Patent Publication Number: US-2021175558-A1

Title: Battery pack manufacturing method

Description:
INCORPORATION BY REFERENCE 
     This application is a divisional of U.S. patent application Ser. No. 16/167,940, filed Oct. 23, 2018, which claims priority to Japanese Patent Application No. 2017-238743 filed on Dec. 13, 2017. The disclosures of the prior applications are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a battery pack manufacturing method. 
     2. Description of Related Art 
     A battery pack including a cell stack, such as a fuel cell, generates heat by reaction and therefore needs to be cooled. Japanese Patent Application Publication No. 2013-033668 (JP 2013-033668 A) discloses a configuration in which a cooled portion that is at least a part of a surface of a cell stack extending along a stacking direction of single cells (a bottom surface of the cell stack) is in contact with a cooler having a refrigerant flow passage, with a heat-conducting sheet interposed between the cooled portion and the cooler. 
     SUMMARY 
     For manufacturing reasons, cell stacks of battery packs are subject to some variations in the surface roughness of the cooled portion. When the surface roughness of the cooled portion of the cell stack is high, the configuration disclosed in JP 2013-033668 A may fail to secure a sufficient area of contact between the cooled portion and the heat-conducting sheet, resulting in lower cooling efficiency. 
     As a countermeasure, the inventor of the present application considered using a viscous body layer, instead of a heat-conducting sheet, in order to allow close contact with the cooled portion of single cells even when the surface roughness of this cooled portion is relatively high. It was found, however, that air getting entrapped between the viscous body layer and the single cells in the process of disposing the cell stack onto the viscous body layer might interfere with securing a sufficient area of contact between the cell stack and the viscous body layer, resulting in lower cooling efficiency. 
     The present disclosure provides a battery pack manufacturing method that can reduce entrapment of air between a cooled portion of a cell stack and a viscous body layer. 
     A first aspect of the present disclosure is a manufacturing method of a battery pack which includes a cell stack that is a stack of a plurality of single cells and a cooler that includes a refrigerant flow passage, and in which a cooled portion that is at least a part of a surface of the cell stack extending along a stacking direction of the single cells is attached to the cooler, with a viscous body layer interposed between the cooled portion and the cooler. The manufacturing method includes: forming the viscous body layer on the cooler such that an opposed surface of the viscous body layer facing the cooled portion has a vertex at which a perpendicular distance from a surface of the viscous body layer opposed to the opposed surface is longest, and that the perpendicular distance decreases continuously and monotonically in directions away from the vertex, as seen in a sectional view of the viscous body layer from a long-side direction of the viscous body layer; and before hardening of the viscous body layer on the cooler is completed, attaching the cell stack to the cooler while pressing the opposed surface of the viscous body layer with the cooled portion. 
     When the viscous body layer is thus configured such that the opposed surface facing the cell stack has the vertex at which the perpendicular distance from the surface on the opposite side from the opposed surface is longest, and that the perpendicular distance decreases continuously and monotonically in the directions away from the vertex, as seen in a sectional view of the viscous body layer from the long-side direction, a vertex portion of the opposed surface of the viscous body layer comes in contact with the cooled portion of the cell stack first when the cell stack is attached to the cooler. Then, in the process of being squeezed, the viscous body layer gradually increases the area of contact with the cooled portion of the cell stack while pushing out air toward both sides with respect to the vertex portion of the opposed surface of the viscous body layer. Thus, entrapment of air between the cooled portion of the cell stack and the viscous body layer can be reduced. 
     In forming the viscous body layer in the first aspect, the viscous body layer may be formed on the cooler such that the opposed surface protrudes in an arc shape having a central angle within a range of 10° to 35° as seen in a sectional view of the viscous body layer from the long-side direction. 
     When the viscous body layer is thus configured such that the opposed surface facing the cell stack protrudes in an arc shape having a central angle within a range of 10° to 35° as seen in a sectional view of the viscous body layer from the long-side direction, the vertex portion of the arc of the opposed surface of the viscous body layer comes in contact with the cooled portion of the cell stack first when the cell stack is attached to the cooler. Then, in the process of being squeezed, the viscous body layer gradually increases the area of contact with the cooled portion of the cell stack while pushing out air toward both sides with respect to the vertex of the arc of the opposed surface of the viscous body layer. Thus, entrapment of air between the cooled portion of the cell stack and the viscous body layer can be reduced. 
     In the first aspect, the viscous body layer may be formed on the cooler such that the opposed surface protrudes in the arc shape having the central angle within a range of 20° to 35° as seen in the sectional view. Thus, entrapment of air between the cooled portion of the cell stack and the viscous body layer can be reduced. 
     A second aspect of the present disclosure is a manufacturing apparatus of a battery pack which includes a cell stack that is a stack of a plurality of single cells and a cooler that includes a refrigerant flow passage, and in which a cooled portion that is at least a part of a surface of the cell stack extending along a stacking direction of the single cells is attached to the cooler, with a viscous body layer interposed between the cooled portion and the cooler. The manufacturing apparatus includes a viscous body layer forming device configured to form the viscous body layer on the cooler by discharging a viscous body from a nozzle, and an attaching device that attaches the cell stack to the cooler. An opening of a discharge port of the nozzle is in an arc shape having a central angle within a range of 10° to 35° toward a downstream side in a film formation direction in forming the viscous body layer. The attaching device is configured to attach the cell stack to the cooler while pressing an opposed surface of the viscous body layer facing the cooled portion of the cell stack with the cooled portion, before hardening of the viscous body layer on the cooler is completed. 
     In the viscous body layer forming device of this battery pack manufacturing apparatus, the opening of the discharge port of the nozzle is shaped so as to protrude in an arc shape having a central angle within a range of 10° to 35° toward the downstream side in the film formation direction. Discharging a coating liquid from this discharge port can form the viscous body layer such that the opposed surface facing the cooled portion of the cell stack protrudes in an arc shape having a central angle within a range of 10° to 35° as seen in a sectional view from the film formation direction. In the process of being squeezed, the viscous body layer thus formed gradually increases the area of contact with the cooled portion of the cell stack while pushing out air toward both sides with respect to the vertex of the arc of the opposed surface of the viscous body layer. Thus, entrapment of air between the cooled portion of the cell stack and the viscous body layer can be reduced. 
     In the second aspect, the opening of the discharge port may be in the arc shape having the central angle within a range of 20° to 35°. With the discharge port of the nozzle thus configured, entrapment of air between the cooled portion of the cell stack and the viscous body layer can be reduced more favorably. 
     A third aspect of the present disclosure is a battery pack including a cell stack that is a stack of a plurality of single cells and a cooler that includes a refrigerant flow passage. A cooled portion that is at least a part of a surface of the cell stack extending along a stacking direction of the single cells is attached to the cooler, with a viscous body layer interposed between the cooled portion and the cooler. A surface of the viscous body layer in contact with the cooled portion has a first width in a direction parallel to a short side of the surface. A surface of the viscous body layer in contact with the cooler has a second width in the direction parallel to the short side. The first width is less than the second width. The refrigerant flow passage includes a side surface in the direction parallel to the short side and an opposite side surface opposed to the side surface in the direction parallel to the short side. A distance from the side surface to the opposite side surface is corresponding to a width that is greater than the first width but less than the second width. 
     In this battery pack, the first width, in the direction parallel to the short side, of the surface of the viscous body layer in contact with the cooled portion is less than the second width, in the direction parallel to the short side, of the surface of the viscous body layer in contact with the cooler. When the viscous body layer is thus formed, entrapment of air between the viscous body layer and the cooled portion is reduced to such a level as not to cause trouble in practical use. As a result, a decrease in cooling efficiency attributable to entrapment of air between the viscous body layer and the cooled portion can be avoided. Moreover, the refrigerant flow passage includes a side surface in the direction parallel to the short side and an opposite side surface opposed to the side surface in the direction parallel to the short side. A distance from the side surface to the opposite side surface is corresponding to a width that is greater than the first width, in the direction parallel to the short side, of the surface of the viscous body layer in contact with the cooled portion but less than the second width, in the direction parallel to the short side, of the surface of the viscous body layer in contact with the cooler. Thus, cooling energy in the refrigerant flow passage can be kept from leaking out, and this cooling energy in the refrigerant flow passage can be reliably delivered to the viscous body layer. 
     The present disclosure can reduce entrapment of air between a cooled portion of a cell stack and a viscous body layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is an exploded perspective view showing the general configuration of a battery pack manufactured by a battery pack manufacturing method according to an embodiment; 
         FIG. 2  is a schematic view showing the general configuration of a battery pack manufacturing apparatus used in the battery pack manufacturing method according to the embodiment; 
         FIG. 3  is a flowchart showing the flow of processing in the battery pack manufacturing method according to the embodiment; 
         FIG. 4  is a schematic view illustrating details of a viscous body layer forming step in the battery pack manufacturing method according to the embodiment; 
         FIG. 5  is a schematic view illustrating details of the viscous body layer forming step in the battery pack manufacturing method according to the embodiment; 
         FIG. 6  is a schematic view illustrating details of the viscous body layer forming step in the battery pack manufacturing method according to the embodiment; 
         FIG. 7  is a schematic view showing viscous body layers formed on coolers by the viscous body layer forming step; 
         FIG. 8  is a sectional view taken along line VIII-VIII in  FIG. 7 ; 
         FIG. 9  is a schematic view illustrating details of a cell stack mounting step in the battery pack manufacturing method according to the embodiment; 
         FIG. 10  is a schematic view illustrating details of the cell stack mounting step in the battery pack manufacturing method according to the embodiment; 
         FIG. 11  is a schematic view illustrating details of the cell stack mounting step in the battery pack manufacturing method according to the embodiment; 
         FIG. 12  is a schematic view showing details of a configuration around the viscous body layer in the battery pack manufactured by the battery pack manufacturing method according to the embodiment; 
         FIG. 13A  is a schematic view illustrating a battery pack manufacturing method according to a comparative example; 
         FIG. 13B  is a schematic view illustrating the battery pack manufacturing method according to the comparative example; 
         FIG. 13C  is a schematic view illustrating the battery pack manufacturing method according to the comparative example; 
         FIG. 14  is a schematic view illustrating the procedure of a test that confirmed effects of the battery pack manufacturing method according to the embodiment; 
         FIG. 15  is a schematic view showing the configuration of a mounting device used in the test; 
         FIG. 16  is a schematic view showing a state where an acrylic plate is pressed against a viscous body layer in the test; and 
         FIG. 17  is a table showing the result of the test. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present disclosure will be described below with reference to the drawings. Where appropriate, the following description and the drawings are abbreviated and simplified for clarity. The same components in the drawings are denoted by the same reference signs, while redundant description thereof is omitted as necessary. The right-handed xyz-coordinate system shown in the drawings is for the convenience of illustrating the positional relationships of the components. 
     First, the general configuration of a battery pack manufactured by a battery pack manufacturing method according to the embodiment will be described with reference to  FIG. 1 .  FIG. 1  is an exploded perspective view showing the general configuration of a battery pack  60 . As shown in  FIG. 1 , the battery pack  60  includes a cell stack  61 , coolers  62 , and a lower case  63 . 
     The cell stack  61  has a configuration in which a plurality of single cells  61   a  is stacked in a long-side direction (X-axis direction). Here, for example, the single cell  61   a  is a secondary battery, such as a lithium-ion battery or a nickel-metal hydride battery. The single cells  61   a  are electrically connected in series by a bus bar etc. The cell stack  61  is at least partially covered by a resin frame  61   b.    
     The cooler  62  is a device that cools the cell stack  61 , and has a substantially rectangular parallelepiped shape elongated along a stacking direction of the single cells  61   a  (X-axis direction). On a surface of the cooler  62  facing the cell stack  61 , a viscous body layer  64  elongated along the stacking direction of the single cells  61   a  (X-axis direction) is formed. The shape of the viscous body layer  64  will be described in detail later. A cooled portion  61   c  that is at least a part of a surface of the cell stack  61  extending along the stacking direction of the single cells  61   a  (X-axis direction) is mounted on the cooler  62 , with the viscous body layer  64  interposed between the cooled portion  61   c  and the cooler  62 . The lower case  63  is a case that houses the cell stack  61  mounted on the cooler  62  with the viscous body layer  64  interposed between the cell stack  61  and the cooler  62 . 
     Next, the general configuration of a battery pack manufacturing apparatus  1  used in the battery pack manufacturing method according to the embodiment will be described with reference to  FIG. 2 .  FIG. 2  is a schematic view showing the general configuration of the battery pack manufacturing apparatus  1 . As shown in  FIG. 2 , the battery pack manufacturing apparatus  1  includes a viscous body layer forming device  30  and a mounting device  40  (attaching device). 
     The viscous body layer forming device  30  is a device that forms the viscous body layer  64  on the surface of the cooler  62  facing the cell stack  61 . The viscous body layer forming device  30  includes a coating liquid storage  32 , a dispenser  33 , a static mixer  35 , a nozzle  31 , and a stage  20 . In this embodiment, a type of coating liquid that is obtained by mixing two materials, a base compound and a hardening agent, (two-component mixed coating liquid) is used as the coating liquid for forming the viscous body layer  64 . The coating liquid is made of a resin having viscoelasticity, for example, a silicon-based resin. The coating liquid storage  32  is a tank that stores the materials of the coating liquid, and is composed of a first coating liquid storage  32   a  that stores the base compound and a second coating liquid storage  32   b  that stores the hardening agent. 
     The dispenser  33  is a device that suctions the materials of the coating liquid from the coating liquid storage  32  and supplies the materials in predetermined amounts to the static mixer  35  to be described later. The dispenser  33  is composed of a first dispenser  33   a  and a second dispenser  33   b . The first dispenser  33   a  suctions the base compound of the coating liquid from the first coating liquid storage  32   a  and supplies the base compound in predetermined amounts to the static mixer  35 . Similarly, the second dispenser  33   b  suctions the hardening agent of the coating liquid from the second coating liquid storage  32   b  and supplies the hardening agent in predetermined amounts to the static mixer  35 . 
     The static mixer  35  is a device that stirs and mixes the materials of the coating liquid supplied from the dispenser  33 . The static mixer  35  has no driving unit, and a plurality of elements having a twisted part is spirally formed inside the static mixer  35 . Through the arrangement of these elements, the materials of the coating liquid having entered inside the static mixer  35  are sequentially stirred and mixed to produce the coating liquid. The produced coating liquid is delivered to the nozzle  31 . 
     The nozzle  31  is a flat nozzle that can discharge the coating liquid in a ribbon shape onto the cooler  62  to be coated. The nozzle  31  has a discharge port  31   a  from which the coating liquid is discharged. The nozzle  31  is disposed such that the discharge direction of the coating liquid is oriented at 90° to a surface to be coated. The shape of the discharge port  31   a  will be described later. The nozzle  31  includes a flow rate adjustment mechanism that adjusts the amount of coating liquid to be discharged. 
     The stage  20  is a part on which the cooler  62  is placed. The stage  20  includes a driving mechanism  70 . For example, having a ball screw  70   a  and a servomotor  70   b  that drives the ball screw  70   a  to rotate, the driving mechanism  70  can move the stage  20  so as to reciprocate in the X-axis direction. Thus, the coating liquid can be applied to the cooler  62  placed on the stage  20  at a predetermined application speed, to form the viscous body layer  64  on the surface of the cooler  62 . Alternatively, the viscous body layer forming device  30  may be configured to apply the coating liquid by moving the nozzle  31  while the stage  20  is fixed. 
     The mounting device  40  is a device that disposes the cell stack  61  onto the viscous body layer  64  formed on the cooler  62 . The mounting device  40  includes a grabber unit  41  and a conveyor unit  42 . The grabber unit  41  is configured to be able to grab the cell stack  61 . Specifically, the grabber unit  41  includes a pair of fingers and an actuator (such as a servomotor) that drives the fingers. The conveyor unit  42  conveys the cell stack  61  grabbed by the grabber unit  41  to a predetermined position at which the cell stack  61  faces the viscous body layer  64 , and disposes the cell stack  61  onto the viscous body layer  64 . For example, the conveyor unit  42  is configured to be able to shift in the directions of three axes by an actuator, a combination of a ball screw and a motor, etc. 
     Next, the battery pack manufacturing method according to the embodiment will be described. For the configuration of the battery pack manufacturing apparatus  1 ,  FIG. 2  will be referred to as necessary in the following description.  FIG. 3  is a flowchart showing the flow of processing in the battery pack manufacturing method according to the embodiment. As shown in  FIG. 3 , first, a viscous body layer forming step of forming the viscous body layer on the cooler  62  by the viscous body layer forming device  30  of the battery pack manufacturing apparatus  1  is performed (step  51 ). Then, before hardening of the viscous body layer  64  formed on the cooler  62  is completed, a cell stack mounting step (attaching step) of mounting the cell stack  61  onto the cooler  62  by the mounting device  40  of the battery pack manufacturing apparatus  1  is performed (step S 2 ). 
       FIG. 4  to  FIG. 6  are schematic views illustrating details of the viscous body layer forming step. As shown in  FIG. 4 , when the coating liquid is discharged from the discharge port  31   a  while the stage  20  is moved in the direction of an arrow P 1  (toward a negative side in the X-axis direction) at a predetermined moving speed, the viscous body layer  64  is formed on the cooler  62  at a predetermined application speed (film formation speed). Here, the application speed is equal to the moving speed of the stage  20 . 
       FIG. 5  is a view from the direction of an arrow D 1  in  FIG. 4 . As shown in  FIG. 5 , the opening of the discharge port  31   a  of the nozzle  31  of the viscous body layer forming device  30  is shaped so as to protrude in an arc shape having a central angle θ of not smaller than 10° but smaller than 35° toward a downstream side in a film formation direction (the negative side in the X-axis direction). In other words, the opening of the discharge port  31   a  is shaped so as to protrude in an arc shape having the central angle θ of not smaller than 10° but smaller than 35° toward the downstream side in the film formation direction (the negative side in the X-axis direction), in a state where the discharge port  31   a  faces the cooler  62 . The “arc shape” here is not limited to an arc of an exact circle but also includes a slightly deformed arc and an arc having more than one center points.  FIG. 6  is a view from the direction of an arrow D 2  in  FIG. 4 . Since the opening of the discharge port  31   a  is shaped as shown in  FIG. 5 , as shown in  FIG. 6 , an opposed surface  64   a  of the viscous body layer  64  discharged from the discharge port  31   a  onto the cooler  62  that is a surface on the side facing the cooled portion, protrudes in an arc shape having a central angle of not smaller than 10° but smaller than 35° as seen in a sectional view of the viscous body layer  64  from the film formation direction. 
       FIG. 7  is a schematic view showing the viscous body layers  64  formed on the coolers  62  by the viscous body layer forming step.  FIG. 8  is a sectional view taken along line VIII-VIII in  FIG. 7 . As shown in  FIG. 7  and  FIG. 8 , the viscous body layer  64  can be formed such that the opposed surface  64   a  of the viscous body layer  64  that is the surface on the side facing the cooled portion  61   c  of the cell stack  61  protrudes in an arc shape having a central angle of not smaller than 10° but smaller than 35° as seen in a sectional view of the viscous body layer  64  from the long-side direction (film formation direction). This means that the viscous body layer  64  is formed such that the opposed surface  64   a  has a vertex at which a perpendicular distance from a surface on the opposite side from the opposed surface  64   a  is longest, and that this perpendicular distance decreases continuously and monotonically in directions away from the vertex, as seen in a sectional view of the viscous body layer  64  from the long-side direction. 
     It is preferable that the opposed surface  64   a  of the viscous body layer  64  protrude in an arc shape having a central angle of larger than 20° but smaller than 35° as seen in a sectional view of the viscous body layer  64  from the long-side direction (film formation direction). In other words, it is preferable that the opening of the discharge port  31   a  (see  FIG. 5 ) of the nozzle  31  of the viscous body layer forming device  30  be shaped so as to protrude in an arc shape having the central angle θ of larger than 20° but smaller than 35° toward the downstream side in the film formation direction (the negative side in the X-axis direction). 
     As shown in  FIG. 8 , the cooler  62  has refrigerant flow passage  62   a  inside. The refrigerant flow passage  62   a  may include a plurality of refrigerant flow passage. To allow cooling energy in the refrigerant flow passage  62   a  to be reliably delivered to the viscous body layer  64 , it is preferable that a width B-B (L 2 ), in a direction parallel to a short side (Y-axis direction), of the surface of the viscous body layer  64  to be formed on the cooler  62  that is the surface in contact with the cooler  62  be greater than a width C-C (L 3 ), in the direction parallel to the short side (Y-axis direction), of a part at which the refrigerant flow passage  62   a  is present inside the cooler  62  (L 2 &gt;L 3 ). 
       FIG. 9  to  FIG. 11  are schematic views illustrating details of the cell stack mounting step.  FIG. 10  and  FIG. 11  correspond to  FIG. 8 . As shown in  FIG. 9 , in the cell stack mounting step, the cell stack  61  is grabbed by the grabber unit  41  of the mounting device  40 , and is conveyed by the conveyor unit  42  to a position at which the cooled portion  61   c  of the cell stack  61  faces the viscous body layer  64 . 
     Then, as shown in  FIG. 10 , the position of the cell stack  61  is moved in the direction of an arrow P 2  (toward a negative side in a Z-axis direction) by the conveyor unit  42 , and the cooled portion  61   c  of the cell stack  61  is brought into contact with the opposed surface  64   a  of the viscous body layer  64 . Here, a vertex  64   a A of the arc of the opposed surface  64   a  of the viscous body layer  64  comes in contact with the cooled portion  61   c  of the cell stack  61  first. Since the cooled portion  61   c  is not covered by the resin frame  61   b , the cooled portion  61   c  can be brought into direct contact with the opposed surface  64   a  of the viscous body layer  64 . While the vertex  64   a A of the arc is located at the center of the viscous body layer  64  in the Y-axis direction in  FIG. 10 , the vertex  64   a A may be located at an off-center position. 
     Then, as shown in  FIG. 11 , the opposed surface  64   a  of the viscous body layer  64  is pressed with the cooled portion  61   c  of the cell stack  61 , and thus the viscous body layer  64  is squeezed until a ratio of squeezing X reaches a desired value. Here, the ratio of squeezing X is expressed by X=D/h×100 [%], where D is the amount of viscous body layer  64  squeezed by pressing (the length of the viscous body layer  64  squeezed in a pressing direction), and h is the thickness of the viscous body layer  64  before the viscous body layer  64  is pressed. 
       FIG. 12  is a schematic view showing details of the configuration around the viscous body layer  64  in the battery pack  60  manufactured by the battery pack manufacturing method according to the embodiment.  FIG. 12  corresponds to  FIG. 8 . As shown in  FIG. 12 , a width A-A (L 1 , first width), in the direction parallel to the short side (Y-axis direction), of the surface of the viscous body layer  64  in contact with the cooled portion  61   c  is set to be less than the width B-B (L 2 , second width), in the direction parallel to the short side, of the surface of the viscous body layer  64  in contact with the cooler  62 . The ratio of squeezing X is set to such a value that the ratio of the width A-A (L 1 ) to the width B-B (L 2 ) (L 1 /L 2 ×100 [%]) is within a range of not lower than 90% but lower than 100%. Thus, the viscous body layer  64  is squeezed until being substantially rectangular as seen in a sectional view of the viscous body layer  64  from the X-axis direction. 
     As described above, in a state before mounting, the viscous body layer  64  is formed such that the opposed surface faces the cooled portion  61   c  protrudes in an arc shape having a central angle of not smaller than 10° but smaller than 35° as seen in a sectional view from the direction parallel to the long side of the surface of the viscous body layer  64  in contact with the cooler  62  (X-axis direction). Thus, entrapment of air between the viscous body layer and the cooled portion is reduced to such a level as not to cause trouble in practical use. As a result, a decrease in cooling efficiency attributable to entrapment of air between the viscous body layer and the cooled portion can be avoided. 
     In a state after mounting, the ratio of the width, in the direction parallel to the short side (Y-axis direction), of the surface of the viscous body layer  64  in contact with the cooled portion  61   c  to the width, in the direction parallel to the short side (Y-axis direction), of the surface of the viscous body layer  64  in contact with the cooler  62  is not lower than 90% but lower than 100%. When the viscous body layer is thus formed, the viscous body layer is unlikely to flow down onto both sides of the cooler, and moreover, the cooling energy supplied from the refrigerant flow passages can be reliably delivered from the viscous body layer to the cooled portion. 
     Further, the viscous body layer  64  may be formed such that, with respect to the width C-C (L 3 ), in the direction parallel to the short side (Y-axis direction), of the part at which the refrigerant flow passage  62   a  are present inside the cooler  62 , the width C-C (L 3 ) is greater than the width A-A (L 1 ) but less than the width B-B (L 2 ) (L 1 &lt;L 3 &lt;L 2 ). When the viscous body layer  64  is thus formed, the cooling energy in the refrigerant flow passage  62   a  can be reliably delivered to the viscous body layer  64 , and this cooling energy in the refrigerant flow passage  62   a  can be kept from leaking out. As a result, the cooling efficiency can be improved. 
     Here, a mechanism of how the battery pack manufacturing method according to the embodiment can reduce entrapment, i.e., catching, of air between the cooled portion of the cell stack and the viscous body layer will be described. 
     First, a problem that can arise in a manufacturing method according to a comparative example will be described with reference to  FIG. 13A  to  FIG. 13C .  FIG. 13A  to  FIG. 13C  are schematic views illustrating the battery pack manufacturing method according to the comparative example. When an opposed surface  564   a  of a viscous body layer  564  that is a surface on the side facing the cooled portion  61   c  of the cell stack  61  is flat as shown in  FIG. 13A , air is entrapped in the process of bringing the cooled portion  61   c  of the cell stack  61  into contact with the opposed surface  564   a  of the viscous body layer  564  as shown in  FIG. 13B . As a result, air layers  565  are formed between the viscous body layer  64  and the cell stack  61 . The cooling efficiency decreases when the air layers  565  are formed between the viscous body layer  64  and the cell stack  61 , as the air layers  565  hinder the exchange of cooling energy between the viscous body layer  64  and the cell stack  61 . 
     When the amount of viscous body layer  564  squeezed is increased as shown in  FIG. 13C  to expel the air layers  565  from between the viscous body layer  564  and the cell stack  61 , some of the air layers  565  can be expelled but the viscous body layer  564  undesirably flows down onto both sides of the cooler  62 . When the opposed surface  564   a  that is the surface on the side facing the cooled portion  61   c  of the cell stack  61  is flat, increasing the amount of squeezing cannot sufficiently expel the air layers  565 . 
     By contrast, in the battery pack manufacturing method according to the embodiment, as shown in  FIG. 10 , the opposed surface  64   a  of the viscous body layer  64  protrudes in an arc shape having a central angle of not smaller than 10° but smaller than 35° as seen in a sectional view of the viscous body layer  64  from the long-side direction (film formation direction). As shown in  FIG. 11 , therefore, in the process of being squeezed, the viscous body layer  64  gradually increases the area of contact with the cooled portion  61   c  of the cell stack  61  while pushing out air toward both sides (in the directions indicated by arrows P 3 , P 4 ) with respect to the vertex  64   a A of the arc of the opposed surface  64   a  of the viscous body layer  64 . Thus, entrapment of air between the cooled portion of the cell stack and the viscous body layer can be reduced. 
     Next, a test that confirmed the effects of the battery pack manufacturing method according to the embodiment will be described below. For the configuration of the battery pack manufacturing apparatus  1 ,  FIG. 2  will be referred to as necessary. 
     First, the procedure of an experiment will be described.  FIG. 14  is a schematic view illustrating the procedure of the experiment. As shown in  FIG. 14 , in this experiment, an acrylic plate  161  was used as a dummy test piece for the cell stack  61 . An aluminum plate  162  was used as a dummy test piece for the cooler  62 . The viscous body layer  64  was formed on the aluminum plate  162 , and the acrylic plate  161  is disposed on the viscous body layer  64 . After the acrylic plate  161  is disposed on the viscous body layer  64 , a ratio of air entrapment area was measured. Here, the ratio of air entrapment area is the ratio of the area of a region where air is entrapped to the total area of contact between the viscous body layer  64  and the acrylic plate  161  (including the region where air is entrapped). 
     The configuration of the battery pack manufacturing apparatus  1  used in this experiment is basically the same as the configuration of the battery pack manufacturing apparatus  1  shown in  FIG. 2 , but the mounting device used is simpler than the mounting device  40  of  FIG. 2 .  FIG. 15  is a schematic view showing the configuration of a mounting device  140  used in the experiment. As shown in  FIG. 15 , the mounting device  140  includes an acrylic plate holding mechanism  141 , an elevation mechanism  142 , a laser displacement meter  143 , and a base  144 . The acrylic plate holding mechanism  141  is a device that holds the acrylic plate  161 . The acrylic plate holding mechanism  141  is mounted on the elevation mechanism  142 . The elevation mechanism  142  is a device that moves the acrylic plate holding mechanism  141  upward and downward. Thus, the acrylic plate  161  held by the acrylic plate holding mechanism  141  is moved downward by the elevation mechanism  142 , and the acrylic plate  161  is pressed against the viscous body layer  64  formed on the aluminum plate  162 . The laser displacement meter  143  is an instrument that measures the ratio of air entrapment area, and is disposed on an upper side of the acrylic plate  161 . The base  144  is a platform on which the aluminum plate  162  with the viscous body layer  64  formed thereon is placed, and the elevation mechanism  142  is installed on the base  144 . 
       FIG. 16  is a schematic view showing a state where the acrylic plate  161  is pressed against the viscous body layer  64 . As shown in  FIG. 16 , the width, in the direction parallel to the short side (Y-axis direction), of the surface of the viscous body layer  64  in contact with the acrylic plate  161  is defined as a width A-A (L 1 ), and the width, in the direction parallel to the short side, of the surface of the viscous body layer  64  in contact with the aluminum plate  162  is defined as a width B-B (L 2 ) (these widths correspond to the width A-A and the width B-B in  FIG. 12 ). 
     In this experiment, an evaluation was made for five cases in which the central angle θ of the viscous body layer  64  as seen in a sectional view from the long-side direction (X-axis direction) (hereinafter referred to as the arc central angle θ) was 0° (flat), 10°, 20°, 30°, and 40°, respectively. The ratio of squeezing X (X=D/h×100 [%]) was set such that, at each of these arc central angles, the ratio of the width A-A (L 1 ) to the width B-B (L 2 ) (L 1 /L 2 ×100 [%]) was in the range of not lower than 90% but lower than 100%. Since the ratio of the width A-A (L 1 ) to the width B-B (L 2 ) is 100% in a state where the opposed surface is not squeezed, the evaluation was made based on this ratio being 100% in the case where the arc central angle θ is 0°. 
       FIG. 17  is a table showing the result of the experiment. In  FIG. 17 , Example 1, Example 2, Examples 3 to 5, Examples 6, 7, and Examples 8, 9 represent the results of the cases where the arc central angle θ is 10°, 20°, 30°, 35°, and 40°, respectively. Comparative Example 1 represents the result of the case where the arc central angle θ is 0° (flat). As shown in  FIG. 17 , the ratio of air entrapment area is 67% in Comparative Example 1 in which the arc central angle θ is 0° (flat). By contrast, the ratios of air entrapment area in Examples 1 to 9 are lower than the value in Comparative Example 1. This result confirms that air entrapment in Examples 1 to 9 is reduced compared with that in Comparative Example 1. 
     In Examples 1 to 5, the ratios of air entrapment area are not higher than 15%. This result confirms that air entrapment can be favorably reduced when the arc central angle θ is within the range of not smaller than 10° but smaller than 35°. Moreover, when the arc central angle θ is 30° as shown in Examples 3 to 5, the ratios of air entrapment area are not higher than 5%. This result confirms that air entrapment can be especially favorably reduced when the arc central angle θ is 30°. When the arc central angle θ is 35° or larger, the ratios of air entrapment area are much higher than 15%. A possible explanation for this result is that a significant distortion occurred in the viscous body layer in the process of squeezing the viscous body layer. Specifically, it is presumed that, when the arc central angle θ is 35° or larger, air cannot be sufficiently pushed out toward both sides of the viscous body layer due to the distortion caused in the viscous body layer, resulting in a large amount of air being entrapped. 
     As has been described above, the battery pack manufacturing method according to the embodiment can reduce entrapment of air between the cooled portion of the cell stack and the viscous body layer. 
     The present disclosure is not limited to the above embodiment but can be modified as necessary within the scope of the gist of the disclosure. In the embodiment, the configuration in which the cell stack  61  includes the resin frame  61   b  (see  FIG. 1 ) has been described. However, the present disclosure is not limited to this example, and the battery pack may have a configuration in which the cell stack does not include the resin frame. 
     In the embodiment, the opposed surface of the viscous body layer that is the surface on the side facing the cooled portion of the cell stack has been described as having an arc shape as seen in a sectional view of the viscous body layer from the film formation direction. However, the present disclosure is not limited to this example. The opposed surface of the viscous body layer may have any shape, provided that the opposed surface has a vertex at which the perpendicular distance from the surface on the opposite side from the opposed surface is longest, and that this perpendicular distance decreases continuously and monotonically in the directions away from the vertex, as seen in a sectional view of the viscous body layer from the film formation direction. For example, the opposed surface may have a shape of an elliptical arc or an inverted V-shape as seen in a sectional view of the viscous body layer from the film formation direction. Moreover, the opposed surface of the viscous body layer that is the surface on the side facing the cooled portion of the cell stack may protrude in an arc shape having a central angle of 35° or larger as seen in a sectional view of the viscous body layer from the long-side direction.