Abstract:
A weld-free, frameless battery design is provided. The design reduces the number of parts and the weight of the battery pack, simplifies the assembly operation, and keeps the battery pack reparable and remanufacturable with minimal effort and cost. The battery pack includes a stack of battery cells and cooling fins, and a removable restraint is placed around the stack. The positive and negative tabs of the battery cells comprise a pair of sub-tabs which are bent over the faces of the cell. One type of cell can have an extended portion on one of the positive and one the negative sub-tabs which are on opposite faces of the cell. The sub-tabs are used to make the necessary series and parallel connections.

Description:
BACKGROUND 
       [0001]    The invention relates generally to battery packs, and more particularly to battery packs which are easy to assemble and disassemble. 
         [0002]    The battery assembly for hybrid or plug-in electric vehicles (EV) may consist of a plurality of battery cells  10 , cooling fins  15 , repeating frames  20 , and foam sheets  25 , which are stacked and joined together to form a module or pack as shown in  FIG. 1 . Although the battery cells are the only energy and power source of the battery, the assembly needs the other components to form a fully functional and reliable system. All of them add weight and complexity to the battery pack. For example, the total weight of the repeating frames can be as much as 10% of the total weight of the battery pack. 
         [0003]    The battery for a long driving range EV may contain more than 200 battery cells. The cells are preferably in prismatic shape for better spatial energy density and thermal efficiency. The individual number of cooling fins, repeating frames, and foam sheets can be half as many as the number of battery cells. Each part possesses different material properties and a different geometric shape. The battery cells are semi-rigid and laminated in a pouch. The cooling fins, which can be made of a single piece of aluminum or two aluminum sheets welded together with coolant passages inside for example, are prismatic and thin, but stiffer than the battery cells. The foam sheets, which provide space for thermal expansion and compression, are relatively soft and have rubber gasket borders. The plastic repeating frames, which are narrow and hollow, have intricate interlocking details and coolant seals on both sides of the frame. The large number of different parts makes a fast stacking operation challenging enough. The differences in physical form and properties further increase the difficulty and complexity of automated assembly, requiring costly assembly equipment or causing a slower assembly operation. 
         [0004]    Typically, after stacking and assembling the battery components into a module, every two or three adjacent battery cells are welded together to form parallel electric connections. Each cell has at least two tabs or electric terminals (one positive, one negative) for such welding. An interconnect board can be added and welded to the cells to complete the serial connection of the battery pack, if desired. The welding operations require proper welding machines and tooling, and they are expensive and time consuming operations. Furthermore, due to the difficulty of precise tab bending and height control, the protrusion of any tabs after welding poses the risk of an electric short with the battery monitor board attached on top of the battery pack. 
         [0005]    Finally, due to the irreversible nature of current welding techniques, such as spot welding and ultrasonic welding, removing a bad cell from a fully assembled battery pack requires cutting all the tab connections in the module. Thus, the good battery cells are unable to be re-welded, which is an expensive product and manufacturing problem. 
         [0006]    Therefore, there is a need for an improved battery pack that can be easily assembled and disassembled. 
       SUMMARY OF THE INVENTION 
       [0007]    A weld-free, frameless battery design is provided. The design significantly reduces the number of parts and the weight of a battery pack, simplifies the assembly operation, and keeps the battery pack repairable and remanufacturable with minimal effort and cost. In addition, the battery design maintains flexibility in serial/parallel connection of battery cells, the allowance for thermal expansion, and the freedom of liquid or air cooling/heating. 
         [0008]    In one embodiment, the battery pack includes at least two battery cells in electrical contact, the battery cells having first and second faces, the battery cells having a positive tab and a negative tab on an edge of the battery cell, the positive tab comprising a pair of sub-tabs bent over the first and second faces of the cell, and the negative tab comprising a pair of sub-tabs bent over the first and second faces of the cell; a positive terminal in electrical contact with one of the positive sub-tab of one of the battery cells; a negative terminal in electrical contact with one of the negative sub-tabs of another one of the battery cells; at least one cooling fin positioned between battery cells; and a removable restraint around the first and second battery cells and the fin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an illustration of the parts in a prior art battery pack. 
           [0010]      FIG. 2  is a cross-section of one embodiment of a battery pack. 
           [0011]      FIG. 3  is a view of a cooling fin and part of the battery cell of the battery pack of  FIG. 2 . 
           [0012]      FIG. 4  is a top view of the battery pack of  FIG. 2 . 
           [0013]      FIGS. 5A-E  are an illustration of two types of battery cells and one embodiment of a fin. 
           [0014]      FIG. 6  is a cross-section of one embodiment of a battery pack. 
           [0015]      FIG. 7  is a top view of the battery pack of  FIG. 6 . 
           [0016]      FIG. 8  is a schematic electric flow of the embodiment of  FIG. 4 . 
           [0017]      FIG. 9  is a schematic electric flow of an alternate embodiment. 
           [0018]      FIGS. 10A-B  are schematic electric flows of an alternate embodiment. 
           [0019]      FIGS. 11A-D  illustrate the battery cells of the embodiment of  FIG. 10 . 
           [0020]      FIGS. 12A-C  illustrate different embodiments of removable restraints. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    As shown in  FIGS. 2-5 , the battery pack  100  includes two types of battery cells  105 ,  110  and cooling/heating fins  115 . The battery cells  105 ,  110  are generally the same as the commercially available products. They can be contained in a pouch or a hard case, if desired. No internal chemicals or materials need to be changed. Each battery cell  105  has a positive tab  120  and a negative tab  125 , and each battery cell  110  has a positive tab  130  and a negative tab  135 . Each positive or negative tab has two separable sub-tabs  140 ,  145  that can be bent toward the two faces of the battery cell package respectively. The sub-tabs form two electric terminals of the same polarity, positive or negative. 
         [0022]      FIGS. 5A-B  are the front and rear faces of battery cell  105 , while  FIGS. 5D-E  show the front and back of battery cell  11 . The two types of battery cells  105  and  110  are essentially identical. The only significant difference is that for cell type  105 , the positive tab  120  has an extended portion  122  on one face and the negative tab  125  has an extended portion  127  on the other face of the cell. As shown in  FIG. 4 , a parallel cell connection can be arranged by stacking the same type of cells, type  105  or type  110 , next to each other. Any switch from type  105  cells to type  110  cells or vice versa in the stack changes the battery connection to be in series. Therefore, the combination of parallel and serial connections in a battery pack is unlimited. The battery cells are connected by the direct mechanical contact in the stack. No welding or interconnect frame is needed. 
         [0023]    Instead of stacking the battery pack with separate pieces of foam between the cells as in typical prior art designs, the battery cell can have at least one, pre-attached foam strips  150  on each face of the cell. In addition to assisting with assembly compliance, the strips  150  closest to the positive and negative tabs  120 ,  125  also serve as the mounting pads for fixing the bent sub-tabs  140 ,  145  on the strips  150  at specified locations. 
         [0024]    As shown in  FIG. 5 , the thin, but stiff cooling/heating fins  115  have two slots  155 A and  155 B to fit the foam strips  150  of adjacent cells. For instance, the fin  115  may stack on top of the front face of the cell  110  with one of its foam strips fitting into the slot  155 A, then the next cell, either  105  or  110  will have one of its foam strips on the back face fitting into the slot  155 B of the same fin. As such, the slots  155 A and  155 B help align the relative position between the battery cells  105 ,  110  and the fins  115 . The fins  115  can have a thickness such that when the cell stack is compressed, the foam strips can be squeezed to let the cells be in solid contact with the fins for best heat transfer efficiency. 
         [0025]    The fins  115  can be air or liquid cooled/heated. For air cooling/heating, the fin can be as simple as a flat metal sheet. It can optionally include slots  155  as discussed above, and/or a flange around the edge in a tray-like geometry (not shown) that can help position the cells more positively. For liquid cooling/heating, each fin  115  has coolant channels  160  in between two welded metal plates as well as coolant inlets  165  and coolant outlets  170 . The coolant inlets  165  and outlets  170  can be individually connected to a manifold (not shown) for coolant circulation, or they can extend from the fins in ear-shaped features  175  and then be stacked together as shown in  FIG. 2 . To fill up the gap between the fins and to provide proper coolant sealing, the ear-like extensions can be molded with plastic  180  that is sealable or having rubber seals (not shown) around the openings for coolant. The coolant can thus be easily fed into and removed from end plates (not shown) of the stack. 
         [0026]    Because of higher stiffness, the fins  115  can be the primary structural and locating members of the stack to hold battery cells. After compression, the stack can be contained in removable restraints, including, but not limited to, clamping with bolts or tie rods  190  ( FIG. 12A ), wrapping with metal straps  195  ( FIG. 12B ), or boxing in a hard case  200  ( FIG. 12C ), for the final assembly, which saves the weight and cost of repeating frames, as well as assembly time. In remanufacturing, the stack can be easily disassembled because of the weld-free assembly, and any bad cells or other components can be replaced with minimal time and cost. 
         [0027]    It is known that the electric resistance of a mechanical contact is inversely proportional to the contact force at the interface. Higher contact force will deform more microscopic surface asperities of the metal, and thus generate a larger contact area between the two surfaces, which in turn reduces the constriction resistance of electric current flow. Meanwhile, a higher contact force helps break down the oxidation films on metal surfaces and enhances the electric conductivity at the interface. However, when the electric terminals are positioned on the back of the cell as shown in  FIGS. 2-5 , this limits the contact or compression force when the stack is packaged at the end of assembly. Excessive compression could crush and damage the battery cells.  FIGS. 6-7  show another embodiment in which a nonconductive compression bar  185  is embedded in the cell pouch to serve as a mechanical support of the electric terminals for higher contact force in the assembly. However, this approach requires making a change to the battery cell itself. 
         [0028]      FIG. 8  shows the schematic electric flow diagram of  FIG. 4  with expanded number of cells stacked together. Due to the extended tabs and proximity of tab positions, there are several circled areas, five per repeating parallel—serial connection of cells in this particular example, that may have the risk of electrical short if there is any misalignment between cells during the stacking operation. 
         [0029]      FIG. 9  shows an embodiment which mitigates the risk of electric short by rearranging the tab positions. For the cell  105 , the extended portion  122  of the positive tab  120  and the extended portion  127  of the negative tab  125  point to the opposite directions, instead of the same direction as in  FIG. 8 , which result in tabs  120  and  125  closer to each other. On the other hand, for the cell  110 , the positive tab  130  and negative tab  135  are separated further apart. With this tab arrangement, the risk of electric short is reduced from five to three places for the same pattern of parallel-serial cell connection. Nevertheless, the risk still exists. 
         [0030]      FIG. 10  shows an embodiment that can eliminate the risk of electric short by separating the positive and negative tabs to the opposite edges of the cell. The diagram of  FIG. 10  is essentially the same as that of  FIG. 9 , except the right half of the tabs is on the bottom edge of the cells, eliminating the chance of electric short due to stacking misalignment of cells.  FIG. 11  illustrates the tab arrangement of  FIG. 10 . 
         [0031]    It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. 
         [0032]    For the purposes of describing and defining the present invention it is noted that the term “device” is utilized herein to represent a combination of components and individual components, regardless of whether the components are combined with other components. For example, a “device” according to the present invention may comprise an electrochemical conversion assembly or fuel cell, a vehicle incorporating an electrochemical conversion assembly according to the present invention, etc. 
         [0033]    For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0034]    Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.