Patent Publication Number: US-11641125-B2

Title: Vehicular power supply device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2020-155184 filed on Sep. 16, 2020, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The disclosure relates to vehicular power supply devices equipped in vehicles. 
     Vehicles, such as electric automobiles and hybrid vehicles, are equipped with power supply devices having electric storage units, such as batteries (e.g., see PCT International Publication No. WO 2017/017786, Japanese Unexamined Patent Application Publication (JP-A) No. 2017-77158, and JP-A No. 2006-210244). Moreover, vehicles, such as electric automobiles, are equipped with onboard chargers for charging the electric storage units. 
     SUMMARY 
     An aspect of the disclosure provides a vehicular power supply device to be applied to a vehicle. The vehicular power supply device includes an electric storage unit group, a charger, a first switch, a second switch, and a switch controller. The electric storage unit group includes a first electric storage unit and a second electric storage unit configured to have higher internal resistance than the first electric storage unit. The charger is configured to be coupled to the electric storage unit group and charge one or both of the first electric storage unit and the second electric storage unit. The first switch is provided between the first electric storage unit and the charger and is configured to be controlled between an on state and an off state. The second switch is provided between the second electric storage unit and the charger and is configured to be controlled between an on state and an off state. The switch controller is configured to control the first switch and the second switch based on a temperature of the electric storage unit group. 
     An aspect of the disclosure provides a vehicular power supply device to be applied to a vehicle. The vehicular power supply device includes an electric storage unit group, a charger, a first switch, a second switch, and circuitry. The electric storage unit group includes a first electric storage unit and a second electric storage unit configured to have higher internal resistance than the first electric storage unit. The charger is configured to be coupled to the electric storage unit group and charge one or both of the first electric storage unit and the second electric storage unit. The first switch is provided between the first electric storage unit and the charger and is configured to be controlled between an on state and an off state. The second switch is provided between the second electric storage unit and the charger and is configured to be controlled between an on state and an off state. The circuitry is configured to control the first switch and the second switch based on a temperature of the electric storage unit group. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate an embodiment and, together with the specification, serve to explain the principles of the disclosure. 
         FIG.  1    schematically illustrates a configuration example of a vehicular power supply device according to an embodiment of the disclosure; 
         FIG.  2    illustrates an example of the disposition of battery stacks in a battery pack; 
         FIG.  3    schematically illustrates an example of a control system equipped in the vehicular power supply device; 
         FIG.  4    illustrates an operating state of the battery pack in a basic operation mode; 
         FIG.  5    is a flowchart illustrating an example of a procedure of stack switching control; 
         FIG.  6 A  and  FIG.  6 B  each illustrate an operating state of the battery pack in the stack switching control; 
         FIG.  7    is a flowchart illustrating an example of a procedure of temperature-increase suppression control; 
         FIG.  8 A  illustrates an operating state of the battery pack in the basic operation mode, and  FIG.  8 B  illustrates an operating state of the battery pack in a temperature-increase suppression mode; 
         FIG.  9    is a flowchart illustrating an example of a procedure of cruising-distance extension control; 
         FIG.  10 A  illustrates an operating state of the battery pack in the basic operation mode, and  FIG.  10 B  illustrates an operating state of the battery pack in a distance extension mode; 
         FIG.  11    is a flowchart illustrating an example of a procedure of plug-in charging control; 
         FIG.  12    is a flowchart illustrating the example of the procedure of the plug-in charging control; 
         FIG.  13    is a flowchart illustrating the example of the procedure of the plug-in charging control; 
         FIG.  14 A  and  FIG.  14 B  each illustrate a state where a stack group is charged by an external power source; 
         FIG.  15    is a flowchart illustrating an example of a procedure of automatic supplementary charging control; 
         FIG.  16    is a flowchart illustrating the example of the procedure of the automatic supplementary charging control; 
         FIG.  17 A  illustrates an operating state of the battery pack after the vehicle has stopped, and  FIG.  17 B  illustrates an operating state of the battery pack in an automatic supplementary charging mode; and 
         FIG.  18    illustrates an example of the disposition of battery cells. 
     
    
    
     DETAILED DESCRIPTION 
     Because the internal resistance of an electric storage unit increases with decreasing temperature of the electric storage unit, it is desirable to increase the temperature of the electric storage unit to enhance the charging efficiency of the electric storage unit in a low-temperature environment. Accordingly, in order to efficiency charge an electric storage unit at low temperatures, it is desirable that the electric storage unit be warmed before being charged. 
     It is desirable to warm an electric storage unit before charging the electric storage unit. 
     In the following, an embodiment of the disclosure is described in detail with reference to the accompanying drawings. Note that the following description is directed to an illustrative example of the disclosure and not to be construed as limiting to the disclosure. Factors including, without limitation, numerical values, shapes, materials, components, positions of the components, and how the components are coupled to each other are illustrative only and not to be construed as limiting to the disclosure. Further, elements in the following embodiment which are not recited in a most-generic independent claim of the disclosure are optional and may be provided on an as-needed basis. The drawings are schematic and are not intended to be drawn to scale. Throughout the present specification and the drawings, elements having substantially the same function and configuration are denoted with the same numerals to avoid any redundant description. 
     Battery Pack 
       FIG.  1    schematically illustrates a configuration example of a vehicular power supply device  10  according to an embodiment of the disclosure. As illustrated in  FIG.  1   , the vehicular power supply device  10  to be equipped in a vehicle has a battery pack (electric storage unit group)  11  having a stack group A 1  and a stack group B 1 . The stack group A 1  is provided with a plurality of parallel-coupled battery stacks (first electric storage unit) A 2 . Each battery stack A 2  is constituted of a plurality of series-coupled battery cells A 3 . The battery cells A 3  are battery cells manufactured as new products, that is, battery cells not previously used in another device. The stack group B 1  is provided with a plurality of parallel-coupled battery stacks (second electric storage unit) B 2 . Each battery stack B 2  is constituted of a plurality of series-coupled battery cells (electric storage cell) B 3 . The battery cells B 3  are battery cells manufactured as recycled products, that is, battery cells previously used in another device. The battery stacks A 2  and B 2  are also called battery modules. 
     Main Switches 
     The positive terminal of each battery stack A 2  is provided with a main switch SW 1   a , and the negative terminal of each battery stack A 2  is provided with a main switch SW 1   b . By switching on the main switches SW 1   a  and SW 1   b , positive terminals  13   a  of the battery stacks A 2  can be coupled to a positive terminal  12   a  of the stack group A 1 , and negative terminals  13   b  of the battery stacks A 2  can be coupled to a negative terminal  12   b  of the stack group A 1 . That is, by switching on the main switches SW 1   a  and SW 1   b , the battery stacks A 2  can be coupled to a power supply circuit  14  in the battery pack  11 . On the other hand, by switching off the main switches SW 1   a  and SW 1   b , the positive terminals  13   a  of the battery stacks A 2  can be isolated from the positive terminal  12   a  of the stack group A 1 , and the negative terminals  13   b  of the battery stacks A 2  can be isolated from the negative terminal  12   b  of the stack group A 1 . 
     Likewise, the positive terminal of each battery stack B 2  is provided with a main switch SW 2   a , and the negative terminal of each battery stack B 2  is provided with a main switch SW 2   b . By switching on the main switches SW 2   a  and SW 2   b , positive terminals  16   a  of the battery stacks B 2  can be coupled to a positive terminal  15   a  of the stack group B 1 , and negative terminals  16   b  of the battery stacks B 2  can be coupled to a negative terminal  15   b  of the stack group B 1 . That is, by switching on the main switches SW 2   a  and SW 2   b , the battery stacks B 2  can be coupled to the power supply circuit  14  in the battery pack  11 . On the other hand, by switching off the main switches SW 2   a  and SW 2   b , the positive terminals  16   a  of the battery stacks B 2  can be isolated from the positive terminal  15   a  of the stack group B 1 , and the negative terminals  16   b  of the battery stacks B 2  can be isolated from the negative terminal  15   b  of the stack group B 1 . 
     Selector Switches 
     The battery pack  11  is coupled to a motor generator (driving motor)  21  via an inverter  20 . The inverter  20  is constituted of a plurality of switching elements and has a function for performing conversion between alternating-current power of the motor generator  21  and direct-current power of the battery pack  11 . The battery pack  11  is also coupled to an electric device group  24  constituted of electric devices  23 , such as actuators and controllers, via a converter  22 . The converter  22  is a DC-DC converter constituted of a plurality of switching elements and has a function for reducing the direct-current power of the battery pack  11  and outputting the direct-current power to the electric device group  24 . 
     Furthermore, in order to control the coupling state of the inverter  20  and the converter  22  relative to the battery pack  11 , the vehicular power supply device  10  is provided with selector switches SW 3   a , SW 3   b , SW 4   a , and SW 4   b . The selector switch SW 3   a  has a positive terminal  33   a  to be coupled to the positive terminal  12   a  of the stack group A 1 , an inverter terminal  33   b  to be coupled to the inverter  20 , and a converter terminal  33   c  to be coupled to the converter  22 . The selector switch SW 3   a  operates at any one of an inverter position where the positive terminal  33   a  and the inverter terminal  33   b  are coupled to each other, a converter position where the positive terminal  33   a  and the converter terminal  33   c  are coupled to each other, and a neutral position where the positive terminal  33   a  is isolated from both the inverter terminal  33   b  and the converter terminal  33   c.    
     The selector switch SW 3   b  has a negative terminal  43   a  to be coupled to the negative terminal  12   b  of the stack group A 1 , an inverter terminal  43   b  to be coupled to the inverter  20 , and a converter terminal  43   c  to be coupled to the converter  22 . The selector switch SW 3   b  operates at any one of an inverter position where the negative terminal  43   a  and the inverter terminal  43   b  are coupled to each other, a converter position where the negative terminal  43   a  and the converter terminal  43   c  are coupled to each other, and a neutral position where the negative terminal  43   a  is isolated from both the inverter terminal  43   b  and the converter terminal  43   c.    
     By controlling the selector switches SW 3   a  and SW 3   b  to the inverter positions, the stack group A 1  is coupled to the inverter  20  via the selector switches SW 3   a  and SW 3   b . On the other hand, by controlling the selector switches SW 3   a  and SW 3   b  to the converter positions, the stack group A 1  is coupled to the converter  22  via the selector switches SW 3   a  and SW 3   b . Furthermore, by controlling the selector switches SW 3   a  and SW 3   b  to the neutral positions, as illustrated in  FIG.  1   , the stack group A 1  is isolated from both the inverter  20  and the converter  22 . 
     The selector switch SW 4   a  has a positive terminal  34   a  to be coupled to the positive terminal  15   a  of the stack group B 1 , an inverter terminal  34   b  to be coupled to the inverter  20 , and a converter terminal  34   c  to be coupled to the converter  22 . The selector switch SW 4   a  operates at any one of an inverter position where the positive terminal  34   a  and the inverter terminal  34   b  are coupled to each other, a converter position where the positive terminal  34   a  and the converter terminal  34   c  are coupled to each other, and a neutral position where the positive terminal  34   a  is isolated from both the inverter terminal  34   b  and the converter terminal  34   c.    
     The selector switch SW 4   b  has a negative terminal  44   a  to be coupled to the negative terminal  15   b  of the stack group B 1 , an inverter terminal  44   b  to be coupled to the inverter  20 , and a converter terminal  44   c  to be coupled to the converter  22 . The selector switch SW 4   b  operates at any one of an inverter position where the negative terminal  44   a  and the inverter terminal  44   b  are coupled to each other, a converter position where the negative terminal  44   a  and the converter terminal  44   c  are coupled to each other, and a neutral position where the negative terminal  44   a  is isolated from both the inverter terminal  44   b  and the converter terminal  44   c.    
     By controlling the selector switches SW 4   a  and SW 4   b  to the inverter positions, the stack group B 1  is coupled to the inverter  20  via the selector switches SW 4   a  and SW 4   b . On the other hand, by controlling the selector switches SW 4   a  and SW 4   b  to the converter positions, the stack group B 1  is coupled to the converter  22  via the selector switches SW 4   a  and SW 4   b . Furthermore, by controlling the selector switches SW 4   a  and SW 4   b  to the neutral positions, as illustrated in  FIG.  1   , the stack group B 1  is isolated from both the inverter  20  and the converter  22 . 
     Charging Switches 
     The vehicular power supply device  10  is provided with an onboard charger (charger)  50  for charging the battery pack  11  by using an external power source  51 . The onboard charger  50  is constituted of, for example, a plurality of switching elements and has a function for converting alternating-current power from the external power source  51  into direct-current power and outputting the direct-current power to the battery pack  11 . Moreover, the onboard charger  50  has a function for performing voltage adjustment on the direct-current power of the stack group A 1  and outputting the direct-current power to the stack group B 1 , as well as a function for performing voltage adjustment on the direct-current power of the stack group B 1  and outputting the direct-current power to the stack group A 1 . 
     When the battery pack  11  is to be charged by using the external power source  51 , a charging plug  53  of the external power source  51  is coupled to an inlet  52  of the onboard charger  50 . Accordingly, the external power source  51  can be coupled to the battery pack  11  via the onboard charger  50 , so that the electric power from the external power source  51  can be supplied to the battery pack  11 . Furthermore, in order to control the coupling state of the onboard charger  50  relative to the battery pack  11 , the vehicular power supply device  10  is provided with charging switches (first switch) SW 5   a  and SW 5   b  and charging switches (second switch) SW 6   a  and SW 6   b.    
     The charging switch SW 5   a  has a positive terminal  55   a  to be coupled to the positive terminal  12   a  of the stack group A 1  and a charging terminal  55   b  to be coupled to the onboard charger  50 . Furthermore, the charging switch SW 5   b  has a negative terminal  65   a  to be coupled to the negative terminal  12   b  of the stack group A 1  and a charging terminal  65   b  to be coupled to the onboard charger  50 . By switching on the charging switches SW 5   a  and SW 5   b , the onboard charger  50  can be coupled to the stack group A 1  of the battery pack  11 . 
     The charging switch SW 6   a  has a positive terminal  56   a  to be coupled to the positive terminal  15   a  of the stack group B 1  and a charging terminal  56   b  to be coupled to the onboard charger  50 . The charging switch SW 6   b  has a negative terminal  66   a  to be coupled to the negative terminal  15   b  of the stack group B 1  and a charging terminal  66   b  to be coupled to the onboard charger  50 . By switching on the charging switches SW 6   a  and SW 6   b , the onboard charger  50  can be coupled to the stack group B 1  of the battery pack  11 . 
     Disposition of Battery Stacks A 2  and B 2   
       FIG.  2    illustrates an example of the disposition of the battery stacks A 2  and B 2  in the battery pack  11 . In  FIG.  2   , the shaded battery stacks are the battery stacks B 2 . As illustrated in  FIG.  2   , the battery stacks A 2  and the battery stacks B 2  are alternately disposed adjacent to each other. As mentioned above, the battery cells A 3  constituting each battery stack A 2  are battery cells A 3  manufactured as new products, whereas the battery cells B 3  constituting each battery stack B 2  are battery cells B 3  manufactured as recycled products. Therefore, the internal resistance of each battery cell B 3  that is a recycled product is higher than the internal resistance of each battery cell A 3  that is a new product. That is, in the battery pack  11 , the battery stacks A 2  with the lower internal resistance and the battery stacks B 2  with the higher internal resistance are disposed adjacent to each other. 
     Control System 
       FIG.  3    schematically illustrates an example of a control system equipped in the vehicular power supply device  10 . As illustrated in  FIG.  3   , the vehicular power supply device  10  has a plurality of controllers  70  to  74  each constituted of a microcomputer. The controllers  70  to  74  are a battery controller  70  that controls the battery pack  11 , a motor controller  71  that controls the motor generator  21  coupled to vehicle wheels  80 , a converter controller  72  that controls the converter  22 , a charging controller  73  that controls the onboard charger  50 , and a main controller  74  that integrally controls the controllers  70  to  73 . The controllers  70  to  74  are coupled to one another in a communicable manner via an onboard network  75 , such as a controller area network (CAN). 
     The main controller  74  is coupled to an accelerator sensor  81  that detects the operation status of an accelerator pedal, a brake sensor  82  that detects the operation status of a brake pedal, and a vehicle speed sensor  83  that detects the travel speed of the vehicle. The main controller  74  is also coupled to an activation switch  84  that is to be operated by the driver when a vehicle control system is to be activated and stopped. Furthermore, the main controller  74  is provided with a main-switch operation setting unit  85  that sets a target operating state for each of the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b , a selector-switch operation setting unit  86  that sets a target operating position for each of the selector switches SW 3   a , SW 3   b , SW 4   a , and SW 4   b , and a charging-switch operation setting unit  87  that sets a target operating state for each of the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b.    
     The main-switch operation setting unit  85  of the main controller  74  outputs a control signal according to the target operating states of the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  to the battery controller  70 , so as to control the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  via the battery controller  70 . The selector-switch operation setting unit  86  of the main controller  74  outputs a control signal according to the target operating positions of the selector switches SW 3   a , SW 3   b , SW 4   a , and SW 4   b  to the battery controller  70 , so as to control the selector switches SW 3   a , SW 3   b , SW 4   a , and SW 4   b  via the battery controller  70 . Furthermore, the charging-switch operation setting unit  87  of the main controller  74  outputs a control signal according to the target operating states of the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  to the battery controller  70 , so as to control the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  via the battery controller  70 . Accordingly, the main-switch operation setting unit  85 , the selector-switch operation setting unit  86 , the charging-switch operation setting unit  87 , and the battery controller  70  function as a switch controller that controls the various switches SW 1   a , SW 1   b , . . . , SW 6   a , and SW 6   b.    
     The battery controller  70  is coupled to battery sensors  88  and  89 . The battery sensor  88  has a function for detecting, for example, the temperature, the charge-discharge electric current, and the terminal voltage of each battery stack A 2 . The battery sensor  89  has a function for detecting the temperature, the charge-discharge electric current, and the terminal voltage of each battery stack B 2 . The battery controller  70  is provided with an SOC calculator  90  that calculates the state of charge (SOC) of each of the battery stacks A 2  and B 2 , as well as an SOH calculator  91  that calculates the state of health (SOH) indicating the state of degradation of each of the battery stacks A 2  and B 2 . 
     The SOC of each of the battery stacks A 2  and B 2  is a percentage indicating the amount of stored electricity remaining in the battery stack A 2  or B 2 , and is a percentage of the amount of stored electricity relative to the full charge capacity of the battery stack A 2  or B 2 . That is, the SOC is calculated to be higher with increasing amount of stored electricity in the battery stack A 2  or B 2 , whereas the SOC is calculated to be lower with decreasing amount of stored electricity in the battery stack A 2  or B 2 . The SOC also referred to as a charged state is periodically calculated by the SOC calculator  90  of the battery controller  70  based on the charge-discharge electric current and the terminal voltage of the battery stack A 2  or B 2 . 
     The SOH of each of the battery stacks A 2  and B 2  is an indicator indicating the state of degradation of the battery stack A 2  or B 2 . The SOH indicating the state of degradation can be calculated as, for example, a capacity retention rate of the battery stack A 2  or B 2 . That is, in a case where there is no progression in the degradation of the battery stack A 2  or B 2 , the current capacity retention rate is higher than that in the initial state, so that the SOH is calculated to be higher as the battery stack A 2  or B 2  is in a better condition. In contrast, in a case where there is progression in the degradation of the battery stack A 2  or B 2 , the current capacity retention rate is lower than that in the initial state, so that the SOH is calculated to be lower as the battery stack A 2  or B 2  degrades. 
     The SOH of each of the battery stacks A 2  and B 2  is periodically calculated by the SOH calculator  91  of the battery controller  70  based on the charge-discharge electric current and the terminal voltage of the battery stack A 2  or B 2 . Furthermore, as mentioned above, the battery cells A 3  constituting each battery stack A 2  are battery cells A 3  manufactured as new products, whereas the battery cells B 3  constituting each battery stack B 2  are battery cells B 3  manufactured as recycled products. Therefore, at the time of manufacture of the vehicle, the battery stacks B 2  are degraded more than the battery stacks A 2 , such that the SOH of each battery stack B 2  is calculated to be lower than the SOH of each battery stack A 2 . 
     Basic Operation Mode 
     Next, a basic operation mode of the battery pack  11  will be described.  FIG.  4    illustrates an operating state of the battery pack  11  in the basic operation mode. The basic operation mode is to be executed in a case where the vehicle control system is activated in response to an activation operation performed by the driver in a state where the battery stacks A 2  are not excessively degraded. 
     As illustrated in  FIG.  4   , in the basic operation mode, the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched on, so that the battery stacks A 2  and B 2  are coupled to the power supply circuit  14  in the battery pack  11 . Furthermore, in the basic operation mode, the selector switches SW 3   a  and SW 3   b  are controlled to the inverter positions, so that the stack group A 1  constituted of the battery stacks A 2  is coupled to the inverter  20 . Moreover, in the basic operation mode, the selector switches SW 4   a  and SW 4   b  are controlled to the converter positions, so that the stack group B 1  constituted of the battery stacks B 2  is coupled to the inverter  20 . Accordingly, in the basic operation mode, the stack group A 1  is coupled to the inverter  20  having high electric power consumption and high regenerative electric power, whereas the stack group B 1  is coupled to the converter  22  having low electric power consumption. 
     As mentioned above, the battery cells A 3  constituting each battery stack A 2  are battery cells A 3  manufactured as new products, whereas the battery cells B 3  constituting each battery stack B 2  are battery cells B 3  manufactured as recycled products. The battery cells B 3  that are recycled products have lower output and lower capacity than the battery cells A 3 , but can greatly contribute to cost reduction, as compared with the battery cells A 3 . That is, by coupling the stack group B 1  constituted of the battery stacks B 2  that are recycled products to the converter  22  having low electric power consumption, the vehicular power supply device  10  can be reduced in cost. 
     Stack Switching Control 
     As mentioned above, in the basic operation mode, the stack group A 1  equipped with the battery cells A 3  that are new products is coupled to the inverter  20 , and the stack group B 1  equipped with the battery cells B 3  that are recycled products is coupled to the converter  22 , but the coupling destinations for the stack groups A 1  and B 1  are to be switched in accordance with the degradation statuses of the battery cells A 3  and B 3 . The following description relates to stack switching control that is to be executed after the vehicle control system is activated by the driver and that is to be performed for switching the coupling destinations for the stack groups A 1  and B 1  in accordance with the degradation statuses of the battery cells A 3  and B 3 . 
       FIG.  5    is a flowchart illustrating an example of a procedure of the stack switching control, and  FIG.  6 A  and  FIG.  6 B  each illustrate an operating state of the battery pack  11  in the stack switching control. In  FIG.  6 A  and  FIG.  6 B , a charging-discharging state is indicated by using an arrow. As illustrated in  FIG.  5   , in step S 10 , average SOC values (SOCa and SOCb) of the battery stacks A 2  and B 2  are read, and average SOH values (SOHa and SOHb) of the battery stacks A 2  and B 2  are read. The average SOC value (SOCa) is an average value obtained by dividing the total SOC value of the battery stacks A 2  by the number of stacks, and the average SOC value (SOCb) is an average value obtained by dividing the total SOC value of the battery stacks B 2  by the number of stacks. The average SOH value (SOHa) is an average value obtained by dividing the total SOH value of the battery stacks A 2  by the number of stacks, and the average SOH value (SOHb) is an average value obtained by dividing the total SOH value of the battery stacks B 2  by the number of stacks. 
     In the following description, the “average SOC value (SOCa)” will be referred to as “SOCa”, the “average SOC value (SOCb)” will be referred to as “SOCb”, the “average SOH value (SOHa)” will be referred to as “SOHa”, and the “average SOH value (SOHb)” will be referred to as “SOHb”. In the flowcharts from  FIG.  5    and onward, the “main switches SW 1   a  and SW 1   b ” will be referred to as “main switches SW 1 ”, and the “main switches SW 2   a  and SW 2   b ” will be referred to as “main switches SW 2 ”. Furthermore, in the flowcharts from  FIG.  5    and onward, the “selector switches SW 3   a  and SW 3   b ” will be referred to as “selector switches SW 3 ”, the “selector switches SW 4   a  and SW 4   b ” will be referred to as “selector switches SW 4 ”, the “charging switches SW 5   a  and SW 5   b ” will be referred to as “charging switches SW 5 ”, and the “charging switches SW 6   a  and SW 6   b ” will be referred to as “charging switches SW 6 ”. 
     As illustrated in  FIG.  5   , in step S 11 , it is determined whether the SOCa exceeds a predetermined lower limit value Xa and the SOCb exceeds a predetermined lower limit value Xb. If it is determined in step S 11  that the SOCa is lower than or equal to the lower limit value Xa or the SOCb is lower than or equal to the lower limit value Xb, the routine is exited without performing the stack switching control since the SOC of the battery pack  11  is greatly reduced. If the SOC of the battery pack  11  is greatly reduced, a message urging plug-in charging, to be described later, is displayed toward the driver. 
     If it is determined in step S 11  that the SOCa exceeds the lower limit value Xa and the SOCb exceeds the lower limit value Xb, the process proceeds to step S 12  where it is determined whether a degradation indicator difference (SOHa−SOHb) between the battery stacks A 2  and B 2  exceeds a predetermined threshold value Xc. If it is determined in step S 12  that the degradation indicator difference (SOHa−SOHb) exceeds the threshold value Xc, that is, if the battery stacks B 2  have degraded beyond a predetermined value relative to the battery stacks A 2 , the process proceeds to step S 13  where the selector switches SW 3   a  and SW 3   b  are controlled to the inverter positions so that the stack group A 1  constituted of the battery stacks A 2  is coupled to the inverter  20 . Moreover, the process proceeds to step S 14  where the selector switches SW 4   a  and SW 4   b  are controlled to the converter positions so that the stack group B 1  constituted of the battery stacks B 2  is coupled to the converter  22 . Then, the process proceeds to step S 15  where the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched on. 
     In contrast, if it is determined in step S 12  that the degradation indicator difference (SOHa−SOHb) is smaller than or equal to the threshold value Xc, the process proceeds to step S 16  where it is determined whether a degradation indicator difference (SOHb−SOHa) between the battery stacks B 2  and A 2  exceeds a predetermined threshold value Xd. If it is determined in step S 16  that the degradation indicator difference (SOHb−SOHa) exceeds the threshold value Xd, that is, if the battery stacks A 2  have degraded beyond a predetermined value relative to the battery stacks B 2 , the process proceeds to step S 17  where the selector switches SW 3   a  and SW 3   b  are controlled to the converter positions so that the stack group A 1  constituted of the battery stacks A 2  is coupled to the converter  22 . Moreover, the process proceeds to step S 18  where the selector switches SW 4   a  and SW 4   b  are controlled to the inverter positions so that the stack group B 1  constituted of the battery stacks B 2  is coupled to the inverter  20 . Then, the process proceeds to step S 15  where the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched on. 
     If it is determined in step S 16  that the degradation indicator difference (SOHb−SOHa) is smaller than or equal to the threshold value Xd, the degradation statuses of the battery stacks A 2  and B 2  are balanced. Thus, the process proceeds to step S 19  where the selector switches SW 3   a , SW 3   b , SW 4   a , and SW 4   b  are controlled to the most recent operating positions. That is, the coupling destinations for the stack groups A 1  and B 1  set at the time of previous activation of the vehicle control system are maintained. 
     Accordingly, in the stack switching control, the degradation statuses of the battery stacks A 2  and B 2  are compared. The less-degraded battery stacks, that is, the battery stacks A 2  (or B 2 ) with higher output, are coupled to the inverter  20 , whereas the degraded battery stacks, that is, the battery stacks B 2  (or A 2 ) with lower output, are coupled to the converter  22 . 
     As mentioned above, the battery cells A 3  constituting each battery stack A 2  are battery cells A 3  manufactured as new products, whereas the battery cells B 3  constituting each battery stack B 2  are battery cells B 3  manufactured as recycled products. That is, at the time of manufacture of the vehicle, the battery stacks B 2  are degraded more than the battery stacks A 2 . Therefore, since the battery stacks A 2  have not degraded more than the battery stacks B 2  if a predetermined period (e.g., several years) has not elapsed from when the vehicle is manufactured, the basic operation mode illustrated in  FIG.  4    and  FIG.  6 A  is basically executed. That is, as illustrated in  FIG.  6 A , if the battery stacks A 2  have not degraded more than the battery stacks B 2 , the stack group A 1  constituted of the battery stacks A 2  is coupled to the inverter  20 , and the stack group B 1  constituted of the battery stacks B 2  is coupled to the converter  22 . 
     In contrast, if a predetermined period (e.g., several years) has elapsed from when the vehicle is manufactured, the battery stacks A 2  may be degraded more than the battery stacks B 2 , depending on the usage condition of the battery stacks A 2 . If the battery stacks A 2  are degraded more than the battery stacks B 2  in this manner, a time-related degradation mode illustrated in  FIG.  6 B  is executed. That is, as illustrated in  FIG.  6 B , if the battery stacks A 2  are degraded more than the battery stacks B 2 , the stack group A 1  constituted of the battery stacks A 2  is coupled to the converter  22 , and the stack group B 1  constituted of the battery stacks B 2  is coupled to the inverter  20 . 
     Accordingly, even in a case where the output of the battery stacks A 2  is greatly reduced due to time-related degradation, the less-degraded battery stacks B 2  are coupled to the inverter  20 , so that the motor generator  21  is cause to appropriately operate, thereby ensuring minimum driving performance. 
     Temperature-Increase Suppression Control 
     Next, temperature-increase suppression control to be executed in the aforementioned basic operation mode to suppress an excessive temperature increase in the battery stacks A 2  will be described.  FIG.  7    is a flowchart illustrating an example of a procedure of the temperature-increase suppression control.  FIG.  8 A  illustrates an operating state of the battery pack  11  in the basic operation mode, and  FIG.  8 B  illustrates an operating state of the battery pack  11  in a temperature-increase suppression mode. In  FIG.  8 A  and  FIG.  8 B , a charging-discharging state is indicated by using an arrow. 
     As illustrated in  FIG.  7   , in step S 20 , an average temperature Ta of the battery stacks A 2  is read. The average temperature Ta is an average value obtained by the total temperature value of the battery stacks A 2  by the number of stacks. In step S 21 , it is determined whether the average temperature Ta exceeds a predetermined threshold value Xe. If it is determined in step S 21  that the average temperature Ta is lower than or equal to the threshold value Xe, the routine is exited without executing the temperature-increase suppression mode to be described below since the temperature of the battery stacks A 2  is appropriate. In contrast, if it is determined in step S 21  that the average temperature Ta exceeds the threshold value Xe, the temperature of the battery stacks A 2  has increased beyond an appropriate range. Thus, the process proceeds to step S 22  where the temperature-increase suppression mode for decreasing the temperature of the battery stacks A 2  commences. 
     In step S 22 , the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched on. In step S 23 , the onboard charger  50  is controlled to an energized state, so that the stack groups A 1  and B 1  are electrically coupled to each other via the onboard charger  50 . That is, as illustrated in  FIG.  8 B , the temperature-increase suppression mode is executed so that the inverter  20  is electrically coupled to the stack group A 1  and also to the stack group B 1  via the onboard charger  50 . Accordingly, in a case where the electric power consumption and the regenerative electric power of the inverter  20  have increased, both the stack group A 1  and the stack group B 1  can be charged and discharged. That is, the charging and discharging of the stack group A 1  can be suppressed, so that the temperature of the battery stacks A 2  constituting the stack group A 1  can be decreased. 
     When the temperature-increase suppression mode is executed in this manner, the process proceeds to step S 24  where the average temperature Ta of the battery stacks A 2  is read. Then, the process proceeds to step S 25  where it is determined whether the average temperature Ta falls below a predetermined threshold value Xf. If it is determined in step S 25  that the average temperature Ta is higher than or equal to the threshold value Xf, the temperature of the battery stacks A 2  is not sufficiently decreased. Thus, the process returns to step S 23  to continue executing the temperature-increase suppression mode. In contrast, if it is determined in step S 25  that the average temperature Ta falls below the threshold value Xf, the temperature of the battery stacks A 2  is sufficiently decreased, so that the process proceeds to step S 26  where the onboard charger  50  is controlled to a stopped state. Then, the process proceeds to step S 27  where the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched off. 
     Cruising-Distance Extension Control 
     Next, cruising-distance extension control to be executed in the above-described basic operation mode for supplying electric power from the battery stacks B 2  to the battery stacks A 2  will be described.  FIG.  9    is a flowchart illustrating an example of a procedure of the cruising-distance extension control.  FIG.  10 A  illustrates an operating state of the battery pack  11  in the basic operation mode, and  FIG.  10 B  illustrates an operating state of the battery pack  11  in a distance extension mode. In  FIG.  10 A  and  FIG.  10 B , a charging-discharging state is indicated by using an arrow. 
     As illustrated in  FIG.  9   , in step S 30 , the SOCa and SOCb of the battery stacks A 2  and B 2  are read. In step S 31 , it is determined whether the SOCa falls below a predetermined threshold value Xg. If it is determined in step S 31  that the SOCa is higher than or equal to the threshold value Xg, that is, if the amount of stored electricity in the battery stacks A 2  is sufficiently ensured, the distance extension mode to be described below is not to be performed. Thus, the routine is exited without executing the distance extension mode. In contrast, if it is determined in step S 31  that the SOCa falls below the threshold value Xg, the process proceeds to step S 32  where it is determined whether the SOCb exceeds a predetermined threshold value Xh. If it is determined in step S 32  that the SOCb is lower than or equal to the threshold value Xh, that is, if the amount of stored electricity in the battery stacks B 2  is not sufficiently ensured, it is difficult to execute the distance extension mode. Thus, the routine is exited without executing the distance extension mode. 
     In contrast, when the process proceeds to step S 32  from step S 31  and it is determined in step S 32  that the SOCb exceeds the threshold value Xh, that is, when the amount of stored electricity in the battery stacks A 2  is insufficient and the amount of stored electricity in the battery stacks B 2  is sufficiently ensured, the process proceeds to step S 33  where the distance extension mode for enhancing the SOCa of the battery stacks A 2  commences. In step S 33 , the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched on. In step S 34 , the onboard charger  50  is controlled to an energized state, so that electric power is supplied to the stack group A 1  from the stack group B 1  via the onboard charger  50 . That is, as illustrated in  FIG.  10 B , the distance extension mode is executed so that electric power can be supplied to the stack group A 1  from the stack group B 1 , whereby the SOCa of the battery stacks A 2  constituting the stack group A 1  can be enhanced. 
     Subsequently, in step S 35 , the SOCa and SOCb of the battery stacks A 2  and B 2  are read. In step S 36 , it is determined whether the SOCa exceeds a predetermined threshold value Xi. If it is determined in step S 36  that the SOCa exceeds the threshold value Xi, the amount of stored electricity in the battery stacks A 2  is sufficiently restored. Thus, the process proceeds to step S 37  where the onboard charger  50  is controlled to a stopped state. The process then proceeds to step S 38  where the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched off. By executing the distance extension mode in this manner, electric power can be supplied from the battery stacks B 2  to the battery stacks A 2 , so that the amount of stored electricity in the battery pack  11  can be effectively utilized to supply electric power to the motor generator  21 , whereby the cruising distance of the vehicle can be extended. 
     In contrast, if it is determined in step S 36  that the SOCa is lower than or equal to the threshold value Xi, the process proceeds to step S 39  where it is determined whether the SOCb falls below a predetermined threshold value Xj. If it is determined in step S 39  that the SOCb falls below the threshold value Xj, the amount of stored electricity in the battery stacks B 2  has decreased and it is difficult to continue with the distance extension mode. Thus, the process proceeds to step S 37  where the onboard charger  50  is controlled to a stopped state. The process then proceeds to step S 38  where the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched off. In contrast, if it is determined in step S 39  that the SOCb is higher than or equal to the threshold value Xj, the amount of stored electricity in the battery stacks B 2  is ensured. Thus, the process returns to step S 34  to continue executing the distance extension mode. 
     Plug-In Charging Control 
     The following description relates to plug-in charging control for charging (referred to as “plug-in charging” hereinafter) the battery pack  11  using the onboard charger  50  by coupling the charging plug  53  of the external power source  51  to the inlet  52  of the onboard charger  50 .  FIG.  11    to  FIG.  13    are flowcharts illustrating an example of a procedure of the plug-in charging control. In  FIG.  11    to  FIG.  13   , the flowcharts are coupled to each other at sections indicated by reference signs A and B.  FIG.  14 A  illustrates a state where the stack group B 1  is charged by the external power source  51 , and  FIG.  14 B  illustrates a state where the stack group A 1  is charged by the external power source  51 . In  FIG.  14 A  and  FIG.  14 B , a charging state is indicated by using an arrow. 
     As illustrated in  FIG.  11   , in step S 40 , the average temperature Ta of the battery stacks A 2  is read. In step S 41 , it is determined whether the average temperature Ta falls below a predetermined threshold value Xk. If it is determined in step S 41  that the average temperature Ta falls below the threshold value Xk, the process proceeds to step S 42  where the battery stacks B 2  are plug-in charged to warm the battery stacks A 2 . If it is determined in step S 41  that the average temperature Ta is higher than or equal to the threshold value Xk, the process proceeds to step S 48  to be described later without commencing the plug-in charging of the battery stacks B 2 . 
     In order to start plug-in charging the battery stacks B 2 , the main switches SW 1   a  and SW 1   b  are switched off and the main switches SW 2   a  and SW 2   b  are switched on in step S 42 . Then, in step S 43 , the charging switches SW 5   a  and SW 5   b  are switched off, and the charging switches SW 6   a  and SW 6   b  are switched on. When the stack group B 1  is coupled to the onboard charger  50  in this manner, the process proceeds to step S 44  where the onboard charger  50  is controlled to an energized state for charging the stack group B 1 , so that electric power is supplied to the stack group B 1  from the external power source  51  via the onboard charger  50 . That is, as illustrated in  FIG.  14 A , the battery stacks B 2  are plug-in charged before the battery stacks A 2  are plug-in charged, so that the battery stacks A 2  can be warmed with heat generated from the battery stacks B 2 . By warming the battery stacks A 2  in this manner, the internal resistance of the battery stacks A 2  can be reduced, thereby enhancing the charging efficiency. 
     As mentioned above, the battery cells A 3  constituting each battery stack A 2  are battery cells A 3  manufactured as new products, whereas the battery cells B 3  constituting each battery stack B 2  are battery cells B 3  manufactured as recycled products. Therefore, the internal resistance of the battery cells B 3  that are recycled products is higher than the internal resistance of the battery cells A 3  that are new products. That is, by plug-in charging the battery cells B 3  that generate heat more easily due to the higher internal resistance, the battery stacks A 2  can be actively warmed. In addition, since the battery stacks A 2  and B 2  are adjacent to each other, the battery stacks A 2  can be actively warmed by the heat of the battery stacks B 2 . 
     When the battery stacks A 2  are warmed by the battery stacks B 2  in this manner, the process proceeds to step S 45  where the average temperature Ta of the battery stacks A 2  is read and the SOCb of the battery stacks B 2  is read. Then, the process proceeds to step S 46  where it is determined whether the average temperature Ta exceeds a predetermined threshold value Xm, and whether the SOCb exceeds a predetermined target charging value Xo. If it is determined in step S 46  that the average temperature Ta is lower than or equal to the threshold value Xm and that the SOCb is lower than or equal to the target charging value Xo, the process returns to step S 44  to continue plug-in charging the battery stacks B 2 . That is, if the battery stacks A 2  are not sufficiently warm and the battery stacks B 2  are still plug-in chargeable, the process returns to step S 44  to continue plug-in charging the battery stacks B 2 . 
     In contrast, if it is determined in step S 46  that the average temperature Ta exceeds the threshold value Xm or that the SOCb exceeds the target charging value Xo, the process proceeds to step S 47  where the onboard charger  50  is controlled to a stopped state to stop plug-in charging the stack group B 1 . That is, when the battery stacks A 2  are sufficiently warm and it is difficult to plug-in charge the battery stacks B 2 , the process proceeds to step S 47  where the onboard charger  50  is controlled to a stopped state to stop plug-in charging the stack group B 1 . Then, in order to start plug-in charging the stack group A 1 , the main switches SW 1   a  and SW 1   b  are switched on and the main switches SW 2   a  and SW 2   b  are switched off in step S 48 , as illustrated in  FIG.  12   . Then, in step S 49 , the charging switches SW 5   a  and SW 5   b  are switched on, and the charging switches SW 6   a  and SW 6   b  are switched off. 
     When the stack group A 1  is coupled to the onboard charger  50  in this manner, the process proceeds to step S 50  where the onboard charger  50  is controlled to an energized state for charging the stack group A 1 , so that electric power is supplied to the stack group A 1  from the external power source  51  via the onboard charger  50 . Furthermore, in a case where the stack group A 1  is to be plug-in charged in step S 50 , the target charging power is set to a larger value than in the case where the stack group B 1  is to be plug-in charged in step S 44  described above. That is, as illustrated in  FIG.  14 B , when the stack group A 1  is to be plug-in charged, the battery stacks A 2  are in a warmed state such that the battery stacks A 2  have low internal resistance and high charging efficiency, thereby executing high-rate charging in which the charging electric power is increased and the battery stacks A 2  are quickly charged. 
     When the plug-in charging of the stack group A 1  commences in this manner, the process proceeds to step S 51  where the SOCa of the battery stacks A 2  is read. Then, the process proceeds to step S 52  where it is determined whether the SOCa exceeds a predetermined target charging value Xn. If it is determined in step S 52  that the SOCa is lower than or equal to the target charging value Xn, that is, if the battery stacks A 2  are not completely plug-in charged yet, the process returns to step S 50  to continue plug-in charging the battery stacks A 2 . 
     In contrast, if it is determined in step S 52  that the SOCa exceeds the target charging value Xn, the process proceeds to step S 53  where the onboard charger  50  is controlled to a stopped state to stop plug-in charging the battery stacks A 2 . Subsequently, the process proceeds to step S 54  where the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched off. The process then proceeds to step S 55  where the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched off. 
     When the stack group A 1  is isolated from the onboard charger  50  in this manner, the process proceeds to step S 56  where the SOCb of the battery stacks B 2  is read, as illustrated in  FIG.  13   . Then, the process proceeds to step S 57  where it is determined whether the SOCb exceeds the predetermined target charging value Xo. If the SOCb exceeds the target charging value Xo in step S 57 , the battery stacks B 2  are not to be plug-in charged any further. Thus, the routine is exited without executing plug-in charging. 
     In contrast, if it is determined in step S 57  that the SOCb is lower than or equal to the target charging value Xo, that is, if the battery stacks B 2  are not completely plug-in charged yet, the process proceeds to step S 58  where the main switches SW 1   a  and SW 1   b  are switched off and the main switches SW 2   a  and SW 2   b  are switched on. Subsequently, in step S 59 , the charging switches SW 5   a  and SW 5   b  are switched off, and the charging switches SW 6   a  and SW 6   b  are switched on. 
     When the stack group B 1  is coupled to the onboard charger  50  in this manner, the process proceeds to step S 60  where the onboard charger  50  is controlled to an energized state for charging the stack group B 1 , so that electric power is supplied to the stack group B 1  from the external power source  51  via the onboard charger  50 . Subsequently, the process proceeds to step S 61  where the SOCb of the battery stacks B 2  is read. The process then proceeds to step S 62  where it is determined whether the SOCb exceeds the target charging value Xo. If it is determined in step S 62  that the SOCb is lower than or equal to the target charging value Xo, that is, if the battery stacks B 2  are not completely plug-in charged yet, the process returns to step S 60  to continue plug-in charging the battery stacks B 2 . 
     In contrast, if it is determined in step S 62  that the SOCb exceeds the target charging value Xo, that is, if the battery stacks B 2  are sufficiently charged, the process proceeds to step S 63  where the onboard charger  50  is controlled to a stopped state to stop plug-in charging the battery stacks B 2 . Subsequently, the process proceeds to step S 64  where the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched off. The process then proceeds to step S 65  where the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched off. 
     Automatic Supplementary Charging Control 
     The following description relates to automatic supplementary charging control in which the battery stacks A 2  are charged by the battery stacks B 2  when the vehicle is in a stopped state. A vehicle stopped state during which the automatic supplementary charging control is to be executed corresponds to a state where the vehicle is stopped when the activation switch  84  serving as a power switch is turned off. That is, a vehicle stopped state during which the automatic supplementary charging control is to be executed is a state where a vehicle travel control system is aborted and the vehicle is continuously stopped until the activation switch  84  is turned on again. 
       FIG.  15    and  FIG.  16    are flowcharts illustrating an example of a procedure of the automatic supplementary charging control. In  FIG.  15    and  FIG.  16   , the flowcharts are coupled to each other at sections indicated by reference signs C and D.  FIG.  17 A  illustrates an operating state of the battery pack  11  after the vehicle has stopped, and  FIG.  17 B  illustrates an operating state of the battery pack  11  in an automatic supplementary charging mode. In  FIG.  17 A  and  FIG.  17 B , a charging-discharging state is indicated by using an arrow. 
     As illustrated in  FIG.  15   , in step S 70 , the SOCa and SOCb of the battery stacks A 2  and B 2  are read. Subsequently, the process proceeds to step S 71  where it is determined whether plug-in charging using the external power source  51  has not been performed yet. In step S 72 , it is determined whether the SOCa falls below a predetermined threshold value Xp. In step S 73 , it is determined whether the SOCb exceeds a predetermined threshold value Xq. If the determination result obtained in step S 71  indicates that plug-in charging is being performed, the determination result obtained in step S 72  indicates that the SOCa is higher than or equal to the threshold value Xp, or the determination result obtained in step S 73  indicates that the SOCb is lower than or equal to the threshold value Xq, the routine is exited without executing the automatic supplementary charging mode to be described below, as illustrated in  FIG.  16   . That is, in a case where plug-in charging is being performed, the amount of stored electricity in the battery stacks A 2  is sufficiently ensured, or the amount of stored electricity in the battery stacks B 2  is not sufficiently ensured, the routine is exited without executing the automatic supplementary charging mode. 
     As illustrated in  FIG.  15   , in a case where the determination results obtained in step S 71  to step S 73  indicate that plug-in charging has not been performed yet, the SOCa falls below the threshold value Xp, and the SOCb exceeds the threshold value Xq, the process proceeds to step S 74  where the automatic supplementary charging mode commences. In step S 74 , the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched on. In step S 75 , the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched on. 
     When the stack groups A 1  and B 1  are coupled to the onboard charger  50  in this manner, the process proceeds to step S 76  where the onboard charger  50  is controlled to an operating state so that electric power is supplied to the stack group A 1  from the stack group B 1  via the onboard charger  50 , as illustrated in  FIG.  16   . That is, as illustrated in  FIG.  17 B , the automatic supplementary charging mode is executed so that the battery stacks A 2  can be charged using the battery stacks B 2  while the vehicle is stopped, so that the amount of stored electricity in the battery stacks A 2  can be increased, thereby ensuring minimum driving performance immediately after subsequent boarding. 
     When the charging of the stack group A 1  commences in accordance with the automatic supplementary charging mode, the process proceeds to step S 77  where the SOCa and SOCb of the battery stacks A 2  and B 2  are read. Subsequently, in step S 78 , it is determined whether the SOCa exceeds a predetermined target supplementary charging value Xr. In step S 79 , it is determined whether the SOCb falls below a predetermined lower-limit supplementary charging value Xs. If the determination results obtained in step S 78  and step S 79  indicate that the SOCa is lower than or equal to the target supplementary charging value Xr and the SOCb is higher than or equal to the lower-limit supplementary charging value Xs, the process returns to step S 76  to continue with the automatic supplementary charging mode. 
     In contrast, if the determination result obtained in step S 78  indicates that the SOCa exceeds the target supplementary charging value Xr or the determination result obtained in step S 79  indicates that the SOCb falls below the lower-limit supplementary charging value Xs, the process proceeds to step S 80  where the onboard charger  50  is controlled to a stopped state to stop the automatic supplementary charging mode. Subsequently, the process proceeds to step S 81  where the main switches SW 1   a , SW 1   b , SW 2   a , and SW 2   b  are switched off. Then, in step S 82 , the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched off. By executing the automatic supplementary charging mode in this manner, the battery stacks A 2  can be charged by the battery stacks B 2 , thereby ensuring minimum driving performance immediately after subsequent boarding. 
     Other Embodiments (Disposition of Battery Cells A 3  and B 3 ) 
     The following description relates to another example of the disposition of the battery cells A 3  and B 3  in the battery pack  11 .  FIG.  18    illustrates an example of the disposition of the battery cells A 3  and B 3 . In  FIG.  18   , the shaded battery cells are the battery cells B 3 . 
     In the above description, the battery stacks A 2  and the battery stacks B 2  are alternately disposed, as illustrated in  FIG.  2   . However, the disposition is not limited to the illustrated example. For example, as illustrated in  FIG.  18   , the battery cells A 3  constituting each battery stack A 2  and the battery cells B 3  constituting each battery stack B 2  may be alternately disposed adjacent to each other. Even in the case where the battery cells A 3  and the battery cells B 3  are alternately disposed in this manner, the battery stacks A 2  and B 2  can be disposed adjacent to each other, so that the battery stacks A 2  can be efficiently warmed by the heat of the battery stacks B 2  in the above-described plug-in charging control, whereby the battery stacks A 2  can be quickly warmed. 
     CONCLUSION 
     The vehicular power supply device  10  according to this embodiment has the battery pack (electric storage unit group)  11  that includes the battery stacks (first electric storage unit) A 2  and the battery stacks (second electric storage unit) B 2  having the higher internal resistance than the battery stacks A 2 . The battery pack  11  is coupled to the onboard charger (charger)  50  that charges at least either of the battery stacks A 2  and the battery stacks B 2 . The vehicular power supply device  10  also has the charging switches (first switch) SW 5   a  and SW 5   b  provided between the battery stacks A 2  and the onboard charger  50  and the charging switches (second switch) SW 6   a  and SW 6   b  provided between the battery stacks B 2  and the onboard charger  50 . Moreover, the vehicular power supply device  10  controls the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  based on the temperature of the battery pack  11 . 
     The charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are controlled based on the temperature of the battery pack  11  in this manner, so that the battery stacks A 2  can be warmed by using the battery stacks B 2  even in a low-temperature environment. Accordingly, the battery stacks A 2  can be warmed even in a low-temperature environment, so that the charging efficiency of the battery stacks A 2  can be enhanced. Furthermore, as illustrated in  FIG.  2   , the battery stacks A 2  and B 2  are disposed adjacent to each other, so that the battery stacks A 2  can be actively warmed with heat generated from the battery stacks B 2 . 
     As described above with reference to step S 41  and step S 43  in  FIG.  11   , as well as  FIG.  14 A , if the average temperature Ta of the battery stacks A 2  falls below the threshold value Xk during plug-in charging, the battery stacks A 2  are isolated from the onboard charger  50  by switching off the charging switches SW 5   a  and SW 5   b , and the battery stacks B 2  are coupled to the onboard charger  50  by switching on the charging switches SW 6   a  and SW 6   b . If the temperature of the battery stacks A 2  has decreased during plug-in charging in this manner, the battery stacks B 2  are charged by the onboard charger  50 . Accordingly, the battery stacks B 2  can generate heat, so that the battery stacks A 2  can be warmed by the battery stacks B 2 . In addition, the battery stacks A 2  can be warmed without using, for example, an electric heater, thereby enhancing the energy efficiency of the vehicle. 
     Furthermore, as described above with reference to step S 41  in  FIG.  11   , step S 49  in  FIG.  12   , and  FIG.  14 B , if the average temperature Ta of the battery stacks A 2  exceeds the threshold value Xk during plug-in charging, the battery stacks A 2  are coupled to the onboard charger  50  by switching on the charging switches SW 5   a  and SW 5   b , and the battery stacks B 2  are isolated from the onboard charger  50  by switching off the charging switches SW 6   a  and SW 6   b . If the temperature of the battery stacks A 2  has increased during plug-in charging in this manner, the battery stacks A 2  are charged by the onboard charger  50 . Accordingly, the battery stacks A 2  can be efficiently charged. 
     Furthermore, as illustrated in  FIG.  8 B , the vehicular power supply device  10  is provided with the motor generator (driving motor)  21  coupled to the battery stacks A 2  via the inverter  20  and the electric device group  24  coupled to the battery stacks B 2  via the converter  22 . As described above with reference to step S 21  and step S 22  in  FIG.  7   , as well as  FIG.  8 B , if the average temperature Ta of the battery stacks A 2  exceeds the threshold value Xe while the vehicle is running, the charging switches SW 5   a , SW 5   b , SW 6   a , and SW 6   b  are switched on, so that the battery stacks A 2  and the battery stacks B 2  are coupled in parallel with each other via the onboard charger  50 . Accordingly, the load of the battery stacks A 2  can be reduced, thereby decreasing the temperature of the battery pack  11 . 
     Although the above description relates to an example where plug-in charging is performed in a low-temperature environment and control is performed for warming the battery stacks A 2  by using the heat of the battery stacks B 2 , this control is not limited to plug-in charging. For example, in a series hybrid vehicle equipped with a power generating engine, the battery stacks A 2  may be warmed by using the heat of the battery stacks B 2  during series power generation in a low-temperature environment. Furthermore, as an alternative to the above description in which the temperature-increase suppression control and the plug-in charging control are executed by using the average temperature Ta of the battery stacks A 2  as the temperature of the battery pack  11 , for example, the temperature-increase suppression control and the plug-in charging control may be executed based on the temperature of a specific battery stack A 2 , or the temperature-increase suppression control and the plug-in charging control may be executed based on the temperature of the battery pack  11 . 
     The present disclosure is not limited to the above embodiment, and permits various modifications so long as they are within the scope of the disclosure. In the above description, the vehicle to which the vehicular power supply device  10  is applied is an electric automobile equipped with the motor generator  21  alone as a power source, but is not limited thereto and may alternatively be a hybrid vehicle equipped with the motor generator  21  and an engine as power sources. Furthermore, the main switches SW 1   a  to SW 2   b , the selector switches SW 3   a  to SW 4   b , and the charging switches SW 5   a  to SW 6   b  may each be a switch constituted of a semiconductor element, such as a metal oxide semiconductor field-effect transistor (MOSFET), or a switch that mechanically opens and closes a contact by using an electromagnetic force. The various switches, such as the main switches, are also called relays and contactors. 
     The battery cells A 3  and B 3  constituting the battery stacks A 2  and B 2  may be lithium ion batteries, but are not limited thereto and may alternatively be other kinds of batteries or capacitors. Moreover, the battery cells A 3  and B 3  may be the same kind of batteries or capacitors, or may be different kinds of batteries or capacitors. Although a single battery stack A 2  and a single battery stack 1  B 2  are alternately disposed in the example illustrated in  FIG.  2   , the configuration is not limited to this. For example, a stack group having two or more battery stacks A 2  and a stack group having two or more battery stacks B 2  may be formed, and these stack groups may be alternately disposed adjacent to each other. 
     In the above description, the SOH indicating the state of degradation of each of the battery stacks A 2  and B 2  is the current capacity retention rate with respect to the electric storage capacity or the terminal voltage at the time of manufacture, but is not limited thereto. For example, the SOH indicating the state of degradation of each of the battery stacks A 2  and B 2  may be the current resistance increase rate with respect to the internal resistance at the time of manufacture. In a case where the resistance increase rate is to be used as the SOH in this manner, the internal resistance becomes lower as the battery stack A 2  or B 2  is in a better condition, so that the SOH is calculated to be a lower value. Furthermore, the SOH indicating the state of degradation of each of the battery stacks A 2  and B 2  may be a value obtained by integrating the temperatures of the battery stacks A 2  and B 2 . 
     According to the embodiment of the disclosure, the first switch and the second switch are controlled based on the temperature of the electric storage unit group, so that the first electric storage unit can be charged after the first electric storage unit is warmed by the second electric storage unit. 
     The vehicular power supply device  10  illustrated in  FIG.  1    and  FIG.  3    can be implemented by circuitry including at least one semiconductor integrated circuit such as at least one processor (e.g., a central processing unit (CPU)), at least one application specific integrated circuit (ASIC), and/or at least one field programmable gate array (FPGA). At least one processor can be configured, by reading instructions from at least one machine readable tangible medium, to perform all or a part of functions of the vehicular power supply device  10  including the battery pack  11 , the onboard charger  50 , the charging switches SW 5   a  and SW 5   b , the charging switches SW 6   a  and SW 6   b , and the controllers  70  to  74 . Such a medium may take many forms, including, but not limited to, any type of magnetic medium such as a hard disk, any type of optical medium such as a CD and a DVD, any type of semiconductor memory (i.e., semiconductor circuit) such as a volatile memory and a non-volatile memory. The volatile memory may include a DRAM and a SRAM, and the non-volatile memory may include a ROM and a NVRAM. The ASIC is an integrated circuit (IC) customized to perform, and the FPGA is an integrated circuit designed to be configured after manufacturing in order to perform, all or a part of the functions of the modules illustrated in  FIG.  1    and  FIG.  3   .