Electric power storage system

The electric power storage device of an electric power storage system includes plural electric power storage bodies and a switching relay. The switching relay is capable of being switched between a first state where the plural electric power storage bodies are connected in series, and a second state where the plural electric power storage bodies are connected in parallel. The switching relay allows each of the plural electric power storage bodies to be electrically disconnected from the rest of the electric power storage bodies. The control unit controls the switching relay into the all-off state where the plural electric power storage bodies are electrically disconnected from each other, when the main relay is in an opened state.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2018-008334 filed on Jan. 22, 2018, incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to an electric power storage system capable of charging an electric power storage device mounted on a vehicle by using a power supply provided on the outside of the vehicle.

2. Description of Related Art

In Japanese Patent Application Publication No. 2013-81316 (JP 2013-81316 A), a vehicle that includes an electric power storage device configured to be charged by receiving electric power supplied from a power supply on the outside of the vehicle has been disclosed. The electric power storage device provided in the vehicle includes: plural battery packs; and a relay capable of being switched to ore of a fa it state where the plural battery packs are connected in series and a second state v the plural battery packs are connected in parallel. The state of the relay is switched on the basis of a temperature, a state of charge (SOC) or the like of the electric power storage device.

SUMMARY

Upon charging of the electric power storage device that is mounted on an electrically-driven vehicle (hereinafter also simply referred to as the “vehicle”) such as an electric vehicle or a plug-in hybrid vehicle (hereinafter also referred to as “charging of the vehicle”), there is a case where a voltage diagnosis is made to diagnose whether a voltage between terminals is abnormal before initiation of charging.

In the vehicle disclosed in JP 2013-81316 A, the state of the relay is switched to the first state or the second state in accordance with a state of the electric power storage device. Thus, the state of the relay at the time of the voltage diagnosis is not determined. The voltage between the terminals of the electric power storage device is higher in the first state of the relay than that in the second state. In other words, the voltage between the terminals of the electric power storage device possibly varies in accordance with the state of the relay.

Accordingly, it is considered that a voltage range within which the voltage is diagnosed as normal in the voltage diagnosis (hereinafter also referred to as a “normal range”) is set as a wide range that includes ranges of the voltage between the terminals in both of the first state and the second state. However, when the voltage range is set as the wide range, accuracy of the voltage diagnosis is possibly degraded.

The present disclosure improves accuracy of a voltage diagnosis that is made when an in-vehicle electric power storage device is charged by using a power supply on the outside of a vehicle.

An electric power storage system according to this disclosure includes: an electric power storage device configured to be charged by receiving electric power supplied from a power supply on the outside of a vehicle; a main relay provided between the electric power storage device and an electrical load of the vehicle; and a control unit that controls charging of the electric power storage device. The electric power storage device includes: plural electric power storage bodies; and a switching relay capable of being switched between a first state and a second state. The first state is a state where the plural electric power storage bodies are connected in series. The second state is a state where the plural electric power storage bodies are connected in parallel. The switching relay allows each of the plural electric power storage bodies to be electrically disconnected from the rest of the plural electric power storage bodies. The control unit brings the switching relay into an all-off state where the plural electric power storage bodies are electrically disconnected from each other when the main relay is in an opened state.

According to the above configuration, the plural electric power storage bodies are electrically disconnected from each other in the case where the main relay is in the opened state. When the main relay is in the opened state, the electric power cannot be supplied from the electric power storage device to the electrical load (a drive unit) of the vehicle. Thus, the vehicle is brought into a non-drivable state (hereinafter also referred to as a “READY-OFF state”). That is, in the READY-OFF state, the plural electric power storage bodies are electrically disconnected from each other. For this reason, a state of the switching relay before initiation of charging is set to the all-off state. Accordingly, before initiation of external charging to charge the electric power storage device by using the power supply on the outside of the vehicle, a voltage diagnosis can be made for each of the electric power storage bodies. That is to say, a voltage between terminals of each of the electric power storage body can be diagnosed. In addition, the diagnosed voltages do not include both the voltage between the terminals of the electric power storage body connected in series and the voltage between the terminals of the electric power storage body connected in parallel. Thus, a normal range can be set under the assumption that the voltage diagnosis is made for each of the electric power storage bodies. Just as described, the voltage diagnosis in the all-off state can be set as a precondition. Thus, accuracy of the voltage diagnosis can be improved. According to the present disclosure, it is possible to improve the accuracy of the voltage diagnosis at the time when the in-vehicle electric power storage device is charged by using the power supply on the outside of the vehicle.

The control unit may further bring the switching relay into the all-off state in the case where the main relay is in a closed state and a preparation operation is performed to charge the electric power storage device.

According to the above configuration, even when the main relay is in the closed state, the plural electric power storage bodies are disconnected from each other with the preparation operation for charging the electric power storage device being a trigger. Also, in this case, the accuracy of the voltage diagnosis can be improved as described above.

The electric power storage system may further include plural voltage sensors, each of which defects a voltage corresponding to one of the plural electric power storage bodies. The control unit may permit the electric power storage device to be charged by using the power supply in a case where, each of the voltages of the plural electric power storage bodies falls within a specified range when the switching relay is in the all-off state.

According to the above configuration, the voltage diagnosis is made in the all-off state. Thus, the specified range (the normal range) can be set under the assumption that the voltage diagnosis is made for each of the electric power storage bodies. In this way, an abnormal electric power storage body whose voltage between the terminals is out of the normal range can appropriately be detected. In addition, charging of the vehicle can be avoided when the abnormal electric power storage body is present.

The control unit may learn the voltages of the plural electric power storage bodies in the all-off state of the switching relay. The specified range may be defined by the learned voltages.

According to the above configuration, the specified range is defined by the voltage between the terminals of each of the plural electric power storage bodies that is learned when the switching relay is in the all-off state. Each of the learned voltages is not mixed with the voltage between the terminals of corresponding ones of the electric power storage bodies in another state of the switching relay. Thus, the voltages of the plural electric power storage bodies can be learned accurately. Therefore, the specified range can be set accurately.

The electric power storage system may further include plural voltage sensors, each of which detects a voltage corresponding to one of the plural electric power storage bodies. When, of the plural electric power storage bodies, an abnormal electric power storage body whose voltage does not fall within a specified range in the all-off state of the switching relay is present, the control unit may control the switching relay such that the abnormal electric power storage body is disconnected from the power supply, and may permit the electric power storage device to be charged by using the power supply.

According to the above configuration, in the case where, of the plural electric power storage bodies, the abnormal electric power storage body is present, the external charging is permitted in the state where the abnormal electric power storage body is disconnected from the power supply. In this way, even when of the plural electric power storage bodies, the abnormal electric power storage body is present, the external charging is not prohibited, and only the abnormal electric power storage body is disconnected to permit the external charging of the other electric power storage bodies.

When initiating charging of the electric power storage device by using the power supply, the control unit may bring the switching relay into the first state.

According to the above configuration, at the initiation of charging of the electric power storage device, the switching relay is brought into the first state. That is, the plural electric power storage bodies are connected in series. In the case where the plural electric power storage bodies are connected in series, the voltage between the terminals of the electric power storage device becomes higher than that in the case where the plural electric power storage bodies are connected in parallel. Accordingly, in the case where the vehicle is charged by supplying the same charging electric power in the state where the plural electric power storage bodies are connected in series, a charging voltage becomes higher than that in the case where the vehicle is charged in the state where the plural electric power storage bodies are connected in parallel. Thus, a magnitude of a flowing current (a charging current) can be reduced. Therefore, charging efficiency can be improved by suppressing loss of the electric power that is resulted from heat generation by a cable, a component, or the like (hereinafter also referred to as a “current-carrying component”) through which the flowing current flows during charging.

The electric power storage bodies may be secondary batteries. When, of the plural electric power storage bodies, the electric power storage body whose electric power storage amount is smaller than a prescribed value is present at the initiation of charging of the electric power storage device by using the power supply, the control unit may control the switching relay such that the electric power storage body whose electric power storage amount is smaller than the prescribed value is connected to the power supply and that the electric power storage body whose electric power storage amount is equal to or larger than the prescribed value is disconnected from the power supply.

When charging efficiency of a secondary battery (hereinafter also simply referred to as a “battery”) is considered, it has been known that the charging efficiency thereof is increased as the electric power storage amount thereof is reduced. According to the above configuration, in the case where the battery whose electric power storage amount is smaller than the prescribed value is present, such a battery is connected to the power supply, and the battery whose electric power storage amount is equal to or larger than the prescribed value is disconnected from the power supply. In this way, the battery that can be charged at the high charging efficiency can be charged preferentially, and thus the charging efficiency can be improved. As the prescribed value, a value of the electric power storage amount with which the battery can be charged at the charging efficiency that is equal to or higher than certain charging efficiency.

DETAILED DESCRIPTION

A detailed description will hereinafter be made on this embodiment with reference to the drawings. Note that the same or corresponding portions in the drawings are denoted by the same reference numerals and the description thereon will not be repeated.

FIG. 1is an overall configuration diagram if a charging system that includes: a vehicle1on which an electric over storage system according to this embodiment is mounted; and a DC charging facility200. With reference toFIG. 1, the vehicle1includes an electric power storage device10, a power control unit (hereinafter also referred to as a “PCU”)40a power output device50, drive wheels60, an auxiliary machine load70, an inlet90, an electronic control unit (ECU)100, a main relay device20, a charging relay device30, and a monitoring unit80.

The electric power storage device10includes two battery packs11,12and switching relays R1, R2, R3. In the battery pack11, plural batteries are stacked. Each of the batteries is a rechargeable DC power supply and is a secondary battery such as a nickel-metal hydrogen battery or a lithium-ion battery. Alternatively, each of the batteries may be a solid-state battery that includes solid electrolytes between a positive electrode and a negative electrode. The battery pack11stores electric power that is supplied from the DC charging facility200and received from the inlet90, and also stores electric power generated by the power output device50. The same configuration as the battery pack11is applied to the battery pack12. Note that, in this embodiment, a description will be made on the example in which the electric power storage device10includes the two battery packs11,12; however, the number of the battery packs provided in the electric power storage device10is not limited to two. The number of the battery packs provided in the electric power storage device10may be three or more. In addition, each of the battery packs is not limited to the battery pack in which the plural batteries are stacked, and each of the battery packs may be configured to include the single battery. Furthermore, a capacitor of large capacitance can also be adopted for the battery packs11,12.

Each of the switching relays R1, R2, R3can independently control opened/closed states thereof. In this embodiment, each of the switching relays R1, R2, R3can be switched to any one of a first state, a second state, an all-off state, and a one-off state. The first state is a state where the two battery packs11,12are connected in series. The second state is a state where the two battery packs11,12are connected in parallel. The all-off state is a state where the two battery packs11,12are electrically disconnected from each other. The one-off state is a state where both ends of one of the battery packs11,12are electrically connected to the main relay device20but both ends of the other of tile battery packs11,12are electrically disconnected from the main relay device20.

The switching relay R2is provided between a main relay21of the main relay device20and a positive electrode terminal of the battery pack11. The switching relay R3is provided between a main relay22of the main relay device20and a negative electrode terminal of the battery pack12. The switching relay R1is provided between a node N1and a node N2. The node N1is provided between the switching relay R2and the positive electrode terminal of the battery pack11. The node N2is provided between the switching relay R3and the negative electrode terminal of the battery pack12. The opened/closed state of each of the switching relays R1, R2, R3in each of the first state, the second state, the all-off state, and the one-off state will be described later. Here, a transistor such as an insulated-gate bipolar transistor (IGBT) or a metal-oxide-semiconductor field-effect transistor (MOSFET), a mechanical relay, or the like is used for each of the switching relays R1, R2, R3.

Electric power converters used to receive electric power from the electric power storage device10and drive the power output device50are collectively illustrated as the PCU40. For example, the PCU40includes: an inverter that drives a motor provided in the power output device50; a converter that boosts the electric power output from the electric power storage device10and supplies the boosted electric power to the inverter: and the like.

Devices used to drive the drive wheels60are collectively illustrated as the power output device50. For example, the power output device50includes the motor, an engine, and the like that drive the drive wheels60. In addition, when the motor that drives the drive wheels60is operated in a regenerative mode, the power output device50generates the electric power during braking of the vehicle and outputs the generated electric power to the PCU40. In the following description, the PCU40, the power output device50, and the drive wheels60will also collectively be referred to as a “drive unit”. The drive unit is an electrical load of the vehicle1.

The auxiliary machine load70is connected to a positive electrode line PL and a negative electrode line NL, each of which is connected to the electric power storage device10. Auxiliary machines that are operated during external charging are collectively illustrated as the auxiliary machine toad70. For example, the auxiliary machine load70includes a DC/DC converter that lowers a voltage of the positive electrode line PL and generates an auxiliary machine voltage; an electric air conditioner; and the like.

The inlet90can be connected to a charging connector300of the DC charging facility200that supplies DC power to the vehicle1. During DC charging, the inlet90receives the electric power supplied from the DC charging facility200.

The main relay device20is provided between the electric power storage device10and the drive unit. The main relay device20includes the main relay21and the main relay22. The main relay21and the main relay22are connected to the positive electrode line PL and the negative electrode NL, respectively.

When both of the main relays21,22are in an opened state, the electric power cannot be supplied from the electric power storage device10to the drive unit. Thus, the vehicle1is brought into a READY-OFF state where the vehicle1cannot travel. Meanwhile, when both of the main relays21,22are in a closed state, the electric power can be supplied from the electric power storage device10to the drive unit. Thus, the vehicle1is brought into a READY-ON state where the vehicle1can travel.

The charging relay device30is connected between the main relay device20and the drive unit. The charging relay device30includes a charging relay31and a charging relay32. One end of the charging relay31is connected to the positive electrode line PL, and the other end thereof is connected to the inlet90. One end of the charging relay32is connected to the negative electrode line NL, and the other thereof is connected to the inlet90. Both of the charging relays31,32are brought into a closed state when the vehicle1is charged by using the DC charging facility200.

When the main relays21,22are brought into the closed state and the charging relays31,32are brought into the closed state, a state where the electric power storage device10can be charged by using the DC charging facility200(hereinafter also referred to as an “external charging state”) is realized.

Note that, in this embodiment, the charging relay device30is connected between the main relay device20and the drive unit; however, the charging relay device30may be connected between the electric power storage device10and the main relay device20.

The monitoring unit80includes voltage sensors81,82,83and current sensors84,85,86. The voltage sensor81detects a voltage VB between terminals of the electric power storage device10and outputs a detection value to the ECU100. The voltage sensor82detects a voltage V11between terminals of the battery pack11and outputs a detection value to the ECU100. The voltage sensor83detects a voltage V12between terminals of the battery pack12and outputs a detection value to the ECU100.

The current sensor84detects a current113flowing through the electric power storage device10and outputs a detection value to the ECU100. More specifically, the current sensor84detects: a charging current that is supplied from the DC charging facility200to charge the electric power storage device10; and a discharging current that is supplied from the electric power storage device10to the drive unit and the like. The current sensor85detects a current111flowing through the battery pack11and outputs a detection value to the ECU100. The current sensor86detects a current112flowing through the battery pack12and outputs a detection value to the ECU100.

The ECU100includes a central processing unit (CPU), memory, and input/output buffer, none of which are shown, receives a signal from each of the sensors, outputs a control signal to each device, and controls each of the devices. Note that these types of the control are not only processed by software but can also be processed by building dedicated hardware (an electronic circuit).

More specifically, the ECU100controls charging of the electric power storage device10. The ECU100switches the state of each of the switching relays R1, R2, R3to one of the first state, the second state, the all-off state, and the one-off state by controlling opening/closing of the switching relays R1, R2, R3provided in the electric power storage device10. In addition, the ECU100controls opening/closing of the main relays21,22provided in the main relay device20. Furthermore, the ECU100controls opening/closing of the charging relays31,32provided in the charging relay device30.

The ECU100performs predetermined calculation by using the detection values acquired from the monitoring unit80and executes various types of processing. More specifically, for example, the ECU100learns the voltages V11, V12between the terminals of the battery packs11,12in the all-off state of the switching relays R1, R2, R3, and makes a voltage diagnosis, which will be described below, of the battery packs11,12by using the learned values. In addition, the ECU100stores the detection values acquired from the monitoring unit80. Note that the ECU100according to this embodiment corresponds to one example of the “control unit” according to the present disclosure.

The DC charging facility200supplies charging electric power (DC) to the vehicle1via the charging connector300. The DC charging facility200according to this embodiment can change a magnitude of a supply voltage (a charging voltage) when supplying the same charging electric power. For example, when supplying the same charging electric power, the DC charging facility200can supply the charging electric power at a high voltage (for example, 800 V) or can supply the charging electric power at a low voltage (for example, 400 V) in accordance with a request from the vehicle1.

More specifically, as one example, the DC charging facility200supplies the charging electric power of 160 kW. In such a case, the DC charging facility200supplies the charging electric power of 800 V-200 A when the electric power storage device10of the vehicle1can be charged at 800 V. The DC charging facility200supplies the charging electric power of 400 V-400 A when the electric power storage device10of the vehicle1can be charged at 400 V. In a specification of the DC charging facility200in the above example, the maximum charging electric power is 160 kW, the maximum charging voltage is 800 V, and the maximum charging current is 400 A.

In this embodiment, as one example, the DC charging facility200performs the DC charging by supplying the charging electric power (for example, of 160 kW) equal to or higher than certain electric power. However, the DC charging facility200can supply any of various magnitudes of the charging electric power. Note that the DC charging performed by supplying the charging electric power equal to or higher than the certain electric power will also be referred to as “rapid charging”.

FIG. 2is a schematic diagram of the electric power storage device10at the time when the switching relays R1, R2, R3are in the second state. As shown inFIG. 2, the battery pack11and the battery pack12are connected in parallel when the switching relay R1is brought into the opened state, and the switching relays R2, R3are brought into the closed state.

FIG. 3is a schematic diagram of the electric power storage device10at the time when the switching relays R1, R2, R3are in the first state. As shown inFIG. 3, the battery pack11and the battery pack12are connected in series when the switching relay R1is brought into the closed state, and the switching relays R2, R3are brought into the opened state.

FIG. 4is a schematic diagram of the electric power storage device10at the time when the switching relays R1, R2, R3are in the all-off state. As shown inFIG. 4, the battery pack11and the battery pack12are electrically disconnected from each other when the switching relays R1, R2, R3are brought into the opened state.

FIG. 5is a schematic diagram of the electric power storage device10at the time when the switching relays R1, R2, R3are in the one-off state. As one example,FIG. 5shows the one-off state in the case where both of the ends of the battery pack11are electrically disconnected from main relay device20and both of the ends of the battery pack12are electrically connected to the main relay device20. As shown inFIG. 5, only both of the ends of the battery pack12are electrically connected to the main relay device20when the switching relays R1, R2are brought into the opened state, and the switching relay R3is brought into the closed state.

Although not shown, in the one-off state where both of the ends of the battery pack12are electrically disconnected from the main relay device20and both of the ends of the battery pack11are electrically connected to the main relay device20, only both of the ends of the battery pack11are electrically connected to the main relay device20when the switching relays R1, R3are brought into the opened state, and the switching relay R2is brought into the closed state.

Upon charging of the vehicle1, there is a case where the voltage diagnosis is made to diagnose whether the voltage between the terminals of the electric power storage device10is abnormal before the initiation of charging.

In the case where the state of none of the switching relays R1R2, R3is determined during the voltage diagnosis, the voltage between the terminals of the electric power storage device10possibly contains the voltage between the terminals in the plural states of the switching relays R1, R2, R3. As a result, the voltage between the terminals of the electric power storage device10possibly varies.

Accordingly, it is considered that a voltage range (a normal range) within which the voltage is diagnosed as normal in the voltage diagnosis is set as a wide range that includes voltage ranges of the voltage between the terminals in the plural states of the switching relays R1, R2, R3. However, when the voltage range is set as the wide range, itis anticipated that accuracy of the voltage diagnosis is degraded.

In view of the above, when both of the main relays21,22are in the opened state, the electric power storage system according to this embodiment brings the switching relays R1, R2, R3into the all-off state. In this way, in the READY-OFF state, the switching relays R1, R2, R3are fixed in the all off state. Thus, when the voltage diagnosis is made in the READY-OFF state, the state of each of the switching relays R1, R2, R3is fixed to the all-off state. Accordingly, the voltage diagnosis can be made to diagnose the voltage between the terminals of each of the battery packs11,12, and the normal range can be set under the assumption that the voltage diagnosis is made for each of the battery packs11,12. Just as described, the voltage diagnosis in the all-off state can be set as a precondition. Thus, compared to the case where the state of none of the switching relays R1, R2, R3is determined upon the voltage diagnosis, the accuracy of the voltage diagnosis can be improved.

In addition, since the voltage diagnosis is made for each of the battery packs11,,12, it is possible to identify an abnormal battery whose voltage between the terminals is out of the normal range. Furthermore, the switching relays R1, R2, R3can be brought into the all-off state. Thus, when the abnormal battery is present, the battery pack can be replaced per pack unit.

When charging of the vehicle1is initiated, the switching relays R1, R2, R3are brought into the first state (the battery packs11,12are connected in series). In the case where the battery packs11,12are connected in series, the voltage VB between the terminals of the electric power storage device10becomes higher than that in the case where the battery packs11,12are connected in parallel. Accordingly, in the case where the vehicle1is charged by supplying the same charging electric power in the state where the battery packs11,12are connected in series, the charging voltage becomes higher than that in the case where the vehicle1is charged in the state where the battery packs11,12are connected in parallel. Thus, a magnitude of a flowing current can be reduced. As a result, when the vehicle1is charged in the first state, a current-carrying component of the vehicle1generates less heat than the case where the vehicle1is charged in the second state. Therefore, charging efficiency can be improved by suppressing loss of the electric power that is resulted from the heat generation by the current-carrying to component.

FIG. 6is a timing chart that shows a change in the voltage VB between the terminals of the electric power storage device10. InFIG. 6, a horizontal axis represents time, and a vertical axis represents the voltage. The “second state” indicated in the vertical axis represents a voltage VB2between the terminals of the electric power storage device10at the time when the battery packs11,12are connected in parallel. The “first state” represents a voltage VB1between the terminals of the electric power storage device10at the time when the battery packs11,12are connected in series. The “all off state” represents a voltage VB0between the terminals of the electric power storage, device10at the time when the battery packs are disconnected from each other.

In an example shown inFIG. 6, the vehicle1is used (the main relays21,22are in the closed state) until time T1. At the time T1, a use end operation of the vehicle1is performed. The use end operation is an operation that is performed for the vehicle1by a user in order to switch the vehicle1from the READY-ON state to the READY-OFF state.

In conjunction with the use end operation performed at the time T1the main relays21,22are brought into the opened state at time T2, and the switching relays R1, R2, R3are switched from the second state to the all-off state.

Just as described, when the main relays21,22are brought into the opened state, that is, the vehicle1is brought into the READY-OFF state in conjunction with the use end operation, the switching relays R1, R2, R3are brought into the all-off state.

At time T3, a plug-in operation is performed to connect the charging connector300of the DC charging facility200to the inlet90. Note that the plug-in operation according to this embodiment corresponds to one example of the “preparation operation for charging” according to the present disclosure. Other examples of the preparation operation for charging are an operation to open a charging lid (not shown) that covers the inlet90and an operation to unlock the charging lid when the charging lid has a lock function.

When the plug-in operation is performed, in a period between the time T3and time T4, initial checking and diagnostic processing of the electric power storage device10are executed. The initial checking is processing to check whether charging can be performed normally. The diagnostic processing is processing to diagnose whether the electric power storage device10is normal. In this embodiment, as the diagnostic processing, the voltage diagnosis and a discharge diagnosis are made.

Since the state of each of the switching relays R1, R2, R3is fixed to the all-off state in the READY-OFF state, the diagnostic processing of the electric power storage device10is executed in the all-off state. Thus, the diagnostic processing is executed for each of the battery packs11,12. Just as described, the battery packs11,12are set in advance to be electrically disconnected from each other during the diagnostic processing. Thus, the diagnostic processing can be executed for each of the battery packs11,12. A detailed description on the initial checking and the diagnostic processing will be made later.

In conjunction with completion of the initial checking and the diagnostic processing of the electric power storage device10at the time T4, at time T5, the switching relays R1, R2, R3are switched from the all-off state to the first state, and charging of the vehicle1is initiated.

The voltage VB1between the terminals of the electric power storage device10in the first state is higher than the voltage VB2between the terminals of the electric power storage device10in the second state (VB1>VB2). Accordingly, in the case where the vehicle1is rapidly charged by supplying the same charging electric power, the charging voltage for rapid charging in the first state is higher than the charging voltage for rapid charging in the second state. Thus, the magnitude of the flowing current can be reduced. Therefore, the charging efficiency can be improved by suppressing the loss of the electric power that is resulted from the heat generation by the current-carrying component.

FIG. 7is a flowchart of processing that is executed by the ECU100in a period from use of the vehicle1to the use end of the vehicle1. This processing is repeatedly executed by the ECU100when the vehicle1is switched from the READY-OFF state to the READY-ON state. Note that each step in the flowchart shown inFIG. 7is realized when the ECU100processes software; however, some of the steps may be realized by the hardware (the electronic circuit) that is built in the ECU100. The same applies toFIG. 8,FIG. 10, andFIG. 12.

The ECU100initiates the processing when the vehicle1is switched from the READY-OFF state to the READY-ON state. The ECU100determines whether the use end operation of the vehicle1, specifically, an operation to switch the vehicle1to the READY-OFF state has been operated (step100, hereinafter step will be abbreviated as “S”).

If the ECU100determines that the use end operation has not been performed (NO in S100), the processing returns.

On the other hand, if the ECU100determines that the use end operation has been performed (YES in S100), the ECU100brings the main relays21,22into the opened state (S110). When the main relays21,22are brought into the opened state, the electric power can no longer be supplied from the electric power storage device10to the drive unit, Thus, the vehicle1is brought into the READY-OFF state.

Next, the ECU100brings the switching relays R1, R2, R3into the all-off state (S120). In this way, in the READY-OFF state, the switching relays R1, R2, R3are brought into the all-off state. Thus, in the case where the plug-in operation is performed in the READY-OFF state of the vehicle1, the switching relays1, R2, R3are in the all-off state during the initial checking and the diagnostic processing, each of which is executed before charging of the vehicle1is initiated.

The ECU100acquires and stores the voltage between the terminals (hereinafter also referred to as an “end voltage”) of each of the battery packs11,12at the use end of the vehicle1(S130). The end voltage is used to diagnose a self-discharge amount of each of the battery packs11,12, which will be described later.

FIG. 8is a flowchart of processing that is executed by the ECU100when the plug-in operation is performed. This processing is executed every time the plug-in operation is performed. Note that the following description ofFIG. 8will be made on an example in which the plug-in operation is performed in the READY-OFF state of the vehicle1.

When the plug-in operation, specifically, when the charging connector300of the DC charging facility200is connected to the inlet90of the vehicle1, the ECU100executes the initial checking (S200). For example, the initial checking includes contact checking between the inlet90and the charging connector300, self-checking of whether there is no electric failure in the vehicle1, and the like.

If the ECU100determines that there is no abnormality in the initial checking (YES in S200), the ECU100makes the voltage diagnosis to diagnose the voltages V11, V12between the terminals of the battery packs11,12. As the voltage diagnosis, the ECU100acquires the voltages V11, V12between the terminals of the battery packs11,12from the monitoring unit80and determines whether each of the voltages V11, V12between the terminals falls within a specified range (S205). In the following description, the voltages V11, V12between the terminals of the battery packs11,12that are acquired in S205will collectively be referred to as “initial voltages”. In this embodiment, in the READY-OFF state, the switching relays R1, R2, R3are in the all-off state. Thus, the switching relays R1, R2, R3at the time of acquiring the initial voltages are in the all-off state. Since the state of each of the switching relays is set to the all-off state in advance, an appropriate specified range can be set with the assumption of the all-off state.

The ECU100acquires and stores values of each of the initial voltages (hereinafter also referred to as “learning”) and defines the specified range by using an stored value of each of the initial voltages. The specified range in S205is defined by the initial voltages that have been learned in the processing up to the last processing. In addition, the current initial voltages learned by the ECU100in S210, which will be described below, are reflected to the specified range used in S205of the next processing. As a specific example of defining the specified range, the specified range is defined as follows. The ECU100learns the initial voltage of each of the battery packs11,12per SOC at the time of acquiring the initial voltage. Then, the ECU100determines a certain range by using an average value of each of the learned initial voltages per the SOC and sets the certain range from the average value as the specified range. When the specified range is set by learning, just as described, an accurate range for each of the battery packs11,12can be set as the specified range by considering an individual characteristic of each of the battery packs11,12. Note that a certain range from a fixed value per the SOC may be set as the specified range.

If the ECU100determines that each of the initial voltages falls within the specified range (YES in S205), the ECU100learns the initial voltages (S210).

Next, the ECU100reads the end voltages that are stored at the use end of the vehicle1(S215) and makes the discharge diagnosis to diagnose the self-discharge amount of each of the battery packs11,12(S220). The discharge diagnosis is processing to diagnose the self-discharge amount of each of the battery packs11,12in a resting period from the use end of the vehicle1to time at which charging of the vehicle1is initiated. As a specific example, the resting period is a period from the time T2to the time T3inFIG. 6.

In S220, as the discharge diagnosis, the ECU100determines whether a voltage drop amount per unit time by self-discharge is equal to or smaller than a a reference value. The voltage drop amount is expressed by the following formula (1), for example.
Voltage drop amount=(end voltage−initial voltage)/resting period   (1)

If the voltage drop amount is equal to or smaller than the reference value (YES in S220), the ECU100communicates with the DC charging facility200and acquires the specification of the DC charging facility200(S230). In this embodiment, as the specification of the DC charging facility200, the maximum charging voltage that can be supplied by the DC charging facility200is acquired.

The ECU100determines whether the maximum charging voltage of the DC charging facility200is equal to or higher than a threshold (S235). The threshold is an arbitrarily set value between the voltage VB between the terminals of the electric power storage device10at the time when the battery packs11,12are connected in series and the voltage VB between the terminals of the electric power storage device10at the time when the battery packs11,12are connected in parallel. That is, the processing in S235is executed to determine whether the maximum charging voltage of the DC charging facility200corresponds to a voltage at which the electric power storage device10can be charged in a state of the in-series connection of the battery packs11,12.

If the ECU100determines that the maximum charging voltage of the DC charging facility200is higher than the threshold (YES in S235), the ECU100switches the switching relays R1, R2, R3to the first state (connects the battery packs11,12in series) (S240), and the processing proceeds to S250.

If the ECU100determines that the maximum charging voltage of the DC charging facility200is equal to or lower than the threshold (NO in S235), the ECU100switches the switching relays R1, R2, R3to the second state (connects the battery packs11,12in parallel) (S245), and the processing proceeds to S250.

In S250, the ECU100brings the main relays21,22and the charging relays31,32into the closed state. Then, the ECU100initiates charging of the vehicle1(S255).

If the ECU100determines that there is an abnormality in the initial checking (NO in S200), if the ECU100determines that each of the initial voltages does not fall within the specified range (NO in S205), or if the ECU100determines that the voltage drop amount is not equal to or smaller than the reference value (NO in S220), the ECU100executes error processing (S225). For example, as the error processing, the ECU100executes processing to prohibit charging of the vehicle1.

In addition, as the error processing that is executed when the ECU100determines that either one of the initial voltages does not fall within the specified range (NO in S205) or when the ECU100determines that the voltage drop amount is not equal to or smaller than the reference value (NO in S220), the ECU100may control the switching relays R1, R2, R3so as to electrically disconnect the determined battery pack from the main relay device20. For example, if the ECU100determines that the initial voltage of the battery pack11does not fall within the specified range (NO in S205), the ECU100brings the switching relays R1, R2into the opened state and brings the switching relay R3into the closed state. In this way, the battery pack11is electrically disconnected from the main relay device20. As for the battery pack12whose initial voltage fails within the normal range, the subsequent processing is executed to allow charging of the vehicle1.

As it has been described so far, in the case where the main relays21,22are in the opened state, the electric power storage system according to this embodiment brings the switching relays R1, R2, R3into the all-off state. In this way, in the READY-OFF state, the switching relays R1, R2, R3are fixed in the all-off state. Accordingly, the state of each of the switching relays R1, R2, R3in the diagnostic processing, which is executed before charging of the vehicle1is initiated, is fixed to the all-off state. Thus, the voltage diagnosis can be made to diagnose the voltage between the terminals of each of the battery packs11,12, and the normal range can be set under the assumption that the voltage diagnosis is made for each of the battery packs11,12. Just as described, the voltage diagnosis in the all-off state can be set as the precondition. Thus, compared to the case where the state of none of the switching relays R1, R2, R3is determined in the voltage diagnosis, the accuracy of the voltage diagnosis can be improved.

In addition, since the voltage diagnosis is made for each of the battery packs11,12, it is possible to identify the abnormal battery whose voltage between the terminals is out of the normal range. Furthermore, the switching relays R1, R2, R3can be brought into the all-off state. Thus, when the abnormal battery is present, the battery pack can be replaced per pack unit.

In addition, when charging of the vehicle1is initiated, the state of each of the switching relays R1, R2, R3is brought into the first state. In the case where the battery packs11,12are connected in series, the voltage VB between the terminals of the electric power storage device10becomes higher than that in the case where the battery packs11,12are connected in parallel. Accordingly, in the case where the vehicle1is charged by supplying the same charging electric power in the state where the battery packs11,12are connected in series, the charging voltage becomes higher than that in the case where the vehicle1is charged in the state where the battery packs11,12are connected in parallel. Thus, the magnitude of the flowing current can be reduced. As a result, when the vehicle1is charged in the first state, the current-carrying component of the vehicle1generates less heat than the case where the vehicle1is charged in the second state. Therefore, charging efficiency can be improved by suppressing the loss of the electric power that is resulted from the heat generation by the current-carrying component,

First Modified Example

In the above embodiment, an electric power storage amount such as the SOC of each of the battery packs11,12before the initiation of charging is not considered, and the states of the switching relays R1, R2, R3at the initiation of charging are determined on the basis of a relationship with the maximum charging voltage of the DC charging facility200(more specifically, S235to S245inFIG. 8). However, the states of the switching relays R1, R2, R3may be determined in consideration of the electric power storage amount such as the SOC of each of the battery packs11,12. In this way, as will be described below, the vehicle1can be charged at the high charging efficiency.

FIG. 9includes graphs indicative of a relationship between the SOC of each of the battery packs and an allowable charging current. InFIG. 9, a horizontal, axis of art upper graph represents the SOC of the battery packs, and a vertical axis represents the allowable charging current.

The graph in an upper section ofFIG. 9indicates that the large charging current is allowed in a low SOC region A1and the small charging current is allowed in a high SOC region A2. Thus, in the case where a battery pack whose SOC is located in the region A1is present, it is considered to switch the states of the switching relays R1, R2, R3at the initiation of charging so as to allow such a battery pack to be charged preferentially. That is, the states of the switching relays R1, R2, R3at the initiation of charging are switched in accordance with the SOC of each of the battery packs11,12.

In First Modified Example, a prescribed value is set to determine whether the batteries can be charged at the charging efficiency that is equal to or higher than a certain degree of the charging, efficiency. More specifically, when the SOC is lower than the prescribed value, the batteries are charged at the charging efficiency in the region A1(at the high charging efficiency). On the other hand, when the SOC is equal to or higher than the prescribed value, the batteries are charged at the charging efficiency in the region A2(at the low charging efficiency). The prescribed value is a value that is defined experimentally.

For example, as shown in the left graph in a lower section ofFIG. 9, a situation where the battery pack11is fully charged and the SOC of the battery pack12is lower than the prescribed value is assumed. One example of such a situation is a case where the vehicle1travels for a short distance by disconnecting the battery pack11as one of the battery packs and using the battery pack12only in consideration of the later charging efficiency. In this case, the battery pack12can be charged at the charging efficiency in the region A1. Accordingly, the switching relays R1, R2, R3are brought into the one-off state where both of the ends of the battery pack11are electrically disconnected from the main relay device20and both of the ends of the battery pack12are electrically connected to the main relay device20, and only the battery pack12is charged. In this way, the charging efficiency can be improved.

Meanwhile, as shown in the right graph in the lower section ofFIG. 9, in the case where both of the battery packs11,12are used in balance and both of the battery packs11,12can be charged at the charging efficiency in the region A2, it is assumed that the charging efficiency can be improved by charging both of the battery packs11,12.

Thus, in First Modified Example, the states of the switching relays R1, R2, R3are switched in accordance with the SOC of each of the battery packs11,12at the initiation of charging of the vehicle1.

FIG. 10is a flowchart of processing that is executed by the ECU100when the plug-in operation is performed. This processing is executed every time the plug-in operation is performed for the vehicle1. Because processing in S400to S430inFIG. 10is the same as that in S200to S230inFIG. 8, the description thereon will not be made.

The ECU100determines whether the SOC of each of the battery pack11and the battery pack12is lower than the prescribed value (S435). Note that the ECU100calculates the SOCs by using the voltages V11, V12between the terminals of the battery packs11,12, which are acquired from the monitoring unit80. More specifically, due to a correlation between the SOC and an open circuit voltage (OCV), the ECU100can calculate the SOC by using the OCV (a voltage with no load) of each of the battery packs11,12. Alternatively, the ECU100may calculate a SOC of the electric power storage device10by integrating a charge/discharge amount. A known technique only has to be used to calculate the SOC, and thus a detailed description will not be made on the calculation of the SOC.

If the ECU100determines that the SOCs of both of the battery packs11,12are equal to or higher than the prescribed value (NO in S435), the ECU100determines whether the maximum charging voltage of the DC charging facility200is higher than the threshold (S440).

If the ECU100determines that the maximum charging voltage of the DC charging facility200is higher than the threshold (YES in S440), the ECU100switches the switching relays R1, R2, R3to the first state (S445), and the processing proceeds to S480.

If the ECU100determines that the maximum charging voltage of the DC charging facility200is equal to or lower than the threshold (NO in S440), the ECU100switches the switching relays R1, R2, R3to the second state (S450), and the processing proceeds to S480.

If the ECU100determines that the SOC of at least one of the battery packs11,12is lower than the prescribed value (YES in S435), the ECU100determines whether the SOCs of both of the battery packs11,12are lower than the prescribed value (S455).

If the ECU100determines that the SOC of only one of the battery packs11,12is equal to or higher than the prescribed value (NO in S455), the ECU100brings the switching relays R1, R2, R3into the one-off state where both of the ends of the battery pack whose SOC is lower than the prescribed value are electrically connected to the main relay device20and both of the ends of the other battery pack are electrically disconnected from the main relay device20(S460). Then, the processing proceeds to S480.

If the ECU100determines that the SOCs of both of the battery packs12are lower than the prescribed value (YES in S455), the ECU100determines whether the maximum charging voltage of the DC charging facility200is higher than the threshold (S465).

If the ECU100determines that the maximum charging voltage of the DC charging facility200is higher than the threshold (YES in S465), the ECU100switches the switching relays R1, R2, R3to the first state (S470), and tine processing proceeds to S480.

If the ECU100determines that the maximum charging voltage of the DC charging facility200is equal to or lower than the threshold (NO in S465), the ECU100switches the switching relays R1, R2, R3to the second state (S475), and the processing proceeds to S480.

In S480, the ECU100brings the main relays21,22and the charging relays31,32into the closed state. Then, the ECU100initiates charging of the vehicle1(S490).

As it has been described so far, in First Modified Example, the states at the switching relays R1, R2, R3are switched in accordance with the SOC of each of the battery packs11,12at the initiation of charging. When the battery pack whose batteries can be charged at the charging efficiency equal to or higher than the certain degree of the charging efficiency is present, such a battery pack is charged preferentially. In this way, the charging efficiency of the electric power storage device10can be improved.

Second Modified Example

In the embodiment and First Modified Example, the description has been made on the example in which the plug-in operation is performed in the READY-OFF state. That is, the description has been made on the example in which the vehicle1is switched from the READY-OFF state to the external charging state by the plug-in operation. However, in the electrically-driven vehicle, the plug-in operation may be performed in the READY-ON state of the vehicle1. That is, the vehicle1may be switched front the READY-ON state to the external charging state by the plug-in operation. In Second Modified Example, a description will be made on an example in which the plug-in operation is performed in the READY-ON state.

FIG. 11is a timing chart that shows the change in the voltage V13between the terminals of the electric power storage device10. InFIG. 11, a horizontal axis represents the time, and a vertical axis represents the voltage. The “second state” indicated in the vertical axis represents the voltage VB between the terminals of the electric power storage device10at the time when the battery packs11,12are connected in parallel. The “first state” represents the voltage VB between the terminals of the electric power storage device10at the time when the battery packs11,12are connected in series. The “all-off state” represents the voltage VB between the terminals of the electric power storage device10at the time when the battery packs11,12are disconnected from each other.

In an example shown, inFIG. 11, the vehicle1is used (the main relays21,22are in the closed state) until time T10. At the time T10, the plug-in operation is performed. More specifically, for example, a case where the plug-in operation is performed at the time T10in a state where the vehicle1stops traveling and remains in the READY-ON state is assumed.

When the plug-in operation is performed at the time T10, with the plug-in operation being a trigger, the switching relays R1, R2, R3area switched from the second state to the all-off state at time T11. Just as described, also in the case where the plug-in operation is performed in the READY-ON state, the switching relays R1, R2, R3are switched to the all-off state. Note that, in the READY-ON state, the switching relays R1, R2, R3are appropriately switched from the first state or the second state depending on a travel situation of the vehicle1. Thus, in the READY-ON state, there is a case where the switching relays R1, R2, R3are in the first state when the plug-in operation is performed. In such a case, similarly, the switching relays R1, R2, R3are switched from the first state to the all-off state with the plug-in operation being the trigger.

Next, in a period between the time T11and time T12, the initial checking and the diagnostic processing of the electric power storage device10are executed. The diagnostic processing of the electric power storage device10is executed in the all-off state of the switching relays R1, R2, R3. Just as described, the battery packs11,12are set in advance to be electrically disconnected from each other during the diagnostic processing. Thus, the diagnostic processing can be executed for each of the battery packs11,12.

In conjunction with the completion of the initial checking and the diagnostic, processing of the electric power storage device10at the time T12, at time T13, the switching relays R1, R2, R3are switched from the all-off state to the first state, and charging of the vehicle1is initiated.

The voltage VB1between the terminals of the electric power storage device10in the first state is higher than the voltage VB2between the terminals of the electric power storage device10in the second state (VB1>VB2). Accordingly, in the case where the vehicle1is rapidly charged by supplying the same charging electric power, the charging voltage for rapid charging in the first state is higher than the charging voltage for rapid charging in the second state. Thus, the magnitude of the flowing current can be reduced. Therefore, the charging efficiency can be improved by suppressing the loss of the electric power that is resulted from the heat generation by the current-carrying component.

FIG. 12is a flowchart of processing that is executed by the ECU100when the plug-in operation is performed. This processing is executed every time the plug-in operation is performed for the vehicle1. Note that the following description ofFIG. 12will be made on an example in which the plug-in operation is performed in the READY-ON state of the vehicle1.

In the flowchart shown inFIG. 12, S300is added to the flowchart inFIGS. 8, and S215and S220are eliminated from the flowchart inFIG. 8. Since the rest of steps in the flowchart shown inFIG. 12are the same as those in the flowchart inFIG. 8, a description thereon will not be made.

Once the plug-in operation is performed, the ECU100brings the switching relays R1, R2, R3into the all-off state (S300). When the vehicle1is in the READY-ON state, the switching relays R1, R2, R3are either in the first state or in the second state. By the processing in S300, regardless of whether the switching relays R1, R2, R3are in the first state or the second state, the switching relays R1, R2, R3are switched to the all-off state with the plug-in operation being the trigger.

A reason why the discharge diagnosis is not made in Second Modified Example is that, in the case where the plug-in operation is performed in the READY-ON state, the use end operation is not performed, and thus the end voltages are not acquired. For this reason, the self-discharge amounts cannot be diagnosed.

As it has been described so far, in Second Modified Example, in the case where the plug-in operation is performed in the READY-ON state of the vehicle1, the switching relays R1, R2, R3are switched to the all-off state. In this way, when the initial checking and the diagnostic processing of the vehicle1are executed, the switching relays R1, R2, R3are fixed in the all-off state. Accordingly, the voltage diagnosis can be made to diagnose the voltage between the terminals of each of the battery packs11,12, and the normal range can be set under the assumption that the diagnostic processing is executed for each of the battery packs11,12. Just as described, the diagnostic processing in the all-off state can be set as the precondition. Thus, compared to the case where the state of none of the switching relays R1, R2, R3is determined in the diagnostic processing, accuracy of the diagnostic processing can be improved. In addition, since the diagnostic processing is made for each of the battery packs11,12, it is possible to identify the abnormal battery whose voltage between the terminals is out of the normal range. Furthermore, the switching relays R1, R2, R3can be brought into the all-off state. Thus, when the abnormal battery is present, the battery pack can be replaced per pack unit.

It should be understood that the embodiment disclosed herein is illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims rather than the description of the above embodiment, and intends to include all modifications falling within the claims and equivalents thereof.