Abstract:
A temperature-raising device for an in-car battery includes: a battery temperature acquisition unit configured to acquire a temperature of a battery mounted in a vehicle; an environmental temperature acquisition unit configured to acquire an environmental temperature; a heater configured to raise the temperature of the battery; and a controller configured to i) turn ON or OFF the heater based on a result of a comparison between the battery temperature acquired by the battery temperature acquisition unit and a predetermined threshold, and ii) reduce the threshold in accordance with a reduction in the environmental temperature acquired by the environmental temperature acquisition unit.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The disclosure of Japanese Patent Application No. 2014-260879 filed on Dec. 24, 2014 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a temperature-raising device and a temperature-raising method for raising a temperature of a battery mounted in a vehicle. 
         [0004]    2. Description of Related Art 
         [0005]    In the related art, an electric vehicle that uses a rotary electric machine as a power source is widely known, examples of which include a hybrid car and an electric car. In the electric vehicle, a battery is mounted in order to supply electric power to the rotary electric machine. The in-car battery is a secondary battery that can be charged and discharged. The in-car battery can be charged with electric power generated by the rotary electric machine and electric power from an external electric power supply. The in-car battery has a low charging capacity at a low temperature, and then the length of time that is required for charging increases due to a reduction in allowable charging current. In addition, the in-car battery cannot be charged or discharged in a case where the temperature of the in-car battery decreases to the point of reaching a freezing temperature. 
         [0006]    In some cases, the electric vehicle is put into a plug-in connection state, where the in-car battery and the external electric power supply are connected to each other, for charging after being stopped. When an environmental temperature is low in this case, the temperature of the battery decreases with time, and a problem arises in the form of charging using the external electric power supply not being appropriately performed. In this regard, an increase in the temperature of the in-car battery by the use of a heater has been proposed so that the temperature of the in-car battery becomes equal to or higher than a reference value in the plug-in connection state. In addition, calculation of a battery temperature-raising time allowing for the environmental temperature has been proposed. For example, Japanese Patent Application Publication No. 2012-191781 discloses prediction of a battery temperature transition from the battery temperature and the environmental temperature at the current point in time and prediction of the battery temperature-raising time and electric power consumption. According to this technique, charging of the battery or the like can be efficiently performed. 
       SUMMARY OF THE INVENTION 
       [0007]    In the meantime, there are regions across the world that have an extremely low outside temperature. Fairbanks, for example, has an average minimum temperature of lower than −20° C. in some months. In a case where an operation for raising the temperature of the in-car battery similar to those performed in other regions is performed in such a frigid region, the operating time of the heater for heating purposes significantly increases, and then problems could arise such as an increase in electric power consumption and a reduction in the life of an electrical component relating to the heater. 
         [0008]    The invention provides a temperature-raising device and a temperature-raising method to inhibit the shortening of the life of a heater for an in-car battery even in a frigid environment. 
         [0009]    According to an aspect of the invention, there is provided a temperature-raising device for an in-car battery, the temperature-raising device including: a battery temperature acquisition unit configured to acquire a temperature of a battery mounted in a vehicle; an environmental temperature acquisition unit configured to acquire an environmental temperature; a heater configured to raise the temperature of the battery; and a controller configured to i) turn ON or OFF the heater based on a result of a comparison between the battery temperature acquired by the battery temperature acquisition unit and a predetermined threshold, and ii) reduce the threshold in accordance with a reduction in the environmental temperature acquired by the environmental temperature acquisition unit. 
         [0010]    The controller may be configured to i) set the threshold to a standard value defined in advance in a case where the environmental temperature acquired by the environmental temperature acquisition unit is equal to or higher than a reference environmental temperature defined in advance, and ii) set the threshold to a value lower than the standard value in a case where the environmental temperature acquired by the environmental temperature acquisition unit is lower than the reference environmental temperature. 
         [0011]    The battery temperature acquisition unit may have a temperature sensor installed around the in-car battery, and the environmental temperature acquisition unit may be configured to estimate the environmental temperature based on a battery temperature detected by the temperature sensor. The environmental temperature acquisition unit may be configured to i) estimate a battery temperature after a passage of a predetermined period of time based on an estimated environmental temperature at a current point in time and the battery temperature detected by the temperature sensor at the current point in time, and ii) correct the estimated environmental temperature in accordance with an error between the battery temperature detected by the temperature sensor and the estimated battery temperature after the passage of the predetermined period of time. 
         [0012]    The controller may be configured to execute a temperature-raising operation for the in-car battery in a plug-in state where the in-car battery is electrically connected to an external electric power supply. 
         [0013]    According to another aspect of the invention, there is provided a temperature-raising method for a battery mounted in a vehicle, the vehicle including a temperature sensor configured to acquire a temperature of the battery and a controller, the temperature-raising method including: acquiring the temperature of the battery mounted in the vehicle by the temperature sensor; turning ON or OFF the heater raising the temperature of the battery by the controller based on a result of a comparison between the acquired battery temperature and a predetermined threshold; acquiring an environmental temperature of the vehicle by the controller; and reducing the threshold by the controller in accordance with a reduction in the acquired environmental temperature. 
         [0014]    According to the aspects of the invention, the threshold for turning the heater ON or OFF is reduced in accordance with a reduction in the environmental temperature, and thus an excessive increase in heater operation time can be suppressed and shortening of the life of the heater can be suppressed even in a frigid environment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
           [0016]      FIG. 1  is a drawing illustrating a schematic configuration of a hybrid driving system according to an embodiment of the invention; 
           [0017]      FIG. 2  is a flowchart illustrating a flow of a temperature-raising control; 
           [0018]      FIG. 3  is a flowchart illustrating a flow of a temperature-raising operation; 
           [0019]      FIG. 4  is a drawing illustrating a battery temperature and a heater operation timing pertaining to a case where a first ON-OFF temperature is set; 
           [0020]      FIG. 5  is a drawing illustrating the battery temperature and the heater operation timing pertaining to a case where a second ON-OFF temperature is set; 
           [0021]      FIG. 6  is a drawing illustrating the battery temperature and the heater operation timing pertaining to a case where the first ON-OFF temperature is set; 
           [0022]      FIG. 7  is a flowchart illustrating a flow of environmental temperature estimation; and 
           [0023]      FIG. 8  is a drawing illustrating a principle of the environmental temperature estimation. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0024]    Hereinafter, an embodiment of the invention will be described with reference to accompanying drawings.  FIG. 1  is a drawing illustrating a schematic configuration of a hybrid driving system  10  according to the embodiment of the invention. In the hybrid driving system  10 , two rotary electric machines MG 1 , MG 2  and one engine  12  are disposed as power sources. A main battery  20 , which supplies electric power to the rotary electric machines MG 1 , MG 2  or stores electric power generated by the rotary electric machines MG 1 , MG 2 , is disposed in the hybrid driving system  10 . The main battery  20  has a plurality of electric cells that are connected in series. A secondary battery such as a nickel-hydrogen battery and a lithium-ion battery can be used as the electric cell. In addition, an electric double layer capacitor can be used instead of the secondary battery as the electric cell. A plurality of electric cells that are connected in parallel may be included in the main battery  20 . 
         [0025]    A voltage value VB of the main battery  20  is detected by a voltage sensor  22  and is input to a controller  26 . A current value IB of a current of the main battery  20  is detected by a current sensor  23  and is input to the controller  26 . A temperature sensor  24 , which detects a temperature of the main battery  20  (battery temperature Tb), is disposed in the vicinity of the main battery  20 . The temperature sensor  24  functions as a battery temperature acquisition unit that acquires the battery temperature Tb. The battery temperature Tb that is detected by the temperature sensor  24  is input to the controller  26 . The temperature sensor  24  may be one or more in number. In a case where a plurality of the temperature sensors  24  are disposed, it is desirable that the temperature sensors  24  are disposed at different positions. 
         [0026]    During a temperature-raising operation (described later), an environmental temperature α as a temperature of an environment where the main battery  20  is installed also matters. This environmental temperature α is calculated by the controller  26 . In other words, in this embodiment, the environmental temperature α is estimated from the battery temperature Tb as described in detail later. Accordingly, in this embodiment, the controller  26  is an environmental temperature acquisition unit that acquires the environmental temperature α. The environmental temperature α may be actually detected by the use of a temperature sensor or a temperature that is detected by, for example, a temperature sensor which is disposed outside a passenger compartment and a temperature sensor which is disposed in a refrigerant intake path in order to cool the main battery  20  may be used as the environmental temperature α. 
         [0027]    The main battery  20  is connected to an inverter  18  via system main relays  44 . When an ignition switch of a vehicle is switched from OFF to ON, the controller  26  electrically connects the main battery  20  and the inverter  18  to each other by switching the system main relays  44  from OFF to ON. When the ignition switch of the vehicle is switched from ON to OFF, the controller  26  electrically disconnects the main battery  20  and the inverter  18  from each other by switching the system main relays from ON to OFF. 
         [0028]    The inverter  18  converts direct-current electric power that is supplied from the main battery  20  into alternating-current electric power and outputs the alternating-current electric power to the second rotary electric machine MG 2 . The second rotary electric machine MG 2  generates kinetic energy for traveling of the vehicle by receiving the alternating-current electric power output from the inverter  18 . The vehicle travels when the kinetic energy that is generated by the second rotary electric machine MG 2  is transmitted to a driving wheel. In addition, the second rotary electric machine MG 2  converts kinetic energy that is generated during braking of the vehicle into electrical energy. The inverter  18  converts alternating-current electric power (regenerative electric power) that is generated by the second rotary electric machine MG 2  into direct-current electric power and supplies the direct-current electric power to the main battery  20 . The main battery  20  is charged in this manner. 
         [0029]    A power dividing mechanism  14  transmits power of the engine  12  to a driving wheel  16  or transmits the power of the engine  12  to the first rotary electric machine MG 1 . The first rotary electric machine MG 1  generates electric power by receiving the power of the engine  12 . The electric power that is generated by the first rotary electric machine MG 1  is supplied to the second rotary electric machine MG 2  or is supplied to the main battery  20  via the inverter  18 . The main battery  20  is charged by the electric power being supplied to the main battery  20 . 
         [0030]    A booster circuit (not illustrated) is disposed in a current path between the main battery  20  and the inverter  18 . The booster circuit boosts an output voltage of the main battery  20  and outputs electric power after the boosting to the inverter  18 . In addition, the booster circuit steps down an output voltage of the inverter  18  and outputs electric power after the step-down to the main battery  20 . 
         [0031]    A DC/DC converter  30  is also connected to the main battery  20 . The DC/DC converter  30  is connected in parallel to the inverter  18 . An auxiliary machine  36 , an auxiliary machine battery  34 , and a heater  32  are connected to the DC/DC converter  30 . The DC/DC converter  30  steps down the output voltage of the main battery  20  and supplies electric power after the step-down to the auxiliary machine  36  and the auxiliary machine battery  34 . Then, the auxiliary machine  36  can be put into operation or the auxiliary machine battery  34  can be charged. An operation of the DC/DC converter  30  is controlled by the controller  26 . 
         [0032]    The heater  32  is used to raise the temperature of the main battery  20 . In  FIG. 1 , the heater  32  is disposed at a position spaced apart from the main battery  20 . In actuality, however, the heater  32  is disposed in the vicinity of the main battery  20 . In addition, the heater  32  may be one or more in number. A switch  46  is disposed in a current path between the DC/DC converter  30  and the heater  32 . This switch  46  is switched between ON and OFF in response to a control signal from the controller  26 . When the switch  46  is ON, predetermined electric power is supplied from the DC/DC converter  30  to the heater  32  and the heater  32  is allowed to generate heat. The temperature of the main battery  20  is raised when the heater  32  generates heat. Driving of the heater  32  is controlled by the controller  26 . In other words, the heater  32 , the temperature sensor  24 , the controller  26 , and the like constitute a temperature-raising device that raises the temperature of the main battery  20 . 
         [0033]    A charger  38  is also connected to the main battery  20 . A charging relay  42  is disposed between the main battery  20  and the charger  38 . The charging relay  42  is switched between ON and OFF in response to a control signal from the controller  26 . A connector  40  (so-called inlet) is connected to the charger  38 . The connector  40  can be connected to a connector  102  (so-called charging plug) of an external electric power supply  100  (such as a commercial electric power supply). The controller  26  monitors a state of connection between the two connectors  40 ,  102 , that is, whether the two connectors  40 ,  102  are in a plug-in state where the two connectors  40 ,  102  are connected to each other or in a plug-out state where the two connectors  40 ,  102  are disconnected from each other. 
         [0034]    When the connector  40  is connected to the connector  102  and the charging relay  42  is ON, the charger  38  converts alternating-current electric power from the external electric power supply  100  into direct-current electric power and outputs the direct-current electric power. Operations of the charger  38  and the charging relay  42  are controlled by the controller  26 . The direct-current electric power that is output from the charger  38  is supplied to the main battery  20 . Then, the main battery  20  is charged. In the following description, the charging of the main battery  20  by the use of the electric power from the external electric power supply  100  will be referred to as “external charging”. 
         [0035]    In the plug-in state, the electric power from the charger  38  can be supplied to the DC/DC converter  30  as well as the main battery  20 . When the switch  46  is ON in this state, the DC/DC converter  30  can step down the electric power from the charger  38  and supply the electric power after the step-down to the heater  32 . In other words, in the plug-in state, the temperature of the main battery  20  can be raised by the heater  32  being driven by the use of some of the electric power from the external electric power supply  100 . 
         [0036]    Hereinafter, a temperature-raising control for the main battery  20  in the hybrid driving system  10  will be described. The main battery  20  is characterized by having an extended period of charging time due to a reduction in charging capacity and a reduction in allowable charging amount when the temperature of the main battery  20  is reduced. In addition, charging and discharging of the main battery  20  might be impossible in a case where the temperature of the main battery  20  is excessively low. In this regard, the controller  26  executes the temperature-raising operation for the main battery  20  by the heater  32  in the plug-in state even after the vehicle is stopped. 
         [0037]    In a hybrid vehicle, the external charging is available when the vehicle is stationary and a user can set a time when the external charging is terminated. In addition, the user can set a time of initiation of pre-air conditioning for turning air conditioning ON before a restart of the vehicle. In a case where the external charging termination time or the pre-air conditioning initiation time (hereinafter, both collectively referred to as a “set time tc”) is set by the user, the controller  26  drives the heater  32  such that the temperature of the main battery  20  at the set time tc is equal to or higher than battery temperature lower limit value Tb min  that is set. After the heater  32  is driven, the temperature-raising operation is executed in a case where a predetermined period of time (such as 72 hours) has passed since the initial driving of the heater  32  or until a plug-out. 
         [0038]    In a case where the external charging termination time or the pre-air conditioning initiation time (set time tc) is not set, the heater  32  is driven such that the temperature of the main battery  20  becomes equal to or higher than the battery temperature lower limit value Tb min  that is set until a predetermined period of time (such as 72 hours) passes after the plug-in state started. 
         [0039]      FIG. 2  is a flowchart illustrating a flow of the temperature-raising control for the main battery  20 . After the vehicle is stopped, the controller  26  monitors whether or not the vehicle is in the plug-in state (S 10 ). After it is determined as a result of the monitoring that the vehicle is in the plug-in state, the controller  26  checks whether or not the set time tc is set (S 12 ). In a case where the set time tc is not set, the controller  26  proceeds to S 18  and executes the temperature-raising operation. 
         [0040]    In a case where the set time tc is set, the controller  26  calculates a temperature-raising operation initiation time ts (S 14 ). In the first step of the calculation of this temperature-raising operation initiation time ts, the length of time that is required for raising the temperature of the main battery  20  to the prescribed battery temperature lower limit value Tb min , that is, a temperature-raising time tw is calculated. Then, the time that dates back by the temperature-raising time tw from the set time tc is calculated as the temperature-raising operation initiation time ts. The temperature-raising time tw can be obtained from parameters such as the battery temperature lower limit value Tb min  (temperature-raising target temperature), the battery temperature Tb at the current point in time, the environmental temperature α, and the time left tr between the current time and the set time tc. A map or an arithmetic expression that shows a correspondence relationship between these parameters and the temperature-raising time tw is stored in a memory  28  of the controller  26 . The controller  26  calculates the temperature-raising time tw by applying actually detected values of the parameters to the map or the arithmetic expression. The map that shows the correspondence relationship between the parameters and the temperature-raising time tw can be drawn up based on experiment and simulation results or the like. In addition, the following Equation 1 or the like can be used as the arithmetic expression that shows the correspondence relationship between the parameters and the temperature-raising time tw, in which B and C are predetermined constants. 
         [0000]    
       
         
           
             
               
                 
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                   [ 
                   
                     Equation 
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         [0041]    After the calculation of the temperature-raising initiation time ts is allowed, the controller  26  stands by until the temperature-raising initiation time ts (S 16 ). Then, after the temperature-raising initiation time ts is reached, the controller  26  drives the heater  32  and executes the temperature-raising operation (S 18 ). In addition, the controller  26  puts a counter into operation at the point in time when the heater  32  is driven for the first time, and counts the length of time that passes from the initiation of the driving of the heater  32 . 
         [0042]    Then, the controller  26  repeats the temperature-raising operation until the execution of a plug-out in which the connector  40  of the vehicle is removed from the connector  40  of the external electric power supply  100  (Yes in S 20 ) or the passage of a predetermined period of time from the initiation of the driving of the heater  32  (Yes in S 22 ). In the case of the execution of the plug-out or the passage of the predetermined period of time from the initiation of the driving of the heater  32 , the controller  26  stops the temperature-raising operation. The predetermined period of time is not particularly limited. The predetermined period of time is three days (72 hours) in this embodiment. 
         [0043]    Hereinafter, the temperature-raising operation will be described. During the temperature-raising operation, the controller  26  turns ON or OFF the switch  46  of the heater  32  depending on the battery temperature Tb. More specifically, an ON temperature Ton and an OFF temperature Toff that is higher than the ON temperature Ton are stored in the memory  28  of the controller  26 . The controller  26  turns ON the heater  32  when the battery temperature Tb that is detected by the temperature sensor  24  is lower than the ON temperature Ton and turns OFF the heater  32  when the battery temperature Tb is higher than the OFF temperature Toff. When the ON temperature Ton is allowed to be equal to the battery temperature lower limit value Tb min  in this case, the temperature of the main battery  20  can be maintained at the battery temperature lower limit value Tb min  (ON temperature Ton) or a higher temperature. The OFF temperature Toff is a value of the ON temperature Ton to which a certain degree of hysteresis is given. This hysteresis (difference value between the ON temperature Ton and the OFF temperature Toff) is not particularly limited insofar as the hysteresis is a value capable of preventing the switch  46  of the heater  32  or the like from chattering and an operable time of the heater  32  from extending to excess. In other words, when the hysteresis is excessively small, the ON-OFF switching of the heater  32  is repeated within a short period of time, which causes the switch  46  and the relay  42  to be deteriorated. In addition, when the hysteresis is excessively large, more time that is required for the heater  32  to be turned OFF after being turned ON, which eventually leads to an extended operation time of the heater  32  and a deterioration of the heater  32 . In view of these problems, the hysteresis has a value of 6° C. in this embodiment. 
         [0044]    In this embodiment, the ON temperature Ton (battery temperature lower limit value Tb min ) and the OFF temperature Toff are changed in accordance with the environmental temperature α. In other words, relatively high temperatures are set as the ON temperature Ton and the OFF temperature Toff in a case where the environmental temperature α is equal to or higher than a prescribed environmental temperature lower limit value α min  and relatively low temperatures are set as the ON temperature Ton and the OFF temperature Toff in a case where the environmental temperature α is lower than the environmental temperature lower limit value α min . Then, the heater  32  is turned ON or OFF based on the ON temperature Ton and the OFF temperature Toff which are set as described above. This configuration is for the following reason. 
         [0045]    According to the related art, the ON temperature Ton (battery temperature lower limit value Tb min ) and the OFF temperature Toff are constant at all times regardless of the environmental temperature α. For example, Ton is 0° C. and Toff is 6° C. Herein, 0° C. is a lower limit value of the temperature at which the main battery  20  has its performance guaranteed. According to the related art, it is desirable that the temperature of the main battery  20  is equal to or higher than 0° C. at all times. In a case where the vehicle is left unattended in a cryogenic environment, however, the heater  32  has to remain ON for an extended period of time for the temperature of the main battery  20  to be maintained at a temperature of 0° C. or higher, and then the operation time of the heater  32  might show an excessive increase. 
         [0046]    This will be described with reference to  FIGS. 4 and 6 .  FIGS. 4 and 6  are drawings illustrating a change in the battery temperature Tb and the operation time of the heater  32  pertaining to a case where Ton is set to 0° C. and Toff is set to 6° C.  FIG. 4  shows a case where the environmental temperature α is −20° C. and  FIG. 6  shows a case where the environmental temperature α is −35° C. The gray hatching in  FIGS. 4 and 6  stands for a period in which the heater  32  is in operation. 
         [0047]    As is apparent from a comparison between  FIGS. 4 and 6 , the heater  32  has an extending operation period as the environmental temperature α decreases. This is because the temperature is unlikely to increase even by the addition of the same amount of heat in a case where the environmental temperature α is low and the temperature is rapidly reduced in a case where no amount of heat is added. As a result, the ratio of the operation time of the heater  32  increases to approximately 85% at an environmental temperature α of −35° C. whereas the ratio of the operation time of the heater  32  is approximately 50% at an environmental temperature α of −20° C. When the ratio of the operation time of the heater  32  increases as described above, the total operation time of the heater  32  increases, and the heater  32  needs to be replaced relatively early. 
         [0048]    In this embodiment, the value of the ON temperature Ton and the OFF temperature Toff are reduced in a case where the environmental temperature α is lower than the environmental temperature lower limit value α min  so that the above-described problem is avoided. More specifically, in this embodiment, the memory of the controller  26  stores two types of the ON temperature Ton and two types of the OFF temperature Toff, that is, a first ON temperature Ton 1 , a second ON temperature Ton 2 , a first OFF temperature Toff 1 , and a second OFF temperature Toff 2 . The second ON temperature Ton 2  is lower than the first ON temperature Ton 1  (Ton 2 &lt;Ton 1 ). In this embodiment, Ton 1  is 0° C. and Ton 2  is −15° C. In addition, the second OFF temperature Toff 2  is lower than the first OFF temperature Toff 1  (Toff 2 &lt;Toff 1 ). In this embodiment, Toff 1  is 6° C. and Toff 2  is −9° C. 
         [0049]    The driving of the heater  32  is controlled with the first ON temperature Ton1 and the first OFF temperature Toff1 set as the ON temperature Ton and the OFF temperature Toff in a case where the environmental temperature α is equal to or higher than the environmental temperature lower limit value α min  and with the second ON temperature Ton2 and the second OFF temperature Toff2 set as the ON temperature Ton and the OFF temperature Toff in a case where the environmental temperature α is lower than the environmental temperature lower limit value α min . A temperature at which the vehicle can have its performance guaranteed is set as the environmental temperature lower limit value α min . The environmental temperature lower limit value α min  at which the hybrid vehicle according to this embodiment has its performance guaranteed is −25° C. 
         [0050]      FIG. 5  is a drawing illustrating a change in the battery temperature Tb and the operation time of the heater  32  pertaining to a case where Ton is set to −15° C. and Toff is set to −9° C. at an environmental temperature α of −35° C. Both  FIGS. 5 and 6  show a result at an environmental temperature α of −35° C. In  FIG. 5 , however, the values of the ON temperature Ton and the OFF temperature Toff are reduced, and thus the ratio of the operation time of the heater  32  can be significantly reduced in comparison to the case of  FIG. 6 . As a result, the heater  32  can have a shorter total operation time, and it is possible to advance the time for replacement of the heater  32 . 
         [0051]      FIG. 3  is a flowchart illustrating a flow of the temperature-raising operation according to this embodiment. As illustrated in  FIG. 3 , the controller  26  acquires the environmental temperature α in the first step of the temperature-raising operation (S 24 ). A flow of the acquisition of the environmental temperature α will be described in detail later. 
         [0052]    Then, the acquired environmental temperature α and the environmental temperature lower limit value α min  defined in advance are compared to each other, and it is checked whether or not the environmental temperature α is equal to or higher than the environmental temperature lower limit value α min  (S 26 ). In a case where α is equal to or higher than α min  as a result thereof, the first ON temperature Ton1 (0° C.) and the first OFF temperature Toff1 (6° C.) are set as the ON temperature Ton and the OFF temperature Toff (S 28 ). In a case where α is lower than α min , the second ON temperature Ton 2  (−15° C.) and the second OFF temperature Toff2 (−9° C.) are set as the ON temperature Ton and the OFF temperature Toff (S 30 ). 
         [0053]    Then, the controller  26  acquires the battery temperature Tb that is detected by the temperature sensor  24  (S 32 ). In a case where a plurality of the temperature sensors  24  are disposed, the temperature that is detected by one of the temperature sensors  24  may be acquired as the battery temperature Tb. Alternatively, a statistical value such as the average value and the minimum value of a plurality of the temperatures detected by the plurality of temperature sensors  24  may be acquired as the battery temperature Tb. 
         [0054]    After the acquisition of the battery temperature Tb, the battery temperature Tb is compared to the set ON temperature Ton (S 34 ). In a case where the battery temperature Tb is equal to or higher than the ON temperature Ton as a result of the comparison, this flow is terminated without the heater  32  being turned ON. The heater  32  is turned ON in a case where battery temperature Tb is lower than the ON temperature Ton (S 36 ). The temperature of the main battery  20  begins to be raised in this manner. 
         [0055]    Then, the controller  26  acquires the battery temperature Tb again (S 38 ) and compares the battery temperature Tb to the OFF temperature Toff (S 40 ). In a case where the battery temperature Tb exceeds the OFF temperature Toff as a result of the comparison, the heater  32  is turned OFF and this flow is terminated (S 42 ). In a case where the battery temperature Tb is equal to or lower than the OFF temperature Toff, Steps S 38  and S 40  are repeated until Tb becomes higher than Toff. 
         [0056]    As is apparent from the above description, the ON temperature Ton and the OFF temperature Toff, which are thresholds for turning ON or OFF the heater  32 , are changed in accordance with the environmental temperature α in this embodiment. As a result, the lengthening of the operation time of the heater  32  in a low-temperature environment can be prevented, which can eventually prevent the life of the heater  32  from running out early. In a case where the heating temperature is reduced (set to the second ON-OFF temperatures), in the meantime, a problem is predicted in the form of an extended period of the charging time attributable to a reduction in the charging capacity and a reduction in the allowable charging amount. However, this problem that has the form of a somewhat extended charging time is considered to be less burdensome on the user&#39;s part compared to a problem arising from the necessity of frequent exchange of the heater  32 . In other words, according to this embodiment, the number of exchanges associated with the heater  32 , which is more burdensome on the user&#39;s part, can be reduced although the charging time is somewhat extended. 
         [0057]    Hereinafter, a method for acquiring the environmental temperature α will be described. The environmental temperature α may be detected by the temperature sensor  24  that is disposed in a vehicle body, an intake port of the main battery  20 , or the like. In this embodiment, the environmental temperature α is estimated from the battery temperature Tb. This will be described with reference to  FIGS. 7 and 8 .  FIG. 7  is a flowchart illustrating a flow of the estimation of the environmental temperature α from the battery temperature Tb.  FIG. 8  is a drawing illustrating a principle of the estimation of the environmental temperature α. 
         [0058]    In the first step of the estimation of the environmental temperature α, a battery temperature Tb_t 0  at the current point in time (time t 0 ) is acquired (S 44 ). This battery temperature Tb_t 0  is a temperature that is detected by the temperature sensor  24  which is disposed in the main battery  20 . 
         [0059]    Then, an estimated value Tb_es of the battery temperature at time t 1 , which is the predetermined period of time tw after the current point in time (time t 0 ), is calculated based on the battery temperature Tb_t 0  at the current point in time and the estimated environmental temperature α at the current point in time (S 46 ). In other words, a map that shows a correspondence relationship of the estimated temperature Tb_es to the battery temperature Tb_t 0  and the estimated environmental temperature α at the current point in time may be stored in, for example, the memory  28  of the controller  26  and the estimated temperature Tb_es may be acquired by the use of this map. Alternatively, an arithmetic expression that shows the correspondence relationship of the estimated temperature Tb_es to the battery temperature Tb_t 0  and the estimated environmental temperature α at the current point in time may be stored in the memory  28  of the controller  26  and the estimated temperature Tb_es may be calculated based on the application of the current Tb_t 0  and α to the arithmetic expression. The following Equation 2 or the like can be used as the arithmetic expression, in which D is a prescribed constant. 
         [0000]      Tb_ es= (Tb_ t 0−α) e   −D·tw +α  [Equation 2]
 
         [0060]    After the acquisition of the estimated temperature Tb_es of the battery temperature Tb at time t 1 , the controller  26  stands by until time t 1  (S 48 ). Then, at time t 1 , the controller  26  acquires a battery temperature Tb_t 1  detected by the temperature sensor  24  (S 50 ). After the acquisition of the battery temperature Tb_t 1 , the amount of deviation between the battery temperature Tb_t 1  and the estimated temperature Tb_es is calculated, and the environmental temperature α is changed in accordance with the obtained amount of deviation. In other words, the controller  26  calculates ΔT=Tb_t 1 −Tb_es by subtracting the estimated temperature Tb_es from the battery temperature Tb_t 1 , and checks whether or not the difference value ΔT is equal to or higher than a prescribed threshold ΔST (S 52 ). In a case where ΔT is equal to or higher than ΔST, the estimated environmental temperature α at the current point in time is considered to be lower than the actual environmental temperature α, and thus a value that is obtained by adding a prescribed constant value Δα to the current estimated environmental temperature α is obtained as a new estimated environmental temperature α (S 54 ). In a case where ΔT is lower than ΔST, it is checked whether or not the difference value ΔT is equal to or lower than a prescribed threshold −ΔST (S 56 ). In a case where ΔT is equal to or lower than −ΔST, the estimated environmental temperature α at the current point in time is considered to be higher than the actual environmental temperature α, and thus a value that is obtained by subtracting the prescribed constant value Δα from the current estimated environmental temperature α is obtained as a new estimated environmental temperature α (S 58 ). In a case where ΔT exceeds −ΔST, the estimated environmental temperature α at the current point in time and the actual environmental temperature α are considered to have no significant difference from each other, and thus the estimated environmental temperature α at the current point in time is used as it is. Then, the estimated environmental temperature α is learned by the same processing being repeated. 
         [0061]    A case where the threshold ΔST is 1° C., Tb_t 1  is 5° C., Tb_es is 2° C., and ΔT is 3° C. can be considered as a specific example for description. In this case, it is conceivable that the actual battery temperature Tb_t 1  is higher than the estimated temperature Tb_es because the actual environmental temperature is higher than the estimated environmental temperature α. Accordingly, in this case, it is desirable that the estimated environmental temperature α is raised such that the estimated environmental temperature α becomes close to the actual environmental temperature. Accordingly, in a case where Tb_t 1  is 5° C., Tb_es is 2° C., and ΔT is 3° C., a Yes determination is made in Step S 52 , the processing proceeds to Step S 54 , and the estimated environmental temperature α is raised. A case where Tb_t 1  is 0° C., Tb_es is 2° C., and ΔT is −2° C. is also conceivable. In this case, it is conceivable that the actual battery temperature Tb_t 1  is lower than the estimated temperature Tb_es because the actual environmental temperature is lower than the estimated environmental temperature α. Accordingly, in this case, it is desirable that the estimated environmental temperature α is lowered such that the estimated environmental temperature α becomes close to the actual environmental temperature. Accordingly, in a case where Tb_t 1  is 0° C., Tb_es is 2° C., and ΔT is −2° C., a Yes determination is made in Step S 56 , the processing proceeds to Step S 58 , and the estimated environmental temperature α is lowered. A case where Tb_t 1  is 2.5° C., Tb_es is 2° C., and ΔT is 0.5° C. is also conceivable. In this case, the deviation between the actual battery temperature Tb_t 1  and the estimated temperature Tb_es is smaller than the allowable value ΔST=1° C. In this case, the estimated environmental temperature α is considered to have a small deviation from the actual environmental temperature as well. Accordingly, in this case, it is desirable that the estimated environmental temperature α is maintained at the current value without being changed. Accordingly, in this case, No determinations are made in Steps S 52  and S 56  and the estimated environmental temperature α is not changed. When the environmental temperature α is estimated based on the battery temperature Tb as described above, no dedicated temperature sensor  24  for detecting the environmental temperature α is necessary, and thus costs can be reduced. 
         [0062]    Each of the configurations that have been described above is merely an example, and can be appropriately modified insofar as the threshold used as a reference for turning ON or OFF the heater  32  is reduced in accordance with a reduction in the environmental temperature α. For example, the ON temperature Ton and the OFF temperature Toff may be changed in three or more stages although the ON temperature Ton and the OFF temperature Toff are changed in two stages in accordance with the environmental temperature α in this embodiment. In addition, the ON temperature Ton and the OFF temperature Toff may be continuously changed in accordance with the environmental temperature α instead of being changed in stages. For example, the ON temperature Ton and the OFF temperature Toff may be calculated based on an arithmetic expression in which the environmental temperature α is used as a variable. 
         [0063]    In this embodiment, the ON temperature Ton and the OFF temperature Toff have a constant difference value, that is, hysteresis. However, this hysteresis may also be changed in accordance with the environmental temperature α. In addition, only one of the ON temperature Ton and the OFF temperature Toff may be changed without the other one of the ON temperature Ton and the OFF temperature Toff being changed. For example, the ON temperature Ton may be lowered with the OFF temperature Toff being maintained as it is when the environmental temperature α is lowered. 
         [0064]    In this embodiment, both the ON temperature Ton and the OFF temperature Toff are used in the driving control for the heater  32 . However, only either one of the two may be used for the purpose. For example, the heater  32  may be controlled to be turned ON below the ON temperature Ton and then be turned OFF after the passage of a predetermined period of time. In this case, only the ON temperature Ton may be changed in accordance with the environmental temperature α. 
         [0065]    The method for estimating the environmental temperature α may also be appropriately modified from that illustrated in  FIG. 7 . In this embodiment, the constant value Δα is added or subtracted to or from the environmental temperature α when the error ΔT between the estimated temperature Tb_es and the actual battery temperature Tb_t 1  is at least a certain value. However, this value that is added or subtracted may be changed in accordance with the magnitude of the error ΔT. In addition, merely whether or not the current environmental temperature α is equal to or higher than the environmental temperature lower limit value α min  may be estimated without the estimation of the specific numerical value of the environmental temperature α. For example, the environmental temperature α may be determined to be lower than the environmental temperature lower limit value α min  in a case where the battery temperature Tb_t 1  actually detected after the passage of a predetermined period of time is lower than the estimated temperature Tb_es and the environmental temperature α may be determined to be equal to or higher than the environmental temperature lower limit value α min  in a case where the detected battery temperature Tb_t 1  is equal to or higher than the estimated temperature Tb_es with the estimated temperature Tb_es after the passage of the predetermined period of time estimated on the assumption that the current environmental temperature α corresponds to the environmental temperature lower limit value α min . Then, it may be determined, in accordance with a result of the determination, which of the first ON-OFF temperatures and the second ON-OFF temperatures to be adopted. In this embodiment, the temperature-raising device is incorporated into the hybrid vehicle. However, the temperature-raising device may be incorporated to any vehicle including the hybrid vehicle in which a battery (including a fuel cell) is mounted, examples of which include an electric car and a fuel cell vehicle.