Patent Publication Number: US-2023136015-A1

Title: Battery module

Description:
TECHNICAL FIELD 
     The present invention relates to a battery module. 
     BACKGROUND ART 
     For example, as a power source for a vehicle or the like that requires a high output voltage, there has been known a battery module formed by electrically connecting a plurality of batteries to each other. In general, each of the batteries that form the battery module is provided with a valve that opens in response to an increase in internal pressure. When a gas is generated in a battery due to a chemical reaction so that an internal pressure in the battery is increased, a gas having a high temperature and a high pressure is blown off from a valve. With respect to a battery module including such batteries, PTL 1 discloses a battery module which includes: a battery stack in which a plurality of batteries are stacked; and an exhaust duct which is fixed to one surface of the battery stack in such a manner that the exhaust duct is connected to the valves of the respective batteries. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: WO 2013/161655 A 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     For example, in a temperature sensor provided to detect a temperature of a battery in a battery module, even when gas is generated in the battery, an alarm cannot be issued unless heat is transmitted to the temperature sensor. The temperature sensor does not measure the temperature of the gas, and even if the gas is generated, the generation of the gas cannot be detected by the temperature sensor in a case where the temperature of the battery itself does not rise. 
     The present invention has been made in view of such circumstances, and an object of the present invention is to provide a technique for improving safety of a battery module. 
     Solution to Problem 
     A battery module according to an aspect of the present invention includes: a battery stack in which a plurality of batteries provided with a valve that ejects gas are stacked in a first direction; a discharge path that discharges gas ejected from the valve in a second direction intersecting the first direction; a light emitter that is provided on one end in the first direction of the battery stack and emits light to the discharge path; and a light receiver that is provided on another end in the first direction of the battery stack and receives the light emitted from the light emitter. 
     Advantageous Effect of Invention 
     According to the present invention, the safety of a battery module can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view illustrating an appearance of a battery module according to an exemplary embodiment. 
         FIG.  2    is an exploded perspective view of a part of the battery module. 
         FIG.  3 A  is a schematic diagram illustrating a case where batteries are connected in parallel. 
         FIG.  3 B  is a schematic diagram illustrating a case where batteries are connected in series. 
         FIG.  4    is a cross-sectional side view of a region in which a duct plate and a cover plate of the battery module are disposed. 
         FIG.  5    is a schematic diagram for explaining an arrangement of a light emitter and a light receiver. 
         FIG.  6 A  is a schematic cross-sectional view illustrating a configuration of the light emitter. 
         FIG.  6 B  is a schematic cross-sectional view illustrating a configuration of the light receiver. 
         FIG.  7 A  is a schematic cross-sectional view illustrating a configuration of a light emitter according to a modified example. 
         FIG.  7 B  is a schematic cross-sectional view illustrating a configuration of a light receiver according to a modified example. 
         FIG.  8    is a schematic cross-sectional view illustrating a configuration of a light emitter according to another modified example. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, the present invention will be described based on a preferred exemplary embodiment with reference to the drawings. The exemplary embodiment is not intended to limit the invention but is an example, and all features described in the exemplary embodiment and combinations thereof are not necessarily essential to the invention. The same or equivalent constituent elements, members, and processing illustrated in the drawings are denoted by the same reference numerals, and a redundant description will be omitted as appropriate. The scale and the shape of each part illustrated in each figure are set for the sake of convenience in order to facilitate the understanding of the description and should not be interpreted in a limited manner unless otherwise specified. In cases where terms such as “first” and “second” are used in the present description or claims, these terms do not represent any order or importance but are intended to distinguish one configuration from another configuration, unless otherwise specified. From each of the drawings, some of members not important for describing the exemplary embodiments are omitted. 
     Exemplary Embodiment 
       FIG.  1    is a perspective view illustrating an appearance of battery module  100  according to an exemplary embodiment, and  FIG.  2    is an exploded perspective view of a part of battery module  100 . Battery module  100  includes battery stack  2 , rectangular-parallelepiped housing  10 , duct plate  30 , cover plate  40 , light emitter  50 , and light receiver  60 . 
     Housing  10  includes end plates  11  located at both ends in a stacking direction of batteries  20  in battery stack  2 , side plates  12  that sandwich end plates  11  and cover side surfaces of battery stack  2 , and base plate  13  that covers a bottom of battery stack  2 . Battery stack  2  is housed inside housing  10 , and is covered with duct plate  30  and cover plate  40 . In each drawing, a stacking direction of the batteries is defined as an X direction, a direction intersecting an X direction and corresponding to a lateral of the batteries is defined as a Y direction, and a direction intersecting the X and Y directions is defined as a Z direction. The stacking direction (X direction) of the batteries corresponds to the first direction in the claims, and the Y direction intersecting the X direction corresponds to the second direction in the claims. 
     Battery stack  2  is formed by stacking a plurality of batteries  20  in the X direction. A spacer formed of a resin material or the like in a sheet shape or a plate shape may be disposed between the batteries to electrically insulate the batteries from each other. Each battery  20  is a rechargeable secondary battery such as a lithium-ion battery, a nickel-metal-hydride battery, or a nickel-cadmium battery. Additionally, each battery  20  is a so-called prismatic battery, and has outer covering can  21  having a flat rectangular-parallelepiped shape. One surface of outer covering can  21  is provided with an opening having a substantially rectangular shape not illustrated, and an electrode body, an electrolyte, and the like are accommodated in outer covering can  21  through the opening. Sealing plate  21   a  that closes the opening of outer covering can  21  is disposed in the opening. 
     Output terminal  22  of a positive electrode is disposed on sealing plate  21   a  at a position close to one end of sealing plate  21   a  in a longitudinal direction, and output terminal  22  of a negative electrode is disposed on sealing plate  21   a  at a position close to the other end of sealing plate  21   a  in the longitudinal direction. The pair of output terminals  22  is electrically connected to the corresponding one of a positive electrode plate and a negative electrode plate, constituting the electrode assembly. Hereinafter, output terminal  22  of the positive electrode is referred to as positive-electrode terminal  22   a , and output terminal  22  of the negative electrode is referred to as negative-electrode terminal  22   b  as appropriate. When there is no need to distinguish polarities of output terminals  22  from each other, positive-electrode terminal  22   a  and negative-electrode terminal  22   b  are collectively referred to as output terminals  22 . 
     Respective output terminals  22  are inserted into through-holes (not illustrated) formed in sealing plate  21   a . A seal member (not illustrated) having an insulating property is interposed between respective output terminals  22  and respective through-holes. In the following description, for convenience, sealing plate  21   a  is an upper surface of battery  20 , and a bottom surface of outer covering can  21  facing sealing plate  21   a  is a lower surface of battery  20 . 
     Battery  20  has two main surfaces that connect the upper surface and the lower surface of battery  20  to each other. The main surfaces are surfaces having the largest area out of the six surfaces of battery  20 . The main surfaces are long side surfaces connected to the long sides of the upper surface and the long sides of the lower surface. Two remaining surfaces except for the upper surface, the lower surface, and the two main surfaces are referred to as the side surfaces of battery  20 . These side surfaces are a pair of short side surfaces connected to the short sides of the upper surface and the short sides of the lower surface. These directions and positions are defined for the sake of convenience. Therefore, for example, the part defined as the upper surface in the present invention does not necessarily mean a part located above the part defined as the lower surface. 
     Valve  24  is disposed on sealing plate  21   a  between the pair of output terminals  22 . Valve  24  is also referred to as a safety valve. Valve  24  is a mechanism for releasing a gas in each battery  20 . Valve  24  is configured to release an internal gas by opening valve  24  when an internal pressure of outer covering can  21  is increased to a predetermined value or more. For example, valve  24  includes: a thin part that is formed on a part of sealing plate  21   a  and is thinner than other parts of valve  24 ; and a linear groove formed on a surface of the thin part. In this configuration, when an internal pressure of outer covering can  21  increases, valve  24  is opened by tearing the thin wall part with the groove as a tearing starting point. Valve  24  of each battery  20  is connected to discharge path  33  to be described later, and the gas inside the battery is discharged from valve  24  to discharge path  33 . 
     The plurality of batteries  20  are stacked at predetermined intervals with the main surfaces of adjacent batteries  20  facing each other. Note that the term “stack” means that a plurality of members are arranged in any one direction. Thus, stacking batteries  20  also includes arranging the plurality of batteries  20  horizontally. In the present exemplary embodiment, batteries  20  are horizontally stacked, and the X direction, which is the stacking direction of batteries  20 , is a direction extending horizontally. The Y direction is a horizontal direction perpendicular to the X direction, and the Z direction perpendicular to the X and Y directions is a vertical direction. 
     Each battery  20  is disposed such that output terminals  22  are directed in the same direction. In the present exemplary embodiment, each battery  20  is disposed such that output terminals  22  are directed upward in the vertical direction.  FIG.  3 A  is a schematic diagram illustrating a case where batteries  20  are connected in parallel, and  FIG.  3 B  is a schematic diagram illustrating a case where batteries  20  are connected in series.  FIGS.  3 A and  3 B  correspond to views of the upper surfaces of stacked batteries  20 . 
     As illustrated in  FIG.  3 A , when batteries  20  are electrically connected in parallel, one positive-electrode terminal  22   a  of the adjacent batteries  20  and the positive-electrode terminal  22   a  of the other battery  20  are so as to be adjacent to each other, and the positive electrodes or the negative electrodes are connected by bus bar  28 . In  FIG.  3 A , three blocks each including four batteries  20  as a part of 12 batteries  20  electrically connected in parallel are provided, and the blocks are connected in series. In the case that batteries  20  are connected in parallel, sometimes the voltage abnormality is not detected in the whole block even if the voltage abnormality is generated in one battery  20 . 
     As illustrated in  FIG.  3 B , when batteries  20  are electrically connected in series, one positive-electrode terminal  22   a  of adjacent batteries  20  and negative-electrode terminal  22   b  of other battery  20  are stacked so as to be adjacent to each other, and the positive electrode and the negative electrode are connected by bus bar  28 . In  FIG.  3 B , all 12 batteries  20  are connected in series. 
     Duct plate  30  is placed on an upper surface of battery stack  2 . Duct plate  30  is a plate-like member made of resin that covers an upper surface of battery stack  2 , that is, a surface on which valve  24  of each battery  20  is disposed, and is fixed to battery stack  2  by, for example, side plate  12 . 
     Duct plate  30  has base plate  31  extending along the upper surface of battery stack  2 , and has a plurality of openings  32  through which valve  24  is exposed at positions of base plate  31  corresponding to valve  24  of batteries  20 . Duct plate  30  has discharge path  33  that temporarily stores the gas ejected from each battery  20 . Discharge path  33  extends in the stacking direction X of batteries  20  and is connected to valve  24  of each battery  20 . Each valve  24  communicates with discharge path  33  via opening  32 . 
     Discharge path  33  is defined by shielding part  41  of cover plate  40  covering the upper side of the plurality of openings  32  and a pair of walls  34  surrounding the side of each opening  32 . Each of shielding part  41  and the pair of walls  34  has an elongated shape elongated in the X direction. The pair of walls  34  is arranged in the Y direction with the plurality of openings  32  or the plurality of valves  24  sandwiched therebetween, and each wall surface faces the Y direction. The pair of walls  34  is formed so as to protrude from base plate  31  toward cover plate  40 . Shielding part  41  constitutes a top surface of discharge path  33 . 
     In addition, the color of each wall surface of shielding part  41  of cover plate  40 , the pair of walls  34  surrounding the side of each opening  32 , and base plate  31  facing discharge path  33  and defining discharge path  33  is a color such as black for suppressing reflection of light. The color of the wall surface may be the ground color of the constituent material of each component, or may be colored by plating or coating. 
     Duct plate  30  has openings  36  through which output terminals  22  are exposed at positions corresponding to output terminals  22  of respective batteries  20 . Bus bars  28  are placed on respective openings  36 . The plurality of bus bars  28  are supported by duct plate  30  Thus, duct plate  30  also functions as a so-called bus bar plate. Bus bar  28  placed on each opening  36  electrically connects output terminals  22  of adjacent batteries  20 . 
     Cover plate  40  is a plate-like member that covers the upper side of duct plate  30 , and is placed on the upper surface of duct plate  30 . Cover plate  40  is made of, for example, an insulating resin. Cover plate  40  includes shielding part  41  that covers discharge path  33  and wall  34  of duct plate  30 , and terminal cover  42  that further extends from shielding part  41  to both ends in the Y direction of duct plate  30  and covers output terminal  22  of battery  20 . 
     Cover plate  40  of the present exemplary embodiment is a so-called top cover that forms a part of an outer shell of battery module  100 , specifically, the upper surface of battery module  100 . Cover plate  40  suppresses contact of dew condensation water, dust, and the like with output terminal  22 , valve  24 , bus bar  28 , and the like of battery  20 . 
     Both ends of cover plate  40  in the Y direction are fixed to duct plate  30 . Duct plate  30  has a plurality of engagement claws  37  at both ends in the Y direction at intervals in the X direction. Cover plate  40  has engagement holes  47  at positions overlapping respective engagement claws  37  when viewed from the Z direction, and is fixed to duct plate  30  by inserting respective engagement claws  37  into respective engagement holes  47  and engaging with each other. In the present exemplary embodiment, cover plate  40  is fixed to duct plate  30  by so-called snap-fitting, but may be fixed to duct plate  30  using a fastener such as a screw. 
       FIG.  4    is a cross-sectional side view of a region in which duct plate  30  and cover plate  40  of battery module  100  are disposed. In  FIG.  4   , illustration of the internal structure of battery  20  is omitted. Battery module  100  includes flow path parts  70 . Flow path part  70  is a flow path for leaking the gas in discharge path  33  to the outside of battery module  100 . Discharge path  33  is connected to flow path part  70  that causes the gas in discharge path  33  to leak to the outside, thereby discharging the gas in the Y direction. 
     Flow path part  70  is defined by duct plate  30  and cover plate  40 , and extends in the Y direction (that is, the second direction in the claims) from discharge path  33 . In the present exemplary embodiment, flow path part  70  is disposed on both sides in the Y direction with discharge path  33  sandwiched therebetween. Each of flow path parts  70  communicates with discharge path  33  by gap  71  between wall  34  of duct plate  30  and shielding part  41  of cover plate  40 , and is connected to the outside of battery module  100  by flow path outlet  72  disposed at the end in the Y direction. 
     Gap  71  is formed so as to extend to batteries  20  at both ends in the X direction of battery stack  2 . Flow path outlet  72  is formed as a long opening extending in the X direction corresponding to gap  71 . Therefore, flow path portion  70  is a planar flow path extending in the X direction and the Y direction. Gap  71  and flow path outlet  72  may be partitioned into a plurality of gaps and openings in the X direction. 
       FIG.  5    is a schematic diagram for explaining the arrangement of light emitter  50  and light receiver  60 .  FIG.  6 A  is a schematic cross-sectional view illustrating a configuration of light emitter  50 , and  FIG.  6 B  is a schematic cross-sectional view illustrating a configuration of light receiver  60 . Light emitter  50  is provided on one end side in the X direction of discharge path  33  of battery module  100 . Light receiver  60  faces light emitter  50  and is provided on the other end side in the X direction of discharge path  33 . 
     Light emitter  50  is configured by providing light source  52 , emitting lens  53 , and transparent window  54  in housing  51 . Housing  51  has opening  51   a  facing discharge path  33 . Transparent window  54  is fitted in opening  51   a . Transparent window  54  transmits the light flux emitted from light source  52  to discharge path  33 , and prevents the gas emitted from battery  20  to discharge path  33  from flowing into housing  51 . 
     Light source  52  is, for example, a semiconductor laser, and emits light toward discharge path  33 . Emitting lens  53  can adjust the light flux emitted from light source  52  to an arbitrary light flux. That is, the light flux emitted from light source  52  is adjusted to a parallel light flux as an infinite system or a light flux having an arbitrary diffusion angle as a finite system. Note that, in the lens portion of emitting lens  53 , known techniques and optical element components that have been developed in various technical fields related to collimation of emitted light can be used. 
     Light receiver  60  is configured by providing light receiving element  62 , light receiving lens  63 , and transparent window  64  in housing  61 . Housing  61  has opening  61   a  facing discharge path  33 . Transparent window  64  is fitted in opening  61   a . Transparent window  64  transmits the light flux from discharge path  33  into housing  61 , and prevents the gas discharged from battery  20  to discharge path  33  from flowing into housing  61 . 
     Light receiving element  62  is, for example, a photodiode, and detects a light flux transmitted from discharge path  33  into housing  61 . Light receiving lens  63  adjusts the light flux transmitted into housing  61  to an arbitrary light flux and transmits the light flux to light receiving element  62 . For example, light receiving lens  63  condenses the light flux transmitted into housing  61  and transmits the light flux to light receiving element  62 . Note that, for the lens portion in light receiving lens  63 , known techniques and optical element components that have been developed in various technical fields related to condensing of light fluxes and the like can be used. 
     Next, an operation of battery module  100  will be described based on discharge and detection of gas ejected from battery  20 . At least a portion of the gas blown off from battery  20  is a combustible gas. The gas blown off from battery  20  also contains fine particles such as broken pieces of a battery structure. When a combustible gas having a high temperature and fine particles having a high temperature are discharged to the outside of battery module  100 , there is a possibility that a magnitude of a fire outside battery module  100  is increased. On the other hand, in the present exemplary embodiment, the gas jetted from valve  24  is once discharged to the outside of battery module  100  via discharge path  33  and flow path portion  70 . Therefore, the temperature can be lowered until the gas and the fine particles are discharged to the outside of battery module  100 . 
     The light emitted from light emitter  50  is received by light receiver  60  via discharge path  33 , but when the gas is not emitted to discharge path  33 , the light reception level is substantially constant. When the gas is ejected from battery  20  to discharge path  33 , the light emitted from light emitter  50  hits the gas and is reduced, and the light reception level in light receiver  60  is lowered, so that the ejection of the gas can be detected. Battery module  100  outputs an alarm signal when gas ejection is detected, and measures such as limiting and stopping the output of battery module  100  are taken based on the alarm signal, whereby the safety of the battery module can be enhanced. 
     Light emitter  50  is provided on one end side in the X direction that is the stacking direction of battery stack  2 , and light receiver  60  is provided on the other end side, so that gas emission can be detected for all batteries  20  included in battery stack  2 . 
     When a part of the plurality of batteries  20  stacked in battery module  100  is electrically connected in parallel, there is a case where a voltage abnormality is not detected in the entire block even in a situation where a voltage abnormality occurs in one battery  20  included in the blocks connected in parallel. Even in such a situation, when the gas is ejected from battery  20  in which the voltage abnormality occurs, the ejection of the gas can be detected by light emitter  50  and light receiver  60 , and the abnormal state of battery  20  can be detected. 
     By providing transparent window  54 , light emitter  50  can prevent the gas ejected to discharge path  33  from flowing into housing  51  and adhering to light source  52 . In addition, by providing transparent window  64 , light receiver  60  can prevent the gas ejected to discharge path  33  from flowing into housing  61  and adhering to light receiving element  62 . In addition, light emitter  50  can be reduced in size and cost by using, for example, a semiconductor laser as light source  52 . 
     As described above, light emitter  50  is provided with emitting lens  53 , and the light flux emitted from light source  52  can be adjusted to an arbitrary light flux, and light reception in light receiver  60  can be facilitated. By adjusting the light flux emitted from light source  52  to a parallel light flux as an infinite system by emitting lens  53 , the intensity of light can be further increased up to the far point. 
     In addition, by adjusting the light flux emitted from light source  52  to a light flux having an arbitrary diffusion angle as a finite system by emitting lens  53 , it is possible to stably detect the gas even when positional deviation occurs on light receiver  60  side. When battery module  100  is mounted on a vehicle, light incident on light receiver  60  may slightly move due to vibration from the vehicle. In this case, by adjusting the light flux to a light flux having a diffusion angle by emitting lens  53 , it is possible to allow light receiver  60  to always receive light even for slight movement of light. 
     Furthermore, as described above, light receiving lens  63  is provided in light receiver  60 , and the light flux incident on light receiving element  62  can be adjusted to an arbitrary light flux to correspond to the size of the light receiving area of light receiving element  62 . 
     Discharge path  33  is defined by shielding portion  41  of cover plate  40 , a pair of walls  34  surrounding the side of each opening  32 , and each wall surface of base plate  31 . Since each of the wall surfaces facing discharge path  33  and defining discharge path  33  is colored in black or the like for suppressing reflection of light, it is possible to suppress light that has hit the gas from being reflected by the wall surface and reaching light receiver  60 . Battery module  100  can reliably detect a decrease in light intensity at the time of gas ejection in light receiver  60  by suppressing reflection of light that has hit the gas on the wall surface. 
     Modified Example 
       FIG.  7 A  is a schematic cross-sectional view illustrating a configuration of light emitter  50  according to a modified example, and  FIG.  7 B  is a schematic cross-sectional view illustrating a configuration of light receiver  60  according to the modified example. Light emitter  50  includes transparent window  54  having a lens shape capable of adjusting the light flux emitted from light source  52  to an arbitrary light flux. Transparent window  54  transmits the light flux emitted from light source  52  to discharge path  33 , and prevents the gas emitted from battery  20  to discharge path  33  from flowing into housing  51 . 
     In addition, the light flux emitted from light source  52  can be adjusted to an arbitrary light flux by transparent window  54 . The light flux emitted from light source  52  is adjusted to a parallel light flux as an infinite system or a light flux having an arbitrary diffusion angle as a finite system. Light emitter  50  can be downsized by providing transparent window  54  with a function as a lens. 
     Light receiver  60  includes transparent window  64  having a lens shape capable of adjusting a light flux incident on light receiving element  62  to an arbitrary light flux. Transparent window  64  transmits the light flux from discharge path  33  into housing  61 , and prevents the gas discharged from battery  20  to discharge path  33  from flowing into housing  61 . 
     In addition, the light flux transmitted into housing  61  can be adjusted to an arbitrary light flux by transparent window  64 , and for example, the light flux transmitted into housing  61  is condensed and transmitted to light receiving element  62 . Light receiver  60  can be downsized by providing transparent window  64  with a function as a lens. 
       FIG.  8    is a schematic cross-sectional view illustrating a configuration of light emitter  50  according to another modified example. Light emitter  50  includes half mirror  55  and light amount detector  56 . Half mirror  55  reflects a part of the light from light source  52  to be incident on light amount detector  56 . Light amount detector  56  detects a part of the light emitted from light source  52  and detects the intensity of the emitted light of light source  52 . Half mirror  55  transmits light other than a part of the reflected light to the side of discharge path  33 . 
     Half mirror  55  and light amount detector  56  are provided inside light emitter  50  which is outside discharge path  33 , and can detect the intensity of the emitted light by light source  52  without being affected by the gas emitted from battery  20 . 
     The present invention has been described based on the exemplary embodiment of the present invention. As a person skilled in the art understands, the exemplary embodiment is exemplified, and the exemplary embodiment is variously varied and modified within a scope of claims of the present invention. Further, such variations and modified examples fall within the scope of the claims of the present invention. Therefore, it should be understood that the description and the drawings herein are not limitative, but are illustrative. 
     The exemplary embodiment may be defined by the following items. 
     [Item 1] 
     Battery module ( 100 ) including: battery stack ( 2 ) in which a plurality of batteries ( 20 ) provided with valve ( 24 ) that ejects gas are stacked along a first direction; discharge path ( 33 ) that discharges gas ejected from valve ( 24 ) in a second direction intersecting the first direction; light emitter ( 50 ) that is provided on one end side in the first direction of battery stack ( 2 ) and emits light to discharge path ( 33 ); and light receiver ( 60 ) that is provided on another end side in the first direction of battery stack ( 2 ) and receives the light emitted from light emitter ( 50 ). Thus, battery module ( 100 ) can detect gas emitted from battery ( 20 ), and can enhance safety of battery module ( 100 ). 
     [Item 2] 
     Battery module ( 100 ) according to Item 1, in which a part of batteries ( 20 ) is electrically connected in parallel. Thus, when a voltage abnormality occurs in battery ( 20 ) included in the blocks connected in parallel and gas is ejected, battery module ( 100 ) can detect the ejection of gas and detect an abnormal state of battery ( 20 ). 
     [Item 3] 
     Battery module ( 100 ) according to Item 1 or 2, in which each of light emitter ( 50 ) and the light receiver ( 60 ) includes transparent window ( 54 ,  64 ) facing the discharge path. Thus, battery module ( 100 ) can prevent gas ejected to discharge path ( 33 ) from adhering to light source ( 52 ) and light receiving element ( 62 ). 
     [Item 4] 
     Battery module ( 100 ) according to any one of Items 1 to 3, in which light emitter ( 50 ) includes light source ( 52 ) and emitting lens ( 53 ) capable of adjusting a light flux emitted from light source ( 52 ) to an arbitrary light flux. Thus, battery module ( 100 ) can facilitate light reception by light receiver ( 60 ) by adjusting a light flux emitted from light source ( 52 ) to an arbitrary light flux. 
     [Item 5] 
     Battery module ( 100 ) according to Item 4, in which emitting lens ( 53 ) is capable of adjusting a finite light flux having a divergence angle. Consequently, battery module ( 100 ) can stably detect the gas even when the position of the side of light receiver ( 60 ) is shifted. 
     [Item 6] 
     Battery module ( 100 ) according to Item 4 or 5, in which light receiver ( 60 ) includes light receiving element ( 62 ) and light receiving lens ( 63 ) capable of adjusting a light flux incident on the light receiving element ( 62 ) to an arbitrary light flux. Thus, battery module ( 100 ) can adjust the light flux incident on light receiving element ( 62 ) to an arbitrary light flux, and can correspond to the size of the light receiving area of light receiving element ( 62 ). 
     [Item 7] 
     Battery module ( 100 ) according to Item 3, in which light emitter ( 50 ) includes light source ( 52 ), transparent window ( 54 ) included in light emitter ( 50 ) is capable of adjusting a light flux emitted from light source ( 52 ) to an arbitrary light flux, light receiver ( 60 ) includes light receiving element ( 62 ), and transparent window ( 64 ) included in light receiver ( 60 ) is capable of adjusting a light flux incident on the light receiving element ( 62 ) to an arbitrary light flux. Thus, in battery module ( 100 ), transparent window ( 54 ,  64 ) has a function as a lens, so that light emitter ( 50 ) and light receiver ( 60 ) can be downsized. 
     [Item 8] 
     Battery module ( 100 ) according to any one of Items 4 to 7, in which light source ( 52 ) is a semiconductor laser. Thus, battery module ( 100 ) can be reduced in size and cost by using a semiconductor laser as light source ( 52 ). 
     [Item 9] 
     Battery module ( 100 ) according to any one of Items 1 to 8, in which light amount detector ( 56 ) that detects a light amount of emission light from light emitter ( 50 ) is provided outside discharge path ( 33 ). Thus, battery module ( 100 ) can detect the intensity of the light emitted from light source ( 52 ) without being affected by the gas emitted from battery ( 20 ). 
     [Item 10] 
     Battery module ( 100 ) according to any one of Items 1 to 9, in which discharge path ( 33 ) is defined by a wall surface that suppresses reflection of light emitted from light emitter ( 50 ). Thus, battery module ( 100 ) can reliably detect a decrease in light intensity at the time of gas ejection in light receiver ( 60 ) by suppressing reflection of light that has hit the gas on the wall surface. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               2  battery stacks 
               20  battery 
               24  valve 
               33  discharge path 
               50  light emitter 
               52  light source 
               53  emitting lens 
               54  transparent window 
               60  light receiver 
               62  light receiving element 
               63  light receiving lens 
               64  transparent window 
               100  battery module