Patent Publication Number: US-2015069950-A1

Title: Control system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a control system. 
     BACKGROUND ART 
     A battery is charged by power obtained utilizing renewable energy. In Patent Document 1 below, a technology whereby an electricity storage unit is charged by the power generated from a solar power generator and a wind force power generator is recited. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open No. 2009-232668 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     According to the technology recited in Patent Document 1, charging the electricity storage unit can be executed only by on-off control. There is a problem in which charging cannot be controlled in accordance with a change of the output from the solar power generator and the wind force power generator. 
     Therefore, an object of the present disclosure is to provide a control system whereby charging is controlled in accordance with the change of the output from the solar power generator and the wind force power generator. 
     Solutions to Problems 
     To achieve the above-described object, the present disclosure provides, for example, a control system, including a plurality of first devices and at least one second unit connected to each of the plurality of first devices, wherein 
     the first device includes a plurality of conversion units configured to convert first voltage supplied from a power generator to second voltage in accordance with a level of the first voltage, 
     the second unit includes a power storage unit and a charging control unit configured to control charging the power storage unit, 
     the second voltage output from at least one conversion unit out of the plurality of conversion units is supplied to the second device, and 
     the charging control unit controls charging the power storage unit in accordance with fluctuation of the second voltage. 
     Effects of the Invention 
     According to at least one embodiment, charging can be controlled in accordance with output from a solar power generator and a wind force power generator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an exemplary system configuration. 
         FIG. 2  is a diagram illustrating an exemplary connection between a control unit and a battery unit. 
         FIG. 3  is an explanatory diagram for a brief configuration of the control unit. 
         FIG. 4  is an explanatory diagram for the configuration of the control unit. 
         FIG. 5  is an explanatory diagram for a concrete configuration of a conversion unit. 
         FIG. 6  is an explanatory diagram for a configuration related to a power supply system of the control unit. 
         FIG. 7  is an explanatory diagram for a configuration of a battery unit. 
         FIG. 8  is an explanatory diagram for a concrete configuration of a charging control unit. 
         FIG. 9  is an explanatory diagram for a configuration related to a power supply system of the battery unit. 
         FIG. 10A  is a graph illustrating voltage-current characteristics of a solar cell.  FIG. 10B  is a graph (P-V curve) illustrating a relation between terminal voltage of the solar cell and generated power from the solar cell in the case where the voltage-current characteristics of the solar cell is indicated by a specific curve. 
         FIG. 11  is an explanatory diagram for changes of an operating point relative to changes of curves indicating the voltage-current characteristics of the solar cell. 
         FIG. 12A  is an explanatory diagram for changes of the operating point at the time of executing cooperative control in the case where illuminance to the solar cell is reduced.  FIG. 12B  is an explanatory diagram for changes of the operating point at the time of executing cooperative control in the case where load from the standpoint of the solar cell is increased. 
         FIG. 13  is an explanatory diagram for changes of the operating point at the time of executing cooperative control in the case where both illuminance to the solar cell and the load from the standpoint of the solar cell are changed. 
         FIG. 14  is a diagram illustrating an exemplary schedule table. 
         FIG. 15  is a diagram illustrating another exemplary schedule table. 
         FIG. 16  is a flowchart illustrating an exemplary processing flow. 
         FIG. 17  is a flowchart illustrating the exemplary processing flow. 
         FIG. 18  is an explanatory diagram for periods during which the conversion unit is actually turned on. 
         FIG. 19  is a diagram illustrating an exemplary schedule table in which maximum numbers of the conversion units to be turned on are described. 
         FIG. 20  is an explanatory diagram for a modified example. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     In the following, embodiments according to the present disclosure will be described with reference to the drawings. Note that description will be provided in the following order. 
     &lt;1. First Embodiment&gt; 
     &lt;2. Second Embodiment&gt; 
     &lt;3. Third Embodiment&gt; 
     &lt;4. Modified Example&gt; 
     Note that the embodiments described below are preferred examples of the present disclosure, and contents of the present disclosure are not to be limited to these embodiments. 
     1. First Embodiment 
     [1-1. System Configuration] 
       FIG. 1  is an exemplary system configuration according to a first embodiment of the present disclosure. For example, output from a plurality of power generators is supplied to a system  1 . For the power generators, a solar power generator, a wind force power generator, and a biomass power generator are exemplified. In  FIG. 1 , a solar power generator  3  is schematically illustrated by solar panels. A wind force power generator  4  is schematically illustrated by a windmill. A biomass power generator  5  is schematically illustrated by a tank and flame inside the tank. A known solar power generator can be applied as the solar power generator  3 . The same can be applied to the wind force power generator  4  and the biomass power generator  5 . 
     The power generator generates power based on energy existing in the surrounding environment, for example, light, heat, oscillation, radio wave, temperature difference, ion concentration difference, and so on. The power generator may be formed of a device that generates the power by grid-connected power (grid) and human power. Also, the power generator may be formed of a plurality of the same kinds of power generators. 
     Direct current (DC) voltage obtained from each of the power generators is supplied to a block in a latter stage. In the case where alternating current (AC) voltage is obtained from the power generator, the alternating current voltage is converted to the direct current voltage to be supplied to the block in the latter stage. The system  1  includes a plurality of blocks. The plurality of blocks is exemplified as a block BL 1 , a block BL 2 , and a block BL 3 . In the case where there is no need to identify the individual blocks, the block is conveniently referred to as a block BL. Note that the block is a term provided for convenience of explanation and does not have any special meaning. A configuration and the like of the block BL will be described below. 
     The block BL is connected to each of the power generators in parallel. Direct-current voltage V 3  supplied from the solar power generator  3  is supplied to the block BL 1 , block BL 2 , and block BL 3 . Direct-current voltage V 4  supplied from the wind force power generator  4  is supplied to the block BL 1 , block BL 2 , and block BL 3 . Direct-current voltage V 5  supplied from the biomass power generator  5  is supplied to the block BL 1 , block BL 2 , and block BL 3 . Each one of the voltage V 3 , voltage V 4 , and voltage V 5  is an example of first voltage. 
     Values of the voltage V 3 , voltage V 4 , and voltage V 5  may fluctuate in accordance with a device size and the like, but hereinafter, description will given for a case where the voltage V 3 , voltage V 4 , and voltage V 5  may fluctuate within a range from 75 V (volt) to 100 V. In  FIG. 1 , the voltage V 3  is indicated by a solid line, the voltage V 4  by a one-dot chain line, and the voltage V 5  by a two-dot chain line. 
     [1-2. Block Configuration] 
     An exemplary configuration of the block BL will be described by exemplifying the block BL 1 . The block BL 1  is configured to include, for example, one control unit and at least one battery unit. The control unit is an example of a first device, and the battery unit is an example of a second device. 
     A control unit CU 1  is connected to, for example, a battery unit BU 1   a , a battery unit BU 1   b , and a battery unit BU 1   c . In the case where there is no need to identify the individual battery units, the battery unit is conveniently referred to as a battery unit BU 1 . In  FIG. 1 , the battery unit BU 1   a  and the battery unit BU 1   b  are illustrated. 
     The control unit CU 1  includes, for example, a plurality of ports, and the battery unit BU 1  is freely detachably attached to each of the ports. In other words, the number of the battery units BU 1  to be connected to the control unit CU 1  can be suitably changed. For example, when the battery unit BU 1   a , battery unit BU 1   b , and battery unit BU 1   c  are connected to the control unit CU 1 , a new battery unit can be connected to the control unit CU 1 . For example, when the battery unit BU 1   a , battery unit BU 1   b , and battery unit BU 1   c  are connected to the control unit CU 1 , the battery unit BU 1   b  can be detached from the control unit CU 1 . 
     The battery unit BU 1  is connected to the control unit CU 1  via a line L 1 . As illustrated in  FIG. 2 , the line L 1  includes, for example, a power line L 10  through which power is transmitted from the control unit CU 1  to the battery unit BU 1 , and a power line L 11  through which power is transmitted from the battery unit BU 1  to the control unit CU 1 . The line L 1  further includes a signal line SL 12  for communication between the control unit CU 1  and each of the battery units BU 1 . 
     Meanwhile, in the following description, description will be given under the condition that power transmission and communication are executed via wire; however the power transmission and communication may be executed via radio, too. In this case, there is no need to provide a physical line, namely, the line L 1 . 
     Direct-current voltage V 10  is supplied from the control unit CU 1  to the battery unit BU 1  via the power line L 10 . Among the plurality of battery units BU 1 , charging is executed based the voltage V 10  toward a battery unit BU 1  to which a charging instruction is given. Charging may be executed for one battery unit BU 1  and also for the plurality of battery units BU 1 . 
     Charging is not executed for the battery unit BU 1  that is currently discharging. Direct current voltage V 11  is output from a battery unit BU 1  to which a discharging instruction has been given. The voltage V 11  is supplied to an external device, namely, a load, via the control unit CU 1 , for example. The voltage V 11  may also be directly supplied to the external device without passing the control unit CU 1 . 
     Communication between the control unit CU 1  and each of the battery units BU 1  is executed in accordance with the specifications such as SMBus (System Management Bus) and UART (Universal asynchronous Receiver-Transmitter). The signal line SL 12  is a line shared among the battery units BU 1 , and a control command is transmitted through the signal line SL 12 . For instance, the control command is transmitted from the control unit CU 1  to a predetermined battery unit BU 1 . 
     Each of the battery units BU 1  can be individually controlled by the control command. The battery unit BU 1  can identify a port number of a port to which the battery unit itself is connected. For instance, an identifier indicating the port number is described in a header of the control command. The battery unit BU 1  can identify whether the control command is directed to own unit or not by analyzing the header of the control command. 
     Further, the battery unit BU 1  can notify the control unit CU 1  of own information through communication. The battery unit BU 1  can notify the control unit CU 1  of, for example, residual capacity of a battery included in the battery unit BU 1 . The identifier indicating the port number is described in the header of a notification signal from the battery unit BU 1  to control unit CU 1 . This enables the control unit CU 1  to identify which one of the battery units BU 1  has transmitted the notification signal. 
     An application mode described below may be possible, using the plurality of battery units BU 1 , for example. A control command that provides a charging instruction is transmitted to the battery unit BU 1   a  from the control unit CU 1  to execute control for charging the battery unit BU 1   a . A control command that provides a discharging instruction is transmitted to the battery unit BU 1   b  from the control unit CU 1  to execute control for discharging from the battery unit BU 1   b . The battery unit BU 1   c  is used as a standby power source. For instance, when the residual capacity of the battery unit BU 1   b  is reduced, a using battery unit is switched from the battery unit BU 1   b  to the battery unit BU 1   c . The above-described application mode is only an example and the application mode is not limited thereto. 
     A configuration of the block BL 2  is, for example, same as the configuration of the block BL 1 . The block BL 2  is configured to include the control unit CU 2 . The control unit CU 2  is connected to, for example, a battery unit BU 2   a , a battery unit BU 2   b , and a battery unit BU 2   c  via a line L 2 . In  FIG. 1 , the battery unit BU 2   a  and battery unit BU 2   b  are illustrated. 
     The line L 2  includes, for example, a power line L 20  through which the power is transmitted from the control unit CU 2  to a battery unit BU 2 , and a power line L 21  through which the power is transmitted from the battery unit BU 2  to the control unit CU 2 . The line L 2  further includes a signal line SL 22  for communication between the control unit CU 2  and each of the battery units BU 2 . 
     A configuration of the block BL 3  is, for example, same as the configuration of the block BL 1 . The block BL 3  is configured to include the control unit CU 3 . The control unit CU 3  is connected to, for example, a battery unit BU 3   a , a battery unit BU 3   b , and a battery unit BU 3   c  via a line L 3 . In  FIG. 1 , the battery unit BU 3   a  and the battery unit BU 3   b  are illustrated. 
     The line L 3  includes, for example, a power line L 30  through which the power is transmitted from the control unit CU 3  to a battery unit BU 3 , and a power line L 31  through which the power is transmitted from the battery unit BU 3  to the control unit CU 3 . The line L 3  further includes a signal line SL 32  for communication between the control unit CU 3  and each of the battery units BU 3 . 
     Note that any constitutional difference may exist between the respective blocks BL without departing from the scope and sprit of the present disclosure. In the following description, repeating the same description may be avoided by mentioning that the configuration is the same; however, this does not preclude existence of the constitutional difference without departing from the scope and sprit of the present disclosure. 
     [1-3. Configuration of Control Unit] 
       FIG. 3  is an exemplary schematic configuration of the control unit CU 1 . The control unit CU 1  includes a conversion unit  100   a , a conversion unit  100   b , and a conversion unit  100   c . In the case where there is no need to identify the individual conversion units, the conversion unit is conveniently referred to as a conversion unit  100 . The voltage V 3  which is output voltage of the solar power generator  3  is supplied to the conversion unit  100   a . The conversion unit  100   a  converts the voltage V 3  to a voltage V 10  in accordance with a level of the voltage V 3 . As described above, the voltage V 3  is the voltage that fluctuates within a range, for example, from 75 V to 100 V. The voltage V 10  is the direct current voltage that fluctuates within a range, for example, from 45 V to 48 V. 
     When the voltage V 3  is 75 V, the conversion unit  100   a  converts the voltage V 3  such that the voltage V 10  becomes 45 V. When the voltage V 3  is 100 V, the conversion unit  100   a  converts the voltage V 3  such that the voltage V 10  becomes 48 V. In accordance with the change of the voltage V 3  within the range from 75 V to 100 V, the conversion unit  100   a  converts the voltage V 3  to the voltage V 10  such that the voltage V 10  substantially linearly changes in the range from 45 V to 48 V. Various kinds of feedback circuits may also be used instead of linearly changing a changing rate. An output obtained by the feedback circuit may be output from the conversion unit  100   a.    
     The conversion unit  100   b  and conversion unit  100   c  operate same as the conversion unit  100   a . When the voltage V 4  is 75 V, the conversion unit  100   b  converts the voltage V 4  such that the voltage V 10  becomes 45 V. When the voltage V 4  is 100 V, the conversion unit  100   a  converts the voltage V 4  such that the voltage V 10  becomes 48 V. In accordance with the change of the voltage V 4  within the range from 75 V to 100 V, the conversion unit  100   a  converts the voltage V 4  to the voltage V 10  such that the voltage V 10  substantially linearly changes in the range from 45 V to 48 V. Meanwhile, in the case where the voltage V 4  changes in the range from 200 V to 420 V, for example, the conversion unit  100   b  generates the voltage V 10  in the range from 45 V to 48 V by stepping down the voltage V 4 . Thus, each of the conversion units  100  is configured to suitably operate in accordance with the input voltage. 
     When the voltage V 5  is 75 V, the conversion unit  100   c  converts the voltage V 5  such that the voltage V 10  becomes 45 V. When the voltage V 5  is 100 V, the conversion unit  100   c  converts the voltage V 5  such that the voltage V 10  becomes 48 V. In accordance with the change of the voltage V 5  in the range from 75 V to 100 V, the conversion unit  100   a  converts the voltage V 5  such that the voltage V 10  substantially linearly changes in the range from 45 V to 48 V. Meanwhile, in the case where the voltage V 5  changes in the range from 10 V to 40 V, for example, the conversion unit  100   c  boosts the voltage V 5  to generate the voltage V 10  in the range from 45 V to 48 V. Thus, the conversion unit  100  is configured to suitably operate in accordance with the input voltage. 
     The voltage V 10  is output from each of the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c , and one of those is supplied to the battery unit BU 1  via the power line L 10 . For instance, the largest voltage V 10  is supplied to the battery unit BU 1  via the power line L 10 . In the case where power consumption in the battery unit BU 1  is large, the outputs from the plurality of conversion units may be combined and supplied to the battery unit BU 1 . 
     Note that the output to be supplied to the battery unit BU 1  can be selected from among the plurality of outputs from the plurality of conversion units  100 . As described below, a variable resistor (volume) is provided in each of the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c , for example. The voltage V 10  output from a predetermined conversion unit  100  can be supplied to the battery unit BU 1  by suitably setting a value of the variable resistor. 
       FIG. 4  is an exemplary schematic configuration of the control unit CU 1 . The conversion unit  100   a  of the control unit CU 1  includes a DC-DC converter  101   a  that converts (steps down) the voltage V 3  to the voltage V 10 . In the case where the voltage V 3  is lower than 45 V, for example, the DC-DC converter  101   a  is configured as a boost DC-DC converter. For a configuration of the DC-DC converter  101   a , a known configuration can be applied. Further, in the case where alternating current voltage is supplied as the voltage V 3 , an AC-DC converter may be provided in a former stage of the DC-DC converter  101   a.    
     A voltage sensor, an electronic switch, and a current sensor are connected to each of an input stage and an output stage of the DC-DC converter  101   a . Further, the variable resistor is connected to the output stage of the DC-DC converter  101   a . In  FIG. 4  and  FIG. 7  later described, the voltage sensor is indicated by a rectangular mark, the electronic switch by a circle mark, the current sensor by a circle mark with diagonal lines, and the variable resistor by a triangular mark, respectively in a simple manner. 
     A voltage sensor  101   b , an electronic switch  101   c , and a current sensor  101   d  are sequentially connected to the input stage of a DC-DC converter  101   a . A current sensor  101   e , an electronic switch  101   f , a voltage sensor  101   g , and a variable resistor  101   h  are sequentially connected to the output stage of the DC-DC converter  101   a.    
     The conversion unit  100   b  and the conversion unit  100   c  have the configuration same as the conversion unit  100   a , for example. The conversion unit  100   b  includes a DC-DC converter  102   a . A voltage sensor  102   b , an electronic switch  102   c , and a current sensor  102   d  are sequentially connected to the input stage of the DC-DC converter  102   a . A current sensor  102   e , and an electronic switch  102   f , a voltage sensor  102   g , and a variable resistor  102   h  are sequentially connected to the output stage of the DC-DC converter  102   a.    
     The conversion unit  100   c  includes a DC-DC converter  103   a . A voltage sensor  103   b , an electronic switch  103   c , and a current sensor  103   d  are sequentially connected to the input stage of the DC-DC converter  103   a . A current sensor  103   e , an electronic switch  103   f , a voltage sensor  103   g , and a variable resistor  103   h  are sequentially connected to the output voltage of the DC-DC converter  103   a . The output from the conversion unit  100  can be stopped by switching off the electronic switch in each of the conversion units  100 . For instance, the output from the conversion unit  100   a  can be stopped by switching off at least one of the electronic switch  101   c  and the electronic switch  101   f.    
     The resistance values of the variable resistor  101   h , variable resistor  102   h , and variable resistor  103   h  may be adjusted, instead of controlling the electronic switch. The output of the DC-DC converter  101   a  and the like can be limited by adjusting the resistance value of the variable resistor. For example, the resistance values of the variable resistor  102   h  and the variable resistor  103   h  are set to a predetermined value by setting the resistance value of the variable resistor  101   h  to zero or nearly zero. 
     The output voltage of the DC-DC converter  102   a  is stepped down by the variable resistor  102   h , and the output voltage of the DC-DC converter  103   a  is stepped down by the variable resistor  103   h . The voltage V 10  which is the largest voltage and output from the conversion unit  100   a  is supplied to the power line L 10 , and the voltage V 10  is supplied to the battery unit BU 1 . Thus, one of the outputs from the three conversion units (conversion unit  100   a , conversion unit  100   b , conversion unit  100   c ) can be selected by suitably adjusting the resistance values of the three variable resistors (variable resistor  101   h , variable resistor  102   h , variable resistor  103   h ). 
     The control unit CU 1  further includes a CPU (Central Processing Unit)  110 . A memory  111 , a D/A (Digital to Analog) converter  112 , an A/D (Analog to Digital) converter  113 , and a temperature sensor  114  are connected to the CPU  110  via a bus  115 . The bus  115  is formed of an I 2 C bus, for example. 
     The CPU  110  controls respective sections of the control unit CU 1 . The CPU  110  executes, for example, on-off control for the electronic switch of the conversion unit  100 , and executes control in the conversion unit  100  in accordance with sensor information supplied from the voltage sensor and current sensor. 
     Note that arrows indicating the voltage sensors and current sensors show that the sensor information obtained by the sensors is supplied to the CPU  110  via the A/D convertion unit  113 . Further, the arrows directed to the marks indicating the electronic switches and variable resistors show that the electronic switches and variable resistors are controlled by the CPU  110 . 
     The CPU  110  further controls the battery unit BU 1  connected to the control unit CU 1 . For instance, the CPU  110  generates a control command to turn on a power source of a predetermined battery unit BU 1 , and a control command that provides an instruction for charging or discharging with respect to a predetermined battery unit BU 1 . Then, the CPU  110  transmits the generated control command to the signal line SL 12 . Further, the CPU  10  acquires information transmitted from each of the battery units BU 1  (e.g., residual capacity in the battery of the battery unit BU 1 ), and executes control in accordance with the acquired information. 
     The memory  111  is a general term that represents a ROM (Read Only Memory) where programs executed by the CPU  110  are stored, a RAM (Random Access Memory) used as a work memory when the CPU  110  executes processing, a non-volatile memory such as an EEPROM (Electrically Erasable and Programmable Read Only Memory) where various kinds of data (e.g., schedule tables later described), and so on. 
     The D/A converter  112  converts digital data to analog data. The A/D convertion unit  113  converts the analog data to the digital data. The analog-data sensor information is supplied from, for example, the voltage sensor and current sensor to the A/D convertion unit  113 . The A/D convertion unit  113  converts the analog-data sensor information to digital-data sensor information. The digital-data sensor information is supplied to the CPU  110 . 
     The temperature sensor  114  measures ambient temperature. For instance, inner temperature of the control unit CU 1  and the ambient temperature of the control unit CU 1  are measured. Temperature information obtained by the temperature sensor  114  is supplied to the CPU  110  after being converted to the digital data by the A/D convertion unit  113 . 
     The control unit CU 1  may be configured to communicate with other device. For instance, the CPU  110  may be configured to have a communication function, and communication is held between the CPU  110  and the other device  118 . Examples of the other device  118  may be devices such as a personal computer (PC), a tablet computer, and a smartphone. 
     The communication may be executed via the internet and near field communication. Examples of the near field communication include communication using infrared ray, communication based on “ZigBee (registered trademark)” standard, communication based on “Bluetooth (registered trademark)” standard, and communication based on “Wi-Fi (registered trademark)” whereby network can be easily formed. 
       FIG. 5  is an example of concrete configuration of the conversion unit  100   a . As illustrated in  FIG. 5 , the conversion unit  100   a  includes a DC-DC converter  101   a  and a feedforward control system later described. In  FIG. 5 , the voltage sensor  101   b , electronic switch  101   c , current sensor  101   d , current sensor  101   e , electronic switch  101   f , voltage sensor  101   g , and variable resistor  101   h  are not illustrated. 
     The DC-DC converter  101   a  is formed of, for example, a primary circuit  121  including a switching device, a transformer  122 , and a secondary circuit  123  including a rectifying device. The DC-DC converter  101   a  illustrated in  FIG. 5  is a current resonance converter (LLC resonant converter), for example. 
     A feedforward control system includes an operational amplifier  124 , a transistor  125 , a resistance Rc 1 , a resistance Rc 2 , and a resistance Rc 3 , and output from the feedforward control system is received at, for example, a control terminal provided at a driver of the primary circuit  121  of the DC-DC converter  101   a . The DC-DC converter  101   a  adjusts output voltage from the conversion unit  100   a  such that input voltage to the control terminal becomes constant. 
     Since the feedforward control system is provided at the conversion unit  100   a , the voltage V 10  which is the output voltage of the conversion unit  100   a  is adjusted to be a voltage value within a preset range. Therefore, the control unit CU 1  including the conversion unit  100   a  includes, for example, a function of a voltage converter that changes the output voltage (voltage V 10 ) in accordance with the change of the input voltage (voltage V 3 ) from the solar power generator  3 . 
     As illustrated in  FIG. 5 , the output voltage is taken out from the conversion unit  100   a  via the primary circuit  121 , transformer  122 , and secondary circuit  123 . The output from the control unit CU 1  is transmitted to the battery unit BU 1  via the power line L 10 . Note that the AC-DC converter is connected to a former stage of the primary circuit  121  in the case where the voltage V 3  is the alternating current voltage. The AC-DC converter is, for example, a power factor correction circuit. 
     In the following, the feedforward control system included in the conversion unit  100   a  will be described. 
     The voltage obtained by multiplying the input voltage (voltage V 3 ) to the conversion unit  100   a  by kc (kc: approximately a several tenths to a hundredth) is received at a non-inverted input terminal of the operational amplifier  124 . On the other hand, the voltage obtained by multiplying predetermined constant voltage Vt 0  by kc is received at an inverted output terminal c 1  of the operational amplifier  124 . The input voltage (kc×Vt 0 ) to the inverted output terminal c 1  of the operational amplifier  124  is applied to, for example, from the D/A converter  112 . A value of the voltage Vt 0  is held in, for example, an embedded memory in the D/A converter  112 , and the value of voltage Vt 0  can be changed, if necessary. The value of the voltage Vt 0  is held in the memory  111  connected to the CPU  110  via the bus  115 , and may be transferred to the D/A converter  112 . The value of the voltage Vt 0  may be a fixed value. 
     An output terminal of the operational amplifier  124  is connected to a base of the transistor  125 , and current-voltage conversion is executed by the transistor  125  in accordance with a difference between the input voltage received at the non-inverted input terminal and the input voltage received at the inverted output terminal in the operational amplifier  124 . 
     A resistance value of the resistance Rc 2  connected to an emitter of the transistor  125  is set so as to be larger than a resistance value of the resistance Rc 2  connected in parallel with the resistance Rc 1 . 
     For instance, it is assumed that the input voltage to the conversion unit  100   a  is the voltage sufficiently higher than the preset constant value Vt 0 . At this point, the transistor  125  in turned on and a combined resistance value of the resistance Rc 1  and resistance Rc 2  becomes smaller than a resistance value of the resistance Rc 1 . Therefore, electrical potential at f point illustrated in  FIG. 5  becomes close to ground potential. 
     Then, the input voltage to the control terminal connected via a photocoupler  126  and provided at the driver of the primary circuit  121  is decreased. The DC-DC converter  101   a  that has detected decrease of the input voltage to the control terminal boosts the output voltage from the conversion unit  100   a  such that the input voltage to the control terminal becomes constant. 
     In contrast, it is assumed that the terminal voltage of the solar cell connected to the control unit CU 1  is decreased and the input voltage to the conversion unit  100   a  becomes close to the preset constant voltage Vt 0 , for example. 
     When the input voltage to the conversion unit  100   a  is decreased, the transistor  125  becomes almost an on state from an off state. Due to this change of the transistor  125  from the on state to the off state, current hardly flows at the resistance Rc 1  and resistance Rc 2 , thereby increasing the electrical potential at the f point illustrated in  FIG. 5 . 
     Then, the input voltage to the control terminal provided at the driver of the primary circuit  121  cannot be kept constant. As a result, the DC-DC converter  101   a  steps down the output voltage from the conversion unit  100   a  such that the input voltage to the control terminal is kept constant. 
     In other words, the conversion unit  100   a  boosts the output voltage in the case where the input voltage is sufficiently higher than the preset constant voltage Vt 0 . Further, the conversion unit  100   a  steps down the output voltage when the terminal voltage of the solar cell is decreased and the input voltage becomes close to the preset constant voltage Vt 0 . Thus, the control unit CU 1  including the conversion unit  100   a  dynamically changes the output voltage in accordance with the level of the input voltage. 
     Further, as described below, the conversion unit  100   a  dynamically converts the output voltage in accordance with changes of the voltage required on the output side of the control unit CU 1 . 
     For instance, it is assumed that the number of the battery units BU 1  to be charged and electrically connected to the control unit CU 1  is increased while the solar power generator  3  is generating power. In other words, it is assumed that a load from the standpoint of the solar power generator  3  is increased. 
     In this case, the terminal voltage of the solar cell connected to the control unit CU 1  is decreased because of the battery unit BU 1  newly electrically connected to the control unit CU 1 . Then, a state of the transistor  125  becomes the off state from the on state due to the decrease of the input voltage to the conversion unit  100   a , thereby stepping down the output voltage from the conversion unit  100   a.    
     On the other hand, when the number of the battery units BU 1  to be charged and electrically connected to the control unit CU 1  is reduced while the solar power generator  3  is generating power, the load from the standpoint of the solar cell in the solar power generator  3  is reduced, and the terminal voltage of the solar cell connected to the control unit CU 1  is increased. When the input voltage to the conversion unit  100   a  becomes sufficiently higher than the present constant voltage Vt 0 , the input voltage to the control terminal provided at the driver of the primary circuit  121  is decreased and the output voltage from the conversion unit  100   a  is boosted. 
     Meanwhile, the resistance values of the resistance Rc 1 , resistance Rc 2 , and resistance Rc 3  are suitably selected such that a value of the output voltage from the conversion unit  100   a  becomes a voltage value within the preset range. In other words, an upper limit of the output voltage from the conversion unit  100   a  is determined by the resistance values of the resistance Rc 1  and the resistance Rc 2 . The transistor  125  is disposed in order that the value of the output voltage from the conversion unit  100   a  does not exceed the preset upper limit voltage value when the input voltage to the conversion unit  100   a  exceeds the predetermined value. 
     On the other hand, as described later, a lower limit of the output voltage from the conversion unit  100   a  is determined by the input voltage to the inverted output terminal of the operational amplifier of the feedforward control system in the charging control unit in the battery unit BU 1 . 
     Configurations of the conversion unit  100   b  and the conversion unit  100   c  are same as the configuration of the conversion unit  100   a , for example. The conversion unit  100   b  and the conversion unit  100   c  operate same as the conversion unit  100   a , for example. 
     A power source of each of the control unit CU 1 , control unit CU 2 , and control unit CU 3  can be independently turned on/off.  FIG. 6  is a diagram illustrating mainly an exemplary configuration of the power supply system of the control unit CU 1 . 
     A backflow prevention diode  130   a  is connected to the output stage of the conversion unit  100   a . A backflow prevention diode  130   b  is connected to the output stage of the conversion unit  100   b . A backflow prevention diode  130   c  is connected to the output stage of the conversion unit  100   c . The conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c  are OR connected by the diode  130   a , diode  130   b , and diode  130   c  in parallel. 
     The outputs from the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c  are combined and supplied to the battery unit BU 1 . Actually, the output having the highest voltage among the outputs from the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c  is supplied to the battery unit BU 1 . Note that the outputs from the plurality of conversion units  100  may be supplied in accordance with the power consumption in the battery unit BU 1 . 
     A main switch SW 1  that can be operated by a user is provided at the control unit CU 1 . When the main switch SW 1  is turned on, the power is supplied to the CPU  110 , thereby starting the control unit CU 1 . The main switch SW 1  may be operated by remote control such as on-off switching operation by a remote control device. 
     For example, the power from a battery  133  incorporated inside the control unit CU 1  is supplied to the CPU  110 . The battery  133  is, for example, a lithium ion secondary battery. The direct current voltage supplied from the battery  133  is converted to voltage corresponding to the CPU  110  by a DC-DC converter  134 . The converted voltage is supplied to the CPU  110  as power supply voltage. The battery  133  is used at the time of starting the control unit CU 1 . The battery  133  is controlled, for example, by the CPU  110  (e.g., for charging and discharging). 
     The battery  133  can be charged based on the voltage supplied from the battery unit BU 1 , for example. The battery  133  may also be charged based on the voltage supplied from the conversion unit  100   a , the conversion unit  100   b , and so on. 
     For instance, the voltage V 11  supplied from the battery unit BU 1   a  is supplied to a charging control unit  135 . The charging control unit  135  converts the voltage V 11  to appropriate voltage, and charges the battery  133  based on the converted voltage. Charging by the charging control unit  135  is executed based on CVCC (Constant Voltage Constant Current) mode. 
     Meanwhile, the CPU  110  may be configured to operate based on the voltage V 11  supplied from the battery unit BU 1  or the voltage supplied from the conversion unit  100   a , the conversion unit  100   b , and so on. The voltage V 11  supplied from the battery unit BU 1  is converted to voltage having a predetermined level by a DC-DC converter  136 . The converted voltage is supplied to the CPU  110  as the power supply voltage, thereby operating the CPU  110 . 
     After the control unit CU 1  is started, the CPU  110  turns on at least one conversion unit out of the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c , and at least one voltage of the voltage V 3 , voltage V 4 , and voltage V 5  is received at the conversion unit corresponding to the control unit CU 1 , and the voltage V 10  is output from the conversion unit. The voltage V 10  is supplied to the battery unit BU 1  via the power line L 10 . 
     The CPU  110  communicates with the battery unit BU 1  by using the signal line SL. By this communication, the CPU  110  outputs a control command that provides an instruction to start the battery unit BU 1  to execute discharging. The CPU  110  turns on a switch SW 2 . The switch SW 2  is formed of, for example, an FET (Field Effect Transistor). The switch SW 2  may also be formed of an IGBT (insulated Gate Bipolar Transistor). When the switch SW 2  is turned on, the voltage V 11  is supplied to the control unit CU 1  from the predetermined battery unit BU 1 . 
     When the voltage V 11  supplied from the battery unit BU 1  is supplied to an external device, the CPU  110  turns on a switch SW 3 . When the switch SW 3  is turned on, voltage V 12  based on the voltage V 11  is supplied to the external device via a power line L 12 . The voltage V 12  may be the voltage V 11  itself, or may be obtained by converting the voltage V 11  so as to accept the external device. The power line L 12  is connected to various kinds of external devices which are loads. Meanwhile, power based on the voltage V 12  is supplied to a battery unit BU 1  different from the battery unit BU 1  that is discharging so as to charge the battery unit BU 1  to which the power is supplied. 
     A backflow prevention diode  130   d  is connected to an output side (cathode side) of the switch SW 2 . By connecting the diode  130   d , non-stabilized power supplied from the solar power generator  3  and the wind force power generator  4  can be prevented from being supplied to the external device which is the load. The power supplied from the battery unit BU 1  can be stably supplied to the external device. Of course, a diode may be provided at a final stage of the battery unit BU 1  for safety. 
     An exemplary configuration of the control unit CU 1  in the block BL 1  has been described above. Note that configurations of the control units (e.g., control unit CU 2  and control unit CU 3 ) in other blocks BL have the configuration same as the control unit CU 1 , for example, and also operate same as the control unit CU 1 . 
     Meanwhile, an example of determining which output voltage is to be prioritized among the output voltage from the conversion unit  100   a , output voltage from the conversion unit  100   b , and output voltage from the conversion unit  100   c  by suitably adjusting the resistance value of the variable resistor has been described, but the voltage to be prioritized may be determined by other methods, too. For instance, the output voltage to be preferentially supplied may be determined by adjusting the resistance values of the resistance Rc 1 , resistance Rc 2 , and resistance Rc 3  in each of the conversion units  100 . 
     The output voltage of the conversion unit may be changed as well. For instance, it is assumed that power supplied from the solar power generator  3  is preferentially used. In this case, the range from 45 V to 48 V of the output voltage of the conversion unit  100   a  is changed to a slightly higher range. The range can be changed by suitably setting the resistance values of the resistance Rc 1 , resistance Rc 2 , and resistance Rc 3  as described above. This enables the output of the conversion unit  100   a  to be supplied to the battery unit BU more preferentially than other conversion units (conversion unit  100   b  and conversion unit  100   c ). 
     The range of the output voltage is exemplified as follows. 
     (1) Both the upper limit (48 V) and the lower limit (45 V) are increased (e.g., range from 45.5 V to 48.5V).
 
(2) Only the lower limit is increased (e.g., range from 45.5 V to 48V).
 
(3) Only the upper limit is increased (e.g., range from 45 V to 48.5 V).
 
     In the case of the exemplary setting (1), the output of the conversion unit  100   a  can be constantly prioritized. In the case of the exemplary setting (2), the output of the conversion unit  100   a  can be prioritized when the value of the voltage V 3  is small (e.g., approximately from 75 V to 80 V), for example. The output of the conversion unit  100   a  is treated equal to the outputs of other conversion units (conversion unit  100   b  and conversion unit  100   c ) when the value of the voltage V 3  is large. In the case of the exemplary setting (3), the output of the conversion unit  100   a  can be prioritized when the value of the voltage V 3  is large (e.g., approximately 100 V). In the case where the value of the voltage V 3  is small (e.g., approximately from 75 V to 80 V), the output of the conversion unit  100   a  is treated equal to the outputs of other conversion units (conversion unit  100   b  and conversion unit  100   c ). Thus, the output voltage from a predetermined conversion unit can be preferentially supplied to the battery unit. In the same manner, the outputs from the conversion unit  100   b  and conversion unit  100   c  can be preferentially supplied to the battery unit BU. 
     [1-4. Configuration of Battery Unit] 
     Next, the battery unit BU connected to the control unit CU will be described. In the following, description will be described by exemplifying the battery unit BU 1   a  connected to the control unit CU 1 . 
       FIG. 7  is a diagram illustrating an exemplary configuration of the battery unit BU 1   a . The battery unit BU 1   a  is configured to include a charging control unit  140 , a discharging control unit  141 , and a battery Ba. The voltage V 10  is supplied from the control unit CU 1  to the charging control unit  140 . The voltage V 11  which is output from the battery unit BU 1   a  is supplied to the control unit CU 1  via the discharging control unit  141 . A power line L 14  different from the power line L 11  is provided in the battery unit BU 1   a . The voltage V 11  is directly supplied from the discharging control unit  141  to the external device via the power line L 14 . The power line L 14  may be omitted. 
     The battery Ba as the example of the power storage unit is a chargeable battery such as the lithium ion secondary battery, for example. The charging control unit  140  and the discharging control unit  141  have the configurations corresponding to the kinds of the battery Ba. 
     The charging control unit  140  includes a DC-DC converter  142   a . The voltage V 10  to be input to the charging control unit  140  is converted to predetermined voltage by the DC-DC converter  142   a . The voltage output from the DC-DC converter  142   a  is supplied to the battery Ba, and the battery Ba is charged. The predetermined voltage value is varied by the kinds of the battery Ba. A voltage sensor  142   b , an electronic switch  142   c , and a current sensor  142   d  are connected to an input stage of the DC-DC converter  142   a . A current sensor  142   e , an electronic switch  142   f , and a voltage sensor  142   g  are connected to an output stage of the DC-DC converter  142   a.    
     The discharging control unit  141  includes a DC-DC converter  143   a . The DC-DC converter  143   a  generates the voltage V 11  based on direct-current voltage supplied from the battery Ba to the discharging control unit  141 . The voltage V 11  is output from the discharging control unit  141 . A voltage sensor  143   b , an electronic switch  143   c , and a current sensor  143   d  are connected to the input stage of the DC-DC converter  143   a . A current sensor  143   e , an electronic switch  143   f , and a voltage sensor  143   g  are connected to the output stage of the DC-DC converter  143   a.    
     The battery unit BU 1   a  includes a CPU  145 . The CPU  145  control respective sections of the battery unit BU 1   a . For example, on-off operation of the electronic switches in the charging control unit  140  and the discharging control unit  141  is controlled. The CPU  145  may be configured to execute safety ensuring processing such as overcharge preventing function and overcurrent preventing function. The CPU  145  communicates with the CPU  110  of the control unit CU 1  via the signal line SL to transmit and receive control commands and data. 
     A memory  146 , an A/D convertion unit  147 , and a temperature sensor  148  are connected to the CPU  145  via a bus  149 . The bus  149  is formed of, for example, an I 2 C bus. 
     The memory  146  is a general term that represents a ROM where programs executed by the CPU  145  are stored, a RAM used as a work memory when the CPU  145  executes processing, a non-volatile memory, such as an EEPROM, where various kinds of data, and so on. 
     The analog-data sensor information is supplied from, for example, the voltage sensor and current sensor to the A/D convertion unit  147 . The A/D convertion unit  147  converts the analog-data sensor information to digital-data sensor information. The digital-data sensor information is supplied to the CPU  145 . 
     The temperature sensor  148  measures the ambient temperature. For instance, the temperature inside the battery unit BU 1   a  and the ambient temperature of the battery unit BU 1   a  are measured. The temperature information obtained by the temperature sensor  148  is converted to the digital data by the A/D convertion unit  147 , and then supplied to the CPU  145 . 
       FIG. 8  is a diagram illustrating an exemplary configuration of the charging control unit  140  in the battery unit BU 1   a . As illustrated in  FIG. 8 , the charging control unit  140  includes a DC-DC converter  142   a  and a feedforward control system and a feedback control system later described. Note that, in  FIG. 8 , the voltage sensor  142   b , electronic switch  142   c , current sensor  142   d , current sensor  142   e , electronic switch  142   f , and voltage sensor  142   g  are not illustrated. 
     The DC-DC converter  142   a  is formed of, for example, a transistor  151 , a coil  152 , control IC (Integrated Circuit)  153 , and so on. The transistor  151  is controlled by the control IC  153 . 
     The feedforward control system includes an operational amplifier  155 , a transistor  156 , a resistance Rb 1 , a resistance Rb 2 , and a resistance Rb 3 . An output from the feedforward control system is received at, for example, a control terminal provided at the control IC  153  in the DC-DC converter  142   a . The control IC  153  in the DC-DC converter  142   a  adjusts output voltage from the charging control unit  140  such that input voltage to the control terminal becomes constant. 
     In other words, the feedforward control system included in the charging control unit  140  acts same as the feedforward control system included in the conversion unit  100   a.    
     The charging control unit  140  includes the feedforward control system, thereby adjusting a value of the output voltage from the charging control unit  140  so as to be a voltage value within a preset range. The value of the output voltage from the charging control unit  140  is adjusted to the voltage value within the preset range, thereby adjusting charging current to each of the batteries B electrically connected to the control unit CU 1  in accordance with change of input voltage (voltage V 10 ) from the conversion unit  100   a . Therefore, the battery unit BU 1   a  including the charging control unit  140  has a function as a battery charger that changes a charging rate toward the battery Ba. 
     By changing the charging rate toward each of the batteries B electrically connected to the control unit CU 1 , the value of the input voltage to the charging control, unit  140  of each of the battery units BU 1  (may be referred to as the value of the output value from at least one of the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c ) is adjusted to the voltage value within the preset range. 
     As illustrated in  FIG. 8 , the output voltage is taken out from the charging control unit  140  via the DC-DC converter  142   a , a current sensor  154 , and a filter  159 . The output voltage is supplied to the battery Ba. 
     As described later, the value of the output voltage from the charging control unit  140  is adjusted to be the voltage value within the preset range in accordance with the kinds of battery connected to the charging control unit  140 . The range of the output voltage from the charging control unit  140  is adjusted by suitably selecting the resistance values of the resistance Rb 1 , resistance Rb 2 , and resistance Rb 3 . 
     Thus, since the range of the output voltage from the charging control unit  140  is individually determined in accordance with the kinds of the battery B connected to the charging control unit  140 , the kinds of the battery B included in the battery unit BU 1  are not limited. The reason is that it is only to suitably select the resistance values of the resistance Rb 1 , resistance Rb 2 , and resistance Rb 3  inside the charging control unit  140  in accordance with the kinds of the battery B to be connected. 
     Meanwhile, in  FIG. 8 , the configuration in which the output of the feedforward control system is received at the control terminal of the control IC  153  is exemplified, but the CPU  145  of the battery unit BU 1  may be configured to give the input to the control terminal of the control IC  153 . For example, the CPU  145  of the battery unit BU 1  may be configured to receive information related to the input voltage toward the battery unit BU 1  from the CPU  110  of the control unit CU 1  via the signal line SL. The CPU  110  of the control unit CU 1  can receive the information related to the input voltage toward the battery unit BU 1  based on measurement results by the voltage sensor  101   g , voltage sensor  102   g , and voltage sensor  103   g.    
     In the following, the feedforward control system provided in the charging control unit  140  will be described. 
     An input to a non-inverted input terminal of an operational amplifier  155  has voltage obtained by multiplying the input voltage (voltage V 10 ) to the charging control unit  140  by kb (kb: approximately a few tenths to a few hundredth). On the other hand, an input to an inverted output terminal b 1  of the operational amplifier  155  has voltage obtained by multiplying voltage Vb by kb. The voltage Vb is intended to be set as a lower limit of output voltage of the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c . The input voltage (kb×Vb) toward the inverted output terminal b 1  of the operational amplifier  155  is applied from the CPU  145 , for example. 
     Therefore, in the case where the input voltage toward the charging control unit  140  is sufficiently higher than the preset constant voltage Vb, the feedforward control system included in the charging control unit  140  boosts the output voltage from the charging control unit  140 . Further, when the input voltage toward the charging control unit  140  becomes close to the preset constant voltage Vb, the feedforward control system steps down the output voltage from the charging control unit  140 . 
     The transistor  156  is disposed such that a value of the output voltage from the charging control unit  140  does not exceed the preset upper limit when the input voltage toward the charging control unit  140  exceeds a predetermined value in the same manner as the transistor  125  illustrated in  FIG. 5 . Note that the range of the value of the output voltage from the charging control unit  140  is determined by combination of the resistance values of the resistance Rb 1 , resistance Rb 2 , and resistance Rb 3 . Accordingly, the resistance values of the resistance Rb 1 , resistance Rb 2 , and resistance Rb 3  are adjusted in accordance with the kinds of the battery B connected to the charging control unit  140 . 
     As described above, the charging control unit  140  further includes the feedback control system. The feedback control system is formed of, for example, the current sensor  154 , an operational amplifier  157 , and a transistor  158 , and so on. 
     When an amount of current supplied to the battery Ba exceeds a preset specified value, the output voltage from the charging control unit  140  is stepped down by the feedback control system, and the amount of current supplied to the battery Ba is restricted. A restriction level for the amount of current supplied to the battery Ba by the feedback control system is determined by rating of the battery Ba connected to the charging control unit  140 . 
     When the output voltage from charging control unit  140  is stepped down by the feedforward control system or the feedback control system, the amount of current supplied to the battery Ba is restricted. As a result of restricting the amount of current supplied to the battery Ba, charging the battery Ba connected to the charging control unit  140  is slowed down. 
       FIG. 9  is a diagram mainly illustrating an exemplary configuration related to the power supply system of the battery unit BU 1   a . The battery unit BU 1   a  is not provided with a main switch. A switch SW 5  and a DC-DC converter  160  are connected between the battery Ba and the CPU  145 . A switch SW 6  is connected between the battery Ba and the discharging control unit  141 . A switch SW 7  is connected to the input stage of the charging control unit  140 . A switch SW 8  is connected to the output stage of the discharging control unit  141 . The respective switches SW are formed of, for example, FET. 
     The battery unit BU 1   a  is started by, for example, a control command from the control unit CU 1 . For example, a high-level signal is constantly supplied from the control unit CU 1  via a predetermined signal line. For this reason, the high-level signal is supplied to the switch SW 5  only by connecting a port of the battery unit BU 1   a  to the predetermined signal line, thereby turning on the switch SW 5 . The battery unit BU 1   a  is started by turning on the switch SW 5 . The voltage from the battery Ba is supplied to a DC-DC converter  160  by turning on the switch SW 5 . The power-supply voltage based on the voltage from the battery Ba is generated by the DC-DC converter  160 . The power-supply voltage is supplied to the CPU  145  to actuate the CPU  145 . 
     The CPU  145  executes processing in accordance with the control command of the control unit CU 1 . For example, the control command providing a charging instruction is supplied to the CPU  145  from control unit CU 1 . In accordance with the command providing the charging instruction, the CPU  145  turns off the switch SW 6  and the switch SW 8 , and then turns on the switch SW 7 . The voltage V 10  supplied from the control unit CU 1  is supplied to the charging control unit  140  by turning on the switch SW 7 . The voltage V 10  is converted to a predetermined value by the charging control unit  140 , and the battery Ba is charged with the converted voltage. Note that charging method to the battery Ba may be suitably changed in accordance with the kinds of the battery Ba. 
     The control command providing a discharging instruction, for example, is supplied to the CPU  145  from control unit CU 1 . In accordance with the control command providing the discharging instruction, the CPU  145  turns off the switch SW 7  and turns on the switch SW 6  and switch SW 8 . For example, the switch SW 8  is turned on after a predetermined passed from the switch SW 6  is turned on. The voltage is supplied from the battery Ba to the discharging control unit  141  by turning on the switch SW 6 . The voltage supplied from the battery Ba is converted to the voltage V 11  by the discharging control unit  141 . The converted voltage V 11  is supplied to the control unit CU 1  via the switch SW 8 . Meanwhile, a diode may be added to a latter stage of the switch SW 8  so as to avoid collision with outputs from other battery units BU 1 . 
     Meanwhile, the discharging control unit  141  can be turned on/off by the control of the CPU  145 . A control command for turning on/off the discharging control unit is supplied to the discharging control unit  141  via an on-off signal line directed to the discharging control unit  141  from the CPU  145 . In accordance with the control command, at least one of the electronic switch  143   c  and the electronic switch  143   f  of the discharging control unit  141  is turned on/off. 
     An exemplary configuration of the battery unit BU has been described by exemplifying the battery unit BU 1   a . Note that the battery unit BU 1   b  and battery unit BU 1   c  have the configuration same as the battery unit BU 1   a , for example, and operates same as the battery unit BU 1   a . There may be constitutional difference between the respective battery units BU. For instance, the battery B included in the battery unit BU 1   b  may be a secondary battery other than the lithium ion battery. 
     The battery units of other blocks (e.g., battery unit BU 2   a  and battery unit BU 3   a ) have the configuration same as the battery unit BU 1   a , for example, and operate same as the battery unit BU 1   a.    
     [1-5. Outline of Operation] 
     Next, an exemplary operation in the block BL 1  will be described. The block BL 2  and block BL 3  operate same as the block BL 1 , for example. A description related to the operation of the block BL 2  and block BL 3  will be omitted accordingly. 
     The voltage V 3 , voltage V 4 , and voltage V 5  are supplied to the control unit CU 1  in the block BL 1 . The voltage V 3  is received by the conversion unit  100   a . The voltage V 4  is received by the conversion unit  100   b . The voltage V 5  is received by the conversion unit  100   c . The voltage V 10  fluctuating in the range, for example, from 45 to 48 V is generated by each of the conversion units  100 . 
     Here, note that outputs of the solar power generator  3  and the wind force power generator  4  fluctuate depending on the weather. For instance, it is effective to utilize the output of the solar power generator  3  during a daytime time zone under fine weather, and utilize the output of the wind force power generator  4  during the night-time or when a typhoon is approaching. In other words, preferably, the voltage V 10  are generated by the three conversion units  100  (conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c ), and the predetermined voltage V 10  to be supplied to the battery unit BU 1  side is selected from among the voltage V 10  generated by the conversion units to suit the weather and the like. Otherwise, it is preferably to turn on only the conversion unit whose output is to be used. 
     As described above as an example, one of the voltage V 10  generated by the three conversion units  100  can be selected by suitably adjusting the resistance values of the variable resistors (variable resistor  101   h , variable resistor  102   h , and variable resistor  103   h ). In the following, description will be given for a case in which the voltage V 10  generated by the conversion units  100   a  is selected, and the voltage V 10  generated by the conversion unit  100   a  is supplied to the battery unit BU 1 . 
     In the case where the voltage V 3  supplied from the solar power generator  3  is sufficiently high (e.g., approximately 100 V), the voltage V 10  generated by the conversion unit  100   a  becomes approximately 48 V. At this point, when illuminance to the solar cell of the solar power generator  3  is reduced and the voltage V 3  is decreased, the voltage V 10  is decreased. With the decrease of the voltage V 10 , control to restrict charging is executed in the charging control unit  140  of the battery unit BU 1  being charged (any one of the battery unit BU 1   a , battery unit BU 1   b , battery unit BU 1   c ). In other words, the load is reduced from the standpoint of the solar cell of the solar power generator  3 . 
     With the reduction of the load from the standpoint of the solar cell of the solar power generator  3 , the voltage V 3  which is terminal voltage of the solar cell is increased (recovered). The voltage V 10  is increased due to the increase of the voltage V 3 . The charging control unit  140  of the battery unit BU 1  being charged boosts the output voltage to increase the charging rate. After that, cooperative control by the conversion unit  100   a  of the control unit CU 1  and the battery unit BU 1  is repeated until the voltage V 10  reaches a specific convergence value and balance between demand and supply of the power is achieved. In the following description, the cooperative control by the conversion unit of the control unit CU and the battery unit BU connected to the conversion unit may be referred to as cooperative control. 
     Note that decrease of the terminal voltage of the solar cell is not caused only by reduced illuminance to the solar cell. For example, in the case where the number of the battery units BU 1  to be charged is increased, the load is increased from the standpoint of the solar cell in the solar power generator  3 , thereby decreasing the voltage V 10 . In this case also, control to restrict charging is executed in battery unit BU 1 , and the cooperative control by the conversion unit  100   a  of the control unit CU 1  and the battery unit BU 1  is repeated. Thus, even in the case where the power to be supplied fluctuates, the battery unit autonomously controls charging in accordance with the fluctuations. 
     In the case where the power supplied from the wind force power generator  4  is used, the same control is executed. In other words, in the case where the voltage V 10  output from the conversion unit  100   b  is supplied to the battery unit BU 1 , the cooperative control by the conversion unit  100   b  of the control unit CU 1  and the battery unit BU 1  is executed in the same manner. 
     In the case where the power supplied from the biomass power generator  5  is used, the same control is also executed. In other words, in the case where the voltage V 10  output from the conversion unit  100   c  is supplied to the battery unit BU 1 , the cooperative control by the conversion unit  100   c  of the control unit CU 1  and the battery unit BU 1  is executed in the same manner. The output from the biomass power generator  5  fluctuates less than the outputs from the solar power generator  3  and the wind force power generator  4 . However, in the case where the output of the biomass power generator  5  is used, the voltage V 10  from the conversion unit  100   c  is decreased when the number of the battery units BU 1  to be charged is increased. In this case also, the battery unit BU 1  controls charging in accordance with the output of the biomass power generator  5 , thereby achieving the balance between demand and supply of the power. 
     The cooperative control by the control unit CU and the battery unit BU is executed in other blocks (block BL 2  and block BL 3 ) in the same manner. The balance between demand and supply of the power in the entire system  1  is achieved by the cooperative control executed in the respective blocks BL. 
     Meanwhile, as one of characteristics of the wind force power generator  4 , a motor section includes a large L (reactance) component and has a heavy load, and therefore, there is a characteristic of discharging a certain amount of power even in the case where rotary speed of the motor section is reduced. However, when the heavy load state continues, there may be a case in which the windmill of the wind force power generator  4  is finally stopped and the output of the wind force power generator  4  is stopped. Therefore, it is desirable to keep the rotary speed of the motor section at a predetermined level or higher than that to cope with the load fluctuation. 
     In the above-described example, the conversion unit  100   b  adjusts the value of the voltage V 0  which is the output voltage within the range from 45 V to 48 V in accordance with the voltage V 4  which is the input voltage, but instead of that, the value of the voltage V 10  may be adjusted in accordance with the voltage corresponding to the predetermined rotary speed of the motor section (conveniently referred to as voltage V 50 ), for example. Generally, it is considered that a power generation amount of the wind force power generator is proportional to the rotary speed of the motor section. Therefore, the voltage V 50  corresponding to the predetermined rotary speed can be set. The voltage V 50  may be input to the inverted output terminal c 1  of the operational amplifier  124  instead of a reference voltage (75 V) above described. 
     With this configuration, when the voltage V 4  becomes close to the voltage V 50 , the above-described cooperative control works and reduces the charging rate toward the battery unit BU, for example. Then, the load is reduced and the voltage V 4  can be prevented from becoming lower than voltage V 50 . In other words, the rotary speed of the motor section in the wind force power generator  4  can be prevented from falling below the predetermined rotary speed. 
     [1-6. Detailed Description for Operation] 
     The cooperative control in the case of using outputs from the solar power generator  3  will be described in detail. 
     [1-6-1. MPPT Control] 
     First, an outline of MPPT (Maximum Power Point Tracking) control will be described below. 
       FIG. 10A  is a graph illustrating voltage-current characteristics of the solar cell. In  FIG. 10A , the vertical axis represents terminal, current of the solar cell and the horizontal axis represents the terminal voltage of the solar cell. In  FIG. 10A , Isc indicates output current when terminals of the solar cell are short-circuited during exposure to light, and Voc indicates output voltage when the terminals of the solar cells are opened during exposure to light. The Isc and Voc are referred to short-circuit current and open voltage respectively. 
     As illustrated in  FIG. 10A , the terminal current of the solar cell is maximum when the terminals of the solar cell are short-circuited during exposure to light, and at this point, the terminal voltage of the solar cell is substantially zero V. On the other hand, the terminal voltage of the solar cell is maximum when the terminals of the solar cells are opened during exposure to light, and at this point the terminal current of the solar cell is substantially zero A (ampere). 
     Now, it is assumed that the graph illustrating the voltage-current characteristics of the solar cell is indicated by a curve C 1  illustrated in  FIG. 10A . Here, when a load is connected to the solar cell, the voltage and current taken out from the solar cell is determined by power consumption required by the load connected. A point on the curve C 1  represented by a pair of the terminal voltage and terminal current of the solar cell at this point is referred to as an operating point of the solar cell. Note that, in  FIG. 10A , a position of the operating point is schematically illustrated, and a position of an actual operating point is not illustrated. The same is applied to the operating points illustrated in other drawings of the present disclosure. 
     By changing the operating point on the curve indicating the voltage-current characteristics of the solar cell, the product of the terminal voltage and the terminal current, namely, a pair of terminal voltage Va and terminal current Ia that provides a maximum generated power can be found. A point represented by the pair of terminal voltage Va and terminal current Ia that provides the maximum power obtained by the solar cell is referred to as an optimum operating point of the solar cell. Note that the optimum operating point may also be referred to as a maximum power point, a maximum operating point, and the like. 
     When the graph representing the voltage-current characteristics of the solar cell is indicated by the curve C 1  illustrated in  FIG. 10A , the maximum power obtained from the solar cell can be obtained by the product of VA and Ia providing the optimum operating point. In other words, when the graph representing the voltage-current characteristics of the solar cell is indicated by the curve C 1  illustrated in  FIG. 10A , the maximum power obtained from the solar cell is illustrated by an area (Va×Ia) indicated by a shaded region in  FIG. 10A . Meanwhile, an amount obtained by dividing the (Va×Ia) by (Voc×Isc) is a fill factor. 
     The optimum operating point is changed by the power required by the load connected to the solar cell, and point P A  indicating the optimum operating point moves on the curve C 1  in accordance with the change of the power required by the load connected to the solar cell. In the case where the amount of the power required by the load is little, the current to be supplied to the load may be covered by the current less than the terminal current at the optimum operating point. Therefore, the value of the terminal voltage of the solar cell becomes higher than the voltage value at the optimum operating point. On the other hand, in the case where the amount of required power is larger than the amount of power that can be supplied at the optimum operating point, the power amount exceeds the power that can be provided by illuminance at this point. Accordingly, it can be considered that the terminal voltage of the solar cell is to be decreased up to zero. 
     The curves C 2  and C 3  illustrated in  FIG. 10A  indicate the voltage-current characteristics of the solar cell in the case where illuminance to the solar cell is changed, for example. For instance, the curve C 2  illustrated in  FIG. 10A  corresponds to the voltage-current characteristics in the case where illuminance to the solar cell is increased, and the curve C 3  illustrated in  FIG. 10A  corresponds to the voltage-current characteristic in the case where illuminance to the solar cell is decreased. 
     For instance, when the illuminance to the solar cell is increased and the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 1  to the curve C 2 , the optimum operating point is also changed along with the increase of illuminance to the solar cell. Note that the optimum operating point moves from the point on the curve C 1  to the point on the curve C 2  at this point. 
     The MPPT control is nothing less than the control to obtain the optimum operating point relative to the change of the curve indicating the voltage-current characteristics of the solar cell and control the terminal voltage (or terminal current) of the solar cell such that the power obtained from the solar cell becomes maximum. 
       FIG. 10B  is a graph (P-V curve) illustrating a relation between the terminal voltage of the solar cell and generated power of the solar cell in the case where the voltage-current characteristics of the solar cell is indicated by a specific curve. 
     As illustrated in  FIG. 10B , in the case where the generated output of the solar cell takes a maximum value Pmax at the terminal voltage providing the optimum operating point, the terminal voltage providing the optimum operating point can be acquired by a hill climbing method. A series of steps described below is generally executed by a CPU of a power conditioner connected between the solar cell and the power system. 
     For instance, first, generated power P 0  at the time of receiving from the solar cell is calculated, defining an initial value of the voltage at this point as V 0 . Next, the voltage received from the solar cell is increased by an amount ε, as expressed as follows: V 1 =V 0 +ε (here, ε&gt;0). Next, generated power P 1  at the time of receiving from the solar cell is calculated, defining the voltage at this point as V 1 . Then, the obtained P 0  and P 1  are compared, and in the case of P 1 &gt;P 0 , the voltage received from the solar cell is increased by the amount ε as expressed in V 2 =V 1 +ε. Next, generated power P 2  at the time of receiving from the solar cell is calculated, defining the voltage at this point as voltage V 2 . Then, the obtained P 1  and P 2  are compared, and in the case of P 2 &gt;P 1 , the voltage received from the solar cell is increased by the amount ε as expressed in V 3 =V 2 +ε. Next, generated power P 3  at the time of receiving from the solar cell is calculated, defining the voltage at this point as V 3 . 
     Here, in the case of P 3 &lt;P 2 , the terminal voltage providing the optimum operating point is located between V 2  and V 3 . Thus, the terminal voltage providing the optimum operating point can be acquired with arbitrary accuracy by making adjustment with the amount ε. The bisection method algorithm may also be applied in the above-described steps. Meanwhile, some change is required in the control program because the simple hill climbing method cannot handle the case in which the P-V curve includes two or more peaks, such as when an irradiation surface of the solar cell is partially shadowed. 
     According to the MPPT control, the maximum power can be taken out from the solar cell in the respective weather conditions because the terminal voltage is adjusted such that the load becomes constantly optimum from the standpoint of the solar cell. On the other hand, it takes some time to execute the control because analog-digital conversion (A/D conversion) is required to calculate the terminal voltage providing the optimum operating point and further multiplication is included in this calculation. Therefore, the MPPT control sometimes cannot handle a rapid change of illuminance to the solar cell, such as when the sky is suddenly overcast and illuminance to the solar cell is rapidly changed. 
     [1-6-2. Control by Voltage Tacking Method] 
     Here, comparing the curves C 1  to C 3  illustrated in  FIG. 10A , the change of the open voltage Voc is smaller than the change of the short-circuit current Isc relative to the change of illuminance to the solar cell (may also be referred to as change of the curve indicating the voltage-current characteristics). Further, it is known that any solar cell indicates similar voltage-current characteristics and in the case of a crystal silicon solar cell, the terminal voltage providing the optimum operating point is around approximately 70% to 80% of the open voltage. Therefore, it can be expected that the power can be efficiently taken out from the solar cell by setting an appropriate voltage value as the terminal voltage of the solar cell and adjusting the output current of the converter such that the terminal voltage of the solar cell becomes the set voltage value. The above-described control by current restriction is referred to as a voltage tacking method. 
     In the following, an outline of control according to the voltage tacking method will be described. As a premise, a switching device is disposed between the solar cell and the power conditioner, and a voltage measuring unit is disposed between the solar cell and the switching device. Further, the solar cell is in a state of being exposed to the light. 
     First, the switching device is turned off, and when a predetermined period has passed after the switching device is turned on, the terminal voltage of the solar cell is measured by the voltage measuring unit. The reason for waiting until the predetermined period has passed after the switching device is turned off before measuring the terminal voltage of the solar cell is to wait for the terminal voltage of the solar cell to be stabilized. The terminal voltage at this point is the open voltage Voc. 
     Next, for example, 80% of the voltage value of the open voltage Voc obtained by measurement is calculated as a target voltage value, and the target voltage value is temporarily held inside a memory and the like. Then, the switching device is turned on, and energization to the converter inside the power conditioner is started. At this point, the output current of the converter is adjusted such that the terminal voltage of the solar cell becomes the target voltage value. The above-described steps are executed at optional time intervals. 
     The control according to the Voltage tracking method has a larger loss of the power obtained by the solar cell, compared to the MPPT Control Method, but can be implemented by a simple circuit at low cost, thereby achieving to reduce the cost for the power conditioner including the converter. 
       FIG. 11  is an explanatory diagram for the changes of the operating point relative to the changes of the curves indicating the voltage-current characteristics of the solar cell. In  FIG. 11 , the vertical axis represents the terminal current of the solar cell, and the horizontal axis represents the terminal voltage of the solar cell. Further, a white circle in  FIG. 11  indicates the operating point at the time of executing the MPPT control, and a black circle in  FIG. 1I  indicates the operating point at the time of executing the voltage tracking method. 
     Now, it is assumed that the curve indicating the voltage-current characteristics of the solar cell is a curve C 5 . Next, in the case where the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 5  to a curve C 8  along with change of illuminance to the solar cell, the operating points of the respective control methods are also changed along with the change of the curve indicating the voltage-current characteristics of the solar cell. Note that the target voltage value at the time of executing the voltage tacking method is regarded to be a substantially constant value Vs in  FIG. 11  because the change of the open voltage Voc relative to the change of illuminance to the solar cell is small. 
     As is obvious from  FIG. 11 , in the case where the curve indicating the voltage-current characteristics of the solar cell is a curve C 6 , a divergence degree between the operating point of the MPPT control and the operating point of the voltage tacking method is small. Therefore, in the case where the curve indicating the voltage-current characteristics of the solar cell is the curve C 6 , it can be considered that there is no significant difference in the generated power obtained from the solar cell in either case of control. 
     On the other hand, in the case where the curve indicating the voltage-current characteristics of the solar cell is a curve C 8 , the divergence degree between the operating point of the MPPT control and the operating point of the voltage tacking method is large. For example, as illustrated in  FIG. 11 , comparing a difference ΔV 6  with a difference ΔV 8  between the terminal voltage at the time of applying the MPPT control and the terminal voltage at the time of applying the voltage tacking method, ΔV 6  is smaller than ΔV 8 . Accordingly, in the case where the curve indicating the voltage-current characteristics of the solar cell is a curve C 8 , the difference between the generated power obtained from the solar cell at the time of applying the MPPT control and the generated power obtained from the solar cell at the time of applying the voltage tacking method is large. 
     [1-6-3. Cooperative Control by Control Unit and Battery Unit] 
     Next, the cooperative control by the control unit and the battery unit will be described. 
     Generally, in the case of charging one battery with the power obtained from the solar cell, control according to the above-described MPPT control or voltage tacking method is executed by the power conditioner interposed between the solar cell and the battery. The above battery may include a battery that operates integrally with a plurality of batteries incorporated inside thereof, but the battery is generally formed of a single kind of the plurality of batteries. In other words, the above-described MPPT control or voltage tacking method control is assumed to be executed singularly by the power conditioner connected between the solar cell and one battery. Further, no change is made in the number and configuration (connection mode such as serial connection and parallel connection) of the batteries to be charged while charging, and the number and configuration of the batteries to be charged is fixed during charging. 
     On the other hand, in the cooperative control, each of the control unit CU 1 , plurality of battery unit BU 1   a , battery unit BU 1   b , battery unit BU 1   c , etc. autonomously executes control such that balance between the output voltage of the control unit CU 1  and the voltage required by the plural number of the battery units BU 1  is achieved. As described above, the battery B included in the battery unit BU 1   a , battery unit BU 1   b , battery unit BU 1   c , etc. may be any kind. In other words, the control unit CU according to the present disclosure can execute the cooperative control for the plural kinds of the batteries B. 
     Further, the respective battery units BU 1  are detachably attached to the control unit CU 1  in the system  1 . In other words, while power is generated from the solar cell of the solar power generator  3 , there is possibility that the number of the battery units BU 1  connected to the control unit CU 1  is changed and the number of the battery units BU 1  to be charged is changed. 
     The load from the standpoint of the solar cell may be changed while the power is generated from the solar cell, but according to the cooperative control, it is possible to handle not only the change of the illuminance to the solar cell but also the load change from the standpoint of the solar cell while the power is generated from the solar cell. Further, the cooperative control is executed in each of the plurality of blocks BL, and the balance between demand and supply of power can be achieved in the entire system  1 . 
     By connecting the above-described control unit CU 1  to the battery unit BU 1 , the control system that dynamically changes the charging rate in accordance with supply capacity from the control unit CU 1  can be established. In the following, an exemplary cooperative control will be described. Meanwhile, the following description will be given under the presumption that one battery unit BU 1   a  is connected to the control unit CU 1  in an initial state, but control is the same in the case where the plurality of battery units BU 1  is connected to the control unit CU 1 . 
     For example, the solar cell is connected to the input side of the control unit CU 1  and the battery unit BU 1   a  is connected to the output side thereof. Further, for example, the upper limit of the output voltage of the solar cell is 100 V and the lower limit of the output voltage of the solar cell is desired to be kept at 75 V. In other words, setting is Vt 0 =75 V, and the input voltage to the inverted output terminal of the operational amplifier  124  is (kc×75) V. 
     Further, the upper limit and the lower limit of the output voltage from the control unit CU 1  (voltage V 10 ) are respectively set at, for example, 48 V and 45 V. In other words, setting is Vb=45 V, and the input voltage to the inverted output terminal of the operational amplifier  155  is (kb×45) V. Note that the value 48 V which is the upper limit of the output voltage from the control unit CU 1  is adjusted by suitably selecting the resistance Rc 1  and resistance Rc 2  inside the conversion unit  100   a . In other words, the target voltage value of the output from the control unit CU 1  is set at 48 V. 
     Further, the upper limit and the lower limit of the output voltage from the charging control unit  140  of the battery unit BU 1   a  are respectively set at, for example, 42 V and 28 V. Therefore, the resistance Rb 1 , resistance Rb 2 , and resistance Rb 3  inside the charging control unit  140  are selected such that the upper limit and the lower limit of the output voltage from the charging control unit  140  become 42V and 28V respectively. 
     Meanwhile, a time when the voltage V 10  which is the input voltage to the charging control unit  140  is the upper limit corresponds to a state in which the charging rate toward the battery Ba is 100%, and a time when the voltage V 10  is the lower limit corresponds to a state in which the charging rate toward the battery Ba is 0%. In other words, the time when the voltage V 10  toward the charging control unit  140  is 48 V corresponds to the state in which the charging rate toward the battery Ba is 100%, and the time when the voltage V 10  toward the charging control unit  140  is 45 V corresponds to the state in which the charging rate toward the battery Ba is 0%. The charging rate is set in the range from 0 to 100% in accordance with fluctuation of the voltage V 10  in the range from 45 V to 48 V. 
     Note that control of the charging rate toward the battery may be executed in parallel apart from the cooperative control. In other words, since constant current charge is executed at the beginning of charge, the charging voltage is adjusted so as to keep the charging current equal to or lower than the predetermined level by executing feedback adjustment for the output from the charging control unit  140 , and in the final stage, the charging voltage is kept equal, to or lower than the predetermined level. Here, the charging voltage to be adjusted is equal to or lower than the voltage adjusted by the above-described cooperative control. In this manner, charging processing is executed within the power supplied from the control unit CU 1 . 
     First, the changes of the operating point at the time of executing the cooperative control in the case where illuminance to the solar cell is changed will be described. 
       FIG. 12A  is an explanatory diagram for the changes of the operating point at the time of executing cooperative control in the case where illuminance to the solar cell is reduced. In  FIG. 12A , the vertical axis represents the terminal current of the solar cell, and the horizontal axis represents the terminal voltage of the solar cell. Further, a white circle in  FIG. 12A  indicates the operating point at the time of executing the MPPT control, and a shaded circle in  FIG. 12A  indicates the operating point at the time of executing the cooperative control. The curves C 5  to C 8  illustrated in  FIG. 12A  indicate the voltage-current characteristics of the solar cell in the case where illuminance to the solar cell is changed. 
     Now, it is assumed that power required by the battery Ba is 100 W (watt) and the voltage-current characteristics of the solar cell is indicated by the curve C 5  (state under the finest weather). The operating point of the solar cell at this point is indicated by point a on the curve C 5 , for example, and it is assumed that power (supplied amount) supplied from the solar cell to the battery Ba via the conversion unit  100   a  and the charging control unit  140  exceeds the power (demanded amount) required by the battery Ba. 
     In the case where the power supplied from the solar cell to the battery Ba exceeds the power required by the battery Ba, the voltage V 10  which is the output voltage from the control unit CU 1  to the battery unit BU 1   a  becomes the upper limit 48 V. In other words, since the voltage V 10  which is the input voltage to the battery unit BU 1   a  is 48 V, the output voltage from the charging control unit  140  of the battery unit BU 1   a  is to be the upper limit 42 V, and charging the battery Ba is executed at the charging rate of 100%. While it has been described that the charging the battery is executed at 100%, note that the charging is not limited to 100% and the charging rate can be suitably adjusted in accordance with the characteristics of the battery. 
     When the weather becomes cloudy from this state, the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 5  to the curve C 6 . Since the sky starts to be overcast, the terminal voltage of the solar cell is gradually decreased and the output voltage from the control unit CU 1  to the battery unit BU 1   a  is gradually decreased as well. Therefore, the operating point of the solar cell moves to, for example, point b on the curve C 6  due to change of the curve indicating the voltage-current characteristics of the solar cell from the curve C 5  to the curve C 6 . 
     In the case where the sky is further overcast from this state, the curve indicating the voltage-current characteristics of the solar cell changes from the curve C 6  to the curve C 7 , and the terminal voltage of the solar cell is gradually decreased, thereby decreasing the voltage V 10  which is the output voltage from the control unit CU 1  to the battery unit BU 1   a . When the voltage V 0  is decreased to a certain level, 100% of the power cannot be supplied to the battery Ba. 
     At this point, in the case where the terminal voltage of the solar cell becomes close to the lower limit, namely, Vt 0 =75V from 100V, the conversion unit  100   a  of the control unit CU 1  starts to step down the voltage V 10  toward the battery unit BU 1   a  from 48 V to Vb=45V. 
     When the voltage V 10  is stepped down, the input voltage toward the battery unit BU 1   a  is decreased, and therefore, the charging control unit  140  of the battery unit BU 1   a  starts to step down the output voltage toward the battery Ba. When the voltage V 10  is stepped down, the charging current supplied to the battery Ba is decreased, thereby slowing down charging the battery Ba connected to the charging control unit  140 . In other words, the charging rate toward the battery Ba is decreased. 
     When the charging rate toward the battery Ba is decreased, the power consumption is decreased, thereby reducing the load from the standpoint of the solar cell. Consequently, the terminal voltage of the solar cell is increased (recovered) by the reduced load from the standpoint of the solar cell. 
     When the terminal voltage of the solar cell is boosted, a degree of stepping down the voltage V 10  by the control unit CU 1  is reduced, thereby boosting the input voltage toward the battery unit BU 1   a . Since the input voltage toward the battery unit BU 1   a  is boosted, the charging control unit  140  of the battery unit BU 1   a  boosts the output voltage from the charging control unit  140 , thereby increasing the charging rate toward the battery Ba. 
     When the charging rate toward the battery Ba is increased, the load is increased from the standpoint of the solar cell, thereby stepping down the terminal voltage of the solar cell by the increased amount of the load from the standpoint of the solar cell. When the terminal voltage of the solar cell is stepped down, the conversion unit  100   a  of the control unit CU 1  steps down the output voltage toward the battery unit BU 1   a.    
     After that, the above-described adjustment for the charging rate is automatically repeated until the output voltage from the control unit CU 1  to the battery unit BU 1   a  reaches a specific convergence value and the balance between demand and supply of power can be achieved. 
     The cooperative control is not controlled by software, different from the MPPT control. Therefore, calculation of the terminal voltage providing the optimum operating point is unnecessary in the cooperative control. Also, calculation by the CPU does not intervene when adjusting the charging rate according to the cooperative control. Therefore, power consumption is small in the cooperative control, compared to the MPPT control, and the above-described adjustment of the charging rate is executed in a short period such as about a few nanoseconds to a few hundred nanoseconds. 
     Moreover, the conversion unit  100   a  and the charging control unit  140  are only to detect the input voltage received therein and adjust the output voltage, and therefore, no analog-digital conversion is necessary and no communication between the control unit CU 1  and the battery unit BU 1   a  is necessary. Therefore, the cooperative control does not require any intricate circuit, and the circuit for implementing the cooperative control circuit is to be compact. 
     Now, it is assumed that the control unit CU 1  could supply power of 100 W when the operating point was at the point a on the curve C 5 , and the output voltage from the control unit CU 1  toward the battery unit BU 1   a  has reached a specific convergence value. In other words, it is assumed that the operating point of the solar cell has moved to, for example, point c on the curve C 7 . At this point, the power supplied to the battery Ba falls below the power of 100 W, but as illustrated in  FIG. 12A , the power not inferior to the case of executing the MPPT control can be supplied to battery Ba, depending on a selected value of the voltage Vt 0 . 
     When the sky is further overcast, the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 7  to the curve C 8 , and the operating point of the solar cell moves to, for example, point d on the curve C 8 . 
     As illustrated in  FIG. 12A , under the cooperative control, the balance between demand and supply of power is adjusted. Therefore, the terminal voltage of the solar cell is prevented from falling below the voltage Vt 0 . In other words, even in the case where illuminance to the solar cell is extremely reduced, the terminal voltage of the solar cell is prevented from falling below the voltage Vt 0  under the cooperative control. 
     In the case where illuminance to the solar cell is extremely reduced, the terminal voltage of the solar cell becomes a value close to the voltage Vt 0 , and the current amount supplied to the battery Ba becomes little. Therefore, in the case where illuminance to the solar cell is extremely reduced, it takes quite a time to charge the battery Ba, but the terminal voltage of the solar cell does not drop and the system  1  does not go down because the demand and supply of the power is well balanced. 
     As described above, since the charging rate is adjusted in a short time in the cooperative control, the system  1  can be prevented from going down according to the cooperative control even in the case where the sky is suddenly overcast and illuminance to the solar cell is rapidly reduced. 
     Next, description will be given for the changes of the operating point at the time of executing the cooperative control in the case where the load from the standpoint of the solar cell is changed. 
       FIG. 12B  is an explanatory diagram for the changes of the operating point at the time of executing cooperative control in the case where the load from the standpoint of the solar cell is increased. In  FIG. 12B , the vertical axis represents the terminal current of the solar cell, and the horizontal axis represents the terminal voltage of the solar cell. Further, a shaded circle in  FIG. 12B  indicates the operating point at the time of executing the cooperative control. 
     Now, it is assumed that illuminance to the solar cell has no change and the voltage-current characteristics of the solar cell are indicated by curve C 0  illustrated in  FIG. 12B . 
     Since it can be considered there is almost no power consumption immediately after starting the each of the blocks BL, the terminal voltage of the solar cell is supposed to be almost equal to the open voltage. Therefore, it can be considered that the operating point of the solar cell immediately after starting each of the blocks BL is be at point e on the curve C 0 , for example. Note that the output voltage from the control unit CU 1  toward the battery unit BU 1   a  is the upper limit, 48 V. 
     When power supply to the battery Ba connected to the battery unit BU 1   a  is started, the operating point of the solar cell moves to, for example, point g on the curve C 0 . Note that an area of a shaded region S 1  illustrated in  FIG. 12B  is equal to 100 W because the power required by battery Ba is 100 W in the description of the present embodiment. 
     A time when the operating point of the solar cell is at the point g on the curve C 0  corresponds to a state in which the power supplied from the solar cell toward the battery Ba via the conversion unit  100   a  and charging control unit  140  exceeds the power required by the battery Ba. Therefore, the terminal voltage of the solar cell, the output voltage from the control unit CU 1 , and the power supplied to the battery Ba when the operating point is at the point g on the curve C 0  are slightly lower than 100 V, 48 V, and 42 V respectively. 
     Here, it is assumed that the battery unit BU 1   b  having the same configuration as the battery unit BU 1   a  is newly connected to the control unit CU 1 . Assuming that a battery (conveniently referred to as battery Bb) included in the battery unit BU 1   b  requires the power of 100 W for charging as is the case with the battery Ba connected to the battery unit BU 1   a , power consumption is increased and the load from the standpoint of the solar cell is rapidly increased. 
     To supply power of 200 W in total to the two batteries, the output current is required to be doubled in total while keeping the output voltage from the charging control unit  140  of the battery unit BU 1   a  and the output voltage of the charging control unit  140  of the battery unit BU 1   b , for example. 
     However, in the case where the power generator is the solar cell, the terminal voltage of the solar cell is decreased due to the increase of the output current from the charging control unit  140  included in the battery unit BU 1   a  and the output current from the charging control unit  140  included in the battery unit BU 1   b . Therefore, the total output current is required to be doubled, compared to when the operating point of the solar cell is at the point g. That means, as illustrated in  FIG. 12B , the operating point of the solar cell is required to be at point h on the curve C 0 , for example, and the terminal voltage of the solar cell is rapidly decreased. When the terminal voltage of the solar cell is rapidly decreased, the voltage V 3  may drop and the system  1  may go down. 
     According to the cooperative control, when the terminal voltage of the solar cell is decreased because of the newly connected battery unit BU 1   b , the cooperative control is executed in the block BL 1  and the balance between demand and supply of power is adjusted. More specifically, the charging rate toward the two batteries is automatically decreased such that the power supplied to the battery Ba included in the battery unit BU 1   a  and the battery Bb included in the battery unit BU 1   b  becomes 150 W in total, for example. 
     In other words, when the terminal voltage of the solar cell is decreased because of the newly connected battery unit BU 1   b , the output voltage from the control unit CU 1  to the battery unit BU 1   a  and the battery unit BU 1   b  is also decreased. When the terminal voltage of the solar cell becomes close to the lower limit, namely Vt 0 =75 V, from 100 V, the conversion unit  100   a  of the control unit CU 1  starts to step down the output voltage toward the battery unit BU 1   a  and battery unit BU 1   b  from 48 V to Vb=45 V. 
     When the output voltage from the control unit CU 1  toward the battery unit BU 1   a  and the battery unit BUb is stepped down, the input voltage to the battery unit BU 1   a  and battery unit BUb is decreased. Then, the charging control unit  140  included in the battery unit BU 1   a  and the charging control unit  140  included in the battery unit BU 1   b  start to step down the output voltage toward the battery Ba and battery Bb respectively. When the output voltage from each of the charging control units  140  is stepped down, charging the battery Ba and battery Bb is slowed down. In other words, the charging rate toward the respective batteries is reduced. 
     When the charging rate toward the respective batteries is reduced, the power consumption is reduced as a whole. Therefore, the load from the standpoint of the solar cell is reduced and the terminal voltage of the solar cell is increased (recovered) by an amount of the reduced load. 
     After that, same as the case where the illuminance to the solar cell is rapidly reduced, the charging rate is adjusted until the output voltage from the control unit CU 1  toward the battery unit BU 1   a  and battery unit BU 1   b  reaches a specific convergence value and the balance between demand and supply of power is achieved. 
     Note that the actual convergence voltage value is different depending on the situation. Accordingly, although the actual convergence voltage value is not clear, the convergence voltage value is estimated to be the voltage slightly higher than the lower limit Vt 0  value because charging is stopped when the terminal voltage of the solar cell becomes the lower limit, namely, Vt 0 =75 V. Also, since the respective battery units are not controlled in an interlocking manner, the charging rate is estimated to be different from one another due to variation of devices to be used although the respective battery units have the same configuration. However, consequently, there is no difference in being capable of executing the cooperative control. 
     Since the charging rate is adjusted in an extremely short time according to the cooperative control, the operating point of the solar cell moves to point i from the point g on the curve C 0  when the battery unit BU 1   b  is newly connected. Note that the operating point of the solar cell does not actually move to the point h under the cooperative control while the point h is illustrated in  FIG. 12B  as an example of the operating point on the curve C 0  for the sake of description. 
     Thus, according to the cooperative control, when the load is increased from the standpoint of the solar cell, the charging control unit  140  included in each of the battery units BU 1  detects the input voltage applied to oneself, and the charging control unit  140  in each of the battery units BU 1  automatically controls the amount of current. According to the cooperative control, the system  1  can be prevented from going down even in the case where the load is rapidly increased from the standpoint of the solar cell due to the increased number of battery units BU 1  connected to the control unit CU 1 . 
     Next, description will be given for the changes of the operating point at the time of executing the cooperative control in the case where both the illuminance to the solar cell and the load from the standpoint of the solar cell are changed. 
       FIG. 13  is an explanatory diagram for the changes of the operating point at the time of executing cooperative control in the case where both illuminance to the solar cell and the load in view of the solar cell are changed. In  FIG. 13 , the vertical axis represents terminal current of the solar cell and the horizontal axis represents the terminal voltage of the solar cell. Further, a shaded circle in  FIG. 13  indicates the operating point at the time of executing the cooperative control. The curves C 5  to C 8  illustrated in  FIG. 13  indicate the voltage-current characteristics of the solar cell in the case where illuminance to the solar cell is changed. 
     First, it is assumed that the battery unit BU 1   a  including the battery Ba and requiring the power of 100 W for charging is connected to the control unit CU 1 . Further, the voltage-current characteristics of the solar cell are indicated by the curve C 7 , and the operating point of the solar cell is indicated by point p on the curve C 7 . 
     As illustrated in  FIG. 13 , it is assumed that the terminal voltage of the solar cell at the point p is quite close to the voltage Vt 0  preset as the lower limit of the output voltage of the solar cell. The state in which the terminal voltage of the solar cell is quite close to the voltage Vt 0  indicates that the charging rate is adjusted by the cooperative control and the charging rate is extremely suppressed. In other words, the state in which the operating point of the solar cell is indicated by the point p illustrated in  FIG. 13  indicates that the power supplied to the battery Ba via the charging control unit  140  largely exceeds the power supplied to the conversion unit  100   a  of the control unit CU 1  from the solar cell. Therefore, in the state in which the operating point of the solar cell is indicated by the point p illustrated in  FIG. 13 , the charging rate is considerably adjusted, and the power quite smaller than 100 W is supplied to the charging control unit  140  that charges the battery Ba. 
     Next, it is assumed that illuminance to the solar cell is increased and the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 7  to the curve C 6 . Further, it is assumed that the battery unit BU 1   b  having the same configuration as the battery unit BU 1   a  is newly connected to the control unit CU 1 . At this point, the operating point of the solar cell moves to, for example, point q on the curve C 6  from the point p on the curve C 7 . 
     Since the two battery units are connected to the control unit CU 1 , the power consumption becomes 200 W when the charging control unit  140  included in the battery unit BU 1   a  and the charging control unit  140  included in the battery unit BU 1   b  fully charge the battery Ba and battery Bb. However, in the case where illuminance to the solar cell is not sufficient, the cooperative control is continued and the power consumption is adjusted less than 200 W (e.g., 150 W or the like). 
     Next, it is assumed that the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 6  to the curve C 5  by the sky clearing up and the like. At this point, when the generated power from the solar cell is increased due to the increase of illuminance to the solar cell, the output current from the solar cell is increased. 
     When the illuminance to the solar cell is sufficiently increased and then the generated power from the solar cell is further increased, the terminal voltage of the solar cell reaches to a sufficiently large value at a certain point, compared to the voltage Vt 0 . When the power supplied to the two batteries of the battery unit BU 1   a  and battery unit BU 1   b  exceeds the power required for charging the two batteries, charging rate adjustment according to the cooperative control is eased or automatically cancelled. 
     At this point, the operating point of the solar cell is indicated by, for example, point r on the curve C 5 , and charging the respective battery Ba and battery Bb is executed at the charging rate of 100%. 
     Next, it is assumed that illuminance to the solar cell is reduced and the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 5  to the curve C 6 . 
     Then, the terminal voltage of the solar cell is decreased, and when the terminal voltage of the solar cell becomes close to the preset voltage Vt 0 , the charging rate adjustment is executed again according to the cooperative control. The operating point of the solar cell at this point is indicated by the point q on the curve C 6 . 
     Next, it is assumed that illuminance to the solar cell is reduced and the curve indicating the voltage-current characteristics of the solar cell is changed from the curve C 6  to the curve C 8 . 
     Then, the charging rate is adjusted such that the terminal voltage of the solar cell does not fall below the voltage Vt 0 . Therefore, the terminal current from the solar cell is decreased and the operating point of the solar cell moves to point s on the curve C 8  from the point q on the curve C 6 . 
     According to the cooperative control, the balance between demand and supply of the power is adjusted between the control unit CU 1  and each of the battery units BU 1  such that the input voltage toward each of the battery units BU 1  does not fall below the preset voltage Vt 0 . Therefore, according to the cooperative control, the charging rate toward each of the batteries B can be changed in real time in accordance with input-side supply capacity from the standpoint of each of the battery units BU 1 . Thus, according to the cooperative control, it is possible to handle not only the change of illuminance to the solar cell but also the change of the load from the standpoint of the solar cell. 
     In the case of using the output from the solar power generator  3  in other blocks BL, the cooperative control is executed in the same manner. The balance between demand and supply of power is adjusted in the respective blocks, and as a result, the balance between demand and supply of the power is achieved in the entire system  1 . The system  1  can be prevented from going down even in the case where the output from the solar power generator  3  and wind force power generator  4  is decreased and even in the case where the load is increased from the standpoint of the solar power generator  3  and the like. 
     Note that the voltage corresponding to the predetermined rotary speed of the motor section included in the wind force power generator  4  may be input to the feedback system as described above. This can prevent the rotary speed of the motor section from falling below a predetermined number of times. 
     2. Second Embodiment 
     [2-1. Outline of Second Embodiment] 
     Next, a second embodiment will be described. The second embodiment has a system configuration same as a system  1  according to a first embodiment. A control unit and a battery unit constituting the system have the same configurations and operate same as the first embodiment. The description for the matters same as the first embodiment will be omitted, accordingly. 
     A control unit CU 2  in a block BL 2  includes a conversion unit  200   a , a conversion unit  200   b , and a conversion unit  200   c . Voltage V 3  is received by the conversion unit  200   a . Voltage V 4  is received by the conversion unit  200   b . Voltage V 5  is received by the conversion unit  200   c . The control unit CU 2  includes a CPU and a memory same as a control unit CU 1 . The CPU included in the control unit CU 2  is referred to as a CPU  210 , and the memory included in the control unit CU 2  is referred to as a memory  211 . 
     A control unit CU 3  in a block BL 3  includes a conversion unit  300   a , a conversion unit  300   b , and a conversion unit  300   c . The voltage V 3  is received by the conversion unit  300   a . The voltage V 4  is received by the conversion unit  300   b . The voltage V 5  is received by the conversion unit  300   c . The control unit CU 3  includes a CPU and a memory same as the control unit CU 1 . The CPU included in the control unit CU 3  is referred to as a CPU  310 , and the memory included in the control unit CU 3  is referred to as a memory  311 . 
     As described in the first embodiment, an output from any one of the conversion units can be preferentially used by suitably adjusting a resistance value of a variable resistor provided in each of the conversion units, for example. In other words, among an output from a solar power generator  3 , the output from a wind force power generator  4 , and the output from a biomass power generator  5 , the output from any of them can be preferentially supplied to a battery unit BU. 
     Meanwhile, for example, in the case where the output from the solar power generator  3  is preferentially used, only the conversion unit  100   a  is required to be started in a block BL 1 , and the conversion unit  100   b  and conversion unit  100   c  are not required to be started. According to the second embodiment, on-off control for the conversion unit in each of control units CU is efficiently executed by using a schedule table, for example. 
     [2-2. Operation Based on Schedule Table and Schedule Table] 
       FIG. 14  is a diagram illustrating an exemplary schedule table for two days. The schedule table includes, for example, a schedule table STA 1  for the control unit CU 1 , a schedule table STA 2  for the control unit CU 2 , and a schedule table STA 3  for the control unit CU 3 . In each of the schedule tables STA, on/off periods for each of the conversion units are described. In each of the schedule tables STA, an electronic switch of a relevant conversion unit is turned on in a time zone corresponding to a shaded portion, thereby starting the conversion unit. 
     The schedule table STA 1  is stored in a memory  111  included in the control unit CU 1 . The CPU  110  refers to the schedule table STA 1  and executes on-off control for the conversion unit  100   a , conversion unit  100   b , and conversion unit  100   c.    
     In accordance with the schedule table STA 1 , the CPU  110  turns on the electronic switch (electronic switch  10   c  and electronic switch  101   f ) of the conversion unit  100   a  in a daytime time zone (e.g., from 6 o&#39;clock in the morning to 18 o&#39;clock) during which it can be considered that the output from the solar power generator  3  is increased, and turns on the conversion unit  100   a.    
     The schedule table STA 2  is stored in the memory  211  in the control unit CU 2 . The CPU  210  refers to the schedule table STA 2 , and executes on-off control for the conversion unit  200   a , conversion unit  200   b , and conversion unit  200   c.    
     In accordance with the schedule table STA 2 , the CPU  210  turns on the electronic switch of the conversion unit  200   a  in the daytime time zone (e.g., from 6 o&#39;clock in the morning to 18 o&#39;clock) during which it can be considered that the output from the solar power generator  3  is increased, and turns on the conversion unit  200   a . The CPU  210  turns on the electronic switch of the conversion unit  200   c  in a night-time time zone (e.g., from 18 o&#39;clock to 6 o&#39;clock) during which it can be considered that the output from the solar power generator  3  is substantially zero. 
     The schedule table STA 3  is stored in a memory  311  included in the control unit CU 3 . The CPU  310  refers to the schedule table STA 3 , and executes on-off control for the conversion unit  300   a , conversion unit  300   b , and conversion unit  300   c.    
     In accordance with the schedule table STA 3 , the CPU  310  turns on the electronic switch of the conversion unit  300   a  in a time zone (e.g., from 10 o&#39;clock to 16 o&#39;clock) during which it can be considered that the output from the solar power generator  3  is largely increased, and turns on the conversion unit  300   a . The CPU  310  turns on the electronic switch of the conversion unit  300   b  in the night-time time zone (e.g., from 18 o&#39;clock to 6 o&#39;clock). 
     In the daytime, the conversion unit configured to mainly processes the voltage V 3  which is the output from the solar power generator  3  is started, and in the night time, the conversion unit configured to mainly processes the voltage V 4  and voltage V 5  which are outputs from the wind force power generator  4  and biomass power generator  5  is started. Further, in the time zone (from nearly noon to nearly evening time) during which the output from the solar power generator  3  is expected to be increased, all of the conversion units (conversion unit  100   a , conversion unit  200   a , and conversion unit  300   a ) which process voltage supplied from the solar power generator  3  are to be started, thereby effectively using the output from the solar power generator  3 . Furthermore, the conversion unit is prevented from being started needlessly. 
     Meanwhile, the schedule table STA used for processing can be updated (changed), if necessary. For instance, the respective control units CU keep the schedule table STA 1 , schedule table STA 2 , and schedule table STA 3 . On the first two days, the CPU  110  executes on-off control for the conversion unit  100  based on the schedule table STA 1 , on the next two days, executes on-off control for the conversion unit  100  based on the schedule table STA 2 , and on another next two days, executes on-off control for the conversion unit  100  based on the schedule table STA 3 . 
     On the first two days, the CPU  210  executes on-off control for the conversion unit  200  based on the schedule table STA 2 , on the next two days, executes on-off control for the conversion unit  200  based on the schedule table STA 3 , and on another next two days, executes on-off control for the conversion unit  200  based on the schedule table STA 1 . 
     On the first two days, the CPU  310  executes on-off control for the conversion unit  300  based on the schedule table STA 3 , on the next two days, executes on-off control for the conversion unit  300  based on the schedule table STA 1 , and on another next two days, executes on-off control for the conversion unit  300  based on the schedule table STA 2 . Thus, the respective control units CU keep a plurality of schedule tables STA, and may be configured to switch the using schedule table STA per predetermined period (e.g., per season). 
     Contents of the schedule tables STA may be dynamically changed. For instance, the CPU in each of the control units CU acquires information related to weather conditions (fair, cloudy, rainy, typhoon approach), and may change the schedule table STA to be referred to in accordance with the acquired information related to the weather conditions. The schedule table STA is created for each of the weather conditions. 
       FIG. 15  is a diagram illustrating examples of a schedule table STA 11 , a schedule table STA 12 , and a schedule table STA 13  when typhoon approach is anticipated. The CPU  110  refers to the schedule table STA 11 , and executes on-off control for the conversion unit  100 . The CPU  210  refers to the schedule table STA 12 , and executes on-off control for the conversion unit  200 . The CPU  310  refers to the schedule table STA 13 , and executes on-off control for the conversion unit  300 . 
     Since the wind power is expected to be increased because of the typhoon approach, the conversion units (conversion unit  100   b , conversion unit  200   b , and conversion unit  300   b ) which process the output from the wind force power generator  4  are turned on in all of the schedule table STA 11 , schedule table STA 12 , and schedule table STA 13 . Thus, the schedule table STA to be referred may be changed in accordance with the forecasted weather, for example. The schedule table STA to suit the forecasted weather may be transmitted from an external server to each of the control units CU. The schedule table STA may be also created based on statistical data in a place where the wind force power generator  4  is installed (e.g., time zone when the wind is strong, time zone when the wind is weak, etc.). 
     3. Third Embodiment 
     [3-1. Outline of Third Embodiment] 
     Next, a third embodiment will be described. The third embodiment has a system configuration same as a system  1  according to a first embodiment. A control unit and a battery unit constituting the system have the same configurations and operate same as the first embodiment. The description for the matters same as the first embodiment and a second embodiment will be omitted, accordingly. 
     The outline of the third embodiment will be described. As exemplified in the second embodiment, on-off control for a conversion unit  100  is executed based on a schedule table STA 1 , on-off control for a conversion unit  200  is executed based on a schedule table STA 2 , and on-off control for a conversion unit  300  is executed based on a schedule table STA 3 . For instance, starting the conversion unit  100   a  and conversion unit  200   a  at 6 o&#39;clock is commanded by the schedule table STA 1  and schedule table STA 2 . 
     The conversion unit  100   a  starts a battery unit required to be charged (e.g., battery unit BU 1   a ) and charges the battery unit BU 1   a . The conversion unit  200   a  starts the battery unit required to be charged (e.g., battery unit BU 2   a ) and charges the battery unit BU 2   a . Particularly, when a plurality of conversion units is started at a time and charging processing to battery units BU connected to each of the conversion units is executed in the case where voltage V 3  which is an output of a solar power generator  3  is small, the voltage V 3  may drop and system  1  may go down. According to the third embodiment, start of the conversion unit is suitably controlled, considering this point. 
     [3-2. Processing Flow] 
     A processing flow according to the third embodiment will be described. In the case where the plurality of conversion units that processes the output from the same power generator is started based on the schedule table STA, a CPU in each of the control unit CU executes, at a different timing, determining processing on whether to actually start each conversion unit. For instance, the determining processing on whether to actually start the conversion unit is executed in a time sharing manner. The timing of executing the determining processing on whether to actually start the conversion unit may be described in the schedule table STA. In the following description, description will be given by exemplifying a case in which starting the conversion unit  100   a  and conversion unit  200   a  in a same time zone is commanded by the schedule table STA. 
       FIG. 16  is a flowchart illustrating an exemplary processing flow according to the third embodiment. In step ST 1 , determination is made on whether any one of the conversion unit  100   a  and conversion unit  200   a  commanded to be started by the schedule table STA has been started or not. In an initial state, the processing proceeds to step ST 2  because the conversion unit  100   a  and conversion unit  200   a  have not been started yet. 
     The determining processing on whether to start the converting unit is executed by the control unit CU 1  first. The determining processing may also be started from a control unit CU 2 . In step ST 2 , searching processing for a conversion unit that can be started is executed. The conversion unit that can be started means, for example, a conversion unit commanded to be started by the schedule table STA. Here, since starting the conversion unit  100   a  is commanded by the schedule table STA, the conversion unit  100   a  is set as the conversion unit that can be started. Then, the processing proceeds to step ST 3 . 
     In step ST 3 , a CPU  110  communicates with the CPU of the battery unit (e.g., battery unit BU 1   a , battery unit BU 1   b , and battery unit BU 1   c ) connected to the control unit CU 1 . By this communication, the CPU  110  acquires information related to residual capacity of a battery B included in each battery unit BU 1 . 
     The CPU  110  searches for the battery unit BU 1  needed to be charged based on the acquired information related to the residual capacity and determines, based on a searching result, the battery unit to be charged. The CPU  110  determines the battery unit BU 1  having the least residual capacity as the battery unit to be charged, for example. Here, description will be given, assuming that the battery unit BU 1   a  is determined as the battery unit to be charged. Then, the processing proceeds to step ST 4 , and the conversion unit  100   a  is determined as the conversion unit to be started. Meanwhile, in the case where all of the battery units BU 1  connected to the control unit CU 1  have the residual capacity exceeding a threshold, later-described processing (processing indicated by A in  FIG. 16 ) in the control unit CU 2  may be executed. Then, the processing proceeds to step ST 5 . 
     In step ST 5 , determination is made on whether the voltage V 3  which is input voltage to the conversion unit  100   a  is larger than a specified value. The specified value is a voltage value for determining whether to start the conversion unit and is set to 90 V, for example. The voltage V 3  is acquired by a voltage sensor (e.g., voltage sensor  101   b ) included in the conversion unit  100   a , and the acquired sensor information is supplied to the CPU  110 . As a result of the determination, the determining processing in step ST 5  is repeated for a predetermined period in the case where the voltage V 3  does not exceed the specified value. In the case where the voltage V 3  does not exceed 90 V even after the determining processing is repeated for the predetermined period, the processing by the CPU  110  of the control unit CU 1  finishes, and the processing by the CPU  210  of the control unit CU 2  is executed. 
     In step ST 5 , in the case where the voltage V 3  exceeds 90 V, the processing proceeds to step ST 6 . In step ST 6 , the CPU  110  turns on an electronic switch  101   c  and an electronic switch  101   f  to start the conversion unit  100   a . At this point, power is not consumed in the battery unit BU 1 , and therefore, voltage V 10  which is an output of the conversion unit  100   a  becomes substantially 48 V. Then, the processing proceeds to step ST 7 . 
     In step ST 7 , the CPU  110  transmits a control command to a CPU  145  of the battery unit BU 1   a  to turn on and start charging. The CPU  145  starts the charging control unit  140  in accordance with the control command and charges the battery Ba. Then, the processing proceeds to step ST 8 . 
     In step ST 8 , determination is made on whether the voltage V 10  which is the output voltage of the conversion unit  100   a  is larger than a specified value. This specified value is a value indicating whether there is enough power for supply and charging other battery units BU is permitted or not. The specified value is set to, for example, 47 V. The voltage V 10  is acquired by, for example, the voltage sensor  101   g.    
     In the case where the voltage V 10  is 47 V or less, the processing returns to step ST 8 . Meanwhile, in the case where the voltage V 10  does not exceed 47 V although the determining processing in step ST 8  is repeated for a predetermined period, the processing by the CPU  110  of the control unit CU 1  finishes, and the processing by the CPU  210  of the control unit CU 2  is executed. In the case where the voltage V 10  is larger than 47 V, the processing proceeds to step ST 9 . 
     In step ST 9 , determination is made on whether there is other battery unit needed to be charged besides the battery unit BU 1   a . For example, the battery unit having the second least residual capacity is set as the battery unit needed to be charged. In step ST 9 , in the case where there is any battery unit needed to be charged, the processing proceeds to step ST 10 . In step ST 10 , control for charging the battery of the concerning battery is executed in the same manner as step ST 7 . In step ST 9 , in the case there is no battery unit needed to be charged besides the battery unit BU 1   a , the processing proceeds to A. Note that the processing referred to as A in  FIG. 16  indicates continuity to later-described processing in  FIG. 17  and does not indicate any specific processing. 
     The processing may also proceed to the processing A after step ST 8  without executing the processing in step ST 9 . For instance, the number of the battery units that can be charged in each block BL may be limited to one. The processing may be executed in consideration of necessity or urgency of charging the battery units connected to other control units. In this case, in the case where there is enough energy to supply, determining step (algorithm) for the battery unit to be charged is described in a program executed by the CPU in each control unit. 
       FIG. 17  is a flowchart illustrating the processing flow continued from A shown in  FIG. 16 . The processing exemplified in  FIG. 17  is executed by the control unit CU 2 , for example. As described above, the process for determining whether to actually start the conversion unit is executed between the control units CU, for example, in a time sharing manner. There is no need to provide a configuration for executing communication between control units CU. 
     In step ST 20 , searching processing for a conversion unit that can be started is executed. The conversion unit that can be started means, for example, a conversion unit commanded to be started by the schedule table STA. Here, the conversion unit  200   a  is commanded to be started by the schedule table STA, and therefore, the conversion unit  200   a  is set as the conversion unit that can be started. Then, the processing proceeds to step ST 21 . 
     In step ST 21 , determination is made on whether the voltage V 3  which is the input voltage to the conversion unit  200   a  is larger than a specified value. The specified value is a voltage value for determining whether to start the conversion unit and is set to 90 V, for example. The voltage V 3  is acquired by a voltage sensor included in the conversion unit  200   a , and the acquired sensor information is supplied to the CPU  210 . As a result of the determination, the determining processing in step ST 21  is repeated for a predetermined period in the case where the voltage V 3  does not exceed the specified value. In the case where the voltage V 3  does not exceed 90 V even though the determining processing is repeated for the predetermined period, the processing finishes. In other words, it is determined that there is not enough power for supply, and the conversion unit  200   a  is not started. 
     In step ST 21 , in the case where the voltage V 3  exceeds 90 V, the processing proceeds to step ST 22 . In step ST 22 , the CPU  210  turns on the electronic switch included in the conversion unit  200   a  to start the conversion unit  200   a . Then, the processing proceeds to step ST 23 . 
     In step ST 23 , determination is made on whether the output voltage of the conversion unit  200   a  is larger than a specified value. The output voltage of the conversion unit  200   a  is supplied from the conversion unit  200   a  to the battery unit BU 2 . In the following, the output voltage of the conversion unit  200   a  is conveniently referred to as voltage V 20 . 
     This specified value in step ST 23  is a value indicating whether there is enough power for supply and charging the battery unit BU is permitted or not. The specified value is set to, for example, 47 V. In the case where the voltage V 20  is 47 V or less, the processing returns to step ST 23 . In the case where the voltage V 20  does not exceed 47 V even though the determining processing in step ST 23  is repeated for the predetermined period, the processing finishes. In this case, it is determined that there is not enough power for supply and control for executing charge is not executed. In the case where the voltage V 20  is larger than 47 V, the processing proceeds to step ST 24 . 
     En step ST 24 , a predetermined battery unit out of the battery units BU 2  connected to the control unit CU 2  is charged. For instance, among the battery unit BU 2   a , battery unit BU 2   b , and battery unit BU 2   c , a battery unit having the least residual capacity is determined as the battery unit to be charged. In the case where all of the battery unit BU 2   a , battery unit BU 2   b , and battery unit BU 2   c  connected to the battery unit BU 2  are not needed to be charged, the processing finishes without executing charging. 
     The CPU  210  in the control unit CU 2  controls charging the battery unit to be charged. Contents of control are same as the contents of control in step ST 7  and step ST 10  described above. Therefore, repetition of description will be omitted here. Then, the processing proceeds to step ST 25 . 
     In step ST 25 , determination is made on whether the voltage V 20  is larger than 47 V, same as in ST 23 . In the case where the voltage V 20  is 47 V or less, the processing returns to ST 25  and the determining processing in step ST 25  is repeated. In the case where the voltage V 20  does not exceed 47 V even though the determining processing is repeated for the predetermined period, the processing finishes. 
     In the case where the voltage V 20  is larger than 47 V in step ST 25 , the processing proceeds to step ST 26 . In step ST 26 , determination is made on whether there is any battery unit needed to be charged. In the case where there is not any battery unit needed to be charged, the processing finishes. In the case where there is a battery unit needed to be charged, the processing proceeds to step ST 27  to executed the processing to charge the battery unit. 
     Thus, even in the case where starting the plurality of conversion units processing the outputs of the same power generator is commanded by the schedule table STA, the plurality of conversion unit is not started at the same time. The processing to determine whether to actually start the conversion unit is executed, and then on-off control for the conversion unit is executed based on the determining result. 
     By monitoring the output from the power generator (e.g., voltage V 3 ), it is possible to determine whether there is enough power to supply to a load side. In the case where there is enough power for supply, control is executed so as to start the next conversion unit, and therefore, the output of the power generator is prevented from dropping and the system  1  is prevented from going down. 
     Even in the case where starting the conversion unit is commanded by the schedule table STA, the conversion unit is not always actually started. In  FIG. 18 , the time when each of the conversion units is actually started is schematically illustrated. The time when the conversion unit is actually started is schematically illustrated by a reference sign OT. Thus, according to the schedule table STA, starting the conversion unit is permitted only, and whether the conversion unit is actually started or not is suitably controlled by the output from the power generator. 
     The third embodiment is not limited to the start using the schedule table STA. The third embodiment may be further modified as follows. For instance, the battery unit BU 1   a  is charged in accordance with control by the control unit CU 1 . The voltage V 3  is decreased to, for example, 90 V or less. Since the voltage V 3  is decreased to 90 V or less, the conversion unit  200   a  in the control unit CU 2  is not started. 
     The control unit CU 2  acquires voltage V 4  acquired by a voltage switch provided at an input stage of the conversion unit  200   b . In the case where the voltage V 4  is larger than 90 V, the battery unit BU 2  connected to the control unit CU 2  may be charged by using the voltage V 4 . In other words, in the case of determining that there is not enough power for supply from a certain power generator, the conversion unit that processes the output from other power generator may be started. 
     Further, determination on whether to receive inputs from different power generators may be made one after the other. For instance, it is assumed that starting three conversion units that process the output from the solar power generator  3  is permitted, and starting one conversion unit that processes the output from the wind force power generator  4  is permitted by the schedule table STA. For instance, first determination is made on whether the voltage V 3  is 90 V or more in the conversion unit  100   a  of the control unit CU 1 , and then, determination may be made whether the voltage V 4  is 90 V or more in the conversion unit  200   b  of the control unit CU 2 . The determination in the conversion unit  100   a  and the determination in the conversion unit  200   b  may be made at the same time. Note that other processing is executed same as the processing described above. 
     As illustrated in  FIG. 19 , a maximum number of the conversion units that can be started may be described in the schedule table STA. A schedule table STA 21 , a schedule table STA 22 , and a schedule table STA 23  exemplified in  FIG. 19  show schedules for two days. Normally, the schedule table STA 21  is used. The schedule table STA 22  is used in the case where a typhoon comes on the first day, and the weather gets better on the next day. The schedule table STA 23  is used in the case where the weather is cloudy, rainy, and the like. 
     The schedule table STA 21  will be described. Among the three conversion units (conversion unit  100   a , conversion unit  200   a , and conversion unit  300   a ) that process the output (voltage V 3 ) from the solar power generator  3 , the maximum number of the conversion units that can turned on in each time zone is described in the schedule table STA 21 . Among the three conversion units (conversion unit  100   b , conversion unit  200   b , and conversion unit  300   b ) that process the output voltage (voltage V 4 ) from the wind force power generator  4 , the maximum number of the conversion units that can be turned on in each time zone is described in the schedule table STA 21 . Among the three conversion units (conversion unit  100   c , conversion unit  200   c , and conversion unit  300   c ) that process the output voltage (voltage V 5 ) from the biomass power generator  5 , the maximum number of the conversion units that can be turned on in each time zone is described in the schedule table STA 21 . 
     The schedule table STA 22  will be described. The wind is strong before and after the typhoon passes. Therefore, in the schedule table STA 22 , the maximum number (e.g., 3) is set such that as many conversion units as possible that process the output (voltage V 4 ) of the wind force power generator  4  can be used. Further, after the typhoon has passed, the maximum number is set such that the conversion units that process the output voltage (voltage V 3 ) of the solar power generator  3  can be used same as the normal state. 
     The schedule table STA 23  will be described. In the cloudy day or rainy day, it can be considered that the output (voltage V 3 ) of the solar power generator  3  is little. Therefore, among the three conversion units (conversion unit  100   c , conversion unit  200   c , and conversion unit  300   c ) that process the output (voltage V 5 ) of the biomass power generator  5 , the number of the conversion units that can be turned on is set to two such that the voltage V 5  can be used. 
     Of course, the values indicated by the schedule table STA 21 , schedule table STA 22 , and schedule table STA 23  are the maximum numbers of the conversion units that can be turned on, and the number of the conversion units actually turned on does not constantly conforms to the maximum number. The number of the conversion units actually turned on is suitably determined in accordance with the output and the like from each of the power generators. 
     It has been described that the conversion unit  100  acquires the values of the input voltage in a time sharing manner according to the third embodiment, but the embodiment is not limited thereto. For instance, as illustrated in  FIG. 20 , a host controller connected to each of the control units CU may be provided. The host controller is formed of, for example, a personal computer (PC). The control command is transmitted from the personal computer PC to each of the CPUs (e.g., CPU  110 , CPU  210 , and CPU  310 ) of the respective control units CU. The CPU of each of the control units CU may be configured to acquire the value of input voltage (value acquired by a voltage sensor included in a predetermined conversion unit) in accordance with the control command, and determine whether the value of the input voltage is larger than 90 V. 
     4. Modified Example 
     While an embodiment of the present disclosure has been described above, the present disclosure is not limited thereto and various modifications may be made. The configurations, operations, values, etc. according to the above-described plurality of embodiments are examples, and the contents of the present disclosure are not limited to the exemplified configurations and the like. 
     The present disclosure can be implemented not only as the device but also as a method, a program, and a storage medium. 
     Note that the configurations and operation according to the embodiments and the modified example may be suitably combined without departing from the technical spirit and scope of the present disclosure. The sequences of respective processing in the exemplified processing flows may be suitably changed without departing from the technical spirit and scope of the present disclosure. 
     The present disclosure may be applied to a so-called cloud system whereby the exemplified processing is executed by a plurality of devices in a distributed manner. The present disclosure may be implemented as a system whereby the exemplified processing is executed and as a device whereby at least a part of the exemplified processing is executed. 
     The present disclosure may also have the following configurations. 
     (1) 
     A control system including: 
     a plurality of first devices; and 
     at least one second device connected to each of the plurality of first devices, wherein 
     the first device includes a plurality of conversion units configured to convert first voltage supplied from a power generator to second voltage in accordance with a level of the first voltage, 
     the second device includes a power storage unit and a charging control unit configured to control charging the power storage unit, 
     the second voltage output from at least one conversion unit out of the plurality of conversion units is supplied to second device, and 
     the charging control unit controls charging the power storage unit in accordance with fluctuation of the second voltage. 
     (2) 
     The control system according to (1), wherein the conversion unit converts the first voltage such that the second voltage is increased in the case where the first voltage is increased, and converts the first voltage such that the second voltage is decreased in the case where the first voltage is decreased. 
     (3) 
     The control system according to (1) or (2), wherein the charging control unit increases a charging rate toward the power storage unit in the case where the second voltage is increased, and decreases the charging rate toward the power storage unit in the case where the second voltage is decreased. 
     (4) 
     The control system according to any of (1) to (3), wherein the different first voltage is supplied to each of the plurality of conversion units from the different power generator. 
     (5) 
     The control system according to any of (1) to (4), wherein the power generator is at least one of a solar power generator, a wind force power generator, and a biomass power generator. 
     (6) 
     The control system according to any of (1) to (5), wherein the second device is detachably attached to the first device. 
     (7) 
     The control system according to any of (1) to (6), wherein the first voltage is supplied to the second device via a predetermined line. 
     (8) 
     The control system according to any of (1) to (7), wherein the conversion unit includes a sensor configured to acquire a value of the first voltage. 
     (9) 
     The control system according to any of (1) to (8), wherein the charging control unit includes a sensor configured to acquire a value of the second voltage. 
     (10) 
     The control system according to any of (1) to (9), wherein the first device can communicate with the second device. 
     REFERENCE SIGNS LIST 
     
         
           1  System 
           3  Solar power generator 
           4  Wind force power generator 
           5  Biomass power generator 
           100   a ,  100   b ,  100   c  Conversion unit 
           101   b ,  142   b  Voltage sensor 
           140  Charging control unit 
           110 ,  145 ,  210 ,  310  CPU 
         CU Control unit 
         BU Battery unit 
         Ba Battery 
         V 3 , V 4 , V 5 , V 10  Voltage 
         L 10  Power line 
         STA Schedule table