Abstract:
A battery module includes: a plurality of rechargeable batteries; and a connector for connecting a first terminal of one of the plurality of rechargeable batteries and including a first material, to a second terminal of another one of the plurality of rechargeable batteries and including a second material, wherein welded portions connect the connector to the first terminal and the second terminal, respectively, at least one of the welded portions including a nugget zone including a mixture of the respective materials of the connector and a corresponding one of the first terminal or the second terminal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/248,839, filed on Oct. 5, 2009, in the United States Patent and Trademark Office, the disclosure of which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     1. Field 
     The following description relates to a battery module with electrically connected rechargeable batteries. 
     2. Description of Related Art 
     Rechargeable batteries can be charged and discharged. Small capacity rechargeable batteries are used for small portable electronic devices such as mobile phones, laptop computers, and camcorders, while large capacity batteries are used as power sources for driving motors of hybrid vehicles, etc. 
     High power battery modules using a high energy density non-aqueous electrolyte have been developed, and the high power battery modules are formed as large-capacity battery modules by connecting a plurality of rechargeable batteries in series to be used for driving the motors of electric vehicles, etc. 
     Further, one large capacity rechargeable battery is generally composed of a plurality of rechargeable batteries connected in series, in which the rechargeable battery may be formed in a cylindrical shape or a prismatic shape. 
     Prismatic rechargeable batteries include a case having an electrode assembly in which a positive electrode and a negative electrode are disposed with a separator therebetween, and a space where the electrode assembly is disposed, a cap plate sealing the case and having a terminal hole where an electrode terminal is inserted, and an electrode terminal that is electrically connected with the electrode assembly and protrudes outside the case through the terminal hole. 
     The electrode terminal is fixed to the cap plate by a nut, but the nut can be loosened by continuous external vibration or shock. This causes contact resistance inside the rechargeable batteries, such that the output and cycle-life of the rechargeable batteries are reduced. 
     A method of connecting a connecting member to the positive electrode and the negative electrode using resistance welding has been proposed. 
     In general, the positive electrode terminal is made of aluminum and the negative electrode terminal is made of copper. As such, it is difficult to form a connecting member using the same material as both the positive electrode terminal and the negative electrode terminal. When the connecting member is made of a different material from that of at least one of the positive electrode terminal or the negative electrode terminal, it is difficult to connect the connecting member to the terminals with different materials using resistance welding or ultrasonic welding. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention provide a battery module for easily and stably connecting a connecting member with terminals. 
     According to an exemplary embodiment of the present invention, a battery module includes: a plurality of rechargeable batteries; and a connector for connecting a first terminal of one of the plurality of rechargeable batteries and including a first material, to a second terminal of another one of the plurality of rechargeable batteries and including a second material, wherein welded portions connect the connector to the first terminal and the second terminal, respectively, at least one of the welded portions including a nugget zone including a mixture of the respective materials of the connector and a corresponding one of the first terminal or the second terminal. 
     At least one of the welded portions may be formed by friction stir welding. 
     The first material may include aluminum. The second material may include copper. The connector may include the first material or the second material. The first material and the second material may be different. 
     The welded portions may include at least one spot weld connecting the connector to the first terminal and at least one more spot weld connecting the connector to the second terminal. 
     The first terminal, the second terminal, and the connector may be plate shaped. 
     The connector may be positioned on the first terminal and the second terminal. The welded portions may include spot welds formed from a side of the connector opposite a side facing the first terminal and the second terminal. The connector may be in contact with the first terminal and the second terminal, and the welded portions may each be formed in a line where the connector and one of the first terminal or the second terminal are in contact. 
     At least one of the first terminal or the second terminal may include a protrusion, and the connector may include a groove sized to receive the protrusion. The welded portions may include spot welds formed from a side of the connector opposite a side facing the first terminal and the second terminal. A corresponding one of the spot welds may be aligned with the protrusion and the corresponding groove. The nugget zone may be formed by melding of the connector with the protrusion while in a solid state to form a melded portion, and dynamic recrystallization of the melded portion. 
     The nugget zone may be formed by melding of the connector with the corresponding one of the first terminal or the second terminal while in a solid state to form a melded portion, and dynamic recrystallization of the melded portion. The battery module may further include a thermo-mechanically affected zone around the nugget zone and formed by plastic deformation and partial recrystallization of the connector with the corresponding one of the first terminal or the second terminal, and a heat affected zone around the thermo-mechanically affected zone and having slanting crystals and air holes. 
     The welded portions may each include a surface and a welding groove recessed from the surface. 
     The connector may contact the corresponding one of the first terminal or the second terminal, wherein the nugget zone traverses the contacting surfaces between the connector and the corresponding one of the first terminal or the second terminal. 
     According to another exemplary embodiment of the present invention, a method of connecting a first terminal of a first rechargeable battery with a second terminal of a second rechargeable battery, includes: providing a connector including a material different than a material of one of the first terminal or the second terminal; and welding the connector to the first terminal and the second terminal, respectively, to form respective welded portions, wherein the welded portion between the connector and the one of the first terminal or the second terminal is formed by friction stir welding. 
     The friction stir welding may form a nugget zone including a mixture of the respective materials of the connector and the one of the first terminal or the second terminal, by melding of the connector with the one of the first terminal or the second terminal while in a solid state to form a melded portion, and dynamic recrystallization of the melded portion. 
     According to exemplary embodiments of the present invention, with a decrease in contact resistance, the output of the battery module can be improved, and the cycle-life of the rechargeable batteries can also be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a battery module according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a partial perspective view of the battery module according to the first exemplary embodiment of the present invention; 
         FIG. 3  is a cross-sectional view taken along the line III-III of  FIG. 2 ; 
         FIG. 4  is a perspective view showing a portion of a battery module according to a second exemplary embodiment of the present invention; 
         FIG. 5  is an exploded perspective view showing a portion of a battery module according to a third exemplary embodiment of the present invention; and 
         FIG. 6A  and  FIG. 6B  are cross-sectional views illustrating a process of welding a connecting member with a terminal of the battery module according to the third exemplary embodiment of the present invention. 
       
         
           
                 
               
                 
                 
                 
                 
               
             
                 
                     
                 
                 
                   Description of Reference Numberals  
                 
                 
                   Indicating Features in the Drawings 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   100:  
                   rechargeable battery 
                   120:  
                   cap plate 
                 
                 
                   100, 100′,  
                   battery module 
                   110, 210:  
                   rechargeable  
                 
                 
                   200: 
                     
                     
                   battery 
                 
                 
                   112:  
                   case 
                   114:  
                   cap plate 
                 
                 
                   116:  
                   vent member 
                   118:  
                   sealing cap 
                 
                 
                   130, 230:  
                   positive electrode  
                   140, 240:  
                   negative electrode  
                 
                 
                     
                   terminal 
                     
                   terminal 
                 
                 
                   150. 175, 250:  
                   welded portion 
                   151, 251:  
                   welding groove 
                 
                 
                   152. 252:  
                   nugget zone 
                   160,  
                   connecting  
                 
                 
                   154, 254:  
                   thermo-mechanically 
                   170, 260: 
                   member 
                 
                 
                     
                   affected zone 
                   181, 271:  
                   shank 
                 
                 
                   156, 256:  
                   heat affected zone 
                   183, 273:  
                   shoulder 
                 
                 
                   180, 270:  
                   tool 
                   265: 
                   support groove 
                 
                 
                   182, 272:  
                   pin 
                     
                     
                 
                 
                   235, 245:  
                   terminal protrusion 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, exemplary embodiments of the present invention have been shown and described by way of illustration. As those skilled in the art will recognize, the described embodiments may be modified in various different ways without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
       FIG. 1  is a perspective view of a battery module according to a first exemplary embodiment of the present invention, and  FIG. 2  is a partial perspective view of the battery module according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 1  and  FIG. 2 , a battery module  100  according to the first exemplary embodiment includes a plurality of rechargeable batteries  110  having a positive electrode terminal  130  and a negative electrode terminal  140 , and connecting members  160  that electrically connect the rechargeable batteries  110 . 
     The battery module  100  according to the present exemplary embodiment is formed by connecting the rechargeable batteries  110  in series. However, the present invention is not limited thereto, and the rechargeable batteries  110  may, for example, be connected in parallel. 
     The rechargeable battery  110  according to the present exemplary embodiment is in a prismatic shape and includes a case  112 , a cap plate  114  connected to the opening of the case  112 , and the positive electrode terminal  130  and the negative electrode terminal  140  that protrude outside the case  112 . Although the prismatic rechargeable battery is exemplified in the present exemplary embodiment, the present invention is not limited thereto, and the battery may be, for example, in a cylindrical shape or other shapes. 
     The terminals  130  and  140  are fixed to the cap plate  114  while protruding outside the cap plate  114 , and a gasket  123  is between the cap plate  114  and each of the terminals  130  and  140  for insulation and sealing. In some embodiments, the positive electrode terminal  130  and the negative electrode terminal  140  are substantially plate shaped, and are electrically connected to an electrode assembly (not shown) inserted in the case  112 . Further, in some embodiments, the positive electrode terminal  130  is made of aluminum and the negative electrode terminal  140  is made of copper. 
     A vent member  116  that is opened when an internal pressure increases and a sealing cap  118  that seals an electrolyte injection inlet may also be disposed on the cap plate  114 . 
     The rechargeable batteries  110  arranged adjacent to each other are connected in series by the connecting members  160 , in which the positive electrode terminals  130  and the negative electrode terminals  140  of adjacent rechargeable batteries  110  are alternately disposed and the connecting members  160  are welded to the positive electrode terminal  130  of one rechargeable battery  110  and the negative electrode terminal of an adjacent rechargeable battery  110 . 
     The connecting member  160  is formed in a plate shape and disposed on the positive electrode terminal  130  and the negative electrode terminal  140  to cover the positive electrode terminal  130  and the negative electrode terminal  140 . The connecting member  160  is bonded to at least one of the positive electrode terminal  130  and the negative electrode terminal  140  by friction stir welding. 
     With the connecting members  160  covering the terminals  130  and  140 , a welded portion  150  is formed. For friction stir welding, a tool  180  is rotated to recrystallize the structure using dynamic flow, to weld the connecting member  160  with either the terminal  130  or  140 . The tool  180  has a pin  182  and a shank  181  where the pin  182  is fixed, and the cross-section where the pin  182  protrudes from the shank  181  is called a shoulder  183 . 
     In the present embodiment, the connecting member  160  and the terminals  130  and  140  are bonded by spot welding, in which a portion where the shoulder  183  contacts the connecting member  160  is a welded portion  150  and a welding groove  151  is formed at a portion where the pin  182  was positioned in the welded portion  150 . 
     As shown in  FIG. 3 , a nugget zone  152  that is formed by dynamic recrystallization, a thermo-mechanically affected zone (TMAZ)  154 , and a heat affected zone (HAZ)  156  are formed in the welded portion  150 . 
     The nugget zone  152  is a region where recovery and recrystallization occur due to high heat and the amount of deformation, such that the nugget zone  152  can also be referred to as a dynamic-recrystallized portion. Unlike general welding in which melting occurs by heat, the nugget zone  152  is formed by dynamic recrystallization of a material melded in a solid state by friction heat and stirring. The diameter of the nugget zone  152  is larger than the diameter of the pin  182  and smaller than the diameter of the shoulder  183 . The size of the nugget zone  152  can vary corresponding to the rotational speed of the tool, where when the rotational speed is high, the size of the nugget zone  152  is reduced. When the rotational speed is too high, the shape of the crystal is incomplete and defects may occur at the incomplete portion. 
     The thermo-mechanically affected zone  154  is a region where partial recrystallization occurs by plastic deformation caused by friction at a contact surface where the shoulder  183  of the tool contacts the connecting member  160 , and where thermal deformation by friction and mechanical deformation by the shoulder  183  occur substantially simultaneously. Crystals softened by excessive plastic flow and deformation of the material are distributed at an angle in the thermo-mechanically affected zone  154 . 
     The heat affected zone  156  is more affected by heat than the thermo-mechanically affected zone  154 . Here, slanting crystals may exist and a plurality of air holes may be formed. 
     When the connecting member  160  is bonded to at least the terminals made of different materials by friction stir welding, as in the present exemplary embodiment, the negative electrode terminal  140  made of copper and the positive electrode terminal  130  made of aluminum can be more effectively bonded using the connecting member  160  made of aluminum. Copper and aluminum have different melting points, such that when they are bonded by resistance welding or ultrasonic welding there is high possibility that defects can occur in the corresponding welded portion, or the welded portion may be more easily separated by external shock or vibration. In particular, when a battery module is used in electric vehicles or hybrid electric vehicles, vibration is applied continuously to the connecting member  160 , such that the continuous vibration can cause contact defects between the connecting member  160  and the terminals  130  and  140 . 
     However, when the connecting member  160  is bonded to the terminals  130  and/or  140  by friction stir welding, as in the present exemplary embodiment, solid-state bonding is achieved such that the connecting member  160  and the terminals  130  and/or  140  which have different melting points can be more stably bonded. Accordingly, not only can the output of the battery module  100  be improved, but the bonds between the connecting member  160  and the terminals  130  and/or  140  can be improved, such that the overall cycle-life of the battery module  100  can also be improved. 
     In particular, the nugget zone  152  is an area where dynamic recrystallization occurs, such that it has a structure that resists external vibration or shock. The thermo-mechanically affected zone  154 , which is an area where two connecting members  160  and the terminals  130  and  140  have been rotated and bonded, has mixed parent metals, such that it has a structural characteristic that resists external shock and vibration. 
     Friction stir welding does not need a heat source, a welding rod, or filler metal, unlike other types of welding, such that it is an environment-friendly type of welding that does not discharge harmful light or substances. Further, since dynamic recombination occurs, solidification cracks that may be formed in, for example, fuse bonding may be minimized or reduced, and there is not much deformation, such that mechanical properties are excellent. 
       FIG. 4  is a perspective view showing a battery module according to a second exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , a battery module  100 ′ according to the second exemplary embodiment includes a plurality of rechargeable batteries  110  and connecting members  170  electrically connecting the rechargeable batteries  110 . The battery module  100 ′ according to the second exemplary embodiment has a similar structure as the battery module  100 ′ according to the first exemplary embodiment, except for the configuration of the welding portion  175 . Accordingly, repeated descriptions of similar configurations will not be provided. 
     In this embodiment, the rechargeable battery  110  has a positive electrode terminal  130  and a negative electrode terminal  140  that are plate shaped and protrude outside a case. The connecting member  170  is formed substantially as a rectangular plate and covers the positive electrode terminal  130  and the negative electrode terminal  140 . The connecting member  170  is bonded to at least one of the positive electrode terminal  130  or the negative electrode terminal  140  by friction stir welding, and a welded portions  175  are formed at the sides of the connecting member  170  and the terminals  130  and  140 . 
     The connecting member  170  and the terminals  130  and  140  are welded with the sides contacting each other, and a tool performs welding while moving along the sides of the terminals  130  and/or  140 , such that the welded portion  175  is formed in a line. 
     By welding the sides of the connecting member  170  and the terminals  130  and/or  140  made of different materials using friction stir welding as in the present exemplary embodiment, different metals can be more stably bonded. 
       FIG. 5  is an exploded perspective view showing a battery module according to a third exemplary embodiment of the present invention, and  FIG. 6A  and  FIG. 6B  are cross-sectional views illustrating a process of welding a terminal and a connecting member according to the third exemplary embodiment of the present invention. 
     Referring to  FIG. 5 ,  FIG. 6A  and  FIG. 6B  a battery module  200  according to the third exemplary embodiment includes a plurality of rechargeable batteries  210  and connecting members  260  electrically connecting the rechargeable batteries  210 . The battery module  200  according to the third exemplary embodiment has a similar structure as the battery module according to the first exemplary embodiment, except for the structure of terminals  230  and  240  and the connecting member  260 . Accordingly, repeated descriptions for similar configurations will not be provided. 
     The rechargeable battery  210  has a positive electrode terminal  230  and a negative electrode terminal  240  that are plate shaped and protrude outside a case  212 . 
     In one embodiment, two terminal protrusions  235  are on an upper surface of the positive electrode terminal  230 , and two terminal protrusions  245  are on an upper surface of the negative electrode terminal  240 . Some embodiments may have more or less than two terminal protrusions on each electrode terminal. 
     The connecting member  260  is formed substantially in a rectangular plate and covers the positive electrode terminal  230  and the negative electrode terminal  240 . Further, support grooves  265  are located on the lower surface of the connecting member  260 , and the terminal protrusions  235  and  245  of the terminals  230  and  240  are inserted in the support grooves  265 . 
     As shown in  FIG. 6A  and  FIG. 6B , in this state, the connecting member  260  and at least one of the terminals  230  or  240  are bonded by friction stir welding using a tool  270 . Here, terminal  230  is illustrated for convenience, but terminal  240  can be similarly configured. The tool  270  has a pin  272  and a shank  271 , and is positioned above the connecting member  260 . The pin  272  protrudes from a shoulder  273  at a lower end of the shank  271 . By pressing the tool  270  against the connecting member  260  and either of the terminals  230  or  240  and rotating the tool  270 , the connecting member  260  and a corresponding one of the terminals  230  or  240  are bonded in a solid state by friction heat and stirring. Accordingly, a welded portion  250  and a welding groove  251  are formed. 
     In this state, since the terminal protrusion  235  in  FIG. 6A  is inserted in the support grooves  265 , as the terminal protrusion  235  is stirred, the terminal  230  and the connecting member  260  are stably bonded. Here, even if heat and friction force cannot influence or reach the lower portions of the terminal  230 , the terminal protrusion  235  is dynamically recombined with the connecting member  260  by the heat and friction force, such that the connecting member  260  and the terminal  230  can be stably bonded. 
     A nugget zone  252  that is formed by dynamic recrystallization, a thermo-mechanically affected zone  254  where partial recrystallization is generated by plastic deformation, and a heat affected zone  256  that is affected by heat are formed in the welded portion  250 . In such an embodiment, the nugget zone  252  is formed approximate to where the terminal protrusion  235  is fitted in the support groove  265 . 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.