Patent Publication Number: US-2010116468-A1

Title: Temperature adjustment mechanism, method for controlling temperature adjustment mechanism, and vehicle

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to temperature adjustment mechanisms, methods for controlling temperature adjustment mechanisms, and vehicles having a temperature adjustment mechanism, which are all adapted to prevent excessive increase and decrease in the temperature of a power source. 
     2. Description of the Related Art 
     Various hybrid vehicles, fuel-cell vehicles, and electric vehicles run on the drive force of an electric motor. Such motor-driven vehicles often use a secondary battery or a capacitor (condenser) for storing electric power to be supplied to the electric motor. The performance and life of a secondary battery largely depends on the environmental temperature. In particular, charging and discharging a secondary battery at a high temperature may deteriorate the secondary battery significantly. 
     In view of the above, for example, various structures for suppressing such deterioration of secondary batteries have been described in Japanese Patent Application Publications No. 09-259940 (JP-A-09-259940), No.  11 -307139 (JP-A-11-307139), No. 2001-319697 (JP-A-2001-319697), and No. 09-167631 (JP-A-09-167631). 
       FIG. 10  shows a battery pack  1100  having a case  1101  containing a secondary battery  1102  and coolant  1103 . The battery pack  1100  is in contact with a vehicle body  200  (e.g., floor panel). According to this structure, the heat generated at the secondary battery  1102  is transferred to the case  1101  via the coolant  1103 , and the heat is then radiated from the case  1101  to the atmosphere and to the vehicle body in contact with the case  1101 , whereby an increase in the temperature of the secondary battery  1102  is suppressed. 
     According to this structure, however, because the battery pack  1100  is in contact with the vehicle body  200 , the following drawbacks are unavoidable. 
     That is, the secondary battery exhibits an adequate power storage performance within a given operation temperature range. Thus, if the temperature of the secondary battery is lower than the lower limit of the operation temperature range or higher than the upper limit of the operation temperature range, the power storage performance of the secondary battery is not adequate. 
     According to the structure shown in  FIG. 10 , because the battery pack  1100  is always in contact with the vehicle body  200 , the battery pack  1100  may be cooled or heated excessively depending upon the environmental temperature. The sentence “the battery back  1100  is excessively cooled or heated” refers to cases where the battery pack  1100  is cooled below the lower limit of its operation temperature range and to cases where the battery pack  1100  is heated beyond the upper limit of its operation temperature range. 
     For example, in winter, the temperature of the vehicle body  200  may become lower than 0° C., and in such cases, the battery pack  1100  (the secondary battery  1102 ) contact with the vehicle body  200  is cooled excessively. On the other hand, in summer, the temperature of the vehicle body  200  increases to a high temperature, whereby the battery pack  1100  in contact with the vehicle body  200  is heated excessively. 
     Thus, with the above structure in which the battery pack  1100  is in contact with the vehicle body  200 , in some case, the battery pack  1100  is cooled or heated excessively and therefore an adequate power storage performance can not be obtained. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the invention to provide temperature adjustment mechanisms capable of suppressing excessive increase and decrease in the temperature of a power source, control methods for controlling such temperature adjustment mechanisms, and vehicles having such temperature adjustment mechanisms. 
     A first aspect of the invention relates to a temperature adjustment mechanism for adjusting the temperature of a power source. The temperature adjustment mechanism includes: a case containing the power source and a first heat transfer medium for cooling the power source and integrated with or contacting a heat transfer portion; and a drive device that delivers the first heat transfer medium from the inside of the case to the outside of the case and from the outside of the case to the inside of the case. The drive device is adapted to deliver the first heat transfer medium from the inside of the case to the outside of the case so as to establish a first state in which a layer of a second heat transfer medium (e.g., air) is created in a region on the heat transfer portion side of the power source in the case, and the drive device is adapted to deliver, in the first state, the first heat transfer medium from the outside of the case to the inside of the case so as to establish a second state in which at least a portion of the region in the case (i.e., the region where the layer of the second heat transfer medium has been created) is filled with the first heat transfer medium. 
     The temperature adjustment mechanism described above may be such that the heat transfer portion is a portion through which heat is transferred between the atmosphere and the power source. 
     Further, the temperature adjustment mechanism described above may be such that the first heat transfer medium is coolant and the second heat transfer medium is gas. 
     Further, the temperature adjustment mechanism described above may be such that the case has a wall portion integrated with or contacting the heat transfer portion and having an inner face formed such that the area of contact between the inner face and the first heat transfer medium in the case varies continuously, or in steps, as the level of the first heat transfer medium in the case changes. 
     Further, the temperature adjustment mechanism described above may be such that the inner face of the wall portion of the case has a conical shape, a polygonal pyramid shape, or a stepped shape. 
     Further, the temperature adjustment mechanism described above may be such that at least one of wall portions of the case other than a wall portion integrated with or contacting the heat transfer portion has a heat-insulating portion. 
     Further, the temperature adjustment mechanism described above may have a controller that controls the drive device. This controller may be adapted to control the drive device based on information regarding the temperature of the power source and the temperature of the heat transfer portion. 
     Further, the temperature adjustment mechanism described above may be such that the drive device has a pump for delivering the first heat transfer medium and a container for storing the first heat transfer medium delivered to the outside of the case via the pump. 
     Further, the temperature adjustment mechanism described above may be such that the controller controls the drive device so as to establish the first state when the temperature of the power source is higher than a first reference temperature and lower than the temperature of the heat transfer portion or when the temperature of the power source is lower than a second reference temperature and higher than the temperature of the heat transfer portion. 
     Further, the temperature adjustment mechanism described above may have a partition dividing the interior of the case into a first region containing the power source and the first heat transfer medium and a second region containing the first heat transfer medium, the layer of the second heat transfer medium being created in the second region. 
     Further, the temperature adjustment mechanism described above may be such that the layer of the second heat transfer medium is in contact with the inner face of the wall portion integrated with or contacting the heat transfer portion. 
     Further, the temperature adjustment mechanism described above may have an agitating member for creating a flow of the first heat transfer medium in the case. 
     A second aspect of the invention relates to a temperature adjustment mechanism for adjusting a power source. This temperature adjustment mechanism has: a case containing the power source and a first heat transfer medium for cooling the power source and integrated with or contacting a heat transfer portion; and a heat transfer medium container provided in the case, adapted to contain a second heat transfer medium (e.g., gas or liquid) having a heat conductivity lower than the heat conductivity of the first heat transfer medium, and being variable in volume according to the amount of the second heat transfer medium contained. The heat transfer medium container is selectively placed in: a first state that is established by increasing the volume of the heat transfer medium container by delivering the second heat transfer medium into the heat transfer medium container such that, in the case, a layer of the second heat transfer medium is created in a region on the heat transfer portion side of the power source; and a second state that is established by reducing the volume of the heat transfer medium container by taking the second heat transfer medium out of the heat transfer medium container so that the first heat transfer medium can move to the region on the heat transfer portion side of the power source (the region where the layer of the second heat transfer medium has been created). 
     The temperature adjustment mechanism described above may have a drive mechanism used to deliver the first heat transfer medium from the inside of the case to the outside as the volume of the heat transfer medium container increases and to deliver the first heat transfer medium from the outside of the case to the inside of the case as the volume of the heat transfer medium container decreases. 
     The heat transfer medium container may be elastic. 
     A third aspect of the invention relates to a method for controlling a temperature adjustment mechanism used to adjust a temperature of a power source. The temperature adjustment mechanism has a case containing the power source and a first heat transfer medium for cooling the power source and integrated with or contacting a heat transfer portion. The method includes: delivering the first heat transfer medium from the inside of the case to the outside of the case such that, in the case, a layer of a second heat transfer medium having a heat conductivity lower than the heat conductivity of the first heat transfer medium is created in a region on the heat transfer portion side of the power source in the case; and delivering, after the layer of the second heat transfer medium has been created, the first heat transfer medium from the outside of the case to the inside of the case such that at least a portion of the region on the heat transfer portion side of the power source is filled with the first heat transfer medium. 
     A fourth aspect of the invention relates to a method for controlling a temperature adjustment mechanism used to adjust the temperature of a power source. The temperature adjustment mechanism has: a case that contains the power source and a first heat transfer medium for cooling the power source and is integrated with or in contact with a heat transfer portion; and a heat transfer medium container provided in the case, adapted to contain a second heat transfer medium having a heat conductivity lower than the heat conductivity of the first heat transfer medium, and being variable in volume according to the amount of the second heat transfer medium contained. The method includes: increasing the volume of the heat transfer medium container by delivering the second heat transfer medium into the heat transfer medium container such that, in the case, a layer of the second heat transfer medium is created in a region on the heat transfer portion side of the power source; and reducing, after the layer of the second heat transfer medium has been created, the volume of the heat transfer medium container by taking the second heat transfer medium out of the heat transfer medium container so that the first heat transfer medium can move to the region on the heat transfer portion side of the power source. 
     A fifth aspect of the invention relates to a vehicle provided with the temperature adjustment mechanism according to the first aspect or the second aspect of the invention. The case may be spaced from the body of the vehicle, and the case and the body of the vehicle may be connected via the heat transfer portion. Further, the heat transfer portion may be a portion of the body of the vehicle. 
     According the first to fifth aspects of the invention, it is possible to suppress excessive increase and decrease in the temperature of the power source, which may be caused by the environmental temperature, while cooling the power source properly. 
     That is, with the temperature adjustment mechanism according to the first aspect of the invention, even when the heat transfer portion is excessively cooled or heated under given environmental conditions, the second heat transfer medium layer created in the case suppresses heat transfer between the heat transfer portion and the first heat transfer medium and thus prevents excessive heating or cooling of the power source in the case. On the other hand, when the power source is producing heat, the region where the second heat transfer medium layer has been created is filled with the first heat transfer medium, so that the heat of the power source is properly radiated via the first heat transfer medium (i.e., the power source is properly cooled via the first heat transfer medium). 
     Next, with the temperature adjustment mechanism according to the second aspect of the invention, even when the heat transfer portion is excessively cooled or heated under given environmental conditions, the second heat transfer medium layer created in the case suppresses heat transfer between the heat transfer portion and the first heat transfer medium and thus prevents excessive heating or cooling of the power source in the case. On the other hand, when the power source is producing heat, the region where the second heat transfer medium layer has been created is filled with the first heat transfer medium, so that the heat of the power source is properly radiated via the first heat transfer medium (i.e., the power source is properly cooled via the first heat transfer medium). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is an exploded perspective view of a portion (battery pack) of a temperature adjustment mechanism of the first example embodiment of the invention; 
         FIG. 2A  is a perspective view of the temperature adjustment mechanism of the first example embodiment, and  FIG. 2B  is a side view of the temperature adjustment mechanism of the first example embodiment. 
         FIG. 3A  and  FIG. 3B  are views schematically showing the internal structure of the temperature adjustment mechanism of the first example embodiment of the invention; 
         FIG. 4  is a block diagram illustrating the system configuration for controlling the operation of the temperature adjustment mechanism of the first example embodiment of the invention; 
         FIG. 5  is a flowchart illustrating an operation routine of the temperature adjustment mechanism of the first example embodiment of the invention; 
         FIG. 6A  to  FIG. 6C  are views schematically showing the internal structure of a temperature adjustment mechanism of a modification example of the first example embodiment of the invention; 
         FIG. 7A  and  FIG. 7B  are views schematically showing the internal structure of a temperature adjustment mechanism of the second example embodiment of the invention; 
         FIG. 8A  and  FIG. 8B  are views schematically showing the internal structure of a temperature adjustment mechanism of the third example embodiment of the invention; 
         FIG. 9A  and  FIG. 9B  are views schematically showing the internal structure of a temperature adjustment mechanism of the fourth example embodiment of the invention; and 
         FIG. 10  is a view schematically showing the arrangement of a related-art battery pack. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Example embodiments of the invention will hereinafter be described. 
     First, a temperature adjustment mechanism according to the first example embodiment of the invention will be described with reference to  FIG. 1  to  FIG. 3 .  FIG. 1  is an exploded perspective view of the temperature adjustment mechanism of the first example embodiment,  FIG. 2A  is a perspective view of the temperature adjustment mechanism of the first example embodiment, and  FIG. 2B  is a side view of the temperature adjustment mechanism of the first example embodiment.  FIG. 3A  and  FIG. 3B  are views schematically showing the internal structure of the temperature adjustment mechanism of the first example embodiment. 
     A first case member  1  has an opening  11  for a battery unit  2 , which will be described later. Fins  12  are formed on the outer face of the first case member  1  in order to facilitate the heat radiation from the first case member  1  (i.e., the heat radiation from the battery unit  2 ). Note that the fins  12  are not necessarily provided. 
     The number of the fins  12  and the interval between the fins  12  are set in consideration of the heat capacity of the battery unit  2 , etc. The first case member  1  is made of a material having a high heat conductivity and a high corrosion resistance. For example, the first case member  1  may be made of a material having a heat conductivity as high as or higher than the heat conductivity of coolant  4  (Refer to  FIG. 3 ), which will be described later. More specifically, the first case member  1  may be made of metal (copper, iron, etc). 
     The battery unit  2  has a battery assembly (power source assembly)  20  constituted of a plurality of battery cells  20   a  and clamp members (end plates)  21  clamping the battery assembly  20  from both sides. The battery cells  20   a  are electrically connected in series by a bus bar (not shown in the drawings). Positive electrode cables and negative electrode cables (both not shown in the drawings) are connected to the battery assembly  20 , and these cables penetrate the first case member  1  and lead to electric components (e.g., motor) outside of the first case member  1 . 
     In the first example embodiment, each battery cell  20   a  is a cylindrical secondary battery. Examples of secondary batteries include nickel-hydrogen batteries, lithium-ion batteries, etc. The shape of the battery cell  20   a  is not necessary cylindrical. That is, it may have various other shapes including square shapes. Further, while a secondary battery is used as the battery cell  20   a  in this example embodiment, it may alternatively be an electric double layer capacitor (condenser) or a fuel cell, for example. Thus, the battery cells  20   a  serve as a power source for the electric components mentioned above. 
     A second case member  3  has a top  31  covering the opening  11  of the first case member  1  and legs  32  extending from the four corners of the top plate  31 . A plurality of frames  31   a  is formed at the top plate  31  to increase the strength of the top plate  31 . 
     The second case member  3  is made of a material having a high heat conductivity and a high corrosion resistance. For example, the second case member  3  may be made of a material having a heat conductivity substantially equal to or higher than the heat conductivity of coolant  4  (Refer to  FIG. 3 ), which will be described later. More specifically, the second case member  3  may be made of metal (copper, iron, etc). 
     The second case member  3  is fixed to the first case member  1  using screws, or the like, whereby the first case member  1  and the second case member  3  together define a closed space in which the battery unit  2  is placed. The ends of the respective legs  32  are fixed to a vehicle body  40  using screws, or the like, (Refer to  FIG. 2B ). The vehicle body  40  is, for example, a floor panel or a body frame of the vehicle. 
     The length of each leg  32  is greater than the height of the first case member  1 . Therefore, the bottom face of the first case member  1  is spaced from the surface of the vehicle body  40  when the first case member  1  and the second case member  3  are joined together and the legs  32  are connected to the vehicle body  40 . 
     The space (the space containing the battery unit  2 ) defined by the first case member  1  and the second case member  3  is filled with the coolant  4  for the battery unit  2  (Refer to  FIG. 3 ). Thus, the first case member  1  and the second case member  3  together constitute a case  30  shown in  FIG. 3 . Note that the space containing the battery unit  2  will hereinafter be referred to as “battery chamber” where necessary. 
     Insulative oil (e.g., silicon oil) or inert liquid (e.g., fluorine-based inert liquid) may be used as the coolant  4 . Examples of fluorine-based inert liquid include Fluorinert, Novec, HFE (hydrofluoroether), and Novec 1230 of 3M. 
     A fan may be provided in the battery chamber  30   a  of the case  30 . In this case, the coolant  4  in the battery chamber  30   a  of the case  30  can be made to flow (circulate) by driving (rotating) the fan, whereby the efficiency of the cooling of the battery unit  2  by the coolant  4  improves. 
     Referring to  FIG. 3 , a liquid level adjustment mechanism  5  is connected to the case  30 . The liquid level adjustment mechanism  5  adjusts the level of the coolant  4  in the battery chamber  30   a  of the case  30  (i.e., the amount of the coolant  4  in the battery chamber  30   a ). The liquid level adjustment mechanism  5  has a pipe  50  connected at both ends to the case  30 , a pump  51 , a valve  52 , and a reserve tank  53 , which are provided on the pipe  50 . 
     On a side wall of the reserve tank  53  are provided two liquid level sensors  54   a,    54   b  that detect the level of the coolant  4  in the reserve tank  53  (i.e., the amount of the coolant  4  in the reserve tank  53 ). The liquid level sensors  54   a,    54   b  are provided at different positions in the reserve tank  53 . 
     Next, the operation of the liquid level adjustment mechanism  5  will be described. 
     In the first example embodiment, first, the battery chamber  30   a  of the case  30  is filled up with the coolant  4  while the pipe  50  and the reserve tank  53  are partially filled with the coolant  4 , as shown in  FIG. 3A . At this time, the level of the coolant  4  in the battery chamber  30   a  of the case  30  and the level of the coolant  4  in the reserve tank  53  are substantially equal to each other. 
     In the state shown in  FIG. 3A , the coolant  4  is in contact with all the inner faces of the battery chamber  30   a  and the outer face of each battery cell of the battery unit  2 . In other words, at this time, the coolant  4  is in contact with the inner face of the top plate  31  of the second case member  3  and the inner faces of the first case member  1 . 
     In the state shown in  FIG. 3A , as the pump  51  is driven to deliver the coolant  4  in the battery chamber  30   a  to the reserve tank  53  via the pipe  50 , the air in the reserve tank  53  moves to the battery chamber  30   a,  whereby the level of the coolant  4  in the battery chamber  30   a  lowers while the level of the coolant  4  in the reserve tank  53  rises. At this time, the valve  52  is open. 
     As a result, an air layer AS is created in the upper area of the battery chamber  30   a  as shown in  FIG. 3B . In this state, the coolant  4  is not in contact with the upper inner face of the battery chamber  30   a  (i.e., the second case member  3 ), and the level of the coolant  4  in the reserve tank  53  coincides with the position of the liquid level sensor  54   a.  Note that the upper inner face of the battery chamber  30   a  is substantially flat (substantially perpendicular to the direction of gravity). 
     In the state shown in  FIG. 3B , the valve  52  is kept closed, preventing the coolant  4  in the pipe  50  from moving into the battery chamber  30   a  via the pipe  50 , whereby the state shown in  FIG. 3B  is maintained. 
     In the state shown in  FIG. 3B , as the pump  51  is driven to deliver the coolant  4  in the reserve tank  53  to the battery chamber  30   a  via the pipe  50 , the air in the battery chamber  30   a  moves to the reserve tank  53 , whereby the level of the coolant  4  in the reserve tank  53  lowers while the level of the coolant  4  in the battery chamber  30   a  rises. At this time, the valve  52  is open. 
     As a result, the level of the coolant  4  in the battery chamber  30   a  reaches the upper inner face of the battery chamber  30   a,  whereby the state shown in  FIG. 3A  is established. At this time, the coolant  4  is in contact with the upper inner face of the battery chamber  30   a  (the second case member  3 ), and the level of the coolant  4  in the reserve tank  53  coincides with the position of the liquid level sensor  54   b.    
     In the first example embodiment, the liquid level sensor  54   a  is located at the position corresponding to the level of the coolant  4  in the reserve tank  53  in the state shown in  FIG. 3A , and the liquid level sensor  54   b  is located at the position corresponding to the level of the coolant  4  in the reserve tank  53  in the state shown in  FIG. 3B . Therefore, the level of the coolant  4  in the battery chamber  30   a  can be confirmed by monitoring the outputs of the two liquid level sensors  54   a,    54   b.    
     That is, when receiving an output from the liquid level sensor  54   b,  it is determined that the coolant  4  is in contact with the upper inner face of the battery chamber  30   a,  and when receiving an output from the liquid level sensor  54   a,  it is determined that the coolant  4  is spaced from the upper inner face of the battery chamber  30   a.    
     The above-described operation of the liquid level adjustment mechanism  5  is controlled by a controller  100  (“controller”).  FIG. 4  shows the structure of the controller  100 . 
     Referring to  FIG. 4 , the controller  100  controls the pump  51  via a pump drive circuit  101  and controls the valve  52  via a valve drive circuit  102 . A first temperature sensor  103  is used to detect the temperature of the battery unit  2  and outputs the detection result to the controller  100 . A second temperature sensor  104  is used to detect the temperature of the vehicle body  40  and outputs the detection result to the controller  100 . The output signals of the liquid level sensors  54   a,    54   b  are input to the controller  100 . 
     The controller  100  may be adapted to serve also as a controller for controlling the running state of the vehicle. 
     The first temperature sensor  103  is adapted to detect directly, or indirectly, the temperature of the battery unit  2 . That is, the first temperature sensor  103  may either be arranged to be in contact with the battery unit  2  to detect the temperature thereof or arranged to be in contact with the coolant  4  in the battery chamber  30   a  to detect the temperature of the battery unit  2  indirectly. 
     Likewise, the second temperature sensor  104  is adapted to detect directly, or indirectly, the temperature of the vehicle body  40 . An existing sensor in the vehicle may be used as the second temperature sensor  104 . Alternatively, the temperature of the vehicle body  40  may be estimated based on the temperature control state of the air conditioner provided in the passenger compartment. In this case, the second temperature sensor  104  may be removed. 
     Next, the driving control of the liquid level adjustment mechanism  5  by the controller  100  will be described with reference to  FIG. 5 . 
     Referring to  FIG. 5 , in step S 1 , the controller  100  detects the temperature of the battery unit  2  based on the output of the first temperature sensor  103  and detects the temperature of the vehicle body  40  based on the output of the second temperature sensor  104 . In step S 2 , the controller  100  determines whether the temperature of the battery unit  2  detected in step S 1  is higher than an upper threshold. If the detected temperature of the battery unit  2  is higher than the upper threshold, the controller  100  then proceeds to step S 3 . On the other hand, if the detected temperature of the battery unit  2  is equal to or lower than the upper threshold, the controller  100  then proceeds to step S 6 . 
     Note that the upper threshold is predetermined in view of suppressing deterioration of the power storage performance that occurs as the temperature of the battery unit  2  increases, and the upper threshold is set to, for example, 40° C. 
     In step S 3 , the controller  100  determines whether the detected temperature of the battery unit  2  is lower than the detected temperature of the vehicle body  40 . If the detected temperature of the battery unit  2  is lower than the detected temperature of the vehicle body  40 , the controller  100  then proceeds to step S 4 . On the other hand, if the detected temperature of the battery unit  2  is higher than the detected temperature of the vehicle body  40 , the controller  100  proceeds to step S 5 . 
     In step S 4 , the controller  100  drives the pump  51  and the valve  52  via the pump drive circuit  101  and the valve drive circuit  102 , respectively, whereby the liquid level adjustment mechanism  5  is placed in the state shown in  FIG. 3B  (“first state”). 
     At this time, if it is determined based on the outputs of the liquid level sensors  54   a,    54   b  that the liquid level adjustment mechanism  5  is already in the state shown in  FIG. 3B , the liquid level adjustment mechanism  5  is simply maintained in the same state. On the other hand, if it is determined based on the outputs of the liquid level sensors  54   a,    54   b  that the liquid level adjustment mechanism  5  is presently in the state shown in  FIG. 3A , the controller  100  then executes the following control. 
     That is, the controller  100  delivers the coolant  4  in the battery chamber  30   a  to the reserve tank  53  via the pipe  50  by driving the pump  51  via the pump drive circuit  101 . At this time, the valve  52  is open. 
     When it is determined based on the outputs of the liquid level sensors  54   a,    54   b  that the coolant  4  has been spaced apart from the upper inner face of the battery chamber  30   a,  the controller  100  stops driving the pump  51 . Then, the controller  100  closes the valve  52  via the valve drive circuit  102 , whereby the liquid level adjustment mechanism  5  is placed in the state shown in  FIG. 3B , after which the controller  100  finishes the present cycle of the routine. 
     In step S 5 , on the other hand, the controller  100  places the liquid level adjustment mechanism  5  in the state shown in  FIG. 3A  (“second state”) by driving the pump  51  and the valve  52  via the pump drive circuit  101  and the valve drive circuit  102 , respectively. 
     At this time, if it is determined based on the outputs of the liquid level sensors  54   a,    54   b  that the liquid level adjustment mechanism  5  is already in the state shown in  FIG. 3A , the liquid level adjustment mechanism  5  is simply maintained in the same state. On the other hand, if it is determined based on the outputs of the liquid level sensors  54   a,    54   b  that the liquid level adjustment mechanism  5  is presently in the state shown in  FIG. 3B , the controller  100  then executes the following control. 
     That is, the controller  100  first opens the valve  52  via the valve drive circuit  102  and then drives the pump  51  via the pump drive circuit  101  so that the coolant  4  in the reserve tank  53  is delivered to the battery chamber  30   a  via the pipe  50 . 
     Then, when it is determined based on the outputs of the liquid level sensors  54   a,    54   b  that the coolant  4  has reached the upper inner face of the battery chamber  30   a,  the controller  100  stops driving the pump  51 . At this time, the valve  52  is kept open. Thus, the liquid level adjustment mechanism  5  is placed in the state shown in  FIG. 3A , and the controller  100  finishes the present cycle of the routine. 
     On the other hand, in step S 6 , the controller  100  determines whether the detected temperature of the battery unit  2  is lower than a lower threshold. If the detected temperature of the battery unit  2  is lower than the lower threshold, the controller  100  then proceeds to step S 7 . If not, conversely, the controller  100  proceeds to step S 5 . 
     The lower threshold is predetermined in view of suppressing deterioration of the power storage performance that occurs as the temperature of the battery unit  2  decreases, and the lower threshold is set to, for example, 10° C. 
     In step S 7 , the controller  100  determines whether the detected temperature of the battery unit  2  is higher than the detected temperature of the vehicle body  40 . If the detected temperature of the battery unit  2  is higher than the detected temperature of the vehicle body  40 , the controller  100  then proceeds to step S 4 . If not, conversely the controller  100  proceeds to step S 5 . 
     Each battery cell  20   a  generates heat as it is charged and discharged, and the generated heat is transferred to the case  30  via the coolant  4 . After transferred to the case  30 , the heat is then released to the outside (atmosphere). 
     Further, because the case  30  (the second case member  3 ) is connected to the vehicle body  40  as described above, the heat of the second case member  3  is transferred to the vehicle body  40  via the paths indicated by the arrows in  FIG. 1  and then released to the outside (atmosphere). In this case, the flames  31   a  the top plate  31  and the legs  32  of the second case member  3  may be regarded as corresponding to “heat transfer portion” of the invention. The “heat transfer portion” is a portion through which heat is transferred between the side of the battery unit  2 , the coolant  4 , and the case  30  and the outside (atmosphere), that is, a portion enabling heat transfer (indirect heat transfer) between the outside (atmosphere) and the battery unit  2 . 
     The heat generated at the battery unit  2  is mostly transferred to the vehicle body  40  via the case  30  (the second case member  3 ). 
     In the state shown in  FIG. 3A , the heat generated at the battery unit  2  is released (radiated) via the heat radiation paths mentioned above, whereby the increase in the temperature of the battery unit  2  is suppressed, thus reducing deterioration of the power storage performance of each battery cell  20   a  that occurs as its temperature increases. 
     On the other hand, in the state shown in  FIG. 3A , if the vehicle body  40  is cooled excessively, the portion of the case  30  connected to the vehicle body  40  (i.e., the second case member  3 ) is cooled, and then the coolant  4  in contact with the second case member  3  is cooled by the second case member  3 , resulting in the battery unit  2  being cooled excessively. This may deteriorate the power storage performance of each battery cell  20   a.  It is to be noted that, in the first example embodiment, the phrase “cooled excessively” refers to states where “YES” is obtained in step S 6  of the routine of  FIG. 5 , that is, states where the battery unit  2  has been cooled to an extent that the detected temperature of the battery unit  2  is lower than the lower threshold. This will be applied also to other example embodiments and modification examples described later. 
     Meanwhile, in the state shown in  FIG. 3A , if the vehicle body  40  is heated excessively, the portion of the case  30  connected to the vehicle body  40  (i.e., the second case member  3 ) is heated, whereby the coolant  4  in contact with the second case member  3  is heated, resulting in the battery unit  2  being heated excessively. This may deteriorate the power storage performance of each battery cell  20   a.  It is to be noted that, in the first example embodiment, the phrase “heated excessively” refers to states where “YES” is obtained in step S 2  of the routine of  FIG. 5 , that is, states where the battery unit  2  has been heated to an extent that the detected temperature of the battery unit  2  is higher than the upper threshold. This will be applied also to other example embodiments and modification examples described later. 
     In view of the above, in the first example embodiment, when the detected temperature of the battery unit  2  is higher than the upper threshold and the detected temperature of the vehicle body  40  is higher than the detected temperature of the battery unit  2 , and when the detected temperature of the battery unit  2  is lower than the lower threshold and the detected temperature of the vehicle body  40  is lower than the detected temperature of the battery unit  2 , the liquid level adjustment mechanism  5  is placed in the state shown in  FIG. 3B  (first state). At this time, the coolant  4  is spaced apart from the upper inner face of the battery chamber  30   a,  whereby the air layer AS is created at the upper area of the battery chamber  30   a.    
     Because the heat conductivity of the air layer AS is lower than the heat conductivity of the coolant  4 , when a portion of the case  30  (the second case member  3 ) is cooled or heated, the air layer AS between the upper inner face of the battery chamber  30   a  (the top plate  31  of the second case member  3 ) and the coolant  4  reduces the extent to which the coolant  4  is cooled or heated, whereby the battery unit  2  is prevented from being excessively cooled and heated via the coolant  4  and thus deterioration of the power storage performance of each battery cell  20   a  can be minimized. 
     Moreover, the structure employed in the example embodiment is simple, just connecting the liquid level adjustment mechanism  5  to the case  30 . 
     Further, in the first example embodiment, because the bottom face of the first case member  1  is spaced from the surface of the vehicle body  40  as shown in  FIG. 2B , the first case member  1  is not directly cooled by the vehicle body  40  when the vehicle body is excessively cooled. That is, the air layer between the first case member  1  and the vehicle body  40  minimizes the extent to which the first case member  1  is cooled as the vehicle body  40  is cooled. 
     As mentioned above, because the first case member  1  and the second case member  3  are joined together, when the second case member  3  is excessively cooled, the first case member  1  may be cooled through the joint between the first case member  1  and the second case member  3 , and the coolant  4  may be cooled by the first case member  1 . 
     However, if heat-insulating layers are formed at the inner faces of the battery chamber  30   a  other than the upper inner face, that is, if heat-insulating layers are formed at the inner faces of the first case member  1 , the cooling of the coolant  4  by the first case member  1  can be suppressed. Note that the first case member  1  may be made of a heat-insulating material. 
     Next, a modification example of the first example embodiment will be described with reference to  FIG. 6A  to  FIG. 6C .  FIG. 6A  to  FIG. 6C  are views each schematically showing the internal structure of a temperature adjustment mechanism of this modification example and corresponding to  FIG. 3A  and  FIG. 3B . Note that the elements identical to those recited in the first example embodiment are denoted by the same numerals. 
     While the upper inner face of the battery chamber  30   a  is flat in the first example embodiment, in this modification example, an upper inner face  30   b  of the battery chamber  30   a  is slanted, as will be described in detail below. 
     The upper inner face  30   b  of the battery chamber  30   a  slants from the side walls of the battery chamber  30   a  toward the pipe  50 . The upper inner face  30   b  slants with respect to the direction of gravity (the vertical direction in  FIG. 6A  to  FIG. 6C ). In other words, the upper inner face  30   b  of the battery chamber  30   a  slants downward (i.e., in the direction of gravity) from the joint between the pipe  50  and the case  30  toward the periphery of the upper inner face  30   b  of the battery chamber  30   a.    
     Note that the upper inner face  30   b  may be formed in any shape as a whole as long as it has a portion slanting with respect to the direction of gravity. For example, the upper inner face  30   b  may be formed in a conical shape or in a polygonal pyramid shape as a whole. Further, the upper inner face  30   b  may alternatively be stepped rather than being made a continuous, slanted face. That is, in this case, the upper inner face  30   b  may be formed in any shape as long as the cross-sectional area of the upper area of the battery chamber  30   a  measured on a plane perpendicular to the direction of gravity decreases continuously, or in steps, toward the pipe  50  side. 
     In this modification example, the thickness of the portion of the case  30  forming the upper inner face  30   b,  that is, the thickness of the top plate  31  of the second case member  3  decreases continuously toward the joint of the pipe  50 . Note that the upper inner face  30   b  of the battery chamber  30   a  may be slanted while maintaining the thickness of the top plate  31  of the second case member  3  substantially uniform. 
     Further, in this modification example, as shown in  FIG. 6A  to  FIG. 6C , the level of the coolant  4  in the battery chamber  30   a  is switched between three positions. More specifically, three liquid level sensors  54   a,    54   b,  and  54   c  are provided in the reserve tank  53 , and the level of the coolant  4  in the battery chamber  30   a  is determined based on the outputs of the liquid level sensor  54   a  to  54   c.    
       FIG. 6B  shows a state where the coolant  4  is spaced from the upper inner face  30   b,  and this state is established to prevent excessive increase and decrease in the temperature of the battery unit  2  due to the vehicle body being excessively cooled and heated.  FIG. 6A  shows a state where the coolant  4  is in full contact with the upper inner face  30   b,  and  FIG. 6C  shows a state where the coolant  4  is in limited contact with the upper inner face  30   b.  These states are established in order to suppress an increase in the temperature of the battery unit  2  by releasing the heat that has been generated at the battery unit  2  through its charging and discharging. 
     Thus, in this modification example, the upper inner face  30   b  of the battery chamber  30   a  is slanted and therefore the area of contact between the upper inner face  30   b  of the battery chamber  30   a  and the coolant  4  is variable. In this way, as the area of contact between the upper inner face  30   b  of the battery chamber  30   a  and the coolant  4  varies, the characteristic of the heat radiation through the upper inner face  30   b  of the case  30  changes accordingly, whereby the battery unit  2  can be cooled more properly according to the temperature of the battery unit  2 . 
     While the level of the coolant  4  in the battery chamber  30   a  is switched between the three states shown in  FIG. 6A  to  FIG. 6C , respectively, in this modification example, the level of the coolant  4  in the battery chamber  30   a  may alternatively be switched between four or more states. Note that in this modification example the liquid level adjustment mechanism  5  may be driven as described in the first example embodiment (Refer to  FIG. 5 ). 
     Further, while the liquid level sensors are provided at the reserve tank  53  in the first example embodiment and the modification example, the liquid level sensors may be provided in the battery chamber  30   a  of the case  30 . In this case, the level of the coolant  4  in the battery chamber  30   a  can be directly monitored. 
     Further, while the level of the coolant  4  in the battery chamber  30   a  is determined based on the outputs of the liquid level sensors in the first example embodiment, it may be determined otherwise. For example, the level of the coolant  4  in the battery chamber  30   a  may be determined by monitoring the driving amount of the pump  51 . More specifically, if the relation between the drive amount of the pump  51  and the amount of the coolant  4  delivered is obtained in advance, the level of the coolant  4  can be determined from the driving amount of the pump  51 . 
     Next, a temperature adjustment mechanism according to the second example embodiment of the invention will be described with reference to  FIG. 7A  and  FIG. 7B .  FIG. 7A  and  FIG. 7B  are views each schematically showing the internal structure of the temperature adjustment mechanism of the second example embodiment. Note that the elements identical to those recited in the first example embodiment are denoted by the same numerals. 
     In the first example embodiment, as described above, the air layer AS is selectively created and eliminated by changing the level of the coolant  4  in the battery chamber  30   a.  Meanwhile, in the second example embodiment, chambers S 1  and S 2  are formed in the case  30 , and the battery unit  2  is disposed in the chamber S 1  and the level of the coolant  4  (the amount of the coolant  4 ) in the chamber S 2  is controlled. The features of the second example embodiment will be described in detail below. 
     The case  30  is supported by support members  60  provided on the vehicle body  40 , whereby the bottom face of the case  30  is spaced from the surface of the vehicle body  40 . 
     Within the case  30  is provided a partition  70  by which the chamber S 1  and the chamber S 2  are partitioned off from each other. The partition  70  may be made of a material having a high heat conductivity and a high corrosion resistance, for example, a material having a heat conductivity as high as or higher than the heat conductivity of the coolant  4  in the case  30 . More specifically, the partition  70  may be formed of metal (copper, iron, etc.). 
     The chamber S 1  contains the battery unit  2  and the coolant  4 . The coolant  4  in the chamber S 1  is in contact with the outer face of each battery cell of the battery unit  2 , the inner face of the case  30 , and the partition  70 . 
     The chamber S 2  contains the coolant  4 . The liquid level adjustment mechanism  5  recited in the first example embodiment is connected to the chamber S 2 . That is, the pipe  50  is connected to the chamber S 1  and the pipe  50  extends to a reserve tank (not shown in the drawing) corresponding to the reserve tank  32  in the first example embodiment. Although not shown in the drawings, a valve and a pump corresponding to the pump  51  and the valve  52  in the first example embodiment are provided on the pipe  50 . 
     Note that although  FIG. 7A  and  FIG. 7B  show that the pipe  50  extends downward into the vehicle body  40 , the pipe  50  is actually located between the case  30  and the vehicle body  40 . That is, the liquid level adjustment mechanism is provided on the vehicle body  40 . 
     A heat transfer plate (“heat transfer portion”)  80  is in contact with the side wall of the case  30  that defines the chamber S 2 . The heat transfer plate  80  may be made of a material having a high heat conductivity and a high corrosion resistance, for example, a material having a heat conductivity as high as or higher than the heat conductivity of the coolant  4  in the case  30 . More specifically, the heat transfer plate  80  may be made of metal (copper, iron, etc.). 
     In the second example embodiment, the chamber S 1  is always filled up with the coolant  4  while the level of the coolant  4  in the chamber S 2  is changed as the liquid level adjustment mechanism is driven. The driving of the liquid level adjustment mechanism is controlled by the controller as in the first example embodiment described above. 
     That is, as the pump of the liquid level adjustment mechanism is driven in the state shown in  FIG. 7A , the coolant  4  in the chamber S 2  is delivered to the reserve tank so that the state shown in  FIG. 7B  is established. At this time, the air in the reserve tank moves to the chamber S 2  via the pipe  50 , whereby an air layer is created in the chamber S 2 . Note that other medium (e.g., other gas) having different constituents may be used instead of air. 
     On the other hand, as the pump of the liquid level adjustment mechanism is driven in the state shown in  FIG. 7B , the coolant  4  in the reserve tank is delivered to the chamber S 2  of the case  30  so that the state shown in  FIG. 7A  is established. At this time, the air in the chamber S 2  moves to the reserve tank via the pipe  50 . 
     As in the first example embodiment described above, if liquid level sensors are provided in the reserve tank or in the case  30  (i.e., the chamber S 2 ), the level of the coolant  4  (i.e., the amount of the coolant  4 ) in the chamber S 2  of the case  30  can be detected using the liquid level sensors. Alternatively, the level of the coolant  4  in the chamber S 2  can be determined by detecting the drive amount of the pump. 
     In the state shown in  FIG. 7A , the chamber S 2  of the case  30  is filled up with the coolant  4 . In this state, if the battery unit  2  produces heat through its charging and discharging, or the like, the produced heat is transferred to the coolant  4  in contact with the battery unit  2 . The heat of the coolant  4  is then transferred to the partition  70  through the natural convection of the coolant  4 , and the heat of the partition  70  is transferred to the coolant  4  in the chamber S 2 . 
     An agitating member may be provided in the chamber S 1  to create a flow of the coolant  4 . In this case, the cooling efficiency of the battery unit  2  further improves. 
     Meanwhile, because the heat transfer plate  80  is in contact with the side wall of the case  30  that defines the chamber S 2 , the heat transferred to the coolant  4  in the chamber S 2  is transferred to the vehicle body  40  via the heat transfer plate  80 . Further, the heat of the coolant  4  in the case  30  is released also to the atmosphere via the case  30 . Note that the heat produced at the battery unit  2  is mostly transferred to the vehicle body  40  via the heat transfer plate  80 . 
     As mentioned above, the heat produced at the battery unit  2  is released (radiated) to the atmosphere via the coolant  4  and the case  30  and radiated through the case  30 , the heat transfer plate  80 , and the vehicle body  40 , thus suppressing an increase in the temperature of the battery unit  2  that occurs as the battery unit  2  is charged and discharged and minimizing deterioration of the power storage performance of each battery cell. 
     If the vehicle body  40  is excessively cooled in the state shown in  FIG. 7A , the case  30  is excessively cooled via the heat transfer plate  80 , and it may result in an excessive decrease in the temperature of the battery unit  2 . On the other hand, if the vehicle body  40  is excessively heated, the case  30  is excessively heated via the heat transfer plate  80 , and it may result in an excessive increase in the temperature of the battery unit  2 . 
     According to the structure of the second example embodiment, because the case  30  is spaced, via the support members  60 , from the surface of the vehicle body  40 , the bottom face of the case  30  is not cooled nor heated excessively. Note that the support members  60  may be made of a heat-insulating material. 
     In the second example embodiment, when the vehicle body  40  is excessively cooled or heated, an air layer is created in the chamber S 2  as shown in  FIG. 7B , whereby the coolant  4  and the battery unit  2  in the chamber S 1  are prevented from being cooled or heated excessively via the chamber S 2 . 
     That is, when there is an air layer in the chamber S 2 , the heat conductivity is lower than when the chamber S 2  is filled with the coolant  4 , and thus the coolant  4  and the battery unit  2  in the chamber S 1  are prevented from being cooled and heated excessively as the heat transfer plate  80  is cooled and heated, whereby deterioration of the power storage performance of each battery cell, which may otherwise be caused as a result of excessive heating and cooling, can be minimized. 
     In the second example embodiment, the liquid level adjustment mechanism can be driven as described in the first example embodiment (Refer to  FIG. 5 ). 
     That is, when the detected temperature of the battery unit  2  is higher than the upper threshold and the detected temperature of the vehicle body  40  is lower than the detected temperature of the battery unit  2 , when the detected temperature of the battery unit  2  is between the upper threshold and the lower threshold, and when the detected temperature of the battery unit  2  is lower than the lower threshold and the detected temperature of the vehicle body  40  is higher than the detected temperature of the battery unit  2 , the liquid level adjustment mechanism is placed in the state shown in  FIG. 7A  (“second state”). On the other hand, when the detected temperature of the battery unit  2  is higher than the upper threshold and the detected temperature of the vehicle body  40  is higher than the detected temperature of the battery unit  2 , and when the detected temperature of the battery unit  2  is lower than the lower threshold and the detected temperature of the vehicle body  40  is lower than the detected temperature of the battery unit  2 , the liquid level adjustment mechanism is placed in the state shown in  FIG. 7B  (“first state”). 
     While the liquid level adjustment mechanism of the second example embodiment is switched between the state where the chamber S 2  is filled up with the coolant  4  and the state where the chamber S 2  is filled up with air, the liquid level adjustment mechanism may be operated otherwise. For example, the level of the coolant  4  in the chamber S 2  may be changed in steps. In this case, the temperature of the battery unit  2  (i.e., the heat radiation from the battery unit  2 ) can be adjusted in steps as in the modification example of the first example embodiment. 
     Further, while the chamber S 1  and the chamber S 2  are both filled with the coolant  4  in the second example embodiment, they may be filled with different coolants. Further, as described in the first example embodiment, heat-insulating layers may be formed at the inner faces of the case  30  other than the inner face of the side wall in contact with the heat transfer plate  80 . In this case, even when the heat transfer plate  80  is excessively cooled or heated, the heat-insulating layers reduce the extent to which the coolant  4  in the chamber Si is cooled and heated via the case  30 . 
     Next, a temperature adjustment mechanism according to the third example embodiment of the invention will be described with reference to  FIG. 8A  and  FIG. 8B .  FIG. 8A  and  FIG. 8B  are views each schematically showing the internal structure of the temperature adjustment mechanism of the third example embodiment. Note that the elements identical to those recited in the first and second example embodiments are denoted by the same numerals. 
     The structure of the temperature adjustment mechanism of the third example embodiment is almost the same as that of the temperature adjustment mechanism of the second example embodiment. That is, in the third example embodiment, too, the case  30  has two chambers  51  and S 2  that are partitioned off from each other. However, the direction in which the chambers S 1 , S 2  are arranged is different from that in the second example embodiment. The features of the third example embodiment will be described in detail below. 
     The case  30  is disposed on the vehicle body (“heat transfer portion”)  40  and the bottom face of the case  30  is in contact with the surface of the vehicle body  40 . 
     Within the case  30  is provided a partition  70  by which the chamber S 1  and the chamber S 2  are partitioned off from each other. The partition  70  may be made of a material having a high heat conductivity and a high corrosion resistance, for example, a material having a heat conductivity as high as or higher than the heat conductivity of the coolant  4  in the case  30 . More specifically, the partition  70  may be formed of metal (copper, iron, etc.). 
     The chamber S 1  contains the battery unit  2  and the coolant  4 . The coolant  4  in the chamber S 1  is in contact with the outer face of each battery cell of the battery unit  2 , the inner faces of the case  30 , and the partition  70 . 
     The chamber S 2  contains the coolant  4 . The chamber S 2  is located on the vehicle body  40  side of the chamber S 1 . A liquid level adjustment mechanism having substantially the same structure as that of the first example embodiment is connected to the chamber S 2 . That is, the pipe  50  is connected to the chamber S 2 , and the pipe  50  extends to a reserve tank corresponding to the reserve tank  53  of the first example embodiment (not shown in the drawings). A pump  51  and a valve  52   a  are provided at one end portion of the pipe  50  while a valve  52   b  is provided at the other end portion of the pipe  50 . 
     In the third example embodiment, the chamber S 1  is always filled up with the coolant  4  while the level of the coolant  4  (i.e., the amount of the coolant  4 ) in the chamber S 2  changes as the liquid level adjustment mechanism is driven. The driving of the liquid level adjustment mechanism is controlled by the controller as in the first example embodiment. 
     As the pump  51  is driven in the state shown in  FIG. 8A , the coolant  4  in the chamber S 2  is delivered to the reserve tank so that the state shown in  FIG. 8B  is established. 
     At this time, the air in the reserve tank moves to the chamber S 2  via the pipe  50 , whereby an air layer is created in the chamber S 2 . At this time, the valves  52   a,    52   b  both remain closed. In the state shown in  FIG. 8B , the portion of the pipe  50  on the chamber S 2  side of the valve  52   a  is occupied by air while the portion of the pipe  50  on the reserve tank side of the valve  52   a  is occupied by the coolant  4 . 
     While an air layer is created in the chamber S 2  in the third example embodiment, a layer of other gas may alternatively be used in place of air. 
     Meanwhile, as the pump  51  is driven in the state shown in  FIG. 8B , the coolant  4  in the reserve tank is delivered to the chamber S 2  of the case  30  so that the state shown in  FIG. 8A  is established. At this time, the air in the chamber S 2  moves to the reserve tank via the pipe  50 . 
     At this time, the valves  52   a,    52   b  are both open. In the state shown in FIG.  8 A, the portion of the pipe  50  on the chamber S 2  side of the valve  52   b  is occupied by the coolant  4  while the portion of the pipe  50  on the reserve tank side of the valve  52   b  is occupied by air. 
     As in the first example embodiment described above, if liquid level sensors are provided in the reserve tank or in the case  30  (i.e., the chamber S 2 ), the level of the coolant  4  (i.e., the amount of the coolant  4 ) in the chamber S 2  of the case  30  can be detected using the liquid level sensors. Alternatively, the level of the coolant  4  in the chamber S 2  may be determined by detecting the drive amount of the pump  51 . 
     In the state shown in  FIG. 8A , the chamber S 2  of the case  30  is filled up with the coolant  4 . In this state, if the battery unit  2  produces heat through its charging and discharging, or the like, the produced heat is transferred to the coolant  4  in contact with the battery unit  2 . The heat of the coolant  4  is then transferred to the partition  70  through the natural convection of the coolant  4 , and the heat of the partition  70  is transferred to the coolant  4  in the chamber S 2 . 
     An agitating member may be provided in the chamber S 1  to create a flow of the coolant  4 . In this case, the cooling efficiency of the battery unit  2  further improves. 
     Further, because the bottom face of the case  30 , which defines the chamber S 2 , is in contact with the vehicle body  40 , the heat transferred to the coolant  4  in the chamber S 2  is transferred to the vehicle body  40  via the case  30 . A portion of the heat transferred to the coolant  4  is released to the outside (atmosphere) via the case  30 . 
     As mentioned above, the heat produced at the battery unit  2  through its charging and discharging is released (radiated) to the atmosphere via the coolant  4  and the case  30  and radiated through the case  30  and the vehicle body  40 , thus suppressing an increase in the temperature of the battery unit  2  and minimizing deterioration of the power storage performance of each battery cell. 
     If the vehicle body  40  is excessively cooled or heated in the state shown in  FIG. 8A , the case  30 , which is in contact with the vehicle body  40 , may be excessively cooled or heated. If the coolant  4  in the case  30  (the chambers S 1 , S 2 ) is cooled excessively, the battery unit  2  in the chamber S 1  may be cooled excessively. 
     In the third example embodiment, when the vehicle body  40  is excessively cooled or heated, an air layer is created in the chamber S 2  as shown in  FIG. 8B , whereby the coolant  4  and the battery unit  2  in the chamber S 1  are prevented from being cooled or heated excessively via the chamber S 2 . 
     That is, when there is an air layer in the chamber S 2 , the heat conductivity is lower than when the chamber S 2  is filled with the coolant  4 . As such, the coolant  4  and the battery unit  2  in the chamber S 1  are prevented from being cooled or heated excessively as the heat transfer plate  80  is cooled or heated, and therefore deterioration of the power storage performance of each battery cell that may otherwise be caused by excessive heating and cooling of the battery unit  2  is minimized. 
     In the third example embodiment, the liquid level adjustment mechanism can be driven as in the first example embodiment (Refer to  FIG. 5 ). 
     That is, when the detected temperature of the battery unit  2  is higher than the upper threshold and the detected temperature of the vehicle body  40  is lower than the detected temperature of the battery unit  2 , when the detected temperature of the battery unit  2  is between the upper threshold and the lower threshold, or when the detected temperature of the battery unit  2  is lower than the lower threshold and the detected temperature of the vehicle body  40  is higher than the detected temperature of the battery unit  2 , the liquid level adjustment mechanism is placed in the state shown in  FIG. 8A  (“second state”). On the other hand, when the detected temperature of the battery unit  2  is higher than the upper threshold and the detected temperature of the vehicle body  40  is higher than the detected temperature of the battery unit  2 , and when the detected temperature of the battery unit  2  is lower than the lower threshold and the detected temperature of the vehicle body  40  is lower than the detected temperature of the battery unit  2 , the liquid level adjustment mechanism is placed in the state shown in  FIG. 8B  (“first state”). 
     According to the structure of the third example embodiment, it is not necessary to take the entire coolant  4  out of the chamber S 2  to create an air layer therein. That is, an air layer can be created in the chamber S 2  by delivering part of the coolant  4  in the chamber S 2  to the reserve tank. Thus, it is possible to prevent excessive increase and decease in the temperature of the battery unit  2  which may otherwise be caused as the vehicle body  40  is excessively cooled or heated. 
     Further, as described in the first example embodiment, heat-insulating layers may be formed at the inner faces of the case  30  other than the inner face of the wall in contact with the vehicle body  40 . In this case, even when the vehicle body  40  is excessively cooled or heated, the heat-insulating layers reduce the extent to which the coolant  4  in the chamber S 1  is cooled or heated via the case  30 . 
     Next, a temperature adjustment mechanism according to the fourth example embodiment of the invention will be described with reference to  FIG. 9A  and  FIG. 9B .  FIG. 9A  and  FIG. 9B  are views schematically showing the internal structure of the temperature adjustment mechanism of the fourth example embodiment. Note that the elements identical to those recited in the first, second, and third example embodiments are denoted by the same numerals. 
     In the fourth example embodiment, the structure of the case  30  is substantially the same as that of the case  30  of the first example embodiment. That is, in the fourth example embodiment, heat radiations are performed through the top of the case  30  as in the case illustrated in  FIG. 3 . 
     The case  30  contains the battery unit  2  and the coolant  4 . Further, a hollow elastic member  90  is provided in the case  30 . The elastic member  90  is fixed to the battery unit  2  (at least one of the battery cells). 
     A tubular member  91  is connected to the elastic member  90 . Air is drawn into and out of the elastic member  90  via the tubular member  91 . The tubular member  91  is elastically deformable and penetrates the case  30  to the outside. The tubular member  91  is connected to a pump  92  that supplies air into the elastic member  90  and sucks air out of the elastic member  90 . The elastic member  90  and the tubular member  91  are made of elastic materials including high-polymer resins. 
     The pipe  50  is connected to the case  30 . The pump  51 , the valve  52 , and a reserve tank  53  are provided on the pipe  50 . Thus, the liquid level adjustment mechanism of the fourth example embodiment is constituted of the elastic member  90 , the pump  92 , the pump  51 , the valve  52 , and the reserve tank  53 . 
     In the fourth example embodiment, as the pump  92  is driven, in other words, as air is supplied to or sucked out of the elastic member  90 , the elastic member  90  expands or contracts. At this time, the pump  51  and the valve  52  are driven according to the expansion or contraction of the elastic member  90  so that the coolant  4  is drawn out of or supplied into the case  30 . The pump  91 , the pump  51 , and the valve  52  are driven under the control of the controller as in the first example embodiment. 
     That is, in the state shown in  FIG. 9A , the pump  92  is driven to supply air into the elastic member  90  while the pump  51  is driven to draw the coolant  4  out of the case  30 , so that the state shown in  9 B is established. 
     The valve  52  is kept open while delivering the coolant  4  in the case  30  to the reserve tank  53  by driving the pump  51 , and the valve  52  is closed when the delivering of the coolant  4  to the reserve tank  53  has been finished. 
     In the state shown in  FIG. 9B , the expanded elastic member  90  is in contact with the inner faces of the case  30 , whereby the interior of the case  30  is divided into two regions. 
     At this time, if the vehicle body is cooled or heated, the case  30 , which is in contact with the vehicle body, may be cooled or heated excessively. 
     In such a case, if the coolant  4  can freely move between the region proximal to the upper inner face of the case  30  and the region around the battery unit  2  (as in the state shown in  FIG. 9A ), the coolant  4  and the battery unit  2  in the case  30  are excessively cooled or heated as the case  30  is excessively cooled or heated. 
     In view of the above, in the fourth example embodiment, when the vehicle body is excessively cooled or heated, the elastic member  90  is expanded as shown in  FIG. 9B , whereby the coolant  4  is prevented from moving between the region proximal to the upper inner face of the case  30  and the region around the battery unit  2 . As such, the coolant  4  in the region around the battery unit  2  is not excessively cooled nor heated although the coolant  4  in the region proximal to the upper inner face of the case  30  may be. 
     That is, when the elastic member  90  is expanded, the heat conductivity becomes lower than when the entire region of the case  30  is filled up with the coolant  4 , and therefore the coolant  4  present on the battery unit  2  side of the elastic member  90  and thus excessive cooling and heating of the battery unit  2  can be prevented. As such, deterioration of the power storage performance of each battery cell due to excessive cooling and heating can be suppressed. 
     From the state in  FIG. 9B , then, the pump  92  is driven to suck the air out of the elastic member  90  while the pump  51  is driven to supply the coolant  4  into the case  30 , whereby the state shown in  FIG. 9A  is established. 
     The valve  52  is kept open while delivering the coolant  4  in the reserve tank  53  to the case  30  by driving the pump  51 , and the valve  52  is closed when the delivering of the coolant  4  to the case  30  has been finished. 
     In the state shown in  FIG. 9A , the elastic member  90  is spaced from the inner faces of the case  30 , and therefore the coolant  4  can move to the upper side of the case  30 . 
     In this state, if the battery unit  2  produces heat through its charging and discharging, or the like; the produced heat is transferred to the coolant  4  in contact with the battery unit  2 . The heat of the coolant  4  is then transferred to the case  30  through the natural convection of the coolant  4 , and the heat of the case  30  is radiated to the outside (atmosphere) through the case  30  and transferred to the vehicle body. An agitating member may be provided to create a flow of the coolant  4 . 
     Through such heat radiations, an increase in the temperature of the battery unit  2  is suppressed, whereby deterioration of the power storage performance of each battery cell is minimized. 
     In the case where the relation between the amount of air supplied to the elastic member  90  via the pump  92  (or the amount of air sucked out of the elastic member  90  via the pump  92 ) and the degree of the resultant expansion of the elastic member  90  (or the degree of the resultant contraction of the elastic member  90 ) has been determined in advance, the elastic member  90  can be controlled between the state shown in  FIG. 9A  (“second state”) and the state shown in  FIG. 9B  (“first state”) by simply monitoring and controlling the drive amount of the pump  92 . Further, in the case where the characteristic of variation of the volume of the elastic member  90  has been determined in advance, the drive amount of the pump  51  (the amount of coolant transferred between the case  30  and the reserve tank  53 ) can be determined based on the variation of volume of the elastic member  90 . 
     While air is used to expand and contract the elastic member  90  in the example embodiment described above, other gas may be used in place of air, or even liquid may be used. As liquid for this use, for example, liquid having a heat conductivity lower than that of the coolant  4  in the case  30  may be used. Thus, even when the vehicle body is excessively cooled or heated, the coolant  4  and the battery unit  2  in the case  30  are not excessively cooled nor heated via the elastic member  90 . 
     While heat radiations are performed through the top of the case  30  in the fourth example embodiment and therefore the elastic member  90  is arranged above the battery unit  2  in the case  30 , the elastic member  90  may be arranged otherwise. 
     For example, if the case  30  is structured as in the second example embodiment (Refer to  FIG. 7A  and  FIG. 7B ), the elastic member  90  may be arranged in the region proximal to the side wall of the case  30  which is in contact with the heat transfer plate  80 . More specifically, the partition  70  and the liquid level adjustment mechanism are removed from the structure shown in  FIG. 7A  and  FIG. 7B , and then the elastic member  90  is arranged, in the case  30 , between the battery unit  2  and the side wall contacting the heat transfer plate  80 . This structure also provides the same effects as those obtained with the fourth example embodiment described above. 
     Further, in the case where the case  30  is structured as in the third example embodiment (Refer to  FIG. 8A  and  FIG. 8B ), the elastic member  90  may be arranged below the battery unit  2  in the case  30 . More specifically, the partition  70  and the liquid level adjustment mechanism are removed from the structure shown in  FIG. 8A  and  FIG. 8B , and the elastic member  90  is arranged, in the case  30 , between the battery unit  2  and the wall contacting the vehicle body  40 . This structure also provides the same effects as those obtained with the fourth example embodiment described above. 
     While the elastic member  90  is used as a container for storing air (medium) in the fourth example embodiment, any other container may be used in place of the elastic member  90  as long as it has an inner space for storing the medium. In the fourth example embodiment, that is, any container (heat transfer medium container) can be used as long as the volume (capacity) of the container varies as the medium flows into and flows out of the container. 
     While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims.