Patent Publication Number: US-2012026690-A1

Title: Heat exchange medium and electric storage device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a heat exchange medium for exchanging heat with an electric storage element, and an electric storage device employing this heat exchange medium. 
     2. Description of the Related Art 
     A secondary battery may generate heat when being charged or discharged, and properties of the secondary battery may deteriorate as a result of the increase in temperature. Approaches for holding a coolant (liquid) in contact with a secondary battery to minimize temperature increases in the secondary battery are described in, for example, Japanese Patent Application Publication No. 2001-060466 (JP-A-2001-060466) and Japanese Patent Application Publication No. 2008-16346 (JP-A-2008-16346). 
     In the assembled battery described in JP-A-2001-060466, a case for accommodating the assembled battery is provided with an inlet and an outlet. Coolant is supplied into the case via the inlet, and discharged from the case via the outlet. An insulating oil or a liquid paraffin may be used as the coolant. 
     Further, in an accommodation device described in JP-A-2008-16346, a cooling liquid is stored together with a secondary battery inside a battery accommodation chamber. Ethylene glycol is used as the cooling liquid. 
     In a structure in which a liquid is held in contact with a secondary battery, high heat conductivity, the presence of electric insulation properties, a remote possibility of deteriorating the secondary battery, and the like may be mentioned as the performances required of the liquid. It should be noted herein that the liquid described in JP-A-2001-060466 or JP-A-2008-16346 may exhibit the aforementioned performances insufficiently. 
     SUMMARY OF THE INVENTION 
     The invention provides a heat exchange medium that is excellent in fluidity and insulation properties, and an electric storage device employing this heat exchange medium. 
     A heat exchange medium according to a first aspect of the invention is a liquid heat exchange medium that is provided in a case together with an electric storage element to exchange heat with the electric storage element. The heat exchange medium is an ester compound of a fatty acid with a carbon number of 6 to 8 and 2-ethyl hexanol, and contains 90 or more volume % of 2-ethylhexyl caprylate. More specifically, the heat exchange medium may be composed of 2-ethylhexyl caprylate alone or a mixture of 2-ethylhexyl caprylate and an ester compound of a fatty acid other than caprylic acid (with a carbon number of 6 to 8) and 2-ethyl hexanol. 
     The heat exchange medium according to the above aspect of the invention may not contain sulfur constituents. Thus, corrosion of the electric storage element and the like clue to sulfur constituents may be avoided. 
     An electric storage device according to a second aspect of the invention includes the heat exchange medium according to the foregoing first aspect of the invention. 
     The electric storage device according to the above aspect of the invention may further include a fan disposed in the case to circulate the heat exchange medium. By circulating the heat exchange medium disposed in the case, the heat exchange medium is caused to flow efficiently with the aid of a driving force of the fan. 
     In the electric storage device according to the above aspect of the invention, the fan may circulate the heat exchange medium to the electric storage element with a laminar flow state. If the fan is driven to generate a laminar flow of the heat exchange medium around the electric storage element, partial dispersion of the temperature within the electric storage element may be minimized. 
     In the electric storage device according to the above aspect of the invention, the fan may have a rotary shaft and a plurality of blades disposed on an outer peripheral surface of the rotary shaft. The fan may be disposed such that the rotary shaft extends in a direction that is substantially parallel to the electric storage element. The length of the plurality of blades may be approximately equal to the length of the electric storage element in a rotational direction of the rotary shaft of the fan. 
     The electric storage device according to the above aspect of the invention may be mounted on a vehicle. 
     According to the invention, the insulating properties and fluidity of the liquid heat exchange medium that exchanges heat with the electric storage element may be enhanced by using an ester compound of a fatty acid with a carbon number of 6 to 8 and 2-ethyl hexanol (containing 90 or more volume % of 2-ethylhexyl caprylate) as the heat exchange medium. The safety in handling the electric storage device may be enhanced by improving the insulating properties of the heat exchange medium. Further, the temperature of the electric storage element may be efficiently adjusted with the aid of the heat exchange medium by enhancing the fluidity thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and/or further objects, features and advantages of the invention will become more apparent from the following description of an example embodiment of the invention with reference to the accompanying drawings, in which like numerals are used to represent like elements and wherein: 
         FIG. 1  is an exploded perspective view showing the structure of a battery pack according to the first embodiment of the invention; 
         FIG. 2  shows the internal sturcture of part of the battery pack according to the first embodiment of the invention; 
         FIG. 3  shows the main flow of a heat exchange medium in the battery pack according to the first embodiment of the invention; 
         FIG. 4  shows the flow directions of the heat exchange medium in the battery pack according to the first embodiment of the invention; 
         FIG. 5  shows a relationship between temperature and kinematic viscosity in the heat exchange medium according to the first embodiment of the invention; and 
         FIG. 6  shows a relationship between ambient temperature and temperature dispersion in a battery module according to the first embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT 
     The structure of a battery pack (an electric storage device) according to the first embodiment of the invention will be described using  FIG. 1 .  FIG. 1  is an exploded perspective view showing the structure of the battery pack according to this embodiment of the invention. 
     A battery pack  1  (an electric storage device) according to this embodiment of the invention is mounted on a vehicle. The vehicle may be a hybrid vehicle or an electric vehicle. The hybrid vehicle may be further equipped with, in addition to the battery pack  1 , another power source that outputs energy used to cause the vehicle to run, such as an internal combustion engine or a fuel cell. Alternatively, the electric vehicle is a vehicle that runs using only the output of the battery pack  1 . The battery pack  1  according to this embodiment of the invention outputs energy used to cause the vehicle to run through discharge, and is charged with kinetic energy generated during the braking of the vehicle as a regenerative electric power. It should be noted that the battery pack  1  may also be charged by supplying an electric power thereto from outside the vehicle. 
     The battery pack  1  includes a battery module  10 , a pack case  20 , and a circulation unit  30 . The pack case  20  includes an accommodation member  21  that forms a space for accommodating the battery module  10  and the circulation unit  30 , and a lid member  22  that closes an opening portion  21   a  of the accommodation member  21 . The lid member  22  is fixed to the accommodation member  21  by a fastening member such as a screw or the like or through welding. Thus, the interior of the pack case  20  is sealed. 
     The accommodation member  21  and the lid member  22  may be made from any material having sufficient heat conductivity, corrosion resistance, and the like, for example, a material with heat conductivity is equal to or higher than that of a later-described heat exchange medium  40  (an ester compound). More specifically, the accommodation member  21  and the lid member  22  can be made of a metal such as aluminum, iron, or the like. The outer wall surfaces of the accommodation member  21  and the lid member  22  are designed as flat surfaces in this embodiment of the invention, however the invention is not restricted to this configuration. More specifically, a plurality of heat radiating fins can be provided on at least one of the outer wall surfaces of the accommodation member  21  and the lid member  22 . Thus, the heat radiation performance of the battery pack  1  may be improved via the heat radiating fins. 
     In addition to the battery module  10  and the circulation unit  30 , the liquid heat exchange medium  40  for exchanging heat with the battery module  10  is accommodated inside the pack case  20 . The constituents of the heat exchange medium  40  will be described later. 
     The heat exchange medium  40  is used to adjust the temperature of the battery module  10  (electric cells  11  (electric storage elements)). It should be noted herein that the amount of the heat exchange medium  40  accommodated inside the pack case  20  may be set as appropriate. More specifically, the liquid surface of the heat exchange medium  40  may be either in contact or out of contact with the lid member  22 . The heat exchange medium  40  preferably maintains contact with the entire surface of the battery module  10 . 
     Next, the structure of the battery module  10  will be described. 
     The battery module  10  is composed of a plurality of the electric cells (secondary batteries or electric storage elements)  11  that are electrically connected to one another in series. The plurality of the electric cells  11  are oriented parallel with one another inside the pack case  20 . Nickel hydride batteries or lithium ion batteries may be employed as the secondary batteries. Further, electric double layer capacitors may also be employed instead of the secondary batteries. In addition, although cylindrical electric cells  11  are employed in this embodiment of the invention, electric cells formed in other shapes, such as rectangular electric cells or the like, can also be employed. 
     Each electric cell  11  includes a power generation element (not shown), and a battery case that accommodates the power generation element in a sealed state. The power generation element may be charged with an electric power and can discharge the electric power therefrom, and can be composed of, for example, electrode elements (a positive electrode element and a negative electrode element) and separators. The positive electrode element is obtained by forming a layer of a positive electrode active material on the surface of a collector plate, and the negative electrode element is obtained by forming a layer of a negative electrode active material on a surface of a collector plate. 
     A positive electrode terminal  11   a  and a negative electrode terminal  11   b  are respectively provided at opposite ends of the e electric cell  11 . The positive electrode terminal  11   a  is electrically and mechanically connected to the positive electrode element of the power generation element, and the negative electrode terminal  11   b  is electrically and mechanically connected to the negative electrode element of the power generation element. The positive electrode terminal  11   a  of each electric cell  11  is electrically connected to the negative electrode terminal  11   b  of an adjacent electric cell  11  via a bus bar  13 . Thus, the plurality of the electric cells  11  are electrically connected to one another in series. 
     Each end of each individual electric cell  11  is supported by a flat support member  12 . The support members  12  are fixed to the pack case  20  (the accommodation member  21 ) by a fastening member (not shown) such as a screw or the like. Further, end surfaces (outer edge portions) of each support member  12  contact the bottom surface and lateral surfaces of the accommodation member  21 . 
     Although two support members  12  are employed in this embodiment of the invention, they can be integrated with each other. Further, if rectangular electric cells  11  are employed, the plurality of the electric cells  11  can be arranged in a certain direction with spacers sandwiched therebetween respectively, and can be sandwiched at both ends thereof in the direction of arrangement by end plates. 
     Cables (not shown) for the positive electrode and the negative electrode are connected to specific ones (two) of the plurality of the electric cells  11 . These cables are connected to devices disposed outside the pack case  20 . These devices may be, for example, a DC/DC converter for raising the voltage of the battery module  10  and an inverter for converting a direct current and an alternating current into each other. 
     The circulation unit  30  is disposed at a corner portion of the battery module  10 . Both ends of the circulation unit  30  are so disposed as to be located on the same plane as the pair of the support members  12 . The structure of the circulation unit  30  will be described using  FIG. 2 . It should be noted herein that  FIG. 2  is a partial schematic view of the structure of the interior of the battery pack  1 . 
     The circulation unit  30  has a fan (a cross flow fan)  31 , a pair of bearings  32  that rotatably support a rotary shaft  31   a  of the fan  31 , and a support plate  33  that supports the bearing  32 . The fan  31  has a plurality of blades  31   b  on the outer peripheral surface of the rotary shaft  31   a . Further, the fan  31  is disposed such that an axis of rotation of the rotary shaft  31   a  extends substantially parallel to the electric cells  11 . The plurality of the blades  31   b  are equidistantly disposed in a circumferential direction of the rotary shaft  31   a , and are each formed in a curved shape. The length of the respective blades  31   b  in the direction of the rotary shaft of the fan  31  is approximately equal to the distance between the pair of the support members  12 . 
     A motor (not shown) is connected to the rotary shaft  31   a , and the fan  31  rotates by receiving a driving force from the motor. A region  33   a  of the support plate  33  is formed along an outer periphery of the fan  31  to allow the heat exchange medium  40  to move smoothly as the fan  31  rotates. 
     A first partition member  34   a  is connected to a second partition member  34   b  and both are disposed between the fan  31  and the battery module  10  (the electric cells  11 ). As shown in  FIG. 2 , the first partition member  34   a  is disposed between the lowest electric cell  11  of the battery module  10  and a bottom surface of the pack case  20  (the accommodation member  21 ). Further, the second partition member  34   b  extends in the direction of gravity (a vertical direction in  FIG. 2 ) along the battery module  10 , and a tip of the second partition member  34   b  is located at an upper portion of the battery module  10 . The widths of the first partition member  34   a  and the second partition member  34   b  are each equal to the distance between the pair of the support members  12 . 
     Next, the flow of the heat exchange medium  40  in the battery pack  1  when the fan  31  is driven, as described above, will be described using  FIGS. 3 and 4 . 
     When the fan  31  is rotated by the driving force of the motor, the heat exchange medium  40  is circulated by the fan  31 . The heat exchange medium  40  circulated by the fan  31  passes a space between the first partition member  34   a  and the bottom surface of the accommodation member  21 , and moves to the battery module  10  side. The plurality of the blades  31   b  of the fan  31  extends along the length of the rotary shaft  31   a , and that the heat exchange medium  40  circulated by the fan  31  hence forms a laminar flow having the length of the blades  31   b.    
     As indicated by arrows in  FIG. 3 , the heat exchange medium  40  circulated the fan  31  moves along the periphery of the battery module  10  and returns to the fan  31 . The arrows in  FIG. 3  indicate the main flow of the heat exchange medium  40 , but the heat exchange medium  40  may flow in other directions as well. It should be noted that the first partition member  34   a  is omitted in  FIG. 3 . 
     In this embodiment of the invention, the distance (the shortest distance) between the battery module  10  (the outermost one of the electric cells  11 ) and an inner wall surface of the pack case  20  is longer than the distance (the shortest distance) between adjacent ones of the electric cells  11 . By setting the distance in this manner, the heat exchange medium  40  sent out from the fan  31  can be moved along the periphery of the battery module  10 . By causing the main flow of the heat exchange medium  40  around the battery module  10 , secondary flow of the heat exchange medium  40  is also generated between adjacent electric cells  11  as well. More specifically, as shown in  FIG. 4 , the heat exchange medium  40  can be caused to circulate through spaces between adjacent ones of the electric cells  11  in a direction from a lower region of the battery module  10  to an upper region thereof. 
     The charging and discharging of the electric cells  11  may generate heat. However, by holding the heat exchange medium  40  in contact with the electric cells  11 , heat is exchanged between the electric cells  11  and the heat exchange medium  40 , and the heat of the electric cells  11  is transmitted to the heat exchange medium  40 . The heated heat exchange medium  40  flows inside the pack case  20  as described above, and comes into contact with inner wall surfaces of the pack case  20 , thereby allowing the heat to be transmitted to the pack case  20 . The heat transmitted to the pack case  20  is then dissipated into the atmosphere. Thus, heat radiation (the cooling) of the battery pack  1  (the electric cells  11 ) can be carried out. 
     In contrast, when the heat exchange medium  40  is warmed, heat may be transmitted to the electric cells  11  through heat exchange between the warmed heat exchange medium  40  and the electric cells  11 . Thus, the electric cells  11  can be warmed. The warming of the electric cells  11  is effective when the temperature of the electric cells  11  has excessively fallen due to an ambient temperature. 
     The heat exchange medium  40  can be directly or indirectly warmed in warming the heat exchange medium  40 . As a method of directly warming the heat exchange medium  40 , for example, it is possible to dispose a heater in the pack case  20  whereby the heater remains in contact with the heat exchange medium  40 . Further, as a method of indirectly warming the heat exchange medium  40 , for example, it is possible to warm the pack case  20  by means of a heater and warm the heat exchange medium  40  via the pack case  20 . 
     In this embodiment of the invention, the heat exchange medium  40  sent out from the fan  31  comes into contact with the electric cells  11  in the laminar flow state. It should be noted herein that the width of a laminar flow of the heat exchange medium  40  is approximately equal to the length of the electric cells  11  in the longitudinal direction. Therefore, the heat exchange medium  40  exchanges heat with substantially entire regions of the electric cells  11 . Thus, partial dispersion of the temperature in the electric cells  11  may be suppressed. Further, as shown in  FIG. 4 , heat exchange with all the electric cells  11  can be carried out by holding the heat exchange medium  40  in contact with all the electric cells  11  constituting the battery module  10 . Thus, the dispersion of the temperature in the plurality of the electric cells  11  constituting the battery module  10  can be suppressed. 
     It should be noted that the circulation unit  30  is disposed within the pack case  20  in this embodiment of the invention. However, the circulation unit  30  may not be disposed. Further, although the cross-flow fan is employed as the fan  31 , any fan having a structure that generates adequate force to circulate the heat exchange medium  40  may be employed. Furthermore, although the circulation unit  30  is disposed along the bottom surface of the pack case  20  in this embodiment of the invention, the invention is not limited to this configuration. That is, the circulation unit  30  may be located at any position as long as the heat exchange medium  40  is appropriately circulated around the battery module  10 . For example, the circulation unit  30  may instead be disposed along an upper surface of the pack case  20 . 
     Next, the concrete constituents of the heat exchange medium  40  will be described. 
     An ester compound of a fatty acid with a carbon number of 6 to 8 and 2-ethylhexanol is used as the heat exchange medium  40 . The ester compound contains 90 or more volume % of 2-ethylhexyl caprylate. The heat exchange medium  40  may be composed of 2-ethylhexyl caprylate alone or contain 10 or less volume % of an ester compound with a fatty acid other than caprylic acid (with a carbon number of 6 to 8). 
     For example, caproic acid, enanthic acid, or caprylic acid can be mentioned as a fatty acid with a carbon number of 6 to 8 (the number of carbons of R 1  is 5 to 7). One of these fatty acids (caprylic acid) can be used alone, or two or more of these fatty acids (including caprylic acid) can be mixed and used. 
     It should be noted herein that the carbon number of the fatty acid is preferably equal to or larger than 6 to ensure appropriate insulative properties of the heat exchange medium (the ester compound)  40 . Further, the carbon number of the fatty acid is preferably equal to or smaller than 8 to maintain appropriate fluidity of the heat exchange medium  40  in the pack case  20 . The fluidity of the heat exchange medium  40  may be enhanced as the kinematic viscosity of the ester compound decreases. On the other hand, the heat exchange medium  40  can be endowed with excellent properties as to fluidity at low temperatures and electric insulating properties by using 2-ethylhexanol. 
     For example, 2-ethylhexyl caprylate or 2-ethylhexyl caproate may be mentioned as the aforementioned ester compound of the fatty acid with the carbon number of 6 to 8 and 2-ethylhexanol. One (2-ethylhexyl caprylate) of these ester compounds may be used alone, or two or more (2-ethylhexyl caprylate is contained) of these ester compounds may be mixed and used. 
     The ester compound used as the heat exchange medium  40  may be manufactured using various esterifying methods. For example, there is a method in which a fatty acid with a carbon number of 6 to 8 and 2-ethylhexanol are caused to react with each other under the presence of an acid or an alkali to be esterified. Further, it is also possible to obtain a transesterified produce by reacting a fatty acid with a carbon number of 6 to 8 and 2-ethylhexanol in the presence of an acid or an alkali. 
     If an ester compound is used, the Prandtl number at 20° C. is preferably 8 to 40000. Thus, the heat transfer coefficient of the heat exchange medium  40  may be increased, and the temperature of the battery module  10  may be efficiently adjusted using the heat exchange medium  40 . 
     As described above, when an ester compound is used as the heat exchange medium  40 , excellent insulating properties can be obtained. Thus, the ester compound may be suitably used for the battery module  10  that generates a high voltage. Further, even if 200 ppm or less of water is added to the ester compound, ester molecules surround water molecules. Therefore, changes in the volume resistivity of the ester compound are minimal. 
     In addition, if an ester compound is used, the heat exchange medium  40  is allowed not to contain sulfur constituents. For example, a catalyst that does not contain sulfur may be used to esterify a fatty acid with a carbon number of 6 to 8 and 2-ethylhexanol. Thus, the risk of the battery module  10  being partially corroded by sulfur may be avoided in comparison with a case where a mineral oil containing sulfur is used. For example, if the bus bar  13  and the electrode terminals  11   a  and  11   b  of each of the electric cells  11  are made of copper, the risk of these members being corroded by sulfur can be avoided. 
     Table 1 shown below shows the kinematic viscosity of the heat exchange medium  40  with respect to its temperature for example 1, in which 2-ethylhexyl caprylate is used alone as the heat exchange medium  40 , and a comparative example that uses a mineral oil as the heat exchange medium  40 . In particular, an automatic transmission fluid (ATF; Toyota Auto Fluid WS) is used as the mineral oil. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 kinematic viscosity [mm 2 /s] 
               
            
           
           
               
               
               
               
               
            
               
                   
                 (−30° C.) 
                 (0° C.) 
                 (40° C.) 
                 (100° C.) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 example 1 
                 2-ethylhexyl 
                 32.64 
                 8.164 
                 2.841 
                 1.174 
               
               
                   
                 caprylate 
               
               
                 comparative 
                 mineral oil 
                 2371.7 
                 142.9 
                 23.6 
                 5.4 
               
               
                 example 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the volume resistivity of 2-ethylhexyl caprylate, mineral oil, and silicon oil. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 volume resistivity [Ω · cm] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 2-ethylhexyl caprylate 
                 5.4 × 10 10   
               
               
                   
                 mineral oil 
                 5.8 × 10 10   
               
               
                   
                 silicon oil 
                 5.0 × 10 10   
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 2, 2-ethylhexyl caprylate is approximately equal in volume resistivity to mineral oil and silicon oil. Thus, 2-ethylhexyl caprylate may be suitably used as the heat exchange medium  40  that is in contact with the battery module  10  designed to generate a high voltage. 
     In contrast,  FIG. 5  shows relationships between the temperature and kinematic viscosity of the heat exchange medium  40  when mineral oil and 2-ethylhexyl caprylate are used as the heat exchange medium  40  respectively. 
     As shown in  FIG. 5 , when 2-ethylhexyl caprylate is used, the kinematic viscosity of the heat exchange medium  40  is unlikely to change even when the temperature of the 2-ethylhexyl caprylate changes. On the other hand, the kinematic viscosity of mineral oil increases as its temperature falls below 0° C. Thus, if the battery pack  1  according to this embodiment of the invention is used in an environment below 0° C., 2-ethylhexyl caprylate is preferably used as the heat exchange medium  40 . 
       FIG. 6  shows the relationships between the ambient temperature and a temperature dispersion in the plurality of the electric cells  11  of the battery module  10  when mineral oil and 2-ethylhexyl caprylate are used as the heat exchange medium  40 , respectively. The temperature dispersion (ΔT) represents a difference in temperature between that one of the plurality of the electric cells  11  constituting the battery module  10  which is at the highest temperature and that one of the plurality of the electric cells  11  constituting the battery module  10  which is at the lowest temperature after the driving of the fan  31  in the battery pack  1  for a predetermined time. Further, the ambient temperature refers to the temperature around the battery pack  1 . 
     As shown in  FIG. 6 , the temperature dispersion in the battery module  10  can be suppressed in the case where 2-ethylhexyl caprylate is used than in the case where mineral oil is used. The dispersion of performance deterioration in the plurality of the electric cells  11  is then be suppressed by suppressing the temperature dispersion. Thus, the plurality of the electric cells  11  that constitute the battery module  10  may be used in a well-balanced manner. As a result, the battery module  10  can be efficiently charged and discharged. 
     Further, in the case where 2-ethylhexyl caprylate is used as the heat exchange medium  40 , even when an electrolytic solution of the electric cells  11  leaks to the heat exchange medium  40  and the concentration of the electrolytic solution becomes equal to or higher than 20 [vol %], the volume resistivity of this liquid can be made equal to or higher than 1.0×10 5  Ω.cm. Furthermore, 2-ethylhexyl caprylate is not decomposed by the electrolytic solution of the electric cells  11  either. It should be noted that when the electric cells  11  generate excessive heat, a gas may be discharged from the electric cells  11  (the battery case) and the electrolytic solution of the power generation elements may leak together with this gas. For example, dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC) is used as the electrolytic solution. 
     Thus, it is preferable to ensure the insulating properties of the heat exchange medium  40  even if the electrolytic solution leaks from the electric cells  11 . As described above, when 2-ethylhexyl caprylate is used as the heat exchange medium  40 , the volume resistivity of the heat exchange medium  40  can be restrained from falling drastically. 
     However, a resinous material or a rubber material may be used for the pack case  20  and a vehicle body on which the battery pack  1  is mounted. For example, acrylonitrile butadiene styrene (ABS), polybutylene terephthalate (PBT), polyamide 6 (PA6), or polyamide 66 (PA66) may be suitably used as the resinous material. Further, the rubber material is used, for example, to ensure sealability. Acrylonitrile butadiene rubber (NBR), Viton®, or polyurethane may be suitably used as the rubber material. 
     It should be noted herein that when 2-ethylhexyl caprylate is used as the heat exchange medium  40 , Based on the following sentences, it seems to me that this should be rewritten as “the degree of change in the volume and weight of the above resinous material or rubber material may be minimized. More specifically, when the resinous material is soaked in the ester compound (2-ethylhexyl caprylate) at 70° C. for two weeks, the degree of change in the volume and weight of the resinous material was equal to or below 0.5%. Further, when the rubber material is soaked in the ester compound (2-ethylhexyl caprylate) at 70° C. for two weeks, the degree of change in volume and weight of the rubber material was equal to or below 20%. In this manner, when 2-ethylhexyl caprylate is used as the heat exchange medium  40 , the battery pack  1  and the vehicle body in which the battery pack  1  is installed may be prevented from being adversely affected. 
     While the invention has been described with reference to the example embodiment thereof, it should be understood that the invention is not limited to the example embodiment or construction. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiment of the invention are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.