Patent Publication Number: US-11646578-B2

Title: Power supply system

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2018-223880 filed on Nov. 29, 2018 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a power supply system. 
     2. Description of Related Art 
     A power supply system that includes a plurality of modules of which each includes a battery and a circuit and performs at least one of outputting of electric power to the outside and storage of electric power which is input from the outside by controlling the plurality of modules is known. For example, a power supply device (a power supply system) described in Japanese Patent Application Publication No. 2018-74709 (JP 2018-74709 A) includes a plurality of battery circuit modules of which each includes a battery, a first switching element, and a second switching element. The plurality of battery circuit modules are connected in series with output terminals interposed therebetween. A control circuit of the power supply device outputs a gate signal for switching the first switching element and the second switching element between ON and OFF to the battery circuit modules at intervals of a predetermined time. Accordingly, a target electric power is output from the plurality of battery circuit modules. 
     SUMMARY 
     The power supply device described in JP 2018-74709 A can be additionally provided with a device that detects a state of the power supply device based on a current value. When the state of the power supply device is monitored using such a device and a current flowing in the power supply device is small, an error in the device that detects the state of the power supply device based on a current value may increase and cause misunderstanding of the state of the power supply device. A plurality of power supply devices can be incorporated into a power distribution device connected to a power system to be parallel to each other. However, when a plurality of power supply devices is incorporated into a power distribution device to be parallel to each other and electric power which is required by the power distribution device is small, a current flowing in one power supply device decreases. This may cause misunderstanding of the state of the power supply device. 
     According to an aspect of the disclosure, there is provided a power supply system including: a power distribution device that is connected to a power system; a plurality of strings that is connected in parallel to the power distribution device; and a control device. Each string includes a main line that is connected to the power distribution device and a plurality of sweep modules that is disposed along the main line. Each sweep module includes a battery module, an input and output circuit that is configured to connect the battery module in series to the main line, and at least one switching element that is provided in the input and output circuit and is configured to switch between connection and disconnection between the battery module and the main line. The control device is configured to control inputting of electric power from the power system connected to the power distribution device to the plurality of strings connected to the power distribution device and outputting of electric power from the plurality of strings to the power system. The control device is configured to perform a first process of determining certain strings out of the plurality of strings connected in parallel to the power distribution device and a second process of performing inputting of electric power to the plurality of strings connected in parallel to the power distribution device or outputting electric power from the plurality of strings to the power distribution device using at least the certain strings. 
     With this power supply system, it is possible to stably easily secure a current value required for a string. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG.  1    is a diagram schematically illustrating a configuration of a power supply system  1 ; 
         FIG.  2    is a diagram schematically illustrating a configuration of a sweep module  20 ; 
         FIG.  3    is a timing chart illustrating an example of a sweep operation; 
         FIG.  4    is a timing chart illustrating an example of a forcible through operation; 
         FIG.  5    is a block diagram illustrating a control device  100  of the power supply system  1 ; and 
         FIG.  6    is a flowchart illustrating an example of a first process and a second process in the control device  100 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an exemplary embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. Details which are not particularly mentioned in this specification and which are required for embodiment can be understood as design details based on the related art by those skilled in the art. The disclosure can be embodied based on details described in this specification and common general technical knowledge in the art. In the following drawings, members and parts performing the same operations will be referred to by the same reference signs. The dimensional relationships in the drawings do not reflect actual dimensional relationships. 
     &lt;Overall Schematic Configuration&gt; 
     The overall configuration of a power supply system  1  according to an embodiment will be schematically described below with reference to  FIG.  1   . The power supply system  1  performs at least one of outputting of electric power to a power distribution device  5  which is connected to a host power system  8  and storage of electric power which is input from the power distribution device  5  (hereinafter simply referred to as “inputting and outputting of electric power”). For example, in this embodiment, a power conditioning subsystem (PCS) is used as the power distribution device  5 . The PCS has a function of converting electric power input from the power system  8  to the power supply system  1  or the like and electric power output from the power supply system  1  or the like to the power system  8  between the power supply system  1  or the like and the power system  8 . 
     When electric power is surplus to the power system  8 , the power distribution device  5  outputs the surplus electric power to the power supply system  1 . In this case, the power supply system  1  stores electric power which is input from the power distribution device  5 . The power supply system  1  outputs electric power stored in the power supply system  1  to the power distribution device  5  in accordance with an instruction from a host system  6  that controls the host power system  8 . In  FIG.  1   , the host system  6  is a system that controls the power system  8  and the power distribution device  5  and is provided separately from the power system  8  and the power distribution device  5 . However, the host system  6  may be incorporated into the power system  8  or the power distribution device  5 . 
     The power supply system  1  includes one or more strings  10 . The power supply system  1  according to this embodiment includes a plurality of (N: N≥2) strings  10  ( 10 A,  10 B, . . . ,  10 N). In  FIG.  1   , for the purpose of convenience, only two strings  10 A and  10 B out of the N strings  10  are illustrated. Each string  10  serves as a unit for inputting and outputting electric power to and from the power distribution device  5 . The plurality of strings  10  is connected in parallel to the power distribution device  5 . Inputting and outputting (supply of power) of electric power between the power distribution device  5  and each string  10  is performed via a main line  7 . 
     Each string  10  includes a string control unit (SCU)  11  and a plurality of (M: M≥2) sweep modules  20  ( 20 A,  20 B, . . . ,  20 M). Each sweep module  20  includes a battery and a control circuit. The SCU  11  is provided for each string  10 . The SCU  11  is a controller that comprehensively controls the plurality of sweep modules  20  included in the corresponding string  10 . Each SCU  11  communicates with a group control unit (GCU)  2  serving as a power control device. The GCU  2  is a controller that comprehensively controls a group including the plurality of strings  10  as a whole. The GCU  2  communicates with the host system  6  and the SCUs  11 . Various methods (for example, at least one of wired communication, wireless communication, and communication via a network) can be employed as a method of communication between the host system  6 , the GCU  2 , and the SCUs  11 . 
     The configuration of the controllers that control the strings  10 , the sweep modules  20 , and the like may be modified. For example, the GCU  2  and the SCUs  11  may not be separately provided. That is, one controller may control the whole group including one or more strings  10  and all the plurality of sweep modules  20  included in the string  10 . 
     &lt;Sweep Module&gt; 
     A sweep module  20  will be described below in detail with reference to  FIG.  2   . The sweep module  20  includes a battery module  30 , a power circuit module  40 , and a sweep unit (SU)  50 . 
     The battery module  30  includes at least one battery  31 . A plurality of batteries  31  is provided in the battery module  30  according to this embodiment. The plurality of batteries  31  is connected in series. In this embodiment, a secondary battery is used as each battery  31 . At least one of various secondary batteries (for example, a nickel-hydride battery, a lithium ion battery, and a nickel-cadmium battery) can be used as the battery  31 . In the power supply system  1 , a plurality of types of batteries  31  may be mixed. The types of the batteries  31  in all the battery modules  30  may be the same. 
     A voltage detecting unit  35  and a temperature detecting unit  36  are provided in the battery module  30 . The voltage detecting unit  35  detects a voltage of the batteries  31  in the battery module  30  (the plurality of batteries  31  connected in series in this embodiment). The temperature detecting unit  36  detects a temperature of the batteries  31  in the battery module  30  or a temperature near the batteries  31 . Various devices (for example, a thermistor) that detect a temperature can be used as the temperature detecting unit  36 . 
     The battery module  30  is provided to be attached to and detached from the power circuit module  40 . Specifically, in this embodiment, with the battery module  30  including a plurality of batteries  31  as one unit, detachment of the battery module  30  from the power circuit module  40  and attachment thereof to the power circuit module  40  are performed. Accordingly, in comparison with a case in which the batteries  31  in the battery module  30  are replaced one by one, the number of operation steps when an operator replaces the batteries  31  decreases. In this embodiment, the voltage detecting unit  35  and the temperature detecting unit  36  are replaced separately from the battery module  30 . However, at least one of the voltage detecting unit  35  and the temperature detecting unit  36  may be replaced along with the battery module  30 . 
     The power circuit module  40  forms a circuit for appropriately realizing inputting and outputting of electric power in the battery module  30 . In this embodiment, the power circuit module  40  includes at least one switching element that switches between connection and disconnection between the battery module  30  and the main line  7 . In this embodiment, the power circuit module  40  includes an input and output circuit  43  that connects the battery module  30  to the main line  7  and a first switching element  41  and a second switching element  42  that are provided in the input and output circuit  43 . The first switching element  41  and the second switching element  42  perform a switching operation in accordance with a signal (for example, a gate signal) which is input form the sweep unit  50 . 
     In this embodiment, as illustrated in  FIG.  2   , the first switching element  41  is attached in series to the main line  7  and in parallel to the battery module  30  in the input and output circuit  43 . The second switching element  42  is attached to a part of the input and output circuit  43  that connects the battery module  30  in series to the main line  7 . A source and a drain of the first switching element  41  are disposed such that a forward direction thereof is set to a direction in which a discharging current flows in the main line  7 . A source and a drain of the second switching element  42  are disposed in the input and output circuit  43  attaching the battery module  30  in series to the main line  7  such that a forward direction thereof is set to a direction in which a charging current flows in the battery module  30 . In this embodiment, the first switching element  41  and the second switching element  42  are MOSFETs (for example, Si-MOSFETs) and include body diodes  41   a  and  42   a , respectively, set to a forward direction. Here, the body diode  41   a  of the first switching element  41  can be appropriately referred to as a first body diode. The body diode  42   a  of the second switching element  42  can be appropriately referred to as a second body diode. 
     The first switching element  41  and the second switching element  42  are not limited to the example illustrated in  FIG.  2   . Various elements that can switch between connection and disconnection can be used as the first switching element  41  and the second switching element  42 . In this embodiment, a MOSFET (specifically an Si-MOSFET) is used as both the first switching element  41  and the second switching element  42 . However, an element (for example, a transistor) other than a MOSFET may be employed. 
     The power circuit module  40  includes an inductor  46  and a capacitor  47 . The inductor  46  is provided between the battery module  30  and the second switching element  42 . The capacitor  47  is connected in parallel to the battery module  30 . In this embodiment, since secondary batteries are used as the batteries  31  of the battery module  30 , it is necessary to curb deterioration of the batteries  31  due to an increase in internal resistance loss. Accordingly, by forming an RLC filter using the battery module  30 , the inductor  46 , and the capacitor  47 , equalization of a current is achieved. 
     A temperature detecting unit  48  is provided in the power circuit module  40 . The temperature detecting unit  48  is provided to detect emission of heat from at least one of the first switching element  41  and the second switching element  42 . In this embodiment, the first switching element  41 , the second switching element  42 , and the temperature detecting unit  48  are assembled into one base. Accordingly, the base is replaced at a time point at which a defect of one of the first switching element  41  and the second switching element  42  has been detected. Accordingly, in this embodiment, by providing one temperature detecting unit  48  near the first switching element  41  and the second switching element  42 , it is possible to decrease the number of components. Here, a temperature detecting unit that detects the temperature of the first switching element  41  and a temperature detecting unit that detects the temperature of the second switching element  42  may be provided separately from each other. Various devices (for example, a thermistor) that detect a temperature can be used as the temperature detecting unit  48 . 
     As illustrated in  FIGS.  1  and  2   , a plurality of battery modules  30  in the string  10  are connected in series to the main line  7  with the power circuit modules  40  interposed therebetween. By appropriately controlling the first switching element  41  and the second switching element  42  of each power circuit module  40 , the corresponding battery module  30  is connected to the main line  7  or is disconnected from the main line  7 . In the example of the configuration of the power circuit module  40  illustrated in  FIG.  2   , when the first switching element  41  is turned off and the second switching element  42  is turned on, the battery module  30  is connected to the main line  7 . When the first switching element  41  is turned on and the second switching element  42  is turned off, the battery module  30  is disconnected from the main line  7 . 
     The sweep unit (SU)  50  is a control unit that is incorporated into the sweep module  20  such that various controls associated with the sweep module  20  are executed, and is also referred to as a sweep control unit. Specifically, the sweep unit  50  outputs a signal for driving the first switching element  41  and the second switching element  42  in the power circuit module  40 . The sweep unit  50  notifies a host controller (the SCU  11  illustrated in  FIG.  1    in this embodiment) of states of the sweep module  20  (for example, the voltage of the battery module  30 , the temperature of the batteries  31 , and the temperature of the switching elements  41  and  42 ). The sweep unit  50  is incorporated into each of a plurality of sweep modules  20  of each string  10 . The sweep units  50  incorporated into the plurality of sweep modules  20  of each string  10  are sequentially connected to each other and are configured to allow a gate signal GS which is output from the SCU  11  to propagate sequentially. As illustrated in  FIG.  2   , in this embodiment, each sweep unit  50  includes an SU processing unit  51 , a delay/selection circuit  52 , and a gate driver  53 . 
     The SU processing unit  51  is a controller that takes charge of various processes in the sweep unit  50 . For example, a microcomputer can be used as the SU processing unit  51 . Detection signals from the voltage detecting unit  35 , the temperature detecting unit  36 , and the temperature detecting unit  48  are input to the SU processing unit  51 . The SU processing unit  51  performs inputting and outputting various signals to and from a host controller (the SCU  11  of the string  10  in this embodiment). 
     The signals which are input from the SCU  11  to the SU processing unit  51  include a forcible through signal CSS and a forcible connection signal CCS. The forcible through signal CSS is a signal for instructing to disconnect the battery module  30  from the main line  7  (see  FIG.  1   ) extending from the power distribution device  5  to the string  10 . That is, the sweep module  20  to which the forcible through signal CSS is input ignores an operation for inputting and outputting electric power to and from the power distribution device  5 . The forcible connection signal CCS is a signal for instructing to maintain connection of the battery module  30  to the main line  7 . 
     A gate signal GS is input to the delay/selection circuit  52 . The gate signal (a PWM signal in this embodiment) GS is a signal for controlling an alternate repeated switching operation between an ON state and an OFF state of the first switching element  41  and the second switching element  42 . The gate signal GS is a pulse-shaped signal in which ON and OFF are alternately repeated. The gate signal GS is first input to the delay/selection circuit  52  in one sweep module  20  from the SCU  11  (see  FIG.  1   ). Subsequently, the gate signal GS propagates sequentially from the delay/selection circuit  52  of one sweep module  20  to the delay/selection circuit  52  of another sweep module  20 . 
     In each string  10 , sweep control which is illustrated in  FIGS.  3  and  4    is executed. Here,  FIG.  3    is a timing chart illustrating an example of a sweep operation. Specifically, in  FIG.  3   , a relationship between a connection state of the sweep modules  20  and a voltage output to the power distribution device  5  when all the sweep modules  20  execute the sweep operation is illustrated as an example.  FIG.  4    is a timing chart illustrating an example of a forcible through operation. Specifically, in  FIG.  4   , a relationship between a connection state of the sweep modules  20  and a voltage output to the power distribution device  5  when certain sweep modules  20  execute the forcible through operation is illustrated as an example. 
     In sweep control which is executed in each string  10 , the number m of sweep modules  20  which are turned on at the same time out of a plurality of (for example, M) sweep modules  20  incorporated into the string  10  is determined. The gate signal GS in sweep control has, for example, a pulse-shaped waveform. In the gate signal GS, for example, a signal waveform for connecting the battery module  30  to the main line  7  and a signal waveform for disconnecting the battery module  30  from the main line  7  may be sequentially disposed. In the gate signal GS, the signal waveform for connecting the battery module  30  to the main line  7  may embed the number of battery modules  30  which are connected to the main line  7  in a predetermined period T in which the string  10  is swept. The signal waveform for disconnecting the battery module  30  from the main line  7  may embed the number of battery modules  30  which are to be disconnected from the main line  7  out of the battery modules  30  incorporated into the string  10 . In the signal waveform for connecting the battery module  30  to the main line  7  and the signal waveform for disconnecting the battery module  30  from the main line  7 , wavelengths thereof and the like are appropriately adjusted. 
     In each string  10  according to this embodiment, M sweep modules  20  are connected in series in the order of sweep modules  20 A,  20 B, . . . ,  20 M from the power distribution device  5 . In the following description, a side which is close to the power distribution device  5  is defined as an upstream side, and a side which is distant from the power distribution device  5  is defined as a furthest downstream side. First, the gate signal GS is input from the SCU  11  to the delay/selection circuit  52  of the sweep unit  50  in the sweep module  20 A which is furthest upstream. Subsequently, the gate signal GS propagates from the delay/selection circuit  52  of the sweep module  20 A to the delay/selection circuit  52  of the sweep module  20 B adjacent thereto downstream. Propagation of the gate signal to the sweep module  20  adjacent thereto downstream is sequentially repeated up to the sweep module  20 M which is furthest downstream. 
     Here, the delay/selection circuit  52  can allow a pulse-shaped gate signal GS which is input from the SCU  11  or the upstream sweep module  20  to propagate to the downstream sweep module  20  with a delay of a predetermined delay time. In this case, a signal indicating the delay time is input from the SCU  11  to the sweep unit  50  (the SU processing unit  51  in the sweep unit  50  in this embodiment). The delay/selection circuit  52  delays the gate signal GS based on the delay time indicated by the signal. The delay/selection circuit  52  may allow the input gate signal GS to propagate to the downstream sweep module  20  without a delay. 
     The gate driver  53  drives the switching operations of the first switching element  41  and the second switching element  42 . The delay/selection circuit  52  outputs a signal for controlling driving of the gate driver  53  to the gate driver  53 . The gate driver  53  outputs control signals to the first switching element  41  and the second switching element  42 . When the battery module  30  is to be connected to the main line  7 , the gate driver  53  outputs a control signal for turning off the first switching element  41  and turning on the second switching element  42 . When the battery module  30  is disconnected from the main line  7 , the gate driver  53  outputs a control signal for turning on the first switching element  41  and the turning off the second switching element  42 . 
     The delay/selection circuit  52  in this embodiment is controlled by a controller such as the SCU  11  and selectively performs a sweep operation, a forcible through operation, and a forcible connection operation. 
     For example, in the sweep operation, the first switching element  41  and the second switching element  42  are operated by the gate signal GS. A plurality of battery modules  30  included in the string  10  are connected to the main line  7  in a predetermined order and is disconnected from the main line  7  in a predetermined order. As a result, the string  10  is driven such that a predetermined number of battery modules  30  are normally connected to the main line  7  while sequentially changing the battery modules  30  connected to the main line  7  in a short control cycle. Through this sweep operation, the string  10  serves as one battery pack in which the predetermined number of battery modules  30  are connected in series while sequentially changing the battery modules  30  connected to the main line  7  in the short control cycle. The sweep modules  20  of the string  10  are controlled by the SCU  11  such that such a sweep operation is realized. In this control, the SCU  11  outputs the gate signal GS to the string  10  and outputs the control signal to the SU processing unit  51  incorporated into the sweep module  20 . Details of an example of the sweep operation will be described later with reference to  FIGS.  3  and  4   . 
     In the sweep operation, the delay/selection circuit  52  outputs the input gate signal GS to the gate driver  53  without any change and causes the gate signal GS to propagate to a downstream sweep module  20  with a delay of a delay time. As a result, the battery modules  30  of the sweep modules  20  under the sweep operation are sequentially connected to the main line  7  and are sequentially disconnected from the main line  7  at different timings in the string  10 . 
     In the forcible through operation, the delay/selection circuit  52  outputs a signal for maintaining the first switching element  41  in the ON state and maintaining the second switching element  42  in the OFF state to the gate driver  53  regardless of the input gate signal GS. As a result, the battery modules  30  of the sweep modules  20  under the forcible through operation are disconnected from the main line  7 . The delay/selection circuit  52  of the sweep module  20  under the forcible through operation causes the gate signal GS to propagate the downstream sweep module  20  without a delay. 
     In the forcible connection operation, the delay/selection circuit  52  outputs a signal for maintaining the first switching element  41  in the OFF state and maintaining the second switching element  42  in the ON state to the gate driver  53  regardless of the input gate signal GS. As a result, the battery modules  30  of the sweep modules  20  under the forcible connection operation are normally connected to the main line  7 . The delay/selection circuit  52  of the sweep module  20  under the forcible connection operation causes the gate signal GS to propagate the downstream sweep module  20  without a delay. 
     The delay/selection circuit  52  may be constituted as a single integrated circuit that performs the above-mentioned necessary functions. The delay/selection circuit  52  may be constituted in combination between a circuit that delays a gate signal GS and a circuit that selectively outputs a gate signal GS to the gate driver  53 . An example of the configuration of the delay/selection circuit  52  in this embodiment will be described below. 
     In this embodiment, as illustrated in  FIG.  2   , the delay/selection circuit  52  includes a delay circuit  52   a  and a selection circuit  52   b . The gate signal GS input to the delay/selection circuit  52  is input to the delay circuit  52   a . The delay circuit  52   a  outputs the gate signal GS to the selection circuit  52   b  with a delay of a predetermined delay time. The gate signal GS input to the delay/selection circuit  52  is output to the selection circuit  52   b  via another route which does not pass through the delay circuit  52   a  without any change. The selection circuit  52   b  receives an instruction signal form the SU processing unit  51  and outputs the gate signal GS in accordance with the instruction signal. 
     When the instruction signal from the SU processing unit  51  instructs to perform a sweep operation, the selection circuit  52   b  outputs the input gate signal GS to the gate driver  53  of the sweep module  20  without any change. The gate driver  53  outputs a control signal to the power circuit module  40 , turns off the first switching element  41 , turns on the second switching element  42 , and connects the battery module  30  to the main line  7 . On the other hand, the selection circuit  52   b  outputs the gate signal GS with a delay to the delay/selection circuit  52  of the sweep module  20  adjacent thereto downstream. That is, when the battery module  30  is connected to the main line  7  in the sweep operation, the gate signal GS with a delay of a predetermined delay time is sent to the sweep module  20  adjacent thereto downstream. 
     When the instruction signal from the SU processing unit  51  is the forcible through signal CSS, the selection circuit  52   b  outputs a signal for ignoring the battery module  30  to the gate driver  53 . By maintaining the forcible through signal CSS, the battery module  30  of the sweep module  20  receiving the forcible through signal CSS is maintained in a state in which it is disconnected from the main line  7 . In this case, the selection circuit  52   b  outputs the gate signal GS, which is input to the selection circuit  52   b  via another route which does not pass through the delay circuit  52   a , to the sweep module  20  adjacent thereto downstream. 
     When the instruction signal from the SU processing unit  51  is the forcible connection signal CCS, the selection circuit  52   b  outputs a signal for connecting the battery module  30  to the main line  7  to the gate driver  53 . That is, the gate driver  53  turns off the first switching element  41 , turns on the second switching element  42 , and connects the battery module  30  to the main line  7 . By maintaining the forcible connection signal CCS, the battery module  30  is maintained in a state in which it is connected to the main line  7 . In this case, the selection circuit  52   b  outputs the gate signal GS, which is input to the selection circuit  52   b  via another route which does not pass through the delay circuit  52   a , to the sweep module  20  adjacent thereto downstream. 
     As illustrated in  FIGS.  1  and  2   , in this embodiment, a plurality of sweep units  50  (specifically a plurality of delay/selection circuits  52 ) included in one string  10  is sequentially connected in a daisy chain manner. As a result, the gate signal GS input form the SCU  11  to one sweep unit  50  propagates sequentially to the plurality of sweep units  50 . Accordingly, processes in the SCU  11  are likely to be simplified and an increase in signal properties is easily curbed. However, the SCU  11  may individually output the gate signal GS to the plurality of sweep units  50 . 
     Each sweep unit  50  includes an indicator  57 . The indicator  57  notifies an operator of, for example, a state of the sweep module  20  including a battery module  30  or a power circuit module  40 . The indicator  57  can notify an operator, for example, that a defect in the battery module  30  of the sweep module  20  (for example, failure or deterioration of the batteries  31 ) has been detected (that is, the battery module  30  should be replaced). 
     For example, an LED which is a kind of light emitting device is used as the indicator  57  in this embodiment. However, a device (for example, a display) other than an LED may be used as the indicator  57 . A device (for example, a speaker) that outputs voice may be used as the indicator  57 . The indicator  57  may notify an operator of the state of the sweep module  20  by driving an actuator (for example, a motor or a solenoid). The indicator  57  may be configured to indicate the state using different methods depending on the state of the sweep module  20 . 
     In this embodiment, the operation of the indicator  57  is controlled by the SU processing unit  51  of the sweep unit  50 . However, a controller (for example, the SCU  11 ) other than the SU processing unit  51  may control the operation of the indicator  57 . 
     In this embodiment, the indicator  57  is provided for each sweep unit  50 . Accordingly, an operator can easily identify the sweep module  20  of which the state has been notified by the indicator  57  out of the plurality of sweep modules  20  which are arranged. However, the configuration of the indicator  57  may be modified. For example, separately from the indicator  57  disposed for each sweep unit  50  or along with the indicator  57 , a state notifying unit that notifies the states of a plurality of sweep modules  20  in a bundle may be provided. In this case, for example, the state notifying unit may display the states of the plurality of sweep modules  20  (for example, whether a defect has occurred) on one monitor. 
     &lt;Sweep Control&gt; 
     Sweep control which is executed in a string  10  will be described below. Here, sweep control is control for causing each battery module  30  of the string  10  to perform a sweep operation. In sweep control which is executed in the string  10 , the SCU  11  outputs a pulse-shaped gate signal GS. The switching elements  41  and  42  in a plurality of sweep modules  20  of the string  10  are driven to switch appropriately between ON and OFF. As a result, connection of the battery module  30  to the main line  7  and disconnection of the battery module  30  from the main line  7  are fast switched to each other for each sweep module  20 . In the string  10 , the gate signal GS which is input to an X-th sweep module  20  from upstream can be delayed with respect to the gate signal GS which is input to an (X−1)-th sweep module  20 . As a result, m (m&lt;M) sweep modules  20  connected to the main line  7  out of M sweep modules  20  in the string  10  are sequentially switched. Accordingly, a plurality of battery modules  30  included in the string  10  is connected to the main line  7  in a predetermined order and is disconnected from the main line in a predetermined order. A predetermined number of battery modules  30  can be normally connected to the main line  7 . Through this sweep operation, the string  10  serves as a single battery pack in which a predetermined number of battery modules  30  are connected in series. 
       FIG.  3    is a timing chart illustrating an example of a relationship between connection states of sweep modules  20  and a voltage which is output to the power distribution device  5  when all the sweep modules  20  included in the string  10  are caused to perform the sweep operation. The number M of sweep modules  20  included in one string  10  can be appropriately changed. In the example illustrated in  FIG.  3   , five sweep modules  20  are included in one string  10  and all of the five sweep modules  20  are caused to perform the sweep operation. 
     In the example illustrated in  FIG.  3   , a VH command signal for setting a voltage VH [V] output to the power distribution device  5  to 100 V is input to the SCU  11  of the string  10 . The voltage Vmod [V] of the battery module  30  in each sweep module  20  is 43.2 V. The delay time DL [μsec] by which a gate signal GS is delayed is appropriately set depending on the specification required for the power supply system  1 . The period T of the gate signal GS (that is, the period in which a sweep module  20  is connected and disconnected) has a value which is obtained by multiplying the delay time DL by the number P of sweep modules  20  (≤M) which are to perform the sweep operation. Accordingly, when the delay time DL is set to be greater, the frequency of the gate signal GS becomes lower. On the other hand, when the delay time DL is set to be less, the frequency of the gate signal GS becomes higher. In the example, illustrated in  FIG.  3   , the delay time DL is set to 2.4 μsec. Accordingly, the period T of the gate signal GS is “2.4 μsec×5=12 μsec.” 
     In this embodiment, a battery module  30  of a sweep module  20  in which the first switching element  41  is turned off and the second switching element  42  is turned on is connected to the main line  7 . That is, when the first switching element  41  is turned off and the second switching element  42  is turned on, the capacitor  47  that is provided in parallel to the battery module  30  is connected to the input and output circuit  43  and electric power is input and output. The sweep unit  50  of the sweep module  20  connects the battery module  30  to the main line  7  while the gate signal GS is in the ON state. On the other hand, a battery module  30  of a sweep module  20  in which the first switching element  41  is turned on and the second switching element  42  is turned off is disconnected from the main line  7 . The sweep unit  50  disconnects the battery module  30  from the main line  7  while the gate signal GS is in the OFF state. 
     When the first switching element  41  and the second switching element  42  are simultaneously turned on, a short-circuit occurs. Accordingly, when the first switching element  41  and the second switching element  42  are driven to switch, the sweep unit  50  switches one element from ON to OFF and switches the other element from OFF to ON after a slightly waiting time has elapsed thereafter. As a result, it is possible to prevent a short-circuit from occurring. 
     A VH command value which is instructed by a VH command signal is defined as VH_com, a voltage of each battery module  30  is defined as Vmod, and the number of sweep modules  20  which are to perform the sweep operation (that is, the number of sweep modules  20  which are to be connected to the main line  7  in sweep control) is defined as P. In this case, a duty ratio of an ON time to the period T in a gate signal GS is calculated as VH_com/(Vmod×P). In the example illustrated in  FIG.  3   , the duty ratio of the gate signal GS is about 0.46. Strictly, the duty ratio varies due to an influence of the waiting time for preventing occurrence of a short-circuit. Accordingly, the sweep unit  50  may perform correction of the duty ratio using a feedback process or a feedforward process. 
     As illustrated in  FIG.  3   , when sweep control is started, first, one of P sweep modules  20  (the sweep module  20  of No. 1 which is furthest upstream in the example illustrated in  FIG.  3   ) is connected. Thereafter, when the delay time DL elapses, a next sweep module  20  (the sweep module  20  of No. 2 which is located the second from upstream in the example illustrated in  FIG.  3   ) is connected. In this state, the voltage VH which is output to the power distribution device  5  is a sum value of the voltages of two sweep modules  20  and does not reach the VH command value. When the delay time DL elapses additionally, the sweep module  20  of No. 3 is connected. In this state, the number of sweep modules  20  connected to the main line  7  is three of Nos. 1 to 3. Accordingly, the voltage VH which is output to the power distribution device  5  is a sum value of the voltages of three sweep modules  20  and is greater than the VH command value. Thereafter, when the sweep module  20  of No. 1 is disconnected from the main line  7 , the voltage VH returns to the sum value of the voltages of two sweep modules  20 . When the delay time DL elapses after the sweep module of No. 3 has been connected, the sweep module  20  of No. 4 is connected. As a result, the number of sweep modules  20  which are connected to the main line  7  through sweep controlare three of Nos. 2 to 4. As described above, m (three in  FIG.  3   ) sweep modules  20  which are connected to the main line  7  out of M (five in  FIG.  3   ) sweep modules  20  are sequentially switched. 
     As illustrated in  FIG.  3   , the VH command value may not be indivisible by the voltage Vmod of each battery module  30 . In this case, the voltage VH which is output to the power distribution device  5  varies. However, the voltage VH is equalized by the RLC filter and is output to the power distribution device  5 . Even when the battery modules  30  of the sweep modules  20  are charged with electric power which is input from the power distribution device  5 , the connection states of the sweep modules  20  are controlled similarly to the timing chart illustrated in  FIG.  3   . 
     &lt;Forcible Through Operation&gt; 
     Control when certain sweep modules  20  are caused to perform a forcible through operation and the other sweep modules  20  are caused to perform a sweep operation will be described below with reference to  FIG.  4   . As described above, the sweep module  20  which has been instructed to perform a forcible through operation maintains a state in which the battery module  30  is disconnected from the main line  7 . The example illustrated in  FIG.  4    is different from the example illustrated in  FIG.  3    in that the sweep module  20  of No. 2 is caused to perform a forcible through operation. That is, in the example illustrated in  FIG.  4   , the number P of sweep modules  20  which are caused to perform a sweep operation (that is, the number of sweep modules  20  which are to be connected to the main line  7 ) out of five sweep modules  20  included in one string  10  is four. The VH command value, the voltage Vmod of each battery module  30 , and the delay time DL are the same as in the example illustrated in  FIG.  3   . In the example illustrated in  FIG.  4   , the period T of the gate signal GS is “2.4 μsec×4=9.6 μsec.” The duty ratio of the gate signal GS is about 0.58. 
     As illustrated in  FIG.  4   , when certain sweep modules  20  (the sweep module  20  of No. 2 in  FIG.  4   ) are caused to perform a forcible through operation, the number P of sweep modules  20  which are caused to perform a sweep operation is less than that in the example illustrated in  FIG.  3   . However, the string  10  adjusts the period T of the gate signal GS and the duty ratio of the gate signal GS with the decrease in the number P of sweep modules  20  which are caused to perform a sweep operation. As a result, the waveform of the voltage VH which is output to the power distribution device  5  is the same as the waveform of the voltage VH illustrated in  FIG.  3   . Accordingly, the string  10  can appropriately output the commanded voltage VH to the power distribution device  5  even when the number P of sweep modules  20  which are caused to perform a sweep operation is increased or decreased. 
     For example, when a defect (for example, deterioration or failure) occurs in a battery  31  in a certain sweep module  20 , the string  10  can cause the sweep module  20  including the battery  31  in which a defect has occurred to perform a forcible through operation. Accordingly, the string  10  can appropriately output the commanded voltage VH to the power distribution device  5  using the sweep modules  20  in which a defect has not occurred. An operator can replace the battery module  30  including the battery  31  in which a defect has occurred (that is, the battery module  30  of the sweep module  20  which is performing a forcible through operation) in a state in which the string  10  is operating normally. In other words, in the power supply system  1  according to this embodiment, it is not necessary to stop the operation of the string  10  as a whole when a battery module  30  is replaced. 
     When a certain sweep module  20  is caused to perform a forcible connection operation, the connection state of the sweep module  20  which is caused to perform a forcible connection operation is a normally connected state. For example, when the sweep module  20  of No. 2 in  FIG.  4    is caused to perform a forcible connection operation instead of a forcible through operation, the connection state of No. 2 is maintained in a “connected state” instead of a “disconnected state.” 
     When the power supply system  1  includes a plurality of strings  10 , the above-mentioned sweep control is executed in each of the plurality of strings  10 . The controller (the GCU  2  in this embodiment) that comprehensively controls the power supply system  1  as a whole controls the operations of the plurality of strings  10  such that a command from the host system  6  is satisfied. For example, when a VH command value required from the host system  6  cannot be satisfied by only one string  10 , the GCU  2  may satisfy the VH command value by causing the plurality of strings  10  to output electric power. 
     &lt;String&gt; 
     The entire configurations of the string  10  and the power supply system  1  will be described below in detail with reference to  FIG.  1   . As described above, the string  10  includes an SCU  11  and a plurality of sweep modules  20  that is connected in series to the main line  7  with a power circuit module  40  interposed therebetween. The main line  7  of the string  10  is connected to a bus line  9  extending from the power distribution device  5 . The string  10  includes a bus line voltage detecting unit  21 , a system breaker (this system breaker is appropriately referred to as a “system main relay (SMR)”)  22 , a string capacitor  23 , a string current detecting unit  24 , a string reactor  25 , and a string voltage detecting unit  26  sequentially from the power distribution device  5  side (upstream) in the main line  7 . Disposition of certain members may be modified. For example, the system breaker  22  may be provided downstream from the string capacitor  23 . 
     The bus line voltage detecting unit  21  detects a voltage of the bus line  9  extending from the power distribution device  5  to the string  10 . The system breaker  22  switches between connection and disconnection between the string  10  and the power distribution device  5 . In this embodiment, the system breaker  22  is driven in accordance with a signal which is input from the SCU  11 . The string capacitor  23  and the string reactor  25  form an RLC filter to achieve equalization of a current. The string current detecting unit  24  detects a current flowing between the string  10  and the power distribution device  5 . The string voltage detecting unit  26  detects a total voltage of voltages of the plurality of sweep modules  20  which is connected in series to the main line  7  in the string  10 , that is, a string voltage of the string  10 . 
     In the example illustrated in  FIG.  1   , the system breaker  22  includes a switch  22   a  and a fuse  22   b . The switch  22   a  is a device that connects or disconnects the string  10  to and from the power distribution device  5 . The switch  22   a  can be appropriately referred to as a string switch. By turning on the switch  22   a , the main line  7  of the string  10  is connected to the bus line  9  of the power distribution device  5 . By turning off the switch  22   a , the string  10  is disconnected from the power distribution device  5 . The switch  22   a  is controlled by the SCU  11  controlling the string  10 . By operating the switch  22   a , the string  10  can be appropriately disconnected from or connected to the power distribution device  5 . The fuse  22   b  is a device that stops an unexpected large current when the large current flows in the main line  7  of the string  10  in view of design of the string  10 . The fuse  22   b  is also appropriately referred to as a string fuse. 
     Here, when batteries incorporated into one battery module  30  have the same standard, the voltage of one battery module  30  increases as the number of batteries incorporated increases. On the other hand, when the voltage of one battery module  30  is high, the battery module is dangerous for an operator to handle and is heavy. In this regard, as many batteries as possible may be be incorporated into one battery module  30  within a range of a voltage with which an operator will not be subjected to a significant accident even with touch of the operator with the fully charged battery module (for example, lower than 60 V and preferably lower than 42 V) and within a range of a weight with which an operator can easily replace the battery module. The battery module  30  which is incorporated into the string  10  does not need to include the same batteries, and the number of batteries which are incorporated into one battery module  30  can be determined depending on types, standards, or the like of the batteries which are incorporated into the battery module  30 . The string  10  is configured to output a necessary voltage by combining sweep modules  20  into which the battery module  30  has been incorporated in series. The power supply system  1  is configured to output electric power required for connection to the power system  8  by combining a plurality of strings  10 . 
     In this embodiment, the power distribution device  5  to which a plurality of strings  10  of the power supply system  1  is connected includes sub power distribution devices  5 A and  5 B that are connected to the strings  10 A and  10 B. The strings  10 A and  10 B connected to the sub power distribution devices  5 A and  5 B are connected in parallel via the sub power distribution devices  5 A and  5 B. The power distribution device  5  controls distribution of electric power which is input to the strings  10 A and  10 B from the power system  8 , combination of electric power which is output from the strings  10 A and  10 B to the power system  8 , and the like through the sub power distribution devices  5 A and  5 B connected to the strings  10 . The power distribution device  5  and the sub power distribution devices  5 A and  5 B are controlled such that the power supply system  1  into which a plurality of strings  10  is incorporated serves as a single power supply device as a whole by cooperation between the GCU  2  connected to the host system  6  and the SCU  11  that controls each string  10 . 
     For example, in this embodiment, a downstream side from the power distribution device  5 , that is, the strings  10 A and  10 B side, is controlled with a direct current. An upstream side from the power distribution device  5 , that is, the power system  8 , is controlled with an alternating current. The voltages of the strings  10 A and  10 B are controlled to be roughly balanced with the voltage of the power system  8  via the power distribution device  5 . When the voltage of each of the strings  10 A and  10 B is controlled to be lower than that of the power system  8 , a current flows from the power system  8  to each of the strings  10 A and  10 B. At this time, when sweep control is executed in the strings  10 A and  10 B, the battery modules  30  are appropriately charged. When the voltage of each of the strings  10 A and  10 B is controlled to be higher than that of the power system  8 , a current flows from each of the strings  10 A and  10 B to the power system  8 . At this time, when sweep control is executed in the strings  10 A and  10 B, the battery modules  30  are appropriately discharged. The power distribution device  5  may maintain the voltages of the strings  10 A and  10 B to be equal to the voltage of the power system  8  such that a current hardly flows in the strings  10 A and  10 B. In this embodiment, this control can be executed for each of the sub power distribution devices  5 A and  5 B to which the strings  10 A and  10 B are connected. For example, by adjusting the voltage for each of the strings  10 A and  10 B, control may be executed such that a current hardly flows in certain string  10  out of a plurality of strings  10 A and  10 B connected to the power distribution device  5 . 
     In the power supply system  1 , the total capacity of the power supply system  1  can be increased by increasing the number of strings  10  which are connected in parallel to the power distribution device  5 . For example, with the power supply system  1 , it is possible to construct a large system that can output electric power such that a sudden increase in demand in the power system  8  can be absorbed or can supplement sudden power shortage in the power system  8 . For example, by increasing the capacity of the power supply system  1 , great surplus electric power of the power system  8  can be appropriately transferred to charging of the power supply system  1 . For example, when output power of a power plant is surplus in a night time zone in which demand for electric power is low or when an amount of electric power generated in a large photovoltaic system increases suddenly, the power supply system  1  can absorb surplus electric power via the power distribution device  5 . On the other hand, when demand for electric power in the power system  8  increases suddenly, necessary electric power can be appropriately output from the power supply system  1  to the power system  8  via the power distribution device  5  in accordance with a command from the host system  6 . Accordingly, with the power supply system  1 , power shortage in the power system  8  is appropriately supplemented. 
     In the power supply system  1 , it is not necessary to normally connect all battery modules  30  out of a plurality of battery modules  30  which is incorporated into a string  10 . Since a forcible through operation can be performed for each battery module  30  as described above, a battery module  30  in which a defect has occurred can be disconnected from sweep control of the string  10  when a defect has occurred in the battery module  30 . Accordingly, in the power supply system  1 , a battery which is used for the battery module  30  does not need to be a new battery which has not been used. 
     For example, a secondary battery which has been used as a driving power source of a motor-driven vehicle such as a hybrid vehicle or an electric vehicle can be appropriately reused. Even when such a secondary battery which has been used as a driving power source is used, for example, for about 10 years, the secondary battery can satisfactorily perform a secondary battery function. In the power supply system  1 , since a battery module  30  in which a defect has occurred can be immediately disconnected, a battery can be incorporated into the battery module  30 , for example, by ascertaining that the battery performs a necessary function. The time for sequentially recovering a secondary battery which has been used as a driving power source of a motor-driven vehicle comes up. With the power supply system  1 , for example, secondary batteries corresponding to 10,000 motor-driven vehicles may be incorporated thereinto and thus considerable recovered secondary batteries can be absorbed. It cannot be seen when a secondary battery which has been used as a driving power source of a motor-driven vehicle deteriorates in performance. When such a secondary battery is reused for a battery module  30  of the power supply system  1 , it is not possible to predict when a defect occurs in the battery module  30 . 
     With the power supply system  1  which has been proposed herein, it is possible to appropriately disconnect a battery module  30  via a sweep module  20 . Accordingly, even when a defect occurs suddenly in a battery module  30  or a secondary battery incorporated into the battery module  30 , it is not necessary to stop the power supply system  1  as a whole. 
     The plurality of strings  10  of the power supply system  1  is connected in parallel to the power distribution device  5  which is connected to the power system  8  as described above. Electric power which is input or output between the power system  8  and the power distribution device  5  can be determined by the host system  6  that controls the power system  8 . For example, the GCU  2  that takes charge of certain or all of the functions of the control device  100  can calculate a predicted value of a current value flowing in the strings  10  to which electric power is distributed by the power distribution device  5  depending on electric power which is input or output to and from the power system  8  and the number of strings  10  to which electric power is distributed. For example, the electric power (input or output) requested for the power distribution device  5  from the power system  8  is determined by the host system  6 . 
     The host system  6  requests the power distribution device  5  to input or output necessary electric power via the GCU  2 . For example, when there is surplus electric power in the power system  8 , the host system  6  requests the power distribution device  5  to take electric power from the power system  8 . In response to this request, the power distribution device  5  controls the voltage of the string  10  side such that this voltage is lower than that on the power system  8  side. When there is a shortage of electric power in the power system  8 , the host system  6  requests the power distribution device  5  to supply electric power to the power system  8 . In response to this request, the power distribution device  5  controls the voltage of the string  10  side such that this voltage is higher than that on the power system  8  side. 
     At this time, states of the string  10 , the battery module  30 , the power circuit module  40 , and the like are normally monitored based on measured values which are detected by the string current detecting unit  24  and the string voltage detecting unit  26  which are provided in the main line  7  of the string  10 , the voltage detecting unit  35  and the temperature detecting unit  36  which are provided in the battery module  30 , the temperature detecting unit  48  which is provided in the power circuit module  40 , and the like. In a device that detects states of the power supply system  1  based on a current value, an error increases as the current value decreases. 
     In the power supply system  1 , the inventor has been aware of knowledge that in particular a decrease in a current flowing in the main line  7 , a current flowing in the input and output circuit  43  of the power circuit module  40 , or the like causes misunderstanding of the states of the power supply system  1 . For example, as described above, when a plurality of strings  10  is connected in parallel to the power distribution device  5  and an amount of electric power requested from the power distribution device  5  is small, the current flowing in the main line  7  of one string  10  is decreased, for example, by uniformly distributing the current to the plurality of strings  10  connected in parallel from the power distribution device  5 . When the current flowing in the main line  7  of the string  10  decreases, an error in the detected current value in the device that detects states of the string  10  based on a current value increases, which may cause misunderstanding of the states of the strings  10 . 
       FIG.  5    is a block diagram illustrating the control device  100  of the power supply system  1 . From the above-mentioned point of view, the control device  100  of the power supply system  1  has only to include a first processing unit  101  and a second processing unit  102  as illustrated in  FIG.  5   . 
     Here, the control device  100  can be a device that controls inputting of electric power from the power system  8  to a plurality of strings  10  connected to the power distribution device  5  and outputting of electric power form the plurality of strings  10  to the power system  8 . In the above-mentioned embodiment, for example, control of the control device  100  can be taken charge of in cooperation by the GCU  2  serving as a power controller that controls the power distribution device  5  or the strings  10 , the SCU  11 , the sweep units  50 , and the like. For example, the control device  100  controls the power distribution device  5  or the strings  10  based on a relationship with the situation of the power system  8  or the like in accordance with a command from the host system  6 . A variety of information which is detected in the power supply system  1  can be managed by an external server which is located remotely by IOT technology. Various processes of the control device  100  can be remotely controlled in cooperation with an external manager computer which is accessibly connected to the power supply system  1  via a communication network by cloud computing technology. 
     The first processing unit  101  is a processing unit that performs a first process of determining certain strings  10  out of a plurality of strings  10  connected in parallel to the power distribution device  5 . The second processing unit  102  is a processing unit that performs a second process of performing inputting of electric power to the plurality of strings  10  connected in parallel to the power distribution device  5  or outputting of electric power from the plurality of strings  10  to the power distribution device  5  using at least certain strings  10  which are determined by the first processing unit  101 . 
     In this case, at least electric power is distributed to certain strings  10  which are determined by the first processing unit  101  from the power distribution device  5 . Accordingly, in the certain strings  10 , electric power which is distributed from the power distribution device  5  is stabilized. Accordingly, in the certain strings  10 , a current flowing in the main line  7  of the string  10  is likely to be stabilized in a state in which a necessary power value is secured. Accordingly, in the device that detects states of the string  10  based on a current value, an error of the detected current value is curbed and the state of the string  10  is likely to be appropriately and easily understood. In the certain strings  10 , it is possible to stably drive the power supply system  1 . 
     For example, the power distribution device  5  may evaluate performance of the strings  10  connected in parallel in advance and the first processing unit  101  may be configured to determine the strings  10  to which electric power is to be distributed based on the result of performance evaluation. For example, the first processing unit  101  may be configured to select the strings  10  with good performance as the certain strings  10  based on the result of performance evaluation. In this case, when the power distribution device  5  is requested to stably input or output electric power from the host system  6 , it is possible to obtain stable performance. The first processing unit  101  may be configured to select strings  10  with poor performance as the certain strings  10 . In this case, for example, a predetermined inspection mode may be executed to inspect the battery module  30  of each sweep module  20  in the string  10 . A predetermined deterioration recovery mode may be executed to recover deterioration of the battery module  30  of each sweep module  20 . The mode in which the strings  10  with poor performance are selected as the certain strings  10  can be executed, for example, when a request from the host system  6  to the power distribution device  5  (for example, a request for input or output of electric power) is not strict. 
     For example, the second process which is performed by the second processing unit  102  is advantageous when electric power input or output between the power distribution device  5  and a plurality of strings  10  is equally distributed to the plurality of strings  10  and a predetermined current value is not obtained in certain strings  10 . In this case, the second processing unit  102  can be configured to distribute electric power for obtaining a predetermined current value in certain strings  10  to the certain strings. The second processing unit  102  may be configured to distribute surplus electric power to at least one string other than the certain strings determined by the first processing unit  101  out of the plurality of strings  10 . Accordingly, a predetermined current value is stably obtained in certain strings  10  determined by the first processing unit  101 . With the certain strings  10 , the power supply system  1  can be stably driven. Since surplus electric power can be transferred to other strings  10 , other strings  10  can be appropriately utilized effectively. 
     In this case, first, the second processing unit  102  can perform a process of obtaining a current value of the main line  7  of each string  10  when electric power input or output between the power distribution device  5  and the plurality of strings  10  is equally distributed to the plurality of strings  10 . Then, the process of determining whether the obtained current value of the main line  7  of each string  10  reaches the predetermined current value can be performed in each string  10 . When it is determined that the predetermined current value is not obtained in certain strings  10  determined by the first processing unit  101 , electric power for obtaining the predetermined current value in the certain strings  10  is calculated. Then, the power distribution device  5  is controlled such that the calculated electric power is distributed to the certain strings  10 . The power distribution device  5  can be configured to additionally distribute surplus electric power to at least one string  10  other than the certain strings  10  determined by the first processing unit  101  out of the plurality of strings  10  connected in parallel to the power distribution device  5 . 
     Here, the predetermined current value is merely, for example, a current value required for accurately detecting a state of a string  10 . The predetermined current value can be determined, for example, for each string  10  connected to the power distribution device  5 . In this case, the predetermined current value may be determined depending on a battery module  30  attached to the string  10  or a battery  31  (see  FIG.  2   ) incorporated into the battery module  30 . The control device  100  can include a storage unit that stores the predetermined current value for each string  10 . The current value which is determined for each string  10  may be appropriately edited. The current value which is determined for each string  10  may be appropriately calculated from an evaluation value for performance of the string  10 , for example, by the control device  100 . 
     The second processing unit  102  may be configured to distribute electric power to at least one string  10  of the certain strings  10  when electric power input or output between the power distribution device  5  and a plurality of strings  10  is distributed to the certain strings  10  and the predetermined current value is not obtained in at least one of the certain strings  10 . 
     For example, when electric power is distributed to certain strings  10  determined by the first processing unit  101  and input or output electric power which is requested to the power distribution device  5  from the host system  6  is small, the current value in the certain strings  10  to which electric power is distributed may be small. For example, the predetermined current value may not be obtained in certain strings  10  out of the certain strings  10  determined by the first processing unit  101 . In this case, the second processing unit  102  can be configured to distribute electric power to at least one string  10  of the certain strings  10 . Accordingly, the current value in the string  10  to which electric power is distributed is likely to be stabilized. 
       FIG.  6    is a flowchart illustrating an example of the first process and the second process in the control device  100 . As illustrated in  FIG.  6   , in the first process, certain strings  10  out of a plurality of strings  10  connected in parallel to the power distribution device  5  are determined (S 11 ). In the second process, for example, a current value A 1  flowing in each string  10  when electric power is distributed to the certain strings  10  determined in the first process is calculated (S 12 ). It is determined whether the calculated current value A 1  is equal to or greater than a predetermined current value A 0  of each string  10  (A 1 &gt;A 0 ) (S 13 ). When it is determined in the process (S 13 ) that the calculated current value A 1  is equal to or greater than the predetermined current value A 0  of each string  10  (YES), electric power is distributed to the certain strings  10  determined in the first process (S 14 ). 
     When it is determined in the process (S 13 ) that the calculated current value A 1  does not satisfy the predetermined current value A 0  of each string  10  (NO), certain strings  10  can be additionally determined out of the certain strings  10  determined in the first process (S 11 ). That is, certain strings  10  are redetermined (S 15 ). This redetermination may be performed, for example, by the function of the first processing unit  101 . Then, a current value A 2  flowing in each string  10  when electric power is distributed to certain strings  10  which are redetermined is calculated (S 16 ). It is determined whether the calculated current value A 2  is equal to or greater than the predetermined current value A 0  of each string  10  (A 2 &gt;A 0 ) (S 17 ). When it is determined that the calculated current value A 2  is equal to or greater than the predetermined current value A 0  of each string  10  (YES), electric power is distributed to the certain strings  10  redetermined in the process of S 15  in the second process by the second processing unit  102  (S 18 ). 
     When it is determined in the process (S 17 ) that the calculated current value A 2  does not satisfy the predetermined current value A 0  of each string  10  (NO), certain strings  10  are additionally redetermined out of the certain strings  10  (S 15 ). The processes from redetermination (S 15 ) to determination (S 17 ) are repeated until the calculated current value A 2  becomes equal to or greater than the predetermined current value A 0  of each string  10  (A 2 &gt;A 0 ). 
     According to the second process, when electric power is distributed to the determined certain strings  10 , the current values A 1  and A 2  flowing in each string  10  becomes equal to or greater than the predetermined current value A 0  of each string  10 . Accordingly, it is possible to stably secure a current value required for a string  10 . 
     For example, electric power which is output from the power system  8  to a plurality of strings  10  connected to the power distribution device  5  can be calculated in accordance with a command from the host system  6  or the GCU  2 . Even when electric power which is calculated in accordance with a command from the host system  6  or the GCU  2  varies, the current value required for a string  10  is stably secured through the first process and the second process which are performed by the control device  100 . Since the current value required for each string  10  is stably secured, it is possible to accurately monitor the state of each string  10  and to stably operate the power supply system  1 . 
     In the processes from redetermination (S 15 ) to determination (S 17 ), when determination of whether the calculated current value A 2  is equal to or greater than the predetermined current value A 0  of each string  10  (A 2 &gt;A 0 ) is repeatedly performed, the number of strings  10  which are determined as certain strings  10  in the redetermination (S 15 ) decreases gradually. Finally, it is conceivable that the number of strings  10  be one. When the calculated current value is not equal to or greater than the predetermined current value of each string  10  even if any string  10  is selected in the processes from the redetermination (S 15 ) to the determination (S 17 ), this can be fed back to the host system  6 . Until electric power requested from the host system  6  to the power distribution device  5  increases stably, the strings  10  connected to the power distribution device  5  may be paused. 
     The power supply system has been described above in various forms. Unless otherwise mentioned, examples or the like of the power supply system according to the embodiment do not limit the disclosure. The power supply system can be modified in various forms and the elements or processes mentioned herein can be appropriately omitted or appropriately combined unless any particular problem is caused.