Patent Publication Number: US-11050247-B2

Title: Power supply system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent No. 2018-222766 filed on Nov. 28, 2018. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present teaching relates to a power supply system including a plurality of modules each including a battery and a circuit. 
     2. Description of the Related Art 
     A power supply device known to date includes a plurality of modules each including a battery and a circuit, and each of the modules is controlled to perform at least one of outputting of electric power to the outside and storing of electric power input from the outside. For example, a power supply device described in Japanese Patent Application Publication No. 2018-74709 includes a plurality of battery circuit modules each including a battery, a first switching device, and a second switching device. The battery circuit modules are connected in series through output terminals thereof. A control circuit for the power supply device outputs gate signals for turning the first switching device and the second switching device on and off, to the battery circuit modules at every given time. In this manner, a target level of electric power is output from the battery circuit modules. 
     SUMMARY 
     In a power supply device including a plurality of modules each including a battery, a plurality of batteries having different performances can be mixed in the power supply device in some cases. If the batteries having different performances are used in the same manner, performance of the entire power supply device can be restricted by batteries having low performance among the plurality of batteries. In this case, it is difficult to fully utilize batteries having high performance. 
     The present teaching typically provides a power supply system capable of fully using a plurality of batteries having different functions. 
     In view of this, a power supply system in an aspect of this disclosure includes: a main line configured to energize at least electric power input from outside; a plurality of sweep modules connected to the main line; and a controller, wherein each of the plurality of sweep modules includes a battery module including at least one battery, and an electric power circuit module including at least one switching device that switches a connection state between the battery module and the main line between connection and disconnection, and the controller is configured to perform a first process of performing sweep control of sequentially switching the battery module connected to the main line among the plurality of battery modules by outputting a gate signal for controlling the switching device to the electric power circuit module, and a second process of performing the sweep control by detecting an SOC in each of the battery modules while electric power is input from outside through the main line, by disconnecting, from the main line, a high SOC module that is the battery module whose detected SOC level satisfies a high SOC condition, and by excluding the sweep module including the high SOC module. 
     The power supply system of the configuration described above performs sweep control of sequentially switching the sweep module connected to the main line. Here, the controller detects an SOC for each battery module while electric power is input from outside through the main line. The controller disconnects, from the main line, the high SOC module that is a battery module whose detected SOC level satisfies a condition for a high SOC level. Consequently, charging of the high SOC module is stopped. The controller excludes the sweep module including the high SOC module, and performs sweep control. In this case, when an SOC of one or more of the battery modules increases while the plurality of battery modules are being charged by sweep control, charging of the battery module having increased the SOC is stopped, and charging of the other battery modules continues. That is, even when the SOC of the battery module having low performance increases, charging of battery modules having high performance continues. In addition, since charging of the battery module having an increased SOC is ignored, the possibility that one or more of the battery modules are overcharged decreases so that the lifetime of the battery module increases. Furthermore, the possibility that the battery module having a low SOC level is overdischarged during discharging of the battery modules decreases. Accordingly, a plurality of batteries having different performances can be fully utilized. 
     In a preferred aspect of the power supply system disclosed here, the switching device of the electric power circuit module includes a first switching device attached to the main line in series and attached to the battery module in parallel, and a second switching device disposed in a circuit that connects the battery modules to the main line in series, the controller is configured to perform the sweep control by sequentially outputting, to each of the plurality of sweep modules, the gate signal for controlling alternate driving of turning the first switching device and the second switching device on and off, at every predetermined delay time, and the controller is configured to output, to the sweep module including the high SOC module, a signal for keeping the first switching device in an on state and the second switching device in an off state in disconnecting the high SOC module from the main line. In this case, sweep control of the plurality of sweep modules and disconnection of the high SOC module in the sweep control can be appropriately performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a configuration of a power supply system  1 . 
         FIG. 2  schematically illustrates a configuration of sweep modules  20 . 
         FIG. 3  is an example of a timing chart in a sweep operation. 
         FIG. 4  is an example of a timing chart in a forced through operation. 
         FIG. 5  is a flowchart of an energizing control process performed by the power supply system  1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One exemplary embodiment of the present disclosure will be described hereinafter in detail with reference to the drawings. Matters not specifically mentioned in the description but required for carrying out the disclosure can be understood as matters of design variation of a person skilled in the art based on related art in the field. The present teaching can be carried out on the basis of the contents disclosed in the description and common general knowledge in the field. In the drawings, members and parts having the same functions are denoted by the same reference numerals. In addition, dimensional relationship in each drawing does not reflect an actual dimensional relationship. 
     &lt;Schematic Overall Configuration&gt; 
     With reference to  FIG. 1 , an overall configuration of the power supply system  1  according to this exemplary embodiment will be schematically described. The power supply system  1  performs either output of electric power to a distribution device  5  connected to a higher-order electric power system  8 , or storage of electric power input from the distribution device  5  (hereinafter also simply referred to as “input/output of electric power”). In this exemplary embodiment, a power conditioning subsystem (PCS) is used as the distribution device  5 , as an example. The PCS has the function of exchanging electric power input from the electric power system  8  to, for example, the power supply system  1  and electric power output from, for example, the power supply system  1  to the electric power system  8 , between the power supply system  1 , for example, and the electric power system  8 . 
     In a case where electric power is redundant in the electric power system  8 , the distribution device  5  outputs the redundant electric power to the power supply system  1 . In this case, the power supply system  1  stores electric power input from the distribution device  5 . In response to an instruction from a higher-order system  6  for controlling the higher-order electric power system  8 , the power supply system  1  outputs the electric power stored in the power supply system  1  to the distribution device  5 . In  FIG. 1 , the higher-order system  6  serves as a system for controlling the electric power system  8  and the distribution device  5  and is disposed separately from the electric power system  8  and the distribution device  5 . Alternatively, the higher-order system  6  may be incorporated in the electric power system  8  or the distribution device  5 . 
     The power supply system  1  includes at least one string  10 . The power supply system  1  of this exemplary embodiment includes a plurality of (N: N≥2) strings  10  ( 10 A,  10 B, . . . ,  10 N). For convenience of illustration,  FIG. 1  shows only two strings  10 A and  10 B of the N strings  10 . Each of the strings  10  is a unit of inputting/outputting of electric power to/from the distribution device  5 . The strings  10  are connected to the distribution device  5  in parallel. Electric power is input and output (electrification) between the distribution device  5  and each of the strings  10  through a main line  7 . 
     Each of the strings  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 of the sweep modules  20  includes a battery and a control circuit. The SCU  11  is provided in each of the strings  10 . The SCU  11  is a controller for integrally controlling the sweep modules  20  included in one string  10 . Each of the SCUs  11  communicates with a group control unit (GCU)  2  serving as an electric power controller. The GCU  2  is a controller for integrally controlling an entire group including the strings  10 . The GCU  2  communicates with the higher-order system  6  and each of the SCUs  11 . Communication among the higher-order system  6 , the GCU  2 , and SCUs  11  can employ various methods (e.g., at least one of communications such as wired communication, wireless communication, and communication through a network). 
     The configuration of the controller for controlling, for example, the strings  10  and the sweep modules  20  may be changed. For example, the GCU  2  may be disposed separately from the SCUs  11 . Specifically, one controller may control both an entire group including at least one string  10 , and the sweep modules  20  included in the string  10 . 
     &lt;Sweep Module&gt; 
     With reference to  FIG. 2 , the sweep modules  20  will be described in detail. Each of the sweep modules  20  includes a battery module  30 , an electric power circuit module  40 , and a sweep unit (SU)  50 . 
     The battery module  30  includes at least one battery  31 . The battery module  30  of the exemplary embodiment includes a plurality of batteries  31 . The batteries  31  are connected to each other in series. In the exemplary embodiment, secondary batteries are used as the batteries  31 . As the batteries  31 , at least one of various secondary batteries (i.e., nickel-metal hydride batteries, lithium ion batteries, or nickel-cadmium batteries) may be used. In the power supply system  1 , a plurality of types of batteries  31  may be mixed. Of course, all the batteries  31  in the battery module  30  may be of the same type. 
     A voltage detector  35  and a temperature detector  36  are attached to the battery module  30 . The voltage detector  35  detects a voltage of the batteries  31  (batteries  31  connected in series in this exemplary embodiment) included in the battery module  30 . The temperature detector  36  detects a temperature of the batteries  31  included in the battery module  30  or a temperature near the batteries  31 . Various types of devices (e.g., a thermistor) for detecting a temperature may be used for the temperature detector  36 . 
     The battery module  30  is detachably attached to the electric power circuit module  40 . Specifically, in this exemplary embodiment, detachment from the electric power circuit module  40  and attachment to the electric power circuit module  40  are performed using the battery module  30  including the plurality of batteries  31 , as one unit. Thus, as compared to a case where the batteries  31  included in the battery module  30  are replaced one by one, the number of jobs in replacing the batteries  31  by an operator can be reduced. In this exemplary embodiment, the voltage detector  35  and the temperature detector  36  may be replaced, separately from the battery module  30 . Alternatively, at least one of the voltage detector  35  and the temperature detector  36  may be replaced together with the battery module  30 . 
     The electric power circuit module  40  constitutes a circuit for appropriately implementing input/output of electric power in the battery module  30 . In this exemplary embodiment, the electric power circuit module  40  includes at least one switching device for switching a connection state between the battery module  30  and the main line  7  between connection and disconnection. In this exemplary embodiment, the electric power circuit module  40  includes an input/output circuit  43  for connecting the battery module  30  to the main line  7 , and a first switching device  41  and a second switching device  42  disposed in the input/output circuit  43 . The first switching device  41  and the second switching device  42  perform switching operations in accordance with signals (e.g., gate signals) input from the sweep unit  50 . 
     In this exemplary embodiment, as illustrated in  FIG. 2 , the first switching device  41  is attached to the main line  7  in series and is attached to the battery module  30  in parallel, in the input/output circuit  43 . The second switching device  42  is attached to a portion of the input/output circuit  43  in which the battery module  30  is connected to the main line  7  in series. The first switching device  41  includes source and drain disposed in a forward direction along a direction in which a discharge current flows in the main line  7 . The second switching device  42  includes source and drain disposed in a forward direction along a direction in which a charge current flows in the battery module  30 , in the input/output circuit  43  through which the battery module  30  is connected to the main line  7  in series. In this exemplary embodiment, the first switching device  41  and the second switching device  42  respectively include body diodes  41   a  and  42   a  that are MOSFETs (e.g., Si-MOSFETs) and oriented in a forward direction. Here, the body diode  41   a  of the first switching device  41  will be referred to as a first body diode as appropriate. The body diode  42   a  of the second switching device  42  will be referred to as a second body diode as appropriate. 
     The first switching device  41  and the second switching device  42  are not limited to the example illustrated in  FIG. 2 . Various devices capable of switching a connection state between conduction and non-conduction may be used as the first switching device  41  and the second switching device  42 . In this exemplary embodiment, MOSFETs (specifically, Si-MOSFETs) are used for both of the first switching device  41  and the second switching device  42 . Alternatively, devices except for MOSFETs (e.g., transistors) may be employed. 
     The electric power circuit module  40  includes an inductor  46  and a capacitor  47 . The inductor  46  is disposed between the battery module  30  and the second switching device  42 . The capacitor  47  is connected to the battery module  30  in parallel. In this exemplary embodiment, since secondary batteries are used as the batteries  31  of the battery module  30 , degradation of the batteries  31  caused by an increase in an internal resistance loss needs to be suppressed. In view of this, an RLC filter is constituted by the battery module  30 , the inductor  46 , and the capacitor  47  in order to level a current. 
     The electric power circuit module  40  includes the temperature detector  48 . The temperature detector  48  is disposed to detect heat generation of at least one of the first switching device  41  and the second switching device  42 . In this exemplary embodiment, the first switching device  41 , the second switching device  42 , and the temperature detector  48  are incorporated in one board. Thus, the board itself is replaced with a new one at the time when a failure is found in one of the first switching device  41  and the second switching device  42 . Thus, in this exemplary embodiment, one temperature detector  48  is disposed near the first switching device  41  and the second switching device  42  so that the number of components is reduced. The temperature detector for detecting a temperature of the first switching device  41  and a temperature detector for detecting a temperature of the second switching device  42  may be provided separately. Various devices for detecting a temperature (e.g., a thermistor) may be used as the temperature detector  48 . 
     As illustrated in  FIGS. 1 and 2 , the battery modules  30  in the strings  10  are connected to the main line  7  in series through the electric power circuit modules  40 . The battery modules  30  are connected to or disconnected from the main line  7  by appropriately controlling the first switching device  41  and the second switching device  42  of the electric power circuit modules  40 . In the example configuration of the electric power circuit module  40  illustrated in  FIG. 2 , when the first switching device  41  is turned off and the second switching device  42  is turned on, the battery module  30  is connected to the main line  7 . When the first switching device  41  is turned on and the second switching device  42  is turned off, the battery module  30  is disconnected from the main line  7 . 
     A sweep unit (SU)  50  is a control unit incorporated in each of the sweep module  20  in order to perform various types of control concerning the sweep module  20 . The sweep unit  50  will be also referred to as a sweep control unit. Specifically, the sweep unit  50  outputs a signal for driving the first switching device  41  and the second switching device  42  in the electric power circuit module  40 . The sweep unit  50  notifies a higher-order controller (SCU  11  illustrated in  FIG. 1  in this exemplary embodiment) of the state of the sweep module  20  (e.g., a voltage of the battery module  30 , temperatures of the batteries  31 , and the temperatures of the switching devices  41  and  42 ). The sweep unit  50  is incorporated in each of the sweep modules  20  of the string  10 . The sweep units  50  incorporated in the sweep modules  20  of the string  10  are sequentially connected, and sequentially propagates a gate signal GS output from the SCU  11 . As illustrated in  FIG. 2 , in this exemplary embodiment, the sweep unit  50  includes an SU processor  51 , a delay/selection circuit  52 , and a gate driver  53 . 
     The SU processor  51  is a controller for various processes in the sweep unit  50 . A microcomputer, for example, may be used as the SU processor  51 . The SU processor  51  receives detection signals from the voltage detector  35 , the temperature detector  36 , and the temperature detector  48 . The SU processor  51  inputs and outputs various types of signals to/from a higher-order controller (the SCU  11  of the string  10  in this exemplary embodiment). 
     A signal input from the SCU  11  to the SU processor  51  includes a forced through signal CSS and a forced connection signal CCS. The forced through signal CSS is a signal for instructing disconnection of the battery module  30  from the main line  7  (see  FIG. 1 ) extending from the distribution device  5  to the string  10 . That is, the sweep module  20  that has received the forced through signal CSS does not perform (passes through) an operation for inputting and outputting electric component to/from the distribution device  5 . The forced connection signal CCS is a signal for instructing maintenance of 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 (PWM signal in this exemplary embodiment) GS is a signal for controlling a repetitive switching operation between an on state and an off state of each of the first switching device  41  and the second switching device  42 . The gate signal GS is a pulse signal in which on and off are alternately repeated. First, the gate signal GS is input from the SCU  11  (see  FIG. 1 ) to the delay/selection circuit  52  in one of the sweep modules  20 . Next, the gate signal GS is sequentially propagated from the delay/selection circuit  52  in one sweep module  20  to the delay/selection circuit  52  in another sweep module  20 . 
     In the string  10 , sweep control shown in  FIGS. 3 and 4  is performed.  FIG. 3  is an example of a timing chart in a sweep operation. Specifically,  FIG. 3  shows an example of a relationship between a connection state of each sweep module  20  and a voltage output to the distribution device  5  in a case where all the sweep modules  20  perform a sweep operation.  FIG. 4  is an example of a timing chart in a forced through operation. Specifically,  FIG. 4  shows an example of a relationship between a connection state of each sweep module  20  and a voltage output to the distribution device  5  in a case where at least one of the sweep modules  20  performs a forced through operation. 
     In the sweep control performed in the string  10 , in the plurality of (e.g., M) sweep modules  20  incorporated in the string  10 , the number (m) of sweep modules  20  that turn on at the same time is defined. The gate signal GS in the sweep control is constituted by, for example, a pulse waveform. In the gate signal GS, signal waveforms for connecting the battery module  30  to the main line  7  and signal waveforms for disconnecting the battery module  30  from the main line  7  are preferably alternately arranged, for example. In the gate signal GS, the signal waveform for connecting the battery modules  30  to the main line  7  preferably includes the number of battery modules  30  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 modules  30  from the main line  7  includes a given number of battery modules  30  that need to be disconnected from the main line  7  out of the battery modules  30  incorporated in the string  10 . The signal waveform for connecting the battery modules  30  to the main line  7  and the signal waveform for disconnecting the battery modules  30  from the main line  7  are adjusted as appropriate with respect to, for example, the shape of the waveforms. 
     In the string  10  of the exemplary embodiment, the M sweep modules  20  are connected in series in the order of sweep modules  20 A,  20 B, . . . ,  20 M from the side toward the distribution device  5 . The side toward the distribution device  5  will be hereinafter referred to as an upstream side, and the side away from the distribution device  5  will be hereinafter referred to as a downstream side. First, a 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 at the most upstream side. Next, the gate signal GS is propagated from the delay/selection circuit  52  in the sweep module  20 A to the delay/selection circuit  52  in the sweep module  20 B adjacent to the sweep module  20 A at the downstream side. Propagation of the gate signal to the adjacent downstream sweep module  20  is sequentially repeated to the most downstream sweep module  20 M. 
     Here, the delay/selection circuit  52  can delay the pulsed gate signal GS input from the SCU  11  or the upstream sweep module  20  by a predetermined delay time and propagate the resulting gate signal GS to the downstream sweep module  20 . In this case, a signal indicating the delay time is input from the SCU  11  to the sweep unit  50  (e.g., the SU processor  51  in the sweep unit  50  in this exemplary embodiment). Based on the delay time indicated by the signal, the delay/selection circuit  52  delays propagation of the gate signal GS. The delay/selection circuit  52  may also propagate the input gate signal GS to the downstream sweep module  20  without delay. 
     The gate driver  53  drives switching operations of the first switching device  41  and the second switching device  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 a control signal to each of the first switching device  41  and the second switching device  42 . In the case of connecting the battery module  30  to the main line  7 , the gate driver  53  outputs control signals for turning the first switching device  41  off and the second switching device  42  on. In the case of disconnecting the battery module  30  from the main line  7 , the gate driver  53  outputs control signals for turning the first switching device  41  on and the second switching device  42  off. 
     The delay/selection circuit  52  of the exemplary embodiment is controlled by a control device such as the SCU  11 , and selectively performs a sweep operation, a forced through operation, and a forced connection operation. 
     For example, in the sweep operation, the first switching device  41  and the second switching device  42  are operated based on the gate signal GS. The battery modules  30  included in the string  10  are connected to the main line  7  in a predetermined order, and are disconnected from the main line  7  in a predetermined order. Consequently, in the string  10 , a predetermined number of battery modules  30  are driven while being always connected to the main line  7  with the battery module  30  connected to the main line  7  being sequentially switched in a short control period. With this sweep operation, in the string  10 , while the battery module  30  connected to the main line  7  is sequentially switched in a short control period, the string  10  functions as if the string  10  is one battery assembly in which a predetermined number of battery modules  30  are connected in series. To obtain the sweep operation, the sweep modules  20  in the string  10  are controlled by the SCU  11 . In this control, the SCU  11  outputs the gate signal GS to the string  10 , and outputs a control signal to the SU processor  51  included in each of the sweep modules  20 . An example of the sweep operation will be described later in detail 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 change, delays the gate signal GS by a delay time, and propagates the resulting gate signal GS to the downstream sweep module  20 . Consequently, 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  while the timings of each of the connection and the disconnection are shifted from one another in the string  10 . 
     In the forced through operation, the delay/selection circuit  52  keeps the first switching device  41  on, independently of the input gate signal GS, and outputs a signal for keeping the second switching device  42  off, to the gate driver  53 . Consequently, the battery module  30  in the sweep module  20  under the forced through operation is disconnected from the main line  7 . The delay/selection circuit  52  of the sweep module  20  under the forced through operation does not delay the gate signal GS and propagates the gate signal GS to the downstream sweep module  20 . 
     During the forced connection operation, the delay/selection circuit  52  keeps the first switching device  41  off, independently of the input gate signal GS, and outputs a signal for keeping the second switching device  42  on, to the gate driver  53 . Consequently, the battery module  30  of the sweep module  20  under the forced connection operation is always connected to the main line  7 . The delay/selection circuit  52  of the sweep module  20  under the forced connection operation does not delay the gate signal GS, and propagates the gate signal GS to the downstream sweep module  20 . 
     The delay/selection circuit  52  may be configured as one integrated circuit having necessary functions as described above. The delay/selection circuit  52  may be a combination of a circuit for delaying the gate signal GS and a circuit for selectively sending the gate signal GS to the gate driver  53 . An example configuration of the delay/selection circuit  52  of the exemplary embodiment will be described below. 
     In the exemplary 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  delays the gate signal GS by a predetermined delay time, and outputs the resulting gate signal GS to the selection circuit  52   b . In another case, the gate signal GS input to the delay/selection circuit  52  is output to the selection circuit  52   b  through another route not passing through the delay circuit  52   a . The selection circuit  52   b  receives an instruction signal from the SU processor  51 , and produces an output in accordance with the instruction signal. 
     In a case where the instruction signal from the SU processor  51  instructs performing a sweep operation, the selection circuit  52   b  outputs the input gate signal GS to the gate driver  53  of this sweep module  20  without change. The gate driver  53  outputs a control signal to the electric power circuit module  40 , turns the first switching device  41  off, turns the second switching device  42  on, and connects the battery module  30  to the main line  7 . On the other hand, the selection circuit  52   b  outputs the delayed gate signal GS to the delay/selection circuit  52  in the sweep module  20  at a downstream side adjacent to the sweep module  20  which input gate signal GS. That is, in a case where the battery module  30  is connected to the main line  7  in the sweep operation, the gate signal GS delayed by the predetermined delay time is sent to the adjacent downstream sweep module  20 . 
     In a case where the instruction signal from the SU processor  51  is the forced through signal CSS, the selection circuit  52   b  outputs a signal for passing through the battery module  30 , to the gate driver  53 . By continuing the forced through signal CSS, the battery module  30  in the sweep module  20  that has received the forced through signal CSS is kept disconnected from the main line  7 . In this case, the selection circuit  52   b  outputs a gate signal GS input to the selection circuit  52   b  through a route not passing through the delay circuit  52   a , to the adjacent downstream sweep module  20 . 
     In a case where the instruction signal from the SU processor  51  is the forced connection signal CCS, the selection circuit  52   b  outputs, to the gate driver  53 , a signal for connecting the battery module  30  to the main line  7 . That is, the gate driver  53  turns the first switching device  41  off, turns the second switching device  42  on, and connects the battery module  30  to the main line  7 . By continuing the forced connection signal CCS, the battery module  30  is kept connected to the main line  7 . In this case, the selection circuit  52   b  outputs a gate signal GS input to the selection circuit  52   b  through a route not passing through the delay circuit  52   a , to the adjacent downstream sweep module  20 . 
     As illustrated in  FIGS. 1 and 2 , in this exemplary embodiment, the plurality of sweep units  50  (specifically a plurality of delay/selection circuits  52 ) included in one string  10  are connected sequentially to each other in a daisy chain mode. Consequently, the gate signal GS input to one sweep unit  50  from the SCU  11  is sequentially propagated among the plurality of sweep units  50 . Thus, processing in the SCU  11  can be easily simplified, and an increase in number of signals can be easily suppressed. Alternatively, the SCU  11  may output a gate signal GS to each of the plurality of sweep units  50  independently of each other. 
     The sweep unit  50  includes an indicator  57 . The indicator  57  notifies an operator of a state of the sweep module  20  including the battery module  30 , the electric power circuit module  40 , and other components, for example. The indicator  57  is capable of notifying the operator that a problem (e.g., a failure or degradation of the batteries  31 ) is detected in the battery module  30  in the sweep module  20  (i.e., the battery module  30  needs to be replaced). 
     As an example, an LED that is a light-emitting device is used for the indicator  57  of the exemplary embodiment. Alternatively, a device except for an LED (e.g., a display) may be used as the indicator  57 . A device for outputting voice (e.g., a loudspeaker) may be used as the indicator  57 . The indicator  57  may notify the operator of the state of the sweep module  20  by driving a member by an actuator (e.g., a motor or a solenoid). The indicator  57  is preferably configured to indicate a state by various methods in accordance with the state of the sweep module  20 . 
     In the exemplary embodiment, operation of the indicator  57  is controlled by the SU processor  51  in the sweep unit  50 . Alternatively, the operation of the indicator  57  may be controlled by a controller (e.g., the SCU  11 ) except for the SU processor  51   
     In the exemplary embodiment, the indicator  57  is disposed for each of the sweep unit  50 . Thus, the operator can easily identify the sweep module  20  whose state is notified by the indicator  57  among the plurality of sweep modules  20 . Alternatively, the configuration of the indicator  57  may be changed. For example, separately from the indicator  57  disposed for each sweep unit  50 , or together with the indicator  57 , a state notifier for notifying a summary of states of the plurality of sweep modules  20  may be disposed. In this case, the state notifier may display a summary of the states of the plurality of sweep modules  20  (e.g. whether a problem occurs or not) on one monitor. 
     &lt;Sweep Control&gt; 
     Sweep control performed in the string  10  will be described. Here, the sweep control is a control for causing the battery modules  30  in the string  10  to perform a sweep operation. In the sweep control performed in the string  10 , the SCU  11  outputs a pulsed gate signal GS. The switching devices  41  and  42  in the sweep modules  20  of the strings  10  are switched between on and off to be driven as appropriate. Consequently, connection of the battery module  30  to the main line  7  and disconnection of the battery module  30  from the main line can be switched from each other at high speed for each of the sweep modules  20 . In addition, the string  10  can delay the gate signal GS input to the X-th sweep module  20  from the upstream side with respect to the gate signal GS input to the (X−1)th sweep module  20 . Consequently, among the M sweep modules  20  included in the string  10 , m (m&lt;M) sweep modules  20  connected to the main line  7  are sequentially switched. Accordingly, the plurality of battery modules  30  included in the string  10  are connected to the main line  7  in a predetermined order and disconnected from the main line  7  in a predetermined order. Then, a state as if a predetermined number of battery modules  30  are always connected to the main line  7  can be obtained. With the sweep operation, the string  10  functions as one battery assembly in which a predetermined number of battery modules  30  are connected in series. 
       FIG. 3  is a timing chart showing an example of a relationship between a connection state of each sweep module  20  and a voltage output to the distribution device  5  in a case where all the sweep modules  20  included in the string  10  are caused to perform a sweep operation. The number M of the sweep modules  20  included in one string  10  may be changed as appropriate. In the example shown in  FIG. 3 , one string  10  includes five sweep modules  20 , and all the five sweep modules  20  are caused to perform the sweep operation. 
     In the example shown in  FIG. 3 , a VH instruction signal for setting a voltage VH [V] to be output to the distribution device  5  at 100 V is input to the SCU  11  of the string  10 . A voltage Vmod [V] of the battery module  30  in each of the sweep modules  20  is 43.2 V. A delay time DL [μsec] for delaying the gate signal GS is set as appropriate in accordance with requirements for the power supply system  1 . A period T (i.e., a period for connection or disconnection of the sweep module  20 ) of the gate signal GS is a value obtained by multiplying, by a delay time DL, the number P (≥M) of sweep modules  20  caused to perform the sweep operation. Thus, if the delay time DL is long, the frequency of the gate signal GS is a low frequency. On the other hand, if the delay time DL is short, the frequency of the gate signal GS is a high frequency. In the example shown in  FIG. 3 , the delay time DL is set at 2.4 μsec. Thus, the period T of the gate signal GS is “2.4 μsec×5=12 μsec.” 
     In this exemplary embodiment, the battery module  30  of the sweep module  20  in which the first switching device  41  is off and the second switching device  42  is on, is connected to the main line  7 . That is, when the first switching device  41  is turned off and the second switching device  42  is turned on, the capacitor  47  connected to the battery module  30  in parallel is connected to the input/output circuit  43 , and electric power is input or 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 on. On the other hand, the battery module  30  of the sweep module  20  in which the first switching device  41  is off and the second switching device  42  is on, 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 off. 
     When the first switching device  41  and the second switching device  42  are turned on at the same time, a short circuit occurs. Thus, in the case of switching the first switching device  41  and the second switching device  42 , the sweep unit  50  switches one of the devices from on to off, and after a lapse of a small standby time, then switches the other device from off to on. As a result, occurrence of a short circuit is prevented. 
     Supposing a VH instruction value instructed by the VH instruction signal is VH_com, voltage of each battery module  30  is Vmod, the number of sweep modules  20  to perform a sweep operation (i.e., the number of sweep modules  20  as connection targets to the main line  7  in the sweep control) is P. In this case, in the gate signal GS, a duty ratio occupied by an on-period in the period T is obtained by “VH_com/(Vmod×P).” In the example shown in  FIG. 3 , the duty ratio of the gate signal GS is about 0.46. Strictly, the duty ratio is shifted under the influence of the standby time for preventing occurrence of a short circuit. Thus, the sweep unit  50  may correct the duty ratio by using a feedback process or a feedforward process. 
     As shown in  FIG. 3 , when the sweep control starts, one (e.g., the sweep module  20  of No. 1 at the most upstream side in the example shown in  FIG. 3 ) of P sweep modules  20  comes to be in a connected state. Thereafter, after a lapse of a delay time DL, the next sweep module  20  (e.g., the second sweep module  20  of No. 2 from the upstream side in the example shown in  FIG. 3 ) also comes to be in a connected state. In this state, a voltage VH output to the distribution device  5  is the sum of voltages of the two sweep modules  20 , and does not reach a VH instruction value. Subsequently, after a lapse of the delay time DL, the sweep module  20  of No. 3 comes to be in a connected state. In this state, the number of sweep modules  20  connected to the main line  7  is three, that is, No. 1 through No. 3. Thus, the voltage VH output to the distribution device  5  is the sum of voltages of the three sweep modules  20 , which is larger than the VH instruction value. Subsequently, when the sweep modules  20  of No. 1 is disconnected from the main line  7 , the voltage VH returns to the sum of the voltages of the two sweep modules  20 . After a lapse of the delay time DL from the start of connection of No. 3, the sweep module  20  of No. 4 comes to be in a connected state. Consequently, the number of sweep modules  20  connected to the main line  7  is three, that is, No. 2 through No. 4. As described above, with the sweep control, m (three in  FIG. 3 ) sweep modules  20  connected to the main line  7  out of the M (five in  FIG. 3 ) sweep modules  20  are sequentially switched. 
     As shown in  FIG. 3 , the VH instruction value is not divisible by the voltage Vmod of each battery module  30  in some cases. In such cases, the voltage VH output to the distribution device  5  varies. The voltage VH, however, is levelled by the RLC filter, and is output to the distribution device  5 . In a case where electric power input from the distribution device  5  is stored in the battery module  30  of each of the sweep modules  20 , the connection state of the sweep module  20  is controlled, in a manner similar to the timing chart of  FIG. 3 . 
     &lt;Forced Through Operation&gt; 
     With reference to  FIG. 4 , description will be given on control in a case where one or more of the sweep modules  20  are caused to perform a forced through operation and the other sweep module(s)  20  is/are caused to perform a sweep operation. As described above, the sweep module  20  instructed to perform a forced through operation keeps a state in which the battery module  30  is disconnected from the main line  7 . The example shown in  FIG. 4  is different from the example shown in  FIG. 3  in that the sweep module  20  of No. 2 is caused to perform a forced through operation. That is, in the example shown in  FIG. 4 , the number of sweep modules  20  caused to perform a sweep operation (i.e., the number of sweep modules  20  as connection targets to the main line  7 ) P is four in the five sweep modules  20  included in one string  10 . The VH instruction value, the voltage Vmod of each battery module  30 , and the delay time DL are the same as those in the example shown in  FIG. 3 . In the example shown in  FIG. 4 , a period T of the gate signal GS is “2.4 μsec×4=9.6 μsec.” A duty ratio of the gate signal GS is about 0.58. 
     As shown in  FIG. 4 , in the case where one or more of the sweep modules  20  (e.g., the sweep module  20  of No. 2 in  FIG. 4 ) are caused to perform the forced through operation, the number P of sweep modules  20  caused to perform the sweep operation is smaller than that in the example shown 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 in accordance with the decrease of the number P of the sweep modules  20  caused to perform the sweep operation. Consequently, a waveform of the voltage VH output to the distribution device  5  is the same as the waveform of the voltage VH shown in  FIG. 3  as an example. Thus, even in the case of increasing or reducing the number P of the sweep modules  20  caused to perform the sweep operation, the string  10  can output an instructed voltage VH to the distribution device  5  as appropriate. 
     In a case where a problem (e.g., degradation or a failure) occurs in the battery  31  of one of the sweep modules  20 , for example, the string  10  is capable of causing the sweep module  20  including the battery  31  suffering from the problem to perform the forced through operation. Thus, the string  10  is capable of outputting an instructed voltage VH to the distribution device  5  appropriately by using the sweep modules  20  suffering from no problems. In addition, it is possible for the operator to replace the battery module  30  including the battery  31  suffering from a problem (i.e., the battery module  30  of the sweep module  20  caused to perform the forced through operation) while allowing the string  10  to operate normally. In other words, in the power supply system  1  of the exemplary embodiment, it is unnecessary to stop an operation of the entire string  10  in replacing the battery module  30 . 
     In a case where one or more of the sweep modules  20  are caused to perform the forced connection operation, the connection states of these sweep modules  20  caused to perform the forced connection operation is an always connection state. For example, in the case of causing the sweep module  20  of No. 2 shown in  FIG. 4  is caused to perform not the forced through operation but the forced connection operation, the connection state of No. 2 is kept not “disconnected” but “connected.” 
     In the case where the power supply system  1  includes a plurality of strings  10 , the sweep control is performed in each of the strings  10 . The controller for integrally controlling the entire power supply system  1  (the GCU  2  in this exemplary embodiment) controls operations of the plurality of strings  10  in order to satisfy an instruction from the higher-order system  6 . For example, in a case where only one string  10  cannot satisfy a VH instruction value required by the higher-order system  6 , the GCU  2  causes the plurality of strings  10  to output electric power so that the VH instruction value is satisfied. 
     &lt;String&gt; 
     With reference to  FIG. 1 , an entire configuration of the strings  10  and the power supply system  1  will be described in detail. As described above, each string  10  includes the SCU  11  and the plurality of sweep modules  20  connected to the main line  7  in series through the electric power circuit modules  40 . The main line  7  of the string  10  is connected to a bus line  9  extending from the distribution device  5 . The string  10  includes, from the side toward the distribution device  5  (upstream side) in the main line  7 , a bus line voltage detector  21 , a system breaker (also referred to as a system main relay (SMR) as appropriate)  22 , a string capacitor  23 , a string current detector  24 , a string reactor  25 , and a string voltage detector  26 . Arrangement of one or more of the members may be changed. For example, the system breaker  22  may be disposed downstream of the string capacitor  23 . 
     The bus line voltage detector  21  detects a voltage on the bus line  9  extending from the distribution device  5  to the string  10 . The system breaker  22  switches the connection state between the string  10  and the distribution device  5  between connection and disconnection. In this exemplary embodiment, the system breaker  22  is driven in accordance with a signal input from the SCU  11 . The string capacitor  23  and the string reactor  25  constitute an RLC filter to thereby level a current. The string current detector  24  detects a current flowing between the string  10  and the distribution device  5 . The string voltage detector  26  detects a voltage as the sum of voltages of the plurality of sweep modules  20  connected to the main line  7  in series in the string  10 , that is, a string voltage of the string  10 . 
     In the configuration 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 for connecting and disconnecting the string  10  to/from the distribution device  5 . The switch  22   a  will be also referred to as a string switch, as appropriate. When the switch  22   a  is turned on, the main line  7  of the string  10  and the bus line  9  of the distribution device  5  are connected to each other. When the switch  22   a  is turned off, the string  10  is disconnected from the distribution device  5 . The switch  22   a  is controlled by the SCU  11  for controlling the string  10 . By operating the switch  22   a , the string  10  is disconnected and connected from/to the distribution device  5 , as appropriate. The fuse  22   b  is a device for stopping a flow of an unexpected large current in terms of design of the string  10  in the main line  7  of the string  10  in a case where the large current flows in the main line  7 . The fuse  22   b  will be referred to as a string fuse, as appropriate. 
     Here, if batteries satisfying the same standard are incorporated in one battery module  30 , the voltage of the battery module  30  increases as the number of incorporated batteries increases. On the other hand, if the voltage of the battery module  30  is high, danger arises in handling by an operator, and the system is heavy. In view of this, one battery module  30  preferably includes a large number of batteries within the range where the voltage of the battery module  30  is at such a level that a touch by a person with the module  30  in a fully charged state does not cause a serious accident (e.g., less than 60 V, preferably less than 42 V) and the battery module  30  has such a weight that one operator can replace the systems. The battery module  30  incorporated in the string  10  does not need to be constituted by exactly the same type of batteries, and the number of batteries incorporated in one battery module  30  may be determined in accordance with the type and standard of batteries incorporated in the battery module  30 . The string  10  is configured such that the sweep modules  20  including the battery modules  30  are connected in series to thereby enable an output of a predetermined voltage. The power supply system  1  is configured to enable an output of a predetermined level of electric power for connection to the electric power system  8  by combining the plurality of strings  10 . 
     In this exemplary embodiment, the distribution device  5  to which the strings  10  of the power supply system  1  are connected includes sub-distribution devices  5 A and  5 B respectively connected to the strings  10 A and  10 B. The strings  10 A and  10 B connected to the sub-distribution devices  5 A and  5 B are connected in parallel through the sub-distribution devices  5 A and  5 B. The distribution device  5  controls distribution of electric power input from the electric power system  8  to the strings  10 A and  10 B, integration of electric power output from the strings  10 A and  10 B to the electric power system  8 , and so forth through the sub-distribution devices  5 A and  5 B connected to the strings  10 . The distribution device  5  and the sub-distribution devices  5 A and  5 B are controlled such that the power supply system  1  including the strings  10  functions as one power supply device as a whole by cooperation of the GCU  2  connected to the higher-order system  6  and the SCUs  11  for controlling the strings  10 . 
     For example, in this exemplary embodiment, the downstream side of the distribution device  5 , that is, a side toward the strings  10 A and  10 B, is controlled by a direct current (DC). The upstream side of the distribution device  5 , that is, the electric power system  8 , is controlled by an alternating current (AC). The voltages of the strings  10 A and  10 B are controlled to be substantially balanced with respect to the voltage of the electric power system  8 , through the distribution device  5 . When the voltages of the strings  10 A and  10 B are controlled to be lower than the voltage of the electric power system  8 , a current flows from the electric power system  8  to the strings  10 A and  10 B. At this time, when sweep control is performed in each of the strings  10 A and  10 B, the battery modules  30  are charged as appropriate. When the voltages of the strings  10 A and  10 B are controlled to be higher than the voltage of the electric power system  8 , a current flows form the strings  10 A and  10 B to the electric power system  8 . At this time, when sweep control is performed in each of the strings  10 A and  10 B, the battery modules  30  are discharged as appropriate. The distribution device  5  may be controlled such that the voltages of the strings  10 A and  10 B are kept to be balanced with respect to the voltage of the electric power system  8  so that substantially no current flows in the strings  10 A and  10 B. In this exemplary embodiment, such control can be performed for each of the sub-distribution devices  5 A and  5 B to which the strings  10 A and  10 B are connected. For example, control may be performed such that the voltage of each of the strings  10 A and  10 B is adjusted so that substantially no current flows in one of the strings  10 A and  10 B connected to the distribution device  5 . 
     In the power supply system  1 , the number of strings  10  connected to the distribution device  5  in parallel is increased so that the capacity of the power supply system  1  as a whole can be increased. For example, in the power supply system  1 , a large-size system capable of producing an output that can absorb an abrupt increase in demand of the electric power system  8  and of compensating for a sudden power shortage of the electric power system  8  can be assembled. For example, an increase in the capacity of the power supply system  1  can use large redundant electric power of the electric power system  8  for charging of the power supply system  1  as appropriate. For example, in a case where an output of an electric power station is redundant in a time zone where electric power demand is low at midnight or a case where the amount of power generation rapidly increases in a large solar power station, the power supply system  1  can absorb redundant electric power through the distribution device  5 . In contrast, in a case where demand for electric power rapidly increases in the electric power system  8 , required electric power can be output from the power supply system  1  to the electric power system  8  through the distribution device  5  as appropriate, in accordance with an instruction from the higher-order system  6 . In this manner, the power supply system  1  compensates for an electric power shortage of the electric power system  8  as appropriate. 
     In the power supply system  1 , all the plurality of battery modules  30  incorporated in the strings  10  do not need to be always connected. Since the forced through operation can be performed for each of the battery modules  30  as described above, when an abnormality occurs in one of the battery modules  30 , this battery module  30  can be disconnected from the sweep control of the string  10 . Thus, in the power supply system  1 , batteries used for the battery modules  30  do not need to be unused new batteries. 
     For example, secondary batteries used as a drive power source for an electric vehicle such as a hybrid vehicle or an electric automobile can be reused. Even if a secondary battery used as such a drive power supply is used for about 10 years, this secondary battery can sufficiently function as a secondary battery. In the power supply system  1 , the battery module  30  showing abnormality can be immediately disconnected, and thus, batteries can be incorporated after confirmation that the batteries have necessary given functions. Secondary batteries used as a power for driving an electric vehicle sequentially reach periods for collection. The power supply system  1  can also incorporate secondary batteries corresponding to 10,000 electric vehicles, and is expected to absorb a considerable amount of collected secondary batteries. It is unexpected when functions of the secondary batteries used as a power supply for driving electric vehicles degrade. In a case where such secondary batteries are reused for the battery modules  30  of the power supply system  1 , it is impossible to expect when a problem occurs in the battery modules  30 . 
     However, in the power supply system  1  proposed here, the battery modules  30  can be appropriately disconnected through the sweep modules  20 . Thus, even when a problem occurs in the battery module  30  or a secondary battery incorporated in the battery module  30 , it is unnecessary to stop the entire power supply system  1 . 
     &lt;Charge Control Process&gt; 
     In the charge control process, to fully exhibit functions of the plurality of battery modules  30  included in one string  10 , charging if the battery modules  30  are performed by sweep control. With reference to  FIG. 5 , a charge control process performed by the power supply system  1  of this exemplary embodiment will be described. 
     The charge control process exemplified in the exemplary embodiment is performed by the SCUs  11  serving as controllers (control units) included in the strings  10 . When each of the SCUs  11  receives, from the GCU  2 , an instruction for starting charging of electric power input from the distribution device  5 , the SCU  11  starts the charge control process shown in  FIG. 5 . The controllers that perform the charge control process are not limited to the SCUs  11 . For example, the GCU  2  may perform the charge control process, or a controller for performing a charge control process may be provided in addition to the SCUs  11  and the GCU  2 . A plurality of controllers (e.g., the SCUs  11  and the sweep units  50 ) may perform a charge control process in cooperation. 
     When the charge control process starts, the SCU  11  sets conditions for sweep control (S 1 ). For example, based on, for example, the number P of sweep modules  20  as connection targets to the main line  7  in the sweep control, the SCU  11  sets conditions for the sweep control, such as a delay time DL and a period T of a gate signal GS. In accordance with the set conditions, the SCU  11  starts sweep control (S 2 ). In the charge control process, electric power is input from the distribution device  5  through the main line  7 . Thus, the battery modules  30  in the sweep modules  20  performing the sweep operation are gradually charged with the input electric power. 
     The SCU  11  detects a state of charge (SOC) for each battery module  30  in the string  10  (S 4 ). A specific method for detecting the SOC of the battery module  30  can be selected as appropriate. For example, the SCU  11  may detect the SOC based on the voltage of the battery module  30  detected by the voltage detector  35 . The SOC may be detected in consideration of the temperature of the battery module  30  detected by the temperature detector  36  together with the voltage thereof. The SOC may be detected in consideration of a current flowing in the battery module  30 . The SCU  11  may detect the SOC based on an open-circuit voltage of the battery module  30  and an integrated value of a current flowing in the battery module  30 . 
     The SCU  11  determines whether the plurality of battery modules  30  in the string  10  include the battery module  30  whose SOC level detected in S 4  satisfies a specific condition (condition for high SOC) or not (S 5 ). The condition for high SOC (high SOC condition) is a condition for an SOC set to determine the sweep module  20  for which charging by sweep control is stopped first among the plurality of sweep modules  20  performing a sweep operation. That is, when charging of the battery modules  30  proceeds to increase the SOC level so that the increased SOC level satisfies the high SOC condition, charging of the battery module  30  satisfying the condition is stopped. The high SOC condition can be set as appropriate. The high SOC condition may be a condition that the detected SOC level is a threshold or more. In this case, the threshold may be previously determined or may be changed in accordance with, for example, an operation instruction by an operator. It may be determined that the high SOC condition is satisfied if a proportion of a detected SOC is greater than or equal to a specific proportion (e.g., α times: α&gt;1) as compared to an SOC level of another battery module  30  in the string  10  (with respect to an average of SOC levels of all the battery modules  30  in the string  10 ). In the plurality of battery modules  30  being charged by sweep control, the SOC of the battery module  30  having lower performance is likely to increase in a shorter time than the SOC of the battery module  30  having higher performance. Unless the battery modules  30  does not include the battery module  30  satisfying the high SOC condition (hereinafter referred to as a “high SOC module”) (that is S 5 : NO), the process proceeds to S 11 . 
     If the battery modules  30  include a high SOC module (S 5 : YES), the SCU  11  outputs the forced through signal CSS to the sweep module  20  including the high SOC module (S 7 ). Consequently, the high SOC module is disconnected from the main line  7 , and charging of the high SOC module is stopped. The SCU  11  excludes the sweep module  20  including the high SOC module from connection targets to the main line  7  in the sweep control (S 8 ). The SCU  11  resets conditions for the sweep control, and continues the sweep control under the reset condition (S 9 ). Consequently, charging of another battery module  30  by the sweep control is performed with the charging of the high SOC module stopped. That is, even when charging of the battery module  30  having lower performance is finished, charging of battery modules  30  having high performance is appropriately performed. Thus, in a case where battery modules  30  having different performances are mixed, the battery module  30  can be fully utilized. In the exemplary embodiment, a time in which no current flows in the high SOC module increases. Thus, if the batteries  31  in the high SOC module are nickel-metal hydride batteries, absorption of hydrogen generated in the batteries  31  progresses while no current flows. Accordingly, an increase in the internal pressure of the batteries  31  can be easily suppressed. In addition, since the time in which no current flows increases, an increase in temperature of the batteries  31  can be easily suppressed. In addition, since charging of the high SOC module is ignored, the possibility that the high SOC module is in an overcharged state decreases, resulting an increase in lifetime of the battery modules. Furthermore, the possibility that the battery module  30  having a low SOC level is overdischarged in discharging the battery modules  30  decreases. 
     Next, the SCUs  11  performs various processes (S 11 ). For example, in a case where an instruction from a higher-order controller (i.e., the GCU  2  in this exemplary embodiment) is changed, the SCU  11  resets driving conditions for the plurality of sweep modules  20  based on the changed instruction. Thereafter, the SCU  11  determines whether an instruction for finishing charging by sweep control is input or not (S 12 ). If the instruction is not input (S 12 : NO), the process returns to S 4 , and charging by the sweep control continues. If the finishing instruction is input (S 12 : YES), the charge control process is finished. 
     The technique disclosed in this exemplary embodiment is merely an example. Thus, the technique exemplified in the above exemplary embodiment may be changed. For example, in the above exemplary embodiment, SOCs of the plurality of battery modules  30  are detected by the SCU  11 . Alternatively, an SOC may be detected by another controller (e.g., the sweep unit  50 ). 
     The process of performing the sweep control in S 2  of  FIG. 5  is an example of “first process.” In S 4 , S 5 , and S 7  through S 9  of  FIG. 5 , the process of continuing charging by sweep control with the high SOC module disconnected from the main line  7  is an example of a “second process.” 
     Specific examples of the present teaching have been described in detail hereinbefore, but are merely illustrative examples, and are not intended to limit the scope of claims. The techniques described in the scope of claims include various modifications and changes of the above described exemplary embodiment.