Patent Publication Number: US-2022231530-A1

Title: Battery module and power supply system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-188509 filed on Oct. 15, 2019 and is a Continuation application of PCT Application No. PCT/JP2020/028307 filed on Jul. 21, 2020. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a battery module and a power supply system. 
     2. Description of the Related Art 
     A power supply system including a converter unit and a battery module has been disclosed (for example, refer to International Publication No. 2017/209238). In the power supply system, the converter unit is configured to convert alternating-current power supplied from a commercial power source into direct-current power and output the direct-current power to a load, and the battery module is configured to assist in the supply of direct-current power to the load. The output terminal of the battery module is connected between the output terminal of the converter unit and the load, and the output voltage of the battery module is controlled so as to be maintained at a constant value equal to a predetermined output voltage target value. The output voltage target value is set so as to increase with an increase in the current value of a current that is output from the converter unit when the current value of a current that is output from the converter unit is equal to or higher than a first threshold, which is lower than a predetermined value. Moreover, when the current value of a current that is output from the converter unit is between a second threshold, which is lower than the predetermined value and higher than the first threshold, and the first threshold, the output voltage target value is set so as to be higher than the output voltage of the converter unit. Accordingly, when the load enters a heavy load state and the output voltage of the converter unit decreases, it is possible to stabilize a direct-current voltage applied to the load by smoothly supplying direct-current power to the load from the battery module. 
     In the power supply system described in International Publication No. 2017/209238, the output voltage of the converter unit rapidly decreases, for example, if a power outage of the commercial power source occurs. In contrast, the battery module performs control such that the direct-current voltage applied to the load is maintained at the target value. In such a case, it is possible that constant voltage control of the battery module does not follow a rapid increase in the output current of the battery module and that the direct-current voltage applied to the load temporarily decreases. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide battery modules and power supply systems that are each able to reduce a variation in voltage applied to a load. 
     A battery module according to a preferred embodiment of the present invention is to be connected to a power supply configured to output a predetermined target voltage to a load, the battery module includes a battery, a DC-DC converter to convert a direct-current voltage that is output from the battery and thereafter output the direct-current voltage to the load, and a controller configured or programmed to perform constant voltage control of the DC-DC converter and receive a stop warning signal providing advance notice that output of the power supply is to stop, and the controller is configured and programmed to, before receiving the stop warning signal, perform control such that an output voltage of the DC-DC converter is lower than the target voltage and, after receiving the stop warning signal and before the output of the power supply stops, increase the output voltage of the DC-DC converter to the target voltage or higher. 
     In a battery module according to a preferred embodiment of the present invention, the controller may be configured or programmed to perform control such that a voltage variation in the output voltage of the DC-DC converter at a time when the output voltage of the DC-DC converter reaches the target voltage is smaller than a maximum voltage variation during a period when the output voltage of the DC-DC converter is increased to a voltage close to the target voltage after the stop warning signal is received. 
     In a battery module according to a preferred embodiment of the present invention, the controller may be configured or programmed to perform control such that the output voltage of the DC-DC converter is maintained at a constant value when a current value of a current flowing from the power supply to the load has become equal to or smaller than a predetermined reference current value. 
     In a battery module according to a preferred embodiment of the present invention, the controller may be configured or programmed to perform control such that the output voltage of the DC-DC converter is maintained at a constant value when the output voltage of the DC-DC converter has become equal to or higher than the target voltage. 
     In a battery module according to a preferred embodiment of the present invention, the controller may be configured or programmed to gradually increase the output voltage of the DC-DC converter such that a variation in a current flowing from the DC-DC converter to the load per unit time is smaller than a predetermined upper limit of the variation. 
     In a battery module according to a preferred embodiment of the present invention, the controller may be configured or programmed to perform control such that the output voltage of the DC-DC converter is maintained at a constant value and thereafter perform control such that the output voltage of the DC-DC converter equals a voltage equal to the target voltage. 
     In a battery module according to a preferred embodiment of the present invention, a voltage upper limit of the output voltage of the DC-DC converter may be set based on a rated voltage for the load. 
     A power supply system according to a preferred embodiment of the present invention includes a power supply including an AC-DC converter and a first controller, the AC-DC converter being configured to convert alternating-current power supplied from an alternating-current power source into direct-current power and output the direct-current power to a load, the first controller being configured or programmed to perform constant voltage control of the AC-DC converter such that a voltage that is output from the AC-DC converter to the load equals or substantially equals a predetermined target voltage, the first controller being configured or programmed to, before stopping voltage output to the load, output a stop warning signal providing advance notice that the voltage output to the load is to stop, a battery, a DC-DC converter to convert a direct-current voltage that is output from the battery and thereafter output the direct-current voltage to the load, and a second controller configured or programmed to perform constant voltage control of the DC-DC converter and receive the stop warning signal providing advance notice that output of the power supply is to stop, and the second controller is configured or programmed to, before receiving the stop warning signal, perform control such that an output voltage of the DC-DC converter is lower than the target voltage and, after receiving the stop warning signal and before the output of the power supply stops, increase the output voltage of the DC-DC converter to the target voltage or higher. 
     A power supply system according to a preferred embodiment of the present invention includes a power supply including a first DC-DC converter and a first controller, the first DC-DC converter being configured to convert direct-current power supplied from a direct-current power source into direct-current power having a different voltage and output the direct-current power having the different voltage to a load, the first controller being configured or programmed to perform constant voltage control of the first DC-DC converter such that a voltage that is output from the first DC-DC converter to the load equals a predetermined target voltage, the first controller being configured or programmed to, before stopping voltage output to the load, output a stop warning signal providing advance notice that the voltage output to the load is to stop, a battery, a second DC-DC converter configured to convert a direct-current voltage that is output from the battery and thereafter output the direct-current voltage to the load, and a second controller configured or programmed to perform constant voltage control of the second DC-DC converter and receive the stop warning signal providing advance notice that output of the power supply is to stop, and the second controller is configured or programmed to, before receiving the stop warning signal, perform control such that an output voltage of the second DC-DC converter is lower than the target voltage and, after receiving the stop warning signal and before the output of the power supply stops, increase the output voltage of the second DC-DC converter to the target voltage or higher. 
     According to a preferred embodiment of the present invention, a controller is configured or programmed to, before receiving the stop warning signal of the power supply, perform control such that an output voltage of the DC-DC converter is lower than the target voltage. Further, the controller is configured or programmed to, after receiving the stop warning signal of the power supply and before the output of the power supply stops, increase the output voltage of the DC-DC converter to the target voltage or higher. In this way, when the source of power supply to the load switches from the power supply to the battery module, the increasing rate of the current supplied from the battery module to the load can be reduced. Accordingly, a variation in the voltage applied to the load is reduced. The variation in the voltage is caused because the constant voltage control of the battery module does not follow the variation in the output current of the battery module. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration showing a schematic configuration of a power supply system according to a first preferred embodiment of the present invention. 
         FIG. 2  is an illustration showing output voltage characteristics of an AC-DC converter according to the first preferred embodiment of the present invention. 
         FIG. 3  is a block diagram of a controller according to the first preferred embodiment of the present invention. 
         FIG. 4  is an illustration showing information stored in a voltage-variation storage unit according to the first preferred embodiment of the present invention. 
         FIG. 5  illustrates characteristics of the output voltage and the output current of a power supply and a battery module according to a comparative example. 
         FIG. 6  illustrates characteristics of the output voltage and the output current of a power supply and a battery module according to the first preferred embodiment of the present invention. 
         FIG. 7  is a flowchart showing an example flow of battery-module control processing performed by the controller according to the first preferred embodiment of the present invention. 
         FIG. 8  is an illustration showing a schematic configuration of a power supply system according to a second preferred embodiment of the present invention. 
         FIG. 9  is a block diagram of a controller according to the second preferred embodiment of the present invention. 
         FIG. 10  is an illustration showing information stored in a voltage-variation storage unit according to the second preferred embodiment of the present invention. 
         FIG. 11  is a flowchart showing an example flow of battery-module control processing performed by the controller according to the second preferred embodiment of the present invention. 
         FIG. 12  is a flowchart showing an example flow of battery-module control processing performed by a controller according to a modification of a preferred embodiment of the present invention. 
         FIG. 13  is a flowchart showing an example flow of battery-module control processing performed by a controller according to a modification of a preferred embodiment of the present invention. 
         FIG. 14  illustrates characteristics of the output voltage of a power supply and a battery module according to a modification of a preferred embodiment of the present invention. 
         FIG. 15  is an illustration showing a schematic configuration of a power supply system according to a modification of a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the drawings. 
     First Preferred Embodiment 
     A first preferred embodiment of the present invention will be described in detail hereinafter with reference to the drawings. Together with a power supply, a battery module according to the present preferred embodiment is connected to a load, the power supply being configured to perform control such that a voltage that is output to the load equals or substantially equals a predetermined first target voltage and, before stopping voltage output to the load, output a stop warning signal providing advance notice that the voltage output to the load is to stop. The battery module includes a battery, a DC-DC converter to convert a direct-current voltage that is output from the battery and thereafter output the direct-current voltage to the load, and a controller configured or programmed to perform constant voltage control such that the output voltage of the DC-DC converter is maintained at a predetermined second target voltage. The controller is configured or programmed to, before the power supply outputs the stop warning signal, perform control of the output voltage of the DC-DC converter such that the second target voltage is lower than the first target voltage. In addition, the controller is configured or programmed to, after the power supply outputs the stop warning signal, perform control of the output voltage of the DC-DC converter such that the second target voltage is equal to or higher than the first voltage. 
     The controller is preferably configured or programmed to perform control of the output voltage of the DC-DC converter such that the second target voltage is equal to or higher than the first target voltage before the output of the power supply stops. The stop warning signal is transmitted from the power supply to the controller of the battery module, and either a high-level signal or a low-level signal is transmitted. For example, if a drop in the alternating-current voltage that is input to the power supply is detected, the power supply outputs a high-level signal to the controller of the battery module. If a drop in the alternating-current voltage that is input to the power supply is not detected, the power supply outputs a low-level signal to the controller of the battery module. The stop warning signal is output within an alternating-current-voltage-drop detection period (for example, about 3 msec). Even when the alternating-current voltage that is input to the power supply is low, electric charge stored in a capacitor (for example, an electrolytic capacitor) included in the power supply enables the output voltage to be maintained at the first target voltage for an output retention period (for example, about 10 msec) that reflects the discharging time constant of the capacitor. Accordingly, the controller only has to perform control of the output voltage of the DC-DC converter such that the second target voltage becomes equal to or higher than the first voltage during the period obtained by subtracting the alternating-current-voltage-drop detection period from the output retention period (the obtained period being, for example, about 7 msec). Performing control in this way enables the voltage that is output to the load to be maintained at a constant value. 
     For example, as shown in  FIG. 1 , a power supply system according to the present preferred embodiment includes two power supplies  12 , two battery modules  13 , and a signal processor  14 . Each power supply  12  is configured to convert alternating-current power that is output from an alternating-current power source  100  into direct-current power and output the direct-current power to a load  101 , and each battery module  13  includes a battery  132 . Examples of the load  101  include a blade server. Multiple loads  101  connected in parallel to each other may be connected to the power supplies  12  and the battery modules  13 . Further, the two power supplies  12  connected in parallel to each other for redundancy are connected to the alternating-current power source  100 . Because of this configuration, even if one of the two power supplies  12  comes to a stop due to a breakdown, the other one of the two power supplies  12  can continue supplying power from the alternating-current power source  100  to the load  101 . 
     Each power supply  12  includes an AC-DC converter  121 , a controller  123  configured or programmed to control the AC-DC converter  121 , and a current measurer  124  to measure a current value of a current that is output from the AC-DC converter  121 . The AC-DC converter  121  is connected between the alternating-current power source  100  and the load  101 . The AC-DC converter  121  is configured to convert alternating-current power (such as alternating current having a voltage of, for example, about 200 V) supplied from the alternating-current power source  100  into direct-current power (such as direct current having a voltage of, for example, about 12.3 V) and output the direct-current power to the load  101 . The current measurer  124  is configured to output to the controller  123  a measurement signal based on a current value obtained by measuring the output current of the AC-DC converter  121 . 
     The controller  123  corresponds to a first controller configured or programmed to perform constant voltage control of the AC-DC converter  121  such that a voltage that is output from the AC-DC converter  121  to the load  101  equals or substantially equals a predetermined first target voltage. The first target voltage is determined in accordance with the specification of the input voltage for the load  101  and set to, for example, about 12.3 V. Further, the controller  123  includes what a current share function and is configured or programmed to output to the controller  123  of the other power supply  12  a current share signal that reflects the magnitude of the output current of the AC-DC converter  121  to be controlled. The controller  123  is configured or programmed to output the current share signal based on the measurement signal that the controller  123  receives from the current measurer  124 . The controller  123  is configured or programmed to control the AC-DC converter  121  so that the output current of the AC-DC converter  121  to be controlled equals or substantially equals the output current of the other AC-DC converter  121 , and the control is based on the measurement signal received from the current measurer  124  and a current share signal received from the other controller  123 . In this way, the output currents of the two AC-DC converters  121  are maintained approximately at the same or substantially the same level. Further, the controller  123  is configured or programmed to control the AC-DC converter  121  so that the output voltage of the AC-DC converter  121  decreases when the current value of the current that is output from the AC-DC converter  121  exceeds a predetermined current threshold. In addition, the controller  123  is configured or programmed to output the current share signal also to the signal processor  14 . 
     For example, as shown in  FIG. 2 , until the current value of the output current of the AC-DC converter  121  becomes equal or substantially equal to a current threshold IoutPb 1 , the controller  123  performs constant voltage control of maintaining the output voltage of the AC-DC converter  121  at a constant value equal or substantially equal to a predetermined first target voltage VoutPT. Then, when the load  101  enters a heavy load state and the output current of the AC-DC converter  121  exceeds the current threshold IoutPb 1 , the controller  123  performs control of the AC-DC converter  121  such that the output voltage of the AC-DC converter  121  decreases with an increase in the output current of the AC-DC converter  121 . Further, when the output current of the AC-DC converter  121  reaches a predetermined current threshold IoutPb 2 , the controller  123  performs constant current control of maintaining the output current of the AC-DC converter  121  at a constant value equal to the current threshold IoutPb 2 . 
     In addition, the controller  123  is configured or programmed to output a stop warning signal to the signal processor  14 . Before the AC-DC converter  121  stops the voltage output to the load  101 , the stop warning signal provides advance notice that the AC-DC converter  121  is to stop the voltage output to the load  101 . The controller  123  outputs the stop warning signal, for example, when the input voltage to the AC-DC converter  121 , which is connected to the alternating-current power source  100  shown in  FIG. 1 , becomes below a reference value (such as an alternating-current voltage of, for example, about 180 V). 
     The signal processor  14  is configured or programmed to output the current share signal to a controller  133  of each of the two battery modules  13  upon receiving the current share signal from either one of the controllers  123 . In addition, the signal processor  14  is configured or programmed to output the stop warning signal to the controller  133  of each of the two battery modules  13  upon receiving the stop warning signal from either one of the controllers  123 . 
     Each battery module  13  includes a bidirectional DC-DC converter  131 , the battery  132 , the controller  133  configured or programmed to control the bidirectional DC-DC converter  131 , and a current measurer  134  configured to measure a current value of an output current of the bidirectional DC-DC converter  131 . The battery  132  is, for example, a lithium-ion battery. The bidirectional DC-DC converter  131  is connected between the battery  132  and the load  101 . The bidirectional DC-DC converter  131  includes, for example, a circuit including step-up and step-down functions and configured to operate either in a discharge mode in which the battery  132  is discharged or in a charge mode in which the battery  132  is charged. The bidirectional DC-DC converter  131  converts a direct-current voltage that is output from the battery  132  and thereafter outputs the direct-current voltage to the load  101  when operating in the discharge mode. In contrast, the bidirectional DC-DC converter  131  converts an output voltage of the power supply  12  and thereafter outputs the output voltage to the battery  132  when operating in the charge mode. 
     The controller  133  includes, for example, an integrated circuit including an MPU and a memory and is configured or programmed to control the output voltage of the bidirectional DC-DC converter  131  by changing a second target voltage. Specifically, the controller  133  is configured or programmed to perform constant voltage control of the bidirectional DC-DC converter  131  such that the output voltage of the bidirectional DC-DC converter  131  is maintained at a predetermined second target voltage. The controller  133  is configured or programmed to change the second target voltage in accordance with the current share signal received from the signal processor  14 . Further, before receiving the stop warning signal providing advance notice that the output of the power supply  12  is to stop, the controller  133  performs control of the output voltage of the bidirectional DC-DC converter  131  such that the output voltage of the bidirectional DC-DC converter  131 , that is, the second target voltage, is lower than the first target voltage described above. For example, if the first target voltage of the power supply  12  is set to about 12.3 V, the second target voltage is set to about 12.2 V. Further, after receiving the stop warning signal described above, the controller  133  increases the output voltage of the bidirectional DC-DC converter  131 , which is the second target voltage, to the first target voltage or higher. Immediately after the power supply  12  outputs the stop warning signal described above, the controller  133  starts to gradually increase the second target voltage, which has been set to a voltage lower than the first target voltage, until the second target voltage reaches a voltage equal or substantially equal to the first target voltage or higher. The term “gradually increase” indicates, for example, slowly increasing the output voltage of the bidirectional DC-DC converter  131  so that the variation in the output current of the bidirectional DC-DC converter  131  per unit time does not exceed a predetermined value. Further, the controller  133  gradually increases the output voltage of the DC-DC converter so that the second target voltage increases to the first voltage or higher during the period obtained by subtracting the alternating-current-voltage-drop detection period (for example, about 3 msec) from the output retention period (for example, about 10 msec), which is, for example, a period of about 7 msec. In this way, when the source of power supply to the load  101  switches from the power supplies  12  to the battery modules  13 , the increasing rate of the current supplied from the battery modules  13  to the load  101  can be reduced. 
     Further, when the current value of the current flowing from the power supply  12  to the load  101 , that is, the current value of the output current of the AC-DC converter  121 , has become equal to or smaller than a predetermined reference current value, the controller  133  maintains the second target voltage at a constant value. The reference current value can be set to a current value, for example, equal to or smaller than about 50% of the rated current value for the load  101 . For example, if the rated current value for the load  101  is about 20 A, the reference current value can be set to about 10 A or about 0 A. 
     As shown in  FIG. 3 , in the controller  133 , for example, the MPU is configured or programmed to execute a program stored in a memory and function as a power-supply-unit (PSU) output-current acquiring unit  331 , a battery-module (BM) output-current acquiring unit  332 , a determiner  333 , a target-voltage setter  334 , a voltage-variation identifier  337 , a difference calculator  336 , a current-variation calculator  335 , and a stop-warning-signal acquirer  339 . The memory includes a target-voltage storage  341 , a reference-value storage  342 , a current-value storage  343 , and a voltage-variation storage  344 . Further, the controller  133  includes a converter driver  338  that is built by using a component such as a digital signal processor (DSP) or a field programmable gate array (FPGA) and that is configured or programmed to drive the bidirectional DC-DC converter  131  by outputting a pulse width modulation (PWM) signal to the bidirectional DC-DC converter  131 . The target-voltage storage  341  is configured to store information that indicates a voltage value of the second target voltage that is set by the target-voltage setting unit  334 . The converter driver  338  is configured to acquire information that is stored in the target-voltage storage  341  and that indicates a voltage value of the second target voltage. When operating the bidirectional DC-DC converter  131  in the discharge mode, the converter driver  338  generates a PWM signal and outputs the PWM signal to the bidirectional DC-DC converter  131  so that the output voltage of the bidirectional DC-DC converter  131  is maintained at the second target voltage. 
     The reference-value storage  342  is configured to store information indicating a predetermined current threshold for the output current of the AC-DC converter  121 . The current-value storage  343  is configured to store pieces of information that each indicate a current value of the output current of the bidirectional DC-DC converter  131  and that are acquired by the BM output-current acquiring unit  332 . The pieces of information indicating current values are stored in chronological order. 
     For example, as shown in  FIG. 4 , the voltage-variation storage  344  is configured to store pieces of information each indicating a target-voltage variation, which is a variation in the second target voltage, in association with ranges of a difference (dIoutBL−dIoutB). Here, dIoutB is a variation in the output current of the bidirectional DC-DC converter  131  per unit time, and dIoutBL is the upper limit of dIoutB, which is predetermined. For example, dIoutBL is set to about 10 A/sec. 
     Referring to  FIG. 3 , the PSU output-current acquirer  331  is configured to acquire a current value of the output current of the AC-DC converter  121 . The output current of the AC-DC converter  121  corresponds to the current share signal, which is received from the signal processor  14 . The BM output-current acquirer  332  is configured to acquire a current value of the output current of the bidirectional DC-DC converter  131 . The output current of the bidirectional DC-DC converter  131  corresponds to a measurement signal that is received from the current measurer  134 . The BM output-current acquirer  332  is configured to save in the current-value storage  343  pieces of information that each indicate a current value of the output current of the bidirectional DC-DC converter  131  and that are acquired by the BM output-current acquirer  332 . The pieces of information indicating current values are saved in chronological order. 
     The determiner  333  is configured to determine whether the output current of the AC-DC converter  121  is larger than a predetermined current threshold for the output current of the AC-DC converter  121 . The current-variation calculator  335  is configured to calculate a variation in the output current of the bidirectional DC-DC converter  131  per unit time in accordance with the pieces of information that each indicate a current value of the output current of the bidirectional DC-DC converter  131  and that are stored in the current-value storage  343 . The difference calculator  336  is configured to calculate a difference obtained by subtracting the variation in the output current of the bidirectional DC-DC converter  131  per unit time from the upper limit of the variation in the output current of the bidirectional DC-DC converter  131  per unit time. The variation in the output current of the bidirectional DC-DC converter  131  per unit time is calculated by the current-variation calculator  335 . 
     The voltage-variation identifier  337  is configured to identify a piece of information indicating the target-voltage variation corresponding to the range to which the difference calculated by the difference calculator  336  belongs. The piece of information is selected from multiple kinds of information that indicate target-voltage variations and that are stored in the voltage-variation storage  344 . The target-voltage setter  334  is configured to determine a new second target voltage in accordance with a piece of information indicating the second target voltage and the voltage variation identified by the voltage-variation identifier  337 . The piece of information indicating the second target voltage is stored in the target-voltage storage  341 . [A1]   
     Upon acquiring a stop warning signal from the signal processor  14 , the stop-warning-signal acquirer  339  sends to the PSU output-current acquirer  331  and the BM output-current acquirer  332  stop-warning-signal acquisition notification reporting the acquisition of a stop warning signal. 
     Next, description will be provided with respect to operation of the power supply system according to the present preferred embodiment in comparison with operation of a power supply system according to a comparative example. The power supply system according to the comparative example is the same or substantially the same as the power supply system according to the present preferred embodiment shown in  FIG. 1  except that the power supply system according to the comparative example does not include the signal processor  14  and that battery modules  13  operate independently of power supplies  12 . In the power supply system according to the comparative example, for example, as shown in  FIG. 5 , it is assumed that a controller  123  of a power supply  12  performs constant voltage control of an AC-DC converter  121  and causes the AC-DC converter  121  to output a first target voltage VT before a time T 91 . It is assumed that a controller  133  of a battery module  13  also performs constant voltage control of a bidirectional DC-DC converter  131  in the discharge mode and causes the bidirectional DC-DC converter  131  to output a second target voltage Vs at this time. The second target voltage Vs is set to a voltage lower than the first target voltage VT. In this case, a current having a current value IT is supplied from the power supply  12  to a load  101 , and the current value of a current supplied from the battery module  13  to the load  101  is maintained at  0 . 
     Then, when power supply from the alternating-current power source  100  is shut off, the output voltage of the AC-DC converter  121  rapidly decreases at the time T 91 , and the output current of the battery module  13  rapidly changes from  0  to IT. At this time, the constant voltage control of the battery module  13  is not able to follow the rapid variation in the output current. In this way, in the power supply system according to the comparative example, when the source of power supply to the load  101  switches from the power supplies  12  to the battery modules  13  because alternating-current power supply from the alternating-current power source  100  stops, an instantaneous drop in the voltage applied to the load  101  occurs. 
     In contrast, the power supply system according to the present preferred embodiment, for example, as shown in  FIG. 6 , in response to an input of the stop warning signal from the signal processor  14 , the controller  133  gradually increases the second target voltage of the bidirectional DC-DC converter  131  from the voltage Vs to the voltage VT between a time T 1  and a time T 2  after the time T 1 . Then, the controller  133  further gradually increases the second target voltage of the bidirectional DC-DC converter  131  after the time T 2 . As shown in  FIG. 6 , the voltage variation (variation in the voltage per unit time) at the time T 2  is smaller than voltage variations between the time T 1  and the time T 2 . At this time, the output current of the AC-DC converter  121  of the power supply  12  gradually decreases as the output voltage of the bidirectional DC-DC converter  131  increases. Then, when the current value of the output current of the AC-DC converter  121  decreases to a reference current value IoutP 1 , which is predetermined and is smaller than a current value IoutB 1 , at a time T 3  after the time T 2 , the controller  133  maintains the second target voltage of the bidirectional DC-DC converter  131  at a constant value. At that time, the current value of the output current of the bidirectional DC-DC converter  131  reaches the current value IoutB 1 , which is smaller than the current value IT. 
     Thereafter, when the output current and the output voltage of the power supply  12  becomes 0 at a time T 4 , the output current of the battery module  13  reaches IT. In short, in the power supply system according to the present preferred embodiment, before alternating-current power supply from the alternating-current power source  100  stops, the current value of the current supplied from the bidirectional DC-DC converter  131  of the battery module  13  to the load  101  is increased in advance to a current value close to the current value IT of the current to be supplied to the load  101 . At that time, the current value of the current supplied from the power supply  12  to the load  101  decreases to the reference current value IoutP 1 , which is close to 0 and is predetermined. In this way, when the alternating-current power supply from the alternating-current power source  100  stops and the output of the AC-DC converter  121  of the power supply  12  stops, a rapid increase in the current supplied from the bidirectional DC-DC converter  131  of the battery module  13  to the load  101  is reduced. Consequently, when the source of power supply to the load  101  switches from the power supplies  12  to the battery modules  13 , the voltage applied to the load  101  can be maintained at a constant value. 
     Next, referring to  FIG. 7 , description will be provided with respect to battery-module control processing of controlling output voltage for transition to backup. The battery-module control processing is performed by the controller  133  according to the present preferred embodiment. The battery-module control processing is started in response to the battery module  13  being powered on. The initial value of the second target voltage is set to, for example, about 12.2 V. The initial value of the voltage variation in the second target voltage is set to, for example, about 0.01 V. First, the stop-warning-signal acquirer  339  determines whether a stop warning signal has been received from the signal processor  14  (step S 101 ). If the stop warning signal has not been received from the signal processor  14  (No in step S 101 ), the stop-warning-signal acquirer  339  repeats the process in step S 101  until the stop warning signal is received. In contrast, if the stop-warning-signal acquirer  339  determines that the stop warning signal has been received from the signal processor  14  (Yes in step S 101 ), the BM output-current acquirer  332  acquires output-current-value information indicating the magnitude of the output current of the bidirectional DC-DC converter  131  (step S 102 ). Next, the PSU output-current acquirer  331  acquires output-current-value information indicating the magnitude of the output current of the AC-DC converter  121  of the power supply  12  (step S 103 ). The output current of the AC-DC converter  121  corresponds to the current share signal, which is received from the signal processor  14 . 
     Subsequently, the determiner  333  acquires from the reference-value storage unit  342  information indicating a current threshold for the output current of the AC-DC converter  121 . Then, the determiner  333  determines whether an output current IoutP indicated by the output-current-value information of the AC-DC converter  121  is larger than a current threshold IoutPth (step S 104 ). If the determiner  333  determines that the output current IoutP of the AC-DC converter  121  is equal to or smaller than the current threshold IoutPth (No in step S 104 ), the battery-module control processing of controlling output voltage for transition to backup ends. 
     It is assumed herein that the determiner  333  determines that the output current IoutP of the AC-DC converter  121  is larger than the current threshold IoutPth (Yes in step S 104 ). In this case, the target-voltage setter  334  acquires information that indicates a second target voltage VoutBT and that is stored in the target-voltage storage  341  and sets a new second target voltage to a voltage (VoutBT+dVoutBT), which is obtained by adding a voltage variation dVoutBT to the second target voltage VoutBT indicated by the acquired information (step S 105 ). Here, the voltage variation dVoutBT is the initial value of the voltage variation or a voltage variation determined by the voltage-variation identifier  337 . 
     Thereafter, the BM output-current acquirer  332  acquires output-current-value information indicating the magnitude of the output current of the bidirectional DC-DC converter  131  (step S 106 ). Next, the current-variation calculator  335  acquires from the current-value storage  343  pieces of information indicating previous current values of the output current of the bidirectional DC-DC converter  131 . Then, the current-variation calculator  335  calculates a variation dIoutB in the output current of the bidirectional DC-DC converter  131  (step S 107 ). 
     Subsequently, the difference calculator  336  calculates a difference (dIoutBL−dIoutB) obtained by subtracting the calculated variation dIoutB in the output current of the bidirectional DC-DC converter  131  per unit time from the upper limit of the variation dIoutBL in the output current of the bidirectional DC-DC converter  131  per unit time (step S 108 ). Thereafter, the voltage-variation identifier  337  identifies a piece of information indicating the target-voltage variation corresponding to the range to which the difference calculated by the difference calculator  336  belongs (step S 109 ). The piece of information is selected from multiple kinds of information that indicate target-voltage variations and that are stored in the voltage-variation storage  344 . Next, the process in step S 103  is repeated. In this way, by performing a series of processes in step S 103  to step S 109 , the controller  133  gradually increases the second target voltage so that the current variation, per unit time, in the current flowing from the bidirectional DC-DC converter  131  to the load  101 , that is, the output current of the bidirectional DC-DC converter  131 , is lower than the predetermined upper limit of the variation. 
     As described above, in the battery module  13  according to the present preferred embodiment, before the power supply  12  outputs the stop warning signal, the controller  133  performs control of the output voltage of the bidirectional DC-DC converter  131  such that the second target voltage is lower than the first target voltage. In addition, after the power supply  12  outputs the stop warning signal, the controller  133  performs control of the output voltage of the bidirectional DC-DC converter  131  such that the second target voltage becomes equal or substantially equal to the first target voltage or higher. In this way, when the source of power supply to the load  101  switches from the power supplies  12  to the battery modules  13 , a rapid increase in the current supplied from the battery modules  13  to the load  101  can be reduced. Accordingly, a variation in the voltage applied to the load  101  is reduced. The variation in the voltage is caused because the constant voltage control of the battery module  13  is not able to follow the variation in the load current. 
     In addition, immediately after the power supply  12  outputs the stop warning signal described above, the controller  133  of the battery module  13  according to the present preferred embodiment starts to gradually increase the second target voltage, which has been set to a voltage lower than the first target voltage. In this way, the output voltage of the bidirectional DC-DC converter  131  of the battery module  13  can be smoothly increased. 
     Further, when the current value of the current flowing from the power supply  12  to the load  101 , that is, the current value of the output current of the AC-DC converter  121 , has become equal to or smaller than the predetermined reference current value, the controller  133  of the battery module  13  according to the present preferred embodiment maintains the second target voltage at a constant value. In this way, a malfunction of the load  101  caused by an excessive increase in the voltage applied to the load  101  is prevented from occurring because an excessive increase in the voltage supplied from the battery module  13  and applied to the load  101  is avoided. 
     In addition, when the second target voltage of the bidirectional DC-DC converter  131  has become equal to or higher than the first target voltage, which is described above, the controller  133  of the battery module  13  according to the present preferred embodiment maintains the second target voltage at a constant value. In this way, operation of the load  101  can be stabilized because the variation in the voltage supplied from the battery module  13  and applied to the load  101  can be reduced. 
     Further, the controller  133  of the battery module  13  according to the present preferred embodiment gradually increases the second target voltage so that the current variation in the output current of the bidirectional DC-DC converter  131  per unit time is smaller than the predetermined upper limit of the variation. In this way, the voltage applied to the load  101  can be stabilized because an instantaneous drop in the output voltage of the bidirectional DC-DC converter  131  is avoided. 
     Second Preferred Embodiment 
     Similarly to the battery module  13  according to the first preferred embodiment, a battery module according to a second preferred embodiment of the present invention includes a bidirectional DC-DC converter, a battery, and a controller. However, the battery module according to the present preferred embodiment differs from the battery module  13  according to the first preferred embodiment in that a current measurer configured to measure the output current of the bidirectional DC-DC converter is not included in the battery module. 
     For example, as shown in  FIG. 8 , a power supply system according to the present preferred embodiment includes two power supplies  12 , two battery modules  2013  each including a battery  132 , and a signal processor  14 . In  FIG. 8 , components that are the same as or similar to those in the power supply system according to the first preferred embodiment are denoted by the same reference numerals as in  FIG. 1 . Each battery module  2013  includes a bidirectional DC-DC converter  131 , the battery  132 , a controller  2133  configured or programmed to control the bidirectional DC-DC converter  131 . 
     Similarly to the controller  133  according to the first preferred embodiment, the controller  2133  includes, for example, an integrated circuit including an MPU and a memory and is configured or programmed to perform constant voltage control such that the output voltage of the bidirectional DC-DC converter  131  is maintained at a predetermined second target voltage. In addition, the controller  2133  is configured or programmed to change the second target voltage in accordance with a current share signal received from the signal processor  14 . Further, before the controller  123  of the power supply  12  outputs the stop warning signal described above, the controller  2133  performs control of the output voltage of the bidirectional DC-DC converter  131  such that the second target voltage is lower than the first target voltage described above. In addition, after the controller  123  outputs the stop warning signal, the controller  2133  performs control of the output voltage of the DC-DC converter such that the second target voltage becomes equal to the first target voltage or higher. 
     As shown in  FIG. 9 , in the controller  2133 , for example, the MPU is configured or programmed to execute a program stored in the memory and function as a PSU output-current acquirer  2331 , a determiner  333 , a target-voltage setter  334 , a voltage-variation identifier  2337 , a difference calculator  2336 , a current-variation calculator  2335 , and a stop-warning-signal acquirer  339 . In  FIG. 9 , components that are the same as or similar to those in the first preferred embodiment are denoted by the same reference numerals as in  FIG. 3 . The memory includes a target-voltage storage  341 , a reference-value storage  342 , a current-value storage  2343 , and a voltage-variation storage  2344 . Further, the controller  2133  includes a converter driver  338  that is configured to drive the bidirectional DC-DC converter  131  by outputting a PWM signal to the bidirectional DC-DC converter  131 . The current-value storage  2343  is configured to store pieces of information that each indicate a current value of the output current of the AC-DC converter  121  of the power supply  12  and that is acquired by the PSU output-current acquirer  2331 . The pieces of information indicating current values are stored in chronological order. 
     For example, as shown in  FIG. 10 , the voltage-variation storage  2344  is configured to store pieces of information each indicating a target-voltage variation, which is a variation in the second target voltage, in association with ranges of a difference (dIoutPL−dIoutP). Here, dIoutP represents a variation in the output current of the AC-DC converter  121  of the power supply  12  per unit time, and dIoutPL represents the lower limit of dIoutP, which is predetermined. For example, dIoutPL is set to about −10 A/sec. 
     Referring to  FIG. 9 , the PSU output-current acquirer  2331  is configured to acquire a current value of the output current of the AC-DC converter  121 . The output current corresponds to the current share signal, which is received from the signal processor  14 . Then, the PSU output-current acquirer  2331  is configured to save in the current-value storage  2343  pieces of information that each indicate an acquired current value of the output current of the AC-DC converter  121 . The pieces of information indicating current values are saved in chronological order. The current-variation calculator  2335  is configured to calculate a variation in the output current of the AC-DC converter  121  per unit time in accordance with the pieces of information that each indicate a current value of the output current of the AC-DC converter  121  and that are stored in the current-value storage  2343 . The difference calculator  2336  is configured to calculate a difference obtained by subtracting the variation in the output current of the AC-DC converter  121  per unit time from the lower limit of the variation in the output current of the AC-DC converter  121  per unit time. The variation in the output current of the AC-DC converter  121  per unit time is calculated by the current-variation calculator  2335 . 
     The voltage-variation identifier  2337  is configured to identify a piece of information indicating the target-voltage variation corresponding to the range to which the difference calculated by the difference calculator  2336  belongs. The piece of information is selected from multiple kinds of information that indicate target-voltage variations and that are stored in the voltage-variation storage  2344 . 
     Next, referring to  FIG. 11 , description will be provided with regard to battery-module control processing of controlling output voltage for transition to backup. The battery-module control processing is performed by the controller  2133  according to the present preferred embodiment. Here, similarly to the first preferred embodiment, the initial value of the second target voltage is set to, for example, about 12.2 V, and the initial value of the voltage variation in the second target voltage is set to, for example, about 0.01 V. First, the stop-warning-signal acquirer  339  determines whether a stop warning signal has been received from the signal processor  14  (step S 2101 ). If the stop warning signal has not been received from the signal processor  14  (No in step S 2101 ), the stop-warning-signal acquirer  339  repeats the process in step S 2101  until the stop warning signal is received. It is assumed herein that the stop-warning-signal acquirer  339  determines that a stop warning signal has been received from the signal processor  14  (Yes in step S 2101 ). In this case, the PSU output-current acquirer  2331  acquires output-current-value information indicating the magnitude of the output current of the AC-DC converter  121  of the power supply  12  (step S 2102 ). The output current corresponds to the current share signal, which is received from the signal processor  14 . 
     Next, the determiner  333  acquires from the reference-value storage unit  342  information indicating a current threshold for the output current of the AC-DC converter  121 . Then, the determiner  333  determines whether an output current IoutP indicated by the output-current-value information of the AC-DC converter  121  is larger than a current threshold IoutPth (step S 2103 ). If the determiner  333  determines that the output current IoutP of the AC-DC converter  121  is equal to or smaller than the current threshold IoutPth (No in step S 2103 ), the battery-module control processing of controlling output voltage for transition to backup ends. 
     It is assumed herein that the determiner  333  determines that the output current IoutP of the AC-DC converter  121  is larger than the current threshold IoutPth (Yes in step S 2103 ). In this case, the target-voltage setter  334  acquires information that indicates a second target voltage VoutBT and that is stored in the target-voltage storage  341  and sets a new second target voltage to a voltage (VoutBT+dVoutBT), which is obtained by adding a voltage variation dVoutBT to the second target voltage VoutBT indicated by the acquired information (step S 2104 ). Here, the voltage variation dVoutBT is the initial value of the voltage variation or a voltage variation determined by the voltage-variation identifier  2337 . 
     Subsequently, the PSU output-current acquirer  2331  acquires output-current-value information indicating the magnitude of the output current of the AC-DC converter  121  (step S 2105 ). Thereafter, the current-variation calculator  2335  acquires from the current-value storage  2343  pieces of information indicating previous current values of the output current of the AC-DC converter  121 . Then, the current-variation calculator  2335  calculates a variation dIoutP in the output current of the AC-DC converter  121  (step S 2106 ). 
     Next, the difference calculator  2336  calculates a difference (dIoutPL−dIoutP) obtained by subtracting the calculated variation dIoutP in the output current of the AC-DC converter  121  per unit time from the lower limit of the variation dIoutPL in the output current of the AC-DC converter  121  per unit time (step S 2107 ). Subsequently, the voltage-variation identifier  2337  identifies a piece of information indicating the target-voltage variation corresponding to the range to which the difference calculated by the difference calculator  2336  belongs (step S 2108 ). The piece of information is selected from multiple kinds of information that indicate target-voltage variations and that are stored in the voltage-variation storage  2344 . Thereafter, the process in step S 2103  is repeated. 
     As described above, in the battery module  2013  according to the present preferred embodiment, the output voltage of the bidirectional DC-DC converter  131  is controlled without using information indicating the current value of the output current of the bidirectional DC-DC converter  131 . In this way, since a current measurer configured to measure the output current of the bidirectional DC-DC converter  131  can be omitted, the configuration of the battery module  2013  can be simplified correspondingly. 
     The preferred embodiments of the present invention have been described as above, but the present invention is not limited to the configurations described in the above-described preferred embodiments. For example, when the battery module  13  gradually increases the output voltage of the bidirectional DC-DC converter  131  after acquiring the stop warning signal from the power supply  12 , the second target voltage may be maintained at a constant value after the second target voltage exceeds a predetermined upper limit of the target voltage. In this case, the reference-value storage  342  is configured to store information indicating a voltage upper limit of the output voltage of the bidirectional DC-DC converter  131  together with the information indicating a predetermined current threshold for the output current of the AC-DC converter  121 . The voltage upper limit is the upper limit of the output voltage of the bidirectional DC-DC converter  131  during operation in the discharge mode and is set based on the rated voltage for the load  101 . 
     Referring to  FIG. 12 , description will be provided herein with respect to battery-module control processing of controlling output voltage for transition to backup, which is performed by the controller  133  according to a modification of a preferred embodiment of the present invention. In  FIG. 12 , processes that are the same as or similar to those in the first preferred embodiment are denoted by the same reference numerals as in  FIG. 7 . First, after the processes in steps S 101  to S 103  are performed, the determiner  333  acquires from the reference-value storage  342  information indicating a current threshold for the output current of the AC-DC converter  121 . Then, the determiner  333  determines whether an output current IoutP indicated by the output-current-value information of the AC-DC converter  121  is larger than a current threshold IoutPth (step S 104 ). It is assumed herein that the determiner  333  determines that the output current IoutP of the AC-DC converter  121  is larger than the current threshold IoutPth (Yes in step S 104 ). In this case, the determiner  333  acquires from the reference-value storage unit  342  information indicating the voltage upper limit of the output voltage of the bidirectional DC-DC converter  131 . In addition, the determiner  333  acquires information that indicates the second target voltage VoutBT and that is stored in the target-voltage storage  341  and calculates a voltage (VoutBT+dVoutBT), which is obtained by adding a voltage variation dVoutBT to the second target voltage VoutBT indicated by the acquired information. Then, the determiner  333  determines whether the calculated voltage (VoutBT+dVoutBT) is lower than the voltage upper limit VoutBTUL described above (step S 3001 ). If it is determined that the voltage calculated by the determiner  333  (VoutBT+dVoutBT) is equal to or higher than the voltage upper limit described above (No in step S 3001 ), the battery-module control processing of controlling output voltage for transition to backup ends. In contrast, if it is determined that the voltage calculated by the determiner unit  333  (VoutBT+dVoutBT) is lower than the voltage upper limit VoutBTUL described above (Yes in step S 3001 ), a series of processes starting from step S 105  are executed. 
     According to the present configuration, a malfunction of the load  101  caused by an excessively high voltage applied to the load  101  is prevented from occurring because an excessive increase that leads to the output voltage of the bidirectional DC-DC converter  131  exceeding the voltage upper limit described above can be avoided. 
     The battery module  13  in the first preferred embodiment may be configured to determine whether the power supply from the power supply  12  to the load  101  has stopped after the source of power supply to the load  101  switches from the power supply  12  to the battery module  13 . Then, after the battery module  13  determines that the power supply from the power supply  12  to the load  101  has stopped, the voltage value of the second target voltage applied to the load  101  may be reset. 
     Referring to  FIG. 13 , description will be provided herein with respect to battery-module control processing of controlling output voltage for transition to backup, which is performed by the controller  133  according to the present modification. In  FIG. 13 , processes that are the same as or similar to those in the first preferred embodiment are denoted by the same reference numerals as in  FIG. 7 . First, after the processes in steps S 101  to S 103  are performed, the determiner  333  acquires from the reference-value storage unit  342  information indicating a current threshold for the output current of the AC-DC converter  121 . Then, the determiner  333  determines whether an output current IoutP indicated by the output-current-value information of the AC-DC converter  121  is larger than a current threshold IoutPth (step S 104 ). It is assumed herein that the determiner  333  determines that the output current IoutP of the AC-DC converter  121  is equal to or smaller than the current threshold IoutPth (No in step S 104 ). In this case, the determiner  333  determines whether a stop completion signal for reporting that the AC-DC converter  121  has stopped operating has been received from the signal processor  14  (step S 4001 ). When the output of the AC-DC converter  121  has stopped, the controller  123  outputs the stop completion signal to the signal processor  14 . Upon receiving the stop completion signal, the signal processor  14  outputs the stop completion signal to the controller  133 . If the determiner  333  determines that the stop completion signal has not been received from the signal processor  14  (No in step S 4001 ), the process in step S 4001  is repeatedly performed. In contrast, if the determiner  333  determines that the stop completion signal has been received from the signal processor  14  (Yes in step S 4001 ), the target-voltage setter  334  resets the voltage value of the second target voltage to a voltage equal to the first target voltage in this case (step S 4002 ). Next, battery-module control processing of controlling output voltage for transition to backup ends. 
     In the present modification, description has been provided with respect to a case where the controller  133  of the battery module  13  according to the first preferred embodiment performs the processes in steps S 4001  and S 4002  by way of illustration and not by way of limitation. For example, in the battery module  2013  according to the second preferred embodiment, after performing the processes in steps S 2101  to S 2103  in  FIG. 11 , the controller  2133  may perform the processes in steps S 4001  and S 4002  upon determining that the output current IoutP indicated by the output-current-value information of the AC-DC converter  121  is equal to or smaller than the current threshold IoutPth (No in step S 2103 ). 
     According to this configuration, the battery module  13  resets the second target voltage to a voltage equal or substantially equal to the first target voltage after the power supply  12  stops operating. In this way, before and after the source of power supply to the load  101  switches from the power supplies  12  to the battery modules  13 , the voltage applied to the load  101  can be maintained at a constant value. 
     The controller  133  of the battery module  13  in the first preferred embodiment may be configured to perform control such that a voltage variation in the output voltage of the bidirectional DC-DC converter  131  at a time when the output voltage of the bidirectional DC-DC converter  131  reaches the first target voltage is smaller than the maximum voltage variation during a period when the output voltage of the DC-DC converter  131  is increased to a voltage close to the first target voltage after the stop warning signal is received. Here, the voltage close to the first target voltage equals a voltage equal to or higher than about 95% and lower than about 100% of the first target voltage. 
     For example, as shown in  FIG. 14 , the controller  133  according to the present modification is configured to vary with time the voltage variation in the output voltage of the bidirectional DC-DC converter  131  (variation in the voltage per unit time) after receiving the stop warning signal. Immediately after receiving the stop warning signal at a time T 61  (refer to LiA in  FIG. 14 ), the controller  133  maintains the output voltage of the bidirectional DC-DC converter  131  with a voltage variation A (=about 0). Next, the controller  133  starts to increase the output voltage of the bidirectional DC-DC converter  131  with a voltage variation B at a time T 62  after the time T 61  (refer to LiB in  FIG. 14 ). Then, the controller  133  starts to maintain the output voltage of the bidirectional DC-DC converter  131  with a voltage variation C(=about 0) at a time T 63  after the time T 62  (refer to LiC in  FIG. 14 ). Subsequently, the controller  133  starts to increase the output voltage of the bidirectional DC-DC converter  131  with a voltage variation D at a time T 64  after the time T 63  (refer to LiD in  FIG. 14 ) and starts to increase the output voltage of the bidirectional DC-DC converter  131  with a voltage variation E at a time T 65  after the time T 64  (refer to LiE in  FIG. 14 ). Then, the controller  133  starts to increase the output voltage of the bidirectional DC-DC converter  131  with a voltage variation F when the output voltage of the bidirectional DC-DC converter  131  exceeds the voltage VT at a time T 65  after the time T 64  (refer to LiF in  FIG. 14 ). For the voltage variations A, B, C, D, E, and F, a relationship B&gt;D&gt;E&gt;F&gt;A=C holds. The voltage variations A and C are both about 0. In this way, the controller  133  sets the voltage variation (voltage variation F) at the time when the output voltage of the bidirectional DC-DC converter  131  reaches the voltage VT such that the voltage variation is smaller than the maximum voltage variation B during the period when the output voltage of the bidirectional DC-DC converter  131  is increased so as to be close to the voltage VT after the stop warning signal is received. 
     According to the present configuration, it is possible to cause the output voltage of the bidirectional DC-DC converter  131  to reach the target voltage in a relatively short period and cause the current value of the current supplied from the battery module  13  to the load  101  to gradually increase. Consequently, when the source of power supply to the load  101  switches from the power supplies  12  to the battery modules  13 , the variation in the voltage applied to the load  101  can be reduced. 
     In each preferred embodiment, description has been provided with respect to a configuration including two power supplies  12  by way of illustration and not by way of limitation. For example, a configuration including only one power supply  12  and a configuration including three or more power supplies  12  are also possible. In each preferred embodiment, description has been provided with respect to a configuration including two battery modules  13  by way of illustration and not by way of limitation. For example, a configuration including only one battery module  13  and a configuration including three or more battery modules  13  are also possible. 
     In each preferred embodiment, description has been provided with respect to a configuration including the signal processor  14  by way of illustration and not by way of limitation. For example, a configuration in which the current share signal, the stop warning signal, or both of these signals, which are output from the controller  123  of the power supply  12 , are directly input to the controller  133  of the battery module  13  is also possible. 
     In each preferred embodiment, description has been provided with respect to an example where each power supply  12  includes the AC-DC converter  121 , which is configured to convert alternating-current power supplied from the alternating-current power source  100  into direct-current power. However, this example is not limiting. For example, as shown in  FIG. 15 , a power supply  5012  may include a DC-DC converter  5121 , which is configured to convert direct-current power supplied from a direct-current power source  5100  into direct-current power having a different voltage. In  FIG. 15 , components that are the same as or similar to those in the first preferred embodiment are denoted by the same reference numerals as in  FIG. 1 . In this case, the power supply  5012  includes a controller  5123  configured or programmed to control the DC-DC converter  5121  and a current measurer  124  configured to measure a current value of a current that is output from the DC-DC converter  5121 . Examples of the direct-current power source  5100  include a direct-current voltage bus connected to a power generation facility by using a PV converter. 
     The DC-DC converter  5121  corresponds to a first DC-DC converter including a boost circuit, a buck circuit, or a buck-boost circuit. The controller  5123  corresponds to a first controller configured or programmed to perform constant voltage control of the DC-DC converter  5121  such that a voltage that is output from the DC-DC converter  5121  to the load  101  equals or substantially equals a predetermined target voltage. In addition, the controller  5123  is configured or programmed to output a stop warning signal to the signal processor  14 . Before the voltage output to the load  101  is stopped, the stop warning signal provides advance notice that the voltage output to the load  101  is to stop. Similarly to the first preferred embodiment, a power supply system according to the present modification includes battery modules  13  each including a battery  132 , a bidirectional DC-DC converter  131 , and a controller  133 . The bidirectional DC-DC converter  131  corresponds to a second DC-DC converter configured to convert a direct-current voltage that is output from the battery  132  and thereafter output the direct-current voltage to the load  101 . Further, the controller  133  corresponds to a second controller configured or programmed to, before receiving the stop warning signal, perform control such that the output voltage of the DC-DC converter  131  is lower than the target voltage and, after receiving the stop warning signal and before the output of the power supply  5012  stops, increase the output voltage of the bidirectional DC-DC converter  131  to the target voltage or higher. 
     Preferred embodiments and modifications according to the present invention have been described above, but the present invention is not limited to these preferred embodiments and modifications. The present invention includes appropriate combinations of the preferred embodiments and the modifications and also includes modifications to such combinations. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.