Abstract:
A method for starting a frozen fuel cell stack includes discontinuing reactant humidification before shutting down the fuel cell stack. The anode and cathode are purged with the dry reactants. The fuel cell stack is soaked at freezing temperatures. During subsequent startup, dry reactants are initially delivered. An outlet temperature of the anode and a current load of the fuel cell stack are measured. The dry reactants are shut off when the temperature of the anode outlet or the current load reach predetermined values. The open circuit voltage potential of the fuel cells is monitored and compared to a first voltage value. When the open circuit voltage exceeds the first value, the fuel cell stack begins supplying current load. The current load of the fuel cell stack is increased or decreased based on a difference between the minimum voltage and a second voltage value.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to fuel cells, and more particularly to a control system and method for starting a frozen fuel cell.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fuel cells generate power that can potentially be used in a variety of different applications. Fuel cells may eventually replace internal combustion engines in automobiles and trucks. Fuel cells may also power homes and businesses. There are many different types of fuel cells. For example, a solid-polymer-electrolyte membrane (PEM) fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H 2 ) is supplied to the anode and air or oxygen (O 2 ) is supplied to the cathode.  
           [0003]    In a first half-cell reaction, dissociation of the hydrogen (H 2 ) at the anode generates hydrogen protons (H + ) and electrons (e − ). Because the membrane is proton conductive and dielectric, the protons are transported through the membrane. The electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O 2 ) at the cathode reacts with protons (H + ) and electrons (e − ) are taken up to form water (H 2 O).  
           [0004]    During development of fuel cell systems for specific applications such as vehicles, an operator such as an engineer or technician monitors the starting and operation of the fuel cell stack under controlled conditions. The operator manually controls stack operating parameters such as reactant and cooling flow rates and temperatures and a current load applied to the fuel cell stack. More recently, fuel cell stack testing equipment has automated some testing of fuel cell stacks. The operator is not required to be present when the fuel cell stack is operated in medium to high output modes. When the fuel cell stack is tested at lower output modes, the operator should be present during operation.  
           [0005]    There is an optimum amount of reactant gas that should be supplied to the fuel cell stack to support a desired current load. Usually control systems deliver an additional amount of reactant gas to the fuel cell stack to account for system leaks and inefficiencies and to allow the fuel cell stack to perform more smoothly. When fuel cell stacks are installed as powerplants in vehicles, the extra reactants that are supplied to the fuel cell stack require larger fuel cell stack components and decrease the efficiency of the fuel cell stack. These factors lead to greater production costs due to the larger components. Operating costs are also increased due to the additional reactant gas. Testing and research is currently being performed on fuel cell stacks to lower the additional reactants that are supplied to the fuel cell stack.  
           [0006]    During testing to determine the precise amount of reactants that are required, the fuel cell stack is often operated in a low or relatively weak performance mode. Sometimes the fuel cell voltage output drops to zero or reverses potential, both of which can cause damage the MEA of the fuel cell stack. The operator usually monitors the fuel cell stack during this type of testing even if automated testing equipment is available. When the fuel cell voltage drops quickly or reverses potential, the operator must act quickly to shut down the fuel cell stack to prevent damage.  
           [0007]    There are other situations where precise control the fuel cell stack is needed. The fuel cell stack can be started easily at or near room temperature. At these temperatures, the fuel cell stack provides full rated current relatively quickly, typically within 5-8 seconds following the delivery of reactants. When the temperature of the fuel cell stack is near or below freezing, care should be taken when starting the fuel cell stack to avoid damage. When the current load increases before thawing, individual fuel cells within the fuel cell stack may fail.  
           [0008]    The failure of the individual fuel cells prevents the fuel cell stack from supplying the full rated current load. The residual frozen water slows reaction rates because fewer active sites are available for reaction. Rapid increases in the current load cannot be handled by the frozen fuel cell stack.  
         SUMMARY OF THE INVENTION  
         [0009]    A method and apparatus according to the present invention controls current that is supplied to a load by a fuel cell stack. A minimum voltage of a plurality of fuel cells in a fuel cell stack is monitored. The current load that is supplied by the fuel cell stack is increased if the minimum fuel cell voltage value exceeds a first voltage value.  
           [0010]    In other features of the invention, the current load that is supplied by the fuel cell stack is decreased if the minimum fuel cell voltage value is less than a second voltage value. The current load that is supplied by the fuel cell is maintained if the minimum fuel cell voltage is between the first and second voltage values. The current load is increased, decreased or maintained based upon a difference between the minimum fuel cell voltage and the first and second voltage values.  
           [0011]    In still other features of the invention, an open circuit voltage of the fuel cell stack is measured before supplying current to the load. The open circuit voltage value is compared to a desired open circuit voltage value. Operation of the fuel cell stack is terminated if the open circuit voltage value does not exceed the desired open circuit value.  
           [0012]    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  
       [0013]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 illustrates a cross-section of a membrane electrode assembly (MEA) of a fuel cell;  
         [0015]    [0015]FIG. 2 is a functional block diagram of an exemplary control system for a fuel cell stack according to the present invention;  
         [0016]    [0016]FIGS. 3A, 3B,  3 C and  3 D are flowcharts illustrating steps for starting a frozen fuel cell stack that are performed by the control system of FIG. 2; and  
         [0017]    [0017]FIGS. 4A and 4B are flowcharts illustrating steps controlling current load based on a minimum fuel cell voltage. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    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.  
         [0019]    The fuel cell control system according to the present invention controls a current load that is supplied by the fuel cell by monitoring a minimum fuel cell voltage of fuel cells in the fuel cell stack. The minimum fuel cell voltage is compared to first and second voltage values. Based on a difference between the minimum fuel cell voltage and the first and second voltage values, the fuel cell control system increases, decreases or maintains the current load that is supplied by the fuel cell stack.  
         [0020]    The fuel cell control system according to the present invention will be described in conjunction with a procedure for starting a frozen fuel cell. The control system, however, may also be used for other purposes. For example, the fuel cell control system can be used to monitor fuel cell stacks during developmental testing. In particular, the fuel cell control system is particularly useful for testing at low output modes to identify a precise amount of reactant gas that is required to operate the fuel cell for optimization of fuel efficiency.  
         [0021]    During startup, rapid increases in the current load cannot be handled by a frozen fuel cell stack. The frozen fuel cell stack, however, can supply lower current loads. The current load that is supplied should be low enough to prevent a reverse bias voltage across any of the fuel cells. As the fuel cell stack supplies the lower current loads, heat spreads through the fuel cell stack. The temperature of the fuel cell stack and the number of available active sites increase over time. The current load that can be supplied by the fuel cell stack also increases over time. The membrane of the fuel cell stack heats up relatively quickly after current is produced. A bipolar plate, however, remains at lower temperatures for a longer duration. Water freezing on the bipolar plate can cause additional problems if not accommodated by the starting method of the fuel cell control system.  
         [0022]    Referring now to FIG. 1, the present invention will be described in conjunction with a fuel cell  10  that includes a membrane electrode assembly (MEA)  12 . Skilled artisans will appreciate that other types of fuel cells are contemplated and may be employed without departing from the invention. Preferably, the MEA  12  is a proton exchange membrane (PEM). The MEA  12  includes a membrane  14 , a cathode  16 , and an anode  18 . The membrane  14  is sandwiched between the cathode  16  and the anode  18 .  
         [0023]    A cathode diffusion medium  20  is layered adjacent to the cathode  16  opposite the membrane  14 . An anode diffusion medium  24  is layered adjacent to the anode  18  opposite the membrane  14 . The fuel cell assembly  10  further includes a cathode flow channel  26  and anode flow channel  28 . The cathode flow channel  26  receives and directs air or oxygen (O 2 ) from a source to the cathode diffusion medium  20 . The anode flow channel  28  receives and directs hydrogen (H 2 ) from a source to the anode diffusion medium  24 .  
         [0024]    In the fuel cell assembly  10 , the membrane  14  is a cation permeable, proton conductive membrane having H +  ions as the mobile ion. The fuel gas is hydrogen (H 2 ) and the oxidant is oxygen or air (O 2 ). The overall cell reaction is the oxidation of hydrogen to water and the respective reactions at the anode  18  and the cathode  16  are as follows:  
         H 2 =2H + +2 e   −   
         0.5O 2 +2H + +2 e   − =H 2 O  
         [0025]    Since hydrogen is used as the fuel gas, the product of the overall cell reaction is water. Typically, the water that is produced is rejected at the cathode  16 , which is a porous electrode including an electrocatalyst layer on the oxygen side. The water may be collected as it is formed and carried away from the MEA  12  of the fuel cell assembly  10  in any conventional manner.  
         [0026]    The cell reaction produces a proton exchange in a direction from the anode diffusion medium  24  towards the cathode diffusion medium  20 . In this manner, the fuel cell assembly  10  produces electricity. An electrical load  30  is electrically connected across a first plate  32  and a second plate  34  of the MEA  12  to receive the electricity. The plates  32  and/or  34  are bipolar plates if a fuel cell is located adjacent to the respective plate  32  or  34  or end plates if a fuel cell is not adjacent thereto.  
         [0027]    Referring now to FIG. 2, an exemplary control system  50  for a fuel cell stack includes a controller  51  that is connected to a load  52 . A fuel cell stack  54  supplies current to the load  52 . The controller  51  controls the current that is supplied by the fuel cell stack  54  during testing, starting a frozen fuel cell stack or in other suitable circumstances based upon a minimum cell voltage performance.  
         [0028]    Reactants  56  are supplied to an anode inlet  58  of the fuel cell stack  54 . In a preferred embodiment, the reactants  56  that are supplied to the anode inlet  58  include hydrogen or reformate. Reactants  56  are supplied to a cathode inlet  60  of the fuel cell stack  54 . In a preferred embodiment, the reactants  56  that are supplied to the cathode inlet  60  include air or oxygen. A humidifier or other device (not shown) selectively controls the humidification of the reactants. The humidifier or other device is typically located between the reactant source  56  and the fuel cell stack  54 .  
         [0029]    A first temperature sensor  62  monitors the temperature of an anode outlet  64 . A second temperature sensor  66  monitors the temperature of a cathode outlet  70 . The first and second temperature sensors  62  and  66  are preferably connected to the controller  51  and provide first and second temperature signals that are related to the temperature of the anode outlet  64  and the cathode outlet  70 , respectively. The output of the fuel cell stack  54  is connected to a DC/DC converter  74  that is connected to the load  52 . A fuel cell voltage monitor  76  monitors the voltage of each fuel cell, groups of fuel cells (such as  4  fuel cells), and/or the entire fuel cell stack  54 . In a preferred mode, the fuel cell voltage monitor  76  monitors the voltage of each fuel cell in the fuel cell stack  54 . A coolant pump  80  pumps coolant from an outlet to an inlet of the fuel cell stack  54 . A temperature sensor  82  is located adjacent to the outlet of the fuel cell stack and is connected to the controller  51 . The controller  51  operates the coolant pump.  
         [0030]    To further illustrate the present invention, a method for starting a frozen fuel cell will be described. The present invention, however, has additional applications such as when testing or in other suitable situations. Referring now to FIGS.  3 A- 3 D, a method for starting a frozen fuel cell stack is illustrated and is generally designated  100 . Control begins with step  102 . In step  104 , the controller  51  terminates normal operation of the fuel cell stack  54 . In step  106 , the controller  51  starts a first timer. In step  108 , the controller  51  discontinues humidification of the reactants  56 . In step  110 , the controller  51  purges the anode and cathode of the fuel cell stack  54  with dry reactants. In a preferred embodiment, the anode and cathode are each purged with 0.1 to 25 sccm/cell/cm 2  for a period of 1 to 600 seconds. In a highly preferred embodiment, the anode and cathode are each purged with 8.71 sccm/cell/cm 2  for approximately one minute. In step  112 , the controller  51  determines whether the first timer is up. If not, control loops to step  110 .  
         [0031]    All of the water is not necessarily removed from the fuel cell stack by the preceding steps. These steps preferably clear out the cell inlet areas that may have become blocked with ice. If the cell inlet areas are blocked by ice, startup is prevented. These steps also remove water from the reactant flow fields. In an alternative embodiment, a cathode air compressor (not shown) is used to deliver air to the anode and cathode (either simultaneously or in succession through the use of specialized plumbing) during the purge steps. Using the cathode air compressor eliminates hydrogen emissions and/or the need to combust the anode reactants downstream from the fuel cell stack  54 , which is typically encountered if only reactants are used for purging. In addition, coolant contained in the flow field channels is also optionally removed at this time.  
         [0032]    If the first timer is up, control continues with step  120  where the controller  51  drains coolant from the fuel cell stack  54 . In step  124 , the fuel cell stack  54  is soaked in freezing conditions. In other words, the fuel cell is turned off for a period at temperatures below 0° C.  
         [0033]    Some time later, the operator of the vehicle restarts the fuel cell. In step  128 , dry reactants are delivered to the fuel cell stack  54 . Preferably the dry reactants are delivered at a level that will support a first predetermined current level. In this step, the dry reactants are preferably provided at a higher stoichiometry than the fuel cell stack would encounter while powering the vehicle. Air (airbleed) or O 2  up to 25% is delivered with the anode reactants. This causes an exothermic reaction with the hydrogen in the presence of the Pt coated MEA. The heat that is given off from this reaction facilitates the thawing of the fuel cell stack, increases reaction rates, and ultimately results in faster start times from frozen temperatures. The oxygen present in the anode side consumes some of the hydrogen that is normally present to dissociate and contribute to the fuel cell reaction that generates electricity. It is therefore necessary to account for this H 2  that is reacting with the O 2  by supplementing the anode supply with additional H 2  or reformate. In step  130 , hydrogen levels are corrected for consumption by the fuel cell stack  54 .  
         [0034]    For example, if acceleration of the vehicle requires a maximum current density of 0.6 A/cm 2 , a suitable current density for the initial reactant flows is approximately 0.8 A/cm 2 . Delivery of the dry reactants is preferably at the lowest pressure that can be provided. The higher flow rate combined with the lower pressure creates maximum gas velocities through the fuel cell stack  54 . The increased gas velocities improve entraining and removal of product and/or incident water. This will minimize the possibility that the product and/or incident water will freeze on the bipolar plate and block the flow of reactants through the fuel cell.  
         [0035]    For example, delivery of the anode reactants with 25% airbleed and hydrogen that is corrected for consumption can be provided to warm the fuel cell in the early stages of the frozen start. The anode air bleed preferably remains present until the anode outlet temperature reaches 0° Celsius at which time the air bleed is closed and hydrogen consumption correction is no longer required. It is desirable to stop the air bleed into the anode at this temperature to ensure that the exothermic reaction ceases. This will prevent overheating and possible damage to the MEA.  
         [0036]    The dry reactants are delivered until the anode outlet temperature reaches 5° C. At this point, the temperature in the fuel cell stack allows humidification of the reactants. Failure to humidify the fuel cell stack at this time reduces performance due to the drying of the membrane of the fuel cell stack. Also, coolant can begin to be circulated in the fuel cell stack when the cathode outlet temperature reaches 50° C. Since the coolant is cold and the fuel cell stack is comparatively warm at this time, it is desirable to stage or gradually build up the coolant flow rate as the coolant outlet temperature warms up. If the coolant pump is simply turned on at full speed, the cold rush of incoming coolant results in a significant decrease in stack voltage performance. It is therefore desirable to limit the initial coolant flow rate such that the coolant outlet temperature does not fall below a prescribed temperature. For example, the initial coolant flow rate should be controlled such that the coolant outlet temperature is always greater than the cathode outlet temperature. If this is not true, the coolant pumps speed and the coolant flow rate are reduced further.  
         [0037]    In step  132 , the controller  51  determines whether the open circuit (OC) fuel cell (FC) voltage is greater than a first value. Experimental data has shown that fuel cell stacks with weak open circuit voltage potential (at this point in the startup process) have a poor likelihood of successfully starting from subzero temperatures. In a preferred embodiment, the first predetermined voltage value is approximately 0.7 V or greater. If the open OC FC voltage is greater than a first value, control ends in step  133 .  
         [0038]    Otherwise, control starts a loop timer in step  134 . In step  135 , the controller  51  compares the fuel cell voltage to first calibration value(s). In step  136 , if the fuel cell voltage is greater than the upper value, the controller  51  increases the current value in step  137 . If the fuel cell voltage is less than a lower value as determined in step  138 , the controller  51  decreases the current value in step  139 .  
         [0039]    In step  140 , the controller  51  determines whether the anode temperature is &gt;0° C. If it is, control continues with step  141  where the controller stops air bleed to the anode and stops hydrogen correction. If the anode temperature is not &gt;0° C., the controller  51  determines whether the anode temperature is &gt;5° C. in step  142 . If it is, control continues with step  143  where the reactants are humidified. If the anode temperature is not &gt;5° C., control continues with step  144  where the controller  51  determines whether the cathode outlet temperature is &gt;50 degrees C. If it is, the controller  51  starts the coolant pump in step  146 . In step  147 , the controller  51  determines whether the coolant outlet temperature is greater than or equal to the cathode outlet temperature. If not, control continues with step  148  and lowers the pumps speed.  
         [0040]    In step  150 , the controller  51  determines whether the current load is greater than a first current value. If not, control determines whether a loop timer is up in step  151 . If not, control continues with step  135 . Otherwise, control continues from steps  150  and  151  to step  152  and starts a second timer. In step  154 , control reduces current load to a steady-state value. In step  156 , control determines whether the air bleed and hydrogen correction are on. If true, the controller  51  stops the anode air bleed and the hydrogen correction in step  158 . Otherwise, control continues with step  160  where the anode and cathode flows are reduced to a level that supports steady-state operation.  
         [0041]    In step  162 , control determines whether the reactants are humidified. If not, the controller continues with step  164  and determines whether the anode outlet temperature is &gt;5° C. If it is, control continues with step  165  where the reactants are humidified. In step  166 , the controller  51  determines whether the coolant pump  80  is on. If not, the controller  51  determines whether the cathode outlet temperature is &gt;50° C. in step  168 . If true, the controller starts the coolant pump in step  170 . In step  171 , the controller determines whether the coolant outlet temperature is greater than the cathode outlet temperature. If not, control lowers the speed of the coolant pump  80  in step  172  and loops back to step  171 .  
         [0042]    In step  174 , the controller  51  determines whether the cathode outlet temperature is at a normal operating temperature. If not, control continues with step  175  where the controller  51  determines whether the second timer is up. If not, control loops back to step  162 . If the cathode outlet temperature is at a normal operating temperature or if the second timer is up, control continues with step  177  where normal operation is resumed. Control continues with step  178  where the controller  51  determines whether there is a soak at freezing temperature. If there is a soak at freezing temperature, control returns in step  179 . Otherwise, control performs normal shutdown in step  180 .  
         [0043]    Referring now to FIGS. 4A and 4B, an exemplary method for increasing and decreasing the current load in incremental steps based on the fuel cell voltage is shown. Control begins with step  200 . In step  202 , a loop timer is started. In step to  204 , the controller  51  compares the fuel cell voltage to a first voltage value V 1  such as 0.4V. If the fuel cell voltage is greater than the first voltage value V 1 , control continues with step  206 . In step  206 , the controller  51  compares the fuel cell voltage to a second voltage value V 2  that is greater than the first voltage value V 1 . If the fuel cell voltage is less than the second voltage value V 2 , the fuel cell stack is operated in step  208 . In step  210 , the controller  51  compares the current load to a maximum current load I MAX . In a preferred embodiment, the maximum current load I MAX  is equal to 160 Amps. If the current load is not greater than the maximum current load I MAX , control continues with step  212  where the controller  51  adds a first load value I 1  such as 4 Amps to the current load. In a preferred embodiment, step  212  adds only part of the first load value if the maximum load value would be exceeded. If the current load is greater than or equal to the maximum current load I MAX , control continues from step  210  to step  216 .  
         [0044]    In step  216 , the controller  51  determines whether the loop timer is up. If not, control continues with step  218  where the current load is compared to a second predetermined current load setpoint (normally I Max ). If the current load is greater than the second predetermined current load setpoint, control continues with step  222  where the fuel cell is operated and the method ends with step  224 . If the current load is less than the first setpoint and the loop timer is not up, control continues with step  204 .  
         [0045]    If the fuel cell voltage is greater than the second voltage value V 2 , control continues with step  230  where the control system  50  compares the cell voltage to a third voltage value V 3 . The third voltage value V 3  is greater than the second voltage value V 2 . If the fuel cell voltage is less than the third voltage value V 3 , the fuel cell stack is operated in step  232 . In step  234 , the controller  51  compares the current load to the maximum current load I MAX . If the current load is less than the maximum current load I MAX , control continues with step  236  where the controller  51  adds a second load value I 2  that is greater than the first load value I 1 . For example, if the first load value I 1  is 4 Amps, the second load value I 2  can be 5 Amps. In a preferred embodiment, step  236  adds only part of the second load value I 2  if the maximum load value I MAX  would be exceeded. If the current load is greater than or equal to the maximum current load I MAX , control continues from step  234  to step  216 .  
         [0046]    If the fuel cell voltage is greater than the third voltage value V 3 , control continues with step  240  where the controller  51  compares the cell voltage to a fourth voltage value V 4 . The fourth voltage value V 4  is greater than the third voltage value V 3 . If the fuel cell voltage is less than the fourth voltage value V 4 , the fuel cell stack  54  is operated in step  242 . In step  244 , the controller  51  compares the current load to the maximum current load I MAX . If the current load is less than the maximum current load I MAX , control continues with step  246  where the controller  51  adds a third load value that is greater than the second load value I 2 . For example, if the second load value I 2  is 5 Amps, the third load value I 3  can be 6 Amps. In a preferred embodiment, step  246  adds only part of the third load value I 3  if the maximum load value I MAX  would be exceeded. If the current load is greater than or equal to the maximum current load I MAX , control continues from step  244  to step  216 .  
         [0047]    If the fuel cell voltage is greater than the fourth voltage value V 4 , control continues with step  250  where the controller  51  compares the cell voltage to a fifth voltage value V 5 . The fifth voltage value V 5  is greater than the fourth voltage value V 4 . If the fuel cell voltage is less than the fifth voltage value V 5 , the fuel cell stack  54  is operated in step  252 . In step  254 , the controller  51  compares the current load to the maximum current load I MAX . If the current load is less than the maximum current load I MAX , control continues with step  256  where the controller  51  adds a fourth load value I 4  that is greater than the third load value I 3 . For example, if the third load value I 3  is 6 Amps, the fourth load value I 4  can be 7 Amps. In a preferred embodiment, step  256  adds only part of the fourth load value I 4  if the maximum load value I MAX  would be exceeded. If the current load is greater than or equal to the maximum current load I MAX , control continues from step  254  to step  216 .  
         [0048]    If the fuel cell voltage is greater than the fifth voltage value V 5 , control continues with step  260  where the controller  51  compares the cell voltage to a sixth voltage value V 6 . The sixth voltage value V 6  is greater than the fifth voltage value V 5 . If the fuel cell voltage is less than the sixth voltage value V 6 , the fuel cell stack is operated in step  262 . In step  264 , the control system  50  compares the current load to the maximum current load I MAX . If the current load is less than the maximum current load I MAX , control continues with step  266  where the control system  50  adds a fifth load value I 5  that is greater than the fourth load value I 4 . For example, if the fourth load value I 4  is 7 Amps, the fifth load value I 5  can be 10 Amps. In a preferred embodiment, step  266  adds only part of the fifth load value I 5  if the maximum load value I MAX  would be exceeded. If the current load is greater than or equal to the maximum current load I MAX , control continues from step  264  to step  216 .  
         [0049]    If the fuel cell voltage is greater than the sixth voltage value V 6 , control continues with step  272  where the fuel cell stack  54  is operated. In step  274 , the controller  51  compares the current load to the maximum current load I MAX . If the current load is less than the maximum current load I MAX , control continues with step  276  where the controller  51  adds a sixth load value I 6  that is greater than the fifth load value I 5 . For example, if the fifth load value I 5  is 10 Amps, the sixth load value I 6  can be 12 Amps. In a preferred embodiment, step  276  adds only part of the sixth load value I 6  if the maximum load value would be exceeded. If the current load is greater than or equal to the maximum current load I MAX , control continues from step  274  to step  216 .  
         [0050]    If the fuel cell voltage is less than the first voltage value V 1 , control continues from step  204  to step  280 . In step  280 , the control system  50  compares the fuel cell voltage to a seventh voltage value V 7 . The seventh voltage value V 7  is less than the first voltage value V 1 . If the fuel cell voltage is greater than the seventh voltage value V 7 , control continues with step  282  where the fuel cell is operated. In step  284 , the control system  50  compares the current load to zero. If the current load is greater than 0, control continues with step  286  where the controller  51  subtracts a seventh load value I 7 . For example, seventh load value I 7  can be 1 Amp. In a preferred embodiment, step  286  subtracts only part of the seventh load value I 7  that is needed to reach zero if necessary. If the current load is equal to zero, control continues from step  284  to step  216 .  
         [0051]    If the fuel cell voltage is less than the first voltage value V 1  and the seventh voltage value V 7 , control continues with step  290 . In step  290 , the controller  51  compares the fuel cell voltage to an eight voltage value V 8 . The eighth voltage value V 8  is less than the seventh voltage value V 7 . If the fuel cell voltage is greater than the eighth voltage value V 8 , control continues with step  292  where the fuel cell is operated. In step  294 , the control system  50  compares the current load to zero. If the current load is greater than zero, control continues with step  296  where the control system  50  subtracts an eighth load value I 8 . For example, eighth load value I 8  can be 3 Amps. In a preferred embodiment, step  296  subtracts only part of the eighth load value I 8  that is needed to reach zero if necessary. If the current load is equal to zero, control continues from step  294  to step  216 .  
         [0052]    If the fuel cell voltage is less than the first voltage value V 1  and the eighth voltage value V 8  control continues with step  300 . In step  300 , the controller  51  compares the fuel cell voltage to a ninth voltage value V 9 . The ninth voltage value V 9  is less than the eighth voltage value V 8 . If the fuel cell voltage is greater than the ninth voltage value V 9 , control continues with step  302  where the fuel cell is operated. In step  304 , the controller  51  compares the current load to zero. If the current load is greater than zero, control continues with step  306  where the control system  50  subtracts a ninth load value I 9 . For example, the ninth load value I 9  can be 5 Amps. In a preferred embodiment, step  296  subtracts only part of the ninth load value I 9  to reach zero if necessary. If the current load is equal to zero, control continues from step  304  to step  216 .  
         [0053]    If the fuel cell voltage is less than the first voltage value V 1  and the ninth voltage value V 9 , control continues with step  310 . In step  310 , the controller  51  compares the fuel cell voltage to a tenth voltage value V 10 . The tenth voltage value V 10  is less than the ninth voltage value V 9 . If the fuel cell voltage is greater than the tenth voltage value V 10 , control continues with step  312  where the fuel cell is operated. In step  314 , the controller  51  compares the current load to zero. If the current load is greater than 0, control continues with step  316  where the control system  50  subtracts a tenth load value I 10 . For example, the tenth load value I 10  can be 12 Amps. In a preferred embodiment, step  296  subtracts only part of the tenth load value I 10  to reach zero if necessary. If the current load is equal to zero, control continues from step  314  to step  216 .  
         [0054]    As can be appreciated, the specific current load increases and decreases can be varied from those described above. Factors that impact the selection of these values include the type of fuel cell, the dimensions of the fuel cell plates, the anticipated reliability and life of the fuel cell and other factors.  
         [0055]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.