Abstract:
A connector and method for electrically connecting to a series of closely spaced edges for use in monitoring individual cells using bipolar plates of a fuel cell stack. The connector includes an elongated elastomeric strip with electrical conductivity for a first side thereof to an opposing side thereof, but not having meaningful electrical conductivity in use along its elongated length. This elongate strip is held at an angle in relation to the closely spaced edges by alignment protrusions. This enables the use of a series of connectors which nest together to contact all the closely spaced edges. A plurality of electrically conductive elements are located on a printed circuit board against the opposing side of the elongated elastomeric strip in corresponding relationship to the closely spaced edges located against the first side of the elongate elastomeric strip. Means is provided for exerting a force to push the first side of the elongated elastomeric strip against the closely spaced edges is also provided.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrochemical fuel cell assembly including a cell voltage monitor; and more particularly to an electrical connecting device which may be used to monitor individual cells or clusters of cells within a stack. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a “membrane electrode assembly” (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution of the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued on Dec. 21, 1993 and May 31, 1994, respectively, and assigned to General Motors Corporation, assignee of the present invention, and having as inventors Swathirajan et al. 
     A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation. 
     In most fuel cell assemblies, current is drawn from the fuel cell stack via a pair of bus plates, one of which is positioned at each end of the fuel cell stack. The fuel cells are stacked between the bus plates, which are typically made of copper or coated copper. Very often individual cells of the stack are contacted for monitoring individual cell voltages or currents, and/or for control or charging/discharging purposes. In most cases, these electrical contacts are not intended to carry the entire stack current, but are capable of providing electrical connection to individual fuel cells or clusters within a stack. 
     In mass production, an electrical connecting device is needed which is easy to handle and to install, and which provides reliable electrical contact with certain components of a fuel cell stack. It may be desirable to provide, in a single device, groups of contacts that always communicate with the same type of fuel cell component within the stack, or which contact the fuel cell stack at regularly spaced intervals along the length of the stack. 
     One problem with monitoring individual fuel cells or clusters of cells within a stack is the difficulty of attaching an electrical connector to the electrically conductive elements. For example, for a fuel cell which is designed to generate significant power output, a large number of bipolar plates are provided which require a large number of connections. Perhaps more importantly these connectors are located in close proximity to each other, making it difficult to make electrical connections without short circuiting with adjacent bipolar plates. The stack may include cells at a spacing, for example, of ten cells per inch. Thus, there is less than about 2.5 millimeters between each bipolar plate. Consequently, making such individual connections can be a slow and tedious process. 
     SUMMARY OF THE INVENTION 
     In a first aspect of the present invention a connector for electrically connecting to a series of closely spaced edges for use in monitoring individual cells using bipolar plates of a fuel cell stack is provided. The connector includes an elongated elastomeric strip with electrical conductivity from a first side thereof to an opposing side thereof, but not having meaningful electrical conductivity in use along its elongated length. The first side of the elongated elastomeric strip is located against a plurality of closely spaced edges. The connector also includes a plurality of electrically conductive elements located against the opposing side of the elongated elastomeric strip in corresponding relationship with the plurality of closely spaced edges located against the first side of the elongated elastomeric strip. The connector also includes means for exerting a force to push the first side of the elongated elastomeric strip against the plurality of closely spaced edges to provide an electrically conductive path associated with each of the plurality of closely spaced edges which flows from the first side of the elongated elastomeric strip, through the elongated elastomeric strip to the opposing side thereof, and through the electrically conductive elements which is isolated from the electrically conductive paths of adjacent closely spaced edges. 
     In another aspect of the invention a connector for electrically connecting to a series of closely spaced edges for use in monitoring individual cells using bipolar plates of a fuel cell stack is provided. The connector includes an elongated elastomeric strip with electrical conductivity from a first side thereof to an opposing side thereof, but not having meaningful electrical conductivity in use along its elongated length; the first side of the elongated elastomeric strip being located against a plurality of closely spaced edges. The connector further includes a housing with an opening adapted to hold the elongated elastomeric strip. Also included is a printed circuit board having a plurality of electrically conductive elements located thereon, the printed circuit board being attached to the housing such that the electrically conductive elements are located against the opposing side of the elongated elastomeric strip in corresponding relationship with the plurality of closely spaced edges located against the first side of the elongated elastomeric strip. Additionally included is means for exerting a force to push the first side of the elongated elastomeric strip against the plurality of closely spaced edges to provide an electrically conductive path associated with each of the plurality of closely spaced edges which flows from the first side of the elongated elastomeric strip, through the elongated elastomeric strip to the opposing side thereof, and through the electrically conductive elements which is isolated from the electrically conductive paths of adjacent closely spaced edges. 
     In another aspect of the invention a method of providing an electrical connection to a series of closely spaced edges for use in monitoring individual cells using bipolar plates of a fuel cell stack is provided. The method includes the step of providing an elongated elastomeric strip with electrical conductivity from a first side thereof to an opposing side thereof, but not having meaningful electrical conductivity in use along its elongated length. Also included is the step of locating the first side of the elongated elastomeric strip against a plurality of closely spaced edges. Further included is the step of locating a plurality of electrically conductive elements against the opposing side of the elongated elastomeric strip in corresponding relationship with the plurality of closely spaced edges located against the first side of the elongated elastomeric strip. Additionally included is the step of exerting a force to push the first side of the elongated elastomeric strip against the plurality of closely spaced edges to provide an electrically conductive path associated with each of the plurality of closely spaced edges which flows from the first side of the elongated elastomeric strip, through the elongated elastomeric strip to the opposing side thereof, and through the electrically conductive elements which is isolated from the electrically conductive paths of adjacent closely spaced edges. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is an exploded perspective view of a preferred connector assembly of the present invention; 
     FIG. 2 is a fragmentary perspective view showing several of the preferred connector assemblies of FIG. 1 in nested position against a fuel cell; 
     FIG. 3 is a cross-sectional view of the connector assembly adjacent the fuel cell as seen in FIG. 2; 
     FIG. 4 is a perspective view of a compression bracket assembly for use in compressing the connector assemblies against the fuel cell; 
     FIG. 5 is an enlarged fragmentary view similar to FIG. 2 showing the compression bracket assembly mounted over the preferred connector assemblies of FIG. 1; and 
     FIG. 6 is a perspective view of a fuel cell stack housing including the connector assemblies compressed in place by the compression bracket assembly and ready for enclosure within the housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to FIG. 1, various components of a preferred connector of the present invention, indicated generally as  10 , is provided. Illustrated is an elastomeric connection strip  12 , a housing  14  therefor, and a printed circuit board  16 . As seen in FIG. 6, and as discussed more fully hereinafter, the preferred connector  10  also generally includes a compression bracket assembly  18 . 
     As indicated above, the components of FIG. 1 include an elastomeric connection strip  12 . The strip  12  includes outer support or insulation barriers  13  which sandwich an internal electrically conductive material  12 ′. This internal electrically conductive material  12 ′ is conductive through its height or thickness (or Y direction), but is not meaningfully electrically conductive in use along its elongated length (or X direction). This electrically conductive material  12 ′ may also be conductive along its depth or width (or Z direction). Not meaningfully having electrical conductivity in use means that at the operating condition (i.e., voltage and power required to accomplish the monitoring) the current will not flow longitudinally along the elastomeric strip  12  such that there would be any unacceptable interference between adjacent electrically conductive paths through the elastomeric strip  12 ′; and preferably, that at the operating conditions there is no electrical current flow between adjacent electrically conductive paths. 
     Although this material  12 ′ is conductive through its thickness (or Y direction) it preferably has resistance through the thickness of the electrically conductive material. The resistance protects the system if an accidental short or similar failure of the voltage monitor were to occur. More preferably, the resistance is from about 100 OHMS to about 2000 OHMS; and more preferably, from about 200 OHMS to about 1000 OHMS. Thus, the current flow is preferably from about 10 milliamps to about 0.5 milliamps; and more preferably, from about 5 milliamps to about 1 milliamp. 
     The selective electrical conductivity may be provided, e.g., by constructing the internally electrically conductive material  12 ′ of alternating cross-sections of electrically conductive elastomeric material with non-conductive elastomeric cross-sections. One particularly preferred connection strip is a solid self supported connector using a carbon based elastomer sold by Fujipoly America Corporation, Carteret, N.J. 07008 under the trade name ZEBRA® Elastomeric Connectors. Of course, other elastomeric connectors may be used, including, e.g., low temperature carbon based elastomers or silver based elastomers. Although generally perpendicular cross-sections are utilized, the cross-sections may alternatively be aligned at an angle to be more closely aligned with the angle of the edge of the bipolar plates  32 , when the connector  10  is in place and connected thereto. 
     The housing  14  holds and retains the elastomeric connection strip  12  within a retention slot  20 . The housing  14  also includes appropriately sized apertures  22  for receiving self-tapping, threaded screws  24  (after passing through apertures  26  in the circuit board  16 ). The threaded screws  24  are used to attach the printed circuit board  16  to the housing  14 . An eight pin connector  28  is provided on the circuit board  16  for attachment to an eight lead cable  30  (as seen, i.e. in FIG.  2 ). As seen in FIG. 3, the circuit board  16  provides electrically conductive elements  17  connecting the cable  30  to various points on the elastomeric strip  12 ′ which oppose the edges of the bipolar plates  32  across the Y direction of the elastomeric connection strip  12 . 
     Also as seen in FIG. 3, alignment protrusions  34  are provided on the bottom surface of the housing  14  to insure proper opposing alignment of the electrically conductive elements  17  with the edges of bipolar plates  32 . Although the edges of the bipolar plates  32  are illustrated as straight edges, many modifications may be made. For example, pins of rectangular or arcuate shape, extending from the edges of the bipolar plates  32  may be provided for the elastomeric strip  12 ′ to press against. This might enable the use of reduced compressive force against the connectors. 
     Referring to FIG. 2, given the close proximity of the edges of the bipolar plates  32 , some accommodation must generally be made to enable multiple connectors  10  to be utilized without missing contact with any of the closely spaced edges of the bipolar plates  32 . This is because clearances past the ends of the elastomeric strip  12  required due to adjacent housings  14  and/or the adjacent printed circuit boards  16  are generally greater than the distance between the edges of the bipolar plates  32 . The illustrated series of connectors  10  align the elastomeric strip  12  at an angle to the edges of the bipolar plates  32 . Additionally, the printed circuit boards  16  and housings  14  are shaped to allow adjacent connectors  10  to nest; thereby allowing each connector  10  to occupy some space above the edge of bipolar plates  32  which are being electrically connected to by an adjacent connector  10  in the series. 
     Of course, many alternative constructions are possible to permit the connectors  10  to contact every edge of the bipolar plates  32  of a fuel cell stack. For example, each connector  10  could be offset from adjacent connectors  10 , such that, e.g., each successive connector  10  alternates between one of two adjacent aligned rows of connectors  10 . Thus, in this alternative version, the elastomeric strip  12  could be aligned perpendicular to the edges of the bipolar plate  32 . As another alternative, a single part elastomeric strip  12 , with a single part housing  14  might be used which extends the entire length of the fuel cell stack  54 . Thus, all of the edges of the bipolar plates  32  could be contacted by a single connector  10 . 
     Referring to FIG. 4, a compression bracket assembly  18  is illustrated. As discussed hereinafter, this compression bracket assembly  18  is utilized to apply a downward force on the elastomeric strip  12  to insure good electrical contact with the edges of the bipolar plates  32 . The bracket assembly  18  includes an elongated metal bar  38  having a generally rectangular cross-section. An elastomeric foam material  40  is provided along one side of the bar which is segmented by grooves extending partially therethrough. Attached to the opposite side of the metal bar  38  are five clips  42 . At each end of the bar  38  is a recess  44  for accommodating a slide latch  46  which is retained within the recess  44  by one of the clips  42 . A locking screw  48  is also provided for locking the latch  46  in position with its distal end extended. Lastly, eight alien head screws  50  are threaded into the bar  38  at equally spaced intervals. These screws  50  are used to adjust the force or pressure exerted on the connectors  10  as discussed hereinafter. 
     Referring to FIG. 5, the compression bracket assembly  18  is attached to the cell voltage monitor housing  53  on the side of the fuel cell stack  54 . The clips  42  of the compression bracket assembly  18  slide into a milled groove  56  in the peripheral wall  52  of the housing  53 . The slide latch  46  is extended such that its distal end extends into slots  58  in the peripheral wall  52  and the locking screws  48  are tightened to maintain the slide latch  46  in this position. Thus, the compression bracket assembly  18  is located over the series of connectors  10  and positioned and retained such that it can provide a compression force on the connectors  10  to force the elastomeric strip  12  against the edges of the bipolar plates  32 . Application of this force may also be utilized to push the opposing side of the electrically conductive elastomeric strip  12 ′ against the electrically conductive elements  17  of the circuit board  16 . 
     Referring to FIG. 6, an elongated rigid assembly tool  58  is temporarily attached to the housing  53 . The assembly tool  58  includes apertures  60  which enable an alien wrench to pass therethrough and into each of the eight alien head screws  50  in the compression bracket assembly  18 . The apertures  60  are small enough in diameter, however, that as the alien head screws  50  are backed out of the metal bar  38  of the compression bracket assembly  18 , the flat surfaces of the heads of these alien head screws  50  press against the bottom surface of the assembly tool  58 . Consequently, by adjusting how much each of the alien head screws  50  are backed out of the metal bar  38 , the force exerted by the compression bracket assembly  18  on the connectors  10  against the edges of the bipolar plates  32  can be controlled. Once the force is appropriately adjusted the assembly tool  18  is removed and a cover (not shown) is attached to the peripheral wall  52  of the cell voltage monitoring housing  53  in sealing relationship, thereby providing the surface against which the alien head screws  50  push during use. 
     Prior to sealing the housing  53 , however, one end of a cable  30  is connected to the eight pin connectors  28  of each electrical connector  10  and the other end is attached to a cell voltage monitor mother board (not shown) via additional connectors (not shown). The mother board may be encased within its own housing  62  and may be attached to the side of the fuel cell stack  54  within the peripheral wall  52  of the monitor housing  53 . The mother board is then electrically connected through the monitor housing  53  through a sealed communication port  64 . Thus, the housing  53  may be sealed from the exterior environment, if desired. Although preferably sealed from the environment, the monitor housing  53  may also include a venting means to insure a build up of gases, including hydrogen, water or air, does not occur within the monitor housing  53 . 
     Although the use of Allen head screws pressing against the assembly tool  18  or housing cover to provide a compressive force on the elastomeric strip  12  has been discussed, many alternative mechanisms of providing this compressive force could be utilized. For example, the housing  53  could include hinged brackets with tabs that lock against a series of detents, similar to the locking structure of common plastic wire bundling strips. Alternatively, an inflatable bladder might be located between the housing  53  cover and the connectors  10 . The bladder would be inflated by a gaseous or rigid material until the proper pressure is applied. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. Accordingly, the present invention covers all modifications within the scope of the appended claims.