Abstract:
A voltage monitoring system for measuring the voltage of the fuel cells in a fuel cell stack that employs optical devices for providing an optical signal of the measured voltages, where one or more of the fuel cells power the optical devices. A surface mount device is electrically coupled to opposing plates in the stack, or opposing plates over a plurality of cells in the stack. The surface mount device includes a bonded contact and a spring contact to provide the electrical connection. A detector is positioned remote from the stack that receives the optical signals and converts them back to electrical signals indicative of the voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a voltage monitoring system for a fuel cell stack and, more particularly, to a voltage monitoring system for a fuel cell stack that employs optical devices for providing an optical signal indicative of the voltage of each fuel cell in the fuel cell stack. 
     2. Discussion of the Related Art 
     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. 
     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. 
     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. 
     The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. The bipolar plates are made of an electrically conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows. 
     Typically, the voltage output of every fuel cell in the fuel cell stack is monitored so that the system knows if a fuel cell voltage is too low, indicating a possible failure. As is understood in the art, because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Certain remedial actions can be taken for a failing fuel cell as a temporary solution until the fuel cell vehicle can be serviced, such as increasing the flow of hydrogen and/or increasing the cathode stoichiometry. 
     The fuel cell voltages are measured by a cell voltage monitoring sub-system that includes a wire connected to each bipolar plate in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell. Therefore, a 400 cell stack will include 401 wires connected to the stack. Because of the size of the parts, the tolerances of the parts, the number of the parts, etc., it may be impractical to provide a physical connection to every bipolar plate in a stack with this many fuel cells. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a voltage monitoring system for measuring the voltage of the fuel cells in a fuel cell stack is disclosed that employs optical devices for providing an optical signal of the measured voltages, where the voltage provides the power to operate the optical devices. A surface mount device is electrically coupled to opposing plates in the stack, or opposing plates over a plurality of fuel cells in the stack. The surface mount device includes a bonded contact and a spring contact to provide the electrical connection. The surface mount device also includes a circuit for measuring the voltage of the cell or cells, which is transmitted as an optical signal by the optical device. A detector is positioned remote from the stack that receives the optical signals and converts them back to electrical signals indicative of the voltage. If a single optical device is provided for more than one fuel cell, then the circuit can scan the fuel cells where an optical signal from one of the fuel cells is being provided at any particular point in time. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a plurality fuel cells in a fuel cell stack that includes a separate optical device for providing an optical signal indicative of the voltage of each fuel cell in the stack, according to an embodiment of the present invention; 
         FIG. 2  is a plan view of a plurality of fuel cells in a fuel cell stack that includes an optical device for providing an optical signal indicative of the voltage of each fuel cell in the stack, where a single optical device provides an optical signal for a plurality of the fuel cells, according to another embodiment of the present invention; and 
         FIG. 3  is a plan view of a plurality of fuel cells in a fuel cell stack that includes a voltage monitoring system that measures the voltage of each fuel cell in the fuel cell stack and transmits an optical signal indicative of the measured voltage, according to an another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a voltage monitoring system that includes an optical device for providing an optical signal indicative of the voltage of the fuel cells in a fuel cell stack is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a plan view of a fuel cell stack  10  including a plurality of fuel cells  12 , according to an embodiment of the present invention. Each fuel cell  12  includes an anode side diffusion media layer  14 , a cathode side diffusion media layer  16  and a membrane  18  therebetween. A bipolar plate  20  is provided at each side of each fuel cell  12 , where the side of the bipolar plate  20  facing the anode side diffusion media layer  14  includes anode side reactant gas flow channels (not shown) and the side of the bipolar plate  20  facing an adjacent fuel cell  12  includes cathode reactant gas flow channels (not shown). Further, the bipolar plates  20  include cooling fluid flow channels (not shown). Also, a resilient seal  22  is provided at an outer edge of each fuel cell  12  to contain the various fluids within the stack  10 . 
     As discussed above, typically the voltage of each fuel cell  12  in the fuel cell stack  10  is monitored by electrically coupling a wire to each bipolar plate  20 , and using appropriate circuitry to measure the voltage potential across the plates  20 . According to the invention, the voltage of each fuel cell  12  is measured by a surface mounted device (SMD)  30  including an optical device  32 , such as a light emitting diode (LED). The SMDs  30  would be sized appropriately to fit in the gap between the bipolar plates  20  without causing the bipolar plates  20  of each fuel cell  12  to bend. The SMD  30  includes a bonded contact  34  electrically coupled to one bipolar plate  20  and a spring contact  36  electrically coupled to an opposing bipolar plate  20  so that the device  30  makes good electrical contact with the opposing bipolar plates  20  for each fuel cell  12 . The bonded contact  34  can be mounted to the bipolar plate  20  by any suitable technique, such as adhesive, soldering, welding, etc. The spring contact  36  would provide an electrical contact so that when the stack  10  is assembled and compressed, a complete circuit would be provided across the bipolar plates  20 . The spring contact  36  provides a good electrical contact while satisfying the variance in tolerances between the bipolar plates  20 . 
     The SMD  30  includes a circuit  38  that measures the voltage potential between the plates  20 . The voltage of the fuel cell  12  is used to power the LEDs  32  to generate an optical signal. Those skilled in the art would readily recognize various circuits that could measure the voltage potential of the fuel cells  12 , and provide an optical signal indicative of the voltage potential. Therefore, as the voltage output of a particular fuel cell  12  changes, the optical signal from the LED  32  would change accordingly. Many techniques exist in the art where the optical signal from the LED  32  can provide an indication of the voltage potential. For example, in an analog version, the intensity of the light from the LED  32  can be an indication of the voltage potential, where the optical signal is proportional to the voltage. The circuit  38  may include a voltage regulator and/or other circuitry to convert the voltage to a level usable by the LED  32 . Also, the circuit  38  can generate a digital optical signal that is frequency modulated, for example, to encode the voltage. The SMDs  30  could be aligned along the edge of the stack  10 , as shown, or could be staggered along the edge of the stack  10  to provide separation to allow each plate to bend and maintain electrical contact. 
     Most LEDs have a minimum forward voltage of about 1.2 volts rendering them impractical for measuring a single cell voltage. Thus, the circuit  38  could include a miniature booster converter to amplify the measured voltage. The booster converters feedback loop could be designed to have its output current follow the input voltage, allowing the intensity of the LED  32  to vary with cell voltage. 
       FIG. 2  is a plan view of a fuel cell stack  40 , similar to the fuel cell stack  10 , where like elements are identified by the same reference numeral, according to another embodiment of the present invention. In this embodiment, every other bipolar plate  20  includes a notch to provide a notched plate  42 . Additionally, the SMDs  30  are replaced with SMDs  44  that span the height of two fuel cells  12 , where the notch in the bipolar plates  42  provides the space for the SMD  44 . Each SMD  44  includes an LED  52 , a bonded contact  46 , a spring contact  48  and a circuit  50 . Additionally, the SMDs  44  include a spring contact  54  in electrical contact with the notched bipolar plate  42 , as shown, so that the voltage of the fuel cells  12  covered by a single SMD  44  can be separately measured. In other embodiments, the spring contacts  54  could be rigid contacts. The circuit  50  would be designed so that it selectively measures the voltage potential across one fuel cell  12  and then the other fuel cell. 
     If the spring contact  54  for the notched plate  42  was eliminated, then the SMD  44  would measure the voltage potential across two of the fuel cells  12 . Thus, if either cell failed, the optical signal from the LED  52  would indicate that one of the cells has failed and there is a potential problem. Thus, a single SMD can be used to measure the voltage potential of more than one fuel cell. In other embodiments, more notched plates can be provided so that the number of fuel cells  12  that the SMD is monitoring can be more. 
     A detector would be required to detect the optical signals from the LEDs  32  and  52 . There are many different techniques for designing such a detector. For example, the detector could be mounted facing the array of SMDs and detect the optical signal from all of the LEDs  32  or  52  at one time. Alternately, the optical signals could pass through various optics, such as mirrors, fiber optics, prisms, etc. so that the light intensity of each LED can be sensed remotely, away from the edge of the bipolar plates  20 . Further, one or more micro-mirrors, such as those used in DLP projectors, coupled with optics could be provided so that one single element detector could be used, where the mirror is used to scan the optical signals. Further, a charge coupled device (CCD) could be used to sense the optical signals. 
       FIG. 3  is a plan view of a fuel cell stack  60  similar to the fuel cell stacks  10  and  40 , where like elements are identified by the same reference numeral.  FIG. 3  illustrates two embodiments for measuring the voltage of the fuel cells  12 . In one embodiment, a series of SMDs  62  are electrically coupled together, where each SMD  62  includes an LED  64 . The series of SMDs  62  extend across several fuel cells  12  and notched plates  42 . A bonded contact  66  is provided at one end of the series of SMDs  62  and a spring contact  68  is provided at the other end of the series of SMDs  62 . Additionally, a spring contact  70  is provided in contact with the SMDs  62  and the notched bipolar plates  42 , as shown. Thus, a separate LED  64  is provided for each fuel cell  12  to provide an optical signal of its voltage output. 
     In the other embodiment, the fuel cell stack  10  includes a series of SMDs  74  having a single LED  76 , and is similar to the SMD  44 . The series of SMDs  70  include a bonded contact  78  electrically coupled to one bipolar plate  20 , a spring contact  80  electrically coupled to another bipolar plate  20 , and spring contacts  82  electrically coupled to notched bipolar plates  42  therebetween. 
     A detector  90  is provided to detect the optical beams from the LEDs  66  and  72 . The detector  90  includes a mirror  92  and a CCD array  94 . Optical beams from the LEDs  64  that are proportional to the voltage of single fuel cells  12  are reflected off of the mirror  92  and directed to a particular pixel or group of pixels on the CCD array  94 . The CCD array  94  converts the light intensity to an electrical signal that is processed to convert it to a cell voltage. The SMDs  74  are designed so that circuitry therein directs the voltage potential sequentially from the fuel cells  12  to the LED  76 , which then directs the optical beam to a pixel or group of pixels on the CCD array  94 . Therefore, at any given moment in time, any one of the fuel cells  12  being detected by the SMDs  74  will be output from the CCD array  94 . The number of cells that power each SMD can vary depending on what voltage is required to power such a device and the expected range of voltage produced by the cells. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.