The inverter  14  drives the motor generator MG 1  on the basis of a control signal received from the controller  30 . In addition, the inverter  22  drives the motor generator MG 2  on the basis of a control signal received from the controller  30 . Each of the inverters  14  and  22  is, for example, formed of a three-phase bridge circuit that has a U-phase arm, a V-phase arm and a W-phase arm. 
     Each of the motor generators MG 1  and MG 2  is an alternating-current electric rotating machine, and is, for example, formed of a three-phase alternating-current synchronous electric motor in which a permanent magnet is embedded in a rotor. The rotary shaft of the motor generator MG 1  is connected to the power split device  3 , and the rotary shaft of the motor generator MG 2  is coupled to the drive wheel  2 . The power split device  3  is formed of a planetary gear that includes a sun gear, pinion gears, a planetary carrier and a ring gear. Then, the rotary shaft of the motor generator MG 1 , the crankshaft of the engine  4  and a drive shaft coupled to the drive wheel  2  are connected to the power split device  3 , and the power split device  3  distributes the output of the engine  4  between the motor generator MG 1  and the drive wheel  2 . 
     The charger  6  converts electric power, supplied from the external power supply  8 , into a predetermined charging voltage on the basis of a control signal received from the controller  30  during external charging. Then, electric power converted in voltage is supplied by the charger  6  to the electrical storage device B 1 . Thus, the electrical storage device B 1  is charged. The charger  6  is, for example, formed of an AC/DC converter. 
     The DC/DC converter  33  steps down the output voltage to an auxiliary voltage lower than the voltage between the positive electrode line PL 1  and the negative electrode line NL (voltage of the electrical storage device B 1 ) on the basis of a control signal PWD received from the controller  30 . The auxiliary battery B 2  is an electric power buffer that temporarily stores auxiliary electric power output from the DC/DC converter  33 , and is, for example, formed of a lead-acid battery. The auxiliary load  35  includes various auxiliaries mounted on the vehicle. Note that only part of auxiliaries, such as the controller  30  that executes charging control and a necessary minimum display function, operate during external charging by the charger  6 , and the magnitude of a load of the auxiliary load  35  during external charging is smaller than a load of the auxiliary load  35  during system operation in which the vehicle can travel. 
     The voltage sensor  10  detects the voltage between the positive electrode line PL 1  and the negative electrode line NL, that is, the voltage VB of the electrical storage device B 1 . The current sensor  11  detects the current IB input to or output from the electrical storage device B 1 . The voltage sensor  13  detects the voltage VH between the positive electrode line PL 2  and the negative electrode line NL. The voltage sensor  36  detects the voltage output from the DC/DC converter  33 , that is, an auxiliary voltage VL. Then, values detected by the sensors are transmitted to the controller  30 . 
     The controller  30  generates a control signal for driving the step-up converter  12  and control signals for driving the motor generators MG 1  and MG 2 , and outputs those generated signals to the step-up converter  12  and the inverters  14  and  22 , respectively. In addition, during external charging, the controller  30  generates a control signal for driving the charger  6 , and outputs the generated signal to the charger  6 . 
     Furthermore, the controller  30  generates a control signal PWD for driving the DC/DC converter  33 , and outputs the generated control signal PWD to the DC/DC converter  33 . Here, during external charging by the charger  6 , the controller  30  generates a control signal PWD so that the voltage output from the DC/DC converter  33  (that is, auxiliary voltage VL) is lower than the voltage output from the DC/DC converter  33  during system operation in which the vehicle can travel. 
     Furthermore, the controller  30  estimates a remaining time Tb up to completion of external charging. When the controller  30  determines that completion of external charging will come soon on the basis of the remaining time Tb, the controller  30  generates a control signal PWD so that the voltage output from the DC/DC converter  33  returns to the level during system operation. Note that the remaining time Tb may be, for example, calculated from the residual capacity (hereinafter, referred to as “state of charge (SOC)”) of the electrical storage device B 1  and the charging rate of the charger  6 . The configuration of the controller  30  will be described later in detail. 
       FIG. 2  is a graph that shows a variation in auxiliary voltage VL during external charging. As shown in  FIG. 2 , during external charging, the auxiliary voltage VL is regulated by the DC/DC converter  33  ( FIG. 1 ) to a voltage V 2  that is lower than a voltage V 1  during system operation in which the vehicle can travel. Note that the voltage V 2  is set to a minimum level at which the auxiliary load  35  ( FIG. 1 ) that operates during external charging is normally operable. By so doing, electric power consumed by the auxiliary load  35  during external charging is reduced and, as a result, the efficiency of external charging improves. 
     Then, at time t 1 , as the remaining time Tb reaches a predetermined threshold Ta that indicates that completion of external charging will come soon, the auxiliary voltage VL is returned to the voltage V 1  during system operation in which the vehicle can travel. By so doing, the auxiliary battery B 2  may be sufficiently charged using electric power supplied from the external power supply  8  ( FIG. 1 ) in preparation for the next traveling. 
       FIG. 3  is a functional block diagram of portions of the controller  30  shown in  FIG. 1 , related to control over the DC/DC converter  33  during external charging. The controller  30  includes an SOC estimation unit  52 , a charge remaining time estimation unit  54  and a DC/DC converter control unit  56 . 
     The SOC estimation unit  52  estimates the SOC of the electrical storage device B 1  on the basis of the detected voltage VB received from the voltage sensor  10  ( FIG. 1 ) and the detected current IB received from the current sensor  11  ( FIG. 1 ). Various known methods may be used as a method of estimating the SOC. 
     The charge remaining time estimation unit  54  estimates the remaining time Tb up to completion of charging of the electrical storage device B 1  by the charger  6  on the basis of the estimated SOC received from the SOC estimation unit  52  and the charging rate Pcg of the charger  6 . For example, a charged amount of electric power up to a fully charged state is calculated on the basis of the capacity and SOC of the electrical storage device B 1 , and then the calculated charged amount of electric power is divided by the charging rate Pcg. By so doing, the remaining time Tb may be calculated. Note that the charging rate Pcg may be a target value or may be a value actually detected by a sensor. 
     When the remaining time Tb received from the charge remaining time estimation unit  54  is longer than the threshold Ta ( FIG. 2 ), the DC/DC converter control unit  56  generates a control signal PWD for driving the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  so that the voltage output from the DC/DC converter  33  (auxiliary voltage VL) is lower than the voltage output from the DC/DC converter  33  during system operation in which the vehicle can travel. 
     In addition, when the remaining time Tb for charging is shorter than or equal to the threshold Ta, the DC/DC converter control unit  56  generates a control signal PWD and then outputs the control signal PWD to the DC/DC converter  33  so that the voltage output from the DC/DC converter  33  (auxiliary voltage VL) returns to the level during system operation in which the vehicle can travel. 
       FIG. 4  is a flowchart for illustrating control, executed by the controller  30 , over the DC/DC converter  33  during external charging. Note that the process of the flowchart is executed during external charging at a regular time interval or each time a predetermined condition is satisfied. 
     The controller  30  determines whether the auxiliary voltage VL detected by the voltage sensor  36  ( FIG. 1 ) is lower than the voltage V 2  ( FIG. 2 ) (step S 10 ). Note that as described with reference to  FIG. 2 , the voltage V 2  is lower than the voltage V 1  during system operation in which the vehicle can travel, and is set to a minimum level at which the auxiliary load  35  ( FIG. 1 ) that operates during external charging is normally operable. 
     When it is determined that the auxiliary voltage VL is lower than the voltage V 2  (YES in step S 10 ), the controller  30  generates a control signal PWD for driving the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  to thereby activate the DC/DC converter  33  (step S 20 ). When the DC/DC converter  33  is activated, the auxiliary voltage VL increases. 
     On the other hand, when it is determined in step S 10  that the auxiliary voltage VL is higher than or equal to the voltage V 2  (NO in step S 10 ), the controller  30  determines whether the remaining time Tb for external charging by the charger  6  is longer than the threshold Ta ( FIG. 2 ) (step S 30 ). Note that, as described with reference to  FIG. 2 , the threshold Ta is set for determining that completion of external charging will come soon. 
     Then, when it is determined in step S 30  that the remaining time Tb is longer than the threshold Ta (YES in step S 30 ), the controller  30  generates a control signal PWD for stopping the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  to thereby stop the DC/DC converter  33  (step S 40 ). As the DC/DC converter  33  stops, the auxiliary voltage VL decreases. Through the processes from step S 10  to step S 40 , when the remaining time Tb is longer than the threshold Ta, the auxiliary voltage VL is controlled to the voltage V 2 . 
     On the other hand, when it is determined in step S 30  that the remaining time Tb is shorter than or equal to the threshold Ta (NO in step S 30 ), the controller  30  generates a control signal PWD for driving the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  to thereby activate the DC/DC converter  33  (step S 50 ). Note that, when the DC/DC converter  33 , for example, receives a control signal PWD for driving the DC/DC converter  33  from the controller  30 , the DC/DC converter  33  controls the output voltage to the voltage V 1  ( FIG. 2 ). Thus, when the remaining time Tb is shorter than or equal to the threshold Ta, the auxiliary voltage VL returns to the voltage V 1 . 
     As described above, in the first embodiment, the remaining time Tb up to completion of external charging is estimated, and, when the estimated remaining time Tb is longer than the threshold Ta, the DC/DC converter  33  is controlled so that the voltage output from the DC/DC converter  33  (auxiliary voltage VL) is lower than the voltage output from the DC/DC converter  33  during system operation in which the vehicle can travel. By so doing, the voltage output from the DC/DC converter  33  is controlled to be low until a predetermined time point before completion of external charging to thereby reduce electric power consumed by the auxiliary load  35  during external charging. Thus, according to the first embodiment, it is possible to further improve the charging efficiency during external charging. 
     A second embodiment of the invention will be described with respect to  FIGS. 5 to 7 . In the first embodiment, the remaining time Tb up to completion of external charging is estimated, and, when the remaining time Tb reaches the threshold Ta, the auxiliary voltage VL is returned to V 1  at the normal level. In the second embodiment, during external charging, as the SOC reaches a predetermined threshold near a fully charged state, the auxiliary voltage VL is returned to V 1  at the normal level. 
     The overall configuration of an electric vehicle according to the second embodiment is the same as that of the electric vehicle according to the first embodiment shown in  FIG. 1 . 
       FIG. 5  is a graph that shows a variation in auxiliary voltage VL during external charging according to the second embodiment. As shown in  FIG. 5 , during external charging, the auxiliary voltage VL is regulated by the DC/DC converter  33  ( FIG. 1 ) to the voltage V 2  that is lower than the voltage V 1  during system operation in which the vehicle can travel. 
     Then, at time t 1 , as the SOC of the electrical storage device B 1  reaches a predetermined threshold Sa that indicates that the electrical storage device B 1  is close to a fully charged state Sm, the auxiliary voltage VL is returned to the voltage V 1  during system operation in which the vehicle can travel. 
       FIG. 6  is a functional block diagram of portions of a controller  30 A according to the second embodiment, related to control over the DC/DC converter  33  during external charging. The controller  30 A includes the SOC estimation unit  52  and a DC/DC converter control unit  56 A. The SOC estimation unit  52  is configured as described with reference to  FIG. 3  in the first embodiment. 
     When the estimated SOC received from the SOC estimation unit  52  is lower than the threshold Sa ( FIG. 5 ), the DC/DC converter control unit  56 A generates a control signal PWD for driving the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  so that the voltage output from the DC/DC converter  33  (auxiliary voltage VL) is lower than the voltage output from the DC/DC converter  33  during system operation in which the vehicle can travel. 
     In addition, when the estimated SOC received from the SOC estimation unit  52  reaches the threshold Sa, the DC/DC converter control unit  56 A generates a control signal PWD and then outputs the control signal PWD to the DC/DC converter  33  so that the voltage output from the DC/DC converter  33  (auxiliary voltage VL) returns to the level during system operation in which the vehicle can travel. 
       FIG. 7  is a flowchart for illustrating control, executed by the controller  30 A according to the second embodiment, over the DC/DC converter  33  during external charging. Note that the process of the flowchart is executed during external charging at a regular time interval or each time a predetermined condition is satisfied. 
     The flowchart includes step S 35  instead of step S 30  in the flowchart shown in  FIG. 4 . When it is determined in step S 10  that the auxiliary voltage VL is higher than or equal to the voltage V 2  (NO in step S 10 ), the controller  30 A determines whether the SOC of the electrical storage device B 1  is lower than the threshold Sa ( FIG. 5 ) (step S 35 ). Note that as described with reference to  FIG. 5 , the threshold Sa is set for determining that the electrical storage device B 1  is close to the fully charged state Sm. 
     Then, when it is determined in step S 35  that the SOC is lower than the threshold Sa (YES in step S 35 ), the process proceeds to step S 40 , and the controller  30 A generates a control signal PWD for stopping the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  to thereby stop the DC/DC converter  33 . 
     On the other hand, when it is determined in step S 35  that the SOC is higher than or equal to the threshold Sa (NO in step S 35 ), the process proceeds to step S 50 , and the controller  30 A generates a control signal PWD for driving the DC/DC converter  33  and then outputs the control signal PWD to the DC/DC converter  33  to thereby activate the DC/DC converter  33 . 
     As described above, in the second embodiment, during external charging, until the SOC of the electrical storage device B 1  reaches the threshold Sa, the DC/DC converter  33  is controlled so that the voltage output from the DC/DC converter  33  (auxiliary voltage VL) is lower than the voltage output from the DC/DC converter  33  during system operation in which the vehicle can travel. By so doing, the voltage output from the DC/DC converter  33  is controlled to be low until a predetermined time point before completion of external charging to thereby reduce electric power consumed by the auxiliary load  35  during external charging. Thus, according to the second embodiment as well, it is possible to further improve the charging efficiency during external charging. 
     Note that the above embodiments describe a series/parallel-type hybrid vehicle that uses the power split device  3  to split the power of the engine  4  to thereby make it possible to transmit the power to the drive wheel  2  and the motor generator MG 1  as an example of the electric vehicle; however, the aspect of the invention may also be applied to a hybrid vehicle of another type. For example, the aspect of the invention may also be applied to a so-called series-type hybrid vehicle that uses the engine  4  for driving only the motor generator MG 1  and that uses only the motor generator MG 2  to generate driving force of the vehicle, a hybrid vehicle that collects only regenerative energy from kinetic energy generated by the engine as electric energy or a motor assist-type hybrid vehicle that uses an engine as a main power source and that, where necessary, uses a motor for assisting the engine. 
     In addition, the aspect of the invention may also be applied to an electric automobile that includes no engine  4  and that travels on only electric power or a fuel cell automobile that further includes a fuel cell in addition to the electrical storage device B 1  as a direct-current power supply. In addition, the aspect of the invention may also be applied to an electric vehicle that includes no step-up converter  12 . 
     Note that, in the above description, the DC/DC converter  33  may be regarded as a “voltage converter” according to the aspect of the invention, the SOC estimation unit  52  may be regarded as an “SOC estimation unit” according to the aspect of the invention, the charge remaining time estimation unit  54  may be regarded as a “remaining time estimation unit” according to the aspect of the invention. In addition, the DC/DC converter control units  56  and  56 A may be regarded as a “control unit” according to the aspect of the invention, and the motor generator MG 2  may be regarded as an “electric motor” according to the aspect of the invention. 
     While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention.