Abstract:
A method for equalizing the measured voltage of each cluster in a fuel cell stack wherein at least one of the clusters has a different number of cells than the identical number of cells in the remaining clusters by creating a pseudo voltage for the different cell numbered cluster. The average cell voltage of the all of the cells in the fuel cell stack is calculated and multiplied by a constant equal to the difference in the number of cells in the identical cell clusters and the number of cells in the different numbered cell cluster. The resultant product is added to the actual voltage measured across the different numbered cell cluster to create a pseudo voltage which is equivalent in cell number to the number of cells in the other identical numbered cell clusters.

Description:
CROSS REFERENCE TO CO-PENDING APPLICATIONS 
     The present application discloses subject matter which is disclosed and claimed in co-pending U.S. patent application Ser. No. 09/436,819, filed on Nov. 9, 1999, and entitled “Fuel Cell Voltage Monitoring and System Control” and U.S. patent application Ser. No. 09/358,080, filed on Jul. 21, 1999, and entitled “Methanol Tailgas Combustor Control Method”, the entire contents of both of which are incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT SUPPORT 
     The Government of the United States of America has right in this invention pursuant to Agreement No. DE-AC02-90CH10435 awarded by the U.S. Department of Energy. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a fuel cell system, and more particularly to a system having a plurality of cells which consume an H 2 -rich gas to produce power for vehicle propulsion. 
     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 air is supplied as the oxidant to the cathode. PEM fuel cells include a “membrane electrode assembly” (MEA) comprising a thin, proton transmissive, 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 the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts. 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, typically adjacent cells, is referred to as a cluster. 
     In PEM fuel cells hydrogen (H 2 ) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O 2 ), or air (a mixture of O 2  and N 2 ). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereto. 
     For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction: CH 3 OH+H 2 O→CO 2 +3H 2 . The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur. 
     Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in the name of William Pettit in November, 1997, and U.S. Ser. No. 09/187,125, filed Nov. 5, 1998, and each assigned to the assignee of the present invention. 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 respectively Dec. 21, 1993 and May 31, 1994, each assigned to the assignee of the present invention. 
     For vehicular power plants, the reaction within the fuel cell must be carried out under conditions which preserve the integrity of the cell and its valuable polymeric and precious metal catalyst components. Since the anode, cathode and electrolyte layers of the MEA assembly are each formed of polymers, it is evident that such polymers may be softened, melted or degraded if exposed to too high a temperature. 
     A typical fuel cell system does not directly monitor the rate of hydrogen flow to the fuel cell; that is, a hydrogen sensor is not located directly upstream of the fuel cell. In such a fuel cell system, it is important to match the load being demanded of the fuel cell with the rate at which hydrogen is supplied to the fuel cell. If more current is attempted to be drawn out of the fuel cell than the fuel cell is capable of supplying because there is not enough hydrogen to create the increased electrical power, then it is possible to significantly degrade the fuel cell stack. 
     If the vehicle propulsion system continues to increase the load and allows the cell voltage to continue to decline, deterioration of one or more individual cells can result and it is also possible to incur a permanent reverse polarity. In this situation, a cell begins acting as a resistor and will start heating up. As the cell continues to heat up, it will adversely affect the cell next to it and, if heat effect is not abated, it is possible to melt components of the fuel cell. 
     Although it is possible to obtain the overall voltage of the fuel cell stack, this does not indicate the existence of one problem cell within the stack. In other words, a small voltage drop occurring at a number of the cells could not be distinguished from a large voltage drop in one problem or weak cell. 
     This is evident by an example where the fuel cell stack might have, for example, 200 cells at 0.7 to 0.8 volts each at a given load. In a circumstance where 3 cells drop from 0.75 volts to 0.0 volts the overall fuel stack voltage changes from 150 volts to 147.75 volts. This latter value is well above the expected voltage if all of the cells were at 0.7 volts, that is, at the lower range indicating a stack voltage of 140 volts which is nominally acceptable. 
     While it would be advantageous to monitor the voltage of each cell in a stack, from an economic view point this is not strictly necessary or desirable. Since a typical PEM fuel cell stack, sized for use in automotive power and voltage ranges, contains approximately 150 to 200 cells, the logistic of reading all of the 150 to 200 cell voltages can become a significant task, with respect to hardware connections. Also, due to the sheer size of the data being processed from each of the 150 to 200 cells, care must be taken to design efficient software to collect and process all of the cell voltage information. 
     Therefore, a typical approach relies on monitoring groups of cells, referred to as “clusters” instead each individual cell. Care must be taken not to group too many cells together in a cluster since the total contribution of each cell&#39;s output to the chosen cluster output must be large enough so that an individual poor performing cell can be resolved from the condition where are cells in the cluster are on the low side of nominal performance. This resolution limitation usually results in either three or four cells being grouped together in a cluster. 
     Historically, when forcing a cluster monitoring requirement on the fuel cell stack design, the result is that the stack must be designed with a total number of cells which have a numerical modulus equal to the number of cells in each monitored cluster in order not to leave any cell(s) unmonitored. This poses a problem when there is a need to change the number of cells in a cluster for a given stack design. Additionally, adding “extra” cells to a new fuel cell design just to make the number of cells convenient for the monitoring system is not considered a good economic practice. 
     Thus, it would be desirable to provide a method for diagnostic monitoring of fuel cell clusters containing any number of cells per cluster which can be interfaced to fuel cell stack designs containing any total number of cells. It would also be desirable to provide a method for diagnostic monitoring of a fuel cell stack which avoids the design and calibration of a unique monitoring approach only for a cluster containing less than the identical number of cells per cluster in the remaining portion of the fuel cell stack. It would also be desirable to provide a monitoring technique for a fuel cell which enables a fixed number of cells to be included in each cluster without requiring rearrangement or rewriting of the monitoring software for the addition or removal of cells from the fuel cell stack. 
     SUMMARY OF THE INVENTION 
     The present invention is a method of monitoring a fuel cell apparatus wherein a hydrogen-rich stream is supplied to a fuel cell stack, wherein the fuel cell stack includes a plurality of first clusters formed of identical number of fuel cells and at least one second cluster formed of a different number of fuel cells. The present method includes the following steps: 
     (a) measuring the voltage across the entire fuel cell stack; 
     (b) measuring the voltage across the second cluster of fuel cells; and 
     (c) creating a pseudo cluster voltage for the second cluster based on the average cell voltage of all of the cells in the fuel cell stack, the actual voltage across the second cluster and the difference between the number of cells in the first clusters and the number of cells in the second cluster. 
     In a specific aspect, the method of the present invention includes the following steps: 
     (a) measuring the total voltage across all of the fuel cells in the stack; 
     (b) determining the average cell voltage for the entire stack; 
     (c) multiplying the average cell voltage for the entire stack by the difference between the number of cells in each of the first clusters and the number of cells in the second cluster; and 
     (d) adding the product of (c) to the actual voltage across the second cluster. 
     In another specific aspect, the inventive method determines the difference between the number of cells in each of the first clusters and the number of cells in the second cluster to form a constant; 
     multiplies the constant and the average cell voltage across the entire stack; and 
     adds the product of step (b) to the actual measured voltage across the second cluster. 
     In a broad aspect of the invention, the present inventive method adjusts the measured voltage across the second cluster by a factor equivalent to the average cell voltage of all the cells in the fuel cell stack for each different fuel cell between the number of fuel cells in the second cluster and the number of fuel cells in the first cluster. 
     The method of the present invention provides significant advantages in fuel cell design and diagnostic monitoring. The present inventive method enables the cells of a fuel cell stack to be monitored in clusters containing a small number of cells regardless of the number of cells forming each cluster. This enables a single monitoring program or circuit to monitor each fuel cell cluster regardless of the number of cells in each cluster. This avoids the necessity of designing a new monitoring control program or circuit just for the clusters containing a different number of cells. More importantly, the monitoring method of the present invention enables a fuel cell stack to be devised with any number of cells to meet particular application requirements without the necessity of adding or eliminating individual fuel cells just for monitoring purposes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The various features, advantages and other uses of the present invention will become more apparent by referring to the following description and drawings in which: 
     FIG. 1 is a flow diagram showing an exemplary vehicle fuel cell system usable with the monitoring method of the present invention; 
     FIG. 2 is a schematic diagram showing the connection of the fuel cell of FIG. 1 in a vehicle; 
     FIG. 3 is a pictorial drawing showing cells of a fuel cell stack arranged in clusters with connections for voltage monitoring according to the present invention; and 
     FIG. 4 is a flow/control diagram of a cluster voltage generating method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is hereafter described in the context of a fuel cell fueled by a methanol (MeOH) fuel processor. However, it is to be understood that the principles embodied herein are equally applicable to fuel cells fueled by other fuels, such as ethanol or gasoline, which utilize a fuel processor for conversion into a hydrogen rich stream. 
     By way of background, as shown in FIG. 1, a fuel cell apparatus usable as an energy source for vehicle propulsion includes a fuel processor  2  for catalytically reacting methanol from a methanol stream  6  and water or steam from a water stream  8  in a recirculating bed  10  and a catalytic bed  12  to form a hydrogen-rich reformate gas stream. A heat exchanger  14  is interposed between the catalytic bed  12  and a preferential oxidation (PROX) reactor  16 . The reformate output gas stream comprises primarily H 2  and CO 2 , but also includes N 2 , CO and water. The reformate stream passes through the preferential oxidation (PROX) reactor  16  to reduce the CO-levels therein to acceptable levels (i.e., below 20 ppm). The H 2  rich reformate  20  is then fed into the anode chamber of a fuel cell  22 . At the same time, oxygen (e.g., air) from an oxidant stream  24  is fed into the cathode chamber of the fuel cell  22 . The hydrogen from the reformate stream  20  and the oxygen from the oxidant stream  24  react in the fuel cell  22  to produce electricity. 
     Exhaust or effluent  26  from the anode side of the fuel cell  22  contains some unreacted hydrogen. The exhaust or effluent  28  from the cathode side of the fuel cell  22  contains some unreacted oxygen. Air for the oxidant stream  24  is provided by a compressor  30  and is directed to the fuel cell  22  by a valve  32  under normal operating conditions. During start-up, however, the valve  32  is actuated to provide air to the input of a combustor  34  used to heat the fuel processor  2 , as will be described in more detail hereinafter. 
     Heat from the heat exchanger  14  heats the catalyst bed(s)  10  and  12  in the fuel processor  2  and also heats the PROX  16 . In this regard, the H 2 O-MeOH mixture supplied to the fuel processor  2  will be vaporized and preferably be recirculated/refluxed several times (e.g., 20×) through the recirculating bed  10  in the fuel processor  2 , the heat exchanger side of the bed  12 , the PROX  16  and the heat exchanger  14  such that the mixture also functions as a heat transfer medium for carrying heat from the heat exchanger  14  into the beds  10  and  12  of the fuel processor  2  and to the PROX  16 . 
     The heat exchanger  14  itself is heated from exhaust gases  36  exiting the catalytic combustor  34 . The gases  36  exiting the heat exchanger  14  are still hot and could be passed through an expander, not shown, which could drive the compressor  30  or utilized in another manner. In the present implementation, as shown in FIG. 1, the exhaust gases from the fuel processor  2  pass through a regulator  38 , a shutoff valve  240  and a muffler  242  before being dumped to atmosphere. 
     MeOH vapor  40  emanates from a vaporizer  41  nested in the exhaust end  44  of the combustor  34 . The vaporizer  41  is a heat exchanger that extracts heat from the combustor  34  exhaust to vaporize a first fuel stream, such as liquid MeOH  46  provided to the vaporizer  41  by fuel metering device  43  from the vehicle&#39;s fuel tank. The MeOH vapor  40  exiting the vaporizer  41  and the anode effluent  26  are reacted in a catalyst section  48  of the combustor  34  lying intermediate the inlet and exhaust ends  42  and  44  respectively of the combustor  34 . Oxygen is provided to the combustor  34  either from the compressor  30  (i.e., via valve  32 ) or from a second air flow stream, such as a cathode effluent stream  28  depending on system operating conditions. A valve  50  permits dumping of the combustor exhaust  36  to atmosphere when it is not needed in the fuel processor  2 . 
     Further details concerning the construction of the combustor  34  can be had by referring to pending U.S. patent applications Ser. Nos. 08/975,422 and 08/980,087 filed in the name of William Pettit in November 1997, the entire contents of which are incorporated herein by reference. 
     An electric heating element  52  is provided upstream of the catalyst bed  48  in the combustor  34  and serves to vaporize the liquid fuel  46  entering the combustor  34 , heat the gas entering the bed  48  as well as preheating the bed  48  during start-up of the combustor  34 . The heating element  52  may or may not be catalyzed. After start-up, as described hereafter, the electric heater  52  is no longer required since the fuel will be vaporized by the exhaust gases emanating from the exhaust end  44  of the combustor  34 . A preferred electric heater  52  comprises a commercially available, uncatalyzed extruded metal monolith resistance element such as is used to light off the catalyst of a catalytic converter used to treat IC engine exhaust gases. 
     The exhaust end  44  of the combustor  34  includes a chamber that houses the vaporizer  41  which is a coil of metal tubing which is used to vaporize liquid fuel to fuel the combustor  34 . More specifically, under normal post-start-up conditions, air or cathode effluent  28  may be introduced into the inlet end of the coil and mixed with liquid fuel sprayed into the inlet end via a conventional automotive type fuel injector. The airborne atomized fuel passes through the several turns of the heated coil tube, and therein vaporizes and exits the tube at an outlet which is located in the cathode effluent supply conduit. This vaporized first fuel stream supplements a second fuel stream or anode effluent  26  as fuel for the combustor  34  as may be needed to meet the transient and steady state needs of the fuel cell apparatus. The vaporizer coil is sized to vaporize the maximum flow rate of fuel with the minimum combustor exhaust flow rate, and is designed to operate at temperatures exceeding the autoignition temperature of the MeOH-air mixture therein throughout its fuel operational range. Autoignition within the vaporizer is avoided, however, by insuring that the velocity of the mix flowing through the coil significantly exceeds the worst-case flame speed of the mixture which varies with the composition of the inlet streams. 
     As shown in FIG. 1, and as described in greater detail hereafter, the amount of heat demanded by the fuel processor  2  which is to be supplied by the combustor  34  is dependent upon the amount of fuel input to the combustor  34  and the desired reaction temperature in the combustor  34 . To supply the heat demand of the fuel processor  2 , the combustor  34  utilizes all anode exhaust or effluent and potentially some liquid fuel. Enthalpy equations are used to determine the amount of cathode exhaust or air to be supplied to the combustor  34  to meet the desired heat requirements of the fuel processor  2 . The oxygen, air or air-like stream provided to the combustor  34  includes one or both of cathode effluent exhaust  28  which is typically a percentage of the total oxygen supplied to the cathode of the fuel cell  22  and a compressor output air stream depending on whether the apparatus is operating in a start-up mode wherein the compressor air stream is exclusively employed or in a run mode using the cathode effluent  28  and/or compressor air. In the run mode, any total air, oxygen or diluent demand required by the combustor  34  which is not met by the cathode effluent  28  is supplied by the compressor  30  in an amount to balance the enthalpy equations to reach the desired reaction temperature within the combustor  34  so as to supply the amount of heat required by the fuel processor  2  at the desired temperature. 
     The air quality control is implemented via an air dilution valve  47  which is a stepper motor driven valve having a variable orifice to control the amount of bleed-off of cathode exhaust supplied to the combustor  34  and potentially the system exhaust, which bled-off air is dumped to atmosphere through the regulator  38 , the valve  40 , and the muffler  42 . A further description of the air dilution valve  47  will be presented hereafter in conjunction with the various modes or sequences of operation of the combustor  34 . 
     The fuel cell apparatus of the present invention operates as follows. At the beginning of operations when the fuel cell apparatus is cold and starting up: (1) the compressor  34  is driven by an electric motor energized from an external source (e.g., a battery) to provide the necessary system air; (2) air is introduced into the combustor  34  as well as the input end of the vaporizer  41 ; (3) liquid fuel  46  (e.g., MeOH) is injected into the inlet end of the vaporizer  41  via a fuel injector, and atomized as fine droplets with the air flowing therein; (4) the air-MeOH droplet mix exits the vaporizer  41  and mixes with compressor air introduced into the combustor  34 , and is then introduced into the input end  42  of the combustor  34 ; (5) the mix passes through a flame arrestor in the front of the combustor  34 ; (6) the mix is then heated by the heater  52  to vaporize the liquid droplets and heat the mixture; (7) the preheated vaporous mix then enters a mixing-media bed for still further intimate mixing before contacting the light-off catalyst bed; (8) upon exiting the mixing-media bed, the mix begins oxidizing on the light-off catalyst bed just before it enters a primary catalyst bed  48 , or reacting section of the combustor  34 , where substantially complete combustion of the fuel is effected; and (9) the hot exhaust gases exiting the catalyst bed are conveyed to the heat exchanger  14  associated with the fuel processor  2 . 
     Once the fuel processor&#39;s temperature has risen sufficiently to effect and maintain the reformation process: (1) valve  32  is activated to direct air to the cathode side of the fuel cell  22 ; (2) MeOH and water are fed to the fuel processor  2  to commence the reformation reaction; (3) reformate exiting the fuel processor  2  is fed to the anode side of the fuel cell  22 ; (4) anode effluent  26  from the fuel cell  22  is directed into the combustor  34 ; (5) cathode effluent  28  from the fuel cell  22  is directed into the combustor  34 ; (6) air is introduced into the vaporizer  41 ; (7) liquid methanol is sprayed into the vaporizer  41 ; (8) the methanol-air mix circulates through the heated vaporizer coil where the MeOH vaporizes; (9) the Air-MeOH v  mix along with the cathode effluent  28  then mixes with the anode effluent  26 ; and (10) the mix is burned on the catalyst bed of the combustor  34 . 
     During normal (i.e., post start-up) operating conditions, the heater  52  is not used as the vaporizer  41  alone vaporizes the MeOH and preheats the MeOH-air mix. Under certain conditions, as described hereafter, the combustor  34  could operate solely on the anode and cathode effluents, without the need for additional MeOH fuel from the vaporizer  41 . Under such conditions, MeOH injection to the combustor  34  is discontinued. Under other conditions, e.g., increasing power demands, supplemental fuel is provided to the combustor  34 . 
     As described above, the combustor  34  receives multiple fuels, such as a methanol-air mix as well as anode effluent  26  from the anode of the fuel cell  22 . Oxygen depleted exhaust air  28  from the cathode of the fuel cell  22  and air from the compressor  30  are also supplied to the combustor  34 . 
     According to the present invention, a controller  150  shown in FIG. 1 controls the operation of the combustor  34 . Anode exhaust or effluent plus a liquid fuel, i.e., methanol, if required, support the energy requirements of the combustor  34 . An enthalpy balance maintains the desired reaction by temperature controlling the amount of air and/or cathode exhaust supplied to the combustor  34  to meet all fuel processor heat requirements. 
     It should be noted that the energy requirements of the apparatus components are expressed herein in terms of power. This is for convenience and is meant to express an energy rate, often in units of kilowatts, rather than BTU per second. 
     The controller  150  may comprise any suitable microprocessor, microcontroller, personal computer, etc., which has central processing unit capable of executing a control program and data stored in a memory. The controller  150  may be a dedicated controller specific to the combustor  34  or implemented in software stored in the main vehicle electronic control module. Further, although the following description describes a software based control program for controlling the combustor  34  in various modes of operation or sequence, it will also be understood that the combustor control can also be implemented in part or whole by dedicated electronic circuitry. 
     According to the present example, the controller  150  controls the operation of the combustor  34  in six different modes or sequences of operation. The separate modes of operation include (1) combustor start-up, (2) combustor operation during fuel processor warm-up, (3) combustor operation during fuel processor start-up, with the fuel cell off-line, (4) combustor operation during fuel processor run mode with the fuel cell stack on-line, (5) combustor shut down, and (6) combustor diagnostics. 
     Further details concerning the operation of the combustor  34  in all modes of operation of the fuel cell can be had by referring to co-pending U.S. patent application Ser. No. 08/358,080 filed in Jul. 21, 1999, in the names of David J. Hart-Predmore and William H. Pettit, entitled “Methanol Tailgas Combustor Control Method”, the entire contents of which are incorporated herein by reference. 
     In a preferred embodiment, the fuel cell system comprises the fuel cell  16  as part of a circuit  60  (see FIG. 2) wherein a portion of the external circuit  60 , comprises a battery  62 , an electric motor  64  and drive electronics  65  constructed and arranged to accept electric energy from a DC/DC converter  61  coupled to the fuel cell  16  and to convert it to mechanical energy produced by motor  64 . The battery  62  is constructed and arranged to accept and store electrical energy supplied by fuel cell  22  and to provide electric energy to motor  64 . The motor  64  is coupled to driving axle  66  to rotate wheels of a vehicle (not shown). An electrochemical engine control module (EECM)  70  and a battery pack module (BPM)  71  monitor various operating parameters, including, but not limited to, the voltage and current of the stack which is done by the battery pack module  71 , for example. The BMP  71  sends an output signal (message) to the vehicle controller  74  based on conditions monitored by the BMP  71 . The vehicle controller  74  controls operation of the battery  62 , the drive electronics  65  and the electric motor  64  in a conventional manner. 
     The term “fuel cell” is also used to refer to a fuel cell stack which contains many individual fuel cells as further illustrated in FIG.  3 . Thus, fuel cell  22  of FIG. 3 in a typical arrangement consists of many cells  84  called a stack  80 . The fuel cell stack  80  consists of a plurality of cells  84 , often on the order of one hundred or more, connected in series. Each cell  84  within the stack  80  comprises the membrane electrode assembly described earlier, and each such cell  84  provides its increment of voltage. 
     A group of cells within the stack is referred to as a “cluster”, with the stack  80  being formed of a plurality of first clusters  86 , each formed of an identical number of cells  84 , and a single endmost or last cluster  87  having a different number of cells than the identical number of cells  84  in the first clusters  86 . Typically, the number of cells per cluster is on the order of three or four cells. Four cells  84  in each first cluster  86  is shown and described by way of example only. Other number of cells could also be selected to form a plurality of like clusters  86 . 
     The second cluster  87  is formed of three cells  84  by example. This results from a particular application power requirement, space consideration, etc., which results in a total number of cells which are not devisable by the common number of cells  84  in the first clusters  86 . One or two cells could also form the second cluster  87 . 
     As described in co-pending patent application filed in July, 1999, in the names of Donald H. Keskula, Bruce J. Clingerman, and Robert W. Chalfant, and entitled “Fuel Cell Voltage Monitoring and System Control”, the entire contents of which are incorporated herein by reference, the voltage of each cluster  86  is monitored and compared with predetermined voltage safety levels to detect and/or provide an early detection of a low cell voltage within the stack  80  before the low voltage cell can degrade adjacent cells or even the entire stack  80 . 
     It is preferred that monitoring method be identical for each cluster  86  to implement economies in hardware and software design. However, according to the present invention, the different number of cells  84  in the second cluster  87  is compensated for by the unique method described hereafter. 
     In FIG. 3, each of the first clusters  86  of individual cells  84  are able to be monitored individually and e ssentially simultaneously by means of conductors  85  which connect a positive electrode and a negative electrode of each cluster  86  to a summing node or voltage measurement device denoted by reference number  91 . The summing node or device  91  provides a cumulative voltage for the cells  84  in the respective cluster  86 . The output of each summing node  91  (clstr_volts(1), clstr_volts(2), etc.) is directed to a monitoring circuit as described in the above-referenced patent application. 
     A positive electrode conductor  85  from one end of the stack  80  and a negative electrode conductor  88  from the opposite end of the stack  80  are connected to a separate summing node or junction  90  to provide a measurement of the total voltage of the stack  80  across all of the individual cells  84  in clusters  86  and  87  which form the stack  80 . 
     According to the unique method of the present invention, the total cell voltage as shown by output  92  from the summing node  90  is used as a dividend labeled total_cell_volts and is divided by the total cell  84  count (K_TOTAL_CELLS_CNT) forming the stack  80  as shown by the constant  96  in step  94 . The output of the division step is the average cell volts (cell_volts_ave) as shown by reference number  98  for the stack  80 . 
     The method of the present invention, as shown in FIGS. 3 and 4, may be implemented in hardware or software. A software implementation is described herein due to its easy adaptability in the controller or EECM  70 . 
     In FIG. 4, a constant  100  labeled K_CLSTR_FILL_CNT is used to represent the difference between the number of cells  84  in each of the first clusters  86  and the number of cells  84  in the second or last cluster  87 . In the present example, the constant  100  equals one ( 4 - 3 ). The constant  100  is multiplied by the average cell voltage (cell_volts_ave)  98  in step  102 . The result is summed with a separate input (act_clstr_volts(n))  103  representing the actual measured voltage across the second, last cluster  87  as generated by the summing node or junction  89  in FIG.  3 . The resulting output  106  labeled “clstr_volts(n)”, represents a padded or pseudo total voltage for the cells  84  in the second cluster  87  as if the last cluster  87  contains the same number of cells  84  as in all of the first clusters  86 . 
     In this manner, each of the cluster voltages output from the respective summer nodes  89  and  91  can be separately input to a standard voltage monitoring circuit and treated identically to detect a low voltage of any one cluster despite the fact that the last cluster  87  contains a different number of cells  84  than the number of cells  84  in the other clusters  86 . 
     The output  106  is generated continuously during operation of the fuel cell  22  to provide continuous monitoring of the second cluster  87  and a recalculation of the pseudo voltage of the cluster  87  for use by the monitoring circuit. 
     In summary, there has been disclosed a unique method of equalizing cluster voltages in a fuel cell for use by a fuel cell monitoring circuit wherein a common monitoring circuit or method can be used with fuel cell clusters containing any identical number of cells, and particularly, where at least one of the cells has a different number of cells than the number of cells in the remaining clusters of the stack. This eliminates the need for a special circuit or monitoring method for the non-identical cluster. The present method is also transparent to the number of cells forming each cluster thereby enabling its use along with the corresponding fuel cell monitoring circuit with fuel cell stacks containing any number of cells and even fuel cell stacks from different manufacturers.