Patent Publication Number: US-2013230761-A1

Title: Battery module and battery pack

Description:
TECHNICAL FIELD 
     The present invention relates to battery modules which include positive electrode bus bars and negative electrode bus bars and are electrically connected to each other in series, and battery packs. 
     BACKGROUND ART 
     In recent years, as power supplies for mobile units including electric motorbikes, all-electric vehicles (PEVs), hybrid electric vehicles (HEVs), etc., battery packs (assembled cells) in which a plurality of battery modules including a large number of cells are connected to each other in series or in parallel have been used. 
     In such a battery pack, as illustrated in Patent Document 1 and Patent Document 2, the battery modules are electrically and mechanically connected to each other by conductive members called bus bars. 
     For example, a positive electrode bus bar connected to a positive electrode current collector plate connected to positive electrode terminals of a plurality of cells included in one battery module is bonded to a negative electrode bus bar connected to a negative electrode current collector plate connected to negative electrode terminals of a plurality of cells included in another battery module, thereby connecting the two battery modules to each other in series (or in series parallel). This process is repeated predetermined number of times to form a battery pack, thereby obtaining a required output voltage. 
     In such a battery pack, in order to reduce resistance at a bonding section between the positive electrode bus bar and the negative electrode bus bar as much as possible, and to improve the physical strength of the bonding section, increasing the area of the bonding section is effective. Thus, the positive electrode bus bar and the negative electrode bus bar are formed as plate-like members, and the positive electrode bus bar is linearly bonded to the negative electrode bus bar. 
     CITATION LIST 
     Patent Document 
     
         
         PATENT DOCUMENT 1: Japanese Patent Publication No. 2004-152706 
         PATENT DOCUMENT 2: Japanese Patent Publication No. 2009-301982 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, various factors in a fabrication process of positive electrode bus bars and negative electrode bus bars and in an assembling process of battery modules by using the bus bars may lead to positional variations at a bonding section between the positive electrode bus bar and the negative electrode bus bar in a bonding process. When the variations occur, a gap, or the like is formed at the bonding section between the positive electrode bus bar and the negative electrode bus bar, and thus the linear bonding may not be successfully performed. 
     Moreover, the battery pack in which the positive electrode bus bar and the negative electrode bus bar of the battery modules are bonded to each other is, in many cases, used as a power supply for a mobile unit such as a HEV, and stress-induced strain may be caused at the bonding section between the positive electrode bus bar and the negative electrode bus bar due to vibration, or the like while the mobile unit is moving. When the stress-induced strain is caused at the bonding section, the battery pack may be broken. 
     In view of the above-discussed problems, the present invention was devised. It is an objective of the present invention to provide battery modules in which a gap at a bonding section between a positive electrode bus bar and a negative electrode bus bar is less likely to be formed in a bonding process, and stress-induced strain is less likely to be caused at the bonding section after the positive electrode bus bar is bonded to the negative electrode bus bar, and a battery pack formed by bonding the battery modules. 
     Solution to the Problem 
     A battery module according to a first aspect of the present invention includes: a plurality of cells arranged with a same polarity oriented in a same direction; a positive electrode bus bar by which positive electrode terminals of the cells are electrically connected to each other in parallel; and a negative electrode bus bar by which negative electrode terminals of the cells are electrically connected to each other in parallel, wherein the positive electrode bus bar has a flat plate-like extended portion which extends laterally to the cells from an edge of the positive electrode bus bar toward the negative electrode terminals of the cells, the negative electrode bus bar has a bent portion which is bent in a direction opposite to the positive electrode terminals of the cells from an edge of the negative electrode bus bar opposite to the extended portion, in the extended portion, a slit is formed at at least one of edges in a width direction perpendicular to an extension direction of the extended portion by cutting out a part of the extended portion in the width direction from the at least one edge toward the other edge, and when the battery module is electrically connected to another adjacent battery module in series to form a battery pack, a top edge of the extended portion of the positive electrode bus bar is bonded to a bent portion of a negative electrode bus bar of the another adjacent battery module. 
     With this configuration, a slit is formed in the flat plate-like extended portion of the positive electrode bus bar by cutting out a part of the extended portion in the width direction from an edge of the extended portion. Thus, in a bonding process in which the positive electrode bus bar is bonded to the negative electrode bus bar, even when a gap is formed at the bonding section due to positional displacement between the positive electrode bus bar and the negative electrode bus bar, the displacement is corrected when the slit formed in the positive electrode bus bar is narrowed due to deformation of the expanded portion by pressing one battery module and the other battery module against each other so that the battery modules are brought closer to each other, thereby eliminating the gap between the positive electrode bus bar and the negative electrode bus bar. 
     Moreover, the slit is formed in the flat plate-like extended portion of the positive electrode bus bar, and thus even when stress, such as torsion, is applied to the bonding section in a process after the positive electrode bus bar is bonded to the negative electrode bus bar, the extended portion warps to distribute the stress, so that stress-induced strain is less likely to be caused at the bonding section between the positive electrode bus bar and the negative electrode bus bar. 
     It is preferable that the slit include slits, and each of the slits be formed at a different one of the edges in the width direction of the extended portion at a different position in the extension direction. 
     With this configuration, even when stress is applied to any of the edges in the width direction of the flat plate-like extended portion of the positive electrode bus bar, the stress can be relieved in an appropriate manner. Moreover, even when positional displacement such as a projection of any of the edges of the extended portion in the width direction is caused in the bonding process, the gap at the bonding section is appropriately eliminated. 
     The extended portion is preferably inclined such that a distance of the extended portion from the cells increases toward the negative electrode terminals. 
     With this configuration, the extended portion extends in a direction oblique to a direction in which the cells are arranged. Thus, in the bonding process in which the positive electrode bus bar is bonded to the negative electrode bus bar, it is easy to eliminate the gap at the bonding section between the positive electrode bus bar and the negative electrode bus bar by pressing one battery module and the other battery module against each other so that the battery modules are brought closer to each other. 
     It is preferable that an inner end of the slit have an arc shape, or both corners in a width direction at the inner end of the slit have an arc shape. 
     With this configuration, even when any stress is applied to the inner end of the slit, it is possible to avoid stress concentration on one portion since the inner end has an arc shape, so that a tear from the inner end is less likely to be made. Thus, the positive electrode bus bar is less likely to be broken. 
     The slit preferably has a length greater than or equal to ¼ and shorter than or equal to ½ of a width of the extended portion. 
     With this configuration, since the length of the slit has a predetermined length, the above-described function of the slit can be ensured, and it is possible to prevent an increase in electrical resistance at a position where the slit is formed in the positive electrode bus bar. 
     Alternatively, a width of the flat plate-like extended portion of the positive electrode bus bar may be equal to a width of the bent portion of the negative electrode bus bar. 
     With this configuration, the width of the extended portion of the positive electrode bus bar is equal to the width of the bent portion of the negative electrode bus bar, and thus a long linear bonding section can be ensured between the positive electrode bus bar and the negative electrode bus bar. 
     It is preferable that in the extended portion, first and second slits be each formed at a different one of the edges in the width direction perpendicular to the extension direction of the extended portion by cutting out a part of the extended portion in the width direction from one edge toward the other edge at a different position in the extension direction, and a distance between tips of the first and second slits be greater than or equal to a distance in a slit cut-out direction from the tip of a longer one of the first and second slits, or from the tip of any one of the first and second slits when the first and second slits have a same length, to the edge of the extended portion opposite to the edge at which the longer slit, or the any one of the first and second slits is formed. 
     With this configuration, a sufficiently long distance between tips of the slits is ensured, and thus an increase in electrical resistance between the tips of the slits can be reduced. 
     The top edge of the extended portion of the positive electrode bus bar is preferably bonded to the bent portion of the negative electrode bus bar of the another adjacent battery module by welding. 
     With this configuration, due to welding heat, the gap at the bonding section between the positive electrode bus bar and the negative electrode bus bar is less likely to be formed. 
     A battery pack according to a second aspect of the present invention includes: at least a first battery module and a second battery module which are combined with each other, wherein each of the battery modules includes a plurality of cells arranged with a same polarity oriented in a same direction, 
     a positive electrode bus bar by which positive electrode terminals of the cells are electrically connected to each other in parallel, and a negative electrode bus bar by which negative electrode terminals of the cells are electrically connected to each other in parallel, wherein the positive electrode bus bar has a flat plate-like extended portion which extends laterally to the cells from an edge of the positive electrode bus bar toward the negative electrode terminals of the cells, the negative electrode bus bar has a bent portion which is bent in a direction opposite to the positive electrode terminals of the cells from an edge of the negative electrode bus bar opposite to the extended portion, in the extended portion, first and second slits are each formed at a different one of edges in a width direction perpendicular to an extension direction of the extended portion by cutting out a part of the extended portion in the width direction from one edge toward the other edge, the first slit is a slit formed at a position closer to the positive electrode terminals of the cells, the second slit is a slit formed at a position closer to the negative electrode terminals of the cells, when the first battery module is electrically connected in series to the second battery module adjacent to the first battery module to form a battery pack, a top edge of the extended portion of the positive electrode bus bar of the first battery module is bonded to the bent portion of the negative electrode bus bar of the second battery module, the first battery module has the first slit at the edge on one side in the width direction perpendicular to the extension direction of the extended portion and the second slit at the edge on the other side in the width direction perpendicular to the extension direction of the extended portion, and the second battery module has the first slit at the edge on the other side in the width direction perpendicular to the extension direction of the extended portion and the second slit at the edge on the one side in the width direction perpendicular to the extension direction of the extended portion. 
     With this configuration, one battery module and the other battery module are bonded with their slits symmetrically arranged, and thus the stress-induced strain at the bonding section is less likely to be caused. 
     The top edge of the extended portion of the positive electrode bus bar of the first battery module is preferably bonded to the bent portion of the negative electrode bus bar of the second battery module by welding. 
     With this configuration, due to welding heat, the gap at the bonding section between the positive electrode bus bar and the negative electrode bus bar is less likely to be formed. 
     Advantages of the Invention 
     According to the present invention, a slit is formed at least one edge of the extended portion of the positive electrode bus bar in the width direction, and thus the gap at the bonding section between the positive electrode bus bar and the negative electrode bus bar in the bonding process is less likely to be formed, and stress-induced strain is less likely to be caused at the bonding section after the positive electrode bus bar is bonded to the negative electrode bus bar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating an external appearance of a battery module of the present invention. 
         FIG. 2  is a cross-sectional view illustrating a configuration of a cell. 
         FIG. 3  is an exploded view illustrating a positional relationship between a positive electrode bus bar, a block spacer, cells, and a battery holder. 
         FIG. 4  is a perspective view illustrating a configuration of a negative electrode bus bar. 
         FIG. 5  is a side view illustrating the battery module. 
         FIG. 6  is a view illustrating how the cells are accommodated in a housing. 
         FIG. 7  is a view schematically illustrating how battery modules according to the present embodiment are bonded. 
         FIG. 8  is a perspective view illustrating how one battery module is bonded to the other battery module. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present disclosure will be specifically described with reference to the attached drawings. The embodiment below is intended for easy understanding of the principle of the present disclosure. The scope of the invention is not limited to the embodiment below, and includes other embodiments expected by those skilled in the art. 
       FIG. 1  is a perspective view illustrating an external appearance of a battery module  800  of the present invention. As illustrated in  FIG. 1 , the battery module  800  includes a plurality of cells  100  arranged with the same polarity oriented in the same direction, a positive electrode bus bar  200  by which positive electrode terminals of the cells  100  are electrically connected to each other in parallel, and a negative electrode bus bar  300  by which negative electrode terminals of the cells  100  are electrically connected to each other in parallel. 
     The plurality of cells  100  are accommodated in a battery holder  150 . The battery holder  150  includes a plurality of battery storage portions  140  in which the cells  100  are accommodated. Each battery storage portion  140  is, for example, a cylindrical hollow portion having a circular cross section so that the battery storage portion  140  can accommodate, for example, a cylindrical cell  100 . 
     The battery storage portions  140  are arranged, for example, in a staggered manner. Here, the expression “staggered manner” means that the plurality of battery storage portions  140  are disposed at regular intervals, and the battery storage portions  140  in adjacent rows are displaced by half the length of the battery storage portion  140  relative to each other. When the battery storage portions  140  are arranged in a staggered manner, space inside the battery holder  150  can effectively be utilized. 
     The battery holder  150  is formed to include, for example, aluminum, or an aluminum alloy. The aluminum alloy is not particularly limited as long as it is lightweight and has satisfactory thermal conductivity, and for example, an Al—Mg-based alloy, an Al—Mg—Si-based alloy, an Al—Zn—Mg-based alloy, an Al—Zn—Mg—Cu-based alloy, etc. can be used. 
     Next, the cell  100  accommodated in each battery storage portion  140  will be described.  FIG. 2  is a cross-sectional view schematically illustrating a configuration of the cell  100 . As the cell  100 , for example, a cylindrical lithium ion secondary battery as illustrated in  FIG. 2  can be used. The lithium ion secondary battery has a safety valve mechanism by which gas is released outside the battery when pressure in the battery increases due to the occurrence of an abnormal situation such as an internal short-circuit. 
     In the cell  100 , an electrode group  104  formed by winding a positive electrode  101  and a negative electrode  102  with a separator  103  interposed between the positive electrode  101  and the negative electrode  102  is accommodated in a battery case  107  together with a nonaqueous electrolyte. Insulating plates  109 ,  110  are respectively disposed above and under the electrode group  104 . The positive electrode  101  is bonded to a filter  112  via a positive electrode lead  105 , and the negative electrode  102  is bonded to a bottom of the battery case  107  via a negative electrode lead  106 , the bottom also serving as a negative electrode terminal. 
     A paste containing a positive electrode active material and a paste containing a negative electrode active material are applied to a surface of the positive electrode and a surface of the negative electrode, respectively. As the positive electrode active material, one or two or more of positive electrode active materials such as SiMn2O4, SiCoO2, and SiNiO3 used for lithium-ion batteries may be used without particular limitation. As the negative electrode active material, one or two or more of negative electrode active materials such as amorphous carbon and graphite carbon used for lithium-ion batteries may be used without particular limitation. As the nonaqueous electrolyte, for example, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like may be used. 
     The filter  112  is connected to an inner cap  113 , and a raised portion of the inner cap  113  is bonded to a valve plate  114  made of metal. Further, the valve plate  114  is connected to a terminal plate  108  also serving as a positive electrode terminal. The terminal plate  108 , the valve plate  114 , the inner cap  113 , and the filter  112  together seal an opening of the battery case  107  via a gasket  111 . 
     When an abnormal situation such as an internal short-circuit occurs in the cell  100 , and pressure in the cell  100  increases, the valve body  114  swells to the terminal plate  108 , and a current path is cut when the bonding between the inner cap  113  and the valve body  114  is broken. When the pressure in the cell  100  further increases, the valve body  114  is broken. Thus, gas generated in the cell  100  is released outside via a through hole  112   a  of the filter  112 , a through hole  113   a  of the inner cap  113 , a slit of the valve body  114 , and an opening portion  108   a  of the terminal plate  108 . 
     Referring back to  FIG. 1 , the positive electrode bus bar  200  includes a top plate portion  210  disposed to face the positive electrode terminals of the cells  100  and having, for example, a flat plate-like shape, and an extended portion  220  extending from an edge of the top plate portion  210  toward the negative electrode terminals of the cells  100 . 
     A plurality of openings  240  corresponding to the cells  100  accommodated in the battery holder  150  are formed in the top plate portion  210 . The gas released via the opening portion  108   a  of the cell  100  is released through a corresponding one of the openings  240 . 
     The extended portion  220  has, for example, a flat plate-like shape, and the dimension of the extended portion  220  may accordingly be determined in consideration of, for example, easiness of bonding the positive electrode bus bar  200  to a negative electrode bus bar  300 . For example, the extended portion  220  has a width of 39-42 mm and a length of 60-80 mm, and preferably a width of 40 mm and a length of 70 mm. 
     A block spacer  400  made of, for example, a resin to prevent electrical conduction between the positive electrode bus bar  200  and the battery holder  150  is provided between (i) the battery holder  150  and (ii) the top plate portion  210  and the extended portion  220 . 
     At an edge of the extended portion  220 , a slit  230  is formed as a cut-out in a width direction perpendicular to a direction in which the extended portion  220  extends. The present embodiment includes two slits  230  each of which is formed, for example, at a different edge of the extended portion  220 , and at a different level by cutting out parts of the extended portion  220  in the width direction from one edge toward the other edge. Here, the width direction in which the slits  230  are formed by the cutting out includes not only a direction parallel to a top edge located in the direction in which the flat plate-like extended portion  220  extends but also a direction oblique to the top edge. Each slit  230  is formed at a different one of the edges of the extended portion  220 . Thus, even when positional displacement which results in, for example, a projection of any of the edges of the extended portion  220  in the width direction is caused in a bonding process, a gap at a bonding section is appropriately eliminated, and even when stress is applied to any of the edges of the extended portion  220  in the width direction in a process after the positive electrode bus bar  200  is bonded to a negative electrode bus bar  300 , the stress can be relieved in an appropriate manner. Moreover, each slit  230  is formed at the different edge of the extended portion  220  and at a different level, so that a predetermined length can be ensured as a distance L between tips of the slits  230 , and an increase in electrical resistance of the positive electrode bus bar  200  caused by forming the slits  230  can be prevented. 
     As illustrated in  FIG. 1 , when the slits  230  have the same length, the distance L between the tips of the slits  230  is preferably greater than or equal to a distance A from the tip of one of the slits  230  to the edge of the extended portion  220  opposite to the edge of the extended portion  220  at which the one of the slits  230  is formed in a cut-out direction of the one of the slits  230 . When the distance L between the tips of the slits  230  is short, electrical resistance between the tips of the slits  230  increases, and heat may be excessively generated. However, when the distance L between the tips of the slits  230  is greater than or equal to the distance A from the tip of the slit  230  to the edge of the extended portion  220  in the cut-out direction of the slit  230 , the increase in electrical resistance between the tips of the slits  230  can be reduced. 
     When the slits  230  have different lengths, the distance L between the tips of the slits  230  is preferably greater than or equal to a distance A from the tip of the longer one of the slits  230  to the edge of the extended portion  220  opposite to the edge of the extended portion  220  at which the longer slit  230  is formed in the cut-out direction of the longer slit  230 . In this case also the increase in electrical resistance between the tips of the slits  230  can be reduced for a similar reason. 
     The length of the slit  230  is, for example, greater than or equal to ¼ and shorter than or equal to ½ of the width of the extended portion  220 . This is because when the length of the slit  230  is greater than ½ of the width of the extended portion  220 , it is possible that the increase in electrical resistance of the positive electrode bus bar  200  at a portion where the slits  230  are formed can no longer be acceptable, whereas when the length of the slit  230  is shorter than ¼ of the width of the extended portion  220 , it is possible that it becomes difficult to correct the positional displacement caused in the bonding process in which the positive electrode bus bar  200  is bonded to a negative electrode bus bar  300 , and to distribute stress, such as torsion, applied to the bonding section after the positive electrode bus bar  200  is bonded to the negative electrode bus bar  300 . 
     The size of the slit  230  can accordingly be determined in consideration of the function of the slit  230 , and is not particularly limited. For example, the slit  230  preferably has a length of 22-26 mm and a width of 2.4-2.6 mm, where the width of the positive electrode bus bar  200  is 40 mm. 
     An inner end of the slit  230  has, for example, an arc-like shape as illustrated in  FIG. 1 . The size of the arc-like shape is not particularly limited as long as stress concentration on one portion of the inner end of the slit  230  can be reduced, and for example, the arc-like shape has a radius of 0.5-1.5 mm. 
       FIG. 3  is an exploded view illustrating positional relationship among the positive electrode bus bar  200 , the block spacer  400 , the cells  100 , and the battery holder  150 . As illustrated in  FIG. 3 , the battery holder  150  has the battery storage portions  140  arranged, for example, in a staggered manner, and in the battery storage portions  140 , the cells  100  are accommodated with their positive electrode terminals facing upward. 
     The block spacer  400  is disposed to face the positive electrode terminals of the cells  100  accommodated in the battery holder  150 . A plurality of openings  410  corresponding to the cells  100  are formed in the block spacer  400 . The gas released via the opening portion  108   a  of the cell  100  is released outside through a corresponding one of the openings  410  in the block spacer  400  and the corresponding one of the openings  240  in the top plate portion  210 . 
     The positive electrode bus bar  200  is disposed on the block spacer  400  with the top plate portion  210  facing the positive electrode terminals of the cells  140 . 
       FIG. 4  is a perspective view illustrating a configuration of the negative electrode bus bar  300 . As illustrated in  FIG. 4 , the negative electrode bus bar  300  includes a bottom plate portion  310  which is disposed to face the negative electrode terminals of the cells  100 , and has, for example, a flat plate-like shape, and a bent portion  320  which is bent from an edge of the bottom plate portion  310  toward the negative electrode terminals. The bent portion  320  is disposed at an end opposite to an end at which the extended portion  210  of the positive electrode bus bar  200  is disposed. The width direction of the bent portion  320  of the negative electrode bus bar  300  is the same as the width direction of the extended portion  220  of the positive electrode bus bar  200  so that the gap at the bonding section is less likely to be formed when the battery module  800  is bonded to another battery module  800 . 
     Next,  FIG. 5  is a side view of the battery module  800 . As illustrated in  FIG. 5 , the extended portion  220  has a predetermined angel of θ to the longitudinal direction of the cells  100 . The extended portion  220  extends in a direction oblique to the longitudinal direction of the cells  100 , and thus in the bonding process in which the positive electrode bus bar  200  is bonded to a negative electrode bus bar  300  of the another battery module  800 , it is easy to eliminate the gap at the bonding section by pressing the battery modules  800  against each other so that the battery modules  800  are brought closer to each other. 
     The predetermined angle θ is not particularly limited, and is, for example, greater than 0° and less than or equal to 5°. Note that the block spacer is omitted in the drawing for easy understanding. Moreover, the predetermined angle θ is exaggerated for convenience of understanding the drawing. 
     Next, disposing the cells  100  accommodated in the battery holder  150  in a housing will be described.  FIG. 6  is a view illustrating the disposing the cells  100  in the housing. 
     As illustrated in  FIG. 6 , in the battery module  800 , the plurality of cells  100 ,  100 , . . . are accommodated in a case. Note that the battery holder  150  is omitted in the drawing for easy understanding. The case is partitioned into a lid body  323  and a housing  325  by the positive electrode bus bar  200 . Space defined by the housing  325  and the positive electrode bus bar  200  is a battery chamber  331 , and space defined by the lid body  323  and the positive electrode bus bar  200  is an exhaust chamber  333  via which gas is released outside. 
     In the battery chamber  331 , the plurality of cells  100 ,  100 , . . . are accommodated in the housing  325  with their opening portions  108   a  facing upward, and are arranged in the longitudinal direction of the case. The openings  240  of the positive electrode bus bar  200  are formed at intervals in the longitudinal direction of the positive electrode bus bar  200 , and the opening portion  108   a  of the cell  100  is exposed in each opening  240 . 
     The exhaust chamber  333  has an exhaust port  329 . Specifically, a cut-out is formed in one end face in the longitudinal direction of the lid body  323 , so that a gap exists between the lid body  323  and the housing  325  at one end in the longitudinal direction of the case, and the gap is the exhaust port  329 . 
     Next, a mode of use of the battery module  800  having the configuration described above will be described. 
       FIG. 7  is a view schematically illustrating how the battery modules  800  according to the present embodiment are bonded. As illustrated in  FIG. 7 , a top edge  221  of an extended portion  220  of a positive electrode bus bar  200  of one battery module  800   b  which is closer to negative electrode terminals is placed to face a bent portion  320  of a negative electrode bus bar  300  of the other battery module  800   a , and as indicated by arrows in the figure, the battery module  800   b  and the battery module  800   a  are pressed against each other so that the battery modules  800   b  and  800   a  are brought closer to each other. 
       FIG. 8  is a perspective view illustrating how the battery module  800   b  is bonded to the battery module  800   a . As illustrated in  FIG. 8 , the battery modules  800   a  and  800   b  each has a first slit  230   a  formed at a position closer to the positive electrode terminals of the cells, and a second slit  230   b  formed at a position closer to the negative electrode terminals of the cells. The battery module  800   a  has a first slit  230   a  at an edge on one side (on the left side in  FIG. 8 ) in the width direction perpendicular to an extension direction of the extended portion  220 , and a second slit  230   b  at an edge on the other side (on the right side in  FIG. 8 ) in the width direction perpendicular to the extension direction of the extended portion  220 . The battery module  800   b  has a first slit  230   a  at an edge on the other side (on the right side in  FIG. 8 ) in the width direction perpendicular to an extension direction of the extended portion  220 , and a second slit  230   b  at an edge on the one side (on the left side in  FIG. 8 ) in the width direction perpendicular to the extension direction of the extended portion  220 . A top edge of the extended portion  220  of the battery module  800   b  which is closer to the negative electrode terminals is bonded to the bent portion  320  of the battery module  800   a . Thus, the battery module  800   b  and the battery module  800   a  are bonded with their slits symmetrically arranged, and in this case, even when the battery module  800   b  is under torsion, the torsion can be corrected by the battery module  800   a , so that stress-induced strain at the bonding section can further be reduced, which is particularly advantageous when a large number of battery modules are bonded to form a battery pack. The bonding is not particularly limited, but may be mechanical bonding or metallurgical bonding, and is preferably metallurgical bonding. As the metallurgical bonding, welding is preferable. This is because due to welding heat, the gap at the bonding section between the positive electrode bus bar  200  and the negative electrode bus bar  300  is much less likely to be formed. The welding is not particularly limited, and for example, arc welding, gas welding, electroslag welding, thermit welding, laser welding, etc. may be used, and among them, TIG welding is preferable. 
     In the bonding process, even when a gap is formed at the bonding section due to positional displacement between the positive electrode bus bar  200  of the battery module  800   b  and the negative electrode bus bar  300  of the battery module  800   a , the displacement is corrected when the slits  230  formed in the positive electrode bus bar  200  is narrowed by pressing the battery module  800   b  and the battery module  800   a  positive electrode bus bar  200  against each other, so that the gap at the bonding section is eliminated. 
     Moreover, the slits  230  are formed in the positive electrode bus bar  200 . Thus, even when stress, such as torsion, is applied to the bonding section after the positive electrode bus bar  200  is bonded to the negative electrode bus bar  300 , the slits  230  distribute the stress, and thus the stress-induced strain is less likely to be caused at the bonding section. 
     In the embodiment described above, each cell  100  has the safety valve mechanism to release gas outside the battery when pressure in the battery increases due to occurrence of an abnormal state, the openings  240  are formed in the positive electrode bus bar  200  to release the gas outside, and the exhaust chamber  333  is provided through which the gas is released outside when the cell is disposed in the housing. However, the embodiment is not intended to limit the scope of the present invention. An embodiment is possible in which the cell  100  has no safety valve mechanism to release gas outside the cell, no opening  240  is formed in the positive electrode bus bar  200 , and no exhaust chamber  333  is provided when the cell is disposed in the housing. 
     Moreover, in the embodiment described above, the extended portion  220  has the predetermined angle of θ to the longitudinal direction of the cells  100 , but such an embodiment is not intended to limit the invention. The extended portion  220  may be provided at a right angle to the top plate portion  210 . 
     INDUSTRIAL APPLICABILITY 
     The battery module according to the present invention is useful for power supplies, or the like of mobile electronic devices, mobile communication devices, or vehicles. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           100  Cell 
           101  Positive Electrode 
           102  Negative Electrode 
           103  Separator 
           104  Electrode Group 
           105  Positive Electrode Lead 
           106  Negative Electrode Lead 
           107  Battery Case 
           108  Terminal Plate 
           109 ,  110  Insulating Plate 
           111  Gasket 
           112  Filter 
           113  Inner Cap 
           114  Valve Body 
           140  Battery Storage Portion 
           150  Battery Holder 
           200  Positive Electrode Bus Bar 
           210  Top Plate Portion 
           220  Extended Portion 
           230  Slit 
           240  Opening 
           300  Negative Electrode Bus Bar 
           400  Block Spacer 
           800  Battery Module