Abstract:
A safety element assembly is disclosed. The safety element assembly comprises a first thin metal sheet coupled to the secondary battery; a safety element coupled to the first thin metal sheet; and a second thin metal sheet coupled to the safety element, wherein the first thin metal sheet comprises a first region on which the safety element and the second thin metal sheet are stacked, and a second region on which the safety element and the second thin metal sheet are not stacked.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2009-0110363 and 2009-0110364, filed on Nov. 16, 2009, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in its entirety by reference. This application is also related to and incorporates herein by reference the entire content of the concurrently filed application: SECONDARY BATTERY AND METHOD OF MANUFACTURING THE SAME Ser. No. 12/844,681. 
     BACKGROUND 
     1. Field 
     The present technology relates to a safety element assembly for protecting a secondary battery from overheat and overcurrent. 
     2. Description of the Related Technology 
     Generally, unlike a primary battery, a secondary battery is a battery that may be charged and discharged. Secondary batteries are typically used as sources of energy for mobile devices, such as a laptop computer or a cellular phone, electric drills, electric vehicles, hybrid electric vehicles, electric bicycles, uninterruptible power supplies (UPS), and the like. Examples of the most generally used secondary batteries include a lithium secondary battery and a nickel-hydride battery. Secondary batteries can be categorized into cylindrical types, rectangular types, and pouch types according to their shapes. 
     A secondary battery typically includes a safety element assembly to secure safety of the secondary battery. Examples of safety element assemblies include a positive temperature coefficient (PTC) assembly, a safety vent, a current interrupt device, a thermal fuse, a shut-down separator, and the like. 
     Electronic devices using secondary batteries as a source of energy may be categorized into devices demanding high capacity and devices demanding high power. 
     High capacity secondary batteries may require use of a PTC assembly for safety. For example, mobile devices, such as a laptop computer or a cellular phone, may employ a secondary battery with a PTC assembly for safety. Meanwhile, a PTC assembly may not be necessary for a high power secondary battery. For example, a secondary battery without a PTC assembly is suitable for devices requiring high power rather than safety, such as an electric drill or an electric vehicle. Therefore, it is necessary to manufacture and manage secondary batteries according to separate standards based on whether high power or high capacity is desired. 
     SUMMARY 
     Embodiments of the present invention provide a safety element assembly for using a standardized secondary battery regardless of conditions for using high capacity and high power electric-electronic devices. 
     According to an aspect of the present invention, a safety element assembly of a secondary battery comprises a first thin metal sheet coupled to the secondary battery; a safety element coupled to the first thin metal sheet; and a second thin metal sheet coupled to the safety element, wherein the first thin metal sheet comprises a first region on which the safety element and the second thin metal sheet are stacked, and a second region on which the safety element and the second thin metal sheet are not stacked. 
     According to another aspect, the first thin metal sheet is larger than the second thin metal sheet. 
     According to another aspect, curvature radiuses of the first thin metal sheet, the safety element, and the second thin metal sheet are the same, and the safety element and the second thin metal sheet are at least partially cut, so that the second region of the first thin metal sheet is exposed. 
     According to another aspect, the safety element comprises a positive temperature coefficient (PTC) element. 
     According to another aspect, the safety element assembly further comprises an insulator interposed between the first thin metal sheet and the second thin metal sheet. 
     According to another aspect, curvature radiuses of the first thin metal sheet, the safety element, and the second thin metal sheet are the same, the first thin metal sheet has a circular shape, the second thin metal sheet has a partial circular shape, the safety element is a PTC element having a partial ring shape, and the insulator has a partial circular shape and is located in a space formed by an inner wall of the PTC element. 
     According to another aspect, curvature radiuses of the first thin metal sheet, the safety element, and the second thin metal sheet are the same, the first thin metal sheet has a circular shape, the second thin metal sheet has a partial circular shape, the safety element is a PTC element having a partial circular shape and a cut end, the insulator has a circular shape of which two portions at opposite sides are cut and have straight edges, and the insulator is arranged such that a straight edge of the insulator contacts the cut end of the safety element. 
     According to another aspect, curvature radiuses of the first thin metal sheet, the safety element, and the second thin metal sheet are the same, the first thin metal sheet has a ring shape, the second thin metal sheet has a partial circular shape, the safety element is a PTC element having a partial ring shape, and the insulator has a partial circular shape and is located in a space formed by an inner wall of the safety element. 
     According to another aspect, curvature radiuses of the first thin metal sheet, the safety element, and the second thin metal sheet are the same, the first thin metal sheet has a circular shape, the second thin metal sheet has a partial circular shape, and the safety element is a PTC element having a partial circular shape. 
     According to another aspect, the safety element is a thermal fuse of which a first end is electrically connected to the first thin metal sheet and a second end is electrically connected to the second thin metal sheet, and the insulator is arranged between the first thin metal sheet and the second thin metal sheet around the thermal fuse. 
     According to another aspect, the insulator is arranged on a sidewall of the safety element, and a surface of the first thin metal sheet is larger than combined surfaces of the safety element and the insulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail certain embodiments with reference to the attached drawings in which: 
         FIG. 1  is an exploded perspective view of a safety element assembly according to an embodiment of the present invention; 
         FIG. 2  is a bottom view showing the safety element assembly of  FIG. 1  attached to a secondary battery; 
         FIG. 3  is a sectional view showing the safety element assembly of  FIG. 1  attached to the secondary battery; 
         FIG. 4  is an exploded perspective view of a safety element assembly according to another embodiment of the present invention; 
         FIG. 5  is a bottom view showing the safety element assembly of  FIG. 4  attached to the secondary battery; 
         FIG. 6  is a sectional view showing the safety element assembly of  FIG. 4  attached to the secondary battery; 
         FIG. 7  is an exploded perspective view of a safety element assembly according to another embodiment of the present invention; 
         FIG. 8  is a diagram showing the safety element assembly of  FIG. 7  attached to the secondary battery; 
         FIG. 9  is an exploded perspective view of a safety element assembly according to another embodiment of the present invention; 
         FIG. 10  is a diagram showing the safety element assembly of  FIG. 4  attached to the secondary battery; 
         FIG. 11  is a diagram showing that a welding jig is arranged on the safety element assembly; 
         FIG. 12  is a diagram showing a safety element assembly according to another embodiment of the present invention attached to the secondary battery; 
         FIG. 13  is a diagram showing a safety element assembly according to another embodiment of the present invention attached to the secondary battery; 
         FIGS. 14A through 14D  are sectional views showing a process of attaching the safety element assembly of  FIG. 1 , where  FIG. 14A  is a diagram showing that a battery can is prepared according to an embodiment of the present invention,  FIG. 14B  is a diagram showing a first thin metal sheet welded to the battery can of  FIG. 14A ,  FIG. 14C  is a diagram showing an insulator interposed between the first and second thin metal sheets in  FIG. 14B , and  FIG. 14D  is a diagram showing an external lead plate welded to the second thin metal sheet of  FIG. 14C . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an exploded perspective view of a safety element assembly  100  according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the safety element assembly  100  can include a safety element  101 , a first thin metal sheet  102 , a second thin metal sheet  103 , and an insulator  104 . The safety element assembly  100  is a safety device against overheat and overcurrent. The safety element assembly  100  protects a secondary battery by blocking current flowing into the secondary battery when overcurrent flows while the secondary battery is being charged or discharged. 
     The safety element  101  may be a positive temperature coefficient (PTC) element. The PTC element  101  is a polymer element containing conductive particles. 
     The PTC element  101  may be formed of a conductive polymer-based composite, and the conductive polymer-based composite may be a mixture of a polymer, a conductive filler, an anti-oxidization agent, and a peroxide coupling agent. The polymer may be, for example, high density polyethylene (HDPE), low density polyethylene (LDPE), vinylidene polyfluoride (PVDF), polypropylene (PP), or ethylene/polypropylene co-polymer. The conductive filler may be formed of carbon black, carbon fiber, a metal (e.g. nickel (Ni)), or a metal oxide. 
     Polymers are generally used as insulation materials. However, the PTC element  101  exhibits excellent conductivity because the conductive particles therein provide conduction paths by being mutually connected at or below room temperature due to the conductive filler. 
     When a temperature of a secondary battery in which the PTC element  101  is used exceeds a certain temperature or overcurrent flows, intervals between the conductive particles can increase as the polymer in the PCT element  101  expands, and conductive paths can thus be blocked. As a result, the conductivity of the PTC element  101  can significantly deteriorate. 
     The first thin metal sheet  102  may be located on a top surface of the PTC element  101 , whereas the second thin metal sheet  103  may be located on a bottom surface of the PTC element  101 . The first thin metal sheet  102  may be electrically connected to a terminal of a secondary battery, whereas the second thin metal sheet  103  may be electrically connected to a terminal of a protective circuit board module or a terminal of an external device. The first thin metal sheet  102  and the second thin metal sheet  103  may be thin nickel sheets, thin sheets each formed by stacking a thin nickel sheet and a thin copper sheet, or a thin nickel-copper alloy sheet. 
     Accordingly, the first thin metal sheet  102 , the PTC element  101 , and the second thin metal sheet  103  may be stacked on each other. When being stacked, the first thin metal sheet  102  may include a first region B on which is stacked the PTC element  101  and the second thin metal sheet  103 , and a second region A on which is not stacked the PTC element  101  and the second thin metal sheet  103  and is exposed. 
     The first thin metal sheet  102  may have a circular shape. The first thin metal sheet  102  may be formed to a size smaller than a portion of a secondary battery  300  to which the first thin metal sheet  102  is to be electrically connected. 
     The second thin metal sheet  103  may have a partial circular shape. The second thin metal sheet  103  may have the partial circular shape to expose the second region A of the first thin metal sheet  102  when the second thin metal sheet  103  is stacked on the first thin metal sheet  102 . 
     Although the second thin metal sheet  103  has a half-circular shape in the present embodiment, the shape of the second thin metal sheet  103  is not limited thereto, and the second thin metal sheet  103  may have any of various shapes as long as the second region A of the first thin metal sheet  102  may be exposed. A curvature radius R 2  of the second thin metal sheet  103  may be substantially the same as a curvature radius R 1  of the first thin metal sheet  102 . 
     The PTC element  101  may be interposed between the first thin metal sheet  102  and the second thin metal sheet  103 . The PTC element  101  can have a partial ring shape. Although the PTC element  101  has a half-ring shape in the illustrated embodiment, the shape of the PTC element  101  is not limited thereto. An outer curvature radius R 3  of the PTC element  101  may be substantially the same as the curvature radius R 1  of the first thin metal sheet  102  and the curvature radius R 2  of the second thin metal sheet  103 . 
     Accordingly, the first thin metal sheet  102 , the PTC element  102 , and the second thin metal sheet  103 , which are stacked on each other, may respectively have a circular shape, a partial ring shape, and a half-circular shape. Although the shapes of the first thin metal sheet  102 , the PTC element  101 , and the second thin metal sheet  103  are different from each other in the illustrated embodiment, the curvature radiuses R 1 , R 3 , and R 2  of the first thin metal sheet  102 , the PTC element  101 , and the second thin metal sheet  103  are the same. 
     Furthermore, an insulator  104  may be interposed between the first thin metal sheet  102  and the second thin metal sheet  103 . The insulator  104  may be employed so that the first thin metal sheet  102  and the second thin metal sheet  103  are not electrically connected to each other. Furthermore, the insulator  104  can prevent the second thin metal sheet  103  from being deformed when an external lead plate is welded to the second thin metal sheet  103 . 
     The insulator  104  may have a partial circular shape. Although the insulator  104  has a half-circular shape in the illustrated embodiment, the shape of the insulator  104  is not limited thereto. A curvature radius R 4  of the insulator  104  may be substantially the same as an inner curvature radius R 5  of the PTC element  101 . The insulator  104  may be positioned in an inner space formed by an inner wall of the PTC element  101 . 
     Furthermore, a thickness t 2  of the insulator  104  may be the same as a thickness t 1  of the PTC element  101 . Therefore, when the insulator  104  is positioned in the inner space formed by the inner wall of the PTC element  101 , the horizontal surfaces of the PTC element  101  and the insulator  104  can form an even horizontal surface with respect to a horizontal surface of the second thin metal sheet  103 . 
       FIG. 2  is a bottom view showing the safety element assembly  100  of  FIG. 1  attached to the secondary battery  300 , and  FIG. 3  is a sectional view showing the safety element assembly  100  of  FIG. 1  attached to the secondary battery  300 . 
     Here, the secondary battery  300  includes a can  301 . An electrode assembly (not shown), may be formed by rolling a positive electrode plate, a separator, and a negative electrode plate into a jelly-roll shape and may be housed in the can  310 , and the can  301  may be electrically connected to either the positive electrode plate or the negative electrode plate to have a positive or negative polarity. 
     Referring to  FIGS. 2 and 3 , the first thin metal sheet  102  may be welded to a bottom surface  302  of the can  301 . The size of the first thin metal sheet  102  may be less than the size of the bottom surface  302  of the can  301 . 
     The PTC element  101  and the insulator  104  may be disposed on a top surface of the first thin metal sheet  102 . The PTC element  101  may be thermally press-attached to the first thin metal sheet  102 . 
     The insulator  104  may be positioned in the inner space formed by the inner wall of the PTC element  101 . Since the thickness t 2  of the insulator  104  is the same as the thickness t 1  of the PTC element  101  in the illustrated embodiment, the horizontal surfaces of the PTC element  101  and the insulator  104  can form an even horizontal surface with respect to the second thin metal sheet  103 . Therefore, a flat surface may be provided for welding an external lead plate  320 , to be formed later. 
     At this point, the insulator  104  may be fixed to the first thin metal sheet  102  by using an adhesive, may be detachably interposed between the first thin metal sheet  102  and the second thin metal sheet  103 , or may be arranged otherwise. 
     The second thin metal sheet  103  is arranged on the horizontal surfaces of the PTC element  101  and the insulator  104 . The second thin metal sheet  103  is thermally press-attached to the PTC element  101 . 
     Here, the size of the first thin metal sheet  102  may be larger than the size of the PTC element  101  and the insulator  104  combined. Furthermore, the size of the first thin metal sheet  102  may be larger than the size of the second thin metal sheet  103 . Therefore, the second region A of the first thin metal sheet  102  is not covered by the PTC element  101 , the insulator  104 , or the second thin metal sheet  103 , and is exposed in the illustrated embodiment. On the other hand, the first region B of the second thin metal sheet  103  is exposed on a surface opposite to the surface of the second thin metal sheet  103  attached to the PTC element  101 . 
     In case of electric/electronic devices requiring high power energy, an end of an external lead plate  320  may be welded to the exposed second region A of the first thin metal sheet  102 . On the other hand, in case of electric/electronic device requiring safe and high capacity energy, an end of the external lead plate  320  as is indicated with a dotted line in  FIG. 3  may be welded to the first region B of the second thin metal sheet  103 . 
     When the external lead plate  310  is connected to the first thin metal sheet  102 , current flows directly to the first thin metal sheet  102  without flowing through the PTC element  101 . When the external lead plate  320  is connected to the second thin metal sheet  103 , current flows through the PTC element  101 . 
     Therefore, based on how the secondary battery  300  is to be used, the secondary battery  300  may be applied to electric/electronic devices requiring the safety element assembly  100  or electric/electronic device not requiring the safety element assembly  100 . In other words, the secondary battery  300  may selectively employ the safety element assembly  100  according to whether electric/electronic devices require high power or high capacity. 
     The safety element assembly  100  may be used in various manners as desired by being arranged not only inside the secondary battery  300 , but also on an exterior of the secondary battery  300  (e.g., a top surface, a bottom surface, or one of side surfaces). 
       FIG. 4  is an exploded perspective view of a safety element assembly  400  according to another embodiment of the present invention. 
     Hereinafter, like reference numerals in the drawings denote like elements. 
     Referring to  FIG. 4 , the safety element assembly  400  includes a safety element  401 , a first thin metal sheet  402 , a second thin metal sheet  403 , and an insulator  404 . 
     The safety element  401  may be a PTC element. 
     The first thin metal sheet  402  may be located on a top surface of the PTC element  401 , whereas the second thin metal sheet  403  may be located on a bottom surface of the PTC element  401 . The insulator  404  may be interposed between the first thin metal sheet  402  and the second thin metal sheet  403 . 
     Accordingly, the first thin metal sheet  402 , the PTC element  401 , and the second thin metal sheet  403  may be stacked on each other. When being stacked, the first thin metal sheet  402  can include a first region B on which the PTC element  401  and the second thin metal sheet  403  are stacked, and a second region A on which the PTC element  401  and the second thin metal sheet  403  are not stacked and thus is exposed. 
     In the illustrated embodiment, the first thin metal sheet  402  has a circular shape and the second thin metal sheet  403  has a partial circular shape. The second thin metal sheet  403  has the partial circular shape to expose the second region A of the first thin metal sheet  402  when the second thin metal sheet  403  is stacked on the first thin metal sheet  402 . 
     Although the second thin metal sheet  403  has a half-circular shape in the illustrated embodiment, the shape of the second thin metal sheet  403  is not limited thereto, and the second thin metal sheet  403  may have any of various shapes as long as the second region A of the first thin metal sheet  402  may be exposed. A curvature radius R 2  of the second thin metal sheet  403  may be substantially the same as a curvature radius R 1  of the first thin metal sheet  402 . 
     In the illustrated embodiment, the PTC element  401  has a circular shape that is substantially the same as the circular shape of the first thin metal sheet  402  except that the PTC element  401  has a partial circular shape. Although the PTC element  401  has the shape of a circle, from which half or more of the entire circle is removed, in the present embodiment, the shape of the PTC element  401  is not limited thereto. A curvature radius R 3  of the PTC element  401  may be substantially the same as the curvature radius R 1  of the first thin metal sheet  402  and the curvature radius R 2  of the second thin metal sheet  403 . 
     In the illustrated embodiment, the insulator  404  has a circular shape substantially the same as the circular shape of the first thin metal sheet  402  except that the insulator  404  has a circular shape of which two portions at opposite sides are cut. A curvature radius R 4  of the insulator  404  is substantially the same as the curvature radius R 1  of the first thin metal sheet  402 . 
     Furthermore, a length l 2  of a straight-cut end  404   a  of the insulator  404  may be the same as a length l 1  of a cut end  401   a  of the PTC element  401 . A thickness t 2  of the insulator  404  may be the same as a thickness t 1  of the PTC element  401 . 
     When the insulator  404  is attached to the PTC element  401 , the cut ends  401   a  and  404   a  may be arranged to contact each other, and horizontal surfaces of the insulator  404  and the  401  form an even horizontal surface. 
       FIG. 5  is a bottom view showing the safety element assembly  400  of  FIG. 4  attached to the secondary battery  300 , and  FIG. 6  is a sectional view showing the safety element assembly  400  of  FIG. 4  attached to the secondary battery  300 . 
     Referring to  FIGS. 5 and 6 , the first thin metal sheet  402  may be welded to the bottom surface  302  of the can  301 . The PTC element  401  and the insulator  404  may be arranged on a top surface of the second thin metal sheet  403  and form an even horizontal surface. The straight-cut portion  404   a  of the insulator  404  may contact the cut end  401   a  of the PTC element  401 . The second thin metal sheet  403  may be arranged on the PTC element  401  and the insulator  404 . 
     Here, the size of the first thin metal sheet  402  may be larger than sum of the size of the PTC element  401  and the size of the insulator  404 . Furthermore, the size of the first thin metal sheet  402  may be larger than the size of the second thin metal sheet  403 . Therefore, the second region A of the first thin metal sheet  402  can be exposed. On the other hand, the first region B of the second thin metal sheet  403  is exposed on a surface opposite to the surface of the second thin metal sheet  403  attached to the PTC element  401 . 
     Therefore, based on how the secondary battery  300  is to be used, ends of the external lead plates  310  and  320  may be selectively welded to the exposed second region A of the first thin metal sheet  402  or the exposed first region B of the second thin metal sheet  403 . 
       FIG. 7  is an exploded perspective view of a safety element assembly  700  according to another embodiment of the present invention, and  FIG. 8  is a diagram showing the safety element assembly  700  of  FIG. 7  attached to the secondary battery  300 . 
     Referring to  FIGS. 7 and 8 , the safety element assembly  700  includes a safety element  701 , a first thin metal sheet  702 , a second thin metal sheet  703 , and an insulator  704 . 
     The safety element  701  may be a PTC element. 
     In the illustrated embodiment, the first thin metal sheet  702  is located on a top surface of the PTC element  701 , whereas the second thin metal sheet  703  is located on a bottom surface of the PTC element  701 . The insulator  704  may be interposed between the first thin metal sheet  702  and the second thin metal sheet  703 . 
     Accordingly, the first thin metal sheet  702 , the PTC element  701 , and the second thin metal sheet  703  are stacked on each other. When being stacked, the first thin metal sheet  702  includes a first region B on which the PTC element  701  and the second thin metal sheet  703  are stacked, and a second region A on which the PTC element  701  and the second thin metal sheet  703  are not stacked and thus is exposed. 
     The PTC element  701 , the second thin metal sheet  703 , and the insulator  704  according to the illustrated embodiment have shapes and curvature radiuses respectively corresponding to those of the PTC element  101 , the second thin metal sheet  103 , and the insulator  104  according to the embodiment shown in  FIG. 1 . Therefore, the detailed descriptions thereof will be omitted here. 
     According to the present embodiment illustrated in  FIG. 7 , as compared to the embodiment shown in  FIG. 1 , the first thin metal sheet  702  has a ring shape instead of a circular shape, and the thickness t 2  of the insulator  704  is the same as the sum of the thickness t 1  of the PTC element  701  and the thickness t 3  of the first thin metal sheet  702 . 
     The first thin metal sheet  702  of the safety element assembly  700  having the configuration as described above may be welded to the bottom surface  302  of the can  301 . The PTC element  701  may be thermally press-attached to the first thin metal sheet  702 . 
     The insulator  704  may be positioned in an inner space formed by the inner wall of the PTC element  701 . Since the thickness t 2  of the insulator  704  is the same as the sum of the thickness t 1  of the PTC element  701  and the thickness t 3  of the first thin metal sheet  702 , the top surface of the insulator  704  can contact the bottom surface  302  of the can  301  and may be fixed thereto. Horizontal surfaces of the PTC element  701  and the insulator  704  may form even horizontal surfaces with respect to a horizontal surface of the second thin metal sheet  703 . 
     The second thin metal sheet  703  may be arranged on the PTC element  701  and the insulator  704 . The second thin metal sheet  703  may be thermally press-attached to the PTC element  701 . 
     Therefore, based on how the secondary battery  300  is to be used, ends of the external lead plates  310  and  320  may be selectively welded to the exposed second region A of the first thin metal sheet  702  or the exposed first region B of the second thin metal sheet  703 . 
       FIG. 9  is an exploded perspective view of a safety element assembly  900  according to another embodiment of the present invention, and  FIG. 10  is a diagram showing the safety element assembly  900  of  FIG. 4  attached to the secondary battery  300 . 
     Referring to  FIGS. 9 and 10 , the safety element assembly  900  includes a safety element  901 , a first thin metal sheet  902 , and a second thin metal sheet  903 . 
     The safety element  901  may be a PTC element. 
     In the illustrated embodiment, the first thin metal sheet  902  is located on a top surface of the PTC element  901 , whereas the second thin metal sheet  903  is located on a bottom surface of the PTC element  901 . 
     Accordingly, the first thin metal sheet  902 , the PTC element  901 , and the second thin metal sheet  903  are stacked on each other. When being stacked, the first thin metal sheet  902  includes a first region B on which the PTC element  901  and the second thin metal sheet  903  are stacked, and a second region A on which the PTC element  901  and the second thin metal sheet  903  are not stacked and thus is exposed. 
     In the illustrated embodiment, the PTC element  901 , the first thin metal sheet  902 , and the second thin metal sheet  903  according to the present embodiment have shapes and curvature radiuses respectively corresponding to those of the PTC element  401 , the first thin metal sheet  402 , and the second thin metal sheet  403  according to the embodiment shown in  FIG. 4 . Therefore, the detailed descriptions thereof will be omitted here. 
     According to the present embodiment illustrated in  FIG. 9 , as compared to the embodiment shown in  FIG. 4 , no insulator is interposed between the first thin metal sheet  902  and the second thin metal sheet  903 . Therefore, a space in which a welding jig may be selectively arranged and no insulator exists may be formed between the first thin metal sheet  901  and the second thin metal sheet  902  in the first region B of the first thin metal sheet  902 , on which the PTC element  901  is disposed. 
     The first thin metal sheet  902  of the safety element assembly  900  having the configuration as described above may be welded to the bottom surface  302  of the can  301 . The PTC element  901  may be thermally press-attached to the first thin metal sheet  902 . The second thin metal sheet  903  may be arranged on the PTC element  901 . The second thin metal sheet  903  may be thermally press-attached to the PTC element  901 . 
     Here, as shown in  FIG. 11 , the external lead plate  320  may be directly welded to the exposed second region A of the first thin metal sheet  903  without using a welding jig. 
     On the contrary, when the external lead plate  320  is welded to the exposed region B of the second thin metal sheet  903 , a welding jig  200  may be positioned in the space between the first thin metal sheet  902  and the second thin metal sheet  903 , in which no insulator exists, and then the external lead plate  320  may be welded to the second thin metal sheet  903  by using a welding rod  210 . Here, a portion of the external lead plate  320 , which is the portion facing the welding jig  200 , may be welded with respect to the second thin metal sheet  903 . 
       FIG. 12  is a diagram showing a safety element assembly  1000  according to another embodiment of the present invention attached to the secondary battery  300 . 
     Referring to  FIG. 12 , the safety element assembly  1000  may include a safety element  1010 , a first thin metal sheet  1020 , a second thin metal sheet  1030 , and an insulator  1040 . 
     Here, unlike the previous embodiments, the safety element  1010  may be a thermal fuse. When a temperature of the secondary battery  300  exceeds a predetermined point, electrical resistance of a PTC element can increase significantly, and thus the PTC element can shut down a circuit. However, when the temperature of the secondary battery  300  drops below the predetermined point, the electrical resistance of the PTC element can be reduced, and thus the PTC element may be re-used. However, according to the present embodiment, when the temperature of the secondary battery  300  exceeds a predetermined point, a conductive line in the thermal fuse  1010  may be physically cut to shut down a circuit. 
     The thermal fuse  1010  can shut off a circuit when the temperature of the secondary battery  300  significantly increases in abnormal environments or under abnormal conditions of using the secondary battery  300 , like a PTC element. However, unlike a PTC element, it is may become necessary to replace the safety element assembly  1000  after the thermal fuse  1010  is blown. 
     A first end of the thermal fuse  1010  may be electrically connected to the first thin metal sheet  1020 , a second end of the thermal fuse  1010  may be electrically connected to the second thin metal sheet  1030 , and the insulator  1040  may be interposed between the first thin metal sheet  1020  and the second thin metal sheet  1030  around the thermal fuse  1010 . 
     As described above, the embodiments shown in  FIGS. 1 through 12  provide safety element assemblies having substantially the same curvature radiuses. In other words, a safety element assembly having a configuration as described above may be applied to a cylindrical type secondary battery. 
     However, the safety element assembly may be applied not only to a cylindrical type secondary battery, but also to a rectangular type battery or a pouch type secondary battery. Here, the safety element assembly may be manufactured to have any non-cylindrical shape, as long as an exposed second region of a first thin metal sheet and an exposed first region of a second thin metal sheet are secured. 
     For example, referring to  FIG. 13 , a safety element assembly  1300  can include a safety element  1310 , a first thin metal sheet  1320 , a second thin metal sheet  1330 , and an insulator  1340 . 
     The safety element  1310  may be a PTC element. 
     The first thin metal sheet  1320  may be located on a top surface of the PTC element  1310 , the second thin metal sheet  1330  may be located on a bottom surface of the PTC element  1310 , and the insulator  1340  may be located on a sidewall of the PTC element  1310 . 
     Accordingly, the first thin metal sheet  1320 , the PTC element  1310 , and the second thin metal sheet  1330  may be stacked on each other. When being stacked, the first thin metal sheet  1320  can include a first region B on which the PTC element  1310  and the second thin metal sheet  1330  are stacked, and a second region A on which the PTC element  1310  and the second thin metal sheet  1330  are not stacked and thus is exposed. 
     The first thin metal sheet  1320  of the safety element assembly  1300  having the configuration as described above may be welded to the bottom surface  302  of the can  301 . The PTC element  1310  may be thermally press-attached to the first thin metal sheet  1320 . The second thin metal sheet  1330  may be thermally press-attached to the PTC element  1310 . 
     Here, an overall length l 1  of the first thin metal sheet  1320  may be longer than a sum of a length l 2  of the PTC element  1310  and a length l 3  of the insulator  1340 . Therefore, even when the PTC element  1310  and the insulator  1340  are stacked on the first thin metal sheet  1320 , the second region A of the first thin metal sheet  1320  may be exposed. 
     Furthermore, the overall length l 1  of the first thin metal sheet  1320  can be longer than a length l 4  of the second thin metal sheet  1330 . Meanwhile, the first region B of the second thin metal sheet  1330  may be exposed. 
     Therefore, based on how the secondary battery  300  is used, ends of the external lead plates  310  and  320  may be selectively welded to the exposed second region A of the first thin metal sheet  1320  or the exposed first region B of the second thin metal sheet  1330 . 
     Hereinafter, referring to  FIGS. 14A through 14D , a method of manufacturing the secondary battery  300 , to which the safety element assembly  100  shown in  FIG. 1  may be installed, will be described. 
     First, as shown in  FIG. 14A , the can  301  is prepared. Here, an electrode assembly may or may not be housed in the can  301 . 
     Next, as shown in  FIG. 14B , the safety element assembly  100 , in which the first thin metal sheet  102 , the PTC element  101 , and the second thin metal sheet  103  are sequentially stacked, may be attached to the bottom surface  302  of the can  301 . 
     Here, forming of the safety element assembly  100  is not limited to a specific sequence. For example, the insulator  104  may be fixed between the first thin metal sheet  102  and the second thin metal sheet  103 , or the insulator  104  may be inserted in a later operation. 
     The first thin metal sheet  102  may be attached with respect to the bottom surface  302  of the can  301  by using any of various methods, e.g., a welding method (e.g. electric resistance welding), a solder-attaching method, or an attaching method using a conductive adhesive. In the case of the electric resistance welding, the bottom surface  302  of the can  301  may be welded to the exposed second portion A of the first thin metal sheet  102 . Here, a number of welding points W 1  may be one. However, the number of the welding points W 1  may be two or more. 
     Next, as shown in  FIG. 14C , the insulator  104  may be positioned between the first thin metal sheet  102  and the second thin metal sheet  103 . The insulator  104  can be inserted into a space formed between the first thin metal sheet  102  and the second thin metal sheet  103 . Here, the insulator  104  may be either positioned on the inner wall of the PTC element  101  or a predetermined interval apart from the inner wall of the PTC element  101 . Alternatively, the insulator  104  may be fixed between the first thin metal sheet  102  and the second thin metal sheet  103  in advance by using a non-conductive adhesive. 
     Next, as shown in  FIG. 14D , the external lead plate  320  may be positioned on the exposed first region B of the second thin metal sheet  103  and may be welded thereto. The number of the welding points W 2  may be two or more. However, the present embodiment is not limited thereto. 
     In the case of the electric resistance welding, a welding rod can press the first region B of the second thin metal sheet  103 . However, since the insulator  104  supports the second thin metal sheet  103 , the second thin metal sheet  103  is not deformed during the welding operation in the illustrated embodiment. 
     Furthermore, damages to the PTC element  101  may be prevented by performing the welding operation with respect to the first region B of the second thin metal sheet  104  corresponding to the insulator  104 , instead of performing the welding operation with respect to a portion of the second thin metal sheet  103  corresponding to the PTC element  101 . 
     Accordingly, in the case where the external lead plate  320  is connected to the second thin metal sheet  103 , a discharging circuitry of the secondary battery  300  includes the PTC element  101 , and thus the secondary battery  300  may be suitable for electric/electronic devices requiring safety and high capacity. 
     Meanwhile, in the case where the external lead plate  310  is connected to the exposed second portion A of the first thin metal sheet  102 , the discharging circuitry of the secondary battery  300  does not include the PTC element  101 , and thus the secondary battery  300  may be suitable for electric/electronic devices requiring high power. 
     The following effects may be obtained from safety element assemblies according to embodiments of the present invention. 
     First, since a plurality of thin metal sheets having different shapes may be formed on the two opposite surfaces of a PTC element, different regions of the plurality of thin metal sheets may be exposed. Therefore, a standardized secondary battery manufactured in compliance with the same standards may be used either for high power or high capacity based on how the secondary battery is to be used. 
     Second, as an insulator may be interposed between a plurality of thin metal sheets, deformation of the plurality of thin metal sheets or a safety element while an external lead plate is being welded to the thin metal sheets may be prevented. 
     Third, a standardized secondary battery manufactured in compliance with the same standards may be used either for high power or high capacity by arranging a safety element assembly on various locations of the exterior of the secondary battery (e.g. a top surface, a bottom surface, or one of side surfaces). 
     While the present invention has been particularly shown and described with reference to certain embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.