Patent Publication Number: US-2012031645-A1

Title: System, method and apparatus for connecting battery cells

Description:
FIELD 
     This invention relates to the field of battery packs and more particularly to a system for connecting battery cells in series and/or parallel within a battery pack. 
     BACKGROUND 
     Multiple battery cells are often integrated into a single battery pack. Such battery packs are often used, for example, in portable computers, power tools and other devices. A majority of such battery packs include rechargeable battery cells such as lithium ion, nickel metal hydride or nickel cadmium battery cells. 
     The battery cells are often arranged in series, parallel or series/parallel arrangements to create battery packs that deliver either a higher voltage (serial), higher current (parallel) or both (series/parallel). To connect the individual cells, often terminal strips are spot welded to terminals of the cells, providing a conductive path between terminals of the battery cells. Spot welders use various technologies to physically and electrically bond the terminal strips to the battery terminals. For example, many spot welders use a capacitive discharge in a technique called fusion welding. This allows welding of highly conductive metals such as copper and brass as well as more resistive metals such as steel and nickel. 
     Another method of interconnecting battery cells in a battery pack is soldering. In this, the terminal is soldered to the terminal strip using a lower melting point metal such as lead, tin or silver. 
     All methods of interconnecting battery cells introduce heat into the battery cells. For some battery cells, in particular lithium ion battery cells, excess heat will damage the battery cells. For example, in lithium ion cells, excess heat applied to the battery terminal causes the lithium and cathode separator within the cell to melt. The separator (between the lithium cathode cap and the cathode) becomes perforated and the anode and cathode become welded together causing an internal short, thereby reducing the open circuit battery cell voltage and/or electrical capacity. Therefore, the battery cell loses functionality and, in some cases, bursts. 
     What is needed is a terminal strip system that will reduce heat transfer to the battery cells during welding. 
     SUMMARY 
     In one embodiment, a battery terminal strip for connecting to a terminal of at least one battery cell is disclosed. The battery terminal strip includes a strip of metal that has at least one weld point formed in the strip of metal. Each of the weld points having a protrusion on a first surface of the strip of metal such that the protrusion contacts a terminal of a battery cell of the at least one battery cell thereby forming an air gap between the first surface of the strip of metal and the terminal of the battery cell, thereby reducing heat transfer from the strip of metal to the battery during welding. 
     In another embodiment, a method of connecting a battery cell to a terminal strip is disclosed including providing a terminal strip comprising a strip of metal having at least one weld point formed in the strip of metal. Each of the weld points having a protrusion on a first surface of the strip of metal such that the protrusion contacts a terminal of a battery cell of the at least one battery cell thereby forming an air gap between the first surface of the strip of metal and the terminal of the battery cell, thereby reducing heat transfer from the strip of metal to the battery during welding. The method continues with positioning the battery terminal strip against the battery terminal of the battery cell such that the protrusions of the weld points contact the battery terminal and fusing the protrusions of the weld points to the battery terminal by discharging a spot welder at the weld points. The above steps are repeated for subsequent weld points. 
     In another embodiment, a battery terminal strip for connecting to a terminal of at least one battery cell is disclosed includes a strip of metal having sets of four weld points and a terminal strip vent formed in the strip of metal. The terminal strip vent is located substantially equidistant from each of the weld points and each of the weld points have a protrusion on a first surface of the strip of metal such that the protrusion contacts a terminal of a battery cell of the at least one battery cell thereby forming an air gap between the first surface of the strip of metal and the terminal of the battery cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a plan view of a typical battery cell interconnection system of the prior art. 
         FIG. 2  illustrates a perspective view of a typical battery cell interconnection system of the prior art. 
         FIG. 3  illustrates a plan view of an improved battery terminal strip. 
         FIG. 4  illustrates a perspective view of the improved battery terminal strip. 
         FIG. 5  illustrates a cross sectional view of the improved battery terminal strip welded to terminals of battery cells. 
         FIG. 6  illustrates a cross sectional view of two of the improved battery terminal strip welded to terminals of the battery cells. 
         FIG. 7  illustrates a perspective view of the improved battery terminal strip welded to terminals of battery cells. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Throughout the following detailed description, the same reference numerals refer to the same elements in all figures. 
     Referring to  FIGS. 1 and 2 , a typical battery cell interconnection system of the prior art is shown. In this example, the battery cell terminals  9  (e.g. positive terminal or negative terminal) of battery cells  8  that make up a battery pack are held together by a plastic sheet  6 . Conductive connections are made between the battery cell terminals  9  using flat strips of metal called terminal strips  10  that are spot welded to the battery cell terminals  9 , typically using capacitive discharge welding techniques. One disadvantage of this interconnection method is heat conduction into the battery cells  8 . The typical spot welding methods such as capacitive discharge spot welding often generate high amounts of heat, heating the terminal strips  10 . After welding, the heat conducts across the terminal strip  10 . Being that the terminal strip  10  is flat; a large bottom surface area of the terminal strip  10  is in contact with the battery cell terminals  9 . During spot welding, heat conducts from the terminal strip  10  to the battery cell terminals  9  and into the battery cell  8 . Many types of battery cells  8  are damaged by certain temperatures, especially lithium ion battery cells  8 . After spot welding, the individual welds pressure points  12  created by the spot welding device are noticeable on the top surface of the terminal strip  10 . 
     Another disadvantage of this interconnection method is the terminal strip  10  blocks the battery cell  8  vent hole (not visible because it is blocked by the terminal strip  10 ). Many battery technologies, such as lithium ion, include a vent hole  6  (see  FIGS. 5 and 6 ) in one of the terminals  9  for venting of gasses that are produced during, for example, over discharging or over charging or other failures. The vent holes  6  provide an escape for these gasses, thereby preventing pressure buildup and potential bursting or explosion. The terminal strips  10  of the prior art cover the vent holes  6  and reduce the amount of gasses that are able to effectively escape during, for example, over discharging or over charging or other failures, leading to potentially bursting or an explosion. 
     Referring to  FIGS. 3 through 7 , an improved battery cell interconnection system is shown. In this example of the improved terminal strips  20 , weld points  22  are provided to improve the process of spot welding while reducing the flow of heat from the welder into the battery cells  8 . The weld points  22  are formed indentations in the terminal strips  20  made either by molding or forming a metal strip made of, for example, steel, nickel, brass, copper or an alloy of such. The weld points  22  have protrusions extending out from a surface that contacts the battery terminal  9 / 11 . The protrusions space the terminal strip  20  from the battery cell terminals  9 / 11 , providing an air gap between the battery cell terminals  9 / 11  and the bottom surface of the terminal strip  20 . In a preferred embodiment, the weld points  22  protrude approximately 0.006 inches from the surface of the terminal strip  20 , though any depth of weld point  22  is anticipated. 
     As the spot welder discharges energy at the weld points  22 , intense heat bonds (welds) the protrusions of the weld points  22  to the battery terminal  9 / 11 . Because the terminal strip  22  is made of metal which is a good conductor of heat, during the welding process, residual heat conducts across the terminal strip  20 . The weld points  22  create an air gap (e.g., a 0.006 inch air gap) between the terminal strip  20  and the battery terminals  9 / 11 . Since air is a poor conductor of heat, more of the heat generated at the weld points  22  conducts down the terminal strip  20  instead of conducting through the battery cell terminal  9 / 11  and into the battery cell  8 , thereby reducing heating of the battery cells  8 . Eventually, the heat conducts into the air and into neighboring battery cells  8 , but the maximum temperature allowed by the manufacture of the battery cells  8  is not exceeded by the welds made at the battery cell  8 . 
     The vent holes  24  in the battery cell terminals  20  provide an escape mechanism for gasses that build up within the battery cells  8  that escape from the battery vent holes  6  when, for example, the battery cell  8  is over charged, over discharged or damaged. In some embodiments, a terminal strip vent hole  24  is formed or molded in the terminal strip  20 . The terminal strip vent holes  24  in the battery terminal strip  20  align with the vent holes  6  in the battery cells  8  to provide maximum exhaust surface area should venting occur. 
     To connect a battery cell  9  to a terminal strip  20 , the battery terminal strip  20  is positioned against at battery cell terminal  9 / 11  of the battery cell  8  such that the protrusions of the weld points  22  contact or touch the battery cell terminal  9 / 11 . If the battery terminal  9 / 11  has a vent hole  6 , the optional terminal strip vent hole  24  is aligned with the vent hole  6  of the battery cell terminal  9 / 11 . An electrode of a spot welder contacts the weld points  22  and discharges to fuse the protrusions of the weld points  22  to the battery cell terminal  9 / 11 . The steps are repeated for subsequent weld points  22 , optionally waiting for the terminal strip  20  and the battery cell terminal  9 / 11  to cool between weld steps. In embodiments in which the terminal strip  20  is a copper strip  20 , the spot welder electrodes are preferred to be resistive electrode. It is preferred that these resistive electrodes be made of molybdenum. Other electrodes made from copper, copper-tungsten (AMPCOLOY) and tungsten do not create as good of a weld as does molybdenum, 
     Equivalent elements can be substituted for the ones set forth above such that they perform in substantially the same manner in substantially the same way for achieving substantially the same result. 
     It is believed that the system and method as described and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely exemplary and explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.