in the following detailed description , only certain 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 . fig1 is a perspective view showing a battery module according to a first exemplary embodiment of the present invention , and fig2 is a perspective view showing a connecting member according to the first exemplary embodiment of the present invention . referring to fig1 and fig2 , a battery module 100 according to the first exemplary embodiment includes a plurality of rechargeable batteries 110 each 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 first exemplary embodiment is formed by connecting the rechargeable batteries 110 in series . however , the present invention is not limited thereto , and in some embodiments the rechargeable batteries 110 may be connected , for example , in parallel . the rechargeable battery 110 according to the first exemplary embodiment is formed in a prismatic shape and includes a case 112 , a cap plate 114 connected to an 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 rechargeable battery is exemplified in the first exemplary embodiment in a prismatic shape , the present invention is not limited thereto , and the battery may be formed , for example , in a cylindrical shape or other shapes . a vent member 116 that is opened when an internal pressure increases and a sealing cap 118 for sealing an electrolyte injection inlet are located on the cap plate 114 . the terminals 130 and 140 are fixed to the cap plate 114 while protruding outside the cap plate 114 , and a gasket 123 is positioned between the cap plate 114 and each of the terminals 130 and 140 for insulation and sealing . in this embodiment , the positive electrode terminal 130 and the negative electrode terminal 140 are formed substantially in a plate shape and are electrically connected to an electrode assembly ( not shown ) inserted in the case 112 . the positive electrode terminal 130 may be made of aluminum and the negative electrode terminal 140 may be made of copper . the rechargeable batteries 110 are arranged to be adjacent to each other and 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 arranged , and the connecting member 160 is 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 plate shaped and covers the positive electrode terminal 130 and the negative electrode terminal 140 . the connecting member 160 is welded to the terminals 130 and 140 , respectively , and a terminal welded portion 165 is formed on the connecting member 160 . the connecting member 160 includes a first connecting bar 161 and a second connecting bar 162 that is bonded to the first connecting bar 161 by friction stir welding . the first connecting bar 161 may be made of aluminum , as may be the positive electrode terminal 130 , and the second connecting bar 162 may be made of copper , as may be the negative electrode terminal 140 . the first connecting bar 161 is fixed to the positive electrode terminal 130 , for example , by welding and the second connecting bar 162 is fixed to the negative electrode terminal 140 , for example , by welding . a first welding protrusion 161 a is formed at a side of the first connecting bar 161 . the first welding protrusion 161 a may extend from a lower side of the first connecting bar 161 and may have a smaller thickness than other portions of the first connecting bar 161 , and is stepped with respect to an upper surface of the first connecting bar 161 . a second welding protrusion 162 a is formed at a side of the second connecting bar 162 . the second welding protrusion 162 a may extend from a lower side of the second connecting bar 162 and may have a smaller thickness than other portions of the second connecting bar 162 , and is stepped with respect to an upper surface of the second connecting bar 162 . the second welding protrusion 162 a is positioned over the first welding protrusion 161 a , such that the second welding protrusion 162 a overlaps with the first welding protrusion 161 a . in this position , the first welding protrusion 161 a and the second welding protrusion 162 a are bonded by friction stir welding . fig3 is a partial cross - sectional view of a connecting portion according to the first exemplary embodiment of the present invention . referring to fig3 , with the welding protrusions 161 a and 162 a overlapping , a welded portion 150 is formed by rotating a tool to recrystallize the structure using dynamic flow . the tool has a pin and a shank where the pin is fixed , and the cross - section where the pin protrudes from the shank is called a shoulder . the first connecting bar 161 and the second connecting bar 162 may be bonded by spot welding , in which the portion where the shoulder contacts the connecting member 160 is the welded portion 150 , and a welding groove 151 is located around a portion of the welded portion 150 where the pin was positioned . as shown in fig3 , 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 portion where recovery and recrystallization occur due to high heat and the amount of deformation , such that the nugget zone 152 may also be called a dynamic - recrystallized portion . unlike general welding in which melting occurs by heat , the nugget zone 152 is formed by dynamic recrystallization of materials that are melded in a solid state by friction heat and stirring . a diameter of the nugget zone 152 is generally larger than a diameter of the pin and smaller than a diameter of the shoulder 183 . the size of the nugget zone 152 varies based on the rotational speed of the tool , such that 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 crystals may be incomplete , and defects may occur approximate the incomplete portion . the thermo - mechanically affected zone 154 is a portion where partial recrystallization occurs by plastic deformation caused by friction at a contact surface where the shoulder of the tool contacts the connecting member 160 , and where thermal deformation by friction and mechanical deformation caused by the shoulder simultaneously occur . 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 , in which slanting crystals exist and a plurality of air holes may be formed . when the first connecting bar 161 contacting the positive electrode terminal 130 is made of the same material as the positive electrode terminal 130 and the second connecting bar 162 contacting the negative electrode terminal 140 is made of the same material as the negative electrode terminal 140 , as in the present exemplary embodiment , contact resistance may be minimized or reduced , and corrosion between the connecting member 160 and the terminals 130 and 140 may also be minimized or reduced . accordingly , not only will the output of the battery module 100 be improved , but bonding between the connecting member 160 and the terminals 130 and 140 is also improved , such that the overall cycle - life of the battery module 100 may also be improved . further , in the connecting member 160 , the portion made of the same material as the positive electrode terminal 130 is welded to the positive electrode terminal 130 , and the portion made of the same material as the negative electrode terminal 140 is welded to the negative electrode terminal 140 , such that respective weldability between the connecting member 160 and terminals 130 and 140 is improved . further , by bonding the first connecting bar 161 and the second connecting bar 162 by friction stir welding , the first connecting bar 161 made of aluminum and the second connecting bar 162 made of copper may be more effectively welded together . copper and aluminum have different melting points , such that when they are bonded by resistance welding or ultrasonic welding there is a greater possibility that defects may occur at the welded portion 150 , or the welded portion may more easily be separated by external shock or vibration . in particular , when a battery module is used in electric vehicles or hybrid electric vehicles , vibration is continuously applied to the connecting member 160 , such that the continuous vibration may cause contact defects between the connecting member and the terminals . however , when the first connecting bar 161 and the second connecting bar 162 are bonded by friction stir welding , as in the present exemplary embodiment , solid - state bonding can be achieved , such that the connecting member 160 and the terminals 130 and 140 , which have different melting points , can be stably bonded together . in particular , the nugget zone 152 formed approximate the center of the welded portion 150 is an area where dynamic recrystallization occurs , such that it has a structure that more effectively resists external vibration and shock . further , the thermo - mechanically affected zone 154 , which is an area where two connecting bars 161 and 162 are rotated and bonded , has mixed parent metals , such that it also has structural characteristics that resist external shock and vibration . further , the friction stir welding does not need a heat source , a welding rod , or a filler metal , unlike other types of welding , such that friction stir welding is a more environmentally - friendly type of welding that does not discharge harmful light or substances . further , since dynamic recombination occurs , solidification cracks that may occur in , for example , fuse bonding , may be minimized or reduced and there is less deformation , such that mechanical properties are excellent . fig4 is a perspective view showing a connecting member according to a second exemplary embodiment of the present invention . referring to fig4 , a connecting member 210 according to the second exemplary embodiment includes a first connecting bar 211 and a second connecting bar 212 attached to the first connecting bar 211 , for example , by welding . the first connecting bar 211 and the second connecting bar 212 are made of different materials , in which the first connecting bar 211 may be made of the same material as the negative electrode terminal 140 and the second connecting bar 212 may be made of the same material as the positive electrode terminal 130 . the second connecting bar 212 is formed substantially in a rectangular plate shape . the first connecting bar 211 has a body portion 211 a arranged in substantially a same plane with the second connecting bar 212 , and a bended portion 212 a bending from the body portion 211 a and arranged to be under the second connecting bar 212 in a plane substantially parallel to the body portion 211 a . the bended portion 212 a is in close contact with the lower side of the second connecting bar 212 and overlaps the second connecting bar 212 . a welded portion 215 is formed by bonding the second connecting bar 212 and the bended portion 212 a by friction stir welding using a tool . a welding groove 216 where a pin of the tool was positioned is located around the center of the welded portion 215 . the first connecting bar 211 can easily overlap with the second connecting bar 212 by bending the first connecting bar 211 according to the second exemplary embodiment . further , since the first connecting bar 211 and the second connecting bar 212 are bonded by friction stir welding , different metals can be more easily and stably bonded together , and the connecting member 210 can also be more easily coupled to the terminals 130 and 140 , for example , by more conventional types of welding . fig5 is a perspective view showing a connecting member according to a third exemplary embodiment of the present invention . referring to fig5 , a connecting member 230 according to the third exemplary embodiment includes a first connecting bar 231 and a second connecting bar 232 attached to the first connecting bar 231 by , for example , welding . the first connecting bar 231 and the second connecting bar 232 may be made of different materials , in which the first connecting bar 231 may be made of the same material as the positive electrode terminal 130 and the second connecting bar 232 may be made of the same material as the negative electrode terminal 140 . the first connecting bar 231 and the second connecting bar 232 are each formed substantially in a rectangular plate shape and arranged such that a side of the first connecting bar 231 and a side of the second connecting bar 232 contact each other . in this position , a tool is positioned at an interface between the first connecting bar 231 and the second connecting bar 232 , and the first connecting bar 231 and the second connecting bar 232 can be fixed together by welding . the tool performs welding while moving along the interface between the first connecting bar 231 and the second connecting bar 232 , such that a welded portion 236 is shaped substantially in a line . a welding groove 235 where the pin was positioned is located around a center of the welded portion 236 . according to the third exemplary embodiment , since a weld line is formed with the first connecting bar 231 and the second connecting bar 232 contacting each other , the welded area is wider such that the first connecting bar 231 and the second connecting bar 232 can be more stably fixed . fig6 is a perspective view showing a battery module according to a fourth exemplary embodiment of the present invention . referring to fig6 , a battery module 300 according to the fourth exemplary embodiment includes a plurality of rechargeable batteries 310 having a positive electrode terminal 330 and a negative electrode terminal 340 , and connecting members 350 that electrically connect the rechargeable batteries 310 . the battery module 300 according to the third exemplary embodiment is formed by connecting the rechargeable batteries 310 in series . the rechargeable batteries 310 according to the fourth exemplary embodiment are each formed in a prismatic shape and include a case 312 , a cap plate 314 connected to an opening of the case 312 , and the positive electrode terminal 330 and the negative electrode terminal 340 protruding outside the case 312 . a vent member 316 that is opened when an internal pressure increases and a sealing cap 318 for sealing an electrolyte injection inlet are located on the cap plate 314 . further , the terminals 330 and 340 protrude outside the cap plate 314 , and lower nuts 323 for supporting the terminals 330 and 340 on the cap plate 314 are fitted on the terminals 330 and 340 . further , a gasket 321 may be arranged between each lower nut 323 and the cap plate 314 for insulation . in this embodiment , the positive electrode terminal 330 and the negative electrode terminal 340 are each substantially in a cylindrical shape , and the outer circumferential surfaces may be threaded to fit nuts . further , the positive electrode terminal 330 and the negative electrode terminal 340 of each battery are electrically connected with an electrode assembly inserted in the case 312 of the battery . the positive electrode terminal 330 may be made of aluminum and the negative electrode terminal 340 may be made of copper . the connecting member 350 is arranged on the lower nuts 323 , and includes a first connecting bar 351 contacting the positive electrode terminal 330 and a second connecting bar 352 contacting the negative electrode terminal 340 . the first connecting bar 351 may be made of the same material as the positive electrode terminal 330 , and may have a terminal hole 351 a in which the positive electrode terminal 330 is inserted . the second connecting bar 352 may be made of the same material as the negative electrode terminal 340 , and may have a terminal hole 352 a in which the negative electrode terminal 340 is inserted . the connecting member 350 can be fixed to the terminals 330 and 340 by inserting the positive electrode terminal 330 and the negative electrode terminal 340 in the terminal holes 351 a and 352 a , respectively , and then respectively fitting upper nuts 324 on the terminals 330 and 340 . in some embodiments , the connecting member 350 is formed in a similar structure as the connecting member 160 according to the first exemplary embodiment . welding protrusions are formed on the first connecting bar 351 and the second connecting bar 352 , and a welded portion 360 is formed on the connecting portion by overlapping and then bonding the welding protrusions using friction stir welding . in other embodiments , the connecting member may have other structures , for example , similar to the connecting members as described above in the second or third embodiments . according to the fourth exemplary embodiment , if the portion of the connecting member 350 contacting the positive electrode terminal 330 is made of the same material as the positive electrode terminal 330 , and the portion of the connecting member 350 contacting the negative electrode terminal 340 is made of the same material as the negative electrode terminal 340 , corrosion between the connecting member 350 and the terminals 330 and 340 can be minimized or reduced . further , since the first connecting bar 351 and the second connecting bar 352 are bonded by friction stir welding , different metals can be more stably fixed together . 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 .