Patent Publication Number: US-2009233127-A1

Title: Fuel cell system

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel cell system including a fuel cell. 
     BACKGROUND ART 
     A fuel cell is expected to be applied to automobiles and cellular phones as a portable power source requiring no charge. However, an internal resistance of the fuel cell is high in an existing technique so that it is impossible to supply a large amount of power instantaneously. For this reason, an assisting power source such as an electrical double layer capacitor is used. In addition, an auxiliary power source such as a secondary battery is used, for supplying electric power during an initial operation of a device for conveying fuel and until an output of the fuel cell stabilizes. Accordingly, a control unit for combining the fuel cell, the assisting power source and the auxiliary power source is required, and a control method thereof is variously studied. 
     For example, although down sizing of a fuel cell power source is required in a personal computer using a type of a fuel cell which directly oxidizes methanol, the peak power required for the personal computer sometimes cannot be supplied when only a down sized fuel cell is used. As a counter measure for such problems, provision of a high-capacity capacitor to an output stage of a fuel cell in a fuel cell system is suggested in Japanese Laid Open Patent Application (JP-P2002-32154A). 
     Further, in a fuel cell, electric power cannot be supplied at start-up until fuel is distributed to the whole of fuel cell. In addition, predetermined electrical power cannot be outputted when a body of the fuel cell is not hot. As a result, a direct methanol type fuel cell needs a power source used for a pump to convey the fuel, a fun, and the like. For such problems, a provision of the auxiliary power source to a fuel cell system is suggested in Japanese Laid Open Patent Application (JP-A-Heisei 11-176454). 
     Considering portability, a battery part has to be reduced in its size and its weight in fields of portable equipment. Consequently, it is required to commoditize an auxiliary power source for the peak power and an auxiliary power source for supplying an initial power at time of start-up, and not to request a complex manipulation for a user for that purpose. 
     In Japanese Laid Open Patent Application (JP-P2004-152741A), a technique for controlling power supply to an external load in the direct methanol type fuel cell is suggested. In addition, in Japanese Laid Open Patent Application (JP-P2004-265787A), a technique for stably supplying power to a load in the direct methanol type fuel cell is suggested. 
       FIG. 1  is a block diagram showing a configuration of a fuel cell system in a related art in which an auxiliary power source is replaceable. As shown in  FIG. 1 , main components in the fuel cell system of the related art are a fuel cell  101 , an auxiliary power source pack  150 , a driving section  105 , a control circuit  110 , an auxiliary unit  108 , a charging circuit  104 , and a connector  106 . The connector  6  is a divisional type connector and the auxiliary power source pack  150  can be arbitrarily attached and detached. The auxiliary power source pack  150  is composed of a secondary battery  102  and a protecting circuit  107 , and supplies electrical power through the connector  106  for a shortage of power from only the fuel cell  101 . Since the secondary battery  102  used as the auxiliary power source has duration of life, the auxiliary power source pack  150  has to have a replaceable configuration. In consideration of safety in malfunction, the protecting circuit  107  is included in the pack  150 . 
     When the secondary battery  102  has only one system of power supply as shown in  FIG. 1 , if an output of the secondary battery  102  is cut down by the protecting circuit  107  because of some malfunctions, the following problems will occur. Specifically, the control circuit  110  may wholly stop in a condition that the protecting circuit  107  is activated and the fuel cell  101  is not able to supply electric power, for example, in the middle of start-up of the fuel cell  101 . 
     DISCLOSURE OF INVENTION 
     Accordingly, an object of the present invention is to provide a fuel cell system in which an auxiliary unit is arranged and which is able to realize an optimal control depending on a condition of a fuel cell when an auxiliary power source is required. 
     In addition, another object of the present invention is to provide a fuel cell system, which can supply electric power always from an auxiliary power source other than a fuel cell not so as to shut off a control circuit. 
     In one exemplary aspect of the present invention, a fuel cell system includes a fuel cell for generating electric power by using fuel; a first assisting power source; a protecting circuit connected to the first assisting power source and configured to detect a failure in the first assisting power source; an auxiliary unit configured to supply the fuel to the fuel cell; a control circuit configured to control operations of the fuel cell and the auxiliary unit; a first electric power converter configured to drive the control circuit by the electric power from the first assisting power source; and a first synthesizing section configured to synthesize a first electric power from the fuel cell and a second electric power from the first assisting power source to supply to a load. 
     Here, the first assisting power source is a secondary battery, and a charging circuit may be further provided between the fuel cell and the protecting circuit for charging the secondary battery based on electric power from the fuel cell. The control circuit controls the charging circuit based on an output of the fuel cell. 
     In addition, the first assisting power source may be a primary battery. 
     In addition, the fuel cell system may further include a dummy load circuit connected to the output of the fuel cell; and a first switch circuit connected to the output of the fuel cell. When it is determined based on a dummy load circuit that the output of the fuel cell is within a predetermined range, the control circuit turns on the first switch circuit. 
     In addition, the dummy load circuit may include a heater for heating the fuel cell; and a second switch circuit connected between the output of the fuel cell and the heater. The control circuit controls ON/OFF of the second switch circuit based on a temperature of the fuel cell. 
     In addition, the fuel cell system may further include a second electric power converter for performing an electric power conversion on electric power from the first assisting power source to supply a second electric power to the first synthesizing section; and a third switch circuit provided in parallel to the second electric power converter to supply electric power from the first assisting power source as the second electric power to the first synthesizing section. The control circuit turns on the third switch circuit and turns off the second electric power converter, in a first predetermined period, and turns off the third switch circuit and turns on the second electric power converter, in a period other than the first predetermined period. 
     In addition, the control circuit may supply the electric power to the auxiliary unit. Or, the fuel cell system may further include a third electric power converter connected to an output of the first synthesis to supply electric power to the auxiliary unit through an electric power conversion. 
     In addition, it is preferable that the first assisting power source, the protecting circuit, and the first electric power converter constitute an electric power source pack which is detachably connected through a connector. 
     In addition, the fuel cell system may further include a DC power source, a charging circuit provided between the fuel cell and the protecting circuit to charge the secondary battery based on electric power from the fuel cell; a third electric power converter connected to an output of the first synthesizing section to supply electric power to the auxiliary unit through electric power conversion; a second synthesizing section connected to an input of the first synthesizing section and the DC power source to synthesize electric power from the fuel cell and electric power from the DC power source to supply to the charging circuit; and a third synthesizing section connected to the output of the first synthesizing section and the DC power source to synthesize electric power from the fuel cell and electric power from the direct current power source to supply to the third electric power converter. Or, the fuel cell system may further include the third electric power converter connected to the DC power source and the output of the first synthesizing section to supply electric power to the auxiliary unit through electric power conversion; and a third synthesizing section connected to the output of the first synthesizing section and the DC power source to synthesize electric power from the fuel cell and electric power from the direct current power source to supply to the third electric power converter. 
     In another aspect of the present invention, a control method of a fuel cell system may be achieved by: turning on a control circuit by a first assisting power source, by supplying electric power to a load from the first assisting power source under a control of the control circuit; by turning on the fuel cell in response to a cell-on instruction; by driving the auxiliary unit to supply fuel to the fuel cell; by determining whether an operation of the fuel cell is normal or not; by stopping a supply of electric power from the first assisting power source when it is determined that the operation of the fuel cell is normal; and by supplying electric power to the load from the fuel cell. 
     Here, a first fuel cell is a secondary battery, and the control method of the fuel cell system may be achieved further by charging the first assisting power source by using the charging circuit when it is determined by the control circuit that there is a margin in electric power supplied to the load from the fuel cell. 
     In addition, the control method of the fuel cell system may be achieved by determining whether the first assisting power source is fully charged or not in response to a cell-off instruction; by fully charging the first assisting power source when being not fully charged; by separating the fuel cell from the load; and stopping supply of fuel to the fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a fuel cell system in a related art; 
         FIG. 2  is a block diagram showing a configuration of a fuel cell system according to a first exemplary embodiment of the present invention; 
         FIG. 3  is a block diagram showing a configuration of the fuel cell system according to a second exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart showing an operation of the fuel cell system according to the first exemplary embodiment of the present invention; 
         FIG. 5  is a block diagram showing a configuration of the fuel cell system according to a third exemplary embodiment of the present invention; 
         FIG. 6  is a block diagram showing a configuration of the fuel cell system according to a fourth exemplary embodiment of the present invention; and 
         FIG. 7  is a block diagram showing a configuration of the fuel cell system according to a fifth exemplary embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, a fuel cell system of the present invention will be described in detail with reference to the attached drawings. 
     First Exemplary Embodiment 
       FIG. 2  is a block diagram showing a configuration of the fuel cell system according to a first exemplary embodiment of the present invention. Referring to  FIG. 2 , the fuel cell system of the first exemplary embodiment includes a fuel cell  1 , a secondary battery  2 , and a DC power source  3  as power sources. An output of the fuel cell  1  is connected to a dummy load circuit  6  and is connected to an information equipment through a first switch circuit  11 , a distributing circuit  12 , a first synthesizing circuit  13 , and an output terminal. In addition, the output of the fuel cell  1  is connected to a third synthesizing circuit  17  through the first synthesizing circuit  13 . The distributing circuit  12  supplies a part of the output of the fuel cell  1  to a second synthesizing circuit  14 . An output of the DC power source  3  is connected to the second synthesizing circuit  14  and the third synthesizing circuit  17 . An output of the third synthesizing circuit  17  is connected to an auxiliary unit  8  through a second DC/DC converter circuit  18 . An output of the second synthesizing circuit  14  is connected to a charging circuit  4 . An output of the secondary battery  2  is connected to a third DC/DC converter  19  and an output of the third DC/DC converter  19  is connected to the control circuit  10 . Furthermore, the output of the second cell  2  is outputted through a protecting circuit  7 . Both of outputs of the charging circuit  4  and the protecting circuit  7  are connected to the first synthesizing circuit  13  through a second switch circuit  15  and a first DC/DC converter circuit  16  which are connected in parallel. 
     In the fuel cell system of the first exemplary embodiment, the fuel cell  1  is a direct methanol type fuel cell. Also, although the secondary battery  2  is, for example, a lithium-ion secondary battery, it is not limited to this. The DC power source  3  may be omitted in general, but it can operate to supplement the secondary battery  2  when an AC adapter for a personal computer as a load is used as the DC power source, and can allow a charging operation to the secondary battery  2  and an operation of the auxiliary unit  8  by the AC adapter. Each of the first to third synthesizing circuits  13 ,  14 , and  17  is a circuit for synthesizing two or more inputs to output a synthesis result. Each of the first DC/DC converter circuit  16 , the second DC/DC converter circuit  17 , and the third DC/DC converter circuit  119  is a circuit for converting an input voltage to supply an output voltage. The auxiliary unit  8  is, for example, an electric fan for conveying fuel to the fuel cell  1 . 
     Based on an output from the secondary battery  2 , the third DC/DC converter circuit  19  generates a conversion voltage to output to the control circuit  10 . As a result, the control circuit  10  can operate. That is to say, the control circuit  15  is always in an operating state regardless of an operating condition of the fuel cell  1 . The control circuit  10  controls operations of respective sections in the fuel cell system. For this purpose, the control circuit  10  may store data regarding values of a voltage and a current in the respective sections and regarding an operation of the auxiliary unit  8 . 
     In this state, under the control of the control circuit  10 , electric power is supplied from the fuel cell  1  and the DC power source  3  to the auxiliary unit  8  through the third synthesizing circuit  17  and the second DC/DC converter circuit  18 , and the auxiliary unit  8  starts its operation. The auxiliary unit  8  is a fan and accordingly the fuel is distributed throughout the fuel cell  1 . The fuel cell  1  starts electric power generation by using the fuel and oxidizing agent. 
     When the fuel is supplied and the fuel cell  1  starts electric power generation, the generated electric power is supplied. The dummy load circuit  6  is a pseudo load, and the electric power is supplied from the fuel cell  1  to the dummy load circuit  6  on trial before actually driving a normal load such as information equipment and the like. A value of electric current flowing supplied to the dummy load circuit  6  at this moment and a value of output voltage of the fuel cell  1  are sent to the control circuit  10 . The control circuit  10  controls ON/OFF of the first switch circuit  11  based on these values. The control circuit  10  drives the dummy load circuit  6  in a state that the first switch circuit  11  is turned OFF, and determines a state of the fuel cell  1  based on the data of an output voltage and an output current of the fuel cell  1 . When the fuel cell  1  can supply sufficient electric power to the dummy load circuit  6 , the control circuit  10  turns ON the first switch circuit  11 , and supplies the electric power from the fuel cell  1  to the first synthesizing circuit  13 . The second switch circuit  15  and the first DC/DC converter circuit  16  are connected in parallel, and connected to the first synthesizing circuit  5 . In such a manner, the electric power synthesized from the electric power of the secondary battery  2  and the electric power of the fuel cell  1  is supplied to a load such as an information equipment. 
     The dummy circuit  6  may operate as a constant current circuit acting so as to hold the output current of the fuel cell  1  constant. In this case, the dummy load circuit  6  is used for determining whether the fuel cell  1  normally started up or not. The dummy load circuit  6  may be a simple resistance load, and may be a circuit thermally connected with the fuel cell  1 . For example, the dummy load circuit  6  may be a heater for warming the fuel cell, but is not limited to the heater. In addition, by detecting data showing a temperature of the fuel cell  1  from a value of an output current of the dummy load circuit  6 , the control circuit  10  may control a value of the output current of the fuel cell  1  depending on temperature data. As described above, when the dummy load circuit  6  is a constant current circuit, controlling of the output current of the fuel cell  1  is easy. 
     Here, while the fuel cell  1  cannot generate a sufficient power because being still in the course of start-up, the control circuit  10  makes the second switch circuit  15  turn ON, and supplies a voltage of the secondary battery  1  to the first synthesizing circuit  13  at almost 100%. On the contrary, when an operation of the fuel cell  1  is stabilized, the control circuit  10  makes the second switch circuit  9  turn OFF, operates the first DC/DC converter circuit  16 , and supplies a voltage which is made by lowering the voltage of the secondary battery  1  through conversion for the first synthesizing circuit  5 . 
     In addition, the fuel cell system according to the first exemplary embodiment of the present invention includes the charging circuit  4  which supplies a charging current to the secondary battery  2  based on the output voltage and the output current of the fuel cell  1  in response to an instruction from the control circuit  10 . If the fuel cell  1  has a margin in the output power, the control circuit  10  controls the charging circuit  4  to charge the secondary battery  2  through the protecting circuit  7 . The control circuit  10  determines whether the fuel cell  1  has such a margin or not based on the output voltage and the output current of the fuel cell  1 . In accordance with an instruction from the control circuit  10 , the charging circuit  4  generates charging power from an output synthesized from an output of the second synthesizing circuit  14 , that is, an output from the fuel cell  1 , and an output from the DC power source  3 , to charge the secondary battery  2 . Further, since a normal load such as an information equipment requires electric power when the output current of the fuel cell  1  is equal to or more than a predetermined value, the control circuit  10  controls the charging circuit  4  to reduce a charging current for charging the secondary battery  2  in order to primarily supply electric power to an outer load. At this time, the control circuit  10  detects the output voltage and the output current of the fuel cell  1  to control a charging current of the charging circuit  4 . Specifically, when the output voltage of the fuel cell  1  is equal to or less than a predetermined value (for example, 4.0 V or less), the charging circuit  4  is controlled to reduce the charging current. In addition, when the output voltage of the fuel cell  1  to an information equipment (an outer load) is equal to or more than a predetermined value, the charging circuit  6  is controlled to reduce the charging current. 
     Second Exemplary Embodiment 
       FIG. 3  is a block diagram showing a configuration of the fuel cell system according to a second exemplary embodiment of the present invention. A configuration of the second exemplary embodiment is the same as that of the first exemplary embodiment. Accordingly, only different points will be described. 
     Referring to  FIG. 3 , in the fuel cell system of the second exemplary embodiment, the dummy load circuit  6  is replaced by a switch  21  and a load  22 . The load  22  is a heater for warming a fuel cell  1 . By detecting data showing the temperature of the fuel cell  1  from a value of a current of the load  22 , the control circuit  10  controls a value of an output current of the fuel cell  1  depending on temperature data. The distributing circuit  12  is omitted in the second exemplary embodiment. In addition, the first synthesizing circuit  13 , the second synthesizing circuit  14 , and the third synthesizing circuit  17  are realized by an OR connection of outputs of two diodes, respectively. 
     The diode may be composed of a schottky barrier diode, a transistor, or an FET depending on an applied load and an application field. Furthermore, instead of diodes, any rectifier circuit may be used, which limits a direction of an electric current flow. As described above, usage of a schottky barrier diode, a transistor, or a FET is intended to reduce losses accompanied by the electric current since forward voltage drop is small. 
     In this way, an operation of the fuel cell system of the second exemplary embodiment is also the same as that of the first exemplary embodiment. 
       FIG. 4  is a flowchart showing an operation of the fuel cell system of the present invention. In the fuel cell system of the present invention, a personal computer is a load, and an output terminal of the fuel cell system is connected to the personal computer. 
     A procedure for starting up the fuel cell system of the present invention will be described. In starting of the fuel cell system, the first switch circuit  11  is turned OFF, the third DC/DC converter circuit  19  is turned ON, and the control circuit  10  is in an operating state. 
     At first, at a step S 2 , the second switch circuit  15  is turned ON, the first DC/DC converter circuit  16  is turned OFF, and the electric power from the secondary battery  2  is supplied to the first synthesizing circuit  13  through the second switch circuit  15  and then is supplied to a personal computer as a load. 
     Next, at a step S 4 , a cell-on instruction is send from a key board of the personal computer to the control circuit  10  to turn on the power source of the fuel cell system of the present invention. At a step S 6 , the control circuit  10  turns on the second DC/DC converter circuit  18  in response to the cell-on instruction, to start an operation of the auxiliary unit  8 . In this manner, supplying of fuel to the fuel cell  1  starts, and an operation of the fuel cell  1  starts. At a step S 8 , the dummy load circuit  6  also operates at the same time. 
     Thus, supply of electric power from the fuel cell  1  to the dummy load circuit  6  is performed. When the control circuit  10  determines that the fuel cell  1  has a problem based on data from the dummy load circuit  6 , an alarm is outputted so as to stop a function of the auxiliary unit  8  at a step S 10 . If there is no problem in an electric power supply from the fuel cell  1  to the dummy load circuit  6 , the control circuit  10  turns off the second switch circuit  15 , turns on the first DC/DC converter circuit  16 , and stops a part of supply of electric power from the secondary battery  2  to the personal computer. Thus, only a part of electric power is supplied to the personal computer. At the same time, the control circuit  10  turns on the first switch circuit  11  at a step S 14 . As a result, the supply of electric power from the fuel cell  1  to the personal computer starts. 
     According to the above-mentioned procedure, the fuel cell system performs a normal operation. The control circuit  10  determines whether there is a margin in an output of the fuel cell  1  or not at a step S 16 . If there is a margin, the control circuit  10  controls the charging circuit  4  to charge the secondary battery  2  through the distributing circuit  12 , the second synthesizing circuit  14 , and the charging circuit  4 . 
     Next, an operation for stopping the fuel cell system will be described. First, an instruction is sent from the keyboard of the personal computer to the control circuit  10  at a step S 18  to turn off the fuel cell system of the present invention. In response to this cell-off instruction, the control circuit  10  determines whether the secondary battery  2  is in a fully charged state or not at a step S 20 . In case where the secondary battery  2  in not in a fully charged state, the control circuit  10  continuously performs charging to the secondary battery  2  through the distributing circuit  12 , the second synthesizing circuit  14 , and the charging circuit  4  at a step S 22 . When the secondary battery  2  becomes a fully charged state at a step S 24 , the control circuit  10  turns off the first switch circuit  11 , and sets an operation of the fuel cell  1  to a cooling down state. At a step S 26 , the control circuit  10  turns off the second DC/DC converter circuit  18  in termination of the cooling down state of the fuel cell  1 . At the same time, the control circuit  10  turns off the auxiliary unit  8 . Subsequently, at a step S 28 , the control circuit  10  turns off the second switch circuit  15 , and turns off the first DC/DC converter circuit  16 . 
     As clearly understood from the above description, the control circuit  10  is always in an operating state regardless of an operating condition of the fuel cell  1 , and controls the system normally. 
     Third Exemplary Embodiment 
     Next, the fuel cell system according to a third exemplary embodiment of the present invention will be described. Referring to  FIG. 5 , the fuel cell system of the third exemplary embodiment includes the fuel cell  1 , a synthesizing circuit  30 , the auxiliary unit  8 , and the control circuit  10 . An assisting power source pack  50  is connected to the fuel cell system through a connector  60 . The synthesizing circuit  30  is formed of diodes, and synthesizes the electric power from the fuel cell  1  and the electric power from the assisting power source pack  50  to supply to a load such as a personal computer. The assisting power source pack  50  includes a primary battery  5 , the protecting circuit  7 , and a DC/DC converter  13 . 
     The fuel cell system of the present exemplary embodiment is a system employing a fuel cell as a power source. The direct methanol type fuel cell is suitable for the fuel cell  1 , but the present invention is not limited to this. The assisting power source pack  50  is connected detachably to the connector  60 . As the protecting circuit  7 , an electric current sensor for detecting an over-current or a temperature sensor is used. When an output of the fuel cell  1  is less than the required electric power, a shortage of electrical power is supplied from the primary cell  5  of the assisting power source pack  50 . In addition, the assisting power source pack  50  performs an ON/OFF control of an output voltage by determining whether an output voltage is lower than a predetermined threshold or not. In addition, a result of the ON/OFF control to the control circuit  10  is sent. Furthermore, the control circuit  10  can perform the ON/OFF control to an output of the assisting power source pack  50 . 
     In the fuel cell system according to the third exemplary embodiment of the present invention, a primary battery  5  having the protecting circuit  7  is used for the assisting power source pack  50 . In case of the direct methanol type fuel cell  1 , it is required to operate the auxiliary unit  8  such as a pump, and the electric power to be used for control thereof is supplied to the control circuit  10  from the DC/DC converter  13  of the assisting power source pack  50 . Thus, even when the protecting circuit  7  operates based on an over-current and an output from the assisting power source pack  50  is turned off, the control circuit  10  can operate, and an appropriate processing can be performed even if in an abnormal value. 
     In addition, in the fuel cell system according to the third exemplary embodiment of the present invention, the electric power from the fuel cell  1  and the electric power from the primary cell  5  are synthesized by the synthesizing circuit  30 , and the synthesis resultant power is supplied to a load such as a personal computer. Furthermore, the synthesis resultant power is supplied to the control circuit  10  from the DC/DC converter  13  through the connecter  60 . According to a configuration of  FIG. 5 , even when the protecting circuit  7  operates based on an over-current and electric power from the assisting power source pack  50  to the synthesizing circuit  30  is turned off, the control circuit  10  can operate by using the DC/DC converter  13 , and an appropriate processing can be performed. 
     Fourth Exemplary Embodiment 
     Next, the fuel cell system according to a fourth exemplary embodiment of the present invention will be described.  FIG. 6  is a block diagram showing a configuration of the fuel cell system of the fourth exemplary embodiment. Referring to  FIG. 6 , the fuel cell system according to the fourth exemplary embodiment has the same configuration as that of the fuel cell system of the third exemplary embodiment. Accordingly, only different points will be described. In the fourth exemplary embodiment, an assisting power source pack  50 ′ is used instead of the assisting power source pack  50 . In the assisting power source pack  50 ′, a lithium-ion secondary battery  2 ′ is used instead of the primary cell  5 . Other components are the same as those of the assisting power source pack  50 . In addition, in the fourth exemplary embodiment, the charging circuit  4  is provided between an output of the fuel cell  1  and an input on the assisting power source pack  50 ′ side of the synthesizing circuit  30  in order to charge the secondary battery  2  in the assisting power source pack  50 ′. The charging circuit  4  is controlled by the control circuit  10 . Furthermore, electric power of the auxiliary unit  8  is supplied from an output of the synthesizing circuit  30  through the DC/DC converter circuit  18 . The DC/DC converter circuit  18  is also controlled by the control circuit  10 . 
     Referring to  FIG. 6 , the electric power from the fuel cell  1  and the electric power from the secondary battery  2  are synthesized by the synthesizing circuit  30 , and the synthesis resultant power is supplied to a load such as a personal computer. In addition, in the fuel cell system of the fourth exemplary embodiment, the secondary battery  2  can be always charged from the fuel cell  1  through the charging circuit  11  and the protecting circuit  7 . Since the auxiliary unit  8  such as a pump is considered to consume relatively large electric power, the electric power for controlling the auxiliary unit  8  is supplied from the fuel cell  1  through the DC/DC converter  13 . 
     The electric power is supplied from the secondary battery  2  to the synthesizing circuit  30  through the protecting circuit  7  and the connector  60 , and the synthesis electric power is supplied to the control circuit  10  from the DC/DC converter  13  through the connector  60 . According to a configuration of  FIG. 6 , even when the protecting circuit  7  operates based on an over-current and the electric power from the assisting power source pack  50 ′ to the synthesizing circuit  30  is turned off, the control circuit  10  can operate based on the electric power from the DC/DC converter  13 , and an appropriate processing can be performed even if in an abnormal state. 
     Fifth Exemplary Embodiment 
     Next, the fuel cell system according to a fifth exemplary embodiment of the present invention will be described.  FIG. 7  is a block diagram showing a configuration of the fuel cell system according to the fifth exemplary embodiment. Referring to  FIG. 7 , the fuel cell system according to the fifth exemplary embodiment has the same circuit configuration as that of the fourth exemplary embodiment. That is to say, the fuel cell system of the fifth exemplary embodiment includes the fuel cell  1 , the charging circuit  4 , the synthesizing circuit  30 , the DC/DC converter circuit  18 , the auxiliary unit  8 , the switch circuit  32 , and the control circuit  10 . An assisting power source pack  50 ″ is connected to the fuel cell system through the connector  60 . The synthesizing circuit  30  is formed of diodes, and synthesizes the electric power from the fuel cell  1  and the electric power from the assisting power source pack  50 ″ to supply to a load such as a personal computer. The assisting power source pack  50 ″ includes a lithium-ion secondary battery  2 ′, the protecting circuit  7 , the DC/DC converter  13 , and a DC/DC converter  33 . 
     The secondary battery  2 ′ is connected to the synthesizing circuit  30  through the protecting circuit  7  and the connector  60 . An output of the synthesizing circuit  30  is connected to the DC/DC converter circuit  18  in addition to a load. The DC/DC converter circuit  18  converts the electric power outputted from the synthesizing circuit  30  to supply to the auxiliary unit  8 . In addition, the charging circuit  4  is provided between an output of the fuel cell  1  and an input on the assisting power source pack  50 ″ side of the synthesizing circuit  30 , and always charges the secondary battery  2 ′. The DC/DC converter circuit  19  converts the electric power from the secondary battery  2 ′ to supply to the control circuit  10 . Thus, the control circuit  10  is in an operating state. The control circuit  10  receives data from the protecting circuit  7 , regarding an existence or a nonexistence of an output from the assisting power source pack  50 ″, and outputs an instruction to the protecting circuit  7  to control an output from the assisting power source pack  50 ″. The DC/DC converter  33  converts the electric power from the secondary battery  2 ′ to supply to the switch circuit  32 . The switch circuit  32  controls ON/Off of the DC/DC converter circuit  19  by being operated. Thus, an operation of the auxiliary unit  8  can be controlled by the control circuit  10 . 
     Referring to  FIG. 7 , the switch circuit  32  of the fuel cell system is provided to suppress the electric power consumption as low as possible when a user does not use a power source. An operation of the DC/DC converter circuit  19  is controlled through ON/OFF of the switch circuit  32 , thereby operation/stop of the control circuit  10  can be controlled. In this case, however, the DC/DC converter circuit  33  is required which is designed to drive the switch circuit  32  of the fuel cell system in low power consumption. The DC/DC converter circuit  33  is provided for the assisting power source pack  50 ″ to allow this electrical power to be taken out from the assisting power source pack  50 ″. 
     In case where it is required to shut down the assisting power source pack  50 ″ in safety depending on a state of the fuel cell system, for example, in case where there is a high risk that the assisting power source pack  50 ″ heats up through overcharging due to a trouble in the charging circuit  4 , a circuit for shutting down an output from the assisting power source pack  50 ″ is required. In the fifth exemplary embodiment, ON/OFF of the protecting circuit  7  can be controlled by the control circuit  10  since there is the protecting circuit  7  in the assisting power source pack  50 ″. In addition, in case where the control circuit  10  and the charging circuit  4  operate, the control circuit  10  cannot distinguish whether the protecting circuit  7  is ON or OFF. The control circuit  10  can know whether the protecting circuit  7  is ON or OFF, depending on a signal from the protecting circuit  7 . 
     It should be noted that the first to fifth exemplary embodiments described above can be combined in a range in which inconsistence is not caused. 
     As described above, according to the fuel cell system of the present invention, an optimum control can be performed depending on a state of the fuel cell in case where the auxiliary unit is arranged and an assisting power source is required. In addition, the electric power can be always supplied from the assisting power source other than the fuel cell so that a control circuit cannot be shut down.