Patent Publication Number: US-2022223943-A1

Title: Battery module, electric power unit, and working machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of International Patent Application No. PCT/JP2019/039153 filed on Oct. 3, 2019, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a battery module, an electric power unit, and a working machine. 
     Description of the Related Art 
     Japanese Patent No. 3742261 discloses an arrangement of a battery pack with a plurality of cells arranged therein, in which an outer case is provided with a suction port and an exhaust port, and a ventilation passage is formed where cooling air entering from the suction port passes around and/or between the plurality of cells and is exhausted from the exhaust port. In the arrangement described in Japanese Patent No. 3742261, a plurality of parts such as ribs for preventing rattling of the plurality of cells accommodated in the outer case, heat dissipation plates (formed with a plurality of fins) for efficiently cooling the outer cells, and flow straightening plates for guiding air to the exhaust port are attached to the plurality of cells. 
     In the arrangement described in Japanese Patent No. 3742261, the plurality of parts attached to the plurality of cells, such as the ribs, the heat dissipation plates, and the flow straightening plates, are independently formed (created). That is, with this arrangement, the number of parts to be independently formed increases. This complicates the device arrangement and can be disadvantageous in terms of device cost. 
     SUMMARY OF THE INVENTION 
     The present invention has as its object to provide a battery module that can efficiently cool a plurality of battery cells and is advantageous in terms of simplification of the device arrangement and the device cost. 
     According to one aspect of the present invention, there is provided a battery module comprising: a cell assembly including a plurality of battery cells arrayed in a second direction different from a first direction while each cell axis is directed in the first direction; and an accommodation case configured to accommodate the cell assembly, wherein in the accommodation case, a suction hole configured to take a gas into a peripheral space located on a side of the cell assembly in the first direction and an exhaust hole configured to exhaust the gas having passed between the plurality of battery cells are provided, and a rib extending in the second direction is provided in the peripheral space, wherein the cell assembly includes a holding member configured to hold the plurality of battery cells, wherein the holding member is formed by two members that sandwich the plurality of battery cells in the first direction, and wherein a member located on a side of the peripheral space out of the two members is provided with a plurality of openings configured to guide the gas in the peripheral space to gaps between the plurality of battery cells. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing an arrangement example of a working machine. 
         FIG. 2  is an external view (top perspective view) of a battery module. 
         FIG. 3  is an external view (top view) of the battery module. 
         FIG. 4  is an external view (bottom perspective view) of the battery module. 
         FIG. 5  is an external view (bottom view) of the battery module. 
         FIG. 6  is an exploded view of the battery module. 
         FIG. 7  is a view of a lower member of an accommodation case when viewed from above (a state in which cell assemblies are not accommodated). 
         FIG. 8  is a view of the lower member of the accommodation case when viewed from above (a state in which the cell assemblies are accommodated). 
         FIG. 9  is an enlarged view of a region R in  FIG. 8 . 
         FIG. 10  is a view showing the flow of a gas in the accommodation case. 
         FIG. 11  is a view showing the cell assembly and the lower member of the accommodation case. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. Note that the drawings are schematic views showing a structure or an arrangement according to the embodiment, and the dimensions of members shown in the drawings do not necessarily reflect real dimensions. The same reference numerals denote the same elements in the drawing, and a description of repetitive contents will be omitted in this specification. In each of the following drawings, the directions orthogonal to each other on a plane parallel to the horizontal direction are the X direction and the Y direction, and the direction parallel to the vertical direction is the Z direction. 
     First Embodiment 
     A working machine  1  of the first embodiment according to the present invention will be described.  FIG. 1  is a schematic view showing an arrangement example of the working machine  1 . The working machine  1  of this embodiment is a working machine (electric working machine) including an electric power unit  10  that includes a battery module  100  and a motor device  11 . Examples of the working machine  1  include a plate compactor, a rammer, a lawn mower, a cultivator, and a snow remover, and  FIG. 1  illustrates a plate compactor. The working machine  1  includes, for example, the electric power unit  10 , a working unit  20  (working mechanism), a power transmission mechanism  30 , and a steering handle  40 . The working unit  20  is a unit for performing predetermined work, and in this embodiment, it is a unit that performs rolling compaction work to compact the ground. 
     The electric power unit  10  includes, for example, the battery module  100  and the motor device  11 . The battery module  100  is a storage battery including a plurality of battery cells, and can be configured to be attachable/detachable to/from the motor device  11 . The specific arrangement of the battery module  100  will be described later. The motor device  11  can include a motor  11   a  that is operated by electric power from the battery module  100 , and a control unit (not shown) that controls driving of the motor. The control unit can be a PDU (Power Drive Unit), but may be configured to include a processor represented by a CPU, a storage device such as a semiconductor memory, an interface with an external device, and the like. 
     The motor device  11  is provided with a cooling fan  11   b  as an exhaust unit for exhausting the gas in an accommodation case  120  from exhaust holes formed in the accommodation case  120  of the battery module  100 . The cooling fan  11   b  may also be used for cooling the motor  11   a , or may be provided separately from a component for cooling the motor  11   a . In this embodiment, the cooling fan  11   b  is rotatably attached to the shaft member of the motor  11   a , and configured to rotate together with the shaft member of the motor  11   a , thereby drawing the gas in the accommodation case  120  of the battery module  100  and exhausting the gas in the accommodation case  120  from the exhaust holes. 
     Next, the arrangement of the battery module  100  of this embodiment will be described. Each of  FIGS. 2 to 5  is an external view of the battery module  100 .  FIG. 2  shows a top perspective view of the battery module  100 ,  FIG. 3  shows a top view of the battery module  100 ,  FIG. 4  shows a bottom perspective view of the battery module  100 , and  FIG. 5  shows a bottom view of the battery module  100 .  FIG. 6  is an exploded view of the battery module  100 . 
     As shown in  FIG. 6 , the battery module  100  can include, for example, cell assemblies  110  ( 110   a ,  110   b ) each including a plurality of cylindrical battery cells  111 , the accommodation case  120  for accommodating the cell assemblies  110 , a circuit board  131  on which a control circuit for controlling charge/discharge of the plurality of battery cells  111  is formed, and a connector  132  as an external interface. The circuit board  131  is arranged on the cell assemblies  110 , and is electrically connected to the cell assemblies  110  (the plurality of battery cells  111 ) and the connector  132  via a cable  133 . The connector  132  is arranged in a connector housing  124  provided in the accommodation case  120 . 
     As shown in  FIG. 6 , each cell assembly  110  includes the plurality of battery cells  111  arrayed with the cell axes directed in one direction, and a holding portion  112  that holds the plurality of battery cells  111 .  FIG. 6  shows an arrangement example of the battery module  100  in which a plurality (two) of cell assemblies  110   a  and  110   b  having the same shape are symmetrically accommodated in the accommodation case  120 .  FIG. 6  shows the left cell assembly  110   a  in a state in which the plurality of battery cells  111  are held by the holding portion  112 , and the right cell assembly  110   b  in a state before the plurality of battery cells  111  are held by the holding portion  112 . 
     Each of the plurality of battery cells  111  has a columnar (cylindrical) shape, and the plurality of battery cells  111  are arrayed in a plurality of rows (ten rows) in the X direction and a plurality of stages (four stages) in the Z direction (second direction) while each cell axis (column axis) is directed in the Y direction (first direction). In this embodiment, the direction in which the cell axis of each battery cell  111  is directed is the Y direction (horizontal direction), but it is not limited to the Y direction (horizontal direction) as long as the cell axes of the respective battery cells  111  are directed in the same direction. Note that in the following description, the direction in which the cell axes of the plurality of battery cells  111  are directed may be referred to as the “first direction”. 
     The holding portion  112  includes a pair of members (a first holding member  112   a  and a second holding member  112   b ) as frame bodies formed with a plurality of insertion ports  113  into which the plurality of battery cells  111  are respectively inserted. The first holding member  112   a  is located on the outer side of the holding portion  112  in the first direction (Y direction) (on the side of a peripheral space  142  to be described later), and the second holding member  112   b  is located on the inner side of the holding portion  112  in the first direction (on the side of the other cell assembly  110 ). In the first direction (Y direction) in which the cell axis of each battery cell  111  is directed, the first holding member  112   a  and the second holding member  112   b  sandwich the plurality of battery cells  111  such that each battery cell  111  is inserted into each insertion port  113 , and in this state, the first holding member  112   a  and the second holding member  112   b  are fixed to each other using fixing members such as screws. Thus, the holding portion  112  can hold the plurality of battery cells  111 . 
     The accommodation case  120  is configured to include two surfaces (for example, an upper surface and a lower surface) that sandwich the cell assemblies  110  in a direction different from the first direction (Y direction) in which the cell axis of each battery cell  111  is directed, more specifically, in the second direction (Z direction) which is a direction perpendicular to the first direction. In this embodiment, as shown in  FIGS. 2 to 6 , the accommodation case  120  includes an upper member  121  including the upper surface and a lower member  122  including the lower surface. A handle member  123  is attached to an upper portion of the upper member  121  using fixing members such as screws. The plurality of cell assemblies  110  are inserted inside the lower member  122  and fixed thereto using fixing members such as screws. Then, the upper member  121  and the lower member  122  are stacked on each other and fixed to each other using fixing members such as screws. 
     Further, as shown in  FIGS. 4 and 5 , suction holes  125  for taking a gas (air) for cooling the plurality of battery cells  111  into the accommodation case  120  and exhaust holes  126  for exhausting the gas having passed between the plurality of battery cells  111  are provided in the lower member  122  (the lower surface of the accommodation case  120 ). Since such the suction holes  125  and exhaust holes  126  are provided in the lower member  122 , when the battery module  100  is attached to the motor device  11 , the cooling fan  11   b  of the motor device  11  can draw the gas in the accommodation case  120  from the exhaust holes  126  of the accommodation case  120  so that the gas can be circulated in the accommodation case  120 . Here, both the suction holes  125  and the exhaust holes  126  may be provided in the upper surface (upper member  121 ) of the accommodation case  120 , but it is preferable to provide them in the lower surface (lower member  122 ) of the accommodation case  120  in terms of preventing rain and the like from entering the inside of the accommodation case  120 . 
     Each of  FIGS. 7 and 8  is a view of the lower member  122  of the accommodation case  120  when viewed from above (+Z direction).  FIG. 7  is a view of the lower member  122  when viewed from above in a state in which the cell assemblies  110   a  and  110   b  are not accommodated.  FIG. 8  is a view of the lower member  122  when viewed from above in a state in which the cell assemblies  110   a  and  110   b  are accommodated.  FIG. 9  is an enlarged view of a region R in  FIG. 8  when viewed from obliquely above as indicated by an arrow B. 
     The accommodation case  120  is configured to include an accommodation space  141  in which the cell assembly  110  is to be accommodated (arranged), and the peripheral space  142  located on the side of the cell assembly  110  in the first direction (Y direction). The suction holes  125  are provided in a surface included in the lower surface of the accommodation case  120  and defining the peripheral space  142 , and the exhaust holes  126  are provided in a surface included in the lower surface of the accommodation case  120  and defining the accommodation space  141 . More specifically, the lower surface of the accommodation case  120  (lower member  122 ) can include a mounting surface  122   a , as the surface defining the accommodation space  141 , on which the cell assembly  110  is mounted, and an inclined surface  122   b , as the surface defining the peripheral space  142 , which is inclined with respect to the mounting surface  122   a  on the side of the mounting surface  122   a  in the first direction. A plurality of the exhaust holes  126  arrayed along the X direction are provided in the mounting surface  122   a , and a plurality of the suction holes  125  arrayed along the X direction are provided in the inclined surface  122   b . Since the plurality of the suction holes  125  are provided in the inclined surface  122   b , when the battery module  100  is attached above the member (for example, the motor device  11 ) including the cooling fan  11   b , it is possible to arrange the plurality of the suction holes  125  so as to be spaced apart from the member. Thus, it is possible to prevent entry of rain and the like as described above without decreasing the suction efficiency. 
     Here, as shown in  FIG. 9 , the accommodation case  120  includes guard members  128 , each of which extends in the longitudinal direction of the suction hole  125  and is provided inside the accommodation case  120  with respect to the suction hole  125 . That is, the guard member  128  is a member that covers the suction hole  125  so as not to block the suction hole  125  in the inside of the accommodation case  120 . By providing the guard member  128  to the suction hole  125  in this manner, the flow path that passes through the suction hole  125  can be formed in a labyrinth structure, so that it is possible to reduce entry of foreign substances such as wires into the accommodation case  120  from the suction hole  125 . Similar to each suction hole  125 , each exhaust hole  126  is provided with the guard member  129  so that a labyrinth structure can be formed. 
     Further, as shown in  FIGS. 6 to 9 , the accommodation case  120  includes, in the peripheral space  142 , a plurality of ribs  127  each extending in the second direction (Z direction) which is the array direction (stacking direction) of the plurality of battery cells  111 . The plurality of ribs  127  can be provided to reinforce the accommodation case  120 , but in this embodiment, they are arranged as baffle plates for guiding the gas taken into the peripheral space  142  from the suction holes  125  to the second direction (Z direction). For example, each of the plurality of ribs  127  is connected to a side surface  122   c , that connects the upper surface and the lower surface of the accommodation case  120 , and the lower surface (inclined surface  122   b ) of the accommodation case  120 , and has a plate shape parallel to the first direction (Y direction) and the second direction (Z direction). Further, the plurality of ribs  127  are provided so as to be spaced apart from each other along the X direction such that at least one suction hole  125  is arranged therebetween. 
     By arranging the plurality of ribs  127  as baffle plates as described above, the gas taken into the peripheral space  142  from the plurality of suction holes  125  can be efficiently guided to the second direction, so that the plurality of battery cells  111  in the cell assembly  110  can be efficiently cooled. In addition, since the number of parts of the battery module  100  is reduced by making the plurality of ribs  127  function as the baffle plates, the device arrangement can be simplified and the device cost can be decreased. 
     Next, the flow of the gas in the accommodation case  120  will be described.  FIG. 10  is a perspective view of the Y-Z section (the section taken along A-Ain  FIGS. 3 and 5 ) of the battery module  100 . In  FIG. 10 , the flow of the gas is indicated by arrows, and the size of the arrow represents the flow rate of the gas. Note that in the actual battery module  100 , the circuit board  131  is arranged on the cell assemblies  110   a  and  110   b  as described above, but the circuit board  131  is not shown in  FIG. 10  for clarity. 
     The gas taken into the peripheral space  142  from the plurality of suction holes  125  is guided to the second direction (+Z direction) toward the upper surface by the plurality of ribs  127  serving as the baffle plates. The gas guided by the plurality of ribs  127  in the peripheral space  142  is guided to gaps between the plurality of battery cells  111  (into the accommodation space  141 ) via a plurality of openings  114  ( 114   a ,  114   b ) formed in the first holding member  112   a  of the cell assembly  110 , and exhausted from the plurality of exhaust holes  126 . In this manner, in the battery module  100  of this embodiment, the suction holes  125  and the exhaust holes  126  are provided in the lower surface of the accommodation case  120  and airflows in opposite directions are generated in the accommodation space  141  and the peripheral space  142 . Thus, the plurality of battery cells  111  arrayed in the accommodation space  141  can be efficiently cooled. 
     Here, the plurality of openings  114  formed in the first holding member  112   a  will be described.  FIG. 11  is a view showing a state in which the cell assembly  110  ( 110   a ) is separated from the lower member  122  of the accommodation case  120  when viewed from obliquely below on the peripheral space  142  side. As shown in  FIGS. 6 and 11 , a plurality of first openings  114   a  and a plurality of second openings  114   b  for guiding the gas from the peripheral space  142  to gaps between the plurality of battery cells  111  are formed in the first holding member  112   a  of this embodiment. Each of the plurality of first openings  114   a  has a shape (opening) larger than each of the plurality of second openings  114   b , and is provided on the downstream side of the gas in the peripheral space  142 , specifically, above the plurality of battery cells  111  (the plurality of insertion holes  113 ), more specifically, above the top battery cells  111 . Further, the plurality of second openings  114   b  are provided on the upstream side of the gas in the peripheral space  142 , specifically, between the plurality of battery cells  111  (the plurality of insertion ports  113 ). In the example shown in  FIG. 11 , the second openings  114   b  are provided at all positions between the plurality of battery cells  111 , but the present invention is not limited to this, and they may be provided at some positions between the plurality of battery cells  111 . 
     By forming such the first openings  114   a  and the second openings  114   b  in the first holding member  112   a , as shown in  FIG. 10 , the gas guided to the second direction (+Z direction) by the plurality of ribs  127  in the peripheral space  142  can be efficiently guided to the first direction (Y direction), that is, to gaps between the plurality of battery cells  111 . For example, the gas guided from the first openings  114   a  of the first holding member  112   a  to gaps between the plurality of battery cells  111  is warmed by the battery cells  111  arranged in the upper portion of the cell assembly  110  before being exhausted from the exhaust holes  126 , so that the battery cells  111  arranged in the lower portion of the cell assembly  110  can be insufficiently cooled. In this embodiment, the second openings  114   b  are provided in the intermediate positions of the first holding member  112   a  in the second direction (+Z direction) to guide the gas from the second openings  114   b  to gaps between the plurality of battery cells  111 , so that the battery cells  111  arranged in the lower portion of the cell assembly  110  can be efficiently cooled. On the other hand, when the second openings  114   b  are provided, the flow rate of the gas guided from the first openings  114   a  to gaps between the plurality of battery cells  111  can decrease. In this embodiment, since the second opening  114   b  has the smaller shape (opening) than the first opening  114   a , the flow rate of the gas guided from the first openings  114   a  to gaps between the plurality of battery cells  111  can be secured. In addition, since the first openings  114   a  and the second openings  114   b  are formed in the frame body in which the insertion ports  113  for the battery cells  111  are formed, the gas guided to the first direction (+Y direction) can be efficiently guided to gaps between the plurality of battery cells  111 . 
     As has been described above, in the battery module  100  of this embodiment, the plurality of ribs  127  in the accommodation case  120  are arranged as the baffle plates for guiding the gas taken in from the suction holes  125  to the second direction (+Z direction) in the peripheral space  142 . With this arrangement, the gas (cooling air) taken in from the suction holes  125  is efficiently guided to the upper surface opposite to the lower surface provided with the suction holes  125 , and is exhausted from the exhaust holes  126  provided in the lower surface on the same side as the suction holes  125 . This can improve the cooling efficiently of the plurality of battery cells  111 . In addition, since the number of parts of the battery module  100  is reduced by making the ribs  127  function as the baffle plates, the device arrangement can be simplified and the device cost can be decreased. 
     OTHER EMBODIMENTS 
     In each of cell assemblies  110   a  and  110   b , a plurality of openings  114  (first openings  114   a  and second openings  114   b ) may be formed in a second holding member  112   b  arranged on the inner side in a battery module  100 , as in a first holding member  112   a . In this case, as shown in  FIG. 10 , it is also possible to guide the gas to a gap between the plurality of cell assemblies  110   a  and  110   b  (between a plurality of accommodation spaces  141 ) via the first openings  114   a  of the second holding member  112   b , and further guide the gas from the second openings  114   b  of the second holding member  112   b  to gaps between a plurality of battery cells  111 . This can further improve the cooling efficiency of the battery cells  111  of the plurality of cell assemblies  110   a  and  110   b.    
     In addition, in the battery module  100 , an insulating member (for example, insulating paper) may be provided on the surface of a cell assembly  110  (first holding member  112   a ) on the peripheral space  142  side. In this case, openings are formed in the insulating member at positions corresponding to the first openings  114   a  and the second openings  114   b  in the first holding member  112   a.    
     Summary of Embodiments 
     1. The battery module according to the above-described embodiment comprises 
     a cell assembly (for example,  110 ) including a plurality of battery cells (for example,  111 ) arrayed in a second direction different from a first direction while each cell axis is directed in the first direction, and 
     an accommodation case (for example,  120 ) configured to accommodate the cell assembly, 
     wherein in the accommodation case, a suction hole (for example,  125 ) configured to take a gas into a peripheral space (for example,  142 ) located on a side of the cell assembly in the first direction and an exhaust hole (for example,  126 ) configured to exhaust the gas having passed between the plurality of battery cells are provided in one surface of two surfaces that sandwich the cell assembly in the second direction, and a rib (for example,  127 ) extending in the second direction is provided in the peripheral space, and 
     the rib is arranged as a baffle plate configured to guide the gas taken in from the suction hole to the second direction in the peripheral space. 
     According to this arrangement, a gas taken in from the suction hole is efficiently guided to a surface opposite to the surface provided with the suction hole, and is exhausted from the exhaust hole (for example,  126 ) provided in a surface on the same side as the suction hole (for example,  125 ). This can improve the cooling efficiency of the plurality of battery cells. In addition, since the number of parts of the battery module is reduced by making the rib function as the baffle plate, the device arrangement can be simplified and the device cost can be decreased. 
     2. In the battery module according to the above-described embodiment, 
     a plurality of the ribs are provided in the accommodation case, and at least one suction hole is provided between the plurality of the ribs. 
     According to this arrangement, the gas taken into the accommodation case (into the peripheral space) from a plurality of the suction holes can be efficiently guided to the second direction by the plurality of ribs. 
     3. In the battery module according to the above-described embodiment, 
     the rib extends in the second direction from a surface (for example,  122   b ) of the accommodation case in which the suction hole is provided. 
     According to this arrangement, the gas taken into the accommodation case (into the peripheral space) from the suction hole can be efficiently guided to the second direction by the rib. 
     4. In the battery module according to the above-described embodiment, 
     the rib is connected to a side surface (for example,  122   c ) connecting the two surfaces in the accommodation case. 
     According to this arrangement, the rib serving as the baffle plate can be integrally formed with the accommodation case, so that the device arrangement can be simplified and the device cost can be decreased. 
     5. In the battery module according to the above-described embodiment, 
     the cell assembly includes a holding member (for example,  112 ) configured to hold the plurality of battery cells, 
     the holding member is formed by a pair of members (for example,  112   a  and  112   b ) that sandwich the plurality of battery cells in the first direction, and 
     a member (for example,  112   a ) located on a side of the peripheral space out of the two members is provided with a plurality of openings (for example,  114 ) configured to guide the gas guided by the rib in the peripheral space to gaps between the plurality of battery cells. 
     According to this arrangement, the gas guided by the rib in the peripheral space can be efficiently guided to gaps between the plurality of battery cells, so that the cooling efficiency of the plurality of battery cells can be improved. 
     6. In the battery module according to the above-described embodiment, 
     the plurality of openings include a plurality of first openings (for example,  114   a ) provided on a downstream side of the gas in the peripheral space, and a plurality of second openings (for example,  114   b ) provided on an upstream side of the gas in the peripheral space, and 
     each of the plurality of first openings is larger than each of the plurality of second openings. 
     According to this arrangement, the plurality of second openings are provided, so that not only the gas warmed by the battery cells arranged on the side of the first openings but also the gas in the peripheral space can be supplied, via the plurality of second openings, to the battery cells arranged on the side (exhaust hole side) opposite to the side of the first openings. In addition, each first opening is formed larger than each second opening, so that the flow rate of the gas guided from the first opening to gaps between the plurality of battery cells can be secured. That is, the plurality of battery cells can be efficiently cooled as a whole. 
     7. In the battery module according to the above-described embodiment, 
     the plurality of first openings and the plurality of second openings are formed in a frame body (for example,  112   a  or  112   b ) in which insertion ports for the plurality of battery cells are formed. 
     With this arrangement, the gas guided to the first direction can be efficiently guided to gaps between the plurality of battery cells. 
     8. In the battery module according to the above-described embodiment, 
     the accommodation case is configured to symmetrically accommodate a plurality of the cell assemblies, and 
     of the pair of members forming each one of the plurality of the cell assemblies, the member (for example,  112   b ) located on a side of the other cell assembly is provided with a plurality of openings configured to guide the gas to a gap between the plurality of the cell assemblies. 
     With this arrangement, the gas is supplied to the inner side of the accommodation case, so that the cooling efficiency of the battery cells of the plurality of the cell assemblies  110   a  and  110   b  can be further improved. 
     9. In the battery module according to the above-described embodiment, 
     the plurality of the cell assemblies have the same shape. 
     With this arrangement, the cooling difference between the plurality of the cell assemblies can be decreased, and the manufacturing cost (design cost) of the holding portion that holds the plurality of battery cells can be reduced. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.