Abstract:
The invention relates to a battery assembly with high thermal conductivity. The battery assembly comprises a metal case having a hollow accommodation cavity formed therein, a plurality of battery cells installed parallel to one another within the metal case, and a common electrode for connection to the other electrode in each of the battery cells. Each of the battery cells has two electrodes, with one of the electrodes that corresponds to those of the rest of the battery cells being connected in a thermally conductive manner to the metal case. The invention takes advantage of high thermal conductivity of metallic material and dissipates heat by connecting the metal case to the battery electrodes. The invention further comprises fixation troughs formed on the metal case, thereby reducing the size of the assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 13/371,426, filed on Feb. 11, 2012, the contents of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high thermal conductivity battery assembly; in particular, it is suitable to serve as building block for drive train battery system for electric vehicles, solar energy storage, and grid storage applications. 
     2. Description of Related Art 
     The invention of high energy density battery technologies initiated the market of cellular phones, notebook computers, electric cars, and grid storage applications. The market demand for smaller form factor and longer operating hours pushing the battery makers to invent higher and higher energy density battery cells. Higher energy density cells require more chemically active materials and inherently less stable and more difficult to design battery systems with high safety factor. 
     Most high energy density battery cells are consumed in cellular phones and portable computers. Total energy required for these devices are small, and relatively few battery cells, up to 8 of the 18650 form factor, are required. Battery life expectation for consumer products is typically 1-2 years. It is relatively easy to design a safe product with only a few battery cells. 
     However, battery pack for electric vehicles requires a lot of battery cells. A small electric car with 21 KW-hour capacity can have a driving range of 100 Km, and would require 3,000 of the 18650 form factor battery cells, each with 7 watt-hour capacity. By necessity these battery cells must be packaged tightly together with only millimeter spacing, and could generate about 1,000-watt heat load while in operation. Without careful thermal design, battery cell temperature could elevate up to 30 Celsius above ambient, with detrimental effect on battery cell life. Battery system for grid storage and vehicle operation is often 5 years or more, thermal management is a tough design issue for vehicle applications. Furthermore, high energy density battery cells become unstable when internal temperature exceeds 80 degree Celsius. It is cell chemistry and process dependent, and the probability of thermal run away, cell venting, and fire and explosion increases dramatically beyond such safe temperature limit, and the battery pack design must not exceed this limit. 
     When one battery cell goes into thermal run away, either through violation of safe temperature limit, manufacturing process induced cell short circuit, over charge, or external impact from vehicle crashes, the amount of energy released may cause adjacent battery cells to also go into thermal run away, this chain reaction destroys the battery pack and place the vehicle passengers in great physical danger. Therefore it has been one of major research subjects for the industry to effectively disperse such generated heat and to prevent thermal runaway. 
     To resolve battery life and safety issues, one common heat dissipation design can be shown in  FIG. 1 , wherein cooling fins for heat sinking are added to the sides of each battery cell  2  and hence gaps can be created between such battery cells  2  thereby, using forced air convections through air blow generated by the fan  20 , it is possible to carry away heat energy from the edges of battery cell  2  and cooling fins. 
     Battery is typically constructed by rolling a sandwich of anode/separator/cathode in a sheet form into a jelly roll for cylindrical cells with superior thermal conduction in the same direction as the conductive anode/cathode sheets. It is due to the fact that anode/cathode sheets are constructed with metal with good thermal and electrical conductivity. The positive and negative connections are brought out in either the top plane  21  or bottom plane  22  in the same direction as the jelly roll. In a direction perpendicular to the sheets, thermal conduction is significantly worse, because heat must traverse metal, non-conductive separator, metal, non-conductive separator, several times before reaching the outer edge. For prismatic battery cells, it is typically constructed by a flattened version of a jelly roll or an interleaved anode/separator/cathode structure that also exhibit the same characteristics in thermal conduction. For pouch cells, construction is similar to prismatic battery cells except the outer enclosure is a soft pouch. 
     In other words, even though the distance between the top surface and the bottom surface is greater than the one between the opposite sides, thermal conductivity for the battery cell toward its top face and bottom surface is more efficient than toward the lateral sidewall. A factor of 12 or more in thermal conductance difference between top/bottom surface and sidewall is found in 18650 form factor battery cells. Consequently, prior art cooling fins or forced convection through the sidewall of a battery is hindered by the poor thermal conductivity of the sidewall and not effective in heat removal of the battery cell. 
     SUMMARY OF THE INVENTION 
     Accordingly, an objective of the present invention lies in providing a high thermal conductivity battery assembly which allows direct thermal joints between the metal electrode of the battery cell and the metal case thereby facilitating efficient heat dissipation. 
     Another objective of the present invention is to provide a high thermal conductivity battery assembly that complies with particular mechanical specification by disposing multiple battery cells of standard specification within an enclosure of particular specification. 
     Yet another objective of the present invention is to provide a high thermal conductivity battery assembly having, within a set of particular mechanical specification, multiple independent battery cells conjunctively installed with fusing devices, thereby removing respective failed battery cell from the circuit upon occurrence of individual battery cell short circuit. 
     Yet another objective of the present invention is to provide a high thermal conductivity battery assembly having, within a set of particular specification, multiple independent battery cells conjunctively installed with the outer metal enclosure as a barrier and containment for internal battery cell thermal runaway from affecting adjacent battery assemblies. 
     A high thermal conductivity battery assembly according to the present invention comprises a metal case having a hollow cavity formed therein; a plurality of battery cells, which are installed parallel to each other within the metal case and respectively have two electrodes with one of the corresponding electrodes connected to the aforementioned metal case with low thermal resistance; and a common electrode for connecting in conductivity the other electrode in each of the aforementioned battery cells. The high thermal conductivity connection from the electrodes to the case enables efficient heat dissipation that keeps the battery cells temperature at a low level to prolong battery cell life. In the event of a single battery cell going into thermal runaway, the high thermal conductivity design helps to channel the thermal energy away, reduces the peak temperature of the thermal runaway event, and minimizes the impact to the adjacent battery cells, and lowers the probability of the adjacent cells going into thermal runaway. Furthermore, the enclosure of the battery assembly significantly improves the safety of operation by containing the battery cell thermal runaway event, with one or more vent hole or exhaust valve design to channel away hot gases, and preventing hot gases from directly hitting adjacent battery assembly, and lowering the temperature impinging on adjacent battery assembly to prevent further thermal runaways. 
     By means of the structure as described above, the multiple battery cells in the high thermal conductivity battery assembly according to the present invention can respectively connect from the electrode to the metal case such that the battery cell is capable of rapidly transferring internally generated heat to the metal case along the axial direction, each of the battery cell is individually installed in connection with a fusing element, in case that any one of the battery cells developed a short circuit, the fusing element will open due to over current, and remove itself from the battery circuit. 
     The present invention also provides a battery assembly structure with improved metal case, wherein a plurality of notches are formed in the metal case and used as fixation troughs facilitating battery cell placement. Herein the fixation trough may be cut open such that the battery cell installed within the battery assembly can be partially exposed outside of the case. Such a metal case design aims to achieve two goals: it provides a means to secure the battery cells within the assembly; it also provides a smaller battery system footprint than otherwise possible. 
     Consequently, the present invention can eliminate heat dissipation problems found in prior art through the aforementioned structure. Heat generated by charging and discharging battery cells can be channeled away to the outer enclosure of the battery assembly efficiently, and from there, the heat can be carried away either via convection, forced air, or liquid cooling means. It provides a secondary protection for battery cell thermal runaway. Additionally, battery packing density is improved with this structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a view for the external case of a prior art battery assembly; 
         FIG. 2  shows a view for major components of the present invention; 
         FIG. 3  shows a view for the metal case of the present invention; 
         FIG. 4  shows a lateral cross-section view of the present invention; 
         FIG. 5  shows a top perspective view for assisting illustration of  FIG. 4 ; 
         FIG. 6  shows a view for the metal case with an insulating outer surface layer in a second preferred embodiment of the present invention; 
         FIG. 7  shows a top view for the metal case with a fixation trough in a third preferred embodiment of the present invention; 
         FIG. 8  shows a stereo view for the embodiment in  FIG. 7 ; 
         FIG. 9  shows a top view for the metal case with pierced fixation troughs in a fourth preferred embodiment of the present invention; 
         FIG. 10  shows a stereo view for the embodiment in  FIG. 9 ; 
         FIG. 11  shows a view for combination of the metal case in a fifth preferred embodiment of the present invention, which illustrates how to form a chamfer edge at the corner of the metal case; 
         FIG. 12  shows a view for the second common electrode of the embodiment in  FIG. 11 ; and 
         FIG. 13  shows a view for the fuse device of the embodiment in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following text, the terms “battery”, “cell”, and “battery cell” may be used interchangeably and may refer to any of a variety of different cell chemistries and configurations including, but not limited to, cylindrical, prismatic, or pouch designs. 
     The present invention discloses a high thermal conductivity battery assembly, and the structure of such a high thermal conductivity battery assembly comprises: a hollow metal case; a plurality of battery cells installed in parallel to each other within the above-said metal case, wherein one electrode from each cell are connected to the aforementioned metal case with very low thermal resistance and electrical resistance; and a separate common electrode for connecting the other electrode in each of the aforementioned battery cells. 
     The battery cell is connected to the metal case, in which the electrode contacting the aforementioned metal case is exemplarily a negative electrode in the present embodiment. Such a design allows full exploitation of high thermal conductivity in metal case for battery cooling, but it is still required to make further efforts on structural improvement for achieving good thermal contact to the metal case by the negative electrode terminal of the battery cell. The diagram for such a structure is shown in  FIG. 2 , wherein the main structure of the high thermal conductivity battery assembly  1  according to the present invention comprises a metal case  10  which accommodates a plurality of battery cells  12  capable of providing electric energy, in which the negative electrode of each battery cell  12  is directly connected to the metal case  10  with good thermal conductivity. There are many ways good thermal conduction contact can be made, by different welding technologies securing electrodes to the sheet metal via metal strips or metal braids. Contrary to the negative electrode, the positive electrode of each battery cell  12  is respectively connected to a common electrode  14  via a set of fuses  16 . 
       FIG. 3  shows a stereo perspective view of the metal case  10 , wherein the metal case  10  includes an interior side  102  and an exterior side  104  facing oppositely to the aforementioned interior side  102 ; the central hollow portion acts as a hollow accommodation cavity  100  for placement of the aforementioned plurality of battery cells  12 , the common electrode  14  as well as the fusing device  16  attached between them. Next, referring conjunctively to  FIGS. 4 and 5 , in the present embodiment, the battery cells  12  are arranged in a fashion of left and right rows, with four battery cells  12  allocated in each row and the battery cells  12  in such two left and right rows positioned correspondingly opposite to each other; the negative electrode of each battery cell  12  is directly connected to the metal case  10  such that the metal case  10  acts as common ground thus becoming zero potential or alternatively referred as a common negative electrode. 
     Since the left and right rows of the battery cells  12  are reversely arranged, positive electrodes of each battery cell are facing toward the center, and each can be connected, via the fusing device  16 , to a common electrode  14  and can be brought to the outside of the metal case  10 . Said fusing device  16  including a plurality of fuses  161  with each fuse  161  connecting the positive electrode of each battery cell to a common electrode  14 ; when one of the battery cell  12  is short circuited, excessive electric current will flow through the fuse and cause it to open, removing the damaged battery cell from the circuit and prevent other healthy cells from injecting current to it. Those skilled ones in the art can conveniently appreciate that the fuse device described as above is by no means limited to be installed at the positive terminal of the battery cell, but can be applicably connected at the negative terminal of the circuit. The fusing device can be constructed with discrete fuse, or can be formed by a stamping process where a narrowed strip of metal would serve as fusing element, or can be formed by spot welding a thin wire from the common electrode to the battery cell. 
       FIG. 6  shows a second preferred embodiment according to the present invention, wherein, in order to eliminate safety concerns about possible short circuit between battery assemblies at different electrical potentials, one can form an insulating outer surface layer  106  on the exterior side  104  of the metal case  10  by means of anodizing treatment or spray painting with an insulating material. As an additional measure for safety, said insulating material can have additional flame retarding properties. 
     Furthermore, in order to meet industry mechanical form factor standard such as DIN SPEC 91252, in a third preferred embodiment of the present invention as shown in  FIGS. 7 and 8 , the interior side  102  of the metal case  10  is cut in on the internal wall to obtain a number of fixation troughs  108  which are used for both securing the battery cell  12  in position and also reducing the volume of the metal case  10  in order to meet standard form factor specification. In a preferred embodiment, the inside of the metal case  10  may be additionally filled with a phase change material  107 , the phase change material can be chosen to have phase change temperature at 40 degrees Celsius. When cell temperature exceeds 40 degrees Celsius, said material will undergo change from solid phase to liquid phase, and in the process absorbs thermal energy to keep cell temperature at a safe temperature with good cell life. The phase change material can also be chosen with phase change temperature at 80 degrees Celsius, to serve as a last line of defense, absorbing thermal energy and prevent battery cells from exceeding safe limit and going into thermal runaway. 
     In another preferred embodiment, the inside of the metal case  10  may be filled with a flame retardant material  107 , said flame retardant can effectively block the thermal energy released from a battery cell under thermal runaway from directly impinge upon the adjacent battery cell, effectively prevent adjacent cells from going into thermal runaway. In another preferred embodiment, the inside of the metal case  10  can be filled with a mixture or slurry of flame retardant material and phase change material. 
     At least one pressure relief valve  101  may be installed on the metal case thereby allowing release of high pressure, high thermal energy gas generated by battery cell under thermal runaway into a place where it could be channeled away, rather than for the thermal energy to impinge upon neighbor battery assembly and cause its battery cell to go into thermal runaway. 
     Another preferred embodiment of the present invention is shown in  FIGS. 9 and 10 . It is possible to further deepen the fixation trough  108  such that it penetrates the exterior side  104  outward from the interior side  102  of the metal case  10  thereby forming a fixation trough  108  having four pierced areas. In this way, the essential thickness of the metal case  10  can be minimized and the integral profile thereof is reduced as well. In addition, a temperature sensor  109  can be further installed inside the metal case  10  that enables external monitoring of internal temperature. 
     Another preferred embodiment in  FIG. 11  shows that the profile of the metal case  10  is substantially of a rectangular structure, and the corners are not right-angled but deliberately chamfered to form an arc-wise chamfer edge  13 . As a result, upon combining more than two sets of the high thermal conductivity battery assembly  1 , spaces in between the battery assembly can be used for electrical signal conduit or for liquid cooling pipes or for mechanical structural member to be located. The chamfer is not limited to be arc-wise but can be an inclined clip plane as well, which does not restrict the formation of heat dissipation channel or pipeline channel and still ensure the safety of assembly personnel. 
     Another preferred embodiment in  FIG. 12  shows that a second common electrode  15  can be placed at each battery cell  12  near the metal case  10 , but not connect to the metal case  10  directly, wherein a hollow area  151  in between allows for at least one low resistance connection  150  be made between each battery cell  12  electrode and second common electrode  15 . Said low resistance connection can be made by resistive welding, ultrasonic welding, or laser welding of thin metal sheet or braids from battery cell  12  electrode to the second common electrode  15 . Additionally, a thermal conductive flexible gasket  17  is inserted between the aforementioned second common electrode  15  and the metal case  10 . As such, when the subassembly of the battery cell  12  and second common electrode  15  is inserted into the metal case  10 , both battery cell  12  and second common electrode  15  are pressure fit to the metal case  15  via the flexible gasket  17 , and forms a high thermal conductivity path that allows battery cell  12  internally generated heat to be transferred to the metal case  10  efficiently. Said flexible gasket  17  also serves as a shock absorber for the battery cell  12  under harsh driving conditions. 
     Furthermore, as shown in  FIG. 13 , as compared to the thick conductive wires  150  formed between each battery cell  12  and the common electrode  15  as described above, a thin conductive wire  140  having a smaller diameter is arranged between each battery cell  12  and the common electrode  14 , such that the thin conductive wire  14  will be melted down in case the electric current passing through exceeds a predetermined threshold current. 
     Based on the fact that, in a battery cell, significantly better thermal conductivity exists in the axial direction of electrodes, the present invention allows better heat removal than prior art designs where heat were attempted to be removed from sidewalls. 
     It should be noticed that, however, the illustrations set forth as above simply describe the preferred embodiments of the present invention which are not to be construed as restrictions for the scope of the present invention; contrarily, all effectively equivalent changes and modifications conveniently made in accordance with the claims and specifications disclosed in the present invention are deemed to be encompassed by the scope of the present invention delineated in the following claims.