Patent Publication Number: US-2023155255-A1

Title: Battery module

Description:
This nonprovisional application is based on Japanese Patent Application No. 2021-187143 filed on Nov. 17, 2021, with the Japan Patent Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a battery module. 
     Description of the Background Art 
     For example, Japanese Patent Laying-Open No. 2013-73916 discloses a battery pack including: a plurality of cells arranged side by side; a case that accommodates the plurality of cells; a spacer disposed between the cells; and a bottom member disposed between the bottom surface of each of the cells and the case, the bottom member being attachable to and detachable from the spacer. 
     WO 2019/021912 discloses a combined battery pack including: a first battery pack and a second battery pack each having the same structure; and combining means for combining the first battery pack and the second battery pack with each other. Each battery pack has a substantially rectangular parallelepiped shape and includes a plurality of stacked prismatic batteries. 
     SUMMARY OF THE INVENTION 
     As disclosed in each of the pieces of the patent literature described above, there has been known a battery module including a plurality of battery cells stacked in one direction, wherein a bus bar is used as means for electrically connecting the plurality of battery cells together. 
     On the other hand, there are a variety of purposes of use, required performances, and the like with regard to battery modules, so that the number of battery cells included in a battery module is also changed accordingly. In each of such conventional battery modules, however, a situation of adjusting the number of battery cells in connection among the plurality of battery cells through the bus bar has not been taken into consideration. 
     In view of the above, an object of the present invention is to solve the above-described problem and provide a battery module in which the number of battery cells can be readily adjusted. 
     A battery module according to the present invention includes: a plurality of battery cells stacked in a predetermined direction; a restraint member that applies, to the plurality of battery cells, restraint force along the predetermined direction; and a plurality of bus bars that electrically connects the plurality of battery cells together, the plurality of bus bars including a first bus bar and a second bus bar. The first bus bar has a first joining portion that joins separated members to each other through a first joining form, and connects, via the first joining portion, between battery cells adjacent to each other in the predetermined direction. The second bus bar has a second joining portion that joins separated members to each other through a second joining form different from the first joining form, and connects, via the second joining portion, between battery cells adjacent to each other in the predetermined direction. 
     According to the battery module thus configured, the different joining forms are used for the first joining portion of the first bus bar and the second joining portion of the second bus bar, so that joining and disjoining of the separated members by one of the first joining portion and the second joining portion can be performed more readily than joining and disjoining of the separated members by the other of the first joining portion and the second joining portion. Thus, the number of battery cells included in the battery module can be readily adjusted by joining and disjoining the members by one of the first joining portion and the second joining portion so as to connect or disconnect the battery cells adjacent to each other in the predetermined direction. 
     Preferably, the second joining form is welding. 
     According to the battery module thus configured, the members can be firmly joined to each other by the second joining portion, thus resulting in improved reliability of the connection between the battery cells by the second bus bar. 
     Preferably, the first joining form is fixation with a bolt, pin, clip or band. 
     According to the battery module thus configured, means allowing for attaching to and detaching from the first bus bar is used as the first joining form, with the result that the members can be readily joined to and disjoined from each other by the first joining portion. 
     Preferably, electric resistance of the first bus bar and electric resistance of the second bus bar are different from each other. 
     According to the battery module thus configured, energy loss can be suppressed to be small in the bus bar having smaller electric resistance. 
     Preferably, a body size of the first bus bar and a body size of the second bus bar are different from each other. 
     According to the battery module thus configured, a wide space can be secured around the bus bar having a smaller body size. 
     Preferably, the battery module includes a plurality of battery cell units arranged side by side in the predetermined direction and fed with the restraint force along the predetermined direction by the restraint member. Each of the battery cell units has the plurality of battery cells arranged side by side continuously in the predetermined direction, and a holding member that collectively holds the plurality of battery cells arranged side by side continuously in the predetermined direction. The first bus bar electrically connects between the battery cells adjacent to each other in the predetermined direction between a first battery cell unit of the plurality of battery cell units and a second battery cell unit of the plurality of battery cell units, the second battery cell unit being adjacent to the first battery cell unit in the predetermined direction. The second bus bar electrically connects between the battery cells adjacent to each other in the predetermined direction in each of the battery cell units. 
     According to the battery module thus configured, the number of battery cells included in the battery module can be readily adjusted by connecting the battery cells or disconnecting them by the first bus bar between the first battery cell unit and the second battery cell unit so as to adjust the number of battery cell units. 
     Preferably, the first bus bar further has a first bus-bar-divided body and a second bus-bar-divided body joined to each other by the first joining portion. The restraint member has an end plate facing, in the predetermined direction, a third battery cell unit of the plurality of battery cell units, the third battery cell unit being disposed at an end portion in the predetermined direction. The battery module further includes a third bus bar provided in the third battery cell unit at a battery cell disposed adjacent to the end plate in the predetermined direction. The third bus bar has the same shape as a shape of one of the first bus-bar-divided body and the second bus-bar-divided body, and is able to be joined to an external terminal using the first joining form. 
     According to the battery module thus configured, by using the third bus bar having the same shape as the shape of one of the first bus-bar-divided body and the second bus-bar-divided body of the first bus bar, the third battery cell unit included in the battery module can be joined to an external terminal by the first joining form. 
     Preferably, each of the battery cells has an output density of 8000 W/L or more. 
     According to the battery module thus configured, the number of battery cells can be readily adjusted in a battery module including battery cells each having a high output density of 8000 W/L or more. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view showing a battery module according to an embodiment of the present invention. 
         FIG.  2    is a perspective view showing an internal structure of the battery module in  FIG.  1   . 
         FIG.  3    is a perspective view showing a battery cell unit included in the battery module in  FIG.  1   . 
         FIG.  4    is a perspective view showing a battery cell included in the battery cell unit in  FIG.  3   . 
         FIG.  5    is an exploded assembly diagram showing a connection structure for a plurality of battery cells. 
         FIG.  6    is a perspective view showing a connection structure for battery cells between adjacent battery cell units. 
         FIG.  7    is a cross sectional view showing the connection structure for the battery cells along a line VII-VII in  FIG.  6   . 
         FIG.  8    is a perspective view showing a connection structure for battery cells in a battery cell unit. 
         FIG.  9    is a cross sectional view showing a connection structure for battery cells when viewed in a direction along a line IX-IX in  FIG.  8   . 
         FIG.  10    is a perspective view showing a modification of a first bus bar shown in  FIG.  6   . 
         FIG.  11    is a diagram showing a first modification of a first joining form at a first joining portion shown in  FIG.  10   . 
         FIG.  12    is a diagram showing a second modification of the first joining form at the first joining portion shown in  FIG.  10   . 
         FIG.  13    is a diagram showing a third modification of the first joining form at the first joining portion shown in  FIG.  10   . 
         FIG.  14    is a perspective view showing a modification of the battery module in  FIG.  1   . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to figures. It should be noted that in the figures referred to below, the same or corresponding members are denoted by the same reference characters. 
       FIG.  1    is a perspective view showing a battery module according to an embodiment of the present invention.  FIG.  2    is a perspective view showing an internal structure of the battery module in  FIG.  1   .  FIG.  3    is a perspective view showing a battery cell unit included in the battery module in  FIG.  1   . 
     Referring to  FIGS.  1  to  3   , a battery module  100  is used as a power supply for driving a vehicle such as a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or a battery electric vehicle (BEV). 
     In the present specification, for convenience of description of the structure of battery module  100 , the “Y axis” represents an axis extending in parallel with a stacking direction of a plurality of below-described battery cells  11 , the “X axis” represents an axis extending in a direction orthogonal to the Y axis, and the “Z axis” represents an axis extending in a direction orthogonal to the Y axis and the X axis. An obliquely rightward upward direction in the plane of sheet of  FIG.  1    is “+Y axis direction”, and an obliquely leftward downward direction in the plane of sheet of  FIG.  1    is “−Y axis direction”. An obliquely rightward downward direction in the plane of sheet of  FIG.  1    is “+X axis direction” and an obliquely leftward upward direction in the plane of sheet of  FIG.  1    is “−X axis direction”. An upward direction in the plane of sheet of  FIG.  1    is “+Z axis direction” and a downward direction in the plane of sheet of  FIG.  1    is “−Z axis direction”. 
     Typically, battery module  100  is mounted on a vehicle in such a posture that the +Z axis direction corresponds to the upward direction and the −Z axis direction corresponds to the downward direction. 
     First, an overall structure of battery module  100  will be described. As shown in  FIG.  1   , battery module  100  includes a plurality of battery cell units  21  ( 21 A,  21 B,  21 C,  21 D,  21 E,  21 F) and a restraint member  41 . The plurality of battery cell units  21  are stacked in the Y axis direction. 
     Battery cell unit  21 A, battery cell unit  21 B, battery cell unit  21 C, battery cell unit  21 D, battery cell unit  21 E, and battery cell unit  21 F are arranged side by side in this order from the negative side to the positive side in the Y axis direction. It should be noted that the number of battery cell units  21  included in battery module  100  is not particularly limited as long as two or more battery cell units  21  are included. 
     The plurality of battery cell units  21  are collectively held by restraint member  41 . Restraint member  41  applies, to the plurality of battery cell units  21 , restraint force along the Y axis direction. Restraint member  41  has a pair of end plates  42  ( 42 P,  42 Q), a pair of first restraint bands  43 , and a second restraint band (not shown). 
     The pair of end plates  42  are disposed on both sides beside the plurality of battery cell units  21  stacked in the Y axis direction. End plate  42 P is disposed to face battery cell unit  21 A in the Y axis direction. End plate  42 Q is disposed to face battery cell unit  21 F in the Y axis direction. Each of end plates  42  is composed of a plate material having a thickness direction corresponding to the Y axis direction. 
     The pair of first restraint bands  43  are disposed on both sides beside the plurality of battery cell units  21  in the X axis direction. The second restraint band (not shown) is provided at a position facing the plurality of battery cell units  21  in the Z axis direction. Each of first restraint bands  43  and the second restraint band extends in the Y axis direction. An end portion of each of first restraint bands  43  and the second restraint band in the −Y axis direction is connected to end plate  42 P. An end portion of each of first restraint bands  43  and the second restraint band in the  1 Y axis direction is connected to end plate  42 Q. 
     Each of first restraint bands  43  is provided with a plurality of openings  44 . 
     The plurality of openings  44  are provided at intervals in the Y axis direction, and each of openings  44  is constituted of a through hole extending through first restraint band  43  in the X axis direction. Opening  44  is provided to expose ventilation ports  32  provided in a case body  31  described later. 
     Battery module  100  further has a pair of collective terminals  91  ( 91   p ,  91   q ), a wiring member  92 , a plurality of voltage detection wires  96 , and an exhaust duct  93 . 
     The pair of collective terminals  91  are disposed on both sides beside the plurality of battery cell units  21  stacked in the Y axis direction. Collective terminal  91   p  is provided at a position overlapping with end plate  42 P when viewed in the Z axis direction. Collective terminal  91   q  is provided at a position overlapping with end plate  42 Q when viewed in the Z axis direction. Each of collective terminals  91  is a terminal for connecting battery module  100  to an external wiring such as a cable disposed outside battery module  100 . 
     Wiring member  92  is provided at a position facing the plurality of battery cell units  21  in the Z axis direction. Wiring member  92  is disposed opposite to the second restraint band (not shown) with the plurality of battery cell units  21  being interposed therebetween. Wiring member  92  extends, in the Y axis direction, through the central portions of battery cell units  21  in the X axis direction. Wiring member  92  is composed of, for example, a flexible printed circuit board. The plurality of voltage detection wires  96  extend from wiring member  92  and are respectively connected to a plurality of below-described bus bars  50 . 
     Exhaust duct  93  extends in the Y axis direction. Exhaust duct  93  extends at a position overlapping with wiring member  92  when viewed in the Z axis direction. Exhaust duct  93  is disposed between each of the plurality of battery cell units  21  and wiring member  92  in the Z axis direction. 
     As shown in  FIGS.  2  and  3   , each of battery cell units  21  has a plurality of battery cells  11  and a holding member  30 . 
     Battery cell unit  21  has four battery cells  11  ( 11   a ,  11   b ,  11   c ,  11   d ). Battery cell unit  21  has an even number of battery cells  11 . It should be noted that the number of battery cells  11  included in each battery cell unit  21  is not particularly limited as long as two or more battery cells  11  are included. An odd number of battery cells  11  may be included. 
     In each battery cell unit  21 , i.e., in each of battery cell unit  21 A, battery cell unit  21 B, battery cell unit  21 C, battery cell unit  21 D, battery cell unit  21 E, and battery cell unit  21 F, the plurality of battery cells  11  are arranged side by side continuously in the Y axis direction. Battery cell  11   a , battery cell  11   b , battery cell  11   c , and battery cell  11   d  are arranged side by side in this order from the negative side to the positive side in the Y axis direction. 
     The stacking direction of the plurality of battery cells  11  in each battery cell unit  21  is the same as the stacking direction of the plurality of battery cell units  21 . Restraint member  41  applies, to the plurality of battery cells  11 , restraint force along the Y axis direction. 
     Holding member  30  collectively holds the plurality of battery cells  11  ( 11   a ,  11   b ,  11   c ,  11   d ). Holding member  30  includes case body  31 . Case body  31  has an external appearance having a rectangular parallelepiped shape. The plurality of battery cells  11  are accommodated in case body  31 . 
     Case body  31  is provided with the plurality of ventilation ports  32 . The plurality of ventilation ports  32  are provided in the both side surfaces of case body  31  orthogonal to the X axis direction. Each of ventilation ports  32  is constituted of a through hole extending through case body  31  in the X axis direction. Ventilation port  32  is provided as a path for introducing cooling air into a space between battery cells  11  adjacent to each other in the Y axis direction and discharging the cooling air from the space. 
       FIG.  4    is a perspective view showing a battery cell included in the battery cell unit in  FIG.  3   .  FIG.  5    is an exploded assembly diagram showing a connection structure for a plurality of battery cells. 
     Referring to  FIGS.  4  and  5   , battery cell  11  is a lithium ion battery. Battery cell  11  has an output density of 8000 W/L or more. Battery cell  11  has a prismatic shape and has a thin plate shape in the form of a rectangular parallelepiped. The plurality of battery cells  11  are stacked such that the Y axis direction corresponds to the thickness direction of each battery cell  11 . 
     Each of battery cells  11  has an exterior package  12 . Exterior package  12  is constituted of a housing having a rectangular parallelepiped shape, and forms the external appearance of battery cell  11 . An electrode assembly and an electrolyte solution are accommodated in exterior package  12 . 
     Exterior package  12  has a first side surface  13 , a second side surface  14 , and a third side surface  15 . Each of first side surface  13  and second side surface  14  is constituted of a flat surface orthogonal to the Y axis direction. First side surface  13  and second side surface  14  are oriented oppositely in the Y axis direction. Each of first side surface  13  and second side surface  14  has the largest area among the areas of the plurality of side surfaces of exterior package  12 . Each of first side surface  13  and second side surface  14  has a rectangular shape when viewed in the Y axis direction. Each of first side surface  13  and second side surface  14  has a rectangular shape in which the X axis direction corresponds to the long-side direction and the Z axis direction corresponds to the short-side direction when viewed in the Y axis direction. Third side surface  15  is constituted of a flat surface orthogonal to the Z axis direction. Third side surface  15  is oriented in the +Z axis direction. 
     Battery cell  11  further includes a gas-discharge valve  17 . Gas-discharge valve  17  is provided in third side surface  15 . When internal pressure of exterior package  12  becomes more than or equal to a predetermined value due to gas generated inside exterior package  12 , gas-discharge valve  17  discharges the gas to the outside of exterior package  12 . The gas from gas-discharge valve  17  flows through exhaust duct  93  in  FIG.  1    and is discharged to the outside of battery module  100 . 
     Battery cell  11  further has electrode terminals  16  including a pair of a positive electrode terminal  16 P and a negative electrode terminal  16 N. Each of electrode terminals  16  is composed of a metal. Electrode terminal  16  is provided on third side surface  15 . Positive electrode terminal  16 P and negative electrode terminal  16 N are provided to be separated from each other in the X axis direction. Positive electrode terminal  16 P and negative electrode terminal  16 N are provided beside wiring member  92  and exhaust duct  93  in the X axis direction. 
     The plurality of battery cells  11  are stacked such that first side surfaces  13  of battery cells  11  adjacent to each other in the Y axis direction face each other and second side surfaces  14  of battery cells  11  adjacent to each other in the Y axis direction face each other. Thus, positive electrode terminals  16 P and negative electrode terminals  16 N are alternately arranged in the Y axis direction in which the plurality of battery cells  11  are stacked. 
     It should be noted that when an odd number of battery cells  11  are included in battery cell unit  21 , the posture of battery cell unit  21  may be turned by 180° with respect to the Z axis between battery cell units  21  adjacent to each other in the Y axis direction. 
     Next, a connection structure for the plurality of battery cells  11  will be described. Referring to  FIGS.  1  to  5   , battery module  100  further has the plurality of bus bars  50 . Each of bus bars  50  is composed of an electric conductor. The plurality of bus bars  50  are provided to electrically connect the plurality of battery cells  11  of battery module  100  together. 
     Each of bus bars  50  extends in the Y axis direction. Bus bar  50  has ends that both extend in the Y axis direction and that are connected to battery cells  11 ,  11  adjacent to each other in the Y axis direction. Bus bar  50  is provided between battery cells  11 ,  11  adjacent to each other in the Y axis direction so as to connect positive electrode terminal  16 P and negative electrode terminal  16 N arranged side by side in the Y axis direction. The plurality of battery cells  11  are electrically connected together in series by the plurality of bus bars  50 . 
       FIG.  6    is a perspective view showing a connection structure for battery cells between adjacent battery cell units.  FIG.  7    is a cross sectional view showing the connection structure for the battery cells along a line VII-VII in  FIG.  6   .  FIG.  8    is a perspective view showing a connection structure for battery cells in a battery cell unit.  FIG.  9    is a cross sectional view showing the connection structure for the battery cells when viewed in a direction along a line IX-IX in  FIG.  8   . 
     Referring to  FIGS.  5  to  9   , the plurality of bus bars  50  include a plurality of first bus bars  51  and a plurality of second bus bars  52 . 
     As shown in  FIGS.  5  to  7   , first bus bar  51  has a first joining portion  66 . First joining portion  66  joins separated members to each other through a first joining form. First bus bar  51  connects, via first joining portion  66 , between battery cells  11 ,  11  adjacent to each other in the Y axis direction. 
     First bus bar  51  electrically connects between battery cells  11 ,  11  adjacent to each other in the Y axis direction between a first battery cell unit of the plurality of battery cell units  21  and a second battery cell unit of the plurality of battery cell units  21 , the second battery cell unit being adjacent to the first battery cell unit in the Y axis direction. For explanation in a range shown in  FIG.  6   , battery cell unit  21 B corresponds to the first battery cell unit, and battery cell unit  21 C corresponds to the second battery cell unit. Between battery cell units  21 B,  21 C, first bus bar  51  connects negative electrode terminal  16 N of battery cell  11   d  in battery cell unit  21 B to positive electrode terminal  16 P of battery cell  11   a  in battery cell unit  21 C. 
     As shown in  FIGS.  5 ,  8 , and  9   , second bus bar  52  has a second joining portion  69 . Second joining portion  69  joins separated members to each other through a second joining form different from the first joining form. Second bus bar  52  connects, via second joining portion  69 , between battery cells  11 ,  11  adjacent to each other in the Y axis direction. 
     In each battery cell unit  21 , second bus bar  52  electrically connects between battery cells  11 ,  11  adjacent to each other in the Y axis direction. For explanation in a range shown in  FIG.  8   , second bus bar  52  connects negative electrode terminal  16 N of battery cell  11   b  to positive electrode terminal  16 P of battery cell  11   c  in battery cell unit  21 C. 
     The following describes a more specific structure of first bus bar  51  while focusing on the range shown in  FIG.  6   . As shown in  FIGS.  6  and  7   , first bus bar  51  has an asymmetrical shape with respect to an X-Z-axes plane that constitutes a boundary between battery cell  11   d  in battery cell unit  21 B and battery cell  11   a  in battery cell unit  21 C. 
     First bus bar  51  has a first bus-bar-divided body  61  and a second bus-bar-divided body  62 . First bus-bar-divided body  61  and second bus-bar-divided body  62  are composed of electric conductors (metal plates) separated from each other. First bus-bar-divided body  61  is connected to negative electrode terminal  16 N of battery cell  11   d  in battery cell unit  21 B. Second bus-bar-divided body  62  is connected to positive electrode terminal  16 P of battery cell  11   a  in battery cell unit  21 C. First bus-bar-divided body  61  and second bus-bar-divided body  62  are joined to each other by first joining portion  66 . 
     First bus-bar-divided body  61  is provided at a position overlapping with battery cell  11   d  in battery cell unit  21 B when viewed in the Z axis direction. First bus-bar-divided body  61  has a first plate portion  77  and a second plate portion  78 . 
     First plate portion  77  has a plate shape having a thickness direction corresponding to the Z axis direction, and is disposed in parallel with an X-Y-axes plane. First plate portion  77  is placed on negative electrode terminal  16 N of battery cell  11   d  in battery cell unit  21 B in the Z axis direction. First plate portion  77  is joined to negative electrode terminal  16 N of battery cell  11   d  in battery cell unit  21 B by a welded portion  64 . Second plate portion  78  has a plate shape having a thickness direction corresponding to the X axis direction, and is disposed in parallel with a Y-Z-axes plane. Second plate portion  78  extends in the +Z axis direction from the end portion of first plate portion  77  in the −X axis direction. 
     When viewed in the Z axis direction, second bus-bar-divided body  62  is provided over a position overlapping with battery cell  11   a  in battery cell unit  21 C and a position overlapping with battery cell  11   d  in battery cell unit  21 B. Second bus-bar-divided body  62  has a third plate portion  71 , a fourth plate portion  72 , a fifth plate portion  73 , a sixth plate portion  74 , an elastic portion  75 , and a seventh plate portion  76 . 
     Third plate portion  71  has a plate shape having a thickness direction corresponding to the Z axis direction, and is disposed in parallel with the X-Y-axes plane. Third plate portion  71  is placed on positive electrode terminal  16 P of battery cell  11   a  in battery cell unit  21 C in the Z axis direction. Third plate portion  71  is joined to positive electrode terminal  16 P of battery cell  11   a  in battery cell unit  21 C by a welded portion  64 . Fourth plate portion  72  has a plate shape having a thickness direction corresponding to the X axis direction, and is disposed in parallel with the Y-Z-axes plane. Fourth plate portion  72  extends in the +Z axis direction from the end portion of third plate portion  71  in the −X axis direction. 
     Each of fifth plate portion  73  and sixth plate portion  74  has a plate shape having a thickness direction corresponding to the Z axis direction, and is disposed in parallel with the X-Y-axes plane. Fifth plate portion  73  extends in the −X axis direction from the end portion of fourth plate portion  72  in the +Z axis direction. Sixth plate portion  74  is provided at a position displaced from fifth plate portion  73  in the −Y axis direction. Fifth plate portion  73  is provided at a position overlapping with battery cell  11   a  in battery cell unit  21 C when viewed in the Z axis direction, and sixth plate portion  74  is provided at a position overlapping with battery cell  11   d  in battery cell unit  21 B when viewed in the Z axis direction. 
     Elastic portion  75  extends between fifth plate portion  73  and sixth plate portion  74 . When viewed in the Z axis direction, elastic portion  75  is provided at a position overlapping with a boundary between battery cell  11   a  in battery cell unit  21 C and battery cell  11   d  in battery cell unit  21 B. Elastic portion  75  extends in the −Z axis direction from the end portion of fifth plate portion  73  in the −Y axis direction, is curved to reverse in the Z axis direction, and further extends in the +Z axis direction so as to be contiguous to the end portion of sixth plate portion  74  in the +Y axis direction. Elastic portion  75  can be elastically deformed to change a distance between fifth plate portion  73  and sixth plate portion  74  in the Y axis direction. 
     Seventh plate portion  76  has a plate shape having a thickness direction corresponding to the X axis direction, and is disposed in parallel with the Y-Z-axes plane. Seventh plate portion  76  extends in the +Z axis direction from an end portion of sixth plate portion  74  in the +X axis direction. Seventh plate portion  76  is placed on second plate portion  78  in the X axis direction. Seventh plate portion  76  is in surface contact with second plate portion  78  in a plane parallel to the Y-Z-axes plane. 
     First joining portion  66  joins second plate portion  78  of first bus-bar-divided body  61  and seventh plate portion  76  of second bus-bar-divided body  62  to each other through fixation with a bolt, which serves as the first joining form. 
     The fixation with a bolt as the first joining form is means allowing for attaching and detaching between first bus-bar-divided body  61  and second bus-bar-divided body  62 , which are targets for joining. 
     First joining portion  66  has a bolt  67  and a nut  68 . Respective bolt holes  79  are formed in first bus-bar-divided body  61  (second plate portion  78 ) and second bus-bar-divided body  62  (seventh plate portion  76 ). Bolt holes  79  are constituted of through holes respectively extending through second plate portion  78  and seventh plate portion  76  in the X axis direction. Bolt  67  is inserted into bolt holes  79  and nut  68  is screwed into a threaded portion of bolt  67 , with the result that second plate portion  78  of first bus-bar-divided body  61  and seventh plate portion  76  of second bus-bar-divided body  62  are fastened to each other. 
     A direction in which first bus-bar-divided body  61  (second plate portion  78 ) and second bus-bar-divided body  62  (seventh plate portion  76 ) are joined to each other is the X axis direction orthogonal to the Y axis direction in which the plurality of battery cells  11  are stacked. 
     Next, the following describes a more specific structure of second bus bar  52  while focusing on the range shown in  FIG.  8   . As shown in  FIGS.  8  and  9   , second bus bar  52  has a symmetrical shape with respect to the X-Z-axes plane that constitutes a boundary between battery cell  11   b  in battery cell unit  21 C and battery cell  11   c  in battery cell unit  21 C. 
     Second bus bar  52  is composed of an electric conductor (metal plate) in one piece. When viewed in the Z axis direction, second bus bar  52  is provided over a position overlapping with battery cell  11   b  in battery cell unit  21 C and a position overlapping with battery cell  11   c  in battery cell unit  21 C. Second bus bar  52  is joined to each of negative electrode terminal  16 N of battery cell  11   b  and positive electrode terminal  16 P of battery cell  11   c  by second joining portion  69 . 
     Second bus bar  52  has a first plate portion  81 , a second plate portion  82 , a third plate portion  83 , a fourth plate portion  84 , an elastic portion  85 , a fifth plate portion  86 , and a sixth plate portion  87 . 
     First plate portion  81 , second plate portion  82 , third plate portion  83 , fourth plate portion  84 , and elastic portion  85  have shapes respectively corresponding to third plate portion  71 , fourth plate portion  72 , fifth plate portion  73 , sixth plate portion  74 , and elastic portion  75  in second bus-bar-divided body  62  of first bus bar  51 . First plate portion  81  is placed on positive electrode terminal  16 P of battery cell  11   c  in battery cell unit  21 C in the Z axis direction. First plate portion  81  is in surface contact with positive electrode terminal  16 P of battery cell  11   c  in battery cell unit  21 C in a plane parallel to the X-Y-axes plane. 
     Sixth plate portion  87  has a plate shape having a thickness direction corresponding to the Z axis direction, and is disposed in parallel with the X-Y-axes plane. Sixth plate portion  87  is placed on negative electrode terminal  16 N of battery cell  11   b  in battery cell unit  21 C in the Z axis direction. Sixth plate portion  87  is in surface contact with negative electrode terminal  16 N of battery cell  11   b  in battery cell unit  21 C in a plane parallel to the X-Y-axes plane. Fifth plate portion  86  has a plate shape having a thickness direction corresponding to the X axis direction, and is disposed in parallel with the Y-Z-axes plane. Fifth plate portion  86  extends in the +Z axis direction from the end portion of sixth plate portion  87  in the −X axis direction and is contiguous to the end portion of fourth plate portion  84  in the +X axis direction. 
     First plate portion  81  and positive electrode terminal  16 P of battery cell  11   c  are composed of electric conductors (metal plates) separated from each other Sixth plate portion  87  and negative electrode terminal  16 N of battery cell  11   b  are composed of electric conductors (metal plates) separated from each other. Second joining portion  69  joins first plate portion  81  to positive electrode terminal  16 P of battery cell  11   c , and joins sixth plate portion  87  to negative electrode terminal  16 N of battery cell  11   b  through welding, which serves as the second joining form. 
     The welding as the second joining form is means not allowing for attaching and detaching between second bus bar  52  and electrode terminal  16 , which are targets for joining. 
     Second joining portion  69  has a welded portion  70 . Welded portion  70  is formed by joining second bus bar  52  and electrode terminal  16  to each other through welding such as laser welding. Welded portion  70  is a portion formed in the following manner: at the time of welding, second bus bar  52  and electrode terminal  16  are melted and are then solidified into one piece with each other. Welded portion  70  extends through second bus bar  52  in the Z axis direction and has a bottom portion inside electrode terminal  16 . 
     It should be noted that the shape of welded portion  70  is not particularly limited, and for example, welded portion  70  may be provided to extend in the form of a line when viewed in the Z axis direction, or may be provided at a plurality of positions separated from each other. Further, the second joining form used for second joining portion  69  is not limited to welding, and may be fixation with a rivet, for example. 
     For a battery module, there has been a demand to adjust the number of battery cells included in the battery module in accordance with a manner of use or required performance of the battery module. There has been also a demand to detach a specific battery cell from the battery module or replace a battery cell at the time of maintenance of the battery module. Further, when all the connections among the plurality of battery cells by bus bars are made through welding, a large-sized welding machine that handles the maximum number of battery cells needs to be prepared as a battery module manufacturing facility, and a battery cell cannot be readily attached or detached at the time of maintenance of the battery module, disadvantageously. 
     To address this, in battery module  100  of the present embodiment, at second bus bar  52  that connects between battery cells  11 ,  11  in battery cell unit  21 , welding is used to join second bus bar  52  and battery cell  11  to each other by second joining portion  69 , whereas at first bus bar  51  that connects between respective battery cells  11 ,  11  of battery cell units  21 ,  21  arranged side by side in the Y axis direction, fixation with a bolt is used to join first bus-bar-divided body  61  and second bus-bar-divided body  62  to each other by first joining portion  66 . 
     According to such a configuration, by an operation of fastening bolt  67  at first joining portion  66 , first bus-bar-divided body  61  and second bus-bar-divided body  62  are joined to each other so as to connect battery cell units  21 ,  21  arranged side by side in the Y axis direction, whereas by an operation of loosening bolt  67  at first joining portion  66 , first bus-bar-divided body  61  and second bus-bar-divided body  62  are disjoined from each other so as to disconnect battery cell units  21 ,  21  arranged side by side in the Y axis direction. Thus, the connecting and disconnecting between battery cell units  21 ,  21  arranged side by side in the Y axis direction can be performed by such simple operations, so that the number of battery cells  11  included in battery module  100  can be readily adjusted by increasing or decreasing the number of battery cell units  21 . Further, the operation of attaching or detaching a specific battery cell unit  21  to or from battery module  100  can be readily performed for the same reason, thus resulting in improved maintainability of battery module  100 . 
     As shown in  FIG.  3   , at the time of manufacturing battery module  100 , the plurality of battery cell units  21  are first assembled. 
     In this step, welding is used to connect first bus-bar-divided body  61  to negative electrode terminal  16 N of battery cell  11   d  disposed at the end portion in the +Y axis direction. By the welding, each second bus bar  52  is connected to positive electrode terminal  16 P and negative electrode terminal  16 N of battery cells  11 ,  11  arranged side by side in the Y axis direction. By the welding, second bus-bar-divided body  62  is connected to positive electrode terminal  16 P of battery cell  11   a  disposed at the end portion in the −Y axis direction. 
     Next, as shown in  FIG.  1   , the plurality of battery cell units  21  arranged side by side in the Y axis direction are collectively held by restraint member  41 , and first bus-bar-divided body  61  and second bus-bar-divided body  62  are fastened to each other using bolt  67  between battery cell units  21  adjacent to each other in the Y axis direction. 
     In this case, the size of the welding machine for connecting bus bars  50  to battery cells  11  is determined by the number of battery cells  11  included in battery cell units  21 , rather than the number of battery cells  11  (battery cell units  21 ) included in battery module  100 . Therefore, it is not necessary to prepare a large-sized welding machine, with the result that a facility for manufacturing battery module  100  can be constructed readily and inexpensively. 
     As shown in  FIGS.  6  to  9   , the body size of second bus bar  52  is smaller than the body size of first bus bar  51  for which the fixation with a bolt is used to join first bus-bar-divided body  61  and second bus-bar-divided body  62  to each other by first joining portion  66 . According to such a configuration, a wide space can be secured around second bus bar  52  just above battery cell unit  21 . 
     Further, the electric resistance of second bus bar  52  is smaller than the electric resistance of first bus bar  51  for which the fixation with a bolt is used to join first bus-bar-divided body  61  and second bus-bar-divided body  62  to each other by first joining portion  66 . According to such a configuration, energy loss in second bus bar  52  can be suppressed to be low. 
       FIG.  10    is a perspective view showing a modification of the first bus bar shown in  FIG.  6   . Referring to  FIG.  10   , first bus bar  51  in the present modification has a symmetrical shape with respect to the X-Z-axes plane that constitutes a boundary between battery cell  11   d  in battery cell unit  21 B and battery cell  11   a  in battery cell unit  21 C. 
     Second bus-bar-divided body  62  is provided at a position overlapping with battery cell  11   a  in battery cell unit  21 C when viewed in the Z axis direction. Second bus-bar-divided body  62  has a first plate portion  121 , a second plate portion  122 , a third plate portion  123 , and a fourth plate portion  125 . 
     First plate portion  121 , second plate portion  122 , and third plate portion  123  have shapes respectively corresponding to first plate portion  81 , second plate portion  82 , and third plate portion  83  in  FIG.  8   . Fourth plate portion  125  has a plate shape having a thickness direction corresponding to the Y axis direction, and is disposed in parallel with the X-Z-axes plane. Fourth plate portion  125  extends in the +Z axis direction from the end portion of third plate portion  123  in the −Y axis direction. 
     First bus-bar-divided body  61  is provided at a position overlapping with battery cell  11   d  in battery cell unit  21 B when viewed in the Z axis direction. First bus-bar-divided body  61  has a fifth plate portion  128 , a sixth plate portion  127 , a seventh plate portion  124 , and an eighth plate portion  126 . 
     Fifth plate portion  128 , sixth plate portion  127 , seventh plate portion  124 , and eighth plate portion  126  respectively have shapes symmetrical to first plate portion  121 , second plate portion  122 , third plate portion  123 , and fourth plate portion  125  with respect to the X-Z-axes plane that constitutes the boundary between battery cell  11   d  in battery cell unit  21 B and battery cell  11   a  in battery cell unit  21 C. 
     Fourth plate portion  125  and eighth plate portion  126  overlap with each other in the Y axis direction. Fourth plate portion  125  and eighth plate portion  126  are provided at positions overlapping the boundary between battery cell  11   d  in battery cell unit  21 B and battery cell  11   a  in battery cell unit  21 C when viewed in the Z axis direction. Fourth plate portion  125  and eighth plate portion  126  are in surface contact with each other in a plane parallel to the X-Z-axes plane. 
     First joining portion  66  joins eighth plate portion  126  of first bus-bar-divided body  61  and fourth plate portion  125  of second bus-bar-divided body  62  to each other by fixation with a bolt, which serves as the first joining form. A direction in which first bus-bar-divided body  61  (eighth plate portion  126 ) and second bus-bar-divided body  62  (fourth plate portion  125 ) are joined to each other is parallel to the Y axis direction in which the plurality of battery cells  11  are stacked. 
     It should be noted that the shape of first bus bar  51  is not particularly limited. For example, in first bus bar  51  in  FIG.  10   , eighth plate portion  126  may be directly connected to fifth plate portion  128  not via sixth plate portion  127  and seventh plate portion  124 , and fourth plate portion  125  may be directly connected to first plate portion  121  not via second plate portion  122  and third plate portion  123 . 
       FIGS.  11  to  13    are diagrams showing modifications of the first joining form at the first joining portion shown in  FIG.  10   . 
     Referring to  FIG.  11   , in the present modification, first joining portion  66  joins first bus-bar-divided body  61  (eighth plate portion  126 ) and second bus-bar-divided body  62  (fourth plate portion  125 ) to each other by fixation with a pin, which serves as the first joining form. 
     First joining portion  66  has a pin  131 . First bus-bar-divided body  61  (eighth plate portion  126 ) and second bus-bar-divided body  62  (fourth plate portion  125 ) are provided with pin holes  136 . Pin holes  136  are constituted of through holes respectively extending through eighth plate portion  126  and fourth plate portion  125  in the Y axis direction. Pin  131  is press-fitted into pin holes  136 . 
     Referring to  FIG.  12   , in the present modification, first joining portion  66  joins first bus-bar-divided body  61  (eighth plate portion  126 ) and second bus-bar-divided body  62  (fourth plate portion  125 ) to each other by fixation with a clip, which serves as the first joining form. First joining portion  66  has a clip  141 . Clip  141  sandwiches eighth plate portion  126  and fourth plate portion  125  in the Y axis direction. 
     Referring to  FIG.  13   , in the present modification, first joining portion  66  joins first bus-bar-divided body  61  (eighth plate portion  126 ) and second bus-bar-divided body  62  (fourth plate portion  125 ) to each other by fixation with a band, which serves as the first joining form. First joining portion  66  has a band  142 . Band  142  is provided to surround eighth plate portion  126  and fourth plate portion  125  in a plane parallel to the X-Y-axes plane. 
     Each of the fixations with pin, clip and band as the first joining form described in the modifications shown in  FIGS.  11  to  13    is means allowing for attaching and detaching between first bus-bar-divided body  61  and second bus-bar-divided body  62 , which are targets for joining. 
       FIG.  14    is a perspective view showing a modification of the battery module in  FIG.  1   . Referring to  FIG.  14   , end plate  42 P faces, in the Y axis direction, battery cell unit  21 A (third battery cell unit) of the plurality of battery cell units  21 . Battery cell  21   a  of battery cell unit  21 A is disposed adjacent to end plate  42 P in the Y axis direction. 
     The battery module according to the present modification further has a third bus bar  53 . Third bus bar  53  is provided at battery cell  21   a  in battery cell unit  21 A. Third bus bar  53  is connected to electrode terminal  16  (positive electrode terminal  16 P) of battery cell  21   a  in battery cell unit  21 A. Third bus bar  53  has the same shape as that of second bus-bar-divided body  62  of first bus bar  51  shown in  FIG.  6   . 
     According to such a configuration, bolt hole  79  shown in  FIG.  7    is formed in third bus bar  53  having the same shape as that of second bus-bar-divided body  62  of first bus bar  51 . Therefore, a bolt  153  can be used to connect third bus bar  53  to a terminal  152  of a cable  151  serving as an external wiring. Thus, collective terminal  91  can be omitted, thereby reducing the number of components of the battery module. 
     It should be noted that since third bus bar  53  having the same shape as that of first bus-bar-divided body  61  of first bus bar  51  is provided in battery cell  11   d  of battery cell unit  21 F facing end plate  41 Q in  FIG.  1   , cable  151  can be directly fixed to third bus bar  53  with a bolt. 
     The above-described structure of battery module  100  according to the embodiment of the present invention is summarized as follows: battery module  100  according to the present embodiment includes: the plurality of battery cells  11  stacked in the Y axis direction serving as the predetermined direction; restraint member  41  that applies, to the plurality of battery cells  11 , restraint force along the Y axis direction; and the plurality of bus bars  50  that electrically connect the plurality of battery cells  11  together, the plurality of bus bars  50  including first bus bar  51  and second bus bar  52 . First bus bar  51  has first joining portion  66  that joins separated members to each other through the first joining form, and connects, via first joining portion  66 , between battery cells  11  adjacent to each other in the Y axis direction. Second bus bar  52  has second joining portion  69  that joins separated members to each other through the second joining form different from the first joining form, and connects, via second joining portion  69 , between battery cells  11  adjacent to each other in the Y axis direction. 
     According to battery module  100  of the embodiment of the present invention thus configured, there can be realized battery module  100  in which the number of battery cells  11  can be readily adjusted by such a configuration that bus bars  50  that electrically connect the plurality of battery cells  11  together includes first bus bar  51  and second bus bar  52  for which the first joining form and the second joining form, which are different from each other, are respectively used. 
     Although the embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.