Abstract:
A battery pack includes a battery housing and electrochemical cells disposed in the battery housing in a stacked configuration. Elastic bladders are disposed between adjacent cells of a cell stack. The elastic bladders are configured to serve as a compression spring that provide a predetermined compression force to each cell while accommodating cell growth during use. The elastic bladders may include surface features such as strategically shaped and/or located protrusions or restrained regions that are configured to permit compliance and can be tuned to address the requirements of a specific application and permit application of varying stiffness characteristics across a surface of a cell.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present application relates to battery packs formed of electrically interconnected cells, and in particular, to lithium (e.g., lithium-ion, lithium-polymer, etc.) cells arranged into modules with elastic bladder elements interposed with the cells. 
         [0003]    2. Description of the Related Art 
         [0004]    Battery packs provide power for various technologies ranging from portable electronics to renewable power systems and environmentally friendly vehicles. For example, hybrid electric vehicles (HEV) use a battery pack and an electric motor in conjunction with a combustion engine to increase fuel efficiency. Battery packs are formed of a plurality of electrochemical cells. Although nickel metal hydride (Ni-MH) cells are commonly used to form battery packs for HEV applications, lithium-ion (Li-ion) cells are increasingly used in HEV applications since they provide roughly twice the power and energy density of a Ni-MH cell. 
         [0005]    Lithium-ion cells are sometimes provided in a cell housing having a cylindrical or prismatic (rectangular) shape. Alternatively, such cells may be in the form of a so-called pouch cell. Regardless of shape, the cell may include electrodes (for example, a cathode, an anode and an intermediate separator provided in a stacked arrangement) that are rolled in the form of a so-called jelly roll and are placed in the cell housing along with an electrolyte. 
         [0006]    To construct a power-producing electrical system, multiple cells are arranged in stacks and are connected electrically in series or in parallel. The voltage of the cell is dependent on the cell chemistry, the current is dependent on the rate of ion transfer between the cathode and anode, and the capacity depends on the total surface area of the cell. To maintain cell capacity over the life of the cell, it is important to maintain a uniform distribution of pressure across a surface of the cell. 
         [0007]    However, some cell configurations are subject to cyclical changes in volume as a consequence of variations in the state of charge of the cell. For example, in some instances, the total cell volume may vary as much five to six percent or more during charge and discharge cycling. Thus a need exists for a module assembly structure that can accommodate time-varying cell dimensional changes as well as provide a specified compression force to each cell. 
       SUMMARY 
       [0008]    In some aspects, a battery stack includes a first cell, and a second cell positioned adjacent the first cell in a stacked arrangement with the first cell. The first and second cells each include a cell housing, a positive electrode, and a negative electrode. The positive electrode and the negative electrode are sealed within the cell housing along with an electrolyte. The battery stack includes an elastic member disposed between the first cell and the second cell. The elastic member includes a first sheet, and a second sheet layered with the first sheet. The first sheet and the second sheet are joined along a first sealed line that forms a peripheral edge of the elastic member, and the first sheet and the second sheet are joined along a second sealed line that is spaced apart from the peripheral edge, the second sealed line forming a closed shape. A first interior space is defined between the first sheet, the second sheet, the first sealed line and the second sealed line. In addition, a second interior space is defined between the first sheet, the second sheet and within the second sealed line. One of the first interior space and the second interior space is at least partially inflated, and the other of the first interior space and the second interior space is less inflated than the one of the first interior space and the second interior space. 
         [0009]    In some aspects, a battery module includes a cell support element; a first cell supported on the cell support element, and a second cell supported on the cell support element. The second cell is positioned adjacent the first cell in a stacked arrangement with the first cell. The first and second cells each include a cell housing, a positive electrode, and a negative electrode. The positive electrode and the negative electrode are sealed within the cell housing along with an electrolyte. The battery module includes an elastic member disposed between the first cell and the second cell. The elastic member includes a first sheet, and a second sheet layered with the first sheet. The first sheet and the second sheet are joined along a first sealed line that forms a peripheral edge of the elastic member, and the first sheet and the second sheet are joined along a second sealed line that is spaced apart from the peripheral edge, the second sealed line forming a closed shape. A first interior space is defined between the first sheet, the second sheet, the first sealed line and the second sealed line. In addition, a second interior space is defined between the first sheet, the second sheet and within the second sealed line. One of the first interior space and the second interior space is at least partially inflated, and the other of the first interior space and the second interior space is less inflated than the one of the first interior space and the second interior space. 
         [0010]    In some aspects, a battery pack includes a battery pack housing; a first cell disposed in the housing, and a second cell disposed in the housing. The second cell is positioned adjacent the first cell in a stacked arrangement. The first and second cells each include a cell housing, a positive electrode, and a negative electrode. The positive electrode and the negative electrode are sealed within the cell housing along with an electrolyte. The battery pack includes an elastic member disposed between the first cell and the second cell. The elastic member includes a first sheet, and a second sheet layered with the first sheet. The first sheet and the second sheet are joined along a first sealed line that forms a peripheral edge of the elastic member, and the first sheet and the second sheet are joined along a second sealed line that is spaced apart from the peripheral edge, the second sealed line forming a closed shape. A first interior space is defined between the first sheet, the second sheet, the first sealed line and the second sealed line. In addition, a second interior space is defined between the first sheet, the second sheet and within the second sealed line. One of the first interior space and the second interior space is at least partially inflated, and the other of the first interior space and the second interior space is less inflated than the one of the first interior space and the second interior space. 
         [0011]    The battery stack, the battery module and/or the battery pack may include one or more of the following features: The first interior space is at least partially inflated and the second sealed line surrounds an opening in the elastic element. The second interior space is at least partially inflated and the first interior space lacks inflation. The first sheet and the second sheet are joined along second sealed lines that are spaced apart from the common peripheral edge, each second sealed line defining a second interior space, wherein all of the all of the second interior spaces have the same shape. The first sheet and the second sheet are joined along second sealed lines that are spaced apart from the common peripheral edge, each second sealed line defining a second interior space, wherein all of the all of the second interior spaces have the same size. The second interior spaces are distributed uniformly across an area defined by the peripheral edge. The second interior spaces are concentrated in a central region of an area defined by the peripheral edge. The elastic member is configured to apply a compression force to a surface of the cell housing of each of the first cell and the second cell. The elastic member is configured such that the applied compression force is greater in a central region of the surface than in a peripheral region of the surface. The elastic member is free of a fluid inlet and a fluid outlet. 
         [0012]    In some aspects, a battery module includes a stacked arrangement of prismatic lithium-ion cells with interposed elastic members that are designed to accommodate cyclic volumetric expansion of the cell as well as provide a specified compression force to each cell. 
         [0013]    The elastic members can be manufactured using common and relatively low cost manufacturing processes including, but not limited to, metal stamping, plastic sheet forming, sheet welding to form fluid filled bladders, etc. In addition, the elastic members provide a low-cost, light-weight compliant structural member in the battery module that permits cell growth while maintaining a required cell compression force over the life of the battery in order to enable maximizing the lifetime of the battery cell. 
         [0014]    In some embodiments, the elastic member is formed of two sheets of material that are bonded together along one or more sealed lines or otherwise connected at strategic locations, and filled with fluid to form an elastic bladder. The elastic bladder is used as a compression spring between adjacent battery cells. The elastic bladder uses the compression of the fluid and the resiliency and elasticity of the sheets of material to provide a predictable compression force on the cells. 
         [0015]    The elastic members are fluid-filled bladders that include features such seal lines that form restrained regions defining protrusions that contact the cell housing when the elastic member is positioned adjacent the cell. In some embodiments, the protrusions may be equally distributed about a surface of the elastic member so as to provide a uniformly distributed compression force to the cell surface. In other embodiments, the elastic member may have protrusions that are strategically arranged over a surface of the cell to accommodate cell housing expansion. For example, in cases where the expansion of the prismatic cell housing is non-uniform across a surface of the cell, the protrusions may be unequally distributed over the surface of the elastic member. In some embodiments, the number of protrusions and/or the stiffness of the protrusions is increased in a central region of the elastic member relative to a peripheral region of the elastic member to address relatively larger expansion in a central region of the surface of the cell. The stiffness of the elastic member may be adjusted by changing a size and/or geometry of the protrusions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is an exploded perspective view of a battery pack. 
           [0017]      FIG. 2  is a perspective view of an elastic member disposed between two cells. 
           [0018]      FIG. 3  is a cross-sectional view of the elastic member of  FIG. 2 . 
           [0019]      FIG. 4  is a schematic view of a cell stack with elastic members interposed between adjacent cells. 
           [0020]      FIG. 5  is a schematic view of a cell stack with elastic members interposed between adjacent cells and disposed on each end of the cell stack. 
           [0021]      FIG. 6  is a perspective view of another embodiment elastic member disposed between adjacent cells. 
           [0022]      FIG. 7  is a cross-sectional view of the elastic member of  FIG. 6 . 
           [0023]      FIG. 8  is a perspective view of another embodiment elastic member. 
           [0024]      FIG. 9  is a cross-sectional view of the elastic member of  FIG. 8  as seen along line  9 - 9  of  FIG. 8 . 
           [0025]      FIG. 10  is a perspective view of another embodiment elastic member. 
           [0026]      FIG. 11  is a cross sectional view of the elastic member of  FIG. 10  as seen along line  11 - 11  of  FIG. 10 . 
           [0027]      FIG. 12  is a perspective view of the elastic member of  FIG. 10  in a partially folded configuration. 
           [0028]      FIG. 13  is a perspective view of another embodiment elastic member. 
           [0029]      FIG. 14  is a cross-sectional view of the elastic member of  FIG. 13  as seen along line  14 - 14  of  FIG. 13 . 
           [0030]      FIG. 15  is a schematic illustration of an alternative arrangement of the protruding regions of  FIG. 13 . 
           [0031]      FIG. 16  is a schematic illustration of another alternative arrangement of the protruding regions of  FIG. 13 . 
           [0032]      FIG. 17  is a schematic illustration of another alternative arrangement of the protruding regions of  FIG. 13 . 
           [0033]      FIG. 18  is a schematic illustration of another alternative arrangement of the protruding regions of  FIG. 13 . 
           [0034]      FIG. 19  is a perspective cross-sectional view of another embodiment elastic member. 
           [0035]      FIG. 20  is a schematic illustration of an alternative arrangement of the protruding regions of  FIG. 19 . 
           [0036]      FIG. 21  is a schematic illustration of another alternative arrangement of the protruding regions of  FIG. 19 . 
           [0037]      FIG. 22  is a perspective view of another embodiment elastic member. 
           [0038]      FIG. 23  is a perspective view of another embodiment elastic member. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Referring to  FIG. 1 , a battery pack  10  used to provide electrical power includes prismatic electrochemical cells  20  that are electrically interconnected and stored in an organized manner within a battery pack housing  12 . The term “prismatic” as used herein refers to having a rectangular shape. The cells  20  are arranged in a side-by-side configuration to form a stack  18 , and several cells  20  in the stacked arrangement are bundled together to form a battery module  15 . Within the battery module  15 , the stacked group of cells  20  may be commonly supported on a support plate  17  and bound together under compression via a band  16 . Although the illustrated embodiment of the battery module  15  includes six cells  20 , battery modules  15  may include a greater or fewer number of cells  20 . Several battery modules  15  are collected into subunits  14 , and several subunits  14  are arranged within the battery pack housing  12 . An elastic member  30  is disposed between each cell  20  of the stack  18  to maintain a compression force on the cells  20  and to allow the cells  20  to expand and contract during charge and discharge in a controlled manner, as discussed further below. 
         [0040]    The cells  20  are prismatic lithium-ion cells. Each cell  20  includes a cell housing  19 . An electrode assembly (not shown) is sealed within the cell housing  19  along with an electrolyte to form a power generation and storage unit. The electrode assembly may be a “jelly roll” electrode assembly that includes a positive electrode, a negative electrode and an intermediate separator provided in a stacked and rolled arrangement. 
         [0041]    Each cell housing  19  includes a first side  21  and a second side  22  opposed to the first side  21 . The first and second sides  21 ,  22  correspond to the broad sides of the rectangular cell housing  19 . The cell housing  19  also includes four relatively narrow end surfaces  23 ,  24 ,  25 ,  26  that extend between the first side  21  and the second side  22 . In the illustrated embodiment, a first end surface  23  and a third end surface  25  are on opposed sides of the cell  20  and are longer than a second end surface  24  and a fourth end surface  26 . The second and fourth end surfaces  24 ,  26  extend perpendicular to the first and third end surfaces  23 ,  25 . Each cell  20  includes terminals  27 ,  28  that protrude from the first end surface  23 . Each cell  20  also includes a vent  29  that opens through the first end surface  23 . 
         [0042]    Referring to  FIGS. 2-3 , the elastic member  30  is a fluid-filled bladder. The elastic member  30  serves as a compression spring, and includes a first sheet  31  and a second sheet  32  layered with the first sheet  31 . The first sheet  31  and the second sheet  32  are joined along a sealed line  34  that forms a peripheral edge of the elastic member  30 . The sealed line  34  encloses an area that is the same shape as the area of the cell first and second sides  21 ,  22 , and is in a range of 80 percent to 120 percent of the area of the cell first and second sides  21 ,  22 . In some embodiments, the sealed line  34  encloses an area that is about 100 percent of the area of the cell first and second sides  21 ,  22 . The sealed line  34  may be formed using known methods including heating, welding or adhesives. An interior space  35  defined within the sealed line  34  is filled with a fluid such as air to an extent that the first sheet  31  is spaced from the second sheet  32  at interior locations spaced apart from the sealed line  34 . The fluid is captured within the interior space  35  at the time of manufacture, and the elastic member  30  is free of fluid inlets and outlets. 
         [0043]    Referring to  FIGS. 4-5 , the elastic member  30  is disposed between each cell  20  of a stack  18  to maintain a compression force on the cells  20  and to allow the cells  20  to expand and contract during charge and discharge in a controlled manner. The elastic member  30  leverages the compression of the fluid and the resiliency and elasticity of the sheet material to provide a predictable and uniform compression force on the cells  20 . For a battery module  15  containing a stack  18  of six cells  20 , the battery module  15  would include five elastic members  30 , one elastic member  30  disposed between each adjacent pair of cells  20  ( FIG. 4 ). In other embodiments, in addition to including one elastic member  30  disposed between each adjacent pair of cells  20 , a battery module  15  may also include an elastic member  30  disposed on an outward facing side of each outermost cell  20  of the stack  18  ( FIG. 5 ). In this configuration, the outermost elastic members  30  may be supported by an adjacent structural member such as a battery module band  17 , the outermost cell  20  of an adjacent battery module  15 , an inner surface of the battery pack housing, etc. 
         [0044]    Referring to  FIGS. 6-7 , an alternative embodiment elastic member  130  is a bladder that is restrained at the perimeter and in strategic locations within the perimeter. For example, the elastic member  130  is formed of a plurality of fluid filled bladders  138  joined by webbing. Like the elastic member  30 , the elastic member  130  serves as compression spring, and includes a first sheet  131  and a second sheet  132  layered with the first sheet  131 . The first and second sheets  131 ,  132  are formed of a resilient, elastic material. The first sheet  131  and the second sheet  132  are joined along a first sealed line  134  that forms a peripheral edge of the elastic member  130 . The first sealed line  134  encloses an area that is the same shape as the area of the cell first and second sides  21 ,  22 , and is in a range of 80 percent to 120 percent of the area of the cell first and second sides  21 ,  22 . In some embodiments, the first sealed line  134  encloses an area that is about 100 percent of the area of the cell first and second sides  21 ,  22 . The first sheet  131  and the second sheet  132  are joined along a second sealed line  136  that is spaced apart from the first sealed line  134 . The second sealed line  136  forms a closed shape such as a circle (shown) or a rectangle. The first and second sealed lines  134 ,  136  may be formed using known methods including heating, welding or adhesives. 
         [0045]    A first interior space  135  is defined between the first sheet  131 , the second sheet  132 , the first sealed line  134  and the second sealed line  136 . In addition, a second interior space  137  is defined between the first sheet  131 , the second sheet  132  and within the second sealed line  136 . In the embodiment illustrated in  FIG. 5 , the second interior space  137  is at least partially inflated forming the fluid filled bladder  138 , and the first interior space  135  is free of inflation providing a webbing that surrounds the fluid filled bladder  138 . The elastic member  130  includes several uniformly and closely spaced second sealed lines  136 , each defining a second interior space  137  corresponding to a fluid filled bladder  138 . 
         [0046]    In the illustrated embodiment, the fluid filled bladders  138  each have the same shape and size, and are uniformly distributed within the area defined by the first sealed line  134 . However, the fluid filled bladders  138  are not limited to this configuration. In some embodiments, the fluid filled bladders  138  are shaped, sized, and/or distributed to provide non-uniform spring characteristics over the area defined by the first sealed line  134 . For example, since the prismatic cell  20  tends to experience greater distortion in a central region of the cell first and second sides  21 ,  22  relative to a periphery of the cell first and second sides  21 ,  22 , the elastic member  130  may be configured to provide a greater concentration of fluid filled bladders  138  in a central region of the elastic member  130  than in a peripheral region. Alternatively, or additionally, the elastic member  130  may be configured to provide fluid filled bladders  130  that are more stiff in a central region than those in a peripheral region, for example by tuning the shape and/or size of the fluid filled bladders  130  according to location. 
         [0047]    In some embodiments, the first sealed line  134  is omitted, and the first interior space  135  is defined between the first sheet  131 , the second sheet  132 , a peripheral edge of the elastic member  130  and the second sealed line  136 . In such embodiments, the first interior space  135  is open along the peripheral edge, and is at atmospheric pressure. 
         [0048]    Referring to  FIGS. 8-9 , another alternative embodiment elastic member  230  is a bladder that is restrained at the perimeter and in strategic locations within the perimeter. For example, the elastic member  230  is a single fluid filled bladder  238  having a plurality of restrained regions within the perimeter. Like the elastic member  30 , the elastic member  230  serves as compression spring, and includes a first sheet  231  and a second sheet  232  layered with the first sheet  231 . The first and second sheets  231 ,  232  are formed of a resilient, elastic material. The first sheet  231  and the second sheet  232  are joined along a first sealed line  234  that forms a common peripheral edge of the elastic member  230 . The first sealed line  234  encloses an area that is the same shape as the area of the cell first and second sides  21 ,  22 , and is in a range of 80 percent to 120 percent of the area of the cell first and second sides  21 ,  22 . In some embodiments, the first sealed line  234  encloses an area that is about 100 percent of the area of the cell first and second sides  21 ,  22 . The first sheet  231  and the second sheet  232  are joined along a second sealed line  236  that is spaced apart from the first sealed line  234 . The second sealed line  236  forms a closed shape such as a circle (shown) or a rectangle and provides a restrained region  239  within the fluid filled bladder  238 . The first and second sealed lines  234 ,  236  may be formed using known methods including heating, welding or adhesives. 
         [0049]    A first interior space  235  is defined between the first sheet  231 , the second sheet  232 , the first sealed line  234  and the second sealed line  236 . In addition, a second interior space  237  is defined between the first sheet  231 , the second sheet  232  and within the second sealed line  236 . One of the first interior space  235  and the second interior space  237  is at least partially inflated, and the other of the first interior space  235  and the second interior space  237  is less inflated than the one of the first interior space  235  and the second interior space  237 . In the embodiment illustrated in  FIG. 8 , the first interior space  235  is at least partially inflated forming the fluid filled bladder  238 , and the second interior space  237  is free of inflation. In addition, the material corresponding to the second interior space  237  may be removed, forming openings  240  in the elastic member  230 . In other embodiments (not shown), the material corresponding to the second interior space  237  remains intact and may lack inflation. The elastic member  230  includes several uniformly spaced second sealed lines  236 , each defining a second interior space  237  corresponding to a restrained region  239 . 
         [0050]    In the illustrated embodiment, the restrained regions  239  each have the same shape and size, and are uniformly distributed within the area defined by the first sealed line  234 . However, the restrained regions  239  are not limited to this configuration. In some embodiments, the restrained regions  239  are shaped, sized, and or distributed to provide non-uniform spring characteristics over the area defined by the first sealed line  234 . For example, the elastic member  230  may be configured to provide a greater concentration of the restrained regions  239  in a central region of the elastic member  230  than in a peripheral region. Alternatively, or additionally, the elastic member  230  may be configured to be more stiff in a central region than those in a peripheral region, for example by tuning the shape and/or size of the restrained regions  239  according to location. 
         [0051]    The first and second sheets  31 ,  32 ,  131 ,  132 ,  231 ,  232  are formed of a resilient, elastic material. For example, the material used to form the first and second sheets may selected from the group including, but not limited to, rubber (natural rubber based material), elastomer (synthetic rubber material), and polymer (plastic). 
         [0052]    Referring to  FIGS. 10-12 , another alternative embodiment elastic member  330  is a bladder that is formed of a pair of plate portions  331 ,  332  that cooperate to form a bellows-type compression spring. The first plate portion  331  is substantially similar in form to the second plate portion  332 , whereby only the first plate portion  331  will be described in detail, and common reference numbers will be used to refer to common elements. 
         [0053]    The first plate portion  331  is rectangular in size and shape to conform to the size and shape of the cell first and second sides  21 ,  22 . The first plate portion  331  defines a plane  335  and includes offset regions  337 ,  339  that are non-coplanar with the plane  335 . In particular, the first plate portion  331  includes a peripheral flange  337  that is offset from and parallel to the plane  335 . The flange  337  surrounds a peripheral edge  336  of the first plate portion  331 . In addition, the first plate portion  331  includes a protrusion  339  that protrudes from the plane  335  a distance corresponding to the offset of the flange  337 , and in the same direction as the offset of the flange  337 . The protrusion  339  is a single protrusion that is centered within the area defined by the peripheral edge  336  of the first plate portion  331 . 
         [0054]    The second plate portion  332  is layered with the first plate portion  331  in a stacked configuration. In addition, the second plate portion  332  is arranged in a mirrored orientation relative to the first plate portion  331  so that the first plate portion flange  337 ( 1 ) contacts the second plate portion flange  337 ( 2 ), the first plate portion protrusion  339 ( 1 ) contacts the second plate portion protrusion  339 ( 2 ), and the first plate portion plane  335 ( 1 ) is spaced apart from the second plate portion plane  335 ( 2 ). This configuration forms a bellows that allows for compliance when the elastic member  330  is disposed between adjacent cells  20 . In addition, the central protrusions  339 ( 1 ),  339 ( 2 ) provide some stiffness to the bellows arrangement, which can be adjusted by adjusting the shape and/or size of the protrusions  339 ( 1 ),  339 ( 2 ). 
         [0055]    Although the embodiment illustrated in  FIGS. 10-12  includes plate portions  331 ,  332  having a single protrusion  339 ( 1 ),  339 ( 2 ) that is centered within a periphery of the respective plate portion  331 ,  332 , the elastic member  330  is not limited to having a single, centered protrusion  339 ( 1 ),  339 ( 2 ). For example, in some embodiments, the elastic member  330  includes plate portions  331 ,  332  having multiple protrusions  339 ( 1 ),  339 ( 2 ) that are uniformly distributed across an area surrounded by the respective flanges  337 ( 1 ),  337 ( 2 ). In other embodiments, the elastic member  330  includes plate portions  331 ,  332  having multiple protrusions  339 ( 1 ),  339 ( 2 ) that are concentrated in a central region of the area surrounded by the respective flanges  337 ( 1 ),  337 ( 2 ). 
         [0056]    In some embodiments, the first plate portion  331  is formed separately from, and is not joined to, the second plate portion  332 . As a result, the first plate portion  331  is movable relative to the second plate portion  332 . 
         [0057]    In other embodiments, the first plate portion  331  is formed separately from, and is subsequently joined to, the second plate portion  332 . For example, the first plate portion  331  may be connected to the second plate portion  332  along all contacting surfaces (e.g., along the flange  337  and protrusion  339 ), or alternatively, at strategic portions of the contacting surfaces for example in a spot welding process. As a result, the contacting surfaces of first plate portion  331  are fixed relative to those of the second plate portion  332 . 
         [0058]    In still other embodiments, the first plate portion  331  is formed along with the second plate portion  332  from a single piece of material, for example in a stamping operation. In this embodiment, the first plate portion  331  shares a portion of a peripheral edge  336  with the second plate portion  332 . The shared edge portion serves as a fold line  340 , and in use the elastic member  330  is folded along the fold line  340  so that the first plate portion  331  overlies the second plate portion  332  (see  FIG. 12 , which illustrates a partially folded configuration of the elastic member  330 ). 
         [0059]    Referring to  FIGS. 13-14 , another alternative embodiment elastic member  430  is a plate (e.g., a single-thickness sheet) that is formed having a curved or wavy contour when seen in cross-section Like the previously described embodiments, the elastic member  430  serves as compression spring. The elastic member  430  defines a continuous (e.g., non-perforated) surface that conforms to the shape and size of the cell first and second sides  21 ,  22 . In particular, the elastic member  430  includes a first side  431 , and a second side  432  that is opposed to the first side  431 . The elastic member  430  includes an array of first protruding regions  433  and an array of second protruding regions  434 . Each first protruding region  433  is a protrusion that protrudes outwardly from the first side  431  and coincides with a depression formed in the second side  432 . Similarly, each second protruding region  434  is a protrusion that protrudes outwardly from the second side  432  that coincides with a depression formed in the first side  434 . 
         [0060]    The elastic member  430  generally resides within a plane  435 . The first protruding regions  433  protrude out of the plane  435  in a first direction (e.g., in a direction normal to the first side  431 ), and the second protruding regions  434  protrude out of the plane  435  in a direction opposed to the first direction. The first protruding regions  433  and the second protruding regions  434  are spaced apart from each other, and the portions of the elastic member  430  intermediate the protruding regions  433 ,  434  reside in the plane  435 , and are referred to as “intermediate regions”  436 . 
         [0061]    In some embodiments, the first protruding regions  433  are arranged in a grid pattern defined by rows and columns, and each second protruding region  434  is disposed along the rows and columns so as to alternate with adjacent first protruding regions. This configuration is shown schematically in  FIG. 15 , in which an “o” represents a first protruding region  433 , and an “x” represents a second protruding region  434 . 
         [0062]    Referring to  FIG. 16 , in other embodiments, the first protruding regions  433  are arranged in a grid pattern defined by rows and columns, and each second protruding region  434  is disposed in an interstitial space between adjacent first protruding regions  433 . 
         [0063]    In the embodiments illustrated in  FIGS. 13-16 , the first and second protruding regions  433 ,  434  are uniformly distributed across an area surrounded by the elastic member periphery. In other embodiments, the elastic member  430  includes first and second protruding regions  433 ,  434  that are concentrated in a central region of the area surrounded by elastic member periphery in order to provide relatively increased stiffness in this region. The non-uniform distribution of the first and second protruding regions  433 ,  434 , in which the density of first protruding regions  433  and second protruding regions  434  is greater in a central region of the elastic member  430  than in a periphery of the elastic member  430 , can be accomplished, for example by arranging a subset of the first protruding regions  433  in an alternating manner along a line with a subset of the second protruding regions  434 . In some embodiments, the line is linear ( FIG. 17 ), whereas in other embodiments, the line is curved ( FIG. 18 ). 
         [0064]    As an alternative to, or in addition to, adjusting the spring stiffness of the elastic member  430  by varying the distribution of the protruding regions  433 ,  434 , it is possible to adjust the spring stiffness of the elastic member  430  by varying the shape of the protruding regions  433 ,  434 . In the illustrated embodiments, the protruding regions  433 ,  434  are generally rectangular with sidewalls  437  that are generally perpendicular to the plane  435 . The spring rate of the elastic member  430  can be decreased, for example, by providing protruding regions  433 ,  434  having sidewalls  437  that are less perpendicular to the plane  435 . In other embodiments, the protruding regions  433 ,  434  have a cylindrical, conical or other shape. In addition, and/or alternatively, by providing an elastic member in which the geometry of the protrusions  433 ,  434  varies across the area surrounded by the elastic member periphery, the spring rate of the elastic member can be made to vary across the area surrounded by the elastic member periphery. 
         [0065]    Referring to  FIGS. 19-20 , another alternative embodiment elastic member  530  is a plate (e.g., a single-thickness sheet) that is formed having a curved or wavy contour when seen in cross-section Like the previously described embodiments, the elastic member  530  serves as compression spring. The elastic member  530  defines a continuous (e.g., non-perforated) surface that conforms to the shape and size of the cell first and second sides  21 ,  22 . In particular, the elastic member  530  includes a first side  531 , and a second side  532  that is opposed to the first side  531 . The elastic member  530  includes an array of protruding regions  533  that protrude outwardly from the second side  532  and coincides with a depression formed in the first side  531 . 
         [0066]    The elastic member  530  generally resides within a plane  535 . The protruding regions  533  are annular and arranged concentrically, and protrude out of the plane  535  in a direction normal to the second side  532 . In some embodiments, the annular, concentric protruding regions  533  are arranged in a uniformly distributed pattern within the area surrounded by the elastic member periphery, for example by providing equal spacing between adjacent protruding regions  533   a,    533   b  ( FIG. 20 ). In other embodiments, the annular, concentric protruding regions  533 ′ are arranged in a non-uniformly distributed pattern within the area surrounded by the elastic member periphery by providing unequal spacing between adjacent protruding regions  533   a ′,  533   b ′. For example, the annular protruding regions  533   a ′,  533   b ′ may more closely spaced in a central region of the elastic member relative to annular protruding regions  533   c ′ in the periphery in order to provide relatively increased stiffness in the central region ( FIG. 21 ). 
         [0067]    Referring to  FIGS. 22-23 , another alternative embodiment elastic member  630  is a plate (e.g., a single-thickness sheet) that is formed having a curved or wavy contour when seen in cross-section and that conforms to the shape and size of the cell first and second sides  21 ,  22 . Like the previously described embodiments, the elastic member  630  serves as compression spring. The elastic member  630  includes a first side  631 , and a second side  632  that is opposed to the first side  631 . The elastic member  630  includes an array of first protruding regions  633  and an array of second protruding regions  634 . Each first protruding region  633  is a protrusion that protrudes outwardly from the first side  631  and coincides with a depression formed in the second side  532 . Similarly, each second protruding region  634  is a protrusion that protrudes outwardly from the second side  532  that coincides with a depression formed in the first side  634 . 
         [0068]    The elastic member  630  generally resides within a plane  635 . The first protruding regions  633  protrude out of the plane  635  in a first direction (e.g., in a direction normal to the first side  631 ), and the second protruding regions  634  protrude out of the plane  635  in a direction opposed to the first direction. The first protruding regions  633  and the second protruding regions  634  are spaced apart from each other, and the portions  636  of the elastic member  630  intermediate the protruding regions  633 ,  634  reside in the plane  635 . 
         [0069]    Unlike the wavy contoured sheets illustrated in  FIG. 13-21 , the elastic member  630  has a perforated surface. In particular, The elastic member  630  includes perforations (e.g., elongated, linear openings or slits)  638  that are formed along a transition between intermediate (e.g. in-plane) portions  636  of the elastic member  630  and the first protruding regions  633  and along a transition between intermediate portions  636  of the elastic member  630  and the second protruding regions  634 . The perforations  638  are formed on opposed sides of each of the first protruding regions  633  and the second protruding regions  644 . By providing an elastic member  630  in which the protruding regions are associated with perforations  638 , the elastic member  630  may have reduced weight and increased compliance relative to a continuous elastic member such as is illustrated in  FIG. 13 . 
         [0070]    In some embodiments, the elastic member  630  may be slightly pleated in an accordion manner. The first protruding regions  633  and the second protruding regions  634  are arranged on alternating fold lines  639  of the pleat, and the perforations  638  are slits that extend transversely across the fold lines  639  of the pleat. In these embodiments, the first and second protrusions  633 ,  634  have a profile corresponding to a triangular prism ( FIG. 22 ). 
         [0071]    In some embodiments, the elastic member  630 ′ may be corrugated (e.g., may have alternating ridges  641  and grooves  640 ). The first protruding regions  633 ′ and the second protruding regions  634 ′ are arranged on alternating grooves  640 , and the perforations  638 ′ are slits that extend transversely across the grooves  640 . In the corrugated embodiment, the first and second protrusions  633 ′,  634 ′ have a profile corresponding to a trapezoid ( FIG. 20 ) and protrude outwardly relative to the ridges  641 . 
         [0072]    The plates, including the first plate portion  331 , the second plate portion  332 , and those used to form elastic members  430 ,  530 ,  630  are formed of a material that is sufficiently elastic to serve as a compression spring and sufficiently rigid and plastic to permit shaping in a press. For example, the material used to form the first plate portion may selected from the group including, but not limited to, metal (steel, aluminum, copper, etc.), polymer (plastic), and elastomer (synthetic rubber material). 
         [0073]    Although the cell  20  is described herein as having a prismatic shape, the cell  20  is not limited to this shape. For example, the cell may have a circular, elliptical, pouch or other shape. 
         [0074]    Although the cell  20  is described herein as being a lithium-ion cell, the cell  20  is not limited to this type. For example, the cell  20  may be an alkaline cell, aluminum-ion cell, nickel metal hydride cell or other type of cell. 
         [0075]    The elastic members  30  are not limited to use between adjacent cells  20   a,    20   b,  and may be adapted to provide support and compliance between adjacent modules  15  and/or subunits  14 , and may also be adapted to permit support and compliance between a cell  20 , a module  15  or a subunit  14  and the battery pack housing  12 . 
         [0076]    The elastic members  30  may include the two sheets of material  31 ,  131 ,  231  and  32 ,  132 ,  232  that may be joined at strategic locations using sealed lines  34 ,  134 ,  136 ,  234 ,  236  formed using known methods including heating, welding or adhesives. In alternative embodiments, the two sheets of material  31 ,  131 ,  231  and  32 ,  132 ,  232  may be joined via an intermediate structure such as webbing. 
         [0077]    Selective illustrative embodiments of the elastic member are described above in some detail. It should be understood that only structures considered necessary for clarifying the elastic member have been described herein. Other conventional structures, and those of ancillary and auxiliary components of the battery system, are assumed to be known and understood by those skilled in the art. Moreover, while working examples of the elastic member been described above, the elastic member is not limited to the working examples described above, but various design alterations may be carried out without departing from the device as set forth in the claims.