Patent Publication Number: US-2022223941-A1

Title: Battery unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-003015 filed on Jan. 12, 2021, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The technology disclosed herein relates to a battery unit. In particular, the technology disclosed herein relates to a battery unit including a cell stack including a plurality of cells stacked in a stacking direction, a cooler, and a heat transfer member. 
     2. Description of Related Art 
     The battery unit described above is disclosed in Japanese Unexamined Patent Application Publication No. 2020-053148 (JP 2020-053148 A). Heat generated from the cell stack (referred to as “plurality of battery cells” in JP 2020-053148 A) is transferred to the cooler via the heat transfer member which is in the form of gel. In JP 2020-053148 A, at a surface of the cooler that faces the cell stack, there is a plurality of main cooling surfaces that holds the heat transfer member with the cell stack, and recesses each located between the main cooling surfaces and spaced farther away from the cell stack than the main cooling surfaces. Thus, the heat transfer member in the form of gel that is pushed out of spaces between the cell stack and the main cooling surfaces enters the recesses. By holding the pushed out heat transfer member in the recesses, the heat transfer member can be prevented from moving to undesired parts. 
     SUMMARY 
     In the battery unit of JP 2020-053148 A, the heat transfer member that enters the recesses does not abut against the cell stack, and does not therefore contribute to cooling of the cell stack. As a result, the mass of the heat transfer member that does not contribute to the cooling increases. In a case where the recesses are filled with the heat transfer member so that the heat transfer member abuts against the cell stack, the amount of the heat transfer member used increases, and the mass increases. Since irregularities are formed on the cooling surfaces that face the cell stack, the surface area of the cooler increases. As a result, the mass of the cooler increases. The present disclosure provides a technology for cooling the cell stack while suppressing the increase in the mass and the increase in the amount of the heat transfer member used. 
     A battery unit disclosed herein includes a cell stack, a plurality of bus bars, a cooler, and a heat transfer member. The cell stack includes a plurality of battery cells stacked along a stacking direction. The cell stack includes a first surface where electrodes of the battery cells are disposed, and a second surface opposite to the first surface. The bus bars are each connected to at least one of the electrodes of the battery cells to electrically connect the battery cells. The cooler includes a cooling surface that faces the second surface of the cell stack to cool the cell stack. The heat transfer member is arranged between the second surface of the cell stack and the cooling surface of the cooler to transfer heat of the cell stack to the cooler. The cooling surface of the cooler has a flat shape. The heat transfer member includes a plurality of bands extending along the stacking direction while facing the battery cells. The bands are located away from each other in a direction perpendicular to the stacking direction. At least one of the bands at least partially overlaps each of the electrodes of the battery cells when viewed in a direction orthogonal to the cooling surface. 
     In each battery cell, a current flows into one electrode, and flows out from the other electrode. Therefore, the current is likely to be dense at the electrodes in each battery cell, and the temperature is likely to be high at the electrodes and at portions near the electrodes. Also, since the bus bars are connected to the electrodes of the battery cell, heat generated by the bus bars is likely to further heat the electrodes up. As a result, the amount of heat generation at the electrodes of the battery cell and at portions near the electrodes is likely to be larger than that at other parts of the battery cell. In the battery unit described above, at least one of the bands of the heat transfer member at least partially overlaps each of the electrodes when viewed in the direction orthogonal to the cooling surface. Thus, the heat generated at the electrodes and at portions near the electrodes is likely to be transferred to the cooler via the heat transfer member at positions which overlap with the electrodes. That is, the heat transfer member is likely to cool portions near the electrodes where the temperature is likely to be high. Also, the bands of the heat transfer member are located away from each other in the direction perpendicular to the stacking direction. Thus, the amount of the heat transfer member used can be reduced as compared to a case where the heat transfer member is arranged over the entire cooling surface. Further, since the cooling surface of the cooler has a flat shape, the mass of the cooler itself can be reduced as compared to the related art in which the cooling surface has irregularities. In the battery unit disclosed in the present disclosure, the connecting portion between the cell and the bus bar having a large amount of heat generation can be cooled by a relatively small amount of the heat transfer member while suppressing an increase in the mass. 
     In the battery unit, the bands may include at least one set of band-shaped portions spaced from each other in a width direction by a first distance, and at least one other set of band-shaped portions spaced from each other in the width direction by a second distance. 
     In the battery unit, the heat transfer member may be in a form of gel. 
     In the battery unit, the heat transfer member may also function as an adhesive for fixing the cooling surface to the second surface. 
     In the battery unit, each of the battery cells may be a rectangular cell having a flat rectangular shape in the stacking direction. 
     In the battery unit, the cooler may include a channel where a coolant for cooling the cell stack circulates. The channel may extend along the stacking direction while facing the battery cells. 
     The battery unit may further include a waterproof plate arranged between the second surface of the cell stack and the cooling surface of the cooler to cover the cooling surface. The heat transfer member may be located at one of a position between the cell stack and the waterproof plate and a position between the waterproof plate and the cooling surface. 
     The battery unit may further include a second heat transfer member located at another of the position between the cell stack and the waterproof plate and the position between the waterproof plate and the cooling surface. The second heat transfer member may be symmetrical to the heat transfer member across a symmetry plane that is the waterproof plate. 
     Details and further improvements of the technology disclosed herein are described in the “DETAILED DESCRIPTION OF EMBODIMENTS” section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG. 1  is a perspective view illustrating the structure of a battery unit  10   a  of a first embodiment; 
         FIG. 2  is a sectional view taken along a line II-II in  FIG. 1 ; 
         FIG. 3  is a sectional view similar to  FIG. 2 , illustrating a battery unit  10   b  of a second embodiment; and 
         FIG. 4  is a sectional view similar to  FIG. 2 , illustrating a battery unit  10   c  of a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In one embodiment of the present disclosure, a plurality of bands may include at least one set of band-shaped portions spaced from each other in a width direction (width direction of the band) by a first distance, and at least one other set of band-shaped portions spaced from each other in the width direction by a second distance. With this structure, the degree of freedom in terms of arrangement of a heat transfer member is improved, thereby facilitating the arrangement of the heat transfer member based on positions near electrodes having a large amount of heat generation. 
     In one embodiment of the present disclosure, the heat transfer member may be in a form of gel. With this structure, the heat transfer member can be arranged relatively easily based on the positions near the electrodes having a large amount of heat generation. 
     In one embodiment of the present disclosure, the heat transfer member may also function as an adhesive for fixing a cooling surface to a second surface. With this structure, misalignment of the cooling surface and the second surface can be suppressed. 
     In one embodiment of the present disclosure, each of the battery cells may be a rectangular cell having a flat rectangular shape in a stacking direction. In other embodiments, each battery cell may be, for example, a cylindrical cell. 
     In one embodiment of the present disclosure, a cooler may include a channel where a coolant for cooling a cell stack circulates. The channel may extend along the stacking direction while facing the battery cells. With this structure, the bands of the heat transfer member extend along the channel of the cooler. As a result, the contact area between the bands of the heat transfer member and the channel of the cooler increases. Thus, heat of the cell stack is transferred to the coolant via the heat transfer member more easily. 
     In one embodiment of the present disclosure, a waterproof plate may be arranged between the second surface of the cell stack and the cooling surface of the cooler to cover the cooling surface. In this case, the heat transfer member may be located at one of a position between the cell stack and the waterproof plate and a position between the waterproof plate and the cooling surface. With this structure, when the coolant leaks out of the cooler, the leaked coolant can be prevented from reaching the cell stack. 
     In this case, a second heat transfer member may be located at another of the position between the cell stack and the waterproof plate and the position between the waterproof plate and the cooling surface. The second heat transfer member may be symmetrical to the heat transfer member across a symmetry plane that is the waterproof plate. With this structure, the heat of the cell stack is first transferred to the heat transfer member located between the second surface and the waterproof plate, and then transferred to the cooler via the heat transfer member located between the cooler and the waterproof plate. Since the heat transfer members are symmetrical across the waterproof plate, the heat is easily transferred between the heat transfer members. 
     EMBODIMENTS 
     Battery units of embodiments are described with reference to the drawings.  FIG. 1  is a perspective view of a battery unit  10   a  of a first embodiment. To facilitate understanding of the structure of the battery unit  10   a ,  FIG. 1  illustrates disassembled components. The battery unit  10   a  is, for example, mounted below a floor panel (not illustrated) of an electric vehicle. The battery unit  10   a  forms a battery that stores electric power for driving the electric vehicle.  FIG. 1  illustrates only a part of the battery unit  10   a . The battery unit  10   a  further extends in a width direction of the electric vehicle (that is, an arrow Rh direction in  FIG. 1 ). The battery unit  10   a  is arranged over the entire area below the floor panel of the electric vehicle. In the present disclosure, an arrow Fr in the drawings indicates a front side in a front-rear direction of the electric vehicle (longitudinal direction), the arrow Rh indicates a right side in a lateral direction of the electric vehicle (width direction), and an arrow Up indicates an upper side in a vertical direction of the electric vehicle (height direction). 
     The battery unit  10   a  includes a cell stack  2 , a plurality of bus bars  4   v  and  4   c , a lower case  6 , a cooler  20 , an under panel  8 , a cell-side heat transfer member  30   a , and a cooler-side heat transfer member  30   b . The cell stack  2  includes a plurality of battery cells  2   c  stacked along the width direction (that is, the arrow Rh direction). Although  FIG. 1  illustrates only four battery cells  2   c  out of the plurality of battery cells  2   c  of the cell stack  2 , more battery cells  2   c  are stacked in actuality. Each battery cell  2   c  has a flat rectangular shape in its stacking direction (that is, the arrow Rh direction). That is, each battery cell  2   c  is a so-called rectangular cell. The battery cells  2   c  forming the cell stack  2  each have the same structure, and therefore one battery cell  2   c  is described below. 
     Although illustration is omitted, the battery cell  2   c  is a secondary battery that houses a negative-pole material, a positive-pole material, a separator that separates the negative-pole material and the positive-pole material and so on. The battery cell  2   c  is a lithium ion battery using an oxide including lithium as the positive-pole material. A positive electrode  2   p  is provided at one end on the upper surface of the battery cell  2   c . A negative electrode  2   n  is provided at the other end on the upper surface of the battery cell  2   c . The battery cell  2   c  stores electric power due to lithium ions (not illustrated) in the battery cell  2   c  moving from the positive electrode  2   p  toward the negative electrode  2   n . The battery cell  2   c  discharges electric power due to the lithium ions (not illustrated) in the battery cell  2   c  moving from the negative electrode  2   n  toward the positive electrode  2   p.    
     As illustrated in  FIG. 1 , when the battery cells  2   c  are stacked in the width direction, the electrodes  2   p  and  2   n  of the battery cells  2   c  are alternately arranged along the stacking direction on an upper surface  2   u  of the cell stack  2 . The bus bars that made of copper plates are connected to the electrodes  2   p  and  2   n  of the battery cells  2   c . A device-side bus bar  4   v  is arranged on the positive electrode  2   p  of the battery cell  2   c  located on the leftmost side in  FIG. 1  (that is, a near side of the drawing sheet). The device-side bus bar  4   v  is connected to a power converter (not illustrated) on its front side. The positive electrode  2   p  and the negative electrode  2   n  of adjacent battery cells  2   c  are connected by a connection bus bar  4   c . That is, the battery cells  2   c  are connected in series. A current flows through the battery cells  2   c  via the device-side bus bar  4   v  and the connection bus bars  4   c . As a result, the cell stack  2  generates heat. 
     A bottom wall of the lower case  6  is arranged below a lower surface  2   d  opposite to the upper surface  2   u  of the cell stack  2 . Although  FIG. 1  illustrates only the bottom wall of the lower case  6 , the lower case  6  is a box-shaped sheet metal component that covers the cell stack  2  from the bottom. Although details are described later, a coolant circulates in the cooler  20  arranged below the lower case  6 . By covering the cell stack  2  with the lower case  6  from the bottom, the coolant can be prevented from adhering to the cell stack  2  even if the coolant leaks out of the cooler  20 . The cell-side heat transfer member  30   a  is arranged between the lower case  6  and the cell stack  2 . The cell-side heat transfer member  30   a  includes one band  31   b  extending in the front-rear direction, and four bands  31   a  extending in the width direction (that is, the stacking direction of the battery cells  2   c ). The band  31   b  extends in the front-rear direction at the left end of an upper surface  6   u  of the lower case  6 . The bands  31   a  extend in the width direction across the battery cells  2   c  of the cell stack  2 . As a result, the bands  31   a  face the plurality of battery cells  2   c . The bands  31   a  extend in the width direction in parallel to each other. That is, the bands  31   a  are spaced from each other in a direction perpendicular to the stacking direction on the upper surface  6   u  of the lower case  6 . The cell-side heat transfer member  30   a  arranged in a band shape on the upper surface  6   u  is a semisolid (that is, a gel). Therefore, the cell-side heat transfer member  30   a  is easily arranged on the upper surface  6   u  of the lower case  6 . Details of the arrangement of the cell-side heat transfer member  30   a  are described later with reference to  FIG. 2 . 
     Since the cell stack  2  generates heat due to the current as described above, the cooler  20  for cooling the cell stack  2  is arranged below the lower case  6 . A coolant circulation channel (not illustrated) where the coolant circulates is connected to the cooler  20 . The cooler  20  includes a left channel  20   s , a right channel (not illustrated), a front channel  20   f , and a rear channel  20   r . Each of the channels  20   s ,  20   f , and  20   r  is hollow, and the coolant circulates through the channels  20   s ,  20   f , and  20   r . The left channel  20   s  located at the left end of the cooler  20  extends in the longitudinal direction (that is, a lateral direction on the drawing sheet of  FIG. 1 ). The front channel  20   f  and the rear channel  20   r  extend in the lateral direction (that is, a depth direction on the drawing sheet of  FIG. 1 ). The front channel  20   f  and the rear channel  20   r  extend in the width direction across the battery cells  2   c  of the cell stack  2 . As a result, the front channel  20   f  and the rear channel  20   r  face the plurality of battery cells  2   c . The coolant flowing into the cooler  20  from the coolant circulation channel (see a dashed arrow F 1 ) flows in a rightward direction through the front channel  20   f  (see a dashed arrow F 2 ). Similarly, the coolant that has flowed through the left channel  20   s  (see a dashed arrow F 4 ) flows in a rightward direction through the rear channel  20   r  (see a dashed arrow F 3 ). The coolant that flowed through the front channel  20   f  and the rear channel  20   r  flows in a forward direction (see a dashed arrow F 5 ) through a right channel (not illustrated) located at the right end of the cooler  20 . The coolant circulates in a ladder pattern through the cooler  20  to cool the cell stack  2 . 
     A cooling surface  20   u  that is the upper surface of the cooler  20  faces the lower surface  2   d  of the cell stack  2 . The cooler-side heat transfer member  30   b  is arranged on the cooling surface  20   u . The cooler-side heat transfer member  30   b  is made of a material similar to that of the cell-side heat transfer member  30   a . Similarly to the cell-side heat transfer member  30   a , the cooler-side heat transfer member  30   b  includes one band  32   b  extending in the front-rear direction, and four bands  32   a  extending in the width direction (that is, the stacking direction of the battery cells  2   c ). The band  32   b  located at the left end of the cooler  20  extends in the front-rear direction. The bands  32   a  extend in the width direction across the battery cells  2   c  of the cell stack  2 . As a result, the bands  32   a  face the plurality of battery cells  2   c . The bands  32   a  extend in the width direction in parallel to each other. That is, the bands  32   a  are spaced from each other in a direction perpendicular to the stacking direction on the cooling surface  20   u . Thus, the bands  32   a  of the cooler-side heat transfer member  30   b  are arranged similarly to the bands  31   a  of the cell-side heat transfer member  30   a . As a result, the bands  32   a  are symmetrical to the bands  31   a  across a symmetry plane that is the lower case  6 . 
     The under panel  8  is arranged below the cooler  20 . The under panel  8  is made of sheet metal component, and covers the cooler  20  from the bottom. 
     Details of the structure of the battery unit  10   a  are described with reference to  FIG. 2 .  FIG. 2  is a sectional view of one battery cell  2   c  out of the plurality of battery cells  2   c  of the cell stack  2  (see  FIG. 1 ). The other battery cells  2   c  have similar structures. To facilitate understanding of the drawings, hatching of the battery cell  2   c  is omitted in  FIG. 2  to  FIG. 4 . 
     For example, when the battery cell  2   c  stores electric power, a current transferred to the positive electrode  2   p  via the connection bus bar  4   c , while moving the lithium ions in the battery cell  2   c  from the positive electrode  2   p  to the negative electrode  2   n , flows to another connection bus bar  4   c  via the negative electrode  2   n . As indicated by dashed arrows I in  FIG. 2 , the current enters the battery cell  2   c  from the positive electrode  2   p , and then temporarily spreads in the vertical direction inside the battery cell  2   c . The spreading current converges at the negative electrode  2   n  again, and flows toward an adjacent battery cell  2   c  via the connection bus bar  4   c . Therefore, the current near the electrodes  2   p  and  2   n  is likely to be denser than that at other parts of the battery cell  2   c . As a result, the temperature near the electrodes  2   p  and  2   n  is more likely to be high as compared to other parts of the battery cell  2   c . The electrodes  2   p  and  2   n  are connected to the connection bus bars  4   c . Since the connection bus bar  4   c  is made of copper plate as described above, the connection bus bar  4   c  generates heat due to the current flow. The heat of the connection bus bars  4   c  is transferred to the battery cell  2   c  via the electrodes  2   p  and  2   n . As a result, the temperature in the battery cell  2   c  is highest near the electrodes  2   p  and  2   n . When the temperature of the battery cell  2   c  partially increases, the lithium ions in the battery cell  2   c  move in an uneven manner. Thus, the life of the battery cell  2   c  may decrease. 
     In the battery unit  10   a  of this embodiment (see  FIG. 1 ), the cell-side heat transfer member  30   a  is arranged between the lower surface  2   d  of the cell stack  2  and the upper surface  6   u  of the lower case  6 . As described with reference to  FIG. 1 , the cell-side heat transfer member  30   a  includes four bands  31   a , and the bands  31   a  are arranged away from each other in the longitudinal direction (that is, a lateral direction on the drawing sheet of  FIG. 2 ). The bands  31   a  are separated into a set of band-shaped portions  35   a  on the positive electrode  2   p  side, and a set of band-shaped portions  35   a  on the negative electrode  2   n  side. The battery unit  10   a  of the first embodiment has a bilaterally symmetrical shape across a symmetry plane at the center in the longitudinal direction. Therefore, the set of band-shaped portions  35   a  on the positive electrode  2   p  side is mainly described below. The set of band-shaped portions  35   a  on the positive electrode  2   p  side may hereinafter be referred to simply as “band-shaped portions  35   a”.    
     As indicated by arcs H in  FIG. 2 , heat near the positive electrode  2   p  is transferred inside the battery cell  2   c  from the positive electrode  2   p  toward the cooler  20  while spreading in the longitudinal direction. A right band  31   a  in the band-shaped portions  35   a  is arranged immediately below the positive electrode  2   p . The right band  31   a  overlaps with the positive electrode  2   p  by a length r 1  when viewed in a direction orthogonal to the upper surface  6   u  of the lower case  6  (that is, the arrow Up direction). In other words, the right band  31   a  is arranged closest to the positive electrode  2   p  on the upper surface  6   u . Therefore, the heat near the positive electrode  2   p  is likely to be transferred to the right band  31   a . Thus, the heat near the positive electrode  2   p  is unlikely to be transferred to other portions in the battery cell  2   c . That is, the heat near the positive electrode  2   p  is unlikely to increase the temperatures of the other portions in the battery cell  2   c.    
     The heat near the positive electrode  2   p  spreads inside the battery cell  2   c  in the manner indicated by the arcs H. Therefore, the temperature near a portion immediately below the positive electrode  2   p  may increase. The band-shaped portions  35   a  include a left band  31   a  spaced from the right band  31   a  by a distance d 1  in the longitudinal direction (that is, a width direction of the band  31   a ). The heat near the positive electrode  2   p  is likely to be transferred to the two bands  31   a  of the band-shaped portions  35   a  arranged near a portion immediately below the positive electrode  2   p.    
     The center of the battery cell  2   c  in the longitudinal direction (that is, the lateral direction on the drawing sheet of  FIG. 2 ) is spaced from the positive electrode  2   p . Therefore, the heat near the positive electrode  2   p  is unlikely to be transferred to the center. The bands  31   a  of a set of band-shaped portions  33   a  located at the center of the battery cell  2   c  in the longitudinal direction are spaced from each other by a distance d 2  in the longitudinal direction (that is, the width direction of the band  31   a ). As illustrated in  FIG. 2 , the distance d 2  is longer than the distance d 1 . In the battery unit  10   a , the bands  31   a  of the cell-side heat transfer member  30   a  are arranged at the short distance d 1  as the set of band-shaped portions  35   a  near the portion immediately below each of the electrodes  2   p  and  2   n  where the temperature is likely to be high, and are arranged at the long distance d 2  as the set of band-shaped portions  33   a  near the center where the temperature is unlikely to be high. In the battery unit  10   a , the bands  31   a  are densely arranged near the portion immediately below each of the electrodes  2   p  and  2   n  where the temperature is likely to be high, and a smaller number of bands  31   a  are arranged at the other portion. Thus, in the battery unit  10   a , the portions near the electrodes  2   p  and  2   n  of the battery cell  2   c  where the temperature is likely to be high can efficiently be cooled by a relatively small amount of the cell-side heat transfer member  30   a.    
     The heat transferred to the bands  31   a  is transferred to the bands  32   a  of the cooler-side heat transfer member  30   b  via the lower case  6 . As described above, the bands  32   a  of the cooler-side heat transfer member  30   b  are symmetrical to the bands  31   a  of the cell-side heat transfer member  30   a  across the symmetry plane that is the lower case  6 . As a result, each band  32   a  is arranged immediately below the symmetrical band  31   a . Therefore, the heat transferred to each band  31   a  is likely to be transferred to the band  32   a  arranged immediately below the band  31   a . As a result, the cooler-side heat transfer member  30   b  can efficiently transfer the heat of the cell-side heat transfer member  30   a  to the cooler  20  via the bands  32   a . The heat transferred to the bands  32   a  of the cooler-side heat transfer member  30   b  is transferred to coolants R 1  and R 2  in the cooler  20  via the cooling surface  20   u  of the cooler  20 . As a result, the heat near each of the electrodes  2   p  and  2   n  is released to the coolants, thereby cooling the battery cell  2   c.    
     As illustrated in  FIG. 2 , the cooling surface  20   u  of the cooler  20  has no irregularities. That is, the cooling surface  20   u  has a flat shape. When the band  32   a  in the form of gel is pressed by the lower surface of the lower case  6  and the cooling surface  20   u  during manufacture of the battery unit  10   a , the band  32   a  easily spreads in the longitudinal direction (that is, the width direction of the band). As a result, the range of interposition of the cooler-side heat transfer member  30   b  between the lower surface of the lower case  6  and the cooling surface  20   u  can be increased without increasing the amount of the cooler-side heat transfer member  30   b  used. Thus, the cooling efficiency can be improved without increasing the amount of the cooler-side heat transfer member  30   b  used. Since the cooling surface  20   u  has the flat shape, an increase in the sectional area of the cooler  20  can be suppressed. As a result, an increase in the mass of the cooler  20  can be suppressed. The “flat shape” mentioned herein is at least a shape without irregularities having a thickness larger than the thicknesses of each of the heat transfer members  30   a  and  30   b.    
     Each of the cell-side heat transfer member  30   a  and the cooler-side heat transfer member  30   b  in the form of gel also functions as an adhesive. Therefore, the lower surface  2   d  of the cell stack  2  (see  FIG. 1 ) and the upper surface  6   u  of the lower case  6  are fixed by the cell-side heat transfer member  30   a . Further, the lower surface of the lower case  6  and the cooling surface  20   u  of the cooler  20  are fixed by the cooler-side heat transfer member  30   b . Since each of the heat transfer members  30   a  and  30   b  also functions as the adhesive, it is unlikely that the cell stack  2 , the lower case  6 , and the cooler  20  are misaligned relatively. 
     In the first embodiment, the distance d 1  is an example of “first distance”, and the distance d 2  is an example of “second distance”. The upper surface  2   u  of the cell stack  2  is an example of “first surface”, and the lower surface  2   d  of the cell stack  2  is an example of “second surface”. The cell-side heat transfer member  30   a  is an example of “second heat transfer member”. The lower case  6  is an example of “waterproof plate”. 
     Battery units  10   b  and  10   c  of other embodiments are described below with reference to  FIG. 3  and  FIG. 4 . The battery unit  10   b  of the second embodiment illustrated in  FIG. 3  differs from the battery unit  10   a  of the first embodiment in terms of arrangement of a cell-side heat transfer member  30   c  and in that the lower case  6  is not provided. The battery unit  10   c  of the third embodiment illustrated in  FIG. 4  differs from the battery unit  10   a  of the first embodiment in that the cell-side heat transfer member  30   a  is changed to a cell-side heat transfer member  30   d . The battery unit  10   b  of the second embodiment and the battery unit  10   c  of the third embodiment are similar to the battery unit  10   a  of the first embodiment in terms of the other parts. 
     In the battery unit  10   b  of the second embodiment illustrated in  FIG. 3 , the lower surface  2   d  of the cell stack  2  faces the cooling surface  20   u  of the cooler  20  without intervention of the lower case  6  (see  FIG. 2 ). As a result, the cell-side heat transfer member  30   c  abuts against both the lower surface  2   d  of the cell stack  2  and the cooling surface  20   u  of the cooler  20 . The cell-side heat transfer member  30   c  includes six bands  31   c . The bands  31   c  are separated into a set of band-shaped portions  35   c  including three bands  31   c  on the positive electrode  2   p  side, and a set of band-shaped portions  35   c  including three bands  31   c  on the negative electrode  2   n  side. The battery unit  10   b  of the second embodiment has a bilaterally symmetrical shape across a symmetry plane at the center in the longitudinal direction. Therefore, mainly the band-shaped portions  35   c  on the positive electrode  2   p  side are described herein simply as “band-shaped portions  35   c”.    
     The bands  31   c  of the band-shaped portions  35   c  are located away from each other by a distance d 3  in the longitudinal direction (that is, a width direction of the band  31   c ). A right-end band  31   c  of the band-shaped portions  35   c  is arranged immediately below the positive electrode  2   p . The right-end band  31   c  overlaps with the positive electrode  2   p  by a length r 2  when viewed in a direction orthogonal to the cooling surface  20   u  of the cooler  20  (that is, the arrow Up direction). Therefore, the right-end band  31   c  can transfer the heat near the positive electrode  2   p  to the cooler  20  before the heat is transferred to other portions of the battery cell  2   c.    
     As illustrated in  FIG. 3 , the bands  31   c  of a set of band-shaped portions  33   c  located at the center of the battery cell  2   c  are arranged away from each other by a distance d 4  in the width direction of the band  31   c . The distance d 4  is longer than the distance d 3 . In the battery unit  10   b  of the second embodiment, the bands  31   c  are densely arranged to be away from each other by the short distance d 3  near the portion immediately below the positive electrode  2   p , and are arranged to be away from each other by the long distance d 4  near the center where the temperature is unlikely to be high. Thus, in the battery unit  10   b , the portions near the electrodes  2   p  and  2   n  of the battery cell  2   c  where the temperature is likely to be high can efficiently be cooled by a relatively small amount of the cell-side heat transfer member  30   c.    
     In the second embodiment, the distance d 3  is an example of “first distance”, and the distance d 4  is an example of “second distance”. 
     The battery unit  10   c  of the third embodiment illustrated in  FIG. 4  includes the cell-side heat transfer member  30   d  in place of the cell-side heat transfer member  30   a  of the battery unit  10   a  of the first embodiment. The cell-side heat transfer member  30   d  includes two heat transfer sheets  31   d . Similarly to the band  32   a , each heat transfer sheet  31   d  extends in the stacking direction (that is, a depth direction on the drawing sheet of  FIG. 4 ). The heat transfer sheets  31   d  are bilaterally symmetrical across a symmetry plane at the center in the longitudinal direction (that is, a lateral direction on the drawing sheet of  FIG. 4 ). Therefore, the heat transfer sheet  31   d  on the positive electrode  2   p  side is mainly described below. 
     The heat transfer sheet  31   d  on the positive electrode  2   p  side is arranged immediately above two bands  32   a  on the positive electrode  2   p  side. As a result, the heat transfer sheet  31   d  on the positive electrode  2   p  side is located immediately below the positive electrode  2   p . That is, the heat transfer sheet  31   d  on the positive electrode  2   p  side overlaps with the positive electrode  2   p  by the length r 1  when viewed in the direction orthogonal to the upper surface  6   u  of the lower case  6  (that is, the arrow Up direction). As described with reference to  FIG. 2 , the heat near the positive electrode  2   p  is transferred to the heat transfer sheet  31   d  on the positive electrode  2   p  side before the heat is transferred to other portions in the battery cell  2   c . The heat transferred to the heat transfer sheet  31   d  on the positive electrode  2   p  side is transferred to the cooler  20  via the lower case  6  and the cooler-side heat transfer member  30   b . Thus, the cell stack  2  is cooled. By changing the arrangements and materials of the cell-side heat transfer member  30   d  and the cooler-side heat transfer member  30   b , the degree of freedom can be improved in the manufacturing process of the battery unit  10   c.    
     In the third embodiment, the cell-side heat transfer member  30   d  is an example of “second heat transfer member”. 
     Although the specific examples of the present disclosure are described above in detail, the examples are only illustrative and are not intended to limit the claims. The technologies described in the claims encompass various modifications and changes to the specific examples described above. The technical elements disclosed herein or illustrated in the drawings exert technical utility solely or in various combinations, and are not limited to the combination described in the claims as filed. The technologies disclosed herein or illustrated in the drawings may simultaneously achieve a plurality of objects, and exert technical utility by achieving one of the objects.