Patent Publication Number: US-2017365827-A1

Title: Battery system

Description:
TECHNICAL FIELD 
     The present invention relates to a battery system including a plurality of battery cells connected in series or in parallel via a busbar, and in particular to a battery system in which a busbar is connected to electrode terminals of battery cells by laser welding. 
     BACKGROUND ART 
     In a battery system, a plurality of battery cells can be connected in series to increase an output voltage and in parallel to increase charging and discharging current. For example, a large-current and high-output battery system used as a power source for a motor that drives a vehicle has a plurality of battery cells connected in series to increase an output voltage. In a battery system to be used in this application, a plurality of battery cells is connected by a busbar made of a metal plate. The busbar is connected to electrode terminals of the battery cells constituting the battery system by laser-welding or screwing. The connection structure in which the busbar is connected to the electrode terminals by welding has a feature that the busbar can be stably connected to the electrode terminals for a long time without applying an excessive rotation torque to the electrode terminals. In particular, a connection structure in which the busbar is weld-joined by irradiation with a laser beam has a feature that stable connection can be carried out. In the battery system having this connection structure, the busbar is irradiated with a laser beam and weld-joined to the electrode terminals (see Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Unexamined Publication No. 2011-60623 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In a battery system in which a plurality of battery cells are connected by using busbars, a potential difference is generated between adjacent busbars. In order to improve insulating property between the busbars having a potential difference, it is important that the battery system secures a sufficient creepage distance between the adjacent busbars. In order to secure an insulating distance, specifically, the creepage distance and spatial distance are considered. The spatial distance corresponds to a linear distance between conductors insulated from each other. The creepage distance corresponds to a distance measured along a surface of an insulated product that separates the conductors from each other. The insulating distance between the busbars can be secured by providing an insulating wall between the busbars. 
     Incidentally, in the battery system in which the busbars are laser-welded to the electrode terminals, in a manufacturing step, a laser beam with which the busbars are irradiated melts the busbars and scatters spatters to the surrounding. A problem of scattering of spatters can be prevented by the insulating wall provided between the busbars. However, in the step of irradiating the busbar with a laser beam, an insulating wall made of plastic absorbs thermal energy from the surrounding, is heated, melted, and further vaporized so as to generate a large amount of gas. The gas generated in this step inhibits weld-joining of the busbar. The vaporized plastic gas enters the weld-joined portion of the busbar and the electrode terminal, thus inhibiting reliable weld-joining. 
     A problem that the insulating wall generates gas and inhibits laser-welding can be solved by forming an insulating wall of a material such as ceramic having excellent heat resistance. However, a ceramic insulating wall has various problems that, for example, the component cost is high, it is difficult to form the ceramic insulating wall into an ideal shape at high accuracy because it is produced by firing, and, furthermore, the ceramic insulating wall is heavy, thus increasing a manufacturing cost, and the like. 
     The present invention has been developed for the purpose of solving such problems. An important object of the present invention is to provide a battery system in which a busbar can be stably laser-welded to the electrode terminals while a creepage distance is secured using an insulating wall made of insulating plastic that can be mass-produced at a low cost. 
     Solution to Problem and Advantageous Effects of the Present Invention 
     The battery system of the present invention includes a plurality of battery cells  1 , busbar  3  that is laser-welded to electrode terminals  2  of adjacent battery cells  1  and electrically connects battery cells  1 , and insulating wall  19  made of plastic disposed between the adjacent electrode terminals  2 . Insulating wall  19  has a surface color that is a heat-ray reflecting color having far-infrared reflectance of 50% or more. 
     The above-mentioned battery system has a feature that the busbar can be stably laser-welded to the electrode terminals using an insulating wall made of insulating plastic that can be mass-produced at a low cost, while a creepage distance between the electrode terminals having a potential difference is secured and insulating property is secured. In a conventional battery system in which a busbar is laser-welded to electrode terminals, in a step of laser-welding the busbar to the electrode terminals, the busbar is irradiated with a laser beam and heated. Therefore, the insulating wall absorbs heat rays and is melted, and furthermore, the surface is vaporized to generate a large amount of gas. The generated gas enters the melting portion of the busbar and the electrode terminals to thus inhibit laser welding. 
     In the battery system of the present invention, a surface of the insulating wall has a heat-ray reflecting color. Therefore, in a step of heating the busbar with a laser beam, the surface of the insulating wall can reflect heat-rays efficiently. Accordingly, while the laser beam heats and weld-joins the busbar, the insulating wall made of plastic can be prevented from being heated and generating gas. Therefore, failure in weld-joining of the busbar due to the gas generated by the heated insulating wall can be prevented, so that the busbar can be weld-joined to the electrode terminals reliably and stably. In particular, since the above-mentioned battery system prevents the insulating wall from absorbing heat and inhibits generation of gas, it is not necessary to form an insulating wall using material such as ceramic having excellent heat resistance. Thus, the busbar can be laser-welded to electrode terminals reliably and stably using the insulating wall made of plastic that can be mass produced at a low cost and processed into an ideal shape with high dimensional accuracy. 
     In the battery system of the present invention, insulating wall  19  can be formed of a resin having a heat-ray reflecting color. 
     In this battery system, since an insulating wall is formed of a resin having the heat-ray reflecting color, surface treatment such as coating is not required after the insulating wall is molded, and the insulating wall can be mass-produced at a low cost. 
     In the battery system of the present invention, insulating wall  19  can include a filler having a heat-ray reflecting color. 
     This battery system has a feature that the insulating wall has a surface having a heat-ray reflecting color and can reduce absorption of thermal energy regardless of material property or a body color of plastic to be molded into the insulating wall. 
     In the battery system of the present invention, a surface of insulating wall  19  can be coated with a coating material that reflects at least one of visible light and infrared rays. 
     In the battery system of the present invention, battery cells  1  are rectangular batteries, and plastic insulating separator  18  stacked between the rectangular batteries is formed unitarily with insulating wall  19 . 
     In this battery system, since the insulating wall is unitarily formed with the insulating separator sandwiched between the rectangular batteries, the insulating wall can be positioned in an ideal position, and a creepage distance between the adjacent busbars can be secured. Furthermore, a structure for disposing the insulating wall to a predetermined position is not required, thus making it possible to simplify the attachment structure of the insulating wall. 
     In the battery system of the present invention, insulating wall  19  can be unitarily formed with plastic busbar holder  20  for disposing busbars  3 . 
     In this battery system, since an insulating wall is unitarily formed with a busbar holder that disposes the busbar to the predetermined position, the adjacent busbars can be insulated from each other in a state in which the relative position between the insulating wall and the busbar is allowed to be in an ideal state. Furthermore, a structure for disposing the insulating wall to a predetermined position is not required, thus making it possible to simplify the attachment structure of the insulating wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a battery system in accordance with one exemplary embodiment. 
         FIG. 2  is a vertical sectional view of the battery system shown in  FIG. 1 . 
         FIG. 3  is a schematic perspective view showing a link structure between battery cells and busbars of the battery system shown in  FIG. 1 . 
         FIG. 4  is an exploded perspective view showing the link structure between the battery cells and the busbars of the battery system shown in  FIG. 3 . 
         FIG. 5  is a schematic enlarged sectional view showing the link structure between an electrode terminal of a battery cell and a busbar. 
         FIG. 6  is an enlarged plan view showing another example of a busbar. 
         FIG. 7  is an enlarged plan view showing still another example of a busbar. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, exemplary embodiments of the present invention are described with reference to the drawings. The exemplary embodiments described below are illustrations of a battery system to give a concrete form to technical ideas of the present invention. The present invention is not specifically limited to a battery system described below. Furthermore, it should be appreciated that the members shown in claims are not specifically limited to members in the exemplary embodiments. 
     The battery system of the present invention is used for various applications, for example, a power source installed in an electric-powered vehicle such as a hybrid car or an electric automobile to supply electric power to a driving motor, a power source for storing natural energy power generated, by for example, solar power and wind power, a power source for storing late-night electric power, or the like, and in particular, is used as a power source suitable for applications for large electric power and a large current. 
     A battery system shown in  FIGS. 1 and 2  includes a plurality of battery cells  1  that are fixed in a state in which battery cells  1  are stacked with insulating separators  18  sandwiched therebetween. Each battery cell  1  is a rectangular battery. Furthermore, each battery cell  1  is a rectangular battery including a lithium ion battery. However, in the battery system of the present invention, battery cell  1  is not particularly limited to a rectangular battery, and not particularly limited to lithium ion secondary battery. As the battery cell  1 , any chargeable batteries, for example, nonaqueous electrolyte secondary battery cells other than lithium ion secondary battery cell, a nickel hydride battery cell can be used. 
     In the rectangular battery, positive and negative electrode terminals  2  are fixed to sealing plate  12  via insulating material  11  as shown in  FIGS. 3 and 4 . Note here that in order to easily understand a connection state between battery cell  1  and busbar  3 ,  FIGS. 3 and 4  do not show insulating separator  18  stacked between the plurality of battery cells  1  and busbar holder  20  for disposing a plurality of busbars  3  in predetermined positions (details are described later). Positive and negative electrode terminals  2  each include protruding portion  2 A and welding surface  2 B provided around protruding portion  2 A. Welding surface  2 B is a plane in parallel to the surface of sealing plate  12 . Welding surface  2 B has protruding portion  2 A in a middle of welding surface  2 B. Electrode terminal  2  shown in the drawings has columnar protruding portion  2 A. The protruding portion is not necessarily limited to a columnar-shape, and may be a polygonal or elliptic cylinder shape although not shown. 
     The plurality of stacked battery cells  1  are fixed to a predetermined position by fixing component  13  to form a rectangular parallelepiped battery block  16 . Fixing component  13  includes a pair of end plates  14  and fastening member  15 . End plates  14  are disposed at both end surfaces of stacked battery cells  1 , and fastening member  15  is coupled at the end parts thereof and fixes stacked battery cells  1  in a state in which pressure is applied. 
     In battery block  16 , battery cells  1  are stacked such that the surfaces having electrode terminals  2  of battery cells  1 , that is, sealing plates  12  in the drawings are flush with each other. The battery systems of  FIGS. 1 and 2  have positive and negative electrode terminals  2  on the upper surface of battery block  16 . In battery block  16 , battery cells  1  are stacked in a state in which the directions of the positive and negative electrode terminals  2  on both of the end parts of sealing plate  12  are opposite in the right and left directions. In battery block  16 , as shown in  FIGS. 3 and 4 , on both of the sides of battery block  16 , adjacent electrode terminals  2  are linked to each other using metal plate busbar  3  and battery cells  1  are connected in series. 
     In cell blocks  16 , battery cells  1  are stacked such that adjacent battery cells  1  are insulated from each other with insulating separator  18  sandwiched between battery cells  1 . Furthermore, cell block  16  is provided with insulating wall  19  between adjacent electrode terminals  2  having a potential difference to increase a creepage distance between adjacent electrode terminals  2  having a potential difference. In cell block  16  shown in a sectional view of  FIG. 2 , insulating wall  19  is unitarily molded with insulating separator  18  made of plastic to form a unitary structure with insulating separator  18 . Insulating wall  19  is disposed in a predetermined position with insulating separator  18  sandwiched between battery cells  1 . 
     Insulating wall  19  is disposed between electrode terminals  2  having a potential difference as shown in a sectional view of  FIG. 2  and a perspective view of  FIG. 3 , and protrudes higher than electrode terminal  2  and preferably higher than the upper end of electrode terminal  2 . Insulating walls  19  are disposed high and adjacent to each other, so that a creepage distance between electrode terminals  2  having a potential difference can be increased. Thus, height (h) of insulating wall  19  at a part protruding from the upper end surface of electrode terminal  2  is, for example, 5 mm or more, and preferably 8 mm or more to secure the creepage distance between electrode terminals  2  having a potential difference. 
     The insulating wall may be unitarily formed with busbar holder  20  (see,  FIG. 1 ) made of plastic for disposing busbars  3  in a predetermined position. For example, in busbar holder  20 , an inside of holder main body  20 A disposing a plurality of busbars  3  is partitioned into a plurality of parts to form partitioned chambers. In the partitioned chambers, busbars  3  are disposed in predetermined positions, respectively. At the same time, a partitioned wall serving as a boundary of the partitioned chambers may be an insulating wall. This insulating wall is disposed between busbars  3  that are adjacent to each other, and insulates between electrode terminals  2  having a potential difference. Since in this structure, the insulating wall is unitarily formed with busbar holder  20  for disposing busbars  3  in a predetermined position, a relative position between the insulating wall and the busbar can be in an ideal state. 
     Since insulating wall  19  is disposed near electrode terminal  2  to which busbar  3  is laser-welded, insulating wall  19  is heated under irradiation with a laser beam. When insulating wall  19  made of plastic is heated, it is melted. Furthermore, a surface of insulating wall  19  is vaporized to generate gas. The generated gas enters welding parts of busbar  3  and electrode terminal  2 , causing the welding strength to be deteriorated. In the step of laser-welding busbar  3  to electrode terminals  2 , busbar  3  and electrode terminal  2  are heated to the melting temperature with a laser beam. Light and infrared rays (electromagnetic wave) are radiated from the heated parts of busbar  3  and electrode terminal  2 . Radiated light is applied to the surface of insulating wall  19  that is located in the vicinity thereof. Many substances have property of absorbing light in the wavelength region of far-infrared rays, the object generates heat by irradiation with far-infrared rays. Furthermore, the object generates heat also by absorption of visible light. Insulating wall  19  is configured to have a surface having reflectance of light including visible light and infrared rays of 50% or more in order to reduce the absorbing thermal energy. 
     In general, infrared rays have a wavelength of 0.78 to 1000 μm. Among them, infrared rays having a wavelength of 4 to 1000 μm are called far-infrared rays. Furthermore, the visible light has a wavelength of 380 to 780 nm. The wavelength region of the infrared rays and the wavelength region of the visible light are continuous. Substances having high reflectance of visible light (the light having a wavelength of 380 to 780 nm) tend to have also high reflectance of infrared rays. Therefore, insulating wall  19  has a heat-ray reflecting color having visible light reflectance of 50% or more. Such substances can be formed of polyester plastic materials such as PBT (polybutylene terephthalate), PP (polypropylene), PA (polyamide/nylon (registered trademark)), and the like. Alternatively, composite materials of these resins and glass fiber, glass beads, and the like, can be used. Insulating wall  19  having this configuration can reduce generation of thermal energy due to absorption of light as mentioned above. Note here that when insulating wall  19  is coated with an infrared ray reflecting coating material having property of reflecting infrared rays, it is possible to suppress heat generation due to absorption of light by insulating wall  19 . 
     As mentioned above, at the time of laser welding, light (electromagnetic wave) is radiated. Examples of laser used at the time of laser welding include fiber laser (wavelength: for example, 1060 to 1070 nm), disk laser (wavelength: for example, 1030 nm), semiconductor laser (wavelength: for example, 808, 825, 880, and 975 nm), YAG laser wavelength: for example, 1064 nm), and the like. When laser welding is carried out using such laser, since visible light and infrared ray are mainly radiated, insulating wall  19  can be expected to suppress heat generation of insulating wall  19  due to absorption of light by increasing visible light reflectance and infrared reflectance. In particular, among the radiated light, the far-infrared ray has a remarkably high effect of applying heat to an object, and it is preferable that insulating wall  19  has far-infrared reflectance of 50% or more. Insulating wall  19  reflects not less than half of the irradiated far-infrared rays, so that an absorption amount of heat-rays can be reduced. Furthermore, in insulating wall  19 , a surface color has reflectance of visible light or infrared rays of preferably 60% or more and further preferably 70% or more, and furthermore, the absorption amount of heat-rays can be effectively reduced and generation of gas can be effectively inhibited. 
     The surface of insulating wall  19  can have a heat-ray reflecting color by molding plastic whose body color is a heat-ray reflecting color. Furthermore, insulating wall  19  can have a body color that is a heat-ray reflecting color by filling plastic with powdery filler. Insulating wall  19  can molded to have a body color that is a heat-ray reflecting color by adding inorganic powder of, for example, silica, calcium carbonate, magnesium carbonate, and alumina, having a white body color as a filler to plastic, and mixing thereof. Insulating wall  19  produced by molding plastic whose body color that is a heat-ray reflecting color can be mass-produced at a low cost. After molding plastic, insulating wall  19  can have a surface having a heat-ray reflecting color by coating the surface of insulating wall  19  with coating material having a heat-ray reflecting color. 
     Busbar  3  is welded to positive and negative electrode terminals  2  at both end portions thereof, and connects battery cells  1  in series. In the battery system, battery cells  1  are connected in series to increase an output voltage. Busbar  3  can connect battery cells  1  in series and in parallel. This battery system can increase an output voltage and an output electrical current. 
     Busbar  3  is provided with cut-away portion  30  for guiding protruding portion  2 A of electrode terminal  2 . Busbar  3  of  FIGS. 3 and 4  is provided with cut-away portions  30  at both end portions thereof. Protruding portions  2 A of electrode terminals  2  of adjacent battery cells  1  are guided to cut-away portions  30 , respectively. In busbars  3  of  FIGS. 3 and 4 , cut-away portion  30  is a through-hole, and protruding portion  2 A is inserted into the inside thereof. Cut-away portion  30  has an inner shape capable of guiding protruding portion  2 A of electrode terminal  2 . Furthermore, cut-away portion  30  is provided with exposure gap  4  between the inner edge and protruding portion  2 A in a state in which protruding portion  2 A is guided, in order to expose welding surface  2 B of electrode terminal  2  in a state in which protruding portion  2 A is guided to cut-away portion  30 . 
     In cut-away portion  30  having exposure gap  4 , to the inner side thereof, protruding portion  2 A is not closely attached. The inner edge of cut-away portion  30  is irradiated with a laser beam so as to melt the inner edge, and welding surface  2 B of electrode terminal  2  can be welded reliably. Consequently, welding to welding surface  2 B of electrode terminal  2  can be carried out reliably with the inner edge of cut-away portion  30  as fillet weld part  31 . Furthermore, in a step of laser-welding busbar  3  to electrode terminals  2 , a laser beam or a position-detection sensor is inserted into exposure gap  4 , so that a position of welding surface  2 B can be detected. When the position of welding surface  2 B can be detected, a position of the surface of busbar  3  can be detected by the laser beam or the position-detection sensor, so that it is possible to determine whether busbar  3  is attached closely to welding surface  2 B. In a step of laser-welding busbar  3  to electrode terminal  2 , when there is a gap between busbar  3  and welding surface  2 B, reliable laser welding cannot be secured. The position of welding surface  2 B is detected and further the position of busbar  3  is detected, so that an interval between busbar  3  and welding surface  2 B can be detected. In the laser welding step, when it is detected that busbar  3  is closely attached to welding surface  2 B and laser welding is carried out, busbar  3  can be reliably laser-welded to welding surface  2 B. When there is a gap between busbar  3  and welding surface  2 B, laser welding is stopped, and busbar  3  is pressed to be closely attached to welding surface  2 B, or busbar  3  is exchanged and closely attached to welding surface  2 B. Thus, laser-welded busbar  3  can be welded to electrode terminal  2  reliably. 
     Exposure gap  4  is preferably more than  1  mm, and more preferably  2  mm or more. Exposure gap  4  having this interval makes it possible to irradiate welding surface  2 B with a laser beam, or to insert the position-detection sensor to reliably detect the position of welding surface  2 B. Furthermore, the inner edge of cut-away portion  30  can be irradiated with a laser beam and fillet weld part  31  can be laser-welded to welding surface  2 B reliably. 
     Busbar  3  of  FIGS. 3 and 4  has cut-away portion  30  as a through-hole. Furthermore, the through-hole is formed in a circular shape whose inner shape is made larger than the outer shape of protruding portion  2 A, and exposure gap  4  is provided between busbar  3  and protruding portion  2 A. In a link structure in which columnar protruding portion  2 A is inserted into cut-away portion  30  as a circular through-hole, the inner edge of the through-hole is welded to welding surface  2 B by fillet weld part  31 , as shown in  FIG. 4 , busbar  3  can be reliably welded to welding surface  2 B by fillet weld part  31  and penetration weld part  32  by irradiation with a focused laser beam in a circular locus. 
     As shown in  FIG. 5 , busbar  3  is welded to welding surface  2 B by fillet weld part  31  that welds the inner edge of cut-away portion  30  to welding surface  2 B and by penetration weld part  32  that welds the boundary with respect to welding surface  2 B of electrode terminal  2 . Busbar  3  is welded to welding surface  2 B in a predetermined welding width (H) by fillet weld part  31  and penetration weld part  32 . In order to weld busbar  3  to electrode terminals  2  with sufficient strength, the welding width (H) is, for example, 0.8 mm or more, preferably 1 mm or more, and further preferably 1.2 mm or more. When the welding width (H) is increased, the welding strength can be increased, but it takes a long time to carry out welding. Therefore, the welding width (H) is, for example, 5 mm or less, preferably 4 mm or less, and further preferably 3 mm or less. 
     Busbar  3  is welded to welding surface  2 B of electrode terminal  2  in a predetermined welding width (H) by fillet weld part  31  and penetration weld part  32  by irradiation with a laser beam, focused on a predetermined radius, at a predetermined pitch (t) in a plurality of lines. Busbar  3  is welded to welding surface  2 B by fillet weld part  31  by irradiation with a laser beam applied in a plurality of lines along the inner edge of cut-away portion  30 . Thereafter, irradiation is carried out by displacing the irradiation positions of laser beam at a predetermined pitch (t), and busbar  3  is welded to welding surface  2 B by penetration weld part  32 . The laser beam, which is irradiated in a plurality of lines and with which busbar  3  is welded to welding surface  2 B by fillet weld part  31  and penetration weld part  32 , is focused on a narrow area, and the busbar  3  is irradiated with the focused laser beam. The focused laser beam is focused on an area that is substantially equal to or larger than the pitch (t) of irradiation carried out in the plurality of lines. The laser beam which is focused on an area larger than the pitch (t) is irradiated in a plurality of lines, so that busbar  3  can be welded uniformly welded to welding surface  2 B in a predetermined welding width (H). 
     The laser beam irradiated at a predetermined pitch (t) in a plurality of lines is irradiated, for example, in three lines or more, preferably in five lines or more, and more preferably ten lines or more, so that busbar  3  can be reliably welded to welding surface  2 B by fillet weld part  31  and penetration weld part  32 . With a welding structure in which busbar  3  is welded by fillet weld part  31  and penetration weld part  32  by irradiation with a laser beam at a predetermined pitch (t) in a plurality of lines, busbar  3  can be welded to welding surface  2 B reliably. Also, by increasing an area into which a laser beam is converged, busbar  3  can be welded to welding surface  2 B by both fillet weld part  31  and penetration weld part  32 . This laser beam is adjusted to energy capable of reliably welding busbar  3  to welding surface  2 B by fillet weld part  31  and penetration weld part  32 . 
     Busbar  3  of  FIG. 6  has cut-away portion  30  as a star-shaped through-hole, and the inner edge of the through-hole is welded to welding surface  2 B by fillet weld part  31  and the outer side is welded to welding surface  2 B as penetration weld part  32 . This welding structure enables busbar  3  to be fixed to welding surface  2 B strongly. Furthermore, busbar  3  of  FIG. 6  has cut-away portion  30  as a concave or recess portion, and the inner edge of the recess portion is welded to welding surface  2 B by fillet weld part  31 , and the outer side of fillet weld part  31  is welded to welding surface  2 B as penetration weld part  32 . 
     Busbars  3  are disposed in the predetermined positions by busbar holder  20  shown in  FIG. 1 . Protruding portions  2 A of electrode terminals  2  are guided to cut-away portions  30 . Busbar holder  20  is molded by an insulating material such as plastic, and disposes busbars  3  in the predetermined positions by fitting structures. Busbar holder  20  is linked to battery block  16 , and disposes busbars  3  to the predetermined positions. Busbar holder  20  is linked to insulating separators  18  stacked between rectangular batteries and disposed to the predetermined positions, or linked to the rectangular batteries and linked to the predetermined positions of battery block  16 . Busbar holder  20  shown in  FIG. 1  is provided with frame-shaped holder main body  20 A for disposing a plurality of busbars  3  to the predetermined positions and cover plate  20 B for closing the upper opening of holder main body  20 A. Holder main body  20 A is disposed in the upper surface of battery block  16  in a state in which a plurality of busbars  3  are fixed to the predetermined positions, and cut-away portion  30  of each busbar  3  is disposed to protruding portion  2 A of electrode terminal  2 . Furthermore, in this state, busbars  3  are weld-joined to electrode terminals  2  by irradiation with a laser beam from the upper opening of holder main body  20 A. After all busbars  3  are weld-joined to electrode terminals  2 , the upper opening of holder main body  20 A is covered with the cover plate  20 B. 
     Busbar  3  of  FIGS. 3 and 4  includes a pair of welding plate portions  33  welded and coupled to electrode terminals  2 , and linking portion  34  linking the pair of welding plate portions  33 . A thickness of linking portion  34  is larger than that of welding plate portion  33 . Busbar  3  of  FIG. 4  is provided with welding plate portion  33  in the vicinity of cut-away portion  30  and in a part that is laser-welded to welding surface  2 B by fillet weld part  31  and penetration weld part  32 . In busbar  3  of  FIG. 3 , cut-away portion  30  is a circular through-hole, and, therefore, circular welding plate portion  33  is provided in the vicinity of the through-hole. Since welding plate portion  33  is laser-welded to welding surface  2 B, it has larger width than welding width (H) at which it is welded to welding surface  2 B by fillet weld part  31  and penetration weld part  32 . 
     Welding plate portion  33  has a thickness that can be reliably laser-welded to welding surface  2 B of electrode terminal  2 . A thickness of welding plate portion  33  is set at a dimension that enables reliable welding both fillet weld part  31  and penetration weld part  32  to be welded to welding surface  2 B with a laser beam irradiated to the surface of welding plate portion  33  as shown in the sectional view of  FIG. 5 . The thickness of welding plate portion  33  is, for example, 0.3 mm or more, and preferably 0.4 mm or more. When the thickness is too large, it is necessary to increase energy for laser-welding penetration weld part  32  to welding surface  2 B. Therefore, the thickness of welding plate portion  33  is set at, for example, 2 mm or less, and preferably 1.6 mm or less. 
     Linking portion  34  of busbar  3  of  FIGS. 3 and 4  includes first connection portion  35  and second connection portion  36  provided at both end parts; first rising portion  37  and second rising portion  38  coupled to first connection portion  35  and second connection portion  36  via bent portions, respectively; and middle linking portion  39  coupled to first rising portion  37  and second rising portion  38  via bent portions, respectively. First connection portion  35  and second connection portion  36  are provided with welding plate portion  33  at the inner side. First rising portion  37  and second rising portion  38  are coupled to first connection portion  35  and second connection portion  36  and disposed in a vertical orientation via bent portions bent at a right angle, with a predetermined radius of curvature. Middle linking portion  39  is coupled to first rising portion  37  and second rising portion  38  and disposed in a horizontal orientation via a bent portion that is bent at a right angle, with a predetermined radius of curvature. Middle linking portion  39  is provided with U-curved portion  40  in the middle portion thereof. In middle linking portion  39 , the width of U-curved portion  40  is narrower than the width of first connection portion  35  and second connection portion  36  and made to be easily deformed. Busbar  3  of  FIG. 3  is provided with cut-away recess portion  41  in the vicinity of the bent portion that links first rising portion  37  and middle linking portion  39 , and the width of U-curved portion  40  is made to be narrower. This busbar  3  is formed by linking two metals having different electrical resistance, and is provided with cut-away recess portion  41  in a bent portion made of metal having smaller electrical resistance, to prevent the electrical resistance from being increased by cut-away recess portion  41 . For example, in busbar  3  in which first connection portion  35 , first rising portion  37  and one end of middle linking portion  39  are formed of a copper plate, and second connection portion  36 , second rising portion  38  and the other end of middle linking portion  39  are formed of an aluminum plate, a cut-away recess portion is provided in the vicinity of the bent portion as the copper plate, and the width of U-curved portion  40  can be reduced and easily deformed while increase in the electrical resistance of busbar  3  is reduced. The above-mentioned busbar is configured of the aluminum plate and the copper plate, but it may be formed of only an aluminum plate or only a copper plate. 
     In the above-mentioned battery system, electrode terminals  2  are connected to busbar  3  by the following steps. 
     (1) Busbar holder  20  in which a plurality of busbars  3  are arranged in the predetermined positions is disposed in the predetermined position of battery block  16 . Protruding portion  2 A of electrode terminal  2  is guided to cut-away portion  30  of busbar  3 . 
     (2) Welding surface  2 B is irradiated with a laser beam from exposure gap  4  so as to detect the position of welding surface  2 B, and further the surface of busbar  3  is irradiated with a laser beam so as to detect the position of busbar  3 , for determining whether or not busbar  3  is brought into contact with welding surface  2 B. When it is determined that busbar  3  is in contact with welding surface  2 B, the step proceeds to the next step. 
     When busbar  3  is apart from welding surface  2 B by a set value, an error message is displayed. When the error message is displayed, busbar  3  is exchanged or a position of busbar  3  is adjusted, so that busbar  3  is brought into contact with welding surface  2 B. 
     (3) A position of the inner edge of cut-away portion  30  of busbar  3  is pattern-recognized in a state in which busbar  3  is brought into contact with welding surface  2 B; the inner edge of cut-away portion  30  is irradiated with a laser beam; the inner edge of cut-away portion  30  as fillet weld part  31  is laser-welded; a position that is apart from fillet weld part  31  at a predetermined pitch is irradiated with a plurality of lines of laser beams along fillet weld part  31 ; busbar  3  is welded to welding surface  2 B in a predetermined width, and welded as penetration weld part  32 . As shown in  FIG. 3 , busbar  3  having cut-away portion  30  as a circular through-hole is irradiated with a laser beam along the inner diameter of the through-hole as shown in  FIG. 4 , is welded to welding surface  2 B using the inner edge of the through-hole as fillet weld part  31 , and then irradiated with a laser beam and welded to welding surface  2 B as penetration weld part  32  while a radius irradiated with a laser beam at the predetermined pitch is increased. Welding portions of fillet weld part  31  and penetration weld part  32  are continuous. Welding plate portion  33  of busbar  3  is welded to welding surface  2 B by fillet weld part  31  and the penetration weld part  32  in a predetermined width. 
     A laser beam heats and melts busbar  3  and welding surface  2 B. In this state, the irradiation region of the laser beam is heated to such a high temperature at which metal busbar  3  and welding surface  2 B are melted. The irradiation region that has been heated to a high temperature radiates far-infrared rays to the surrounding. Insulating wall  19  is irradiated with the radiated far-infrared rays and heated. Insulating wall  19  has far-infrared reflectance of 50% or more, and reflects not less than half of the far-infrared ray. In insulating wall  19  having a surface that reflects the far-infrared ray, a temperature at which insulating wall  19  is heated by absorbing irradiated far-infrared rays is low. The surface is not vaporized by thermal energy of the irradiated far-infrared rays. 
     In an insulating wall having a surface whose far-infrared reflectance is 10%, in the step of laser-welding the busbar, the insulating wall made of plastic is heated, vaporized, and generates such a large amount gas that a welding part cannot be recognized. The gas enters the welding portions of the busbar and the electrode terminal, and weld-joining strength is deteriorated. On the contrary, in insulating wall  19  in which white inorganic powder of plastic is mixed into plastic and the surface color is a heat-ray reflecting color having far-infrared reflectance of 70%, gas is not generated due to heating in the welding step of busbar  3 , thus preventing deterioration of the weld-joining strength due to contamination of gas into the welding portion. Furthermore, also in insulating wall  19  whose surface is coated with milk-white infrared ray reflecting coating material having reflectance of light including visible light and infrared rays of 50%, generation of gas due to heating in the welding step of busbar  3  is very small, and deterioration of the weld-joining strength due to contamination of gas into the welding portion is prevented. 
     In busbar  3  of  FIG. 4 , since cut-away portion  30  is a circular through-hole, both fillet weld part  31  and penetration weld part  32  are formed in a ring shape. However, as shown in  FIG. 6 , in busbar  3  having semicircular cut-away portion  30 , fillet weld part  31  and penetration weld part  32  are formed in a semicircular-shape, and welding plate portion  33  of busbar  3  is welded to welding surface  2 B in a predetermined width. 
     INDUSTRIAL APPLICABILITY 
     In a battery system of the present invention, electrode terminals of battery cells and a busbar are electrically connected reliably and stably. Thereby, the battery system can be suitably used for power sources of electric-powered vehicles or power sources for storing natural energy or late-night power. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           1  . . . battery cell 
           2  . . . electrode terminal 
           2 A . . . protruding portion 
           2 B . . . welding surface 
           3  . . . busbar 
           4  . . . exposure gap 
           11  . . . insulating material 
           12  . . . sealing plate 
           13  . . . fixing component 
           14  . . . end plate 
           15  . . . fastening member 
           16  . . . battery block 
           18  . . . insulating separator 
           19  . . . insulating wall 
           20  . . . busbar holder 
           20 A . . . holder main body 
           20 B . . . cover plate 
           30  . . . cut-away portion 
           31  . . . fillet weld part 
           32  . . . penetration weld part 
           33  . . . welding plate portion 
           34  . . . linking portion 
           35  . . . first connection portion 
           36  . . . second connection portion 
           37  . . . first rising portion 
           38  . . . second rising portion 
           39  . . . middle linking portion 
           40  . . . U-curved portion 
           41  . . . cut-away recess portion