Abstract:
A secondary battery includes: a bare cell having an electrode assembly including two electrodes and a separator arranged between the two electrodes, a cell container having an opening and adapted to contain the electrode assembly, and a cap assembly adapted to cover the opening; and an accessory unit having a protective circuit module and adapted to be electrically connected to the bare cell via at least one electrode from outside of the bare cell. The accessory unit is adapted to be affixed to the bare cell with an adhesive member arranged in at least a portion of facing surfaces of both the bare cell and the accessory unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for SECONDARY BATTERY earlier filed in the Korean Intellectual Property Office on 25 th  of Jun. 2004 and there duly assigned Serial No. 10-2004-0048001.  
       BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a secondary battery, and more particularly, to a secondary battery including a bare cell with an accessory unit having a protective circuit module.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, secondary batteries have been researched and developed in a variety of fields because of many advantages such as rechargeability, small size, and large capacity. Nowadays, Nickel Hydride (Ni-MH), lithium (Li), and lithium ion batteries have been developed and are used in the art.  
         [0006]     In such secondary batteries, most of the bare cells are formed by arranging an electrode assembly in a cell container, covering the cell container with its cover, injecting an electrolyte into the internal space of the cell container, and sealing it. Typically, the electrode assembly has a positive electrode, a negative electrode, and a separator. The cell container can be made of an iron frame, but is preferably made of aluminum or an aluminum alloy due to its intrinsic properties such as a light weight and high long-term erosion-resistance.  
         [0007]     On the other hand, the secondary battery is very dangerous because it has the ability to discharge abundant energy in a short time. In other words, the secondary battery receives and accumulates energy from other energy sources during a charging process, and finally becomes high explosive energy storage when fully charged. If an abnormality such as an internal short occurs in a full charge condition or during a charging process, a high capacity of energy accumulated in the battery would be abruptly discharged in a short time, thereby causing accidents such as fire or explosion.  
         [0008]     The lithium secondary battery widely used in recent years is particularly flammable and ignitable in an abnormal condition because the lithium itself has a high activity. The lithium-ion secondary battery is safer than the lithium battery because it uses only lithium ions instead of lithium metal. However, the lithium ion secondary battery is still dangerous because the material used for fabricating a negative electrode or a non-aqueous electrolyte is flammable.  
         [0009]     For this reason, typical secondary batteries have several kinds of safety devices for preventing fire or explosion caused by an abnormality in a full charge condition or in a charging process. The safety devices are connected to positive and negative electrode terminals of the bare cell via a conductive structure called a lead plate. The safety devices prevent fire or explosion by cutting off electrical currents when a voltage in the battery cell is abruptly raised because the battery is heated to a high temperature or excessively charged/discharged. For example, the safety devices connected to the bare cell can include a protection circuit for cutting off a current flow when abnormal currents or voltages are detected, a Positive Temperature Coefficient (PTC) device which is operable at a high temperature, and a bimetal device.  
         [0010]     Typically, the secondary battery is manufactured by assembling the bare cell with the safety device and packaging the assembled battery in a separate pack. Otherwise, the secondary battery is manufactured by electrically connecting the bare cell and the safety device including the protective circuit module and then mechanically assembling them by filling the space between them with a resin or coating them together to provide a secondary battery.  
         [0011]     A lithium ion battery pack includes a protective circuit module arranged facing a surface of the bare cell on which the electrode terminals are provided. The space between the bare cell and the protective circuit module is filled with a resin to form a battery. The resin can be injected into the space until the outer surface of the protective circuit module is covered but the external electrical terminals are exposed.  
         [0012]     The bare cell has a positive electrode terminal and a negative electrode terminal on its surface facing the protective circuit module. The positive electrode terminal can be a cap plate itself made of aluminum or an aluminum alloy or a metallic plate containing nickel formed on a cap plate. The negative electrode terminal protrudes from the cap plate and is electrically insulated from the cap plate by a gasket interposed therebetween.  
         [0013]     The protective circuit module can be fabricated by forming circuitry on a plastic panel, and has outer electrical terminals on its outer surface. The size and the shape of the protective circuit module are similar to those of a corresponding surface (i.e., the surface of the cap plate) of the bare cell.  
         [0014]     A circuit unit and connection terminals are also provided on the inner surface of the protective circuit module, which is opposite to the side of the outer electrical terminals. The circuit unit has a safety device, i.e, the protection circuit for protecting the battery from over-charging/discharging. The circuit unit is electrically connected to the outer electrical terminals via a conductive structure passing through the protective circuit module.  
         [0015]     Connection leads and an insulation plate are also provided between the bare cell and the protective circuit module. The connection leads are typically made of nickel and formed to provide electrical connection between the cap plate of the bare cell and the connection terminals of the protective circuit module, with an L-shaped or planar structure. For example, resistance spot welding can be used to respectively bond the connection leads to the terminals. A breaker can be provided in the connection lead between the protective circuit module and the negative electrode terminal. If the breaker is provided in the connection lead, the breaker is excluded from the circuit unit of the protective circuit module. The insulation plate is provided to insulate the connection lead connected to the negative electrode terminal from the cap plate which functions as a positive electrode.  
         [0016]     When the bare cell and the accessory unit including the protective circuit module are assembled together and then packaged by molding a resin to form the packaged battery, the bond strength between the bare cell and the plastic molded part can be weak. This is because a resin molded part used to fix the accessories to the bare cell is made of a resin whereas the bare cell or the cap plate is made of metal, and the bonding area is not sufficient.  
         [0017]     In order to increase the bond strength, it can be possible to enlarge the area of the bond structure such as the lead plate or provide a separate reinforcement structure. For example, a separate reinforcement structure can be welded to the cap plate, and then, a space between the reinforcement structure and the bare cell can be filled with a resin to encapsulate the reinforcement structure. However, additional materials and processes such as welding must be performed to form such a reinforcement structure.  
         [0018]     Moreover, a mold facility must be used to inject a resin into the space between the bare cell and the protective circuit module. Then, the molded resin must be cured, and the mold must be blanked after completing the mold process. Therefore, the process becomes cumbersome. Furthermore, when the resin is injected into the space between the protective circuit module and the bare cell, the resin is not always uniform. More particularly, when the reinforcement structure is complicated, it is more difficult to uniformly fill the space with the resin.  
         [0019]     On the other hand, after the resin injected into the space between the bare cell and the protective circuit module has hardened, since the connection port of the terminal, the protective circuit module, and the PTC are buried in the resin, they can not be separated. Therefore, reusable accessories must be discarded together with the used bare cell  100  when the life span of the used bare cell has been reached.  
       SUMMARY OF THE INVENTION  
       [0020]     Accordingly, in order to solve the aforementioned problems, the present invention provides a secondary battery having a structure for safely and easily assembling a bare cell with an accessory unit including a safety device such as a protective circuit module.  
         [0021]     Also, the present invention provides a secondary battery eliminating cumbersome works and problems of a resin mold process, that the resin is not uniformly filled in a space between the accessory unit and the bare cell and a bond strength between the accessory unit and the bare cell is not sufficient.  
         [0022]     According to an aspect of the present invention, a secondary battery is provided comprising: a bare cell having an electrode assembly including two electrodes and a separator arranged between the two electrodes, a cell container having an opening and adapted to contain the electrode assembly, and a cap assembly adapted to cover the opening; and an accessory unit having a protective circuit module and adapted to be electrically connected to the bare cell via at least one electrode from outside of the bare cell; the accessory unit is adapted to be affixed to the bare cell with an adhesive member arranged in at least a portion of facing surfaces of both the bare cell and the accessory unit.  
         [0023]     The protective circuit module preferably comprises at least one safety device; the at least one electrode of the accessory unit is preferably arranged on a side thereof facing the cap assembly, and the accessory unit preferably has at least one outer electrical terminal arranged on another side of the accessory unit.  
         [0024]     The protective circuit module is preferably adapted to be affixed within the accessory unit by a resin mold.  
         [0025]     The side facing the cap assembly preferably has at least one electrical connection member; the another side of the protective circuit module preferably has at least one outer electrical terminal; and the resin mold is preferably adapted to expose the electrical connection member and the outer electrical terminal.  
         [0026]     The accessory unit preferably comprises a resin assembly including a resin board with accessories including the protective circuit module.  
         [0027]     The adhesive member preferably comprises a material selected from a group consisting of a thermosetting adhesive, a photo-curing adhesive, and a double-coated fixing tape.  
         [0028]     According to another aspect of the present invention, a secondary battery is provided comprising: a bare cell having an electrode assembly including two electrodes and a separator arranged between the two electrodes, a cell container having an opening and adapted to contain the electrode assembly, and a cap assembly adapted to cover the opening; and an accessory unit having a protective circuit module and adapted to be electrically connected to the bare cell via at least one electrode from outside of the bare cell; the protective circuit module includes: at least one safety device, at least one mechanical connection member and electrical connection member arranged on one side facing the cap assembly and at least one outer electrical terminal arranged on another side thereof; the accessory unit includes a molding resin adapted to expose the mechanical connection member, the electrical connection member, and the outer electrical terminal; and the mechanical connection member of the accessory unit is mechanically or electrically attached to the cap plate with an adhesive member.  
         [0029]     The mechanical connection member is preferably conductive so as to be adapted to be electrically connected to protective circuit module.  
         [0030]     The electrical connection member is preferably adapted to be electrically connected to the electrical terminal of the cap plate.  
         [0031]     A surface of the electrical terminal of the cap plate preferably includes a coating having excellent electrical conductivity.  
         [0032]     The coating preferably comprises a material selected from a group consisting of gold, silver, and solder.  
         [0033]     The adhesive member is preferably arranged on a surface of the cap plate in a position corresponding to that of the mechanical connection member.  
         [0034]     The adhesive member is preferably selected from a group consisting of a thermosetting adhesive, a photo-curing adhesive, and a double-coated fixing tape.  
         [0035]     The adhesive member is preferably electrically conductive.  
         [0036]     The secondary battery preferably further comprises at least one conductive lead adapted to electrically connect the accessory unit to the bare cell.  
         [0037]     One side of the conductive lead is preferably adapted to be welded to the electrical terminal of the cap plate, and the other side of the conductive lead is preferably adapted to be welded to the cap plate of the cap assembly.  
         [0038]     The conductive lead preferably has one or more folding lines adapted to fold a length of the conductive lead upon assembling the accessory unit with the cap assembly.  
         [0039]     A lower surface of the accessory unit preferably includes a groove adapted to store the conductive lead. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     A more complete appreciation of the present invention, and many of the attendant advantages thereof, will be readily apparent as the present invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
         [0041]      FIG. 1  is an exploded perspective view of an example of a lithium ion battery pack before assembling the bare cell with the safety device;  
         [0042]      FIG. 2  is a perspective view of an assembled lithium ion battery pack;  
         [0043]      FIG. 3  is a cross-sectional view of an accessory unit and a bare cell assembled together according to an embodiment of the present invention;  
         [0044]      FIGS. 4A-4B  are respective cross-sectional views of an assembled accessory unit and bare cell and a separated accessory unit and bare cell of the embodiment of  FIG. 3   
         [0045]      FIG. 5  is a cross-sectional view of an assembly consisting of an accessory unit and a bare cell according to another embodiment of the present invention; and  
         [0046]      FIG. 6  is a cross-sectional view of an assembly consisting of an accessory unit and a bare cell according to yet another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]      FIG. 1  is an exploded perspective view of an example of a lithium ion battery pack before assembling the bare cell with the safety device, and  FIG. 2  is a perspective view of an assembled lithium ion battery pack.  
         [0048]     Referring to  FIGS. 1 and 2 , the protective circuit module  30  is arranged facing a surface of the bare cell on which the electrode terminals  111  and  130  are provided. The space between the bare cell  100  and the protective circuit module  30  is filled with a resin  30  to form a battery as shown in  FIG. 2 . The resin  30  can be injected into the space until the outer surface of the protective circuit module is covered but the external electrical terminals  31  and  32  are exposed.  
         [0049]     The bare cell  100  has a positive electrode terminal  111  and a negative electrode terminal  130  on its surface facing the protective circuit module  30 . The positive electrode terminal  111  can be a cap plate itself made of aluminum or an aluminum alloy or a metallic plate containing nickel formed on a cap plate. The negative electrode terminal  130  protrudes from the cap plate and is electrically insulated from the cap plate  110  by a gasket interposed therebetween.  
         [0050]     The protective circuit module  30  can be fabricated by forming circuitry on a plastic panel, and has outer electrical terminals  31  and  32  on its outer surface. The size and the shape of the protective circuit module  30  are similar to those of a corresponding surface (i.e., the surface of the cap plate) of the bare cell  100 .  
         [0051]     A circuit unit  35  and connection terminals  36  and  37  are also provided on the inner surface of the protective circuit module  30 , which is opposite to the side of the outer electrical terminals  31  and  32 . The circuit unit  35  has a safety device, i.e, the protection circuit for protecting the battery from over-charging/discharging. The circuit unit  35  is electrically connected to the outer electrical terminals  31  and  32  via a conductive structure passing through the protective circuit module  30 .  
         [0052]     Connection leads  41  and  42  and an insulation plate  43  are also provided between the bare cell  100  and the protective circuit module  30 . The connection leads  41  and  42  are typically made of nickel and formed to provide electrical connection between the cap plate  110  of the bare cell  100  and the connection terminals  36  and  37  of the protective circuit module  30 , with an L-shaped or planar structure. For example, resistance spot welding can be used to respectively bond the connection leads  41  and  42  to the terminals  36  and  37 . A breaker can be provided in the connection lead  42  between the protective circuit module and the negative electrode terminal. If the breaker is provided in the connection lead  42 , the breaker is excluded from the circuit unit  35  of the protective circuit module  30 . The insulation plate  43  is provided to insulate the connection lead  42  connected to the negative electrode terminal  130  from the cap plate which functions as a positive electrode.  
         [0053]     When the bare cell  100  and the accessory unit including the protective circuit module  30  are assembled together and then packaged by molding a resin to form the packaged battery as shown in  FIG. 2 , the bond strength between the bare cell  100  and the plastic molded part  20  can be weak. This is because a resin molded part  20  used to fix the accessories to the bare cell  100  is made of a resin whereas the bare cell  100  or the cap plate  110  is made of metal, and the bonding area is not sufficient.  
         [0054]     In order to increase the bond strength, it can be possible to enlarge the area of the bond structure such as the lead plate or provide a separate reinforcement structure. For example, a separate reinforcement structure can be welded to the cap plate, and then, a space between the reinforcement structure and the bare cell can be filled with a resin to encapsulate the reinforcement structure. However, additional materials and processes such as welding must be performed to form such a reinforcement structure.  
         [0055]     Moreover, a mold facility must be used to inject a resin into the space between the bare cell  100  and the protective circuit module. Then, the molded resin must be cured, and the mold must be blanked after completing the mold process. Therefore, the process becomes cumbersome. Furthermore, when the resin is injected into the space between the protective circuit module and the bare cell, the resin is not always uniform. More particularly, when the reinforcement structure is complicated, it is more difficult to uniformly fill the space with the resin.  
         [0056]     On the other hand, after the resin injected into the space between the bare cell and the protective circuit module has hardened, since the connection port of the terminal, the protective circuit module, and the PTC are buried in the resin, they can not be separated. Therefore, reusable accessories must be discarded together with the used bare cell  100  when the life span of the used bare cell  100  has been reached.  
         [0057]     Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings, so that those skilled in the art would understand and embody the present invention without difficulties.  
         [0058]      FIG. 3  is a cross-sectional view of an accessory unit and a bare cell being assembled together according to an embodiment of the present invention, and  FIGS. 4A-4B  are respective cross-sectional views of an assembled accessory unit and bare cell and a separated accessory unit and bare cell of the embodiment of  FIG. 3 .  
         [0059]     As shown in  FIGS. 3 and 4 A- 4 B, the secondary battery comprises an accessory unit  20  and a bare cell  100 . The accessory unit  20  is formed by electrically connecting the protective circuit module  21  to the bimetal switch  23  in series and molding them in a single body with a resin  24 . Since the size of the accessory unit  20  is much smaller than that of the entire secondary battery including the bare cell, even a large number of accessory units can be easily fabricated, and the mold can be easily managed. In comparison with the conventional method in which the space between the accessory unit and the bare cell is filled with a resin, it is not necessary to consider a safety vent (not shown) of the cap plate  110 . Furthermore, it is possible to eliminate the problem of the resin not uniformly filling the space between the accessory unit and the bare cell due to the modification or the reinforcement structure provided to improve the bond strength.  
         [0060]     In this embodiment, it should be noted that the accessory unit  20  can be embodied by mechanically assembling the protective circuit module  21  and the bimetal switch  23  by using a resin board and metallic components, and not by using the conventional resin mold process.  
         [0061]     The other terminals of the protective circuit module  21  and the bimetal switch  23 , not incorporating into the series connection between them, are connected to the connection member  25  or  27  and the electrical connection member  26  provided on the lower surface of the accessory unit  20 . Therefore, the mechanical connection member  25  electrically connected to the protective circuit module  21  also serves as an electrical connection structure.  
         [0062]     In addition, adhesive members  50  and  51  are provided on both longitudinal sides of the rectangular cap plate  110 . The adhesive members  50  and  51  are bonded to the mechanical connection members  25  and  27  of the accessory unit  20 . The adhesive members  50  and  51  can be made of, but are not limited to, a thermosetting adhesive, a photo-curing adhesive, a double-coated fixing tape, or equivalents thereof. If the thermosetting adhesive is used as the adhesive members  50  and  51 , a predetermined amount of heat must be applied when the accessory unit  20  and the bare cell  100  are being bonded with pressure. If the photo-curing adhesive is used, light of a predetermined wavelength must be applied. If the double-coated adhesive is used, a relatively high pressing force must be applied. Furthermore, the adhesive members  50  and  51  are electrically conductive, so that the accessory unit  20  can be also electrically connected to the bare cell  100 . Particularly, the adhesive member  50  bonded to the mechanical connection member  25  which is electrically connected to the protective circuit module  21  is preferably made of an electrically conductive material.  
         [0063]     At the center of the cap plate, an internal electric terminal  130  (e.g., a negative electrode) of the bare cell  100  protrudes from the surface. The internal electrical terminal  130  is insulated from the cap plate of the bare cell  100 . In addition, the internal electrical terminal  130  can have a coating formed on its surface to improve electrical conductivity with the electrical connection member  26  of the accessory unit  20 . The coating can be made of, but is not limited to, gold (Au), silver (Ag), solder (Sn/Pb), or equivalents thereof.  
         [0064]     Preferably, a portion of the mechanical connection members  25  and  27  are buried in the resin plate  24  of the accessory unit  20  to provide sufficient mechanical strength. Though the lower surfaces of the mechanical connection members  25  and  27  are shown to be planar in the drawing, the present invention is not limited thereto. For example, the lower surface of the mechanical connection member  25  and  27  can be embossed (not shown) to enlarge the bond area with the adhesive members  50  and  51  and improve the bond strength. Also, the present invention is not limited to the embossed surface, but can employ various shapes.  
         [0065]     By means of such a construction, the mechanical connection members  25  and  27  can be robustly bonded and fixed to the adhesive members  50  and  51 , so that the accessory unit  20  cannot be easily separated from the bare cell  100 . Accordingly, the accessory unit  20  can be safely assembled with the bare cell  100  from the electrical and mechanical view points.  
         [0066]     One end of the electrical connection member  26 , of which the other end is connected to the bimetal switch  23  of the accessory unit  20 , can be extended to the lower surface of the accessory unit  20  and have a plate-spring like shape protruding from the lower surface. The plate spring can make contact with the internal electrical terminal  130  of the bare cell  100  in a wide area when the bare cell  100  and the accessory unit  20  are assembled together by using the mechanical connection members  25  and  27  and the adhesive members  50  and  51 .  
         [0067]     According to the present embodiment, the safety devices of the accessory unit  21  can be safely assembled with the bare cell  100 . The accessory unit  20  is not permanently assembled but rather temporarily assembled with the bare cell  100 . This means that the accessory unit  20  can be removed from a used bare cell and mounted on a new bare cell for recycling even when the used bare cell  100  must be discarded.  
         [0068]     For example, as shown in  FIGS. 4A-4B , when the accessory unit  20  is pulled from the bare cell  100  by great force over a critical point of a bond strength between the mechanical connection member  25  and  27  and the adhesive member  50  and  51 , the mechanical connection member  25  and  27  can be separated from the adhesive member  50  and  51 , so that the accessory unit  20  can be removed from the bare cell  100 . Though the adhesive member  50  and  51  on the bare cell  100  would be ruined and cannot be recycled, it does not matter because the bare cell  100  will be discarded. For this purpose, the bond strength between the adhesive member  50  and  51  and the bare cell  100  is preferably made to be greater than the bond strength between the mechanical connection member  25  and  27  and the adhesive member  50  and  51 . This is because there is no need to additionally remove the adhesive member  50  and  51  from the accessory unit  20  if the adhesive member  50  and  51  is remained on the cap plate  110  when the accessory unit  20  is removed from the bare cell  100 .  
         [0069]     According to the present embodiment, it is possible to reduce cost of the secondary battery and increase user&#39;s demands. In other words, since the safety devices such as the protective circuit module, the PTC device, and the bimetal switch cannot be negligible in the battery cost, the battery cost can be significantly reduced by recycling the accessory unit.  
         [0070]      FIG. 5  is a cross-sectional view of an assembly structure consisting of an accessory unit and a bare cell according to another embodiment of the present invention. Herein, like reference numerals denote like components having similar structures and functions.  
         [0071]     Similarly, a secondary battery according to another embodiment of the present invention comprises an accessory unit  20  and a bare cell  100 . The accessory unit  20  is fabricated by electrically connecting the protective circuit module  21  to the bimetal switch  23  in series, and molding them in a single body with a resin  24 . Also, the accessory unit  20  can be embodied by mechanically assembling the protective circuit module  21  with the bimetal  23  by using a resin board and metallic components, and not by using the conventional resin mold process.  
         [0072]     The other terminals of the protective circuit module  21  and the bimetal switch  23 , not incorporating into the series connection between them, are connected to connection leads  28  and  29 , respectively, provided on the lower surface of the accessory unit  20 . The connection leads  28  and  29  are electrically connected to a portion of the cap plate  110  and internal electrical terminal (e.g., negative electrode terminal)  130 , respectively, of the bare cell  100  by welding or other bonding methods.  
         [0073]     In addition to the connection leads  28  and  29 , the mechanical connection members  25  and  27  are also provided on the lower surface of the accessory unit  20 . Since the mechanical connection members  25  and  27  do not serve as an electrical terminal, they can be made of a plastic resin or a ceramic material other than metal. In order to improve the bond strength with the accessory unit  20 , a portion of the mechanical connection members  25  and  27  can be buried into the resin board  24  of the accessory unit  24  or made as part of the resin board  24 .  
         [0074]     In the positions on the cap plate  110  surface of the bare cell  100 , corresponding to the mechanical connection members  25  and  27  of the accessory unit  20 , adhesive members  50  and  51  are provided. The adhesive members  50  and  51  will be bonded to the mechanical connection members  25  and  27 , respectively, when they are assembled. The adhesive members  50  and  51  can be made of an insulation material rather than a conductive material.  
         [0075]     The mechanical connection members  25  and  27  and the adhesive members  50  and  51  can be formed in a similar way to those shown in  FIG. 3 . In other words, the mechanical connection members  25  and  27  can be embossed on their surface to increase the bond strength to the bare cell  110 . The present invention is not limited to such mechanical bonding structures shown in  FIGS. 3 and 5 .  
         [0076]     For facilitating welding, the connection leads  28  and  29  extending from the lower surface of the accessory unit  20  are formed to be long and bent in folding lines provided at intervals so that a length of the connection lead  28  and  29  can be easily folded. As the lower surface of the accessory unit  20  approaches to the cap plate  110  of the bare cell  100  to assemble them after the connection leads are welded to the cap plate  110  and the negative electrode terminal  130  of the bare cell  100 , the connection leads  28  and  29  are folded by the folding lines  281  and  291  and then piled up one by one. As a result, the long connection leads  28  and  29  are not popped out and do not cause an electrical short with other structures.  
         [0077]     On the other hand, the thickness of the piled connection leads  28  and  29  can interfere the mechanical jointing between the bare cell  100  and the accessory unit  20 . Therefore, the lower surface of the resin board of the accessory unit  20  preferably has grooves  283  formed around the connection leads  28  and  29  for storing the piled connection leads  28  and  29 .  
         [0078]     According to the present embodiment, since a welding process is used to electrically connect the connection leads  28  and  29  of the accessory unit  20  to the cap plate  110  and the internal electrical terminal  130  of the bare cell  100 , it is possible to provide a safe low resistance electrical connection structure. In addition, even after the electrical terminals are welded, the mechanical jointing structure between the accessory unit and the bare cell can be disassembled by cutting the welded portion of the long connection lead  28  and  29 , so that the accessory unit can be used for a new bare cell.  
         [0079]     On the other hand, when the present invention is applied to a practical usage, it should be considered that factors such as sizes, shapes, and materials of the secondary battery components are different for each manufacturer or each type. Since such factors determine design requirements of the safety device, the safety device of the accessory unit should be optimized to each type of bare cells to be used, unless a standard for the secondary battery is established.  
         [0080]     When a used accessory unit is recycled, it can not fit for a new bare cell to be used. This situation can be prevented by providing an identification structure indicating the sizes, locations, and number of the mechanical jointing structures used to assemble the accessory unit with the bare cell. The mechanical jointing structure can be differently formed depending on the capacity or properties of the bare cell. The identification structure can be an engagement structure formed on corresponding surfaces of both the accessory unit and the bare cell.  
         [0081]     If the identification structure is provided depending on the type of the bare cell, it is possible to assort the accessory units with the bare cells, thereby preventing damages or dangers that can happen by using unsuitable safety devices. Preferably, the identification structure is provided in such a way that the accessory unit is commonly used for even products from different manufacturers when properties of the bare cells are within an allowable range. This would increase compatibility of the accessory unit.  
         [0082]      FIG. 6  is a cross-sectional view of an assembly structure consisting of an accessory unit and a bare cell according to yet another embodiment of the present invention.  
         [0083]     Unlike the embodiment of  FIG. 3 , the cap plate of the bare cell shown in  FIG. 6  has an adhesive member  51  formed on its entire surface excluding the electrical coupling portions. In addition, the accessory unit  130  has a groove for storing the electrical terminal  130  on its lower surface, and has an electrical connection member  26  connected to the protective circuit module  21 . Also, the accessory unit  20  has an electrical connection member  26 ′ provided on a portion of its lower surface. The electrical connection member  26 ′ is connected to the electrical terminal of the protective circuit module  21 . The electrical connection member  26 ′ also makes contact with a portion of the cap plate  110  when the accessory unit  20  is assembled with the bare cell  100 . The electrical connection members  26  and  26 ′ can have a high conductive and erosion-resistant coating made of a silver or gold paste on its surface. Also, the electrical connection members  26  and  26 ′ can be a plate spring that can make contact with the corresponding surface of the bare cell  100  in a wide area with pressure when the accessory unit  20  is assembled with the bare cell  100 . Such a contact structure is to provide a safe assembly structure between the accessory unit  20  and the bare cell  100  rather than recycling of the accessory unit  20 .  
         [0084]     According to the present invention, it is possible to assemble an accessory unit including a protective circuit module and a bimetal switch with a bare cell by using a safe and simple structure.  
         [0085]     Also, it is possible to remove the accessory unit from a used bare cell without damaging the mechanical jointing components, and to recycle the removed accessory unit. Therefore, the cost of a secondary battery can be reduced.  
         [0086]     Also, it is possible to solve problems that can happen during molding a resin in a battery pack in relation to a safe vent.  
         [0087]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various modifications in form and detail can be made therein without departing from the spirit and scope of the present invention as defined by the following claims.