BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a voltage monitoring system for a fuel cell stack in which a plurality of fuel cells are stacked.

2. Description of the Related Art

[0002] The output of a single-cell fuel cell does not reach 1 V, so a fuel cell is generally formed as a fuel cell stack in which a plurality of single cells are serially connected. In the above fuel cell stack, when an abnormality or a failure occurs in any one of the single cells, it is necessary to limit the output over the entire stack or stop operation of the stack. Therefore, generally, the fuel cell stack includes a single cell monitor for monitoring the voltage of each single cell (for example, see Japanese Patent Application Publication No. 2008-103201 (JP-A-2008-103201)).

[0003] However, in order to measure the respective voltages of all the single cells, the configuration of a measurement circuit and a CPU for processing the measured results become relatively large. This problematically leads to an increase in size and cost of the system. Particularly, when a relatively large output is required as in the case of a fuel cell stack used as a power source of a vehicle, the number of single cells that constitute the fuel cell stack is considerably large, so the above problem is remarkable.

SUMMARY OF THE INVENTION

[0004] The invention simplifies the configuration of a voltage monitor for a fuel cell.

[0005] A first aspect of the invention relates to a voltage monitoring system for a fuel cell stack in which a plurality of fuel cells are stacked. The voltage monitoring system includes: voltage transmitting means that are provided respectively for the plurality of fuel cells and that extract respective voltages of the fuel cells to transmit the extracted voltages in an insulated manner; and a detecting circuit that collects the voltages of the plurality of fuel cells, transmitted by the voltage transmitting means, and that outputs at least one of a minimum value and a maximum value among the transmitted voltages. The phrase "to transmit the extracted voltages in an insulated manner" signifies that portions, to which the voltages are input, are not electrically connected to portions from which the voltages are output, and information on the input voltage is transmitted.

[0006] The voltage monitoring system collects the voltages of the plurality of fuel cells, transmitted by the voltage transmitting means in an insulated manner, and outputs at least one of the minimum value and the maximum value among the collected voltages, so it is possible to reduce computing load for obtaining the minimum value or the maximum value. Thus, it is possible to simplify the configuration of the voltage monitoring system.

[0007] The voltage transmitting means may be differential amplifiers, each of which is connected to both anode and cathode of a corresponding one of the plurality of fuel cells, and the detecting circuit may include diodes provided in correspondence with respective outputs of the differential amplifiers in the same orientation, and a capacitor connected between the diodes and a ground.

[0008] The voltage monitoring system is formed of the differential amplifiers, the diodes and the capacitor, so it is possible to simplify the configuration of the voltage monitoring system. In addition, the voltage monitoring system may be formed of general-purpose components, so manufacturing is easy, and cost may be reduced.

[0009] A power supply of the differential amplifiers may be an insulated power supply that is provided independently of the fuel cells.

[0010] By so doing, independently of the state of power generation of the fuel cells, even when, for example, the outputs of the fuel cells become negative voltages, it is possible to detect the maximum value and the minimum value. In addition, because the insulated power supply is used, even when the number of fuel cells is large, that is, when the number of differential amplifiers is large and then a necessary power supply voltage is relatively high, it is possible to desirably supply power.

[0011] The voltage transmitting means may be provided one by one for the plurality of fuel cells, and include switch circuits, each of which is connected to both anode and cathode of a corresponding one of the plurality of fuel cells, and a transformer that sequentially switches connected states of the switch circuits to sequentially input the respective voltages of the fuel cells as input voltages, and the detecting circuit may include a capacitor.

[0012] The voltage monitoring system sequentially switches the connected states of the switch circuits provided one by one for the plurality of fuel cells to input the respective voltages to the transformer, and then uses the output voltages to detect the minimum value or maximum value among the input voltages. Thus, by providing only one switch circuit for each fuel cell, it is possible to detect the minimum value or maximum value among the voltages of the respective fuel cells, so it is possible to simplify the circuit configuration. This contributes to a decrease in cost and size of the voltage monitoring system.

[0013] The detecting circuit may include a minimum value detecting circuit that detects the minimum value and a maximum value detecting circuit that detects the maximum value, and the minimum value detecting circuit and the maximum value detecting circuit may be able to switch connection with the transformer using a switch circuit.

[0014] By so doing, both the minimum value and the maximum value among the voltages of the fuel cells may be detected, so it is possible to desirably control operation of the fuel cell stack. In addition, the minimum value detecting circuit and the maximum value detecting circuit are switched by the switch circuit, so it is possible to simplify the circuit configuration.

[0015] The voltage transmitting means may be current sensors, each of which is connected to both anode and cathode of a corresponding one of the plurality of fuel cells and outputs a voltage according to a flowing electric current, and the detecting circuit may include diodes that are provided in correspondence with outputs of the current sensors and a capacitor that is connected between the diodes and a ground. The current sensor may be magnetic sensors.

[0016] The voltage monitoring system is formed of the current sensors, the diodes and the capacitor, so it is possible to simplify the configuration of the voltage monitoring system. In addition, the voltage monitoring system may be formed of general-purpose components, so manufacturing is easy, and cost may be reduced.

[0017] A power supply of the current sensors may be an insulated power supply that is provided independently of the fuel cells.

[0018] By so doing, independently of the state of power generation of the fuel cells, even when, for example, the outputs of the fuel cells become negative voltages, it is possible to detect the maximum value and the minimum value. In addition, because the insulated power supply is used, even when the number of fuel cells is large, that is, when the number of current sensors is large and then a necessary power supply voltage is relatively high, it is possible to desirably supply power.

[0019] The voltage monitoring- system may further include an overall voltage detecting circuit that detects an overall voltage of the fuel cell stack.

[0020] By so doing, the overall voltage of the fuel cell stack, that is, the mean voltage of the plurality of fuel cells, may be detected, so it is possible to desirably control operation of the fuel cell stack by appropriately determining whether the minimum value or maximum value detected by the detecting circuit is an abnormal value.

[0021] The voltage transmitting means and the detecting circuit may be provided for each of predetermined groups of the plurality of fuel cells, and the voltage monitoring system may further include branch voltage transmitting means that is provided for each of the detecting circuits of the predetermined groups and that transmits an output result of the detecting circuit in an insulated manner; and a branch detecting circuit that collects the output results, transmitted by the branch voltage transmitting means, and that outputs at least one of a minimum value and a maximum value among the transmitted output results. [0022] The branch voltage transmitting means and the branch detecting circuit may be formed in a multi-stage manner.

[0023] The plurality of fuel cells are divided into predetermined groups to output a minimum value or a maximum value in a multi-stage, so it is possible to reduce the necessary capacity of each component. Thus, even when the number of the plurality of fuel cells is large, components may be prepared within the range of a general specification capacity, so manufacturing is easy, and cost may be reduced.

[0024] The voltage transmitting means and the detecting circuit may be provided for each of predetermined groups of the plurality of fuel cells, and the voltage monitoring system may further include branch voltage transmitting means that is provided for each of the detecting circuits of the predetermined groups and that transmits an output result of the detecting circuit in an insulated manner; and a branch computing circuit that computes at least one of a minimum value and a maximum value among the output results, transmitted by the branch voltage transmitting means, on the basis of the transmitted output results.

[0025] By so doing, the plurality of fuel cells are divided into predetermined groups, and the overall minimum value or maximum value is computed by the branch computing circuit on the basis of the output results of the minimum value or maximum value of each group, so it is possible to reduce the computing load in comparison with the case where the minimum value or the maximum value is computed from the respective outputs of the fuel cells.

[0026] The detecting circuit may intermittently output at least one of the minimum value and the maximum value. By so doing, it is possible to reduce power consumption.

[0027] A voltage monitoring system for a fuel cell stack in which a plurality of fuel cells are stacked may include switch circuits that are provided one by one for the plurality of fuel cells and that are connected to both anodes and cathodes of the plurality of fuel cells; a transformer that sequentially switches connected states of the switch circuits to sequentially input the respective voltages of the fuel cells as input voltages; and a detecting circuit that uses the output voltages of the transformer to detect at least one of a minimum value and a maximum value among the input voltages.

[0028] The voltage monitoring system sequentially switches the connected states of the switch circuits provided one by one for the plurality of fuel cells to input the respective voltages to the transformer, and then uses the output voltages to detect the minimum value or maximum value of the input voltages. Thus, by providing only one switch circuit for each fuel cell, it is possible to detect the minimum value or maximum value among the voltages of the respective fuel cells, so it is possible to simplify the circuit configuration. This contributes to a decrease in cost and size of the voltage monitoring system.

[0029] The voltage monitoring system may further include an overall voltage detecting circuit that detects an overall voltage of the fuel cell stack.

[0030] The voltage monitoring system is able to detect the overall voltage of the fuel cell stack, that is, the mean voltage of the plurality of fuel cells, so it is possible to desirably control operation of the fuel cell stack by appropriately determining whether the minimum value or maximum value detected by the detecting circuit is an abnormal value.

[0031] The detecting circuit may include a minimum value detecting circuit that detects the minimum value and a maximum value detecting circuit that detects the maximum value, and the minimum value detecting circuit and the maximum value detecting circuit may be able to switch connection with the transformer using a switch circuit.

[0032] By so doing, both the minimum value and the maximum value among the voltages of the fuel cells may be detected, so it is possible to desirably control operation of the fuel cell stack. In addition, the minimum value detecting circuit and the maximum value detecting circuit are switched by the switch circuit, so it is possible to simplify the circuit configuration.

[0033] The detecting circuit may detect at least one of the minimum value and the maximum value using a capacitor.

[0034] By so doing, at least one of the minimum value and the maximum value is detected using the capacitor, so it is possible to simplify the circuit configuration.

[0035] The switch circuit, the transformer and the detecting circuit may be provided for each of predetermined groups of the plurality of fuel cells, and the voltage monitoring system may further include: a branch switch circuit that is provided for each of the detecting circuits; a branch transformer that sequentially switches connected states of the branch switch circuits to sequentially input voltages detected by the detecting circuits as branch input voltages; and a branch detecting circuit that uses output voltages of the branch transformer to detect at least one of a minimum value and a maximum value among the branch input voltages.

[0036] Furthermore, the branch switch circuits, the branch transformer and the branch detecting circuit may be formed in a multi-stage manner.

[0037] By so doing, the plurality of fuel cells are divided into predetermined groups, and the minimum value or the maximum value is detected by the detecting circuits, or the like, provided in a multi-stage manner in units of the groups. Thus, even when the number of fuel cells that constitute the fuel cell stack is relatively large, it is possible to reduce a difference in time among the voltages input from the respective fuel cells, so it is possible to improve detection accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a view that illustrates the configuration of a voltage monitoring system 20;

FIG. 2 is a chart that illustrates an example of progression of on-off operation of each switch and voltages of capacitors CIl and C12 in maximum value detecting operation;

FIG. 3 is a chart that illustrates an example of progression of on-off operation of each switch arid voltages of the capacitors CIl and C12 in minimum value detecting operation;

FIG. 4A and FIG. 4B are views that illustrate the configurations of voltage detecting circuits according to comparative embodiments;

FIG. 5 is a view that illustrates the configuration of a voltage monitoring system 520 that performs minimum value detecting operation according to a second embodiment;

FIG. 6 is a view that illustrates an alternative embodiment to the voltage monitoring system 520;

FIG. 7 is a view that illustrates the configuration of a voltage monitoring system 620 that performs maximum value detecting operation according to the second embodiment;

FIG. 8 is a view that illustrates the configuration of a voltage monitoring system 720 according to a third embodiment;

FIG. 9 is a view that illustrates the configuration of a voltage monitoring system according to an embodiment of the invention;

FIG. 10 is a view that illustrates the configuration of a voltage monitoring system according to an alternative embodiment;

FIG. 11 is a view that illustrates the configuration of a voltage monitoring system according to an alternative embodiment; and

FIG. 12 is a view that illustrates the configuration of a voltage monitoring system according to an alternative embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS A. First Embodiment A-I. Schematic Configuration of Voltage Monitoring System 20

[0039] FIG. 1 shows the schematic configuration of a voltage monitoring system 20 according to a first embodiment of the invention. The voltage monitoring system 20 monitors the voltage of a fuel cell stack FC in which five fuel cells FCl to FC5 are stacked. The fuel cells FCl to FC5 are so-called single cells and are minimum units for power generation.

[0040] The fuel cells FCl to FC5 each are a polymer electrolyte fuel cell. Each of the fuel cells FCl to FC5 is formed so that a gas diffusion layer, a flow passage member and a separator are laminated on each side of a membrane electrode assembly (not shown). The membrane electrode assembly includes an electrolyte membrane, a cathode and an anode. The electrolyte membrane is a thin membrane made of a solid polymer material that exhibits desirable proton conductivity in a wet condition. The cathode and the anode are provided on the surfaces of the electrolyte membrane. In addition, the fuel cells FCl to FC5 are clamped by terminals, insulators and end plates arranged on both ends in a stacking direction, and supply and exhaust systems for fuel gas, oxidation gas and coolant are connected to the fuel cells FCl to FC5 (not shown). Note that the number of fuel cells that constitute the fuel cell stack FC is not limited to five; the number of fuel cells may be selected.

[0041] The voltage monitoring system 20 includes switch circuits 31 to 35, a transformer 50, a maximum/minimum value detecting circuit 60 and operational amplifiers 81 and 82. The switch circuits 31 to 35 are basically the same circuits. When viewed from the fuel cells FCl to FC5, capacitors Cl to C5 are connected between respective anodes and cathodes, and switches SWl to SW5 and coils 51 to 55 are serially connected in parallel with the capacitors Cl to C5. In this way, the switch circuits 31 to 35 are respectively provided one by one for the fuel cells FCl to FC5 that constitute the fuel cell stack FC.

[0042] Each of the switches SWl to SW5 is a relay that receives a signal from a CPU (not shown) to switch on or off the conductive state between the anode side and cathode side of a corresponding one of the fuel cells FCl to FC5. The transformer 50 includes the coils 51 to 55 as input coils and a coil 57 as an output coil. When all the switches SWl to SW5 are off, the capacitors Cl to C5 are charged by the corresponding fuel cells FCl to FC5. When the capacitors Cl to C5 are charged once, the capacitors Cl to C5 do not consume electric power. In this state, when any one of the switches SWl to SW5 is turned on, electric power stored in the capacitor corresponding to the turned-on switch among the capacitors Cl to C5 flows into the input coil of the transformer 50. A variation in that electric current is considerably large, so an output voltage corresponding to an input voltage is generated between both ends of the coil 57, which is the output coil, of the transformer 57 in accordance with a mutual inductance between the input and output of the transformer 50. Note that, when any one of the switches SWl to SW5 turns on, electric current also flows from a corresponding one of the fuel cells FCl to FC5; however, the fuel cells FCl to FC5 each have internal resistance, so the fuel cells FCl to FC5 do not allow large inrush current to flow through the corresponding coils 51 to 55 in a short period of time. Thus, the capacitors Cl to C5 are used to flow inrush current to the coils 51 to 55, which are the input coils, of the transformer 50. These capacitors Cl to C5 are so-called speed-up capacitors.

[0043] The maximum/minimum value detecting circuit 60 is shared by the fuel cells FCl to FC5. The maximum/minimum value detecting circuit 60 includes capacitors CIl and C12, diodes 63 and 64 and switches SWIl and SW12. The maximum/minimum value detecting circuit 60 operates as a maximum value detecting circuit when the maximum/minimum value detecting circuit 60 receives a signal from a CPU (not shown) and then the switch SWIl is turned on and the switch SW12 is left off. In the maximum value detecting circuit, the maximum value among the output voltages of the transformer 50 is held by the capacitor C12. In addition, the maximum/minimum value detecting circuit 60 operates as a minimum value detecting circuit when the maximum/minimum value detecting circuit 60 receives a signal from the CPU (not shown) and then the switch SWIl is left off and the switch SW12 is turned on. In the minimum value detecting circuit, the minimum value among the output voltages of the transformer 50 is held by the capacitor C12. In this way, by forming the maximum/minimum value detecting circuit 60 to be able to switch between the maximum value detecting circuit and the minimum value detecting circuit using the switches, it is possible to simplify the circuit configuration. Of course, the maximum/minimum value detecting circuit 60 may be formed so that the maximum value detecting circuit and the minimum value detecting circuit are separately provided. Note that the operation of the maximum/minimum value detecting circuit 60 will be described in detail later.

[0044] The operational amplifiers 81 and 82 are voltage followers. The operational amplifier 81 outputs the overall voltage of the fuel cell stack FC to an A/D converter (not shown) at a predetermined timing. Because the number of fuel cells that constitute the fuel cell stack FC is known, a mean voltage may also be detected from the thus output overall voltage. The operational amplifier 82 outputs a maximum voltage or a minimum voltage held by the capacitor C12 to the A/D converter (not shown) at a predetermined timing. Note that the output of the above described A/D converter is input to a CPU (not shown) that controls an operating system for the fuel cell stack FC and the voltage monitoring system 20. A-2. Maximum Value Detecting Operation

[0045] The maximum value detecting operation of the voltage monitoring system 20 will be described. The maximum value detecting operation is an operation for detecting a maximum value among the respective values of the fuel cells FCl to FC5. The above operation is implemented by switch on/off operation via a CPU (not shown).

[0046] In the maximum value detecting operation according to the present embodiment, first, the switch SWIl is turned on, and the switch SW12 is left off. Then, in a state where the switches SW2 to SW5 of the switch circuits 31 to 35 are left off, the switch SWl of the switch circuit 31 is turned on. Then, the voltage Vl of the fuel cell FCl is input to the transformer 50, and the output Vl is applied to the capacitor CIl. Then, the voltage Vl is held by the capacitor C12 via the diode 63.

[0047] Subsequently, the switch SWl is turned off, and the switch SW2 of the switch circuit 32 is turned on. Then, the voltage V2 of the fuel cell FC2 is input to the transformer 50, and the output V2 is applied to the capacitor CIl. At this time, when the voltage V2 is lower than or equal to the voltage Vl, the diode 63 is reverse-biased, so the voltage Vl is still held by the capacitor C12. On the other hand, when the voltage V2 is higher than the voltage Vl, the diode 63 is forward-biased, so electric charge of the capacitor CIl and electric charge of the capacitor C12 are leveled via the diode 63. By repeatedly turning on/off the switch SW2 in that state many times, when the voltage V2 is higher than the voltage Vl, the voltage V2 is ultimately held by the capacitor C12.

[0048] A specific embodiment of the above operation is shown in FIG. 2. In this embodiment, it is assumed that voltage Vl < voltage V2. As described above, when the switch SWl is turned on in a state where the switch SWIl is on, the voltage of each of the capacitors CIl and C12 becomes the voltage Vl. Then, when the switch SWl is turned off, the voltage of the capacitor CIl decreases. During then, the voltage Vl is held by the capacitor C12. Then, when the switch SW2 is repeatedly turned on or off, the voltage V2 is repeatedly applied to the capacitor CIl. Accordingly, the voltage of the capacitor C12 gradually increases while holding the maximum value, and ultimately reaches the voltage V2.

[0049] In this way, when the switch circuits 33 to 35 are also similarly operated, the maximum voltage Vmax, which is the maximum value among the voltages of the fuel cells FCl to FC5, is held by the capacitor C12. The thus detected maximum voltage Vmax, is output to the CPU via the operational amplifier 82. A-3. Minimum Value Detecting Operation

[0050] The minimum value detecting operation of the voltage monitoring system 20 will be described. The minimum value detecting operation is an operation for detecting a minimum value among the respective values of the fuel cells FCl to FC5. The above operation is implemented by switch on/off operation via the CPU (not shown).

[0051] In the minimum value detecting operation according to the present embodiment, first, in a state where the switches SWIl and SW12 are off, a predetermined initial voltage VO is applied to the capacitor Cl 2 in advance. Here, the initial voltage VO is a voltage that is assumed to be absolutely higher than the minimum value among the voltages of the fuel cells FCl to FC5. In the present embodiment, by similar operation to the operation for detecting the above described voltages Vl and V2, the larger one of the voltages Vl and V2 is applied to the capacitor C12 as the initial voltage VO; however, a method of applying a voltage is not specifically limited. For example, another circuit configuration for applying a voltage is applicable.

[0052] Then, in a state where the switches SW2 to SW5 of the switch circuits 32 to 35 are off, the switch SWl of the switch circuit 31 is turned on. Then, as in the case of the maximum value detecting operation, the voltage Vl is held by the capacitor CIl. Then, after that, the switch SW12 is turned on. Then, when the voltage Vl is higher than or equal to the voltage VO, the diode 64 is reverse-biased, so the voltage VO is still held by the capacitor C12. On the other hand, when the voltage Vl is lower than the voltage VO, the diode 64 is forward-biased, so electric charge of the capacitor CIl and electric charge of the capacitor C12 are leveled via the diode 64. By repeatedly turning on or off the switch SWl in that state many times, when the voltage Vl is lower than the voltage VO, the voltage Vl is ultimately held by the capacitor C12. Note that, when the switch SWl is turned off, a counter electromotive force is generated, so it is necessary to once turn off the switch SW12 beforehand in each case.

[0053] A specific embodiment of the above operation is shown in FIG. 3. In this embodiment, it is assumed that initial voltage VO > voltage Vl > voltage V2. As described above, when the switch SWl is turned on in a state where the switch SWIl is off, the voltage of the capacitor CIl becomes the voltage Vl. Here, . when the switch SW12 is turned on, the voltage of the capacitor C12 decreases from the initial voltage VO, and, in addition, when the switch SW12 is turned off, the decreased voltage of the capacitor C12 is held. Then, when the switch SWl is turned off, the voltage of the capacitor CIl decreases. When the above operation is repeated (three times in this embodiment), the voltage Vl is held by the capacitor C12. Then, when the switch SW2 is operated similarly, the voltage V2 is held by the capacitor C12.

[0054] When the switch circuits 32 to 35 are also similarly operated, the minimum voltage Vmin, which is the minimum value among the voltages of the fuel cells FCl to FC5, is held by the capacitor C12. The thus detected minimum voltage Vmin is output to the CPU via the operational amplifier 82. A-4. Comparative Embodiment

[0055] FIG. 4A shows the configuration of a voltage detecting circuit for the fuel cells FCl to FC3 according to a comparative embodiment. As shown in the drawing, in the circuit according to the comparative embodiment, a circuit that includes two switches SWIlO and SW120 is connected to the fuel cell FCl. Similarly, a circuit that includes two switches SW210 and SW220 is connected to the fuel cell FC2, and a circuit that includes two switches SW310 and SW320 is connected to the fuel cell FC3. That is, two switch circuits are provided for each fuel cell. Thus, when only any one of the pair of switches SWIlO and SW120, the pair of switches SW210 and SW220 and the pair of switches SW310 and SW320 are turned on, a corresponding one of the voltages of the fuel cells FCl to FC3 is detectable.

[0056] In addition, FIG. 4B shows the configuration of a voltage detecting circuit according to another comparative embodiment. In this embodiment, as shown in the drawing, four switches SW410 to SW440 are used to detect the voltages of the fuel cells FCl to FC3. In short, the circuit shown in FIG. 4B is simplified from the circuit configuration shown in FIG. 4Aby reducing the number of switch circuits. In the above embodiment, for example, the signs of the detected voltages of the fuel cell FCl and fuel cell FC2 are opposite. That is, the detected values cannot be directly input to the maximum/minimum value detecting circuit 60.

[0057] As described above, with the general circuit configuration, unless two or more switch circuits are provided for each fuel cell, it is impossible to detect voltages that can be directly input to the maximum/minimum value detecting circuit 60. A-5. Advantageous Effects

[0058] The thus configured voltage monitoring system 20 sequentially switches the connected states of the switch circuits 31 to 35 that are provided in one-to-one correspondence with the fuel cells FCl to FC5 to input the voltage to the transformer 50, and detects the minimum value and maximum value among the respective voltages of the fuel cells FCl to FC5 by the maximum/minimum value detecting circuit 60 using the output voltages of the transformer 50. Thus, by providing only one switch circuit for each fuel cell, it is possible to detect the minimum value or maximum value among the respective voltages of the fuel cells, so it is possible to simplify the circuit configuration. This contributes to a decrease in cost and size of the voltage monitoring system. The above advantageous effect is particularly remarkable when the number of fuel cells that constitute the fuel cell stack FC is large. In addition, only the maximum value or the minimum value is output to the above described CPU, so it is possible to greatly reduce the computing load of the CPU in comparison with the case where the respective voltages of all the fuel cells that constitute the fuel cell stack FC are output to the CPU.

[0059] In addition, the above described voltage monitoring system 20 is able to also detect not only the maximum voltage and minimum voltage of the fuel cells FCl to FC5 using the maximum/minimum value detecting circuit 60 but also a mean voltage of the fuel cells FCl to FC5 (the overall voltage of the fuel cell stack FC). Therefore, by appropriately determining whether the detected minimum voltage or maximum voltage is an abnormal value, it is possible to desirably control operation of the fuel cell stack FC. B. Second Embodiment

[0060] FIG. 5 shows the schematic configuration of a voltage monitoring system 520 according to a second embodiment. In FIG. 5, like reference numerals as those of FIG. 1 denote similar components to those of the first embodiment. Hereinafter, only the difference of the voltage monitoring system 520 from that of the first embodiment will be described. In FIG. 5, for the sake of simple description, the voltage monitoring system 520 includes only the configuration that detects the minimum value among the respective voltages of the fuel cells FCl to FC5. As shown in the drawing, the voltage monitoring system 520 includes operational amplifiers 531 to 535 and a minimum value detecting circuit 560. The operational amplifiers 531 to 535 are differential amplifiers. The operational amplifiers 531 to 535 respectively amplify the voltages of the fuel cells FCl to FC5 connected to both input terminals thereof at a predetermined gain (here, the voltages of the fuel cells FCl to FC5 themselves because the gain is set at 1), and output the amplified voltages while insulating the voltages from actual potentials.

[0061] The fuel cells FCl to FC5 are stacked and are electrically serially connected, so the respective voltages of the cathodes of the fuel cells each are increased from the ground level to a potential corresponding to the number of fuel cells stacked to that fuel cell. By using the differential amplifiers, that is, the operational amplifiers 531 to 535, the respective outputs of the operational amplifiers 531 to 535 all are values that represent the respective, voltages of the fuel cells FCl to FC5 with respect to the ground level. In the present embodiment, the output of the fuel cell stack FC is used as the power supply of the operational amplifiers 531 to 535.

[0062] The minimum value detecting circuit 560 detects the minimum value among the outputs of the fuel cells FCl to FC5. The minimum value detecting circuit 560 includes diodes 541 to 545, a capacitor C500, a power supply 550 and a resistor 555. The outputs of the above described respective operational amplifiers 531 to 535 are all connected to the backward diodes 541 to 545. Therefore, the respective outputs of the operational amplifiers 531 to 535 are connected by so-called wired-OR connection. That is, the output of each of the operational amplifiers 531 to 535 does not have any influence on the outputs of the other operational amplifiers 531 to 535. The capacitor C500, of which one end is connected to a ground, is connected to the outputs of the wired-OR connected operational amplifiers 531 to 535. In addition, a predetermined positive power supply is connected to the outputs of the operational amplifiers 531 to 535 via a pull-up resistor 555. The voltage of the predetermined positive power supply is set at a value that is sufficiently higher than expected outputs of the operational amplifiers 531 to 5.35.

[0063] The minimum value detecting operation of the voltage monitoring system 520 will be described. Note that, for the sake of simple description, the following operation will be described on the assumption that the voltage drop of each of the diodes 541 to 545 is 0 volt.

(1) When the operational amplifiers 531 to 535 are not operating, and the respective outputs of the operational amplifiers 531 to 535 are in high-impedance states, no electric current flows into the outputs of the operational amplifiers 531 to 535. Therefore, the capacitor C500 is charged, and the voltage of a terminal MMC is equal to the voltage of the positive power supply connected via the pull-up resistor 555.

[0064] (2) Subsequently, as the operational amplifiers 531 to 535, which are differential amplifiers, are operated at predetermined timings to detect the respective output voltages of the fuel cells FCl to FC5 and output the detected output voltages, electric current flows through the diodes 541 to 545, and the voltage of the terminal MMC of the capacitor C500 decreases. The above operation continues until the voltage of the terminal MMC becomes a minimum voltage Vminl among the outputs of the connected operational amplifiers 531 to 535. When the voltage of the terminal MMC coincides with the voltage Vminl, the other operational amplifiers 531 to 535 (operational amplifiers other than the operational amplifier that outputs the voltage Vminl) have voltages higher than the voltage of the terminal MMC, so no electric current flows into the other operational amplifiers 531 to 535 via the diodes 541 to 545.

[0065] (3) If the voltage of any one of the fuel cells further decreases, and the minimum voltage among the outputs of the connected operational amplifiers 531 to 535 becomes Vmin2 (Vminl > Vmin2), electric current flows into any one of the operational amplifiers 531 to 535 that outputs a voltage lower than the voltage of the terminal MMC of the capacitor C500 via a corresponding one of the diodes 541 to 545, so the voltage of the terminal MMC of the capacitor C500 decreases. The above operation continues until the voltage of the terminal MMC becomes the voltage Vmin2. When the voltage of the terminal MMC coincides with the voltage Vmin2, the other operational amplifiers 531 to 535 have voltages higher than the voltage of the terminal MMC, so no electric current flows into the other operational amplifiers 531 to 535 via the corresponding diodes 541 to 545.

[0066] (4) Conversely, when the voltage of the fuel cell that outputs the minimum voltage till then increases, and then the minimum voltage among the outputs of the connected operational amplifiers 531 to 535 becomes Vmin3 (Vminl < Vmin3), the operational amplifiers 531 to 535 have voltages higher than the voltage of the terminal MMC, so no electric current flows into the operational amplifiers 531 to 535 via the corresponding diodes 541 to 545. Thus, the capacitor C500 is gradually charged by the power supply 550. The above charging continues until the voltage of the terminal MMC of the capacitor C500 becomes the voltage Vmin3. As the voltage of the terminal MMC exceeds the voltage Vmin3, electric current flows toward the operational amplifiers via the diodes 541 to 545, and then the voltage of the terminal MMC of the capacitor C500 decreases.

[0067] Actually, in any case of the above described (2) to (4), each diode has a forward voltage drop (in the case of silicon diode, generally, about 0.7 volt), so the voltage of the terminal MMC is higher by the forward voltage drop than the minimum value of the outputs of the operational amplifiers 531 to 535. However, the forward voltage drop of each diode is already known, so it is easy to detect the minimum voltage of fuel cells FCl to FC5 by detecting the voltage of the terminal MMC. Note that, when transistors are used to perform measurement by reducing the forward voltage drop as compared with the configuration using the diodes, it is possible to detect values further closer to actual voltages of fuel cells. In this way, the minimum output of the diodes 541 to 545, that is, a voltage corresponding to the minimum voltage of the fuel cells FCl to FC5, is held by the capacitor C500, and is output to the CPU (not shown).

[0068] The thus configured voltage monitoring system 520 does not need to use switches, and it is sufficient to connect the diodes by wired-OR connection, so it is possible to simplify the configuration. In addition, the voltage monitoring system 520 may be formed of general-purpose components, so manufacturing is easy, and cost may be reduced. Moreover, there is no interference among the circuits that extract the outputs of the fuel cells FCl to FC5. Furthermore, it is not necessary to switch the connected states by switches unlike the first embodiment, so the voltage monitoring system 520 has a high response to fluctuations in voltages of the fuel cells FCl to FC5. In addition, the capacitor C500 is used, and it means that the voltage monitoring system 520, so to speak, includes an integration circuit. Thus, it is possible to output the minimum voltage by eliminating the influence of noise, or the like.

[0069] In addition, in the above described embodiment, the fuel cell stack FC is used as the power supply of the operational amplifiers 531 to 535. Instead, the power supply of the operational amplifiers 531 to 535 may be provided separately from the fuel cell stack FC. FIG. 6 shows the configuration that uses a power supply 580 independent of the fuel cell stack FC as the power supply of the operational amplifiers 531 to 535. It is assumed that the output of the fuel cell stack FC becomes 0 V or a negative voltage because of flooding, or the like. However, when the power supply of the operational amplifiers 531 to 535 are separately provided, it is possible to reliably monitor the voltage of the fuel cell stack FC in such a case. In addition, in this case, the power supply 580 may be an insulated power supply. By so doing, even when a large number of fuel cells that constitute the fuel cell stack FC are stacked, that is, even when the total value of the respective outputs of the operational amplifiers is large, it is possible to desirably supply power to the operational amplifiers. Note that the power supply 550 and the power supply 580 may be integrated.

[0070] In addition, the above described voltage monitoring system 520 outputs the minimum value among the respective voltages of the fuel cells FCl to FC5. FIG. 7 shows the configuration of a voltage monitoring system 620 for outputting the maximum value. In FIG. 7, like reference numerals as those of FIG. 6 denote similar configuration to that of the voltage monitoring system 520. Hereinafter, only the difference of the voltage monitoring system 620 from the voltage monitoring system 520 will be described. As shown in the drawing, the voltage monitoring system 620 includes operational amplifiers 531 to 535 and a maximum value detecting circuit 660. That is, the voltage monitoring system 620 includes the maximum value detecting circuit 660 instead of the minimum value detecting circuit 560 of the voltage monitoring system 520.

[0071] The maximum value detecting circuit 660 detects the maximum value among the outputs of the fuel cells FCl to FC5. The maximum value detecting circuit 660 includes diodes 641 to 645, a capacitor C500 and a resistor 557. The maximum value detecting circuit 660 differs from the minimum value detecting circuit 560 in that the orientations of the diodes 641 to 645 are opposite to the orientations of the diodes 541 to 545 of the minimum value detecting circuit 560, and the resistor 557 is provided instead of the power supply 550 and resistor 555 of the minimum value detecting circuit 560. The resistor 557 is a so-called pull-down resistor. The resistor 557 discharges electric charge stored in the capacitor C500. When there is no output from the operational amplifiers 531 to 535, the resistor 557 makes the voltage of the terminal MMC of the capacitor C500 become zero.

[0072] The maximum value detecting operation of the thus configured voltage monitoring system 620 will be described. Note that, for the sake of simple description, the following operation is described on the assumption that the voltage drop of each of the diodes 641 to 645 is 0 volt.

(1) As the operational amplifiers 531 to 535, which are differential amplifiers, are operated at predetermined timings to detect the output voltages of the respective fuel cells FCl to FC5 and output the detected output voltages, electric current flows through the diodes 641 to 645, and the voltage of the terminal MMC of the capacitor C500 increases. The above operation continues until the voltage of the terminal MMC becomes a maximum voltage Vmaxl among the outputs of the connected operational amplifiers 531 to 535. When the voltage of the terminal MMC coincides with the voltage Vmaxl, the other operational amplifiers 531 to 535 (operational amplifiers other than the operational amplifier that outputs the voltage Vmaxl) have voltages lower than the voltage of the terminal MMC, so no electric current flows into the other operational amplifiers 531 to 535 via the diodes 541 to 545.

[0073] (2) If the voltage of any one of the fuel cells further increases, and the maximum voltage among the outputs of the connected operational amplifiers 531 to 535 becomes Vmax2 (Vmaxl < Vmax2), electric current flows to the capacitor C500 via any one of the operational amplifiers 531 to 535, which outputs a voltage higher than the voltage of the terminal MMC of the capacitor C500, and a corresponding one of the diodes 541 to 545, so the voltage of the terminal MMC of the capacitor C500 increases. The above operation continues until the voltage of the terminal MMC becomes the voltage Vmax2.

[0074] (3) Conversely, when the voltage of the fuel cell that outputs the maximum voltage till then decreases, and then the maximum voltage among the outputs of the connected operational amplifiers 531 to 535 becomes Vmax3 (Vmaxl > Vmax3), the voltage of the terminal MMC of the capacitor C500 is lower than the outputs of the operational amplifiers 531 to 535, so no electric current flows into the capacitor C500 from the operational amplifiers 531 to 535. As a result, electric charge stored in the capacitor C500 is discharged via the resistor 557 until the voltage of the capacitor C500 changes from Vmaxl to Vmax3. Note that, actually, in any case of the above described (1) to (3), the voltage of the terminal MMC is lower by the forward voltage drop than the maximum value of the outputs of the operational amplifiers 531 to 535. However, as in the case of the minimum value detecting operation, it is easy to detect the maximum voltage of the fuel cells FCl to FC5.

[0075] The thus configured voltage monitoring system 620 has similar advantageous effects to those of the voltage monitoring system 520, and is able to output the maximum value. Note that, when both the minimum value and the maximum value need to be output, it is only necessary that the minimum value detecting circuit 560 and the maximum value detecting circuit 660 are connected in parallel with the operational amplifiers 531 to 535. Note that the configuration that detects the overall voltage of the fuel cell stack FC, as in the case of the first embodiment, may be added to the configuration of the above described second embodiment. C. Third Embodiment

[0076] FIG. 8 shows the schematic configuration of a voltage monitoring system 720 according to a third embodiment. In FIG. 8, like reference numerals as those of FIG. 5 denote similar configuration to that of the second embodiment. Hereinafter, only the difference of the voltage monitoring system 720 from that of the second embodiment will be described. In FIG. 8, for the sake of simple description, the voltage monitoring system 720 includes only the configuration that detects the minimum value among the respective voltages of the fuel cells FCl to FC5. As shown in the drawing, the voltage monitoring system 720 includes resistors 771 to 775, current sensors 731 to 735 and a minimum value detecting circuit 560. That is, the voltage monitoring system 720 differs from the second embodiment in that the resistors 771 to 775 and the current sensors 731 to 735 are provided instead of the operational amplifiers 531 to 535 of the voltage monitoring system 520 according to the second embodiment.

[0077] The resistor 771 and the current sensor 731 are serially connected between the anode and cathode of the fuel cell FCl. Similarly, for the fuel cells FC2 to FC5 as well, the resistors 772 to 775 and the current sensors 732 to 735 are serially connected, respectively. [0078] The current sensors 731 to 735 are noncontact direct current sensors. The current sensors 731 to 735 respectively output voltages having values equal to the output voltages of the fuel cells FCl to FC5 depending on electric current flowing through the resistors 771 to 775 in a state where each of the current sensors 731 to 735 is insulated from the fuel cells other than the corresponding one of fuel cells FCl to FC5. In the present embodiment, the current sensors 731 to 735 are Hall element-type magnetic sensors. However, it is only necessary that the current sensors 731 to 735 include the above described function. For example, various magnetic sensors or current sensors, such as magnetic amplifiers and magnetic multivibrators, may be used. In addition, when it is only necessary to output a voltage only when a steep variation in output occurs in any of the fuel cells FCl to FC5, current transformers may be used as the current sensors 731 to 735. Note that, although not shown in FIG. 8, the fuel cell stack FC is used as the power supply of the current sensors 731 to 735 in the present embodiment.

[0079] The thus configured voltage monitoring system 720 is able to perform minimum value detecting operation as in the case of the second embodiment. That is, in the second embodiment, when the voltage of the capacitor C500 is higher than the minimum output of the current sensors 731 to 735, electric current flows from the capacitor C500 into any of the operational amplifiers 531 to 535, and then the voltage of the capacitor C500 becomes equal to the minimum output of the current sensors 731 to 735. On the other hand, in the third embodiment, when the voltage of the capacitor C500 is lower than the minimum output of the current sensors 731 to 735, the capacitor _, C500 is charged by the power supply 550, and the voltage of the capacitor C500 becomes equal to the minimum output of the current sensors 731 to 735.

[0080] The thus configured voltage monitoring system 520 does not need to use switches, and it is sufficient to connect the diodes by wired-OR connection, so it is possible to simplify the configuration. In addition, the voltage monitoring system 520 may be formed of general-purpose components, so manufacturing is easy, and cost may be reduced. Moreover, there is no interference among the circuits that extract the outputs of the fuel cells FCl to FC5. Furthermore, it is not necessary to switch the connected states by switches unlike the first embodiment, so the voltage monitoring system 520 has a high response to fluctuations in voltages of the fuel cells FCl to FC5. In addition, the capacitor C500 is used, and it means that the voltage monitoring system 520, so to speak, includes an integration circuit. Thus, it is possible to output the minimum voltage by eliminating the influence of noise, or the like.

[0081] In addition, when a power supply independent of the fuel cell stack FC is used as the power supply of the current sensors 731 to 735, similar advantageous effects to those of the configuration in the second embodiment may be obtained. In addition, although not shown in the drawing, the configuration that detects the maximum value - instead of the minimum value or in addition to the minimum value is similar to that of the second embodiment. In addition, the configuration that detects the overall voltage of the fuel cell stack FC may be added to the configuration of the above described third embodiment as in the case of the first embodiment. D. Alternative Embodiments

[0082] Alternative embodiments to the above described embodiments will be described.

D- 1. First Alternative Embodiment

As is apparent from the above described embodiments, it is only necessary that the voltage monitoring system according to the aspect of the invention includes voltage transmitting means that are provided respectively for a plurality of fuel cells and that transmit the respective voltages of the fuel cells in a state where the voltages are insulated from actual voltages of the fuel cells; and a detecting circuit that compares the voltages of the plurality of fuel cells, transmitted by the voltage transmitting means, to detect at least one of a minimum value and a maximum value among the transmitted voltages. FIG. 9 shows the above configuration as a voltage monitoring system 820. As shown in the drawing, the voltage monitoring system 820 includes voltage transmitting means 831 to 835 that are connected respectively to the fuel cells FCl to FC5 and a detecting circuit 850 that is connected to the voltage transmitting means 831 to 835. [0083] The voltage transmitting means 831 to 835 correspond to the switch circuits 31 to 35 and the transformer 50 in the first embodiment, the operational amplifiers 531 to 535 in the second embodiment, and the resistors 771 to 775 and the current sensors 731 to 735 in the third embodiment. The detecting circuit 850 corresponds to the maximum/minimum value detecting circuit 60 in the first embodiment, and the minimum value detecting circuit 560 or the maximum value detecting circuit 660 in the second embodiment and the third embodiment.

[0084] The above voltage monitoring system 820 collects the voltages of the plurality of fuel cells FCl to FC5, transmitted by the voltage transmitting means 831 to 835 in an insulated manner, and outputs at least one of the minimum value and the maximum value among the collected voltages, so it is possible to reduce computing load for obtaining the minimum value or the maximum value. As a result, it is possible to simplify the configuration of the voltage monitoring system 820. Note that the configuration of the voltage transmitting means or the detecting circuit in the voltage monitoring system according to the aspect of the invention is not limited to the above described embodiments; the configuration may be replaced with an equivalent circuit, or the like, having an equivalent function. For example, the diodes 541 to 545 in the second embodiment may be replaced with transistors. D-2. Second Alternative Embodiment

[0085] In the above described first embodiment, the maximum voltage and minimum voltage of the fuel cell stack FC are detected by the single maximum/minimum value detecting circuit 60; instead, the maximum voltage and/or the minimum voltage may be detected in a stepwise manner. For example, as shown in FIG. 10, the maximum voltage and the minimum voltage may be detected by a two-stage configuration formed of first stage circuits 111 and 112 and a second stage circuit 121. The first stage circuits 111 and 112 respectively detect the maximum voltage and minimum voltage in a first group formed of the fuel cells FCl to FC4 and the maximum voltage and minimum voltage in a second group formed of the fuel cells FC5 to FC8. The first stage circuits 111 and 112 are formed of circuits that correspond to the switch circuits 31 to 35, the transformer 50 and the maximum/minimum value detecting circuit 60 described in the above embodiment. In addition, the second stage circuit 121 detects the maximum voltage and the minimum voltage on the basis of the outputs from the first stage circuits 111 and 112. The second stage circuit 121 has similar circuit configuration to those of the first stage circuits 111 and 112. Note that the other configuration is similar to that of the first embodiment, so the description thereof is omitted.

[0086] Alternatively, as shown in FIG. 11, the maximum voltage and the minimum voltage may be detected by a three-stage configuration formed of first stage circuits 211 to 214, second stage circuits 221 and 222 and a third stage circuit 231.

[0087] In this way, the voltage monitoring system 20 is formed of a multi-stage circuit configuration, and the Nth-stage (N is a positive integer) circuits are operated simultaneously. By so doing, it is possible to detect the maximum voltage and the minimum voltage in a further short period of time. Thus, even when the number of fuel cells that constitute the fuel cell stack FC is relatively large, it is possible to reduce a difference in time among the respective input voltages of the fuel cells, so it is possible to improve detection accuracy.

[0088] In addition, of course, the configuration shown in FIG. 10 or FIG. 11 may be applied not only to the first embodiment but also to the second embodiment or the third embodiment. When the above configuration is applied to the second embodiment, the first stage circuits 111 and 112 and the second stage circuit 121 may be formed of circuits that correspond to the operational amplifiers 531 to 535 and the minimum value detecting circuit 560. In addition, when the above configuration is applied to the third embodiment, the first stage circuits 111 and 112 and the second stage circuit 121 may be formed of circuits that correspond to, for example, the resistors 771 to 775, the current sensors 731 to 735 and the minimum value detecting circuit 560.

[0089] In this way, when the voltage monitoring system is formed of a multi-stage circuit configuration, it is possible to reduce the capacity of each component. For example, it is possible to reduce the power supply voltage of each operational amplifier. Thus, even when the number of fuel cells that constitute the fuel cell stack FC is large, components may be prepared within the range of a general specification capacity, so manufacturing is easy, and cost may be reduced.

[0090] In addition, when the voltage monitoring system is formed of a multi-stage circuit configuration, a reference potential may be provided in each fuel cell group. Alternatively, a reference potential may be provided for each of a predetermined number of fuel cell groups or a high-potential fuel cell group. For example, any of A to C points shown in FIG. 10 may be connected to a reference potential. By so doing, it is possible to reduce a difference in potential within a group with respect to a reference potential even in a high-potential fuel cell group, so a low-voltage component may be used, and cost may be reduced. In addition, when, within a fuel cell group, a reference potential is provided near the center of a row of fuel cells that constitute the group, for example, at B point in FIG. 10, it is possible to suppress a measurement error. D-3. Third Alternative Embodiment

[0091] In the above described second alternative embodiment, the first stage circuits 111 and 112, the second stage circuit 121, and the like, having the same circuit configuration are formed in multi-stage configuration. Instead, the configuration of the last-stage circuit may be formed of voltage transmitting means and a computing circuit that computes the maximum value or the minimum value using the voltages transmitted by the voltage transmitting means. FIG. 12 shows an embodiment of the above configuration. In this embodiment, the voltage transmitting means 141 and 142 are respectively connected to the first stage circuits 111 and 112, and the voltage transmitting means 141 and 142 are connected to a microcomputer 150. The voltage transmitting means 141 and 142 are equivalent to the above described voltage transmitting means 831 to 835. The microcomputer 150 uses the outputs of the first stage circuits 111 and 112, input via the voltage transmitting means 141 and 142, to compute the minimum value or the maximum value.. By so doing, in comparison with the case where the outputs of the fuel cells FCl to FC8 are used to compute the minimum value or the maximum value, it is possible to reduce the computing load of the microcomputer 150. D-4. Fourth Alternative Embodiment [0092] The minimum value detecting operation and the maximum value detecting operation described in the above embodiments may be performed intermittently. By so doing, in comparison with the case where those operations are performed continuously, it is possible to reduce power consumption of the fuel cell stack FC or the power supply 580. D-5. Fifth Alternative Embodiment

[0093] In the above described first embodiment, the minimum voltage and maximum voltage of the fuel cells FCl to FC5 are detected. Instead, it may be configured to detect only one of the minimum voltage and the maximum voltage. In addition, of course, the configuration for detecting the mean voltage of the fuel cells FCl to FC5 is not indispensable. D-6. Sixth Alternative Embodiment

[0094] The circuit configuration described in the above first embodiment is only illustrative. As long as the configuration includes switch circuits provided one by one for a plurality of fuel cells and connected to both anodes and cathodes of the plurality of fuel cells and a transformer that sequentially switches the connected states of the switch circuits to sequentially input the respective voltages of the fuel cells, and uses the output voltages of the transformer to detect the minimum voltage and/or maximum voltage of the fuel cells, of course, part of the circuit configuration described in the above first embodiment may be modified into another configuration or another configuration may be added. D-7. Seventh Alternative Embodiment

[0095] In the above first embodiment, the switch circuits are connected one by one to all the fuel cells that constitute the fuel cell stack; however, it is not necessary to provide a switch circuit for each fuel cell. Instead, it is only necessary that a switch circuit is provided for only a fuel cell that needs to be monitored. For example, when the voltages of fuel cells located downstream of fuel gas, which tend to be relatively low, need to be specifically monitored, switch circuits for fuel cells located upstream of fuel gas may be omitted or may be provided for every other fuel cells. In the second embodiment or the third embodiment as well, similarly, diodes or current sensors, which serve as voltage transmitting means, may be provided only for fuel cells that need to be monitored.

[0096] The embodiments of the invention are described above; however, the aspect of the invention is not limited to the above embodiments. Of course, the aspect of the invention may be implemented in various forms without departing from the scope of the invention. For example, the aspect of the invention is not limited to a polymer electrolyte fuel cell as described in the above embodiments; it may be applied to various fuel cells, such as a direct methanol fuel cell and a phosphoric acid fuel cell.