Abstract:
An energy storage unit for storing electrical energy comprises a plurality of stacked flat cells, each having protruding electrodes. A cooling body of the energy storage unit is heat-conductively connected, at least in sections, to the flat cells. The cooling body at least partially consists of a plastic material and has openings through which the electrodes extend.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an energy storage unit for storing electrical energy comprising a plurality of stacked flat cells, each having protruding electrodes. 
         [0002]    Energy storage units comprising a stack of flat cells (so-called prismatic cells or pouch cells) are used, for example, in electrically driven motor vehicles. Due to the high voltages and current intensities occurring during operation, considerable heat amounts can develop which have to be dissipated in order to avoid overheating of the energy storage unit. However, the cooling devices used conventionally for heat dissipation are expensive and require relatively much construction space, which especially in the field of automobile engineering is very limited. 
         [0003]    It is therefore an object of the invention to provide heat dissipation for energy storage units of the above-mentioned type in a manner which is as efficient and as compact as possible. 
       SUMMARY OF THE INVENTION 
       [0004]    This object is achieved by an energy storage unit comprising at least one cooling body which is heat-conductively connected, at least in sections, to the flat cells, wherein the cooling body at least partially consists of a plastic material and has openings through which the electrodes extend. 
         [0005]    The cooling body can dissipate heat losses arising in the flat cells and thus increases the reliability and the service life of the energy storage unit. A configuration made of a plastic material enables simple and inexpensive manufacturing and is, on top of that, advantageous with regard to saving weight. 
         [0006]    The openings can particularly be slots adapted to the electrode shape and size. Due to the openings it is possible to arrange the cooling body on the electrode side of the flat cells, wherein the cooling body is placed as near as possible to the active cell region, thereby enabling especially efficient cooling. Since the electrodes are led through the openings, it is additionally possible to realize a cooling body arrangement which is especially space-saving. 
         [0007]    Depending on the respective requirement, a one-piece cooling body can be provided or a plurality of cooling bodies can be combined to form a cooling body arrangement. 
         [0008]    Further embodiments of the invention are mentioned in the detailed description and the drawings. 
         [0009]    According to one embodiment, the cooling body at least partially consists of an electrically insulating material. Thus the cooling body is capable of fulfilling the function of electrical insulation in addition to heat dissipation. Thereby corresponding separate insulating elements can be omitted, thereby saving costs as well as construction space. 
         [0010]    The energy storage unit can comprise a connection unit connecting the flat cells at least electrically, wherein the cooling body is heat-conductively connected, at least in sections, to the connection unit. Such a connection unit commonly comprises elements for interconnecting the individual flat cells serially and/or in parallel. Further, the connection unit can comprise various interconnection elements and contact elements for electrically contacting the flat cells and for tapping electrical power. During operation of the energy storage unit, these electrical interconnection and contact elements generate Joule&#39;s heat (especially in the case of high-voltage connections), resulting in heating-up of the components of the connection unit. Due to the heat-conductive contact between the connection unit and the cooling body, this heat can be reliably dissipated. 
         [0011]    According to an advantageous embodiment, the cooling body can be arranged between the flat cells and the connection unit. In the case of this embodiment, the energy storage unit thus has a sandwich-like structure, wherein the flat cells, the cooling body and the connection unit are stacked one on top of the other. This structure is enabled due to the fact that the electrodes are led through the openings of the cooling body. Thus the cooling body is capable of taking up and dissipating heat losses radiating from the flat cells as well as arising in the connection unit, i.e., for example, in strip conductors, contacts or interconnection elements, whereby the service life of the energy storage unit is increased. Due to the fact that the cooling body is arranged between the stack of flat cells and the connection unit, i.e. integrated in the energy storage unit, it is possible to configure heat dissipation in a way which saves especially much construction space. Another advantage results from the fact that the cooling body can be used for increasing the mechanical stability of the energy storage unit. 
         [0012]    Preferably, the connection unit and/or the cooling body are embodied in a plate-like shape and extend in a plane which is perpendicular to the respective extension plane of the flat cells. For example, the connection unit and the cooling body can be arranged at an end face of the stack of flat cells and cover the stack of flat cells. This in particular facilitates the heat-conducting connection of the cooling body to all the flat cells of the stack. 
         [0013]    The connection unit and the cooling body can, at least in sections, with their surfaces abut on one another in order to guarantee in this way efficient heat transfer between the two components. 
         [0014]    According to another embodiment, the cooling body is embodied as a carrier element for the connection unit and/or for a cover element of the energy storage unit. In this configuration, the cooling body fulfils an advantageous additional function by increasing the mechanical stability of the overall arrangement. 
         [0015]    The cooling body can comprise fixing elements for fixing the connection unit and/or a cover element of the energy storage unit in order to especially facilitate assembly. 
         [0016]    According to one embodiment, the connection unit, too, can have openings, particularly slots, which are in alignment with the openings of the cooling body and through which the electrodes extend, wherein the electrodes are connected to the connection unit especially at the side of the connection unit facing away from the flat cells. In order to avoid short-circuiting between the two electrodes, the openings can be encircled by insulating material. For example, insulation sleeves can be introduced in the openings. Such a measure can be omitted in the area of the cooling body if the cooling body itself is made of a non-conducting material. 
         [0017]    Cooling elements can be arranged between the flat cells, which cooling elements are heat-conductively connected to the flat cells and the cooling body. Thereby more efficient cooling of the flat cells is achieved. 
         [0018]    The cooling elements can be configured in a plate-like shape, wherein one of the plate-shaped cooling elements is arranged between each pair of adjacent flat cells. In particular, the plate-shaped cooling elements can, over their entire surface, abut on the respective adjacent flat cells in order to ensure uniform and effective heat dissipation. 
         [0019]    The flat cells can be glued to the cooling elements. In order to enable optimal heat transfer from the flat cells to the cooling elements, in particular a heat-conducting adhesive can be used for this. 
         [0020]    According to an advantageous embodiment, the cooling elements are inserted in recesses of the cooling body and in particular glued to the cooling body. In this way, assembly is facilitated and at the same time the mechanical stability of the overall system is increased since the cooling elements form a coherent entity with the cooling body. 
         [0021]    In order to enable sufficient clearance or a reserve for compensating relative movements between the cooling body and the stack of flat cells—for example, due to thermal expansion—, each of the cooling elements can have at least one depression, in particular extending essentially in parallel to the extension plane of the cooling body. The depression can be a protrusion having a C-shaped or an S-shaped cross section and extending over the whole length of the cooling element. 
         [0022]    According to another embodiment, at least one hollow space, which is formed in the cooling body and through which a coolant is able to flow, is assigned to each of the cooling elements, wherein the hollow space in particular extends in parallel to the flat cells. In this way, the cooling capacity can be considerably increased. Depending on the respective embodiment, a separately fed hollow space can be assigned to each of the cooling elements or the individual hollow spaces can be interconnected such that eventually a continuous overall hollow space, which has, for example, a meandering shape, is obtained for the corresponding energy storage unit. The respective hollow space can also be formed by the fact that the cooling body has a depression or a recess covered in a fluid-tight manner by the connection unit. 
         [0023]    The hollow space can have a U-shaped cross section having two leg portions directed towards the assigned cooling element, wherein the cooling element extends in the area between the leg portions. The hollow space, through which a coolant can flow, thus encompasses one end of the associated cooling element and in this way enables especially efficient heat dissipation therefrom. Depending on the respective application requirement, a plurality of hollow spaces can be assigned to each of the cooling elements. 
         [0024]    According to another embodiment, the energy storage unit has at least one coolant duct for supplying the hollow spaces with coolant, wherein the coolant duct opens into a coolant inlet at a first end face of the energy storage unit and into a coolant outlet at a second end face of the energy storage unit. All the hollow spaces associated with the energy storage unit can thus be connected in a simple manner to a common coolant supply. 
         [0025]    The coolant inlet/outlet can be configured for connection to a coolant outlet/inlet of another energy storage unit. Within the framework of a modular concept, a plurality of energy storage unit can thus be combined to form an overall energy storage unit (for example, vehicle battery) and be connected to a common coolant supply, wherein in each case the coolant outlet of one energy storage unit is connected to the coolant inlet of the subsequent energy storage unit such that eventually the supplied coolant flows through all the hollow spaces. 
         [0026]    According to a special embodiment of the invention, the invention is not only suitable for application in lithium-ion accumulators/cells, but can also be used for/in other energy storages, such as NiMH (nickel metal hydride) energy storages/cells and/or capacitor storage cells, in particular double-layer capacitor cells (supercaps). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Hereinafter the present invention will be explained by way of example by means of advantageous embodiments with reference to the accompanying drawings, wherein 
           [0028]      FIG. 1  shows a perspective view of two energy storage units according to the invention; 
           [0029]      FIG. 2  shows a cross-sectional view of a portion of one of the energy storage units according to  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  shows a perspective view of two energy storage units  10 ,  10 ′. The two energy storage units  10 ,  10 ′ are joined together in order to form a superordinate unit. Within the framework of a modular concept, thus, depending on the application requirement, any number of energy storage units can be joined together in a line or be combined to form a two- or three-dimensional matrix in order to eventually constitute a vehicle battery, for example. 
         [0031]    Each of the energy storage units  10 ,  10 ′ comprises a plurality of prismatic cells/flat cells  12  arranged in parallel to one another in a stack. The cells  12  can, for example, be lithium ion accumulator cells or double-layer capacitor cells (“supercaps”). Each of the individual cells  12  has two electrode tabs  14   a ,  14   b  protruding from the narrow side thereof. A plate-shaped interconnection board  18  serves for an electrical through-connection of the cells  12 . In the case of the exemplary embodiment described, the cells  12  are serially interconnected. Depending on the requirement profile, any other interconnection of the cells  12  can be chosen, for example, a parallel interconnection or a mixed form of serial and parallel interconnection. The cells  12  of the respective stack are tensioned against one another by means of two pressure plates  15  as well as tension springs  16 . Due to this, the cells  12  are held tight, but are simultaneously enabled to “breathe”, for example, due to thermal expansion. 
         [0032]    The electrical energy stored in the cells  12  can be tapped via plugs  19  arranged on opposing sides of the interconnection board  18 . The interconnection board  18  is aligned at right angles to the cells  12  and completely covers one end face of the cell stack. 
         [0033]    As can be seen from the cross-sectional view of  FIG. 2 , a plate-shaped cooling body  20  made of a plastic material is arranged between the interconnection board  18  and the flat cells  12 , wherein the plate-shaped cooling body  20  essentially has the same surface extension as the interconnection board  18 . The interconnection board  18  has slots  22  for leading the electrode tabs  14   a ,  14   b  through. Likewise, the cooling body  20  has slots  24 , which are in registry with the slots  22  of the interconnection board  18  and are in alignment therewith. The electrode tabs  14   a ,  14   b  are led through the slots  24  of the cooling body  20  and through the slots  22  of the interconnection board  18  and bent around at their upper ends. Further, the electrode tabs  14   a ,  14   b  are connected to the contact elements  40  in an electrically conducting manner, preferably by welding. The contact elements  40  have bimetallic structures in order to guarantee a reduction of corrosion. 
         [0034]    In the case of the embodiment shown, the cooling body  20  is formed as one piece and forms a carrier for the interconnection board  18 , which is mounted by means of fixing elements not shown, such as pressure domes and spring rings, to the top side  26  of the cooling body  20  in such a way that the interconnection board  18  contacts the cooling body  20  permanently and over its entire surface. Thus, the cooling body  20  is heat-conductively connected to the interconnection board  18 , which means that heat developing in the area of the contact elements  40  can be efficiently transferred to the cooling body  20 . 
         [0035]    Generally, the cooling body  20  forms a barrier between the cells  12 , which generate heat losses, and the contact elements  40 , which also give off heat losses due to the current flow. This structure enables particularly efficient heat dissipation. 
         [0036]    It can be seen from  FIG. 2  that plate-shaped cooling elements  46  are provided between adjacent cells  12 , which serve to cool the cells  12  and additionally can act as distance pieces. They are preferably formed in such a way that they are able to take up/compensate “breathing” of the cells  12  during charging and discharging operations, respectively, and dimensional changes during the service life of the cells  12 . In order to stabilize the overall structure and to improve heat conduction, the cooling elements  46  can essentially be glued over their entire surface to the respective adjacent cells  12 . 
         [0037]    The cooling body  20  has, at its bottom side  28  facing towards the cells  12 , recesses  32  in which the cooling elements  46  are inserted and glued. 
         [0038]    As can be further seen from  FIG. 2 , hollow spaces  34  are formed in the cooling body  20 , through which a coolant can flow in order to increase the cooling capacity in this manner. Each of the cooling elements  46  has a hollow space  34  of its own assigned thereto, extending in parallel to the cells  12  and thus also in parallel to the associated cooling element  46 . The hollow spaces  34  have a U-shaped cross section comprising two leg portions  36   a ,  36   b  directed towards the respective cooling element  46  and are arranged such that they encompass the upper end of the corresponding cooling element  46 . 
         [0039]    The hollow spaces  34  are connected to one another in such a way that as a result coolant serially flows through all the hollow spaces  34 . The coolant is supplied and discharged again via tubular coolant ducts  37  ( FIG. 1 ). Two coolant ducts  37  are provided for each energy storage unit  10 ,  10 ′, wherein the coolant ducts  37  are arranged at the side of the interconnection board  18  facing away from the cells  12  and communicate with the hollow spaces  34  from the top side  26  of the cooling body  20 . In the exemplary embodiment shown, the coolant ducts  37  are aligned in parallel to the cells  12  and are located each at an outer marginal area of the interconnection board  18 . Alternatively, the coolant ducts  37  can also be aligned at right angles to the cells  12 . Each coolant duct  37  has a coolant inlet  38  at one end and a coolant outlet  39  at the opposed end. The configuration is such that the coolant outlet  39  of one energy storage unit  10 ′ can be inserted in the coolant inlet  38  of another energy storage unit  10 . 
         [0040]    During operation of the energy storage unit  10 ,  10 ′, on the one hand, thermal tensions develop due to the heat losses generated, on the other hand, the cells  12  “breathe” due to the charging and discharging operations, respectively. In order to compensate the associated movements of the cells  12  relatively to the interconnection board  18  in a vertical and/or a horizontal direction, the electrode tabs  14   a ,  14   b  have depressions  48  leading in a cross-section to a C-shaped protrusion of a portion of the respective electrode tab  14   a ,  14   b . The cooling elements  46  can also have corresponding depressions if the application requires so.