Abstract:
A battery pack includes a battery cell; a case comprising a first case and a second case coupled together and accommodating the battery cell, wherein the first case includes a first rib extending circumferentially around the first case and generally defining a battery cell accommodation space for the battery cell around an interior of the first rib, the first rib having a ridged section comprising alternating thick portions and thin portions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/745,301, filed on Dec. 21, 2012 in the U.S. Patent and Trademark Office, the entire content of which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    One or more embodiments of the present invention relate to battery packs. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Generally, unlike primary batteries that cannot be recharged, secondary batteries are batteries that can be charged, discharged, and recharged. Secondary batteries are used as power sources for mobile devices, electric cars, hybrid cars, electric bicycles, uninterruptible power supplies, etc. According to the type of external device to which a secondary battery is to be applied, the secondary battery may be used in the form of a single battery cell or a battery pack in which a plurality of battery cells are connected. 
         [0004]    Although a small mobile device, such as a mobile phone, may operate for a period of time with the output and capacity of a single battery cell, a battery pack may be desired for a mobile device that consumes a relatively large amount of power, e.g., a laptop, or a device requiring long operation and high power, e.g., an electric car, a hybrid car, etc., where output voltage or output current of a battery pack may increase as a number of battery cells included therein increases. 
       SUMMARY 
       [0005]    One or more embodiments of the present invention include a battery pack with reduced deformation due to cooling contraction after high-temperature molding of a battery case and reduced noises during assembly of the battery case. 
         [0006]    According to one or more embodiments of the present invention, a battery pack includes a battery cell; a first case and a second case which are attached to face each other to provide a space for accommodating the battery cell, wherein the first case includes a first rib to surround the battery cell, and the first rib includes an uneven portion in which thick portions and thin portions having different thicknesses are alternately arranged. 
         [0007]    For example, the first case includes a first base plate arranged to face a first main surface of the battery cell, and the first rib protrudes along an edge of the first base plate. 
         [0008]    For example, the first base plate includes a flange unit to be attached to a main device. 
         [0009]    For example, the uneven portion is formed at a portion of the first rib forming the boundary against the flange unit. 
         [0010]    For example, the flange unit is formed at a corner of the first base plate. 
         [0011]    For example, the uneven portion is formed along the first rib bent to surround the flange unit. 
         [0012]    For example, the uneven portion is formed at a corner of the first rib splitting into different directions to surround the flange unit. 
         [0013]    For example, the flange unit includes a first flange unit and a second flange unit that are formed at two opposite corners in a direction along a side of the first base plate. 
         [0014]    For example, a strength reinforcement unit is formed at the flange unit. 
         [0015]    For example, a boss protruding toward the main device is formed at the flange unit to be attached to the main device. 
         [0016]    For example, the uneven portions are intermittently formed in the lengthwise direction of the first rib. 
         [0017]    For example, a flat portion is formed between the uneven portions adjacent to each other. 
         [0018]    For example, the flat portion is formed to have a substantially uniform thickness. 
         [0019]    For example, a connecting unit for interconnecting the first case and the second case is formed at the flat portion. 
         [0020]    For example, the connecting units are intermittently formed through the entire length of the first rib. 
         [0021]    For example, the second case includes a second base plate arranged to face a second main surface of the battery cell, and a second rib protruding along an edge of the second base plate. 
         [0022]    For example, the plurality of battery cells are arranged, the battery cells adjacent to each other are stacked to form different layers, and the second main surfaces of the battery cells adjacent to each other form a stepped portion. 
         [0023]    For example, the second base plate has a stepped portion having a shape corresponding to the stepped portion between the second main surfaces of the battery cells adjacent to each other. 
         [0024]    First and second connecting units for mutually attaching the first and second cases are formed at the first and second ribs, respectively. 
         [0025]    For example, the first and second connecting units form hooked connection. 
         [0026]    For example, the first connecting unit includes a connecting plate, which is formed at a location offset from the first rib and includes a connecting opening, and the second connecting unit includes a hook that is inserted into and locked by the connecting opening. 
         [0027]    For example, the first and second connecting units are formed at mutually corresponding locations, and the flange unit of the first base plate is formed at a location offset from the second base plate. 
         [0028]    For example, the accommodation space includes a cell accommodating space for accommodating the battery cell and a circuit accommodating space for accommodating a protection circuit module for controlling operations for charging and discharging the battery cell. 
         [0029]    First and second flange units to be combined with a main device may be formed at two opposite sides of the circuit accommodating space. 
         [0030]    According to the present invention, there is provided a battery pack without a deformation due to cooling contraction after high-temperature molding of a battery case and without noises during assembly of the battery case. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is an exploded perspective view of a battery pack according to an embodiment of the present invention; 
           [0032]      FIGS. 2 and 3  are exploded perspective views of a first case and a second case shown in  FIG. 1 , respectively; 
           [0033]      FIGS. 4 through 6  are perspective views of the sections IV, V, and IV of  FIG. 2  in closer details, respectively; 
           [0034]      FIG. 7  is a perspective view of the section VII of  FIG. 3  in closer detail; 
           [0035]      FIG. 8  is a plan view showing an uneven portion of  FIG. 4  in closer detail; and 
           [0036]      FIGS. 9 through 11B  show shapes of first ribs according to first through third comparative embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0038]    Hereinafter, a battery pack according to an exemplary embodiment of the present invention will be described with reference to the attached drawings. 
         [0039]      FIG. 1  is an exploded perspective view of a battery pack  100  according to an embodiment of the present invention.  FIGS. 2 and 3  are exploded perspective views of a first case  110  and a second case  120  shown in  FIG. 1 , respectively. In one embodiment, as shown in  FIGS. 2 and 3 , the first case  110  and the second case  120  may be assembled to rotate around the assembly axis C and face each other. 
         [0040]    Referring to  FIGS. 1 through 3 , the battery pack  100  includes at least one battery cell  150  and the first case  110  and the second case  120  that are assembled to face each other to provide an accommodation space G of the battery cell  150 . 
         [0041]    The first case  110  and the second case  120  are assembled by inserting the at least one battery cell  150  therebetween in the accommodation space G of the battery cell  150 . For example, the first case  110  may include a first base plate  111  and a first rib  115  which protrudes from the first base plate  111  and defines the accommodation space G. Similarly, the second case  120  may include a second base plate  121  and a second rib  125 , which protrudes from the second base plate  121  and defines the accommodation space G. 
         [0042]    The first base plate  111  and the second base plate  121  may be formed as overall-flat plates and may be arranged to face each other across the battery cells  150 . Generally, the first base plate  111  and the second base plate  121  may have mutually corresponding shapes and each of the first base plate  111  and the second base plate  121  may have a substantially rectangular shape including a pair of long sides extending in a long-side direction (x-axis direction) and a pair of short sides extending in a short-side direction (y-axis direction). 
         [0043]    According to an embodiment of the present invention, the first base plate  111  and the second base plate  121  may be formed to have different shapes. In other words, the first base plate  111  may be formed to have a rectangular shape, whereas the second base plate  121  may have a shape not facing a flange unit  114  of the first base plate  111 . As described below, since the flange unit  114  interconnects the battery pack  100  and a main device, the second base plate  121  and the flange unit  114  may be formed at mutually exclusive locations to prevent the second base plate  121  from being located between the flange unit  114  and the main device. 
         [0044]    In other words, the flange unit  114  of the first base plate  111  may be formed at a location spaced from the second base plate  121 . For example, the flange unit  114  may be formed at two opposite corners of the first base plate  111  along the long-side direction (x-axis direction), and the second base plate  121  may have a shape inwardly receding to avoid the flange unit  114  at the corners, that is, a shape in which the center portion of the long-side (in x-axis direction) protrudes. The protruding portion of the second base plate  121  may form a circuit accommodating unit  122  for forming a circuit accommodating space G 2  together with the first base plate  111 . 
         [0045]    In one embodiment, the first base plate  111  may constitute a portion of the overall outer appearance of a main device by being assembled to the main device, where a rubber pad for supporting the entire main device may be formed on the bottom surface of the first base plate  111 . 
         [0046]    The first rib  115  and the second rib  125  may protrude from edges of the first base plate  111  and the second base plate  121 , respectively. The first rib  115  and the second rib  125  may be formed at mutually corresponding locations to surround the battery cells  150  and to define the accommodation space G. For example, the first rib  115  and the second rib  125  may be arranged to form a closed-loop to seal the accommodation space G. 
         [0047]    The first rib  115  is formed substantially along an edge of the first base plate  111 , where the first rib  115  may be formed around the flange unit  114  and inside of the flange unit  114 . In other words, since the flange unit  114  is arranged to directly face a main device and interconnects the battery pack  100  and the main device, the accommodation space G of the battery pack  100  may be formed at a location by which connection of the flange unit  114  to the main device is not interfered. In other words, the accommodation space G of the battery pack  100  and the flange unit  114  may be formed at mutually exclusive locations. In one embodiment, the second rib  125  may be formed along an edge of the second base plate  121 . 
         [0048]    Referring to  FIGS. 2 and 3 , first and second connecting units  118  and  128  interconnecting the first case  110  and the second case  120  may be formed at the first rib  115  and the second rib  125 , respectively. For example, when the first case  110  and the second case  120  are attached to face each other, the first rib  115  and the second rib  125  may be assembled to contact each other and may be attached to each other via the first and second connecting units  118  and  128  formed at mutually corresponding locations. 
         [0049]      FIGS. 4 through 6  are perspective views of the sections IV, V, and IV of  FIG. 2  in closer details, respectively.  FIG. 7  is a perspective view of the section VII of  FIG. 3  in closer detail. 
         [0050]    Referring to  FIGS. 4 through 7 , the first and second connecting units  118  and  128  may be formed at the first rib  115  and the second rib  125  to interconnect the first case  110  and the second case  120 . For example, the first and second connecting units  118  and  128  may form a hooked connection. As described below, the first connecting unit  118  is arranged at a location offset from the first rib  115  and may include a connecting plate in which a connecting opening  118 ′ is formed. Furthermore, the second connecting unit  128  may include a hook that is inserted into the connecting opening  118 ′ and locked thereby. As the hook (the second connecting unit  128 ) formed at the second rib  125  is stopped and locked by the connecting opening  118 ′ formed at the first rib  115 , the first and second connecting units  118  and  128  may be prevented from being detached from each other. 
         [0051]    Hereinafter, referring to  FIGS. 2 and 4  through  6 , an uneven portion  115   a  of the first rib  115  will be described. 
         [0052]    Referring to  FIGS. 2 and 4  through  6 , the uneven portion  115   a  may be formed at the first rib  115 . For example, the uneven portion  115   a  may be formed on the inner surface of the first rib  115  facing the accommodation space G. 
         [0053]    The uneven portion  115   a  may contribute to prevention of contraction. For example, the first case  110  and the second case  120  may be formed to have appropriate shapes via high-temperature molding. Here, the uneven portion  115   a  may contribute to prevention of cooling contraction. 
         [0054]    As described below, the uneven portion  115   a  includes an uneven pattern in which thick portions and thin portions are alternately and repeatedly arranged to form alternating ridges and grooves, thereby preventing contraction of the first rib  115  and maintaining a thickness sufficient for stably supporting the second rib  125  assembled on the first rib  115 . 
         [0055]    The uneven portion  115   a  may be formed entirely along the length of the first rib  115  or may be selectively formed along a portion of the first rib  115 . In detail, according to an embodiment of the present invention, the uneven portion  115   a  may be selectively formed along a portion of the first rib  115 . According to an embodiment of the present invention, the uneven portion  115   a  may be formed at a portion of the first rib  115  that forms the boundary against the flange unit  114 . In other words, the uneven portion  115   a  may be formed at a portion of the first rib  115  which separates the flange unit  114  and the accommodation space G from each other. 
         [0056]    The flange unit  114  is configured to interconnect the battery pack  100  and a main device, where an assembly unit  114   a  is formed on the flange unit  114 . If the first rib  115  forming the boundary against the flange unit  114  is contracted and deformed, the warpage may prevent the flange unit  114  from being easily assembled to a main device. 
         [0057]    For example, according to an embodiment of the present invention, a boss may be formed at the flange unit  114  as the assembly unit  114   a  to be assembled to a main device, where an attachment hole for screw attachment may be formed in the assembly unit  114   a.  If warpage is formed at the  114 , location or perpendicularity of the boss may be distorted, and thus assembly to a main device may become difficult. Therefore, the uneven portion  115   a  may be selectively formed at a portion of the first rib  115  contacting the flange unit  114 . 
         [0058]    In one embodiment, according to another embodiment of the present invention, the uneven portion  115   a  may be formed entirely along the length of the first rib  115 . In this case, the uneven portion  115   a  may also prevent contraction and deformation of the first rib  115  and reduce assembly noises. 
         [0059]    An orientation between the flange unit  114  and the uneven portion  115   a  will be described below in more detail. For example, the flange unit  114  may include a first flange unit  1141  and a second flange unit  1142  along the long-side direction (x-axis direction) of the first base plate  111 , where the uneven portion  115   a  may be formed at the first rib  115  surrounding the first flange unit  1141  and the second flange unit  1142 . 
         [0060]    Referring to  FIG. 5 , the flange unit  114  may be formed at corners of the first base plate  111 , and the uneven portion  115   a  may be formed along the first rib  115  that is bent to surround the flange unit  114 . In other words, the uneven portion  115   a  is formed along the first rib  115  that is bent to surround the flange unit  114  and may be formed along the first rib  115  extending in two different directions (x-axis direction and y-axis direction). 
         [0061]    The uneven portion  115   a  may be formed at a corner of the first rib  115  splitting into two different directions (x-axis direction and y-axis direction). For example, the uneven portion  115   a  may be selectively formed at intermittent portions in the lengthwise direction of the first rib  115 , where the uneven portion  115   a  may be formed at the corner of the first rib  115 . Contraction stress may be applied to the corner, which splits into two different directions (x-axis direction and y-axis direction), in the two different directions during cooling contraction after high-temperature molding, and thus contraction stress may be concentrated thereto. For example, according to concentration of contraction stress, more contraction of volume may occur at the corner of the first rib  115  as compared to other portions of the first rib  115 . Therefore, the uneven portion  115   a  may be arranged to reduce contraction stress applied to the corner of the first rib  115  and reduce stress transmitted to the flange unit  114  as much as possible. 
         [0062]    In  FIG. 6 , unlike in  FIG. 5 , the uneven portion  115   a  is not formed at the corner of the first rib  115  splitting into two different directions (x-axis direction and y-axis direction). Instead, the uneven portion  115   a  is formed at a portion of the first rib  115  extending in a direction (y-axis direction), whereas a flat portion  115   b  having a substantially uniform thickness is formed at a portion of the first rib  115  extending in another direction (x-axis direction). As described below, the first connecting unit  118  to be attached to the second case  120  is formed at the flat portion  115   b,  the flat portion  115   b  having a relatively smooth surface is formed to not to interfere the attachment. In one embodiment, the reference numeral  117  in  FIG. 6  denotes a clamping member to fix a circuit unit accommodated in the circuit accommodating space G 2 . 
         [0063]    Referring to  FIG. 2 , the uneven portions  115   a  may be intermittently formed in the lengthwise direction of the first rib  115 , and the flat portions  115   b  having a substantially uniform thickness may be formed between the uneven portions  115   a  of the first rib  115 . 
         [0064]    The first connecting unit  118  may be formed at the flat portion  115   b.  The first and second connecting units  118  and  128  clamp the first case  110  and the second case  120 , such that the first case  110  and the second case  120  are not detached from each other. The flat portion  115   b  provides attachment spots for the first and second connecting units  118  and  128 . For example, the flat portion  115   b  may have substantially uniform thickness and relatively smooth surfaces to not to interfere the attachment between the first and second connecting units  118  and  128 . 
         [0065]    The first connecting unit  118  is arranged at a location offset from the flat portion  115   b  and may include a connecting plate in which the connecting opening  118 ′ is formed. For example, the second connecting unit  128  (the hook) may be inserted into the connecting opening  118 ′ between the flat portion  115   b  and the first connecting unit  118  and locked thereby, where entrance friction of the second connecting unit  128  may be reduced by the flat portion  115   b  having a uniform thickness and smooth surfaces. The first connecting unit  118  may extend from a location offset from the flat portion  115   b  in parallel therewith and may be formed as an overall flat plate-like structure. 
         [0066]    The first connecting unit  118  may be formed entirely along the length of the first rib  115 . In other words, the first connecting unit  118  may be not only formed at the portion that forms the boundary against the flange unit  114 , but also formed entirely along the length of the first rib  115  extending to surround the battery cells  150 . 
         [0067]    The first connecting unit  118  may be intermittently formed in the lengthwise direction of the first rib  115 . The first and second connecting units  118  and  128  may be formed at mutually corresponding locations to be attached to each other. To firmly clamp the first case  110  and the second case  120 , the first and second connecting units  118  and  128  may be formed entirely along the lengths of the first rib  115  and the second rib  125 . 
         [0068]    For example, connecting plates (the first connecting unit  118 ) extending in parallel to the first rib  115  may be arranged at intermittent locations along the length of the first rib  115 . Furthermore, the connecting opening  118 ′ by which the second connecting unit  128  (the hook) is locked may be formed in the connecting plate. The second connecting unit  128  may enter between the first rib  115  and the connecting plate and be fixed by the connecting opening  118 ′. 
         [0069]    Referring to  FIG. 1 , the flange unit  114  may be formed in the long-side direction (x-axis direction) of the first base plate  111 . For example, the flange unit  114  may be formed at the two opposite corners in the long-side direction (x-axis direction). For example, the flange unit  114  may include the first flange unit  1141  and the second flange unit  1142  along the long-side direction (x-axis direction) of the first base plate  111 . The first flange unit  1141  may extend longer than the second flange unit  1142  from the corner in the long-side direction (x-axis direction). 
         [0070]    As described below, the circuit accommodating space G 2 , in which a protection circuit module  130  for controlling operations for charging and discharging the battery cells  150  is accommodated, may be arranged between the first flange unit  1141  and the second flange unit  1142 . By accommodating the protection circuit module  130  in the space between the first flange unit  1141  and the second flange unit  1142 , the overall structure of the battery pack  100  may be compact. 
         [0071]    The flange unit  114  is to interconnect the battery pack  100  and a main device. The assembly unit  114   a,  such as a boss, may be formed at the flange unit  114 , where a screw attachment hole for screw attachment may be formed in the boss. For example, the assembly unit  114   a  may protrude upward from the top surface of the flange unit  114  or may be formed in any of various forms to align the battery pack  100  to a main device to which the battery pack  100  is to be attached. 
         [0072]    A strength reinforcement unit  114   b  may be formed at the flange unit  114 . The flange unit  114  is to interconnect the battery pack  100  and a main device, and the assembly unit  114   a,  such as a boss, is formed thereon. Therefore, if contraction and/or deformation occurs at the flange unit  114 , it may become difficult to attach the battery pack  100  to the main device. For example, the strength reinforcement unit  114   b  may be formed as a stripe-pattern rib structure protruding from the top surface of the flange unit  114 . 
         [0073]      FIG. 8  is a plan view showing the uneven portion  115   a  of  FIG. 4  in closer detail. Referring to  FIG. 8 , the uneven portion  115   a  may include an uneven pattern in which thick portions  1151  and thin portions  1152  are alternately and repeatedly arranged in the lengthwise direction of the first rib  115 . For example, the thick portion  1151  and the thin portion  1152  may be formed to have different thicknesses, that is, a first thickness t 1  and a second thickness t 2 , respectively. 
         [0074]    The uneven portion  115   a  may contribute to prevention of contraction. For example, the first case  110  and the second case  120  may be formed to have appropriate shapes via high-temperature molding. Here, the uneven portion  115   a  may contribute to prevention of cooling contraction. 
         [0075]    The uneven portion  115   a  includes an uneven pattern in which the thick portions  1151  and the thin portions  1152  are alternately and repeatedly arranged, thereby preventing contraction of the first rib  115  and maintaining a thickness sufficient for stably supporting the second rib  125  assembled on the first rib  115 . 
         [0076]    The first rib  115  protrudes from the first base plate  111 , where the first rib  115  and the first base plate  111  that are adjacent to each other may apply stress to each other based on thickness relationship therebetween during cooling contraction. As described below, if the first rib  115  is thicker than the first base plate  111 , the first base plate  111  may be pulled as a volume of the first rib  115  contracts, thereby causing warpage. Therefore, a thickness of the uneven portion  115   a,  that is, the first thickness t 1  and the second thickness t 2  of the thick portion  1151  and the thin portion  1152  may be equal to or smaller than the thickness of the first base plate  111 . 
         [0077]    According to an embodiment of the present invention, the first thickness t 1  of the thick portion  1151  and the second thickness t 2  of the thin portion  1152  may be from about 0.6 mm to about 0.8 mm. In detail, the first thickness t 1  of the thick portion  1151  may be about 0.8 mm, whereas the second thickness t 2  of thin portion  1152  may be about 0.6 mm. Here, thickness of the first base plate  111  may be about 0.8 mm. 
         [0078]      FIG. 9  shows the structure of a first rib  15  according to a first comparative embodiment. The first rib  15  protrudes upward from a first base plate  11  and may have a thickness t 02  greater than thickness t 01  of the first base plate  11 . The first rib  15  and the first base plate  11  may experience contraction and deformation as being cooled after high-temperature molding, where the relatively thick first rib  15  receives greater stress than the first base plate  11 . The first rib  15  contracts in the thickness direction, thereby pulling the first base plate  11  upward. 
         [0079]    The first rib  15  and the first base plate  11  may interfere with each other during contraction and deformation, where the relatively thick first rib  15  undergoes volume contraction, thereby influencing the first base plate  11 . As a result, the first base plate  11  has a shape concave toward the first rib  15 , and thus so-called warpage, which is an overall not flat and distorted shape, is formed. 
         [0080]      FIG. 10  shows a shape of a first rib  15 ′ according to a second comparative embodiment. The first rib  15 ′ protrudes from a first base plate  11 ′. The bottom portion  15   a ′ contacting the first base plate  11 ′ is formed to have a relatively small thickness, whereas the upper portion  15   b ′ is formed to have a relatively large thickness. Although the shape takes into consideration a warpage of the first base plate  11 ′ due to contraction-deformation and interference of the first rib  15 ′, if the first rib  15 ′ is formed to have the thick upper portion  15   b ′ and the thin bottom portion  15   a ′ throughout the length of the first rib  15 ′, it is difficult to design a mold therefor and the structural strength of the first rib  15 ′ is deteriorated. 
         [0081]      FIGS. 11A and 11B  show shapes of a first rib  15 ″ according to a third comparative embodiment.  FIGS. 11A and 11B  show shapes of the first rib  15 ″ before and after contraction-deformation, respectively. 
         [0082]    The first rib  15 ″ protrudes from a first base plate  11 ″, where the first rib  15 ″ is formed to have a small thickness overall. Although the shape is in consideration of contraction-deformation of the first rib  15 ″ and mass-production operations including designing of a mold for the first rib  15 ″, the shape causes loose assembly of a battery pack, thereby causing assembly noises and deteriorating assembly strength. 
         [0083]    In detail, the first rib  15 ″ contacts a second rib  25 ″ during assembly of a battery pack and provides a supporting structure that supports the second rib  25 ″. As the first rib  15 ″ is contracted to have a thickness smaller than that of the second rib  25 ″ after contraction-deformation, an area by which the first rib  15 ″ and the second rib  25 ″ contact each other decreases, thereby causing loose attachment. For example, the first rib  15 ″ may not form a precise alignment with the second rib  25 ″ and be contacted to have a thickness smaller than that of the second rib  25 ″. As a result, assembly position of the first rib  15 ″ may not be precisely defined and assembly noises may occur. 
         [0084]    In the embodiment of the present invention shown in  FIG. 8 , instead of forming the first rib  115  to have a large thickness or a small thickness overall, the uneven portion  115   a,  in which the thick portions  1151  and the thin portions  1152  are alternately and repeatedly arranged in the lengthwise direction of the first rib  115 , is formed. Therefore, the problems posed in the first through third comparative embodiments may be completely resolved. Furthermore, cooling contraction of the first rib  115  may be reduced without causing any problems in mass-production, e.g., designing of a mold, and assembly noise during attachment to the second rib  125 . 
         [0085]    Referring to  FIG. 8 , the flat portions  115   b  may be formed at locations nearby the uneven portions  115   a,  that is, between the uneven portions  115   a.  The flat portion  115   b  may have a third thickness t 3  that is substantially uniform in the lengthwise direction of the first rib  115 . 
         [0086]    For example, the third thickness t 3  of the flat portion  115   b  may be identical to the thickness of the thick portion  1151  of the uneven portion  115   a  (the first thickness t 1 ). The flat portion  115   b  may have the relatively large third thickness t 3  to increase an area by which the first rib  115  contacts the second rib  125  and may stably support the contacting portion of the second rib  125 . However, the present invention is not limited thereto. For example, the third thickness t 3  of the flat portion  115   b  may be any of thicknesses between the first thickness t 1  of the thick portion  1151  and the second thickness t 2  of the thin portion  1152 . 
         [0087]    Hereinafter, referring to  FIG. 1 , the overall structure of the battery pack  100  according to an embodiment of the present invention will be described. 
         [0088]    The first case  110  and the second case  120  are assembled to face each other with the one or more battery cells  150  therebetween and to define the accommodation space G for accommodating the battery cells  150 . For example, the first case  110  may include the first base plate  111 , which is arranged to face a first main surface  150   a  of the battery cell  150 , and the first rib  115 , which protrudes from the first base plate  111  and surrounds the battery cell  150 . Similarly, the second case  120  may include the second base plate  121 , which is arranged to face a second main surface  150   b  of the battery cell  150 , and the second rib  125 , which protrudes from the second base plate  121  and surrounds the battery cell  150 . 
         [0089]    If it is assumed that the battery cell  150  is formed to have a substantially hexahedral shape, the first main surface  150   a  and the second main surface  150   b  are surfaces having the largest area and may be arranged at opposite sides. 
         [0090]    The accommodation space G defined by the first case  110  and the second case  120  may include a cell accommodating space G 1  for accommodating the battery cell  150  and the circuit accommodating space G 2  for accommodating a circuit unit for controlling operations for charging and discharging the battery cell  150 . 
         [0091]    For example, the cell accommodating space G 1  may occupy most of the accommodation space G and may accommodate a suitable number of battery cells  150  to satisfy demanded output performance of the overall battery pack  100 . For example, according to an embodiment of the present invention, the six battery cells  150  may be accommodated in the cell accommodating space G 1 . 
         [0092]    The battery cells  150  may be arranged in parallel to each other to form a same level side-by-side or the plurality of battery cells  150  may be stacked to form upper/lower levels. The parallel arrangement and the stack arrangement may be combined. 
         [0093]    For example, according to an embodiment of the present invention, the four battery cells  150  arranged side-by-side may form a first layer, and the two battery cells  150  arranged thereon may form a second layer. In detail, the four battery cells  150  may be arranged on the first case  110  side-by-side, and the two battery cells  150  may be stacked on the two battery cells  150  at two opposite ends. 
         [0094]    When the battery cells  150  adjacent to each other are stacked to form different layers, the second main surfaces  150   b  of the battery cells  150  adjacent to each other may form a stepped portion  150   c.  Furthermore, the second case  120  arranged on the second main surface  150   b  may have a stepped portion  120   a  aligned to the profile of the second main surface  150   b.  In detail, the second case  120  may have the stepped portion  120   a  at which the two opposite ends of the second case  120  in the long-side direction (x-axis direction) are offset upward. The stepped portion  120   a  of the second case  120  is formed to closely contact the battery cells  150  selectively stacked to form the second layer and to firmly fix locations of the battery cells  150  and to make the overall battery pack  100  compact without wasting any space. 
         [0095]    The battery cell  150  is a rechargeable secondary battery and may be a lithium-ion battery, for example. The battery cell  150  may include an electrode assembly and an outer casing member  151  for accommodating the electrode assembly. Although not shown, the electrode assembly may be formed by stacking together a positive electrode plate, a separator, and a negative electrode plate and winding the stacked structure in a jellyroll-like shape. The outer casing member  151  may be formed of aluminum or an aluminum alloy. However, the present invention is not limited thereto. 
         [0096]    A flexible printed circuit board (FPCB)  140  which electrically interconnects the battery cell  150  and the protection circuit module  130  may be arranged outside the battery cell  150 . The FPCB  140  may provide a wiring for extracting power generated by the battery cell  150  or inputting charging voltage from an external charger to the battery cell  150 . The FPCB  140  forms an electrical connection with the electrode assembly of the battery cell  150  and may extract power from the electrode assembly to outside. 
         [0097]    The FPCB  140  electrically interconnects the battery cells  150  arranged adjacent to each other. For example, the FPCB  140  may interconnect the battery cells  150  arranged adjacent to each other by opposite polarities to connect the battery cells  150  in series or may interconnect the battery cells  150  arranged adjacent to each other by a same polarity to connect the battery cells  150  in parallel. The serial connection and the parallel connection may be combined. For example, the battery cells  150  stacked in the vertical direction may be connected in parallel and the battery cells  150  arranged side-by-side may be connected in series at the same time. 
         [0098]    The FPCB  140  may include wirings for electrically interconnect the plurality of battery cells  150  and forming charging and discharging paths and an insulation film for insulation from outside environment. 
         [0099]    The circuit accommodating space G 2  is formed in the accommodation space G. The circuit accommodating space G 2  accommodates the protection circuit module  130  for controlling operations for charging and discharging the battery cells  150 . For example, the protection circuit module  130  for controlling operations for charging and discharging the battery cells  150  may measure condition variables, such as currents and temperatures, regarding the battery cells  150  to monitor condition of each of the battery cell  150  and control operations for charging and discharging the battery cells  150  based on the same. Furthermore, the protection circuit module  130  may detect malfunctions, such as overcharging, overdischarging, and overcurrent, in advance and perform safety operations for preventing overheating and explosion. 
         [0100]    The protection circuit module  130  may be arranged on charging and discharging paths for charging and discharging the battery cells  150  and control opening and closing of charging and discharging paths. Accordingly, the protection circuit module  130  may include a switching device or the like. A connector for supplying power to a main device may be arranged at an end of the protection circuit module  130 . 
         [0101]    The circuit accommodating space G 2  is for accommodating the protection circuit module  130  electrically connected to the battery cell  150 , where the circuit accommodating space G 2  may be arranged at an end of the accommodating space G toward which a terminal of the battery cell  150  faces, for connection with the battery cell  150 . Here, the first flange unit  1141  and the second flange unit  1142  may be arranged at two opposite sides of the circuit accommodating space G 2 . By efficiently arranging the first flange unit  1141 , the second flange unit  1142 , and the circuit accommodating space G 2 , the battery pack  100  may be compact without wasting any space. 
         [0102]    It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments. 
       EXPLANATION OF REFERENCE NUMERALS 
       [0000]    
       
           100 : battery pack  110 : first case 
           111 : first base plate  114 : flange unit 
           1141 : first flange unit  1142 : second flange unit 
           114   a:  assembly unit  114   b:  strength reinforcement unit 
           115 : first rib  115   a:  uneven portion 
           1151 : thick portion  1152 : thin portion 
           115   b:  flat portion  117 : clamping member 
           118 : first connecting unit (connecting plate) 
           118 ′: connecting opening 
           120 : second case  120   a:  stepped portion of second case 
           121 : second base plate  125 : second rib 
           128 : second connecting unit (hook)  130 : protection circuit module 
           140 : flexible printed circuit board  150 : battery cell 
           150   a:  first main surface  150   b:  second main surface 
         G: accommodation space G 1 : cell accommodating space 
         G 2 : circuit accommodating space C: assembly axis