Patent Publication Number: US-2022234467-A1

Title: Server, power management system, and energy management method

Description:
This nonprovisional application is based on Japanese Patent Application No. 2021-011921 filed with the Japan Patent Office on Jan. 28, 2021, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Field 
     The present disclosure relates to a server, a power management system, and an energy management method. 
     Description of the Background Art 
     Japanese Patent Laying-Open No. 2018-161018 discloses an aggregation system that manages energy by demand response (DR). A composite power conversion device in this aggregation system starts up objects to be controlled in the descending order of a response speed upon receiving a control command in connection with DR from a server. The aggregation system corresponds to an exemplary power management system. 
     SUMMARY 
     When a battery (a first battery) that is being charged is fully charged, by starting charging of another battery (a second battery) instead of that battery, sufficient charging power can be secured for a long time period. For example, a server determines whether or not the first battery has fully been charged based on a state of charge (SOC) of the first battery that is being charged, and when the first battery is fully charged, the server transmits a charging start command to a second battery so as to successively charge the first battery and the second battery. The server can manage energy through charging of the batteries as above. 
     The server, however, is not necessarily able to obtain the SOC of the battery. For example, a server capable of obtaining information from a vehicle is limited. In general, a server that is unable to communicate with a vehicle is unable to obtain the SOC of the battery mounted on the vehicle. 
     A method of transmitting a charging start command from a server to a battery in accordance with a predetermined charging plan is also available as a method of successively charging a plurality of batteries as instructed by the server. In such a method, when charging end timing of a first battery shown in the charging plan comes, the charging start command is transmitted from the server to a second battery. The server can specify charging end timing of the first battery by referring to the charging plan without the knowledge of a charging condition of the first battery. Charging of the first battery, however, is not necessarily carried out as planned in the charging plan. When charging of the first battery ends earlier than end timing shown in the charging plan, charging discontinuity may be produced between end of charging of the first battery and start of charging of the second battery. Then, while charging is discontinuous, energy management by charging is not carried out. 
     The present disclosure was made to solve the problem above, and an object thereof is to appropriately manage energy depending on a charging condition of a battery by knowing the charging condition of the battery without relying on an SOC of the battery. 
     A server according to a first point of view of the present disclosure includes a controller that controls charging of a plurality of batteries to successively be carried out. When charging power reduction control is carried out in a subject battery during charging of the subject battery, the controller determines that end of the charging of the subject battery is close. 
     The inventor of the present application proposes the server above, with attention being paid to the fact that charging power reduction control is carried out immediately before end of charging of a battery. Charging power reduction control refers to control for charging with low electric power until end of charging, with charging power being lowered immediately before end of charging. Exemplary charging power reduction control is such control as charging a battery with a low charging current until a battery voltage reaches an upper limit voltage, with the charging current being lowered when the battery is almost fully charged. Such control is also referred to as forced charging control. 
     According to the server, a state that end of charging of the subject battery is close can readily and properly be sensed. The server can determine whether or not charging power reduction control has been carried out in the subject battery based on charging power for the subject battery that is being charged. Therefore, the server can determine whether or not end of charging of the subject battery is close without relying on the SOC of the subject battery that is being charged. The server can perform prescribed processing after it determines that end of charging of the subject battery is close and before the end of charging of the subject battery. For example, before end of charging of the subject battery, the server can compensate for decrease in charging power due to charging power reduction control, or can instruct a battery to be charged following the subject battery to start to be charged or to be ready for charging. The server can thus know the charging condition of the battery without relying on the SOC of the battery and can appropriately manage energy depending on the charging condition of the battery. 
     The controller may be configured to determine that the charging power reduction control has been started in the subject battery when charging power for the subject battery lowers and becomes lower than a first reference value during charging of the subject battery. According to such a configuration, the server can readily and properly sense start of charging power reduction control in the subject battery. 
     The controller may be configured to determine that charging of the subject battery has ended when the charging power for the subject battery becomes lower than a second reference value smaller than the first reference value after start of charging power reduction control. According to such a configuration, the server can readily and properly sense end of charging of the subject battery. The server may change the subject battery at the time when it determines end of charging of the subject battery and start charging control of a new subject battery. 
     The server may further include a storage that stores a charging schedule that shows an order of charging of the plurality of batteries. The plurality of batteries may include the subject battery and a next battery, start of charging of the next battery being determined in the charging schedule to follow the charging of the subject battery. The controller may be configured to successively transmit a charging start command for each of the plurality of batteries for energy management of a power grid. 
     According to the configuration, by starting charging of the next battery instead of the subject battery at the time when charging of the subject battery ends or end of charging is close, sufficient charging power can be secured for a long time period. A new battery may be defined as a new subject battery instead of the subject battery charging of which has ended. Each of the subject battery and the next battery may be a stationary battery or a vehicle-mounted battery. 
     The controller may be configured to carry out charging of a charging resource connected to the power grid to compensate for decrease in charging power due to the charging power reduction control when the controller determines that end of charging of the subject battery is close. In such a configuration, decrease in charging power due to charging power reduction control of the subject battery is compensated for by the charging resource connected to the power grid. Therefore, constant charging power is readily secured. 
     The charging resource is configured to store electric power. Any method of storage of electric power (that is, a charging method) is applicable. The charging resource may store electric power (electric energy) as it is or may convert electric power into another type of energy (that is, liquid fuel or gaseous fuel as an energy source) and store resultant energy. 
     The controller may be configured to perform processing for increasing reserve of the power grid when reserve of the power grid is insufficient at the time when the controller determines that end of charging of the subject battery is close. 
     When the server changes an object to be charged from the subject battery to a next battery, charging power may lower due to charging power reduction control or charging discontinuity. The server may compensate for such decrease in charging power with reserve. When there is no sufficient reserve, however, it is difficult to compensate for decrease in charging power with reserve. In this connection, when reserve of the power grid is insufficient at the time when end of charging of the subject battery is close, the server performs processing for increasing reserve of the power grid. Insufficiency of reserve is thus suppressed. 
     Examples of processing for increasing reserve of the power grid include processing for inviting a user of a charging resource to participate in energy management. The server may invite a user of a vehicle not connected to the power grid to connect the vehicle to the power grid. The server may carry out demand response (DR) for increasing reserve of the power grid. 
     In connection with the server, the subject battery may be a secondary battery mounted on a first vehicle and the next battery may be a secondary battery mounted on a second vehicle. The controller may be configured to determine whether the charging power reduction control has been carried out in the subject battery charged with electric power supplied from the power grid, based on a detection value from a wattmeter that detects electric power supplied from the power grid to the subject battery. 
     The server can manage energy by using a secondary battery mounted on the vehicle. The secondary battery mounted on the vehicle may store electric power for travel of the vehicle. The vehicle may be an electrically powered vehicle. The electrically powered vehicle refers to a vehicle configured to travel with electric power supplied from a secondary battery mounted thereon. The electrically powered vehicle includes not only a battery electric vehicle (BEV) and a plug-in hybrid electric vehicle (PHEV) but also a fuel cell electric vehicle (FCEV) and a range extender BEV. The wattmeter may be a watt-hour meter (for example, a smart meter) that measures an amount of electric power consumed in a building, a wattmeter contained in electric vehicle supply equipment (EVSE), or a current transformer (CT) sensor provided outside EVSE. 
     A power management system according to a second point of view of the present disclosure includes a server that controls charging of a plurality of batteries to successively be carried out. The server is configured to successively transmit a charging start command for each of the plurality of batteries. The plurality of batteries include a first subject battery and a second subject battery, charging of the second subject battery being scheduled to be started following the first subject battery. The server is configured to transmit the charging start command for the second subject battery when charging power reduction control is started in the first subject battery that is being charged. 
     The server transmits a charging start command for the second subject battery when charging power reduction control is started in the first subject battery that is being charged. In other words, the charging start command for the second subject battery is transmitted before end of charging of the first subject battery. Therefore, in the power management system, charging discontinuity is less likely between end of charging of the first subject battery and start of charging of the second subject battery. Each of the first subject battery and the second subject battery may be a stationary battery or a vehicle-mounted battery. 
     In the power management system, the first subject battery may be a secondary battery mounted on a first vehicle and the second subject battery may be a secondary battery mounted on a second vehicle. The first vehicle may include a first controller that starts prescribed first charging control of the first subject battery based on the charging start command from the server. The second vehicle may include a second controller that starts prescribed second charging control of the second subject battery based on the charging start command from the server. 
     In the power management system, the controller mounted on the vehicle carries out charging control of the battery mounted on the vehicle. Therefore, processing load imposed on the server involved with charging control is lessened. 
     The server may be configured to transmit the charging start command to a power feed facility to which a vehicle is connected or an energy management system that manages the power feed facility. Such a server can issue an instruction for starting charging to the first subject battery and the second subject battery via EVSE or an energy management system (EMS). For example, a main body or a charging cable of EVSE may perform a communication function, and the server may transmit a charging start command to EVSE (the main body or the charging cable). 
     The server may be configured to directly transmit the charging start command to a vehicle through wireless communication and to obtain charging power for the battery mounted on that vehicle from a smart meter. According to such a configuration, the first vehicle and the second vehicle can directly be instructed to start charging of the first subject battery and the second subject battery. The server can obtain charging power for the battery mounted on the vehicle from the smart meter. Charging control carried out by the controller mounted on the vehicle may be any of three types of charging control shown below. For example, the first controller may implement any of configurations (a) to (c) shown below. 
     (a) The first controller may be configured to carry out, in the prescribed first charging control, charging control of the first subject battery in an order of first constant power charging, constant voltage charging in which charging power is lowered, and second constant power charging lower in electric power than the first constant power charging. The constant voltage charging and the second constant power charging may be carried out as charging power reduction control. 
     (b) The first controller may be configured to carry out, in the prescribed first charging control, charging control of the first subject battery in an order of constant current charging and constant voltage charging. The first controller may be configured to start the charging power reduction control in making transition from the constant current charging to the constant voltage charging. 
     (c) The first controller may be configured to carry out, in the prescribed first charging control, charging control of the first subject battery in an order of first constant power charging and second constant power charging lower in electric power than the first constant power charging. The first controller may be configured to start the charging power reduction control in making transition from the first constant power charging to the second constant power charging. 
     An energy management method according to a third point of view of the present disclosure is an energy management method of managing energy through charging of a battery. The energy management method includes determining whether charging power reduction control has been carried out in a battery that is being charged and performing processing for compensating for decrease in charging power due to the charging power reduction control when it is determined that the charging power reduction control has been carried out in the battery that is being charged. 
     According to the energy management method, a charging condition of the battery can be known without relying on the SOC of the battery and energy can appropriately be managed depending on the charging condition of the battery. 
     The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a vehicle according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram showing a configuration of a server according to the embodiment of the present disclosure. 
         FIG. 3  is a diagram showing a schematic configuration of a power management system according to the embodiment of the present disclosure. 
         FIG. 4  is a diagram for illustrating charging control (a CP 1  period, a CV period, and a CP 2  period) carried out by a controller of the vehicle according to the embodiment of the present disclosure. 
         FIG. 5  is a flowchart showing charging control carried out by the controller of the vehicle according to the embodiment of the present disclosure. 
         FIG. 6  is a diagram showing a modification of transition of charging power shown in  FIG. 4 . 
         FIG. 7  is a diagram showing a first modification of charging control carried out by the controller of the vehicle. 
         FIG. 8  is a diagram showing a second modification of charging control carried out by the controller of the vehicle. 
         FIG. 9  is a diagram showing an exemplary charging schedule. 
         FIG. 10  is a diagram showing a plurality of vehicles that prepare for charging in accordance with the charging schedule shown in  FIG. 9 . 
         FIG. 11  is a diagram showing exemplary energy management carried out by the server according to the embodiment of the present disclosure. 
         FIG. 12  is a flowchart showing processing involved with energy management carried out by the controller of the server according to the embodiment of the present disclosure. 
         FIG. 13  is a flowchart showing details of processing involved with selection of a resource shown in  FIG. 12 . 
         FIG. 14  is a flowchart showing a modification of the processing shown in  FIG. 12 . 
         FIG. 15  is a diagram showing a first modification of a manner of communication by the server shown in  FIG. 2 . 
         FIG. 16  is a diagram showing a second modification of the manner of communication by the server shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present disclosure will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. An energy management system is denoted as an “EMS” below. An electronic control unit mounted on a vehicle is denoted as an “ECU”. 
       FIG. 1  is a diagram showing a configuration of a vehicle  50  according to this embodiment. Referring to  FIG. 1 , vehicle  50  includes a battery  130  that stores electric power for traveling. Vehicle  50  can travel with electric power stored in battery  130 . Vehicle  50  according to this embodiment is a battery electric vehicle (BEV) not including an engine (internal combustion engine). 
     Battery  130  includes a secondary battery such as a lithium ion battery or a nickel metal hydride battery. In this embodiment, a battery assembly including a plurality of lithium ion batteries is adopted as the secondary battery. The battery assembly is composed of a plurality of secondary batteries (which are generally also referred to as “cells”) electrically connected to one another. Battery  130  according to this embodiment corresponds to an exemplary “battery” according to the present disclosure. 
     Vehicle  50  includes an ECU  150 . ECU  150  carries out charging control and discharging control of battery  130 . ECU  150  controls communication with the outside of vehicle  50 . 
     Vehicle  50  further includes a monitoring module  131  that monitors a state of battery  130 . Monitoring module  131  includes various sensors that detect a state (for example, a voltage, a current, and a temperature) of battery  130  and outputs a result of detection to ECU  150 . Monitoring module  131  may be a battery management system (BMS) that further performs, in addition to the sensor function, a state of charge (SOC) estimation function, a state of health (SOH) estimation function, a function to equalize a battery voltage, a diagnosis function, and a communication function. ECU  150  can obtain a state (for example, a temperature, a current, a voltage, an SOC, and an internal resistance) of battery  130  based on an output from monitoring module  131 . 
     Electric vehicle supply equipment (EVSE)  40  includes a power supply circuit  41  and a charging cable  42 . Power supply circuit  41  is contained in a main body of EVSE  40 . Charging cable  42  is connected to the main body of EVSE  40 . Charging cable  42  may always be connected to the main body of EVSE  40  or may be attachable to and removable from the main body of EVSE  40 . Charging cable  42  includes a connector  43  at its tip end and contains a power line. 
     Vehicle  50  includes an inlet  110  and a charger-discharger  120  for contact charging. Inlet  110  receives electric power supplied from the outside of vehicle  50 . Inlet  110  is configured such that connector  43  of charging cable  42  can be connected thereto. As connector  43  of charging cable  42  connected to the main body of EVSE  40  is connected to (plugged into) inlet  110  of vehicle  50 , vehicle  50  enters a chargeable state (that is, a state in which the vehicle can receive power feed from EVSE  40 ). Though  FIG. 1  shows only inlet  110  and charger-discharger  120  adapted to a power feed type of EVSE  40 , vehicle  50  may include a plurality of inlets so as to adapt to a plurality of types of power feed (for example, an alternating-current (AC) type and a direct-current (DC) type). 
     Charger-discharger  120  is located between inlet  110  and battery  130 . Charger-discharger  120  includes a relay that switches between connection and disconnection of an electric power path from inlet  110  to battery  130  and a power conversion circuit (neither of which is shown). The power conversion circuit may include a bidirectional converter. Each of the relay and the power conversion circuit included in charger-discharger  120  is controlled by ECU  150 . Vehicle  50  further includes a monitoring module  121  that monitors a state of charger-discharger  120 . Monitoring module  121  includes various sensors that detect a state of charger-discharger  120  and outputs a result of detection to ECU  150 . In this embodiment, monitoring module  121  detects a voltage and a current input to and output from the power conversion circuit. Monitoring module  121  detects charging power for battery  130 . 
     Vehicle  50  in the chargeable state is capable of external charging (that is, charging of battery  130  with electric power supplied from EVSE  40 ) and external power feed (that is, power feed from vehicle  50  to EVSE  40 ). Electric power for external charging is supplied, for example, from EVSE  40  through charging cable  42  to inlet  110 . Charger-discharger  120  converts electric power received at inlet  110  into electric power suitable for charging of battery  130  and outputs resultant electric power to battery  130 . Electric power for external power feed is supplied from battery  130  to charger-discharger  120 . Charger-discharger  120  converts electric power supplied from battery  130  into electric power suitable for external power feed and outputs resultant electric power to inlet  110 . When any of external charging and external power feed is performed, the relay of charger-discharger  120  is closed (connected), and when neither of external charging and external power feed is performed, the relay of charger-discharger  120  is opened (disconnected). 
     ECU  150  includes a processor  151 , a random access memory (RAM)  152 , a storage  153 , and a timer  154 . A computer may be adopted as ECU  150 . A central processing unit (CPU) may be adopted as processor  151 . RAM  152  functions as a work memory that temporarily stores data to be processed by processor  151 . Storage  153  can store information that is put thereinto. Storage  153  includes, for example, a read only memory (ROM) and a rewritable non-volatile memory. Storage  153  stores not only a program but also information (for example, a map, a mathematical expression, and various parameters) to be used by a program. As a program stored in storage  153  is executed by processor  151 , various types of control by ECU  150  are carried out in this embodiment. Various types of control by ECU  150  are not limited to control carried out by software but can also be carried out by dedicated hardware (electronic circuitry). Any number of processors may be provided in ECU  150  and a processor may be prepared for each prescribed type of control. 
     Timer  154  notifies processor  151  that the set time has come. As the time set in timer  154  comes, timer  154  transmits a signal to that effect to processor  151 . In this embodiment, a timer circuit is adopted as timer  154 . Timer  154  may be implemented by software instead of hardware (timer circuitry). ECU  150  can obtain current time from a real time clock (RTC) circuit (not shown) contained in. ECU  150 . 
     Vehicle  50  further includes a travel driving unit  140 , an input apparatus  161 , a meter panel  162 , a navigation system (which is referred to as a “NAVI” below)  170 , communication equipment  180 , and a drive wheel W. Vehicle  50  is not limited to a front-wheel-drive vehicle shown in  FIG. 1  and it may be a rear-wheel-drive vehicle or a four-wheel-drive vehicle. 
     Travel driving unit  140  includes a power control unit (PCU) and a motor generator (MG) which are not shown, and allows vehicle  50  to travel with electric power stored in battery  130 . The PCU includes, for example, an inverter, a converter, and a relay (none of which is shown). The relay included in the PCU is referred to as a “system main relay (SMR)” below. The PCU is controlled by ECU  150 . The MG is implemented, for example, by a three-phase AC motor generator. The MG is driven by the PCU and rotates drive wheel W. The PCU drives the MG with electric power supplied from battery  130 . The MG performs regeneration and supplies regenerated electric power to battery  130 . The SMR switches between connection and disconnection of an electric power path from battery  130  to the MG. The SMR is closed (connected) when vehicle  50  travels. 
     Input apparatus  161  accepts an input from a user. Input apparatus  161  is operated by a user and outputs a signal corresponding to the operation by the user to ECU  150 . Examples of input apparatus  161  include various switches, various pointing devices, a keyboard, and a touch panel. Input apparatus  161  may include a smart speaker that accepts audio input. 
     Meter panel  162  shows information on vehicle  50 . Meter panel  162  shows, for example, various types of information on vehicle  50  measured by various sensors mounted on vehicle  50 . Information shown on meter panel  162  may include at least one of an outdoor temperature, a traveling speed of vehicle  50 , an SOC of battery  130 , average electric power consumption, and a travel distance of vehicle  50 . Meter panel  162  is controlled by ECU  150 . ECU  150  may have meter panel  162  show a message for a user or a warning indicator when a prescribed condition is satisfied. 
     NAVI  170  includes a processor, a storage, a touch panel display, and a global positioning system (GPS) module (none of which is shown). The storage stores map information. The touch panel display accepts an input from a user or shows a map and other types of information. The GPS module receives a signal (which is referred to as a “GPS signal” below) from a GPS satellite. NAVI  170  can identify a position of vehicle  50  based on a GPS signal. NAVI  170  conducts a path search for finding a travel route (for example, a shortest route) from the current position of vehicle  50  to a destination based on an input from the user, and shows the travel route found by the path search on a map. 
     Communication equipment  180  includes various communication interfaces (IN). ECU  150  is configured to communicate with an EMS  61  ( FIG. 3 ) which will be described later through communication equipment  180 . ECU  150  is configured to wirelessly communicate with a server  30 B ( FIG. 3 ) which will be described later through communication equipment  180 . 
       FIG. 2  is a diagram showing a configuration of a server according to this disclosure. Referring to  FIG. 2 , a power management system  1  includes a power grid PG, a server  30 A, EVSE  40 , vehicle  50 , and a portable terminal  80 . Server  30 A according to this embodiment corresponds to an exemplary “server” according to the present disclosure. 
     Vehicle  50  is configured as shown in  FIG. 1 . In this embodiment, an AC power feed facility that provides AC power is adopted as EVSE  40 . Charger-discharger  120  includes a circuit adapted to the AC power feed facility. Without being limited to such a form, EVSE  40  may be a DC power feed facility that provides DC power. Charger-discharger  120  may include a circuit adapted to the DC power feed facility. 
     Portable terminal  80  corresponds to a terminal carried by a user of vehicle  50 . In this embodiment, a smartphone equipped with a touch panel display is adopted as portable terminal  80 . Without being limited thereto, any portable terminal can be adopted as portable terminal  80 , and a tablet terminal, a wearable device (for example, a smart watch), an electronic key, or a service tool can also be adopted. 
     Power grid PG is a power grid provided by an electric utility (for example, an electric power company). Power grid PG is electrically connected to a plurality of pieces of EVSE (including EVSE  40 ) and supplies AC power to each piece of EVSE. Power supply circuit  41  contained in EVSE  40  converts electric power supplied from power grid PG into electric power suitable for external charging. Power supply circuit  41  may include a sensor for detecting charging power. 
     As the relay of charger-discharger  120  is closed in vehicle  50  in the chargeable state, battery  130  is electrically connected to power grid PG. As electric power is supplied from power grid PG through power supply circuit  41 , charging cable  42 , and charger-discharger  120  to battery  130 , battery  130  is externally charged. 
     Server  30 A does not directly communicate with vehicle  50 . In other words, server  30 A does not wirelessly communicate with vehicle  50 . Server  30 A communicates with vehicle  50  with EMS  61  being interposed. EMS  61  communicates with vehicle  50  through EVSE  40  in accordance with a command from server  30 A. Communication equipment  180  mounted on vehicle  50  communicates with EVSE  40  through charging cable  42 . Communication between EVSE  40  and vehicle  50  may be of any type, and for example, controller area network (CAN) or power line communication (PLC) may be adopted. Standards of communication between EVSE  40  and vehicle  50  may be ISO/IEC15118 or IEC61851. 
     In this embodiment, communication equipment  180  and portable terminal  80  wirelessly communicate with each other. Communication equipment  180  and portable terminal  80  may communicate with each other through short-range communication such as Bluetooth® (for example, direct communication in a vehicle or within an area around the vehicle). 
     Server  30 A is configured to communicate with portable terminal  80 . Prescribed application software (which is simply referred to as an “application” below) is installed in portable terminal  80 . Portable terminal  80  is carried by a user of vehicle  50  and can exchange information with server  30 A through the application. The user can operate the application, for example, through the touch panel display of portable terminal  80 . The user can transmit, for example, scheduled departure time of vehicle  50  to server  30 A by operating the application. 
     Server  30 A includes a controller  31 , a storage  32 , a communication apparatus  33 , and an input apparatus  34 . A computer may be adopted as controller  31 . Controller  31  includes a processor and a storage, performs prescribed information processing, and controls communication apparatus  33 . Various types of information can be stored in storage  32 . Communication apparatus  33  includes various communication I/Fs. Controller  31  communicates with the outside through communication apparatus  33 . Input apparatus  34  accepts an input from a user. Input apparatus  34  provides the input from the user to controller  31 . 
       FIG. 3  is a diagram showing a schematic configuration of power management system  1  according to this embodiment. In this embodiment, power management system  1  functions as a virtual power plant (VPP). The VPP refers to a scheme in which a large number of distributed energy resources (which are also referred to as “DERs” below) are put together according to a sophisticated energy management technology that makes use of the Internet of Things (IoT) and the DERs are remotely controlled as being integrated as if the DERs functioned as a single power plant. In power management system  1 , the VPP is implemented by energy management using an electrically powered vehicle (for example, vehicle  50  shown in  FIG. 1 ). 
     Power management system  1  is a vehicle grid integration (VGI) system. Power management system  1  includes a plurality of electrically powered vehicles and a plurality of pieces of EVSE (each one of them is shown in  FIG. 3 ). Any independent number of electrically powered vehicles and pieces of EVSE may be included in power management system  1 , and the number may be set to ten or more or one hundred or more. Power management system  1  may include at least one of a POV and a MaaS vehicle. The POV is a personally owned vehicle. The MaaS vehicle is a vehicle managed by a mobility as a service (MaaS) entity. Power management system  1  may include at least one of non-public EVSE that only a specific user is permitted to use (for example, home EVSE) and public EVSE that a large number of unspecified users are permitted to use. Portable terminal  80  shown in  FIG. 2  is carried by each vehicle user. Server  30 A in  FIG. 3  is the same as server  30 A in  FIG. 2 . 
     Referring to  FIG. 3  together with  FIG. 2 , power management system  1  includes an electric power company E 1 , a parent aggregator E 2  that establishes contact with electric power company E 1 , and a resource aggregator E 3  that establishes contact with a demand side. 
     Electric power company E 1  serves as a power generation utility and a power transmission and distribution (T&amp;D) utility. Electric power company E 1  constructs a power grid (that is, power grid PG shown in  FIG. 2 ) with a power plant  11  and a power T&amp;D facility  12  and maintains and manages power grid PG with a server  10 . Power plant  11  includes a power generator that generates electricity and supplies electric power generated by the power generator to power T&amp;D facility  12 . Any system for power generation by power plant  11  is applicable. Any of thermal power generation, hydroelectric power generation, wind power generation, nuclear power generation, and solar photovoltaic power generation may be applicable as the system for power generation of power plant  11 . Power T&amp;D facility  12  includes a power transmission line, a substation, and an electricity distribution line and transmits and distributes electric power supplied from power plant  11 . Each of smart meters  13  and  14  measures an amount of power usage each time a prescribed time period elapses (for example, each time thirty minutes elapse), stores the measured amount of power usage, and transmits the measured amount of power usage to server  10 . The smart meter is provided for each demand side (for example, an individual or a company) that uses electric power. Server  10  obtains the amount of power usage for each demand side from the smart meter of each demand side. Electric power company E 1  may receive an electricity fee in accordance with the amount of power usage from each demand side. In this embodiment, the electric power company corresponds to a manager of power grid PG. 
     An electric utility that puts the DERs together to provide an energy management service is referred to as an “aggregator.” Electric power company E 1 , for example, in coordination with an aggregator, can adjust electric power of power grid PG. Parent aggregator E 2  includes a plurality of servers (for example, servers  20 A and  20 B). Servers included in parent aggregator E 2  belong to different utilities. Resource aggregator E 3  includes a plurality of servers (for example, servers  30 A and  30 B). Servers included in resource aggregator E 3  belong to different utilities. Servers included in parent aggregator E 2  will be referred to as a “server  20 ” below and servers included in resource aggregator E 3  will be referred to as a “server  30 ” below unless they are described as being distinguished from each other. Any independent number of servers  20  and servers  30  may be provided, and the number may be set to five or more or thirty or more. 
     In this embodiment, a single server  10  issues a request for energy management to a plurality of servers  20  and each server  20  that has received the request from server  10  issues a request for energy management to a plurality of servers  30 . Furthermore, each server  30  that has received the request from server  20  issues a request for energy management to a plurality of DER users. Electric power company E 1  can issue a request for energy management to a large number of demand sides (for example, vehicle users) using such a hierarchical structure (tree structure). The request may be issued by demand response (DR). 
     When server  30  receives a request for energy management from server  20 , it selects a DER for meeting the request from among DERs registered in server  30 . The thus selected DER is also referred to as an “EMDER” below. The EMDER may include a vehicle-mounted battery (for example, battery  130 ) or a stationary battery (for example, an ESS  70  which will be described later). 
     Server  30  manages energy in an area under its control. The area under the control by server  30  may be one city (for example, a smart city), a factory, or a university campus. An aggregator closes a contract of energy management with a user of a DER located within the area under the control by server  30 . The user who has closed the contract can receive a prescribed incentive by having the DER manage energy in accordance with the request from the aggregator. A prescribed penalty is imposed based on the contract, on a user who did not meet the request in spite of his/her approval to meet the request. The DER and the user thereof obliged to manage energy in the contract are registered in server  30 . 
     After the EMDER is selected, server  30  transmits a command to each EMDER. In response to this command, energy management in accordance with the request from server  20  (for example, adjustment of demand and supply in power grid PG) is carried out. 
     Server  30  measures an amount of power adjustment (for example, an amount of charging power and/or an amount of discharging power for a prescribed period) for each EMDER with a prescribed watt-hour meter. The amount of power adjustment may be used for calculating an incentive. The prescribed watt-hour meter may be smart meter  13  or  14  or a watt-hour meter (for example, monitoring module  121  shown in  FIG. 1 ) mounted on the vehicle. The watt-hour meter may be provided at any location. The watt-hour meter may be contained in EVSE  40 . The watt-hour meter may be attached to a portable charging cable. 
     In this embodiment, server  30  is configured to receive from server  10 , a detection value obtained by each of smart meters  13  and  14 . Without being limited as such, server  30  may be configured to obtain the detection value from each of smart meters  13  and  14  directly (without server  10  being interposed). 
     Smart meter  13  is configured to measure an amount of electric power supplied from power grid PG (that is, the power grid constructed of power plant  11  and power T&amp;D facility  12 ) shown in  FIG. 2  to EVSE  40 . In this embodiment, EVSE  40  and EMS  61  are provided in one house. EMS  61  is, for example, a home EMS (HEMS). Smart meter  13  measures an amount of electric power (that is, an amount of electric power used in a household) supplied from power grid PG to that house. 
     Smart meter  14  is configured to measure an amount of electric power supplied from power grid PG shown in  FIG. 2  to an energy storage system (ESS)  70 . ESS  70  is a stationary battery configured to be chargeable from and dischargeable to power grid PG. For example, a lithium ion battery, a lead-acid battery, a nickel metal hydride battery, a redox flow battery, or a sodium sulfur (NAS) battery may be adopted as ESS  70 . 
     Server  30 A communicates with ESS  70  through an EMS  62 . In this embodiment, EMS  62  and ESS  70  are provided in one business entity (for example, a factory or a commercial facility). EMS  62  may be, for example, a factory EMS (FEMS) or a building EMS (BEMS). Smart meter  14  measures an amount of electric power (that is, an amount of electric power used in a business entity) supplied from power grid PG to that business entity. 
     When server  30 A receives a request for energy management from server  20 , it manages energy through charging of battery  130  by transmitting a charging start command to vehicle  50  via EMS  61  and EVSE  40 . Server  30 A may be a server belonging to a house building company or an electric machinery manufacturer. Server  30 A may be a server belonging to an automobile manufacturer different from an automobile manufacturer that manufactured vehicle  50 . 
     Server  30 B is configured to wirelessly communicate with vehicle  50 . When server  30 B receives a request for energy management from server  20 , it carries out charging of battery  130  by directly transmitting a charging start command to vehicle  50  through wireless communication. During charging of battery  130 , server  30 B obtains a charging condition (including the SOC) of battery  130  from vehicle  50 . Server  30 B may be a server belonging to the automobile manufacturer that manufactured vehicle  50 . 
     In power management system  1  as above, server  30 B can obtain the charging condition (including the SOC) of battery  130  from vehicle  50 . On the other hand, server  30 A is unable to obtain the charging condition of battery  130  from vehicle  50 . Though details will be described later, server  30 A is configured to know the charging condition of battery  130  without relying on the SOC of battery  130 . 
     In charging battery  130  to full charge, ECU  150  of vehicle  50  carries out CP 1  charging (first constant power charging) until battery  130  is close to full charge. When battery  130  is close to full charge and a voltage of battery  130  is equal to or higher than an open circuit voltage (OCV) at the time of full charge, storage of electricity in battery  130  with high charging power becomes hard. Therefore, when battery  130  is close to full charge, ECU  150  carries out CP 2  charging (second constant power charging) for bringing battery  130  closer to full charge with low charging power after charging power is lowered while it carries out CV charging (constant voltage charging). Periods for which CP 1  charging, CV charging, and CP 2  charging are carried out are also referred to as a “CP 1  period,” a “CV period,” and a “CP 2  period,” respectively. Charging power in CP 1  charging and charging power in CP 2  charging may be denoted as “P 31 ” and “P 32 ”, respectively. P 32  represents an electric power value smaller than P 31 . During the CV period, a charging voltage is constant and charging power gradually lowers from P 31  to P 32 . CV charging and CP 2  charging according to this embodiment correspond to exemplary “charging power reduction control” according to the present disclosure. 
       FIG. 4  is a diagram for illustrating the CP 1  period, the CV period, and the CP 2  period. In.  FIG. 4 , a line Ll represents transition of charging power for battery  130 . A line L 2  represents transition of a voltage of battery  130  (a battery voltage). A line L 3  represents transition of an SOC of battery  130 . Each of t 11  to t 13  represents timing. 
     Referring to  FIG. 4  together with  FIG. 1 , in this timing chart, a period before t 11  corresponds to the CP 1  period. When the SOC (line L 3 ) of battery  130  reaches a threshold value Y 1  at t 11 , transition from the CP 1  period to the CV period is made. In this embodiment, when the voltage (line L 2 ) of battery  130  attains to the OCV at the time of full charge, the SOC of battery  130  attains to threshold value Y 1 . 
     A period from t 11  to t 12  corresponds to the CV period. In the example shown in  FIG. 4 , charging power lowers at a constant rate during the CV period. When charging power (line L 1 ) for battery  130  attains to P 32  at t 12 , transition from the CV period to the CP 2  period is made. Thereafter, when the SOC (line L 3 ) of battery  130  reaches a threshold value Y 2  (for example, 100%) larger than threshold value Y 1  at t 13 , charging ends. In this embodiment, the SOC of battery  130  attains to threshold value Y 2  when the voltage (line L 2 ) of battery  130  attains to a closed circuit voltage (CCV) at the time of full charge. 
       FIG. 5  is a flowchart showing charging control carried out by ECU  150  of vehicle  50 . Processing shown in this flowchart is started by ECU  150 , for example, when vehicle  50  receives a charging start command from the outside. 
     Referring to  FIG. 5  together with  FIGS. 1 and 4 , in a step (which is simply denoted as “S” below)  11 , ECU  150  carries out CP 1  charging of battery  130 . A period immediately after reception of the charging start command by vehicle  50  falls under the CP 1  period. Therefore, CP 1  charging is carried out with charging power P 31 . In succession, in S 12 , ECU  150  determines whether or not the SOC of battery  130  is equal to or higher than a threshold value Y 1 . ECU  150  can obtain the SOC of battery  130 , for example, based on an output from monitoring module  131 . During the CP 1  period, CP 1  charging (S 11 ) is continuously carried out and the SOC of battery  130  increases. When the SOC of battery  130  is equal to or higher than threshold value Y 1  (YES in S 12 ), in S 13 , ECU  150  quits the CP 1  period and makes transition to the CV period. 
     In S 14 , ECU  150  carries out CV charging of battery  130 . In succession, in S 15 , ECU  150  determines whether or not charging power for battery  130  is equal to or lower than P 32 . ECU  150  can obtain charging power for battery  130 , for example, based on an output from monitoring module  131 . During the CV period, CV charging (S 14 ) is continuously carried out and charging power for battery  130  lowers. Then, when charging power is equal to or lower than P 32  (YES in S 15 ), in S 16 , ECU  150  quits the CV period and makes transition to the CP 2  period. 
     In S 17 , ECU  150  carries out CP 2  charging of battery  130 . In succession, in S 18 , ECU  150  determines whether or not the SOC of battery  130  is equal to or higher than a threshold value Y 2 . During the CP 2  period, CP 2  charging (S 17 ) is continuously carried out and the SOC of battery  130  increases. When the SOC of battery  130  is equal to or higher than threshold value Y 2  (YES in S 18 ), in S 19 , ECU  150  quits charging of battery  130  and quits a series of processing in  FIG. 5 . Charging power for battery  130  is thus set to 0 W. 
     Transition of charging power during charging is not limited to the example shown with line L 1  in  FIG. 4 .  FIG. 6  is a diagram showing a modification of transition of charging power shown in  FIG. 4 . Referring to  FIG. 6 , as shown with a line L 10 , a pattern of lowering in charging power during the CV period may be a pattern in which charging power lowers stepwise. 
     In this embodiment, ECU  150  carries out charging control of battery  130  in the order of CP 1  charging, CV charging, and CP 2  charging. Without being limited as such, a manner of control can be modified as appropriate. 
       FIG. 7  is a diagram showing a first modification of charging control carried out by ECU  150  in vehicle  50 . In  FIG. 7 , a line L 20 , a line L 21 , and a line L 22  represent charging power, a charging voltage, and a charging current, respectively. Referring to  FIG. 7 , in this modification, ECU  150  carries out charging control of battery  130  in the order of CC charging (constant current charging) and CV charging (constant voltage charging). A period before t 21  corresponds to the CC period during which CC charging is carried out. A period from t 21  to t 22  corresponds to the CV period during which CV charging is carried out. For example, when the SOC of battery  130  becomes equal to or higher than threshold value Y 1  at t 21 , ECU  150  quits the CC period and makes transition to the CV period. ECU  150  starts charging power reduction control at the time when it makes transition from the CC period to the CV period. CV charging according to this modification corresponds to exemplary “charging power reduction control” according to the present disclosure. When the SOC of battery  130  becomes equal to or higher than threshold value Y 2  at t 22 , charging of battery  130  ends. 
       FIG. 8  is a diagram showing a second modification of charging control carried out by ECU  150  in vehicle  50 . As shown with a line L 30  in  FIG. 8 , in this modification, ECU  150  carries out charging control of battery  130  in the order of CP 1  charging (first constant power charging) and CP 2  charging (second constant power charging) lower in electric power than CP 1  charging. A period before t 31  corresponds to the CP 1  period. A period from t 31  to t 32  corresponds to the CP 2  period. For example, when the SOC of battery  130  is equal to or higher than threshold value Y 1  at t 31 , ECU  150  quits the CPI period and makes transition to the CP 2  period. ECU  150  starts charging power reduction control when it makes transition from the CP 1  period to the CP 2  period. CP 2  charging according to this modification corresponds to exemplary “charging power reduction control” according to the present disclosure. When the SOC of battery  130  is equal to or higher than threshold value Y 2  at t 32 , charging of battery  130  ends. 
     X 1  and X 2  in  FIGS. 4 and 6 to 8  will be described later. 
     Server  30 A according to this embodiment is configured to successively charge batteries (cells) mounted on a plurality of vehicles. The order of charging of the plurality of batteries mounted on the plurality of vehicles is shown in the charging schedule held in server  30 A.  FIG. 9  is a diagram showing an exemplary charging schedule.  FIG. 10  is a diagram showing a plurality of vehicles that prepare for charging in accordance with the charging schedule shown in  FIG. 9 . Vehicles A to H in  FIG. 9  correspond to vehicles  50 A to  50 H shown in  FIG. 10 . As shown in  FIG. 10 , vehicles  50 A to  50 H include batteries  130 A to  130 H, respectively. Vehicles  50 A to  50 H are configured to be connectable to EVSE  40 A to EVSE  40 H, respectively. Each of EVSE  40 A to EVSE  40 H is electrically connected to power grid PG and receives supply of electric power from power grid PG. Vehicles  50 A to  50 H and EVSE  40 A to EVSE  40 H are configured similarly to vehicle  50  and EVSE  40  shown in  FIGS. 1 and 2 , respectively. Each of vehicles  50 A to  50 H is referred to as a “vehicle  50 ” below and each piece of EVSE  40 A to EVSE  40 H is referred to as “EVSE  40 ” below, unless they are described as being distinguished from one another. EMS  61  shown in  FIG. 2  is provided for each piece of EVSE  40 . 
     Referring to  FIG. 9  together with  FIG. 2 , this charging schedule defines simultaneous charging of two batteries. For example, server  30 A creates a charging schedule when it receives a charging request from server  20  (parent aggregator E 2 ). Each battery incorporated in the charging schedule corresponds to the EMDER described previously. In the example shown in  FIG. 9 , each of batteries  130 A to  130 H corresponds to the EMDER. The created charging schedule is stored in storage  32 . In creating the charging schedule, server  30 A may select a battery (EMDER) based on scheduled time of departure of each vehicle  50  and determine the order of charging and charging start timing. After creation of the charging schedule, server  30 A may give a prescribed notification to portable terminal  80  carried by the user of each vehicle incorporated in the charging schedule. 
     In the example shown in  FIG. 9 , server  30 A creates the charging schedule to secure charging power P 1  with a first battery and to secure charging power P 2  with a second battery, with charging power requested from parent aggregator E 2  being divided into charging power P 1  and charging power P 2 . As two batteries (the first battery and the second battery) are simultaneously charged, requested charging power is secured. Requested charging power corresponds to the sum of charging power P 1  and charging power P 2 . 
     In the charging schedule shown in  FIG. 9 , each of batteries  130 A,  130 C,  130 E, and  130 G corresponds to the first battery, and each of batteries  130 B,  130 D,  130 F, and  130 H corresponds to the second battery. In  FIG. 9 , each of t 0  to t 5  represents timing. At t 0 , charging of batteries  130 A and  130 B is started. Thereafter, when charging of battery  130 A ends at t 1 , charging of battery  130 C is started. After t 1  as well, at each of timing t 2 , timing t 3 , timing t 4 , and timing t 5 , charging is successively relayed (end of charging and start of charging). Specifically, at t 2 , when charging of battery  130 B ends, charging of battery  130 D is started. At t 3 , when charging of battery  130 C ends, charging of battery  130 E is started. At t 4 , when charging of battery  130 D ends, charging of battery  130 F is started. At t 5 , when charging of batteries  130 E and  130 F ends, charging of batteries  130 G and  130 H is started. 
     Referring to  FIG. 10  together with  FIG. 9 , the user of the vehicle connects the vehicle in which the SOC of the vehicle-mounted battery is within a prescribed range (which is referred to as a “start range” below) to EVSE so as to be in time for charging start timing shown in the charging schedule. The vehicle that has finished charging may leave the EVSE and start traveling. In the example shown in  FIG. 10 , however, charging of battery  130 F ends earlier than charging end timing shown in the charging schedule, and vehicle  50 F leaves EVSE  40 F. For example, when the SOC of battery  130 F at the time of start of charging is higher than the start range, charging of battery  130 F may end earlier than scheduled. When vehicle  50 F leaves in the middle of the process, charging power secured by server  30 A may not reach charging power requested by parent aggregator E 2 . 
       FIG. 11  is a diagram showing exemplary energy management carried out by server  30 A. Referring to  FIG. 11  together with  FIG. 2 , server  30 A according to this embodiment is configured to sense leaving of vehicle  50 F in the middle of the process. Though details will be described later, controller  31  of server  30 A determines whether or not end of charging of battery  130 F is close based on whether or not charging power reduction control is carried out in battery  130 F that is being charged. Specifically, controller  31  determines that end of charging of battery  130 F is close when charging power reduction control is carried out in battery  130 F that is being charged. When charging of battery  130 F ends earlier than scheduled, controller  31  can sense that charging of battery  130 F is about to end before end of charging of battery  130 F. Therefore, server  30 A can compensate for, at early timing, decrease in charging power due to leaving of vehicle  50 F in the middle of the process. Insufficiency of charging power is thus less likely. 
     Server  30 A according to this embodiment compensates for insufficiency for requested charging power with a replacement resource (a resource that functions as reserve) or a next battery. When battery  130 F is designated as the subject battery, battery  130 H charging of which is determined in the charging schedule to start following battery  130 F corresponds to the next battery. The replacement resource is a resource connected to power grid PG, a charging schedule of which is not determined, among charging resources controllable by server  30 A. in other words, a battery, a charging schedule of which is determined in the charging schedule, does not fall under the replacement resource. At the time when vehicle  50 F shown in  FIG. 10  leaves, charging of batteries  130 A to  130 D scheduled in the charging schedule shown in  FIG. 9  has already ended and hence batteries  130 A to  130 D can serve as the replacement resources. 
       FIG. 12  is a flowchart showing processing involved with energy management carried out by controller  31  of server  30 A. Processing shown in this flowchart is started when the subject battery is designated. For example, controller  31  starts energy management based on the charging schedule shown in  FIG. 9  by simultaneously setting batteries  130 A and  130 B as the subject batteries. When battery  130 A is set as the subject battery, a series of processing shown in  FIG. 12  is performed on battery  130 A. When battery  130 B is set as the subject battery, the series of processing shown in  FIG. 12  is performed on battery  130 B. When batteries  130 A and  130 B are simultaneously set as the subject batteries, processing onto the batteries is performed in parallel and simultaneously proceeds. 
     Referring to  FIG. 12  together with  FIG. 2 , in S 21 , controller  31  transmits a charging start command for the subject battery. In this embodiment, the charging start command for the subject battery is transmitted from server  30 A to EMS  61 , and EMS  61  instructs EVSE  40  to which vehicle  50  including the subject battery is connected to start charging of the subject battery in accordance with the command from server  30 A. Therefore, when controller  31  transmits the charging start command for the subject battery, charging of the subject battery is started in the processing shown in  FIG. 5 . The subject battery is charged with electric power supplied from power grid PG. 
     In S 22 , controller  31  determines whether or not charging power for the subject battery that is being charged has lowered. Controller  31  can determine whether or not charging power for the subject battery has lowered, for example, based on a detection value from smart meter  13 . Smart meter  13  detects electric power supplied from power grid PG to the subject battery. Smart meter  13  according to this embodiment corresponds to an exemplary “wattmeter” according to the present disclosure. Controller  31  may determine whether or not charging power for the subject battery has lowered based on whether or not an amount of lowering in charging power per unit time for the subject battery is equal to or larger than a prescribed value. Determination in S 22  is repeated until charging power for the subject battery lowers. When charging power for the subject battery has lowered (YES in S 22 ), the process proceeds to S 23 . 
     In S 23 , controller  31  determines whether or not charging power for the subject battery that is being charged is lower than a prescribed first reference value (which is denoted as “X 1 ” below). The first reference value (X 1 ) corresponds to X 1  in  FIGS. 4 and 6 to 8 . In this embodiment, X 1  is set to a value slightly smaller than P 31 . X 1  may be set within a range from a value 0.6 to 0.9 time as large as P 31 , and set, for example, to a value 0.7 time as large as P 31 . 
     In S 23 , controller  31  determines whether or not charging power for the subject battery is lower than X 1 , for example, based on a detection value from smart meter  13 . Determination in S 22  and S 23  is repeated until charging power for the subject battery becomes lower than X 1 . When charging power for the subject battery that is being charged lowers and becomes lower than X 1  (YES in both of S 22  and S 23 ), the process proceeds to S 24 . 
     Controller  31  determines that charging power reduction control has been started in the subject battery that is being charged when charging power for the subject battery that is being charged lowers and becomes lower than X 1 . Start of charging power reduction control in the subject battery that is being charged means that end of charging of the subject battery is close. Controller  31  according to this embodiment determines that end of charging of the subject battery is close by sensing charging power reduction control carried out in the subject battery. When determination as YES is made in S 23 , in S 24  and S 25 , controller  31  performs processing for compensating for decrease in charging power due to charging power reduction control. 
       FIG. 13  is a flowchart showing details of S 24  in  FIG. 12 . Referring to  FIG. 13  together with  FIG. 2 , in S 31 , controller  31  determines whether or not reserve of power grid PG is insufficient. Controller  31  may determine whether or not reserve of power grid PG is insufficient based on whether or not reserve charging capacity of a replacement resource is equal to or smaller than a prescribed value. 
     When reserve of power grid PG is sufficient (NO in S 31 ), in S 32 , controller  31  selects a replacement resource for compensation. The replacement resource includes, for example, ESS  70  ( FIG. 3 ). Though  FIG. 3  shows only a single ESS  70 , a plurality of ESSs  70  are connected to power grid PG. Power grid PG includes a large number of replacement resources. In S 32 , controller  31  selects replacement resources in number necessary for compensation from among the large number of replacement resources. The replacement resource for power grid PG may include a vehicle-mounted battery other than batteries  130 A to  130 H ( FIG. 10 ). Since the vehicle-mounted battery is not always connected to power grid PG, in S 32 , the vehicle-mounted battery may preferentially be selected over the ESS. Controller  31  may notify a user of the selected replacement resource of that fact. 
     When reserve of power grid PG is insufficient (YES in S 31 ), in S 33 , controller  31  performs processing for increasing reserve of power grid PG. Controller  31  may give to portable terminal  80  carried by a user of an electrically powered vehicle not connected to power grid PG, a notification inviting the user to connect the electrically powered vehicle to power grid PG. Controller  31  may carry out demand response (DR) for increasing replacement resources. 
     After processing in S 33 , in S 34 , controller  31  selects a replacement resource for compensation. 
     As the replacement resource is selected in S 32  or S 34 , a series of processing shown in  FIG. 13  ends and the process proceeds to S 25  in  FIG. 12 . Referring again to  FIG. 12  together with  FIG. 2 , in S 25 , controller  31  carries out charging with the replacement resource selected in S 32  or S 34 . Controller  31  carries out charging control (remote control) of the replacement resource such that charging power secured by server  30 A attains to charging power (see  FIG. 11 ) requested by parent aggregator E 2 . Charging by the replacement resource compensates for decrease in charging power due to charging power reduction control. Controller  31  thus carries out charging by the replacement resource (the charging resource connected to power grid PG) to compensate for decrease in charging power due to charging power reduction control when controller  31  determines that end of charging of the subject battery is close (YES in S 22  and S 23 ). 
     In S 26 , controller  31  determines whether or not charging power for the subject battery that is being charged becomes lower than a prescribed second reference value (which is denoted as “X 2 ” below). X 2  represents an electric power value smaller than X 1 . The second reference value (X 2 ) corresponds to X 2  in  FIGS. 4 and 6 to 8 . In this embodiment, X 2  is set around 0 W. X 2  may be set within a range not lower than 0 W and not higher than 500 W, and set, for example, to 100 W. 
     In S 26 , controller  31  determines whether or not charging power for the subject battery becomes lower than X 2 , for example, based on a detection value from smart meter  13 . Processing in S 24  to S 26  is repeated until charging power for the subject battery is lower than X 2 . Through processing in S 24  and S 25 , charging by the replacement resource compensates for decrease in charging power due to charging power reduction control carried out in the subject battery. When charging power for the subject battery under charging power reduction control becomes lower than X 2  (YES in S 26 ), the process proceeds to S 27 . 
     Controller  31  determines that charging of the subject battery ends when charging power for the subject battery becomes lower than X 2  (YES in S 26 ) after start of charging power reduction control. When determination as YES is made in S 26 , in S 27 , controller  31  determines whether or not there is a next battery by referring to the charging schedule ( FIG. 9 ) stored in storage  32 . Absence of the next battery (that is, determination as NO in S 27 ) means that energy management based on the charging schedule has ended. When determination as NO is made in S 27 , a series of processing shown in  FIG. 12  ends. 
     When determination as YES is made in S 27 , in S 28 , controller  31  performs processing for compensating for decrease in charging power due to end of charging of the subject battery. Specifically, in S 28 , controller  31  selects a replacement resource and carries out charging by using the selected replacement resource. Processing in S 28  is the same as the processing in S 24  and S 25  described previously. 
     In S 29 , controller  31  determines whether or not preparation for charging of the next battery has been completed. Controller  31  determines that preparation for charging of battery  130  mounted on vehicle  50  has been completed when vehicle  50  is in a chargeable state. When preparation for charging of the next battery has not been completed (NO in S 29 ), the process returns to S 28 . Processing in S 28  and S 29  is repeated until preparation for charging of the next battery is completed. As a result of processing in S 28 , charging by the replacement resource compensates for decrease in charging power due to end of charging of the subject battery. 
     When preparation for charging of the next battery has been completed (YES in S 29 ), in S 30 , controller  31  sets the next battery as the new subject battery. Thereafter, the series of processing shown in  FIG. 12  ends. In other words, as the processing in S 30  is performed, the series of processing shown in  FIG. 12  for the subject battery ends, however, the series of processing shown in  FIG. 12  is newly started for the next battery (new subject battery). 
     In the example shown in  FIG. 10 , when the series of processing shown in  FIG. 12  is performed with battery  130 A of vehicle  50 A (first vehicle) being designated as the subject battery and end of charging of battery  130 A is close, charging power reduction control is carried out in battery  130 A (S 14  in  FIG. 5 ). Charging power becomes lower than X 1  (see  FIG. 4 ) and determination as YES is made in S 23 . Then, as a result of processing in S 24  and S 25 , charging by the replacement resource compensates for decrease in charging power due to charging power reduction control. Thereafter, when charging of battery  130 A ends, determination as YES is made in S 26 , and in S 29 , whether or not preparation for charging of battery  130 B (next battery) of vehicle  50 B (second vehicle) has been completed is determined. Since vehicle  50 B is connected to power grid PG before end of charging of battery  130 A, determination as YES is made in S 29  when charging of battery  130 A ends, and in S 30 , battery  130 B is set as the new subject battery. Then, the series of processing shown in  FIG. 12  is started with battery  130 B being designated as the subject battery. 
     When the series of processing shown in  FIG. 12  is performed with battery  130 F of vehicle  50 F (first vehicle) being designated as the subject battery, charging of battery  130 F ends earlier than charging end timing shown in the charging schedule. Therefore, at the time of end of charging of battery  130 F, vehicle  50 H (second vehicle) has not yet been connected to power grid PG. Therefore, determination as NO is made in S 29 , and charging by the replacement resource compensates for decrease in charging power due to end of charging of battery  130 F until vehicle  50 H is connected to power grid PG (S 28 ). Then, when vehicle  50 H is connected to power grid PG (YES in S 29 ), in S 30 , battery  130 H (next battery) is set as the new subject battery and the series of processing shown in  FIG. 12  for battery  130 H is started. 
     As described above, server  30 A according to this embodiment includes controller  31  that has a plurality of batteries (for example, batteries  130 A to  130 H) successively charged. Controller  31  is configured to successively transmit charging start commands for the plurality of batteries for energy management of power grid PG. Then, controller  31  is configured to sense that end of charging of the subject battery is close when charging power reduction control is carried out in the subject battery that is being charged (YES in S 22  and S 23  in  FIG. 12 ). After server  30 A determines that end of charging of the subject battery is close, it can perform prescribed processing before end of charging of the subject battery. For example, server  30 A compensates for decrease in charging power due to charging power reduction control before end of charging of the subject battery (S 24  and S 25  in  FIG. 12 ). Server  30 A according to this embodiment can know a charging condition of the battery without relying on the SOC of the battery and appropriately manage energy depending on the charging condition of the battery. 
     The energy management method according to this embodiment includes determining whether or not charging power reduction control is carried out in the battery that is being charged (S 22  and S 23  in  FIG. 12 ) and performing processing for compensating for decrease in charging power due to charging power reduction control (S 24  and S 25  in  FIG. 12 ) when it is determined that charging power reduction control is carried out in the battery that is being charged (YES in S 22  and S 23 ). According to the energy management method in this embodiment, the charging condition of the battery can be known without relying on the SOC of the battery and energy can appropriately be managed depending on the charging condition of the battery. 
     In the embodiment, controller  31  of server  30 A determines whether or not charging power reduction control is carried out in the subject battery that is being charged based on a detection value from smart meter  13 . Without being limited as such, controller  31  of server  30 A may determine whether or not charging power reduction control is carried out in the subject battery that is being charged based on a detection value from a wattmeter contained in EVSE  40  or a detection value from a CT sensor provided outside EVSE  40 . 
     Controller  31  of server  30 A may be configured to perform processing shown in  FIG. 14  instead of the processing shown in  FIG. 12 .  FIG. 14  is a flowchart showing a modification of the processing shown in  FIG. 12 . In the processing shown in  FIG. 14 , S 24 A and S 25 A are adopted instead of S 24  and S 25  in  FIG. 12  and S 27  to S 30  in  FIG. 12  are omitted. In this modification, the charging schedule is not used. For example, server  30 A selects a first subject battery when it receives a charging request from server  20  (parent aggregator E 2 ) and performs a series of processing shown in  FIG. 14  on the first subject battery. Server  30 A selects the first subject battery from among replacement resources for power grid PG. As the series of processing shown in  FIG. 14  is performed on the first subject battery, energy of power grid PG is managed through charging of the first subject battery. Processing ( FIG. 14 ) according to this modification will be described below with a difference from the processing shown in  FIG. 12  being focused on. 
     Referring to  FIG. 14  together with  FIG. 2 , in S 21 , controller  31  of server  30 A transmits a charging start command for the first subject battery. The first subject battery may be battery  130  of the first vehicle configured similarly to vehicle  50  shown in  FIG. 1 . ECU  150  (a first controller) of the first vehicle starts prescribed first charging control of the first subject battery based on the charging start command from server  30 A. Prescribed first charging control is, for example, control shown in  FIGS. 4 and 5 . Without being limited as such, prescribed first charging control may be control shown in  FIG. 7 or 8 . 
     When charging power reduction control is started in the first subject battery that is being charged, determination as YES is made in S 22  and S 23 , and the process proceeds to S 24 A. In S 24 A, controller  31  selects a second subject battery (a new subject battery) from among the replacement resources for power grid PG. The second subject battery corresponds to a battery (that is, a battery that is charged in succession to the first subject battery) charging of which is to be started following the first subject cell in relay charging. Then, in S 25 A, controller  31  performs the series of processing shown in  FIG. 14  onto the second subject battery. Processing ( FIG. 14 ) involved with charging of the first subject battery and processing ( FIG. 14 ) involved with charging of the second subject battery are performed in parallel and simultaneously proceed. 
     In processing involved with charging of the first subject battery, after processing in S 25 A, in S 26 , controller  31  determines whether or not charging power for the first subject battery becomes lower than X 2 . While charging power for the first subject battery is not lower than X 2  (NO in S 26 ), controller  31  determines that charging of the first subject battery is continuing. When charging power becomes lower than X 2  (YES in S 26 ), controller  31  determines that charging of the first subject battery has ended. When determination as YES is made in S 26 , processing ( FIG. 14 ) involved with charging of the first subject battery ends. 
     Processing involved with charging of the second subject battery is started before end of charging of the first subject battery. Then, in S 21 , controller  31  transmits the charging start command for the second subject battery. The second subject battery may be battery  130  of the second vehicle configured similarly to vehicle  50  shown in  FIG. 1 . ECU  150  (a second controller) of the second vehicle may be started up in response to reception of the charging start command for the second subject battery. ECU  150  of the second vehicle starts prescribed second charging control of the second subject battery based on the charging start command from server  30 A. Prescribed second charging control is, for example, control shown in  FIGS. 4 and 5 . Without being limited as such, prescribed second charging control may be control shown in  FIG. 7 or 8 . 
     Immediately after start of charging of the second subject battery, charging is carried out in both of the first subject battery and the second subject battery. Controller  31  may indicate in the charging start command for the second subject battery that rise of charging power be made gentler such that the sum of charging power for the first subject battery and charging power for the second subject battery does not exceed charging power requested by parent aggregator E 2 . 
     Thereafter, when charging power reduction control is started in the second subject battery that is being charged, determination as YES is made in S 22  and S 23  and the process proceeds to S 24 A. Processing in S 24 A or later is similar to processing involved with charging of the first subject battery. In other words, a new subject battery (a third subject battery) is selected in S 24 A also in processing involved with charging of the second subject battery. 
     Server  30 A according to the modification is configured to transmit the charging start command for the second subject battery when charging power reduction control is started in the first subject battery that is being charged (YES in S 22  and S 23 ). In such a configuration, the charging start command for the second subject battery is transmitted before end of charging of the first subject battery, and therefore discontinuity of charging is less likely. The power management system including server  30 A according to the modification corresponds to an exemplary “power management system” according to the present disclosure. 
     In the embodiment and the modification, server  30 A is configured to transmit the charging start command to the energy management system (EMS) that manages EVSE (power feed facility) to which the vehicle is connected ( FIG. 2 ). Without being limited as such, the server may transmit the charging start command to EVSE or to a vehicle directly (without the EMS and the EVSE being interposed). 
       FIG. 15  is a diagram showing a first modification of a manner of communication by the server shown in  FIG. 2 . Referring to  FIG. 15 , a server  30 C is configured to directly transmit the charging start command to EVSE  40 . Server  30 C includes a communication apparatus  33 C for communication with EVSE  40 . EVSE  40  includes a communication apparatus (not shown) for communication with server  30 C. The communication apparatus of EVSE  40  may be mounted on the main body of EVSE  40  or may be provided in charging cable  42 . Communication between server  30 C and EVSE  40  may be wired or wireless. Server  30 C transmits the charging start command for the subject battery (battery  130 ) to EVSE  40  connected to vehicle  50  including the subject battery, for example, in S 21  in  FIG. 12 or 14 . EVSE  40  instructs vehicle  50  to start charging of the subject battery in accordance with the command from server  30 C. Server  30 C obtains charging power for the subject battery mounted on vehicle  50  from smart meter  13 . EVSE  40  may communicate with an EVSE management cloud. A protocol of communication between EVSE  40  and the EVSE management cloud may be open charge point protocol (OCPP). 
       FIG. 16  is a diagram showing a second modification of the manner of communication by the server shown in  FIG. 2 . Referring to  FIG. 16 , a server  30 D is configured to directly transmit the charging start command to vehicle  50  through wireless communication. Server  30 D includes a communication apparatus  30 D for wireless communication with vehicle  50 . Communication equipment  180  of vehicle  50  includes a communication I/F for communication with server  30 D. Communication equipment  180  may include a data communication module (DCM). Server  30 D transmits, for example, in S 21  in  FIG. 12 or 14 , the charging start command for the subject battery (battery  130 ) directly to vehicle  50  while vehicle  50  including the subject battery is connected to EVSE  40 . ECU  150  of vehicle  50  starts prescribed charging control of the subject battery in accordance with the charging start command from server  30 D. Server  30 D obtains charging power for the subject battery mounted on vehicle  50  from smart meter  13 . 
     In the embodiment and the modifications, when charging power for the subject battery that is being charged lowers and becomes lower than the first reference value (X 1 ), the server determines that charging power reduction control has been started in the subject battery that is being charged. A method of determining whether or not charging power reduction control is carried out in the subject battery, however, is not limited to the method described above. For example, the server may determine whether or not charging power reduction control is carried out based on a pattern of lowering in charging power (a behavior in lowering of charging power). The server may learn behaviors of charging power while charging power reduction control is carried out. Alternatively, the server may determine whether or not charging power reduction control is carried out based on a behavior of at least one of a charging current and a charging voltage. The server may learn behaviors of at least one of the charging current and the charging voltage while charging power reduction control is carried out. The server may determine whether or not charging power reduction control is carried out by using a trained model obtained by machine learning using artificial intelligence (AI). Learning may be done for each battery (for each vehicle). According to such a method, even when charging power is not stable, erroneous sensing of charging power reduction control due to fluctuation in charging power is less likely. 
     The embodiment and various modifications may be carried out as being combined in any manner. For example, controller  31  may accept an input from a user so as to allow the user to adopt any control mode. Controller  31  may be configured to allow the user to select any of control shown in  FIG. 12  (a first control mode) and control shown in  FIG. 14  (a second control mode) through input apparatus  34 . 
     The electric power company may be divided for each business sector. A power generation utility and a power T&amp;D utility may belong to companies different from each other. One aggregator may serve as both of the parent aggregator and the resource aggregator. The server may receive a request for energy management from a power market. 
     It is not essential that a plurality of vehicles that successively carry out charging of batteries in a relayed manner are similarly configured. The plurality of vehicles different in model may successively carry out charging of batteries in the relayed manner. 
     A configuration of the vehicle is not limited to the configuration shown in  FIG. 1 . For example, the vehicle may be capable only of external charging, of external charging and external power feed. The vehicle may be configured to be wirelessly chargeable. The vehicle is not limited to a passenger car, and a bus or a truck may be applicable. The vehicle is not limited to a BEV, and a PHEV may be applicable. The vehicle may be an autonomous vehicle or may perform a flying function. The vehicle may be a vehicle that can travel without human intervention (for example, an automated guided vehicle (AGV) or an agricultural implement). 
     Though an embodiment of the present disclosure has been described, it should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.