Patent Publication Number: US-2015079456-A1

Title: Battery having a connecting element comprised of plural individual wires

Description:
The invention relates to a battery with a plurality of battery cells and at least one connecting element which connects an electric pole of a first battery cell with the electric poles of further battery cells. The connecting element includes plural individual wires which are compacted in a respective attachment zone. Furthermore, the invention relates to a method for manufacturing such a battery. 
     In electric vehicles and hybrid vehicles, electrochemical energy stores in the form of batteries are used to date, which oftentimes are configured as lithium-ion batteries. To realize sufficient output and capacity of the batteries, a plurality of battery cells are connected in series or parallel. The battery may also include a plurality of battery modules in which in turn a plurality of battery cells are arranged and connected in series and/or parallel. 
     Interconnection or electric connection of the battery cells with one another to form a battery module or a battery system is normally implemented using rigid conductor bars which are made of copper or aluminum. These conductor bars are connected with the respective electric poles of the battery cells by interference fit or material joint so as to establish an electrically conducting connection. 
     The relative position of the battery cells to one another is subject however to certain fluctuations during manufacture and installation. To compensate these, i.e. to provide a tolerance compensation, constructive measures are applied. This may be necessary in terms of process, for example when the battery cells and the conductor bars must be positioned precisely in relation to one another so as to realize an electric connection by a welding process. Furthermore, measures using a tolerance compensation minimize static mechanical stress such as for example tensile forces upon the electric poles of the battery cells that could otherwise adversely affect the service life of the battery cells or the connection. 
     U.S. Pat. No. 3,706,955 A describes an electrical cable made form a flat strip of sheet metal. A region between axial ends of the strip of sheet metal is provided with a plurality of slits so that the strip of sheet metal defines in this region a plurality of individual wires. Such a cable can be used for connection to the terminals of a common car battery. 
     U.S. Pat. No. 5,541,380 A describes a cable having a plurality of individual fibers which are braided. The fibers are solidified in end regions of the cables, e.g. by welding. A first end of the cable can be coupled with a current originating device and a second end with a current receiving device. 
     DE 27 45 189 A1 describes a current-carrying conductor for connection between the battery and the starter of combustion engines, which conductor is made from a band material. A number of longitudinal slits is provided between a first end, in which the band material is folded to form a lug, and a second end which forms a battery clamp, so that the current-carrying conductor is split into plural individual wires in this region. 
     It is known in the art, for example from the DE 10 2007 063 177 A1, to use for tolerance compensation a compensating element between a cell connector in the form of a conductor bar and the electric pole, with the compensating element being connected by a material joint with the cell connector on one hand, and with the electric pole on the other hand. 
     This has the drawback that an additional component is required in the form of the compensating element necessitates. This causes added expense to the production, storage, provision and installation, especially in terms of positioning of the compensating element upon the electric pole and the cell connector. In addition, there is the risk of a short-circuit, for example when the compensating element drops. 
     The provision of the compensating element increases in addition the weight of the battery. Furthermore, it is disadvantageous that each connection point has two joints, namely between the electric pole and the compensating element on one hand, and between the compensating element and the cell connector on the other hand. Experience has shown that this leads to an increase of the overall electric resistance of the connection. 
     The provision of compensating elements is able to prevent static loads. However, the rigid connection of the battery cells results in movements of the battery cells in relation to another during operation, directly causing mechanical stress at the electric poles of the battery cells. Such relative movements can be caused in particular by heat expansion but also when subjecting the battery system to bending or torsion in the vehicle. 
     It is further known, for example from DE 10 2009 058 723 A1 or EP 208 0232 A2, to provide flexible elements within the conductor bar for tolerance compensation. This, however, causes a weakening of the cross section of the conductor bar and thus increased resistances and a reduced current carrying capacity. In the WO 2011 045 088 A1, the provision of flexible elements results within the conductor bar in an increase in length of the conductor bar and thus to an increased weight. Furthermore, the flexibility of such flexible constructions is oftentimes limited to one spatial direction. 
     It is further known to use a flexible conductor in the form of a litz wire, instead of a rigid conductor bar, for connection of the battery cells. The litz wire is provided with a press-welded sleeve in a region intended for attachment onto the electric pole of the battery cell and then threadably engaged with the battery poles. A drawback hereby is the fact that the pressed sleeves result in added weight and the attachment of the pressed sleeves is very complicated. 
     WO 2010/142679 A1 describes a battery cell connector in which respective terminal parts of the battery cell connector are made from the same material as the electric poles of the battery cells with which the terminal parts should be connected by a material joint. A connecting part is provided between the terminal parts and is formed from a litz wire or a braid. The litz wire is compacted by brief compaction and heating in the area of attachment of this litz wire to the terminal parts. These compacted regions are connected by a material joint with the respective terminal part. 
     It has been considered hereby as disadvantageous that the battery cell connector is of relatively complex configuration due to the fact that both the terminal parts and the litz wire have compacted end regions to electrically connect the terminal parts with one another. In addition, the presence of joints between the compacted regions of the litz wire and the terminal parts as well as between the terminal parts and the electric poles of the battery cells can increase the overall resistance of the connection. 
     Object of the present invention is the provision of both a battery of the afore-mentioned type and a method of manufacturing such a battery to realize a particularly simple structure. 
     This object is attained by a battery with the features of patent claim  1  and by a method with the features of patent claim  10 . Advantageous configurations with appropriate refinements of the invention are set forth in the dependent patent claims. 
     In the battery according to the invention, the connecting element in the respective attachment zone contacts the respective electric pole of the battery cell. The individual wires are compacted in evenly spaced-apart attachment zones, and the distances of the attachment zones correspond to the distances of the electric poles of the battery cells in the cell assembly of the battery. Due to the compaction of the attachment zones, the latter can be connected directly with the electric pole of the battery cell in an especially good way, just like the case when an attachment zone of an overall rigid conductor bar is involved. Furthermore, the connecting element is especially simple in structure so that the configuration of the entire battery is simplified. 
     In the afore-described battery, the connecting element provides in addition that the tolerances can be easily compensated in all directions because of the flexibility in its non-compacted regions. Both the attachment zones and the electric poles of the battery cell remain therefore substantially free of mechanical stress during operation of the battery. The high flexibility of the connecting element in its non-compacted regions is based on the plurality of the thin individual wires from which the connecting element is made. 
     Furthermore, the properties of the compacted attachment zones correspond substantially to those of a solid conductor bar so that the attachment zones allow a simple and reliable attachment of the connecting element—still having a flexibility—onto the respective electric pole of the battery cell. Moreover, a particularly slight transition resistance is encountered in the region of the attachment of the connecting element onto the electric poles because of the compacted attachment zones. 
     As the respective attachment zones directly contact the respective electric pole of the battery cell, the battery is of simple structure and also the connecting element can be produced in a particularly simple manner. There is only need for a compaction of the connecting element at defined locations, i.e. in the attachment zones respectively provided for attachment onto the electric poles of the battery cells, and these compacted regions are then connected with the electric poles of the battery cell. 
     According to an advantageous configuration of the invention, the connecting element is compacted in the respective attachment zone by welding the individual wires. The thus fused individual wires represent then a particularly compact solid attachment zone. This is advantageous in terms of realizing a low transition resistance of the connection between the attachment zone and the electric pole of the battery cell. 
     To melt the individual wires at the margins, resistance welding may be employed in particular to form the compact attachment zone. In this way, a particular intimate bond of the individual wires with one another can be realized with very good electric properties. However, also a pressure welding process may be applicable as well, for example ultrasonic welding, in which the individual wires are welded by friction and pressure. Ultrasonic welding is especially beneficial, when individual wires of the connecting element are involved which are made of aluminum. 
     It has been shown as further advantageous, when the connecting element is configured substantially cuboid in shape in the respective attachment zone, with a planar contact area of the attachment zone in contact with a planar contact area of a base portion of the electric pole. Such an attachment zone with rectangular cross section enables in terms of dimensions the formation of an especially precisely defined attachment zone which allows realization of a simple and reliable installation of the battery. The complementing planar contact surfaces of the attachment zone and the base portion further provide a particularly good electrically conducting connection between the connecting element and the electric poles of the battery cells. 
     In the respective attachment zone, the connecting element can have a through opening which receives a substantially pin-shaped component of the respective electric pole. This simplifies positioning of the attachment zones in relation to the electric poles. The through opening may, for example, be formed by making a bore in the attachment zone. 
     In particular, the connecting element can be connected in the respective attachment zone with the electric pole through threaded engagement. In this way, the contact pressure by which the attachment zone contacts the respective electric pole of the battery cell can be adjusted especially well. Such threaded engagement of the attachment zone with the electric pole is appropriate for example, when a pin-shaped component of the respective electric pole has a screw thread onto which a screw nut can be threadably engaged. 
     A particularly intimate connection of the connecting element with the electric poles can, however, be implemented when the connecting element is connected by a material joint with the electric pole in the respective attachment zone. The provision of the compacted attachment zones on locales of the connecting element that complement the electric poles enables a particularly precise positioning of the parts to be joined by a material joint. In addition, the connection by material joint enables realization of an especially high and particularly consistently high bonding quality. Connection by material joint may in particular be implemented by a beam welding process, such as laser beam welding, allowing a very high welding rate. 
     When providing a through opening in the attachment zone for receiving a pin-shaped component of the respective electric pole, realization of a connection by material joint can be especially easy when producing butt welds and/or fillet welds during welding. 
     In addition or as an alternative, the connecting element can be welded with the electric pole in an overlap zone in which the respective attachment zone contacts a base portion of the electric pole. In this case, welding is performed through the compacted attachment zone into the base portion of the electric pole. In this way, the flat contact of the attachment zone upon the base position can be utilized in a particularly good manner so as to ensure a reliable bond between the connecting element and the electric poles. The flat contact can be ensured during the welding process by a clamping apparatus. 
     The connecting element may be formed from a fabric band or a litz wire, with a flat litz wire being used in particular. Such fabric tapes or litz wires can be compacted in the designated regions in a particularly simple manner. 
     The individual wires of the connecting element are made preferably of copper or aluminum to realize a good electrically conducting connection to the electric poles of the battery cells. 
     In the method according to the invention for manufacturing a battery having at least two battery cells and provided in particular for a vehicle, individual wires of a connecting element having plural individual wires are compacted in a respective attachment zone. At least one electric pole of a first battery cell is connected by the connecting element with an electric pole of at least one further battery cell of the battery. The connecting element is hereby brought into contact in the respective attachment zone with the respective electric pole of the battery cell for electrically connecting the poles with one another. The individual wires are compacted in evenly spaced-apart attachment zones, wherein the distances of the attachment zones correspond to the distances of the electrical poles of the battery cells in the cell assembly of the battery. This direct contacting of the electric poles of the battery cell with the compacted attachment zone of the connecting element enables realization of a particularly simple structure of the battery, and at the same time a beneficial, low transition resistance is established in the area of the connection of the connecting element with the electric poles. 
     The advantages described for the battery according to the invention and preferred embodiments also apply to the method according to the invention, and vice versa. 
     The features and feature combinations mentioned above in the description and the features and feature combinations mentioned hereinafter in the figure description and/or shown alone in the figures are applicable not only in the respectively described combination but also in other combinations or taken alone, without departing the scope of the invention. 
    
    
     
       Further advantages, features and details of the invention become apparent from the claims, the following description of preferred embodiments and from the drawings. It is shown in: 
         FIG. 1  schematically a battery with a plurality of battery cells, in which the electric poles are connected to one another by a cell connector in the form of a flexible flat litz wire which is compacted in the respective attachment zones onto the electric poles; 
         FIG. 2  by way of example, a fabric band suitable as starting material for the production of a cell connector, as shown in  FIG. 1 ; and 
         FIG. 3  a round litz wire with attachment zone compacted by welding and intended for attachment thereof to the electric pole of a battery cell according to  FIG. 1 . 
     
    
    
       FIG. 1  shows schematically a battery  10 , which can be used as traction battery in an electric vehicle or hybrid vehicle. The battery  10  includes a plurality of battery cells  12  which in particular may be formed as a lithium-ion cells. Electric poles  14  of the respective battery cells  12  are electrically interconnected by a connecting element in the form of a flat litz wire  16 . The flat litz wire  16  has plural individual wires  18  which due to their flexibility are able to compensate deviations of an actual position of the battery cells  12  from a desired position in all spatial directions. 
     The individual wires  18  of the flat litz wire  16  are compacted in evenly spaced-apart attachment zones  20 , i.e. connected to form a solid unit. The distances correspond hereby to the distances of the electric poles  14  of the battery cells  12  in the cell assembly of the battery  10 . Here, the flat litz wire  16  is compacted in the attachment zones  20  by resistance welding, causing a melting at the margins and connection of the individual wires  18  of the flat litz wire  16 . 
     The connection of the flat litz wire  16  with the electric poles  14  of the battery cells  12  is implemented in these attachment zones  20 . As a result of the regions provided between the compacted attachment zones  20  with the flexible individual wires  18 , both the connecting areas of the attachment zones  20  with the electric poles  14  and the electric poles  14  remain substantially free of mechanical stress even during operation of the battery  10 . 
     The flat litz wire  16  has in the compacted attachment zones  20  properties like those of a solid, rigid conductor bar. Thus, the flat litz wire  16  can be connected with the electric poles  14  in these compacted attachment zones  20  like a solid conductor bar. 
     The respective electric pole  14  includes here a base  22  and a pin or bolt  24  extending out from the base  22 . When threadably engaging the flat litz wire  16  with the electric poles  14 , this bolt  24  can be configured as threaded bolt. 
     The bolt  24  is inserted here in a bore which is formed in the compacted attachment zone  20  of the flat litz wire  16 . This insertion of the bolt  24  into the bore provided in the respective attachment zone  20  is followed in the production of the battery  10 , shown in  FIG. 1 , by the connection of the attachment zone  20  with the respective electric pole  14  through welding, in particular laser beam welding. Respective laser beams  26  are indicated schematically in  FIG. 1 . The laser beam welding results in a material joint of the bolt  24  with the flat litz wire  16  via a circumferential butt weld  28  in the attachment zone  20 . 
     In addition, a planar, flat underside of the attachment zone  20  is in contact with a planar, flat topside of the base  22 . In an alternative embodiment, it is also possible to weld in these regions in which the attachment zone  20  and the base  22  overlap through the compacted attachment zone  20  into the base  22  to realize a material joint of the flat litz wire  16  to the electric poles  14  of the battery cells  12 . 
     Instead of the flat litz wire  16 , also a fabric band  30 , shown by way of example in  FIG. 2 , may be used as starting material for the production of the connecting element for connecting the electric poles  14 . Such a fabric band  30  can—just like the flat litz wire  16 —be made especially from copper or aluminum. By welding, in particular resistance welding, this fabric band  30  can be provided with compacted attachment zones  20  at locations that are intended for contacting the electric poles  14 , and subsequently, the fabric band  30  is connected with the electric poles  14  in the compacted attachment zones  20 . Hereby, the use of beam welding processes has proven promising because the parts to be joined can be positioned in a particularly precise manner. 
       FIG. 3  shows a round litz wire  32  as alternative starting material for a connecting element for electrically connecting the electric poles  14  of the battery cells  12 . The individual wires  18  of such a round litz wire  32  are also compacted in the attachment zone  20 . 
     As can be seen especially well in particular in  FIG. 3 , the litz wire has an attachment zone  20  which is cuboid in shape, i.e. a rectangular cross section with precise dimensions. Compaction realizes in the attachment zone  20  a planar, uniform surface which can then be brought into contact with the electric pole  14  of the respective battery cell  12  especially well. 
     The actual connection of the attachment zone  20  with the respective electric pole  14  of the battery cell may be implemented by threaded engagement or, as described above, by welding.