Abstract:
A battery having a can containing an electrode assembly and an electrolyte and a cap assembly with a cap plate having an electrolyte injection unit, the cap plate is coupled to the can and an upper cover is coupled to the cap plate. The battery further includes a gas sensor located within the cap assembly to detect leakage of the electrolyte into the cap assembly and a protective circuit module mounted to the upper cover and electrically coupled to the gas sensor, the protective circuit module is adapted to stop a charge/discharge operation of the battery and discharge the battery upon receipt of a signal from the gas sensor indicating detection of leakage of the electrolyte into the cap assembly.

Description:
CLAIM OF PRIORITY 
     This application claims priority to and the benefit of Provisional Application No. 61/413,044, filed on Nov. 12, 2010, in the United States Patent and Trademark Office, the entire content of which is incorporated herein by reference. 
    
    
     BACKGROUND THE INVENTION 
     1. Field of the Invention 
     The general inventive concept generally relates to a battery pack, and more particularly, to a battery pack containing an electrolyte. 
     2. Description of the Related Art 
     Due to the development of technologies on and the increase in production of mobile devices such as mobile phones and laptop computers, secondary batteries are highly demanded as an energy source. Currently, research is actively conducted on an alternative energy source for replacing fossil fuel and to be used in electric and hybrid vehicles. 
     The above information disclosed in this Related Art Section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     One or more exemplary embodiments of the present invention include a battery pack capable of promptly and accurately sensing a leakage of an electrolyte. 
     One or more exemplary embodiments of the present invention include a battery pack capable of performing a protection operation according to a leakage of an electrolyte. 
     According to an exemplary embodiment of the present invention, a battery having a can containing an electrode assembly and an electrolyte and a cap assembly with a cap plate having an electrolyte injection unit, the cap plate may be coupled to the can and an upper cover may be coupled to the cap plate. The battery may include a gas sensor located within the cap assembly to detect leakage of the electrolyte into the cap assembly and a protective circuit module mounted to the upper cover and electrically coupled to the gas sensor, the protective circuit module may be adapted to stop a charge/discharge operation of the battery and discharge the battery upon receipt of a signal from the gas sensor indicating detection of leakage of the electrolyte into the cap assembly. 
     An aspect of the present invention battery provides for the gas sensor to may be a pressure sensor that detects a leakage of the electrolyte into the cap assembly by detecting an increase in vapor pressure in the cap assembly caused by the leakage of the electrolyte into the cap assembly. 
     An aspect of the present invention battery provides for the gas sensor detects a leakage of the electrolyte into the cap assembly by a change in resistance or electric capacity of the gas sensor caused by contact of the electrolyte with the gas sensor. 
     Another aspect of the present invention battery provides for the protection circuit module to further include a circuit substrate having a plurality of external connection terminals on an upper surface of the circuit substrate that protrude through a plurality of terminal holes in the upper cover; and a protection device coupled to a lower surface of the circuit substrate. The gas sensor may be affixed to the lower surface of the circuit substrate immediately above the electrolyte injection unit of the cap plate. 
     Another aspect of the present invention battery provides for the gas sensor to be coupled to a surface of the cap plate immediately adjacent to the electrolyte injection unit. 
     Another aspect of the present invention battery provides for the protection circuit module to further include a circuit substrate having a plurality of external connection terminals on an upper surface of the circuit substrate that protrude through a plurality of terminal holes in the upper cover; and a protection device coupled to a lower surface of the circuit substrate. The gas sensor may be affixed to the lower surface of the circuit substrate immediately above an electrode terminal that protrudes through the cap plate, said electrode terminal may be connected to a negative tap that may be connected to the electrode assembly. 
     Another aspect of the present invention battery provides for the can to be a case that may be flexible and includes an upper case and a lower case which are sealed to contain the electrode assembly and electrolyte, a positive tap and a negative tap that may be connected to the electrode assembly protrude from the case. 
     Another aspect of the present invention battery provides for the protection circuit module to further include a circuit substrate; a protection device coupled to a lower surface of the circuit substrate; a first terminal affixed to the lower surface of the circuit substrate and electrically connected to the positive tap; and a second terminal affixed to the lower surface of the circuit substrate and electrically connected to the negative tap. The gas sensor may be composed of a pair of gas sensors affixed to the lower surface of the circuit substrate in which one of the pair of gas sensors may be immediately adjacent to the first terminal and another of the pair of gas sensors may be immediately adjacent to the second terminal. 
     Another aspect of the present invention battery provides for the protection circuit module further includes a circuit substrate; a protection device coupled to a lower surface of the circuit substrate; a first terminal affixed to the lower surface of the circuit substrate and electrically connected to the positive tap; and a second terminal affixed to the lower surface of the circuit substrate and electrically connected to the negative tap. The gas sensor may be composed of a pair of gas sensors affixed to a surface of the case in which one of the pair of gas sensors may be immediately adjacent to the positive tap and another of the pair of gas sensors may be immediately adjacent to the negative tap. 
     Another aspect of the present invention battery provides for the protection circuit module further includes a first circuit unit electrically connected to the gas sensor to determine that a leakage of the electrolyte has occurred and to stop a charge/discharge operation of the battery and to forcibly discharge the battery; and a second circuit unit that monitors the charge/discharge state of the battery and controls the charge/discharge operation of the battery. 
     Another aspect of the present invention battery provides for the first circuit unit to further include a signal amplification unit that receives and amplifies the signal from gas sensor indicating the occurrence of the electrolyte leaking; an electrolyte leakage determination unit electrically connected to the signal amplification unit having a comparator that compares an output voltage of the signal of the gas sensor to a reference voltage and generates an output signal according to a result of the comparison; and a forcible discharge unit electrically connected to the electrolyte leakage determination unit having a first switch that is turned on or off according to the output signal of the comparator and a load resistor that forms a closed circuit together with the battery according to the on/off operation of the first switch. Upon the output signal of the comparator of the electrolyte leakage determination unit changes from a first level representing a normal state to a second level representing a leakage of the electrolyte, the battery may be forcibly discharged. 
     Another aspect of the present invention battery provides for the second circuit unit to further include a pair of external connection terminals that contact the battery to an external device; a switching device unit formed on a current path between the pair of external connection terminals and the battery; and a protection integrated circuit that controls the switching device unit. 
     Another aspect of the present invention battery provides for the protection integrated circuit to calculate an open circuit voltage of the battery and outputs a control signal to the switching device to turn off the switching device before the battery is overcharged or turn off the switching device before the battery is over discharged. 
     Another aspect of the present invention battery provides for the can to be a case that may be flexible and includes an upper case and a lower case which are sealed to contain the electrode assembly and electrolyte, a positive tap and a negative tap protrude from the case and may be connected to the electrode assembly. 
     Another aspect of the present invention battery provides for an air tight cover having an upper cover and a lower cover to completely encapsulate the case, positive and negative taps and the protection circuit module. 
     Another aspect of the present invention battery provides for the protection circuit module to further include a circuit substrate; a protection device coupled to a lower surface of the circuit substrate; a first terminal affixed to the lower surface of the circuit substrate and electrically connected to the positive tap; and a second terminal affixed to the lower surface of the circuit substrate and electrically connected to the negative tap. The gas sensor may be composed of a pair of gas sensor affixed to the lower surface of the circuit substrate in which one of the pair of gas sensors may be immediately adjacent to the first terminal and another of the pair of gas sensors may be immediately adjacent to the second terminal. 
     Another aspect of the present invention battery provides for the protection circuit module to further include a circuit substrate; a protection device coupled to a lower surface of the circuit substrate; a first terminal affixed to the lower surface of the circuit substrate and electrically connected to the positive tap; and a second terminal affixed to the lower surface of the circuit substrate and electrically connected to the negative tap. The gas sensor may be composed of a pair of gas sensors affixed to a surface of the case in which one of the pair of gas sensors may be immediately adjacent to the positive tap and another of the pair of gas sensors may be immediately adjacent to the negative tap. 
     Another aspect of the present invention battery provides for the protection circuit module to further include a first circuit unit electrically connected to the gas sensor to determine that a leakage of the electrolyte has occurred and to stop a charge/discharge operation of the battery cell and to forcibly discharge the battery cell; and a second circuit unit that monitors the charge/discharge state of the battery cell and controls the charge/discharge operation of the battery cell. 
     Another aspect of the present invention battery provides for the first circuit unit to further include a signal amplification unit that receives and amplifies the signal from gas sensor indicating the occurrence of the electrolyte leaking; an electrolyte leakage determination unit electrically connected to the signal amplification unit having a comparator that compares an output voltage of the signal of the gas sensor to a reference voltage and generates an output signal according to a result of the comparison; and a forcible discharge unit electrically connected to the electrolyte leakage determination unit having a first switch that is turned on or off according to the output signal of the comparator and a load resistor that forms a closed circuit together with the battery cell according to the on/off operation of the first switch. Upon the output signal of the comparator of the electrolyte leakage determination unit changes from a first level representing a normal state to a second level representing a leakage of the electrolyte, the battery cell may be forcibly discharged. 
     Another aspect of the present invention battery provides for the second circuit unit to further include a pair of external connection terminals that contact the battery cell to an external device; a switching device unit formed on a current path between the pair of external connection terminals and the battery cell; and a protection integrated circuit that controls the switching device unit. 
     Another aspect of the present invention battery provides for the protection integrated circuit to calculate an open circuit voltage of the battery cell and outputs a control signal to the switching device to turn off the switching device before the battery cell is overcharged turn off the switching device before the battery cell is over discharged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same 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: 
         FIGS. 1 and 2  are exploded perspective views of a battery pack according to an embodiment of the present invention; 
         FIG. 3  is a cross-sectional view cut along a line illustrated in  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of a battery pack according to another embodiment of the present invention; 
         FIG. 5  is a cross-sectional view cut along a line V-V illustrated in  FIG. 4 ; 
         FIG. 6  is an exploded perspective view of a battery pack according to another embodiment of the present invention; 
         FIG. 7  is a cross-sectional view cut along a line VII-VII illustrated in  FIG. 6 ; 
         FIG. 8  is an exploded perspective view of a battery cell according to another embodiment of the present invention; 
         FIG. 9  is an exploded perspective view of a battery pack including the battery cell illustrated in  FIG. 8 ; 
         FIG. 10  is an exploded perspective view of a battery pack according to another embodiment of the present invention; and 
         FIG. 11  is a circuit diagram of a protection circuit module applicable to a battery pack according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. 
     Recognizing that sizes and thicknesses of constituent members shown in the accompanying drawings are arbitrarily given for better understanding and ease of description, the present invention is not limited to the illustrated sizes and thicknesses. 
     In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. Alternatively, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     In order to clarify the present invention, elements extrinsic to the description are omitted from the details of this description, and like reference numerals refer to like elements throughout the specification. 
     In several exemplary embodiments, constituent elements having the same configuration are representatively described in a first exemplary embodiment by using the same reference numeral and only constituent elements other than the constituent elements described in the first exemplary embodiment will be described in other embodiments. 
     In a secondary battery containing an electrolyte, although sealing is performed to prevent a leakage of the electrolyte, if overcharge is repeated, battery temperature is increased, an effort occurs in sealing of a case, etc., the electrolyte may leak out of the secondary battery. 
     According to the leakage of the electrolyte, a short or a malfunction of an electric circuit may occur, or a large amount of a current may flow along the leaked electrolyte to cause an accident such as heating, smoking, or firing. 
     According to one or more embodiments of the present invention, a battery pack capable of promptly and accurately sensing a leakage of an electrolyte by installing a gas sensor for detecting the leakage of the electrolyte, and thus performing a protection operation according to the leakage of the electrolyte is provided. Thus, in a secondary battery containing an electrolyte, a leakage of the electrolyte may be promptly detected by disposing a gas sensor at a location vulnerable to the leakage of the electrolyte, e.g., on an electrolyte injection unit or where electrode taps protrude. 
     Also, after a leakage of an electrolyte is sensed, an appropriate protection operation for stopping a charge/discharge operation of a battery cell and forcibly discharging the battery cell to exhaust charges may be performed. 
     Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. 
       FIGS. 1 and 2  are exploded perspective views of a battery pack according to an embodiment of the present invention.  FIG. 3  is a cross-sectional view cut along a line illustrated in  FIG. 2 . 
     Referring to  FIGS. 1 through 3 , the battery pack may include a battery cell  100  that is chargeable/dischargeable, a protection circuit module  150  mounted on the battery cell  100  to control a charge/discharge operation, and an upper cover  160  combined with the battery cell  100  to accommodate the protection circuit module  150 . 
     The battery cell  100  is a rechargeable secondary battery, may be a lithium-ion battery, and may be formed by sealing in a can  20  an electrolyte (not shown) and an electrode assembly  10  including positive and negative plates  11  and  13 , and a separator  15 . For example, the battery cell  100  includes the electrode assembly  10  in which a stack of the positive and negative plates  11  and  13 , and the separator  15  is wound in a jelly-roll type, the can  20  of which an upper portion is open to accommodate the electrode assembly  10  and the electrolyte, and a cap plate  30  for sealing the upper portion of the can  20 . Contact portions between the cap plate  30  and the can  20  may be laser-welded to form an airtight structure. 
     Positive and negative taps  17  and  19  may be respectively connected to the positive and negative plates  11  and  13 . For example, the positive tap  17  may be connected to the cap plate  30  and the negative tap  19  may be connected to an electrode terminal  31  protruding above the cap plate  30 . The electrode terminal  31  may be insulatively connected to the cap plate  30 , and may protrude above the cap plate  30 . For example, an insulating gasket  33  may be interposed between the electrode terminal  31  and the cap plate  30 . 
     The protection circuit module  150  controls the charge/discharge operation of the battery cell  100 , and may block a flowing current to protect the battery cell  100  if overcharge/overdischarge occurs, if an over current greater than a threshold value flows, or if the battery cell  100  is heated to a temperature greater than a setup value. Also, the protection circuit module  150  may sense a leakage of the electrolyte to perform an appropriate protection operation. For example, the protection circuit module  150  may stop the charge/discharge operation of the battery cell  100  and may forcibly discharge the battery cell  100  to exhaust charges. 
     The protection circuit module  150  may include a circuit substrate  140  including a charge/discharge protection circuit and a sensing circuit for detecting state information such as a current or a voltage, and a protection device  120  connected to the circuit substrate  140  to restrict a charge/discharge current according to an increase in temperature. 
     The protection device  120  may be formed on a charge/discharge path of the battery cell  100 . In more detail, the protection device  120  forms a charge/discharge current path between the circuit substrate  140  and the electrode terminal  31  on the cap plate  30 . The protection device  120  increases an electric resistance if the temperature of the battery cell  100  exceeds a set threshold value so as to forcibly reduce a charge/discharge current. The protection device  120  may include a positive temperature coefficient (PTC) device or a temperature fuse. 
     The protection device  120  formed of a PTC device may include a variable resistor  120   c  of which an electric resistance value varies according to temperature, and first and second connection members  120   a  and  120   b  connected to the variable resistor  120   c  and extending in opposite directions. For example, the first connection member  120   a  may be connected to the electrode terminal  31  on the cap plate  30  while the second connection member  120   b  may be connected to the circuit substrate  140 . 
     The positive and negative taps  17  and  19  of the battery cell  100  may be connected to the circuit substrate  140 . For example, the negative tap  19  of the battery cell  100  may be connected to the circuit substrate  140  via the electrode terminal  31  and the protection device  120 , and the positive tap  17  of the battery cell  100  may be connected to the circuit substrate  140  via the cap plate  30  and a lead member  132 . The lead member  132  may be in the form of stairs to include a lower portion connected onto the cap plate  30 , and an upper portion extending to contact the circuit substrate  140 . For example, the lead member  132  may be welded onto the cap plate  30  or may be coupled onto the cap plate  30  by using a coupling member (not shown) that penetrates the cap plate  30 . 
     Insulating members such as an insulating tape  111  and an insulating spacer  112  may be interposed between the protection circuit module  150  and the battery cell  100 . For example, the insulating tape  111  and the insulating spacer  112  may be interposed between the protection circuit module  150  and the cap plate  30 . The insulating tape  111  and the insulating spacer  112  basically performs an insulating function for preventing an electric short between the protection circuit module  150  and the cap plate  30 . Also, the insulating tape  111  and the insulating spacer  112  may also fix the protection circuit module  150  on the cap plate  30 . The insulating spacer  112  may support a portion of the protection circuit module  150  at a certain height from the cap plate  30 , and may support a height step formed between the first and second connection members  120   a  and  120   b  at two sides of the protection device  120 . 
     The upper cover  160  may be assembled on the battery cell  100  on which the protection circuit module  150  may be mounted, so as to accommodate the protection circuit module  150 . In the upper cover  160 , terminal holes  160 ′ may be formed in an opening pattern to expose and connect external connection terminals  145  of the circuit substrate  140  to an external device. 
     An insulating label sheet (not shown) may surround and be adhered onto an outer circumferential surface of the battery cell  100 , and an adhesive means  171  such as double-sided tape may be disposed on a bottom surface of the battery cell  100  to combine a lower cover  170 . 
     An electrolyte injection unit  131  may be formed on the cap plate  30 . The electrolyte injection unit  131  may include a sealing member  135  for sealing an injection hole  35  that penetrates the cap plate  30 . For example, the electrolyte may be injected into the can  20  through the injection hole  35  of the cap plate  30  and, after the electrolyte is completely injected, the injection hole  35  may be sealed by using the sealing member  135 . The sealing member  135  may be, for example, a plug fitted into the injection hole  35  or a resin material filled in the injection hole  35 . In more detail, the plug may be ball-shaped base metal formed of aluminum (Al) or Al-containing metal, and may be put on the injection hole  35  and then mechanically pressed into the injection hole  35 . Also, the plug may be welded around the injection hole  35  of the cap plate  30 . 
     A gas sensor  180  may be disposed adjacent to the electrolyte injection unit  131 . The gas sensor  180  senses a leakage of the electrolyte to prevent an unexpected accident. 
     The gas sensor  180  determines whether the electrolyte leaks by sensing an increase in gas pressure due to a leakage of the electrolyte. However, the current embodiment is not limited thereto and the gas sensor  180  senses, for example, the gas sensor  180  may determine whether the electrolyte leaks, by sensing a variation in electrical characteristics such as a resistance or an electric capacity, due to direct contact of the electrolyte. 
     The gas sensor  180  may be disposed adjacent to the electrolyte injection unit  131 . As the gas sensor  180  may be disposed adjacent to the electrolyte injection unit  131  through which the electrolyte may possibly leak, a leakage of the electrolyte may be promptly and accurately sensed. Since the electrolyte may leak due to poor sealing of the electrolyte injection unit  131 , corrosion of the sealing member  135  due to a long-time contact with the electrolyte, etc., a leakage of the electrolyte may be promptly and accurately sensed by disposing the gas sensor  180  adjacent to the electrolyte injection unit  131 . 
     For example, a signal output terminal (not shown) of the gas sensor  180  may be electrically connected to the protection circuit module  150 , and the gas sensor  180  may output to the protection circuit module  150  raw data for determining whether the electrolyte leaks. The protection circuit module  150  determines whether the electrolyte leaks, based on an output signal of the gas sensor  180 . If it is determined that the electrolyte leaks, the protection circuit module  150  performs a protection operation for stopping a charge/discharge operation of the battery cell  100  and forcibly discharging the battery cell  100 . 
     When the gas sensor  180  may be disposed adjacent to the electrolyte injection unit  131 , as illustrated in  FIG. 3 , the gas sensor  180  may be disposed on the circuit substrate  140  facing the electrolyte injection unit  131 . As the gas sensor  180  may be mounted on the circuit substrate  140 , the connection between the gas sensor  180  and the circuit substrate  140  may be simplified and shortened. For example, the output signal of the gas sensor  180  may be input to the protection circuit module  150  and may be transmitted to a corresponding region of the protection circuit module  150  via a wiring pattern (not shown) formed on the circuit substrate  140 . 
       FIG. 4  is an exploded perspective view of a battery pack according to another embodiment of the present invention.  FIG. 5  is a cross-sectional view cut along a line V-V illustrated in  FIG. 4 . Referring to  FIGS. 4 and 5 , a gas sensor  280  is disposed adjacent to the electrolyte injection unit  131  formed on the cap plate  30 . For example, the gas sensor  280  detects a leakage of the electrolyte by sensing an increase in gas pressure due to the leakage of the electrolyte. As the gas sensor  280  is disposed adjacent to the electrolyte injection unit  131  that is relatively vulnerable to a leakage of the electrolyte, the leakage of the electrolyte may be promptly and accurately detected from the electrolyte injection unit  131 . 
     In the current embodiment, the gas sensor  280  may be mounted on the cap plate  30  adjacent to the electrolyte injection unit  131 . Although not shown in  FIGS. 4 and 5 , for example, a signal output terminal (not shown) of the gas sensor  280  may be electrically connected to the protection circuit module  150 , and the protection circuit module  150  may sense a leakage of the electrolyte based on an output signal of the gas sensor  280  and may perform a protection operation corresponding to the leakage of the electrolyte. For example, the protection circuit module  150  starts an operation for stopping a charge/discharge operation of the battery cell  100  and forcibly discharging the battery cell  100 . 
     As the gas sensor  280  may be mounted on the cap plate  30 , a design modification or redesigning of the circuit substrate  140  according to adding of the gas sensor  280  may be avoided or minimized, and a mounting space of the circuit substrate  140  on which various circuit devices directly related to the performance of the battery pack are mounted may be efficiently utilized. 
       FIG. 6  is an exploded perspective view of a battery pack according to another embodiment of the present invention.  FIG. 7  is a cross-sectional view cut along a line VII-VII illustrated in  FIG. 6 . Referring to  FIGS. 6 and 7 , the battery pack includes the battery cell  100  including the electrode assembly  10  impregnated with an electrolyte (not shown), the protection circuit module  150  mounted on the battery cell  100 , and the upper cover  160  assembled on the battery cell  100  to accommodate the protection circuit module  150 . 
     The electrode terminal  31  electrically connected to an electrode tap, e.g., the negative tap  19 , of the electrode assembly  10  is exposed on the cap plate  30  of the battery cell  100 . The electrode terminal  31  may be insulatively assembled with the cap plate  30  by interposing the insulating gasket  33  therebetween, and may be externally exposed to be electrically connected to the protection circuit module  150 . The insulating gasket  33  may prevent a leakage of the electrolyte through a gap between the electrode terminal  31  and the cap plate  30 , and may be formed around the electrode terminal  31  to insulatively combine the electrode terminal  31  and the cap plate  30 . Meanwhile, the electrolyte injection unit  131  for injecting the electrolyte may be formed on the cap plate  30 , and may include the injection hole  35  that penetrates the cap plate  30 , and the sealing member  135  for sealing the injection hole  35 . 
     A gas sensor  380  for sensing a leakage of the electrolyte may be disposed adjacent to the electrode terminal  31 . For example, the gas sensor  380  determines whether the electrolyte leaks, by sensing an increase in gas pressure. A signal output terminal (not shown) of the gas sensor  380  may be electrically connected to the protection circuit module  150 , and may output to the protection circuit module  150  raw data for determining whether the electrolyte leaks. The protection circuit module  150  may determine whether the electrolyte leaks, based on an output signal of the gas sensor  380 . For example, if it is determined that the electrolyte leaks, the protection circuit module  150  performs a protection operation for stopping a charge/discharge operation of the battery cell  100  and forcibly discharging the battery cell  100 . 
     The electrode terminal  31  is connected to the protection circuit module  150 . For example, the electrode terminal  31  may be welded to the first connection member  120   a  protruding from the protection device  120 . In this case, due to heat generated when the electrode terminal  31  is welded, the insulating gasket  33  closely contacting the electrode terminal  31  may be damaged and thus the electrolyte may leak adjacent to the electrode terminal  31 . Therefore, as the gas sensor  380  may be disposed adjacent to the electrode terminal  31  through which the electrolyte may possibly leak, a leakage of the electrolyte may be promptly and accurately detected. A leakage of the electrolyte adjacent to the electrode terminal  31  may occur when the welding of the electrode terminal  31  becomes poor, sealing characteristics of the insulating gasket  33  around the electrode terminal  31  are reduced, a chemical resistance of the insulating gasket  33  is reduced due to a long-time contact with the electrolyte, etc., and may be promptly and accurately sensed by disposing the gas sensor  380  adjacent to the electrode terminal  31 . 
     The gas sensor  380  may be mounted adjacent to the electrode terminal  31 , and more particularly, on the protection circuit module  150  facing the electrode terminal  31 . For example, as illustrated in  FIGS. 6 and 7 , the gas sensor  380  may be mounted on a surface the circuit substrate  140  facing the electrode terminal  31 . As the gas sensor  380  may be directly mounted on the circuit substrate  140 , an additional connection structure for connecting the signal output terminal of the gas sensor  380  to the circuit substrate  140  may not be formed, and an output signal of the gas sensor  380  may be directly transmitted to a corresponding region of the protection circuit module  150  via a wiring pattern (not shown) formed on the circuit substrate  140 . 
     Although the gas sensor  380  may be mounted on the circuit substrate  140  facing the electrode terminal  31  as illustrated in  FIGS. 6 and 7 , the current embodiment is not limited thereto and the gas sensor  380  may be disposed, for example, on the cap plate  30  adjacent to the electrode terminal  31 . As the gas sensor  380  may be mounted on the cap plate  30 , a design modification or redesigning of the protection circuit module  150 , e.g., the circuit substrate  140 , according to adding of the gas sensor  380  may be avoided or minimized. Meanwhile, the signal output terminal of the gas sensor  380  may be electrically connected to the protection circuit module  150  via an appropriate wiring (not shown). 
       FIG. 8  is an exploded perspective view of a battery cell  300  according to another embodiment of the present invention.  FIG. 9  is an exploded perspective view of a battery pack including the battery cell  300  illustrated in  FIG. 8 . Referring to  FIGS. 8 and 9 , the battery pack includes the battery cell  300 , and a protection circuit module  410  electrically connected to the battery cell  300 . 
     Referring to  FIG. 8 , the battery cell  300  includes an electrode assembly  310 , and a case  320  for accommodating the electrode assembly  310 . The electrode assembly  310  includes positive and negative plates  311  and  312 , and a separator  313  interposed between the positive and negative plates  311  and  312 . Electrode taps for forming a charge/discharge current path are formed on the electrode assembly  310 . The electrode taps may include positive and negative taps  314  and  315  that may be electrically connected to the positive and negative plates  311  and  312 . 
     Insulating tapes  316  and  317  may be respectively wrapped around the positive and negative taps  314  and  315 . The insulating tapes  316  and  317  may be formed to contact a sealing surface  324  of the case  320  so as to insulate the positive and negative taps  314  and  315  from the case  320 , and may be thermally melted together with the sealing surface  324  of the case  320  to increase sealing characteristics of the case  320 . 
     The case  320  includes an upper case  321  and a lower case  322  and, for example, at least one-side surfaces of the upper and lower cases  321  and  322  may be integrally combined. A portion where the upper and lower cases  321  and  322  are integrally combined may form a folding part, and the upper and lower cases  321  and  322  may be folded in facing directions by using the folding part such that a space  323  for accommodating the electrode assembly  310  is sealed. The case  320  may be a flexible pouch-type case including a metal foil  320   a , and insulating films  320   b  and  320   c  stacked on two surfaces of the metal foil  320   a.    
     The electrode assembly  310  may be located in the space  323  provided by the case  320 . The upper and lower cases  321  and  322  for accommodating the electrode assembly  310  may be folded with respect to the folding part in facing directions. Facing sealing parts of the upper and lower cases  321  and  322  may be thermally melted and thus bonded to each other, and the insulating tapes  316  and  317  wrapped around the positive and negative taps  314  and  315  may be thermally melted together with the case  320  between the sealing parts of the upper and lower cases  321  and  322 . Ends of the positive and negative taps  314  and  315  may protrude from the sealed case  320 . 
     As illustrated in  FIG. 9 , the positive and negative taps  314  and  315  protruding from the case  320  may be electrically connected to first and second terminals  414  and  415  formed on a circuit substrate  411 . For example, the positive tap  314  may be electrically connected to the first terminal  414  of the circuit substrate  411 , and the negative tap  315  may be electrically connected to the second terminal  415  of the circuit substrate  411 . 
     The protection circuit module  410  electrically connected to the battery cell  300  may include a circuit substrate  411  including a charge/discharge protection circuit and a sensing circuit for detecting state information such as a current or a voltage, and a protection device  413  connected to the circuit substrate  411  to restrict a charge/discharge current according to an increase in temperature. 
     The protection device  413  may be formed on a charge/discharge path of the battery cell  300 . In more detail, the protection device  413  may be formed on a current path induced by the positive and negative taps  314  and  315 , and may be connected to the second terminal  415  connected to the negative tap  315  via, for example, a circuit wiring. 
     The protection device  413  increases an electric resistance if the temperature of the battery cell  300  exceeds a set threshold value so as to forcibly reduce a charge/discharge current. The protection device  413  may include a PTC device or a temperature fuse. 
     Gas sensors  480  may be disposed adjacent to the positive and negative taps  314  and  315  of the battery cell  300 . The gas sensors  480  convert a variation in gas pressure into an electrical signal and output the electrical signal, and the output signal of the gas sensors  480  may be input to the protection circuit module  410  connected to an output terminal (not shown) of the gas sensors  480 . The gas sensors  480  detect whether an electrolyte (not shown) leaks, by measuring a variation in gas pressure. However, the current embodiment is not limited thereto and, for example, the gas sensors  480  may determine whether the electrolyte leaks, by sensing a variation in electrical characteristics such as a resistance or an electric capacity, due to direct contact of the electrolyte. 
     The positive and negative taps  314  and  315  of the battery cell  300  may protrude externally between the combined sealing parts of the upper and lower cases  321  and  322 , and the electrolyte filled in the battery cell  300  may leak externally through a gap between the positive and negative taps  314  and  315  and the upper and lower cases  321  and  322 . In this case, the gas sensors  480  may be disposed adjacent to the positive and negative taps  314  and  315  to promptly and accurately sense a leakage of the electrolyte. 
     For example, a leakage of the electrolyte may occur due to deterioration of step coverage characteristics regarding whether the combined upper and lower cases  321  and  322  may completely seal stepped portions where the positive and negative taps  314  and  315  protrude externally, or deterioration in sealing characteristics between the upper and lower cases  321  and  322  and the positive and negative taps  314  and  315  due to a long-time operation. In the current embodiment, a leakage of the electrolyte may be promptly and accurately sensed by disposing the gas sensors  480  adjacent to the positive and negative taps  314  and  315 . 
     The gas sensors  480  may be formed adjacent to the positive and negative taps  314  and  315 , and more particularly, on the circuit substrate  411  facing the positive and negative taps  314  and  315 . As the gas sensors  480  may be directly mounted on the circuit substrate  411 , an additional signal wiring for transmitting an output signal of the gas sensors  480  is not required, and the output signal of the gas sensors  480  may be directly transmitted to a corresponding region of the protection circuit module  410  via a wiring pattern (not shown) on the circuit substrate  411 . 
     The gas sensors  480  may be formed in a pair to correspond to the positive and negative taps  314  and  315  protruding from the case  320 . However, the current embodiment is not limited thereto and, for example, one gas sensor  480  may be formed to correspond one of the positive and negative taps  314  and  315  or at a location between the positive and negative taps  314  and  315 , so as to sense a leakage of the electrolyte. 
     As illustrated in  FIG. 9 , a cover may include upper cover  490   a  and lower cover  490   b , and the upper and lower covers ( 490   a  and  490   b ) would be assembled to each other along the facing direction with the batter cell  300  with protection circuit module  410  therebetween, and such configuration may provide a sealed space accommodating the gas sensor. 
       FIG. 10  is an exploded perspective view of a battery pack according to another embodiment of the present invention. Referring to  FIG. 10 , gas sensors  580  for sensing a leakage of the electrolyte may be disposed adjacent to the positive and negative taps  314  and  315  protruding from the case  320 . The gas sensors  580  may be disposed at a side of the case  320 , where the positive and negative taps  314  and  315  protrude. For example, as illustrated in  FIG. 10 , the gas sensors  580  may be disposed at a side of sealing parts of the case  320 , which may be thermally melted together by interposing the positive and negative taps  314  and  315  therebetween. Although not shown in  FIG. 10 , output signal terminals (not shown) of the gas sensors  580  may be electrically connected to the protection circuit module  410 , and appropriate signal wirings (not shown) for the electrical connection may be formed between the gas sensors  580  and the protection circuit module  410 . 
     As the gas sensors  580  may be mounted at a side of the case  320  instead of on the protection circuit module  410 , a design modification or redesigning of the protection circuit module  410 , e.g., the circuit substrate  411 , according to adding of the gas sensors  580  may be avoided or minimized, and a mounting space of the protection circuit module  410  on which a plurality of electric devices directly related to the performance of the battery pack may be mounted may be saved. 
     As illustrated in  FIG. 10 , a cover may include upper cover  490   a  and lower cover  490   b , and the upper and lower covers ( 490   a  and  490   b ) would be assembled to each other along the facing direction with the batter cell  300  with protection circuit module  410  therebetween, and such configuration may provide a sealed space accommodating the gas sensor. 
       FIG. 11  is a circuit diagram of a protection circuit module applicable to a battery pack according to an embodiment of the present invention. The protection circuit module includes a first circuit unit for sensing a leakage of an electrolyte to stop a charge/discharge operation of a battery cell  610  and to forcibly start to discharge the battery cell  610 , and a second circuit unit for forming a charge/discharge path with an external device (e.g., an external power supply device or an external load) and controlling the charge/discharge operation of the battery cell  610  according to a charge/discharge state. Although the first and second circuit units are separated in  FIG. 11  for convenience of explanation, the first and second circuit units may be mixed with each other and thus may not be clearly separated in a functional or structural way. 
     The first circuit unit senses a leakage of the electrolyte and is involved in a protection operation according to the leakage of the electrolyte. The second circuit unit monitors the charge/discharge state of the battery cell  610  to control the charge/discharge operation. For example, the second circuit unit may sense overheating of the battery cell  610  to control the charge/discharge operation regardless of a leakage of the electrolyte. 
     The first circuit unit includes an electrolyte leakage determination unit for determining whether the electrolyte leaks, based on an output signal of a gas sensor  680 , a forcible discharge unit for forcibly discharging the battery cell  610  if it is determined that the electrolyte leaks, and a charge/discharge switch  690  for stopping a normal charge/discharge operation performed in connection with an external device. 
     The gas sensor  680  converts information regarding a gas pressure sensed adjacent to, for example, an electrolyte injection unit, into an electrical signal. The output signal of the gas sensor  680  may be input to the electrolyte leakage determination unit via a signal amplification unit  620 . 
     The electrolyte leakage determination unit determines whether the electrolyte leaks, by comparing an output voltage of the gas sensor  680  to a reference voltage. In more detail, the electrolyte leakage determination unit may include a comparator  630  for comparing the output voltage of the gas sensor  680  to the reference voltage, and generating an output signal according to the comparison result. The comparator  630  outputs a signal of a high level or a low level according to a sign of the difference between the output voltage and the reference voltage. 
     The output voltage of the gas sensor  680  may be input to a non-inverting terminal of the comparator  630 , and the reference voltage may be input to an inverting terminal of the comparator  630 . For example, if the output voltage of the gas sensor  680  is lower than the reference voltage, the comparator  630  may output a low-level signal representing that the gas pressure is normal. If the output voltage of the gas sensor  680  is higher than the reference voltage, the comparator  630  may output a high-level signal representing that the gas pressure is abnormal. Accordingly, immediately when the gas sensor  680  senses a leakage of the electrolyte, the output signal of the comparator  630  may be transited from a low level to a high level. Alternatively, the terminals to which the output signal of the gas sensor  680  and the reference voltage are input may be switched or, according to operation methods of the gas sensor  680  and the comparator  630 , the output signal of the comparator  630  may be transited from a high level to a low level immediately when a leakage of the electrolyte is sensed. 
     A reference voltage generation unit generates and outputs a predetermined reference voltage to the comparator  630 . Although not shown in  FIG. 11 , the reference voltage generation unit may be realized by using reference resistance elements for distributing a power voltage. 
     The output signal of the comparator  630  may be input to the forcible discharge unit. The forcible discharge unit includes a first switch  680  to be turned on/off according to the output signal of the comparator  630 , and a load resistor for forming a closed circuit together with the battery cell  610  according to the on/off operation of the first switch  680 . 
     The first switch  680  may be connected to an output terminal of the comparator  630  by interposing a resistor R 1  therebetween. For example, if the output signal of the comparator  630  is in a high level, the first switch  680  is turned on. As such, a current flows through the load resistor connected to the first switch  680 , the battery cell  610  and the load resistor form a closed circuit, and thus the battery cell  610  starts to be forcibly discharged. That is, if the output signal of the electrolyte leakage determination unit is transited from a first level representing a normal state to a second level representing a leakage of the electrolyte, the battery cell  610  starts to be forcibly discharged. 
     As described above, when a leakage of the electrolyte is sensed, the protection circuit module forcibly discharges the battery cell  610  and stops a normal charge/discharge operation performed in connection with an external device such as an external power supply device or an external load. In more detail, if forcible discharge starts, a voltage drop occurs due to the load resistor, the charge/discharge switch  690  (or the second switch) connected below the load resistor is turned off, and thus the charge/discharge path of the battery cell  610  connected to external connection terminals  605  and  606  is blocked. 
     The charge/discharge switch  690  is associated with the first switch  680  for opening/closing a current path for forcible discharge, and blocks the charge/discharge path of the battery cell  610  connected to the external connection terminals  605  and  606  immediately when forcible discharge starts. Consequently, if the output signal of the electrolyte leakage determination unit is transited from a first level representing a normal state to a second level representing a leakage of the electrolyte, the battery cell  610  starts to be forcibly discharged and the charge/discharge path of the battery cell  610  connected to the external connection terminals  605  and  606  is blocked. The first switch  680  and the charge/discharge switch  690  may be various switch devices such as field effect transistors (FETs) or semiconductor switches. 
     The second circuit unit forms the charge/discharge path with the external device and controls the charge/discharge operation of the battery cell  610 . The second circuit unit includes the external connection terminals  605  and  606  that contact the external device, a switching device unit formed on a current path between the external connection terminals  605  and  606  and the battery cell  610 , and a protection integrated circuit (IC)  650  for controlling the switching device unit. 
     The protection IC  650  operates by using a voltage between terminals Vdd and Vss as power, the terminal Vss may be connected to a negative terminal of the battery cell  610 , and the terminal Vdd may be connected to a positive terminal Vcc of the battery cell  610  by using a pull-up resistor R 2 . A capacitor C 1  for removing a noise signal may be connected between the terminals Vdd and Vss. Also, a resistor R 3  may be connected between a terminal V− of the protection IC  650  and the negative terminal of the battery cell  610 . The protection IC  650  includes terminals DOUT and COUT for controlling the switching device unit. The terminal COUT may be connected to a gate terminal of a charge FET F 2 , and the terminal DOUT may be connected to a gate terminal of a discharge FET F 1 . 
     The switching device unit includes a charge switching device (F 2 , D 2 ) and a discharge switching device (F 1 , D 1 ). The charge switching device (F 2 , D 2 ) includes the charge FET F 2  and a parasitic diode D 2  for the charge FET F 2 . The charge FET F 2  has a drain and a source disposed on the charge/discharge path of the battery cell  610 . Also, the charge FET F 2  has a gate to be turned on or off by a control signal input from the protection IC  650 . If an external power supply device (not shown) may be connected to the external connection terminals  605  and  606 , the charge FET F 2  is turned on to apply a charge current to the battery cell  610 . The parasitic diode D 2  may be electrically connected the charge FET F 2  in parallel. 
     The discharge switching device (F 1 , D 1 ) includes the discharge FET F 1  and a parasitic diode D 1  for the discharge FET F 1 . The discharge FET F 1  has a drain and a source disposed on the charge/discharge path of the battery cell  610 . Also, the discharge FET F 1  has a gate electrically connected to the protection IC  650  and to be turned on or off by the control signal input from the protection IC  650 . The discharge FET F 1  is turned on to supply a discharge current of the battery cell  610  to an external load (not shown) connected to the external connection terminals  605  and  606 . The parasitic diode D 1  may be electrically connected the discharge FET F 1  in parallel. 
     For example, the protection IC  650  calculates an open circuit voltage (OCV) of the battery cell  610 , and thus outputs the control signal to the charge switching device (F 2 , D 2 ) and the discharge switching device (F 1 , D 1 ). For example, the protection IC  650  may turn off the charge switching device (F 2 , D 2 ) before the battery cell  610  is overcharged, such that a charge current does not flow from the external power supply device, and may turn off the discharge switching device (F 1 , D 1 ) before the battery cell  610  is overdischarged, such that a discharge current does not flow to the external load. 
     Therefore, the battery pack is capable of promptly and accurately sensing a leakage of an electrolyte by installing a gas sensor for detecting the leakage of the electrolyte, and thus performing a protection operation according to the leakage of the electrolyte is provided. Thus, a leakage of the electrolyte may be promptly detected by disposing a gas sensor at a location vulnerable to the leakage of the electrolyte, e.g., on an electrolyte injection unit or where electrode taps protrude. 
     Also, in the battery pack after a leakage of an electrolyte is sensed, an appropriate protection operation for stopping a charge/discharge operation of a battery cell and forcibly discharging the battery cell to exhaust charges may be performed. 
     Exemplary embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure as set forth in the following claims.