Abstract:
The present invention relates to an electric cell unit for a secondary battery as well as to such a secondary battery module. The electric cell unit comprises:
       a first electric cell ( 12 ) enclosed by a first casing ( 13 ),   a second electric cell ( 14 ) enclosed a second casing ( 15 ),       
 
     wherein at least one of first and second casings ( 13, 15 ) comprises a recessed portion ( 16, 18 ) extending along a side edge ( 11 ) thereof to form a receptacle ( 30 ), which is adapted to receive at least one thermal transfer element ( 28 ).

Description:
This application claims priority from European Patent Application No. 12169776.7 filed May 29, 2012, the entire disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of secondary, hence rechargeable batteries, and particularly refers to space-efficient thermal management as well as to space-and weight-optimized housing concepts for such batteries. 
     BACKGROUND AND PRIOR ART 
     Electric energy storage may be provided by secondary batteries, such like lithium-ion batteries. Such batteries but nearly all kind of secondary batteries should be operated within a predefined temperature range. For instance, at very low temperature, e.g. lower than −10° C., battery performance declines and efficiency may drop remarkably. At higher operating temperatures, e.g. around 40° C. and above, the economic life-time as well as performance and capacity of such batteries typically reduces. 
     These thermal conditions require that such batteries and battery modules have either to be heated or cooled appropriately. Consequently, there is a need for an adequate thermal management for batteries. 
     Electrochemical cell shapes are generally classified as either prismatic or cylindrical. Cylindrical cells have cylindrical housings. Prismatic cells have prismatic housing shapes, such as parallelepipeds. Common examples of prismatic cells include standard 12 V car batteries. An electrochemical cell can be, for example, a lithium ion cell. 
     Lithium-ion batteries are often packaged in so-called “pouch cells”. An advantage of pouch cells is that the traditional metallic cylinder and glass to metal electrical feed-through is replaced by a relatively inexpensive foil packaging similar to what is used in the food industry. The foil is often a lamination film made from aluminum. The electrical contacts generally consist of conductive tabs that are welded to the electrode and sealed in the pouch material. 
     Document WO 2010/071463 A1 describes electrochemical cells placed inside a housing which are in thermal contact with a Peltier cell. The Peltier cell provides heat transfer into or out of a cell pack, whereas thermal-conducting separating plates in contact with at least one sidewall are placed inside said housing with the electrochemical cells placed between the plates. There, a sidewall of the housing is in contact with said separating plates and is further in thermal contact with the Peltier cell. 
     Moreover, the separating plates sandwiched between electrochemical cells have back edges bent at an angle of 90° contacting with the sidewall. Those back edges are further riverted together with said sidewall. 
     Since the separating plates protrude from lateral edges of the electrochemical cells, the cells have to be positioned and assembled at a particular distance from the sidewall of the housing. As a consequence, the interior space of the housing cannot be completely filled with electrochemical cells. 
     It is therefore an object of the present invention to provide a space-optimized housing for an electric battery, which provides improved thermal management. It is a further aim to optimize size and weight of a secondary battery together with an efficient cooling or thermal management of the battery. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides an electrochemical cell unit for a secondary battery. The electrochemical cell unit comprising:
         a first prismatic pouch cell comprising a first electrochemical cell enclosed by a first prismatic pouch made of flexible foil,   a second prismatic pouch cell comprising a second electrochemical cell enclosed by a second prismatic pouch made of flexible foil,       

     each prismatic pouch cell further comprising current collecting tabs attached to electrodes of the electrochemical cell, leaving the pouch through a recessed portion extending along a side edge of the prismatic pouch, and sealed between layers of the flexible foil, 
     wherein the first and second prismatic pouch cells are stacked in such a way as to maintain an empty space between the recessed portions thereof, so as to form a receptacle between the first and second prismatic pouch cells, the receptacle being adapted to receive at least one thermal transfer element, 
     wherein the electrochemical cell unit further comprises a frame at least partially laterally enclosing the first and the second prismatic pouch cells and comprising a strut extending across the frame, along an outside surface of the recessed portion of one of the first and second prismatic pouch cells, the outside surface facing away from the receptacle. 
     By providing a recessed portion at a lateral side of first and second prismatic pouch cells, the thermal transfer element can be arranged in such a way, that it at least partially overlaps with the first and/or with the second prismatic pouch cell. The thermal transfer element typically extends along the recessed portion and hence along the side edge of first and second prismatic pouch. It may be embedded in or may be surrounded by the at least one recessed portion. 
     The at least one thermal transfer element may comprise a tubing providing thermal energy transfer by way of circulating a heat exchanging medium, such like a gaseous or liquid fluid. The thermal transfer element may alternatively comprise a strip of a material generically providing sufficient heat transfer or transfer of thermal energy. For instance, the thermal transfer element may comprise a metal sheet, e.g. of copper, aluminum or of comparable metals or alloys providing sufficient thermal energy transfer. 
     By providing a receptacle in a structure comprising first and second prismatic pouch cells stacked to one another, the overall volume of a battery comprising a plurality of prismatic cells or electrochemical cell units can be reduced. At the same time, a thermal transfer element can be arranged in an overlapping configuration with a cell, thereby providing a thermal transfer means in direct contact or in direct vicinity with those portions of first and/or second pouch cells from which respective thermal energy may originate and dissipate when the secondary battery is in operation mode. 
     The recessed portion of first and/or second prismatic pouch is typically obtained by a region of the first and/or second pouch having a reduced thickness along a side edge of the prismatic pouch. Typically, the receptacle can be formed by the recessed portions of first and second prismatic pouches. The recessed portions feature a reduced thickness compared to a residual portion of the respective prismatic pouch. By stacking the respective prismatic pouch with another prismatic pouch of either identical or different geometry but at least covering the recessed portion, a receptacle can be formed between respective first and second pouches which is for instance accessible from a side edge of first and/or second prismatic pouches. 
     In another preferred aspect, the recessed portion of either one of the prismatic pouch cells extends flush with one of the main surfaces of the pouch. Here, first and second prismatic pouch cells are arranged such, that respective recesses of the first and second cells face towards each other to form the receptacle there between. In case the first and second recessed portions of first and second cells are located at or form side edges of respective cells having a reduced thickness, then a U-shaped receptacle can be provided when first and second recessed portions of first and second cells are arranged in a substantially overlapping manner. 
     The recessed portions of first and/or second casings may comprise a stepped-down profile, such that the recess or the receptacle formed by two overlapping recesses is of substantially rectangular shape. Alternatively, a transition from a residual or active portion of the casing towards the recessed portion may also be cone shaped or slanted in order to provide a receptacle of corresponding shape. 
     In a further preferred embodiment, at least one thermal coupling element thermally coupled with the at least one thermal transfer element is arranged in the receptacle between first and second electrochemical cells to fill a gap space between first and second casings and/or between first and second receptacles, respectively. 
     By means of at least one thermal coupling element, efficient transfer of thermal energy between the at least one recessed portion of first and/or second casing and the thermal transfer element extending through the receptacle can be provided. By means of the at least one thermal coupling element, inside facing surface portions of the receptacle can be effectively thermally coupled with the thermal transfer element. 
     A heat exchange between the thermal transfer element and the at least one recessed portion of first and/or second electrochemical cells can therefore be improved. Preferably, the at least one thermal coupling element provides a geometric interface between the geometric structure of the receptacle and the geometric structure of the thermal transfer element. The at least one thermal coupling element or a plurality of thermal coupling elements are designed to fill the entire gap between the recessed portions of first and second casings thereby almost entirely filling the receptacle formed between first and second recessed portions. 
     In a further preferred embodiment, first and second cells of the electrochemical cell unit comprise substantially identical geometries. Preferably, first and second electrochemical cells comprise a substantially planar geometry, which allows to assemble first and second cells in a stack to form an electrochemical cell unit comprising a pair of electrochemical cells. It is of particular benefit, when first and second cells of substantially identical geometry are arranged face to face in such a way, that recessed portions of first and second electrochemical cells face towards each other in stacking direction (z), such that laterally protruding portions integrally formed with active portions of respective battery cells are separated from each other with respect to the stacking direction (z) such that the receptacle is formed there between to receive the at least one thermal transfer element. 
     According to another preferred aspect the frame is preferably made of a plastic material, in particular of an injection moldable plastic material. The frame material may therefore comprise thermoplastic material, which may be structurally enhanced or reinforced, e.g. by way of fibers. The frame laterally enclosing at least one of first and second electrochemical cells may directly form part of a modular housing of the secondary battery. Preferably, each electrochemical cell may be preassembled in a respective frame to provide a corresponding frame-electrochemical cell-preassembly. The electrochemical cells may be positively or frictionally engaged with the surrounding frame. 
     The at least one thermal transfer element typically intersects the frame in order to provide a sufficient thermal energy transport through the frame. 
     In a further preferred embodiment, the frame comprises at least one frame element to receive one of first and second electrochemical cells. Preferably, the frame comprises two frame elements, each of which being adapted to receive and to assemble one of first and second electrochemical cells therein. Hence, a first frame element is adapted to receive and to mount the first electrochemical cell whereas a second frame element is adapted to receive and to mount the second electrochemical cell element of the electrochemical cell unit. 
     The frame, in particular its frame element comprises mutually engaging frame portions, by way of which a series of frames can be stacked on one another in a well-defined, structurally stable and durable way. Since each frame element typically provides a mount and a mechanical support for an electrochemical cell, respective first and second electrochemical cells of an electrochemical cell unit can be mutually assembled simply by arranging first and second electrochemical cells in respective first and second frame elements and by mutually assembling first and second frame elements to establish and to provide an electrochemical cell unit. 
     During this assembly, recessed portions of first and second cells can be thermally coupled with the at least one thermal transfer element and optionally with the at least one thermal coupling element. 
     In a further preferred embodiment, a thickness (D) of the frame element exceeds a thickness (d) of the electrochemical cell assembled therein by at least 3%, 5%, 8%, 10% or even by 15%. The thickness of the frame typically coincides with the stacking direction of various frame elements. Since the frame element is thicker in stacking direction (z) compared to the electrochemical cell assembled therein, various electrochemical cells adjacently located in a stack of frame-electrochemical cell-pre-assemblies are positioned at a predefined gap size there between. This way, the electrochemical cells may expand in thickness, hence in stacking direction without transferring mechanical tension to each other, to the surrounding frame structure and/or to respective end structures of a housing of a secondary battery. 
     Therefore, electrochemical cells assembled in a stack of frame elements may “breathe” and may freely expand in stacking direction (z) without imposing mechanical stress in a stack. 
     Typically, it is only the electrically active portion of an electrochemical cell that differs in thickness compared to the surrounding frame element and which comprises coated metal electrodes and non-conductive separators. In the region where electric contact portions (current collecting tabs) of the electrochemical cells are provided, adjacently located electrochemical cells are preferably densely and tightly packed or squeezed in stacking direction when surrounding frame elements are stacked on one another. 
     Since the electric contact portions of first and second electrochemical cells undergo geometric deformations or expansion to a much lesser extend compared to active portions of the electrochemical cells, a comparatively lose fitting of the recessed portion is neither required nor intended. Moreover, by establishing a rather tight fitting of recessed portions with a thermal transfer element squeezed in a respective receptacle, a sufficient thermal coupling can be established and sustained. 
     According to the invention the frame comprises at least one strut extending across the frame. The strut serves to structurally enhance or to reinforce the frame. Moreover, since electrodes as well the at least one thermal transfer element have to intersect the frame element, the frame itself either comprises an interrupted structure along its outer circumference or the frame may comprise recessed portions to guide electric contact electrodes and/or the at least one thermal transfer element there through. Such recesses or interruptions of the frame structure which naturally impose mechanical weakening can be mechanically and/or structurally compensated by means of the at least one strut. 
     In a preferred embodiment the at least one strut substantially flushes with the frame element as seen in stacking direction (z) of first and second electrochemical cells. Hence, as seen in stacking direction and perpendicular to the circumference of the frame element, the strut does not protrude from the frame element. Moreover, the strut extends inwardly as seen in stacking direction (z) to provide a kind of a spacer for supporting a tight fitting of recessed portions of the electrochemical cells and their at least one thermal transfer element arranged there between. 
     According to the invention, the strut of the frame element extends along an outside oriented portion of the recessed portion on the respective prismatic pouch of an electrochemical cell arranged in said frame. Hence, the strut typically extends along a back side of the recessed portion of an electrochemical cell casing that faces away from the thermal transfer element located between pair wise arranged electrochemical cells of an electrochemical cell unit. 
     It is of particular benefit, when according to another preferred embodiment the strut extends substantially parallel to the receptacle and/or substantially parallel to the thermal transfer element. This way, a pre-fitting or squeezing of the thermal transfer element between recessed portions of pair wise and face to face oriented electrochemical cells can be structurally enhanced and reinforced. 
     Moreover, by means of the at least one strut, the electrochemical cell can be easily and intuitively secured and fixed in the frame. This way, a plurality of frame-electrochemical cell-pre-assemblies can be preassembled to form a plurality of electrochemical cell units, each of which comprising a pair of electrochemical cells preassembled in a corresponding pair of frame elements. 
     Regarding the strut, its position and dimension with respect to the frame, it is of particular benefit and according to another preferred embodiment, when the at least one thermal transfer element is in tight thermal contact with at least one recessed portion of first and/or second prismatic pouch cell when a first frame element containing the first electrochemical cell is stacked with a second frame element containing the second electrochemical cell. 
     Even though the recessed portions of first and second electrochemical cells are then tightly fitted or tightly squeezed in stacking direction by means of at least one strut, a residual and electrochemically active portion of the electrochemical cells may still feature a particular gap size in stacking direction (z) due to a reduced thickness compared to the respective frame element. 
     This way, load-dependent or operation-condition-dependent geometric expansion of electrochemical cells may have a reduced or even eliminated impact on a housing of a secondary battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, a preferred embodiment of the invention will be described by making reference to the drawings, in which: 
         FIG. 1  schematically illustrates two isolated electrochemical cells having a thermal transfer element arranged there between, 
         FIG. 2  shows a side view of the electrochemical cells of  FIG. 1 , 
         FIG. 3  shows the electrochemical cells according to  FIG. 1  assembled in a frame according to a first perspective view, 
         FIG. 4  shows the electrochemical cell unit according to  FIG. 3  in a different perspective view, 
         FIG. 5  is illustrative of the electrochemical cell unit according to  FIGS. 3 and 4  as seen from the top, 
         FIG. 6  shows a cross section A-A according to  FIG. 5 , 
         FIG. 7  is illustrative of a stack of electrochemical cell units as seen from a first perspective and 
         FIG. 8  shows the stack of  FIG. 7  from another perspective. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIGS. 1 and 2  first and second cells  12 ,  14  of an electrochemical cell unit  10  are shown in a perspective illustration and in a side view, respectively. The first electrochemical cell  12  comprises a recessed portion  16  towards an upper current collecting tab  32 . Accordingly, also the second electrochemical cell  14  comprises a correspondingly shaped recessed portion  18  at its upper end. In a lower portion as illustrated in  FIGS. 1 and 2 , the electrochemical cells  12 ,  14  each comprise an active portion  20 ,  22 , where layers of coated metal electrodes separated by non-conductive separators are arranged. 
     Each electrochemical cell  12 ,  14  is enclosed by a prismatic pouch  13 ,  15  surrounding the upper recessed portions  16 ,  18  as well as the lower electrically active portions  20 ,  22  of respective electrochemical cells  12 ,  14 . 
     The electrochemical cells  12 ,  14  and their respective prismatic pouches  13 ,  15  comprise substantially identical geometries. As illustrated in  FIGS. 1 and 2 , the prismatic pouch cells  12 ,  14  are arranged face to face, such that a receptacle  30  is provided between a first recessed portion  16  and a second recessed portion  18  of first and second electrochemical cells  12 ,  14 , respectively. The receptacle  30  thus formed at an upper side edge  11  of the electrochemical cell unit  10  is adapted to receive at least one thermal transfer element  28 , which according to the embodiment as shown in  FIGS. 1 to 8  may comprise a tube for circulating a heat exchanging medium, such like a coolant. 
     Since the thermal transfer element  28  is of smaller dimensions compared to the gap between the stepped down recessed portions  16 ,  18 , a residual space between the recessed portions  16 ,  18  is filled and stuffed with thermal coupling elements  24 ,  26 , which provide sufficient, constant and persistent exchange of thermal energy between the recessed portion  16 ,  18  and the thermal transfer element  28 . 
     As illustrated in  FIGS. 1 and 2 , the recessed portions  16 ,  18  of respective pouches  13 ,  15  provide an electric contact portion of the cells  12 ,  14 . Accordingly, at a free and upper end of the recessed portions  16 ,  18  various current collecting tabs  32 ,  34 ,  36 ,  38  extend from the side edge  11  of the electrochemical cells  12 ,  14 . Apart from the design and shape of the various current collecting tabs  32 ,  34 ,  36 ,  38 , first and second electrochemical cells  12 ,  14  are here rather identical. 
     The electrochemical cells  12 ,  14  are further of planar and even prismatic geometry. Hence, an electrically active portion  20 ,  22  substantially extends in a transverse plane, as illustrated by the x- and y-axes according to  FIG. 1 . 
     For providing a secondary battery module  50  as illustrated for instance in  FIGS. 7 and 8 , numerous electrochemical cell units  10 , each of which comprising first and second electrochemical cells  12 ,  14  are to be assembled in a stacked configuration in a stacking direction (z) as becomes apparent from the sketch of  FIGS. 7 and 8 . 
     According to the illustrated example, the identical electrochemical cells are electrically connected in series. As each cell unit  10  is formed of a pair of electrochemical cells  12 ,  14  facing each other, the positive tab of a first cell in each pair faces the negative tab of the second cell, and the negative tab of the first cell faces the positive tab of the second cell. Referring again to  FIG. 1 , one can observe that the L-shaped current collecting tabs  36  and  38  of cells  12  and  14  overlap each other so as to be in contact. In contrast, current collecting tab  32  of electrochemical cell  12  angles away from current collecting tab  34  of electrochemical cell  14 . Tabs  32  and  34  are therefore not in contact with each other. As visible in  FIGS. 7 and 8 , tab  32  extends towards the next electrochemical cell in the stack, so as to make contact with the next pair of cells. 
     For providing a universal and adaptable design of a battery module  50 , the single electrochemical cells  12 ,  14  of an electrochemical cell unit  10  are each arranged in respective frame elements  42 ,  44  as shown in  FIGS. 3 to 6 . Here, a pair of frame elements  42 ,  44  forms a frame  40  of the electrochemical cell unit  10 , which can be preassembled as illustrated in  FIGS. 3 and 4 . The frame elements  42 ,  44  are preferably made of a plastic material. They may comprise a thermoplastic injection moldable material allowing for cost efficient mass production. 
     The frame elements  42 ,  44  are to be intersected by various contact tabs  32 ,  34 ,  36 ,  38  of first and second electrochemical cells  12 ,  14 . Moreover, first and second frame elements  42 ,  44  are intersected by the thermal transfer element  28 , extending through and protruding from the left side of the frame  40  as shown in  FIG. 3 . The frame  40 , in particular its two frame elements  42 ,  44  are adapted to receive and to hold a respective electrochemical cell  12 ,  14 . By substantially enclosing the electrochemical cells  12 ,  14  in circumferential transverse direction (x, y), the electrochemical cell  12 ,  14  can be sufficiently fixed and mounted with regard to the transverse plane (x, y). 
     In stacking direction (z), the frame elements  42 ,  44  are open. Here, the first frame element  42  comprises a kind of through opening which is adapted to receive the entirety of first and second electrochemical cells  12 ,  14 , whereas the second frame element  44  comprises a strut  46  extending across the respective frame element  44 . The strut  46  extends substantially parallel to the side edge  11  and therefore extends substantially parallel to the elongation of the thermal transfer element  28 . 
     Furthermore, as can be seen from  FIGS. 3 and 6 , the strut  46  does not protrude from the circumferential frame element  44  but flushes with the plane of the frame element  44  as shown in  FIG. 3 . Here, the strut  46  provides mechanical reinforcement of the frame element  44  in the region, where the frame elements  42 ,  44  are intersected by current collecting pads  32 ,  34 ,  36 ,  38  and/or by the thermal transfer element  28 . An inevitable structural weakening of the frame elements  42 ,  44  due to the intersections may be structurally compensated by means of the strut  46 . 
     Moreover, the strut  46  also serves to provide a squeezing and tight fit of the thermal transfer element  28  and the thermal coupling elements  24 ,  26  inside the receptacle  30 . Furthermore, the strut  46  also serves to compensate slight geometrical variations of the thickness of the frame elements  42 ,  44  compared to the electrochemical cells  12 ,  14 . In typical configurations, the thickness (d) of the active portions  20 ,  22  of first and second cells  12 ,  14  is less compared to the corresponding thickness (D) of first and second frame elements  42 ,  44  as seen in stacking direction (z). These geometric differences allow to assemble a stack  50  of electrochemical cell units  10 , wherein neighboring and adjacently arranged electrochemical cells  12 ,  14  are separated by a particular gap, at least in the region of their active portions,  20 ,  22 . 
     As schematically illustrated in  FIGS. 5 and 6 , at least the frame element  44  comprises a thickness (D) in stacking direction (z). In comparison thereto, a thickness (d) of an active portion  22  of an electrochemical cell  14  is at least slightly reduced, such that at least a small gap in stacking direction between a border of the respective frame element  44  and the plane of the surface of the active portion  22  arises. 
     This way, the electrically active portions  20 ,  22  may expand in stacking direction (z) under operating conditions and/or under varying thermal conditions. The varying thickness and the gap in stacking direction between surrounding frame elements  42 ,  44  and active portions  20 ,  22  of electrochemical cells  12 ,  14  allows the active portions  20 ,  22  to “breathe” at least to a predefined extend. 
     By providing a strut  46  with a predefined thickness in stacking direction (z), such “breathing” and a lose fitting of recessed portions  16 ,  18  can be effectively prevented. This way, the strut  46  also serves as a kind of spacer, by way of which recessed portions  16 ,  18 , thermal coupling elements  24 ,  26  as well as the thermal transfer element  28  arranged there between can be tightly fitted and squeezed in stacking direction (z) irrespective of the operation mode and operation conditions of the electrochemical cell unit, when a plurality of cell units are arranged in a stack  50  as shown in  FIGS. 7 and 8 . 
     From  FIG. 4  it follows, that the first frame element  42  is strutless. However, the assembly of first and second electric cells  12 ,  14  with thermal coupling elements  24 ,  26  and a thermal transfer element  28  sandwiched there between is fixed with respect to the frame  40  in stacking direction (z) as soon as the frame  40  as shown in  FIG. 4  is stacked with a substantially identical frame in such a way, that the strut  46  of the adjacently located frame  40  also abuts with a neighboring electrochemical cell  12  of an adjacently arranged electrochemical cell unit  10 . 
     Furthermore, as illustrated in  FIGS. 3 to 6 , the frame elements  42 ,  44 , comprise separating members  41 ,  43 ,  45  protruding outwardly from the frame structure but extending substantially parallel to the plane defined by the frame elements  42 ,  44 . Such separating members  41 ,  43 ,  45  provide a mechanical support for various current collecting tabs  34 ,  36 ,  38  extending through the respective frame elements  42 ,  44 . Moreover, the separating members  41 ,  43 ,  45  provide electrical insulation for the tabs  34 ,  36 ,  38  and help to prevent that neighboring tabs  36 ,  38  of adjacently arranged electrochemical cell units  10  get in direct contact with each other, e.g. during assembly of a stack  50 . 
     This way, a risk of electric shortcuts during assembly but also in the event of e.g. an impact-induced deformation can be effectively reduced or even eliminated. The separating members  41 ,  43 ,  45  are preferably integrally formed with the frame elements  42 ,  44  and may further enhance their mechanical stability, stiffness and/or rigidity. 
     In effect, by providing a recessed portion  16 ,  18  at a side edge  11  of first and second electrochemical cells  12 ,  14  folded onto one another in a face to face configuration, an improved thermal management can be directly introduced into the area of electrochemical cell contacting, thereby allowing to reduce the overall size of a housing of a secondary battery module. Moreover, by means of the plastic frame  40  a rather simple, light weight and modular stacking system can be provided allowing to universally assemble a variety of electrochemical cell units to provide differently configured secondary battery modules  50 . 
     LIST OF REFERENCE NUMERALS 
     
         
           10  electrochemical cell unit 
           11  side edge 
           12  prismatic pouch cell 
           13  prismatic pouch 
           14  prismatic pouch cell 
           15  prismatic pouch 
           16  recessed portion 
           18  recessed portion 
           20  active portion 
           22  active portion 
           24  thermal coupling element 
           26  thermal coupling element 
           28  thermal transfer element 
           30  receptacle 
           32  current collecting tab 
           34  current collecting tab 
           36  current collecting tab 
           38  current collecting tab 
           40  frame 
           41  separating member 
           42  frame element 
           43  separating member 
           44  frame element 
           45  separating member 
           46  strut 
           50  stack