Abstract:
A battery pack assembly interleaves pouch cells with thin flexible heater elements to provide for distributed low-power heating with reduced thermal resistance between the heater elements and the cells. The heater elements may include terminals for direct attachment to pouch cell electrodes and to each other to facilitate power distribution among the heater elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application 61/977,802 filed Apr. 10, 2014, and hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to electrical batteries and in particular to electrical batteries used for vehicular enemy storage. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electric vehicles and hybrid electric vehicles make use of batteries for energy storage. In this application, the batteries may be subject to a range of storage temperatures including subzero temperatures. At low temperatures, the available power for many types of batteries, including lithium ion batteries, is substantially reduced and the battery efficiency decreased. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention provides a heater fig electric vehicle batteries that can be applied directly to the flat film walls of an individual “pouch cell”, the latter of which are assembled together in a compact stack to provide a battery of suitable voltage. By attaching a thin heating element directly to the pouch wall, the volume of heater structure is substantially reduced with a corresponding reduction in cost and weight. Equally important, heat may be applied locally to each cell to rapidly warm the battery in a way that would be difficult with a heater external to the battery subject to a larger isolating thermal resistance. Each heater may include a thermostatic control to substantially eliminate hot spots that might otherwise be expected to develop based on the different locations of the cells within the battery. 
         [0005]    In one embodiment, the invention provides a battery pack assembly including at least one pouch cell providing opposed polymeric film walls holding battery electrodes and providing exposed battery terminals. A flexible film heater having exposed heater terminals is applied in thermal contact to at least one polymeric film wall to heat the same when the current passes between the heater terminals. 
         [0006]    It is thus a feature of at least one embodiment of the invention to provide a compact heater with reduced structure that may be placed in direct contact with the walls of the pouch cell. 
         [0007]    The battery pack assembly may include multiple pouch cells each having an attached flexible film heater and stacked so that each pouch cell contacts a flexible film heater on both opposed polymeric film walls and so that at least one film heater contacts two adjacent pouch cells. 
         [0008]    It is thus a feature of at least one embodiment of the invention to provide a compact heater structure that may be interleaved with pouch cells to reduce the thermal resistance between the heater and the battery components. 
         [0009]    The exposed heater terminals for successive film heaters may extend generally along the plane of the film heater at different locations for different film heaters so heater terminals of the first film heater may attach to the battery terminals of an adjacent pouch cell and heater terminals of other film heaters may wrap around pouch cells to attach to an adjacent film heater without contacting other heater terminals. 
         [0010]    It is thus a feature of at least one embodiment of the invention to permit the film heaters to be flexibly connected in series or parallel to provide the desired trade-off between resistance and power dissipation. 
         [0011]    The exposed heater terminals overlie and contact the battery terminals. 
         [0012]    It is thus a feature of at least one embodiment of the invention to permit the film heaters to piggyback on the electrical distribution conductors for the pouch cells. 
         [0013]    The battery terminals may be disposed along a seam between the opposed polymeric film walls to extend generally parallel to the polymeric film walls, and the exposed heater terminals may extend along a plane of the flexible film heater to overlie and contact the battery terminals. 
         [0014]    It is thus a feature of at least one embodiment of the invention to provide an attachment mechanism between the film heater and the pouch cells that works with standard pouch cell designs having extended tab terminals. 
         [0015]    The exposed heater terminals may be flexible and coated with a conductive adhesive for attachment to the battery terminals. 
         [0016]    It is thus a feature of at least one embodiment of the invention to provide a simple attachment mechanism between the pouch cell terminals and the film heater terminals that works with existing pouch cell designs. 
         [0017]    The film heater may employ a positive temperature coefficient material. 
         [0018]    It is thus a feature of at least one embodiment of the invention to provide a simple method of thermoregulation that is responsive to different temperatures that may exist within a bank of pouch cells. 
         [0019]    Alternatively, the film heater may provide a thermostatic element switching current through the film heater according to a temperature of the film heater. 
         [0020]    It is thus a feature of at least one embodiment of the invention to provide a thermoregulation that fully removes the heater load from the battery at threshold temperatures. 
         [0021]    The thermostatic element may be selected from a bimetallic element and a memory metal element. 
         [0022]    It is thus a feature of at least one embodiment of the invention to provide a low-cost thermostat system suitable for film heaters. 
         [0023]    The film heater may provide a set of polymeric conductors extending between flanking distribution conductors extending along edges of the pouch cells in the assembled battery pack assembly, and the distribution conductors may be flexible metallic films or foil. 
         [0024]    It is thus a feature of at least one embodiment of the invention to provide a high-power dissipation heater by parallel connection of polymeric conductors. 
         [0025]    The width of the distribution conductors at a point along the distribution conductors may be proportional to a sum of the widths of the polymeric conductors joined by the distribution conductors downstream from that point with respect to the battery terminals. 
         [0026]    It is thus a feature of at least one embodiment of the invention to minimize material costs for distribution of current among the polymeric conductors while promoting uniform heating over the surface of the film heater. 
         [0027]    The film heater may provide an electrically insulating polymer film substrate to which the film heater is attached. 
         [0028]    It is thus a feature of at least one embodiment of the invention to permit the addition of film heaters to existing pouch cell designs. 
         [0029]    In this case, the polymeric substrate may be coated with a pressure sensitive adhesive for attachment to a polymeric film wall. 
         [0030]    It is thus a feature of at least one embodiment of the invention to provide a simple method of integrating the film heaters into a battery pack while ensuring close thermal proximity with the flexible polymeric film walls of the pouch cell. 
         [0031]    Alternatively, the film heater may provide a conductive material adhered directly to the polymeric film wall, for example, by direct printing. 
         [0032]    It is thus a feature of at least one embodiment of the invention to minimize the structure of the film heater by employing the outer polymeric film wall of the pouch cell for reduced weight and cost. 
         [0033]    The pouch cells may be lithium ion batteries. 
         [0034]    It is thus a feature of at least one embodiment of the invention to permit on-demand, cold-weather operation of standard lithium-ion batteries used in hybrid vehicles and the like. 
         [0035]    Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIG. 1  is an exploded perspective view of a pouch cell having an integrated heater element attached to the cell wall according to the present invention; 
           [0037]      FIG. 2  is a figure similar to that of  FIG. 1  showing a fully assembled pouch cell; 
           [0038]      FIG. 3  is a cross-sectional view through a tab electrode of the pouch cell showing a thermostatic control element operating to disconnect the heater in a warm state; 
           [0039]      FIG. 4  is a figure similar to that of  FIG. 3  showing the thermostatic control element in the cold state connecting the heater to the battery; 
           [0040]      FIG. 5  is a schematic representation of the interconnection of multiple pouch cells and heaters to provide a battery system for use in a hybrid or electric vehicle; 
           [0041]      FIG. 6  is an exploded perspective view of four stages in the assembly of a three cell battery subsystem having interleaved heaters according to a second embodiment of the invention; and 
           [0042]      FIG. 7  is a figure similar to that of  FIG. 2  showing three fabrication processes for attaching the film heater to a pouch cell. 
       
    
    
       [0043]    Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Referring now to  FIG. 1 , a pouch cell  10 , suitable for assembly into a battery for use in an electric vehicle such as a car or the like, may have a generally flattened prismatic form factor having upper and lower rectangular pouch walls  12   a  and  12   b . The upper and lower rectangular pouch walls  12   a  and  12   b  will typically be constructed of a flexible, insulating polymer sheet that may be heat-sealable around a seam periphery  14  to provide a pouch defining an enclosed volume  16 . 
         [0045]    The enclosed volume  16  may hold various plates, separators, and electrolytes selected to provide electrochemical storage and release electrical power. Specifically, the volume  16  may hold an upper current collector plate  18   a  such as a metal foil or other conductor having a plate section  20  to tit within the volume  16  and an extending tab electrode  22   a  to project beyond the seam periphery  14  of the upper and lower rectangular pouch walls  12   a  and  12   b  for external connection to the upper current collector plate  18   a . The upper current collector plate  18   a  will be positioned adjacent to the upper rectangular pouch wall  12   a.    
         [0046]    A similar, lower collector plate  18   b  may be positioned adjacent to the lower rectangular pouch wall  12   b  and may likewise have a plate section  20  fitting within the volume  16  and tab electrode  22   b  projecting beyond the seam periphery  14  and displaced from the tab electrode  22   a , for example, on opposite left and right sides of one edge of the seam periphery  14 . 
         [0047]    The upper and lower collector plates  18   a  and  18   b  may flank a stack comprising a negative electrode material  26  adjacent to the upper current collector plate  18   a , a positive electrode material  28  adjacent to the lower current collector plate  18   b , and a separator  30  between the negative electrode material  26  and the positive electrode material  28 . Generally an individual pouch cell  10  will hold a single positive electrode material  28  and negative electrode material  26 . 
         [0048]    Construction of a pouch cell as described above may be according to the description of US patent application 2012/0263987 entitled “High-Energy Lithium-Ion Secondary Batteries” assigned to Envia Systems. Inc. and hereby incorporated by reference. 
         [0049]    Referring now to  FIGS. 1 and 2 , attached to the outer upper surface of the upper rectangular pouch wall  12   a  may be an electrical resistance heater  31  in the form of an electrode array  32 . The electrode array  32  has attachment tabs  34   a  and  34   b  aligned with and attached to electrodes  22   a  and  22   b , respectively (as will be described in detail below). The attachment tabs  34   a  and  34   b  lead to left and right power rails  36  extending along the full length of two opposed sides of the upper rectangular pouch wall  12   a  having a planar extent that narrows as one moves away from the attachment tabs  34   a  and  34   b . Left and right power rails  36  may be, for example, a metal foil or metalized layer or other conductive material. 
         [0050]    Spanning heater conductors  38  extend between the left and right power rails  36  like rungs of a ladder to provide a path of current flow between attachment tab  34   a  and  34   b . Generally, a width of the left and right power rails can be made proportional to the sum of the widths of all downstream spanning heater conductors  38  so that the left and right power rails  36  become narrower as they proceed away from the attachment tabs  34 . 
         [0051]    The electrode array  32  may be constructed, for example, of a high resistance conductive material providing low current draw and generating heat over the entire surface of the upper rectangular pouch wall  12   a . Preferably, the spanning heater conductors  38  are polymer heaters being, for example, a conductive polyester material exhibiting a positive temperature coefficient (PTC) to provide for temperature driven current limiting and thus a first level of temperature feedback preventing hotspots when the pouch cells  10  are assembled together into a battery. Positive temperature coefficient (PTC) heaters, suitable for the present invention, are also disclosed in U.S. Pat. Nos. 4,857,711 and 4,931,627 to Leslie M. Watts hereby incorporated in their entireties by reference. 
         [0052]    The electrode array  32  may be printed directly on the upper rectangular pouch wall  12   a  or die cut and adhered on the upper rectangular pouch wall  12   a  using well-understood techniques. Conductive polymers may, for example, be polymers having fine particulate filler of a conductive material such as silver. 
         [0053]    The power rails  36  and spanning heater conductors  38  may be supported on a flexible insulating polymer carrier layer  37 . 
         [0054]    Referring now to  FIGS. 2 and 3 , each of the attachment tabs  34  may be parallel to but spaced away from respective tab electrodes  22  which they overlie by a thermostat structure  40  operating to connect and disconnect one or both of the attachment tabs  34  from their respective tab electrodes  22  as a function of temperature. In this regard, each attachment tab  34  is laminated on top of a memory metal strip  42  (or bimetallic strip or the like) by means of a conductive adhesive layer  43 . The memory metal strip  42  is supported away from the tab electrode  22  at its left and right edges by means of flexible insulating (dielectric) pads  44  so that a center portion of the memory metal strip  42  is spaced from the tab electrode  22  by open gap  46  when the memory metal strip  42  is warm (for example, at a positive Fahrenheit temperature). 
         [0055]    Referring now to  FIG. 4 , at subzero temperatures, the memory metal strip  42  will curve to close the gap  46  causing the memory metal strip  42  to directly contact the tab electrode  22  allowing current flow between the tab electrode  22  and the attachment tab  34  energizing the electrical resistance heater  31 . 
         [0056]    Referring now to  FIG. 5 , multiple of the pouch cells  10  may be assembled together in a stack  50  providing a battery  52 , the battery  52  exchanging current with a power management system  54  of the electric vehicle communicating with a motor/generator  56  to provide current to the motor and receive current from the generator. Each of the pouch cells  10  is placed in the stack  50  with its upper rectangular pouch wall  12   a  abutting the lower rectangular pouch wall  12   b  of adjacent pouch cell  10  so as to sandwich each electrical resistance heater  31  between insulating surfaces of different pouch cells  10  and to provide each pouch cell  10  with the separate thermostatically controlled heater element. The pouch cells  10  may be connected in parallel to the power management system  54  and the stack  50  placed in a housing (not shown). 
         [0057]    Referring now to  FIG. 6 , a multi-pouch cell subsystem  58  may be assembled from pouch cells  10   a - 10   c  interleaved with electrical resistance heaters  31   a - 31   c . In this regard, electrical resistance heaters  31   a  and  31   b  flank opposite sides of pouch cell  10   a , electrical resistance heaters  31   b  and  31   c  flank opposite sides of pouch cell  10   b , and electrical resistance heaters  31   c  and  31   d  flank opposite sides of pouch cell  10   c  for assembly in a stack as shown in stage  62 . 
         [0058]    Electrical communication between the various electrical resistance heaters  31   a - 31   c  may be provided by conductive tabs  60  positioned in lateral left and right extensions, respectively, from the left and right power rails  36  of each of the electrical resistance heaters  31 . Electrical resistance heater  31   a  may receive electrical power through tabs  34  as described above and may communicate that electrical power down the left and right power rails  36  to the left and right tabs  60   a . As shown in stage  64 , electrical resistance heaters  31   a  and  31   b  may be pressed against opposite sides of a pouch cell  10   a  and the tabs  60   a  folded around the pouch cell  10   a  to attach to, and to electrically communicate with, the power rails  36  of the electrical resistance heater  31   a  by means of a conductive adhesive material, for example, but not limited to, a pressure sensitive conductive adhesive such as an acrylic adhesive. The tabs  60   a  thus provide an electrical path from the tabs  34  to the left and right power rails of electrical resistance heater  31   b.    
         [0059]    Electrical resistance heater  31   b  may likewise have left and right extending tabs  60   b  (displaced upward slightly with respect to tabs  60   a  to prevent interference) so that when electrical resistance heater  31   b  and electrical resistance heater  31   c  are pressed against the opposite sides of pouch cell  10   b  tabs  60   b  may be folded around the pouch cell  10   b  to contact and conductively adhere to the power rails  36  of the electrical resistance heater  31   c  to provide electrical power thereto. This is shown in stage  66 . 
         [0060]    Finally electrical resistance heater  31   c  may likewise have left and right extending tabs  60   c  (displaced slightly downward with respect to tabs  60   a ) so that when electrical resistance heater  31   c  and electrical resistance heater  31   d  are pressed against opposite sides of pouch cell  10   c , tabs  60   c  may be folded around the pouch cell  10   c  to contact and conductively adhere to the power rails  36  and around electrical resistance heater  31   d  to provide power thereto, as shown in stage  68 . 
         [0061]    It will be appreciated that this technique may be used with direct printing of the spanning heater conductors  38  to the pouch material with an adhesive connection between the spanning heater conductors  38  and foil that may implement the various tabs  60 . 
         [0062]    In this way, the three pouch cells  10   a - 10   c  may have electrical heat applied to both sides with power received only through a pair of tabs  34  which may provide for the thermostatic control described above reducing the cost of such thermostatic control. The tabs  60  also serve to hold and stabilize the assembly for later additional assembly operations. Alternatively, thermostatic control may be provided on each electrical resistance heater  31  for more precise temperature control. 
         [0063]    An exposed metal surface of power rails  36  and tabs  60   a  for electrical resistance heater  31   a  are generally continuous metal conductive material and face an insulating surface of pouch cell  10   a . Likewise exposed metal surfaces of power rails  36  and tabs  60   b  and  60   c  for pouch cells  10   b  and  10   c , respectively, face insulating surfaces of pouch cells  10   b  and  10   c , respectively. 
         [0064]    Referring now to  FIG. 7 , the electrical resistance heater  31  may be attached to an upper or lower pouch wall  12  by direct printing of one or both of the power rails  36  and spanning heater conductors  38  directly on the polymer material of the pouch wall  12 . This printing can use, for example, screenprinting techniques well known in the art. 
         [0065]    Alternatively, one or both of the power rails  36  and spanning heater conductors  38  may be attached to the pouch wall  12  by means of the pressure sensitive or other type of adhesive layer  70 . The adhesive layer  70  may be a conductive adhesive of the type described above. In this case, one or both of the power rails  36  and spanning heater conductors  38  may be prepared on a transfer substrate (not shown) and then applied to the pouch wall  12  for transfer in the manner of a decal. 
         [0066]    In a hybrid system, the spanning heater conductors  38  may be printed directly on the pouch wall  12  and the power rails  36  may be adhesively applied foil members using conductive adhesive. 
         [0067]    Alternatively, one or both of the power rails  36  and spanning heater conductors  38  may be fabricated on a supporting flexible film substrate of polymer carrier layer  37 , the latter of which may then be attached by adhesive layer  72  to the pouch wall  12  of the pouch cell  10 . The terminals  34  the polymer carrier layer  37  underlying the terminals  34  may be peeled back and removed prior to attachment. 
         [0068]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0069]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, an, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0070]    Various features of the invention are set forth in the following claims. It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention. 
         [0071]    All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.