DETERMINATION METHOD

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to an electrical storage system, a battery system and a failure determination method for a battery system, which are allowed to operate a plurality of relays together.

2. Description of Related Art

[0002] Japanese Patent Application Publication No. 2005-222871 (JP 2005-222871 A) describes a relay that operates a plurality of pairs of contacts with the use of a single drive mechanism (solenoid). Each pair of contacts is formed of a movable contact and a fixed contact. By switching between energized and non-energized states of the solenoid, each movable contact is able to contact a corresponding one of the fixed contacts or move away from a corresponding one of the fixed contacts in the plurality of pairs of contacts.

[0003] JP 2005-222871 A also describes a power supply circuit that connects a battery to an inverter and a motor. In this power supply circuit, a plus-side main relay and a minus-side main relay are used. These main relays are the above-described pairs of contacts.

SUMMARY OF THE INVENTION

[0004] In order to switch between the energized and non-energized states of the solenoid, generally, a switch element is used. Specifically, when the switch element is set to an energized state (on state), it is possible to cause current to flow through the solenoid. When the switch element is set to a non-energized state (off state), it is possible to interrupt energization of the solenoid.

[0005] Here, if the switch element remains in the energized state because of a failure of the switch element, current continues flowing through the solenoid. If current continues flowing through the solenoid, each movable contact continues contacting a corresponding one of the fixed contacts in the plurality of pairs of contacts. In the power supply circuit described in JP 2005-222871 A, the plus-side main relay and the minus-side main relay remain in the energized state. Therefore, the battery remains connected to the inverter and the motor. In other words, it is not possible to interrupt connection of the battery with the inverter. If such a state occurs, for example, there is a concern that electric power continues being supplied from the inverter to the battery and then the battery becomes an overcharged state.

[0006] On the other hand, when the switch element remains in the energized state because of a failure of the switch element, it is also required to determine the failure.

[0007] A first aspect of the invention provides an electrical storage system. The electrical storage system includes a first relay, a second relay, a drive circuit configured to drive the first relay and the second relay, and a controller configured to control operation of the drive circuit. The first relay is provided in a positive electrode line that connects a positive electrode terminal of an electrical storage device to a load. The second relay is provided in a negative electrode line that connects a negative electrode tenninal of the electrical storage device to the load. The first relay and the second relay are configured to be mechanically interlocked with each other. The drive circuit is configured to operate the first relay and the second relay.

[0008] The drive circuit includes a coil, a plurality of switch elements, at least one sensor and a controller. The coil is configured to generate electromagnetic force upon reception of electric power supplied from a power supply. The first relay and the second relay are configured to be switched from a non-energized state to an energized state by the electromagnetic force. The plurality of switch elements are provided in a current path between the power supply and the coil and connected in series with each other. The at least one sensor is configured to change an output signal in response to an energized state or non-energized state of each of the plurality of switch elements. The controller is configured to control operation of the drive circuit. The controller is configured to output a control signal for setting one of the switch elements to the non-energized state and determine whether the one of the switch elements is in the energized state on the basis of the output signal of the at least one sensor.

[0009] In the above aspect, the plurality of switch elements are provided in the current path between the power supply and the coil. Therefore, even when part of the switch elements fails and remains in the energized state, it is possible to switch the remainder of the switch elements (that is, non-failed switch elements) between the energized state and the non-energized state. Thus, it is possible to supply electric power to the coil or interrupt supply of electric power to the coil.

[0010] When electric power is continuously supplied to the coil, the first relay and the second relay remain in the energized state, with the result that the electrical storage device and the load remain connected to each other. In the above aspect, as described above, it is possible to interrupt supply of electric power to the coil with the use of the non-failed switch elements. Therefore, it is possible to prevent the first relay and the second relay from remaining in the energized state. Here, when the load supplies electric power to the electrical storage device, it is possible to prevent an overcharged state of the electrical storage device by preventing the electrical storage device and the load from remaining connected to each other. When the control signal for setting one of the switch elements is output and it is checked whether the one of the switch elements is in the energized state, it is possible to determine that the one of the switch elements has a failure in the energized state.

[0011] When it is determined whether one of the switch elements has a failure, the control signal for setting the one of the switch elements to the non-energized state is output. Therefore, when the one of the switch elements has no failure, no electric power is supplied to the coil. Thus, it is possible to determine whether one of the switch elements has a failure without operating the first relay or the second relay. Here, it is possible to determine that the one of the switch elements has no failure. [0012] As the number of times the first relay and the second relay operate increases, the first relay and the second relay abrade. In the above aspect, it is possible to determine whether one of the switch elements has a failure without operating the first relay and the second relay. Therefore, it is possible to suppress abrasion (degradation) of the first relay and the second relay.

[0013] In the above aspect, the plurality of switch elements may include a first switch element and a second switch element. One end of the first switch element may be connected to the power supply, and the other end of the first switch element may be connected to one end of the second switch element. The other end of the second switch element may be connected to the coil. The at least one sensor may include a first voltage sensor and a second voltage sensor. The first voltage sensor is configured to detect a voltage value between a ground and a connection point at which the first switch element and the second switch element are connected to each other. The second voltage sensor is configured to detect a voltage value between the ground and a connection point at which the second switch element and the coil are connected to each other. The controller may be configured to determine whether one of the first switch element and the second switch element is in the energized state by comparing the one of the voltage value detected by the first voltage sensor and the voltage value detected by the second voltage sensor with a voltage value of the power supply.

[0014] In the above aspect, the controller may be configured to output a control signal for setting the first switch element to the energized state and the control signal for setting the second switch element to the non-energized state. The controller may be configured to determine that the second switch element has a failure in the energized state when the voltage value detected by the second voltage sensor is higher than or equal to the voltage value of the power supply.

[0015] In the above aspect, the controller may be configured to output the control signals for setting the first switch element and the second switch element to the non-energized state. The controller may be configured to determine that the first switch element has a failure in the energized state when the voltage value detected by the first voltage sensor is higher than or equal to the voltage value of the power supply.

[0016] In the above aspect, the controller may be configured to output the control signal for setting one of the plurality of switch elements to the non-energized state and a control signal for setting the other one of the plurality of switch elements to the energized state. The controller may be configured to determine whether the one of the plurality of switch elements is in the energized state on the basis of the output signal of the at least one sensor.

[0017] Specifically, when the control signal for setting the first switch element to the energized state and the control signal for setting the second switch element to ,the non-energized state are output, the voltage value detected by the second voltage sensor reaches the voltage value of the power supply when the second switch element has a failure in the energized state. Therefore, by determining whether the voltage value detected by the second voltage sensor is higher than or equal to the voltage value of the power supply, it is possible to determine whether the second switch element has a failure in the energized state. That is, when the voltage value detected by the second voltage sensor is higher than or equal to the voltage value of the power supply, it is determined that the second switch element has a failure in the energized state.

[0018] When the control signals for setting the first switch element and the second switch element to the non-energized state are output, the voltage value detected by the first voltage sensor reaches the voltage value of the power supply when the first switch element has a failure in the energized state. Therefore, by determining whether the voltage value detected by the first voltage sensor is higher than or equal to the voltage value of the power supply, it is possible to determine whether the first switch element has a failure in the energized state. That is, when the voltage value detected by the first voltage sensor is higher than or equal to the voltage value of the power supply, it is determined that the first switch element has a failure in the energized state.

[0019] As described above, when it is determined whether any one of the first switch element and the second switch element has a failure, it is just required to switch only the first switch element between the energized state and the non-energized state. Here, by switching each of the first switch element and the second switch element between the energized state and the non-energized state, it is possible to determine whether any one of the first switch element and the second switch element has a failure.

[0020] In the above aspect, the at least one sensor may be a current sensor configured to detect a current value flowing through the coil. The controller may be configured to detennine that the one of the switch elements is in the energized state when the controller determines that current is flowing through the coil on the basis of the output signal of the current senson

[0021] In the above aspect, each switch element may be one of a switch element having a movable contact and a fixed contact, and a semiconductor switch element. The load may be configured to supply electric power to the electrical storage device.

[0022] For example, when it is determined whether the first switch element has a failure, the control signal for setting the first switch element to the non-energized state and the control signal for setting the second switch element to the energized state are output, and then it is possible to determine whether the first switch element is in the energized state. When it is determined whether the second switch element has a failure, the control signal for setting the first switch element to the energized state and the control signal for setting the second switch element to the non-energized state are output, and then it is possible to determine whether the second switch element is in the energized state. Here, by determining whether current is flowing through the coil with the use of the current sensor, it is also possible to determine whether one of the above-described switch elements is in the energized state. Specifically, when current is flowing through the coil, it may be determined that the one of the switch elements is in the energized state.

[0023] However, when it is determined whether any one of the first switch element and the second switch element has a failure by switching only the first switch element between the energized state and the non-energized state, it is possible to reduce the number of times the switch elements operate, so it is possible to reduce a time required for failure determination. Here, when the switch element having a movable contact and a fixed contact is used as each switch element, it is possible to suppress abrasion (degradation) of the switch elements by reducing the number of times the switch elements operate. A semiconductor switch may also be used as each switch element.

[0024] A second aspect of the invention provides a battery system. The battery system includes a first relay, a second relay, a coil, a plurality of switch elements, at least one sensor, and a controller. The first relay is provided in a positive electrode line that connects a positive electrode terminal of an electrical storage device to a load. The second relay is provided in a negative electrode line that connects a negative electrode terminal of the electrical storage device to the load, and is configured to be mechanically interlocked with the first relay. The coil is configured to generate electromagnetic force upon reception of electric power supplied from a power supply. The first relay and the second relay are configured to be switched from a non-energized state to an energized state by the electromagnetic force. The plurality of switch elements are provided in a current path between the power supply and the coil in series with each other. The plurality of switch elements are configured to switch the current path between an energized state and a non-energized state. The at least one sensor is configured to change an output signal in response to an energized state or non-energized state of each of the plurality of switch elements. The controller is configured to control operation of the plurality of switch elements. The controller is configured to set one of the plurality of switch elements to the non-energized state. The controller is configured to compare the output value of the at least one sensor at this time with the output value of the at least one sensor at the time when the plurality of switch elements are in the energized state, and determine whether the one of the plurality of switch elements has a failure in the energized state.

[0025] In the above aspect, the plurality of switch elements may include a first switch element and a second switch element. One end of the first switch element is connected to the power supply, and the other end of the first switch element is connected to one end of the second switch element. The other end of the second switch element is connected to the coil. The at least one sensor may include a first voltage sensor and a second voltage sensor. The first voltage sensor is configured to detect a voltage value between a ground and a connection point at which the first switch element and the second switch element are connected to each other. The second voltage sensor is configured to detect a voltage value between the ground and a connection point at which the second switch element and the coil are connected to each other. The controller may be configured to output a control signal for setting the first switch element to the energized state and the control signal for setting the second switch element to the non-energized state. The controller may be configured to detennine that the second switch element has a failure in the energized state when the voltage value detected by the second voltage sensor is higher than or equal to a voltage value of the power supply.

[0026] In the above aspect, the controller may be configured to output a control signal for setting one of the plurality of switch elements to the non-energized state and a control signal for setting the remainder of the plurality of switch elements to the energized state. The at least one sensor may be a current sensor configured to detect a current value flowing through the coil. The controller may be configured to determine that the one of the plurality of switch elements has a failure in the energized state when the controller determines that current is flowing through the coil on the basis of the current value that is obtained at the time when the one of the plurality of switch elements is set to the non-energized state.

[0027] A third aspect of the invention provides a failure determination method for a battery system. The battery system includes a first relay, a second relay, a coil, a first voltage sensor, a second voltage sensor, and a controller. The first relay is provided in a positive electrode line that connects a positive electrode terminal of an electrical storage device to a load. The second relay is provided in a negative electrode line that connects a negative electrode terminal of the electrical storage device to the load, and is configured to be mechanically interlocked with the first relay. The coil is configured to generate electromagnetic force for switching the first relay and the second relay from a non-energized state to an energized state upon reception of electric power supplied from a power supply. One end of the first switch element is connected to the power supply. One end of the second switch element is connected to the other end of the first switch element, and the other end of the second switch element is connected to the. coil. The first voltage sensor is configured to detect a voltage value between a ground and a connection point at which the first switch element and the second switch element are connected to each other. The second voltage sensor is configured to detect a voltage value between the ground and a connection point at which the second switch element and the coil are connected to each other. The failure determination method includes: setting the first switch element to an energized state and setting the second switch element to a non-energized state by the controller; comparing the voltage value of the second voltage sensor with a voltage value of the power supply by the controller; and determining that the second switch element has a failure in the energized state by the controller when the voltage value of the second voltage sensor is higher than or equal to the voltage value of the power supply.

[0028] In the above aspect, the failure determination method may further include setting the first switch element to a non-energized state by the controller when the voltage value of the second voltage sensor is lower than the voltage value of the power supply; comparing the voltage value of the first voltage sensor with the voltage value of the power supply by the controller; and determining that the first switch element has a failure in the energized state by the controller when the voltage value of the first voltage sensor is higher than or equal to the voltage value of the power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a view that shows the configuration of a battery system;

FIG. 2 is a view that shows the configuration of a circuit that drives system main relays;

FIG. 3 is a view that shows the configuration of another battery system;

FIG. 4 is a flowchart that shows the process of determining whether any one of switch elements has a failure; FIG. 5 is a timing chart that shows the behaviors of the switch elements and system main relays in the process shown in FIG. 4;

FIG. 6 is a flowchart that shows the process of determining whether any one of the switch elements has a failure;

FIG. 7 is a timing chart that shows the behaviors of the switch elements and system main relays in the process shown in FIG. 6;

FIG. 8 is a view that shows the configuration of a circuit that drives system main relays; and

FIG. 9 is a flowchart that shows the process of determining whether any one of switch elements has a failure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0030] Hereinafter, embodiments of the invention will be described.

[0031] A battery system according to a first embodiment of the invention will be described with reference to FIG. 1 . FIG. 1 is a schematic view that shows the configuration of the battery system according to the present embodiment.

[0032] A positive electrode line PL is connected to the positive electrode terminal of a battery pack. 10. A negative electrode line NL is connected to the negative electrode terminal of the battery pack 10. The battery pack 10 may be regarded as an electrical storage device recited in the appended claims. The battery pack 10 includes a plurality of single cells. The number of the single cells is set as needed. Here, the plurality of single cells that constitute the battery pack 10 may be electrically connected in series with each other or may be electrically connected in parallel with each other.

[0033] Instead of the battery pack 10, only one single cell may be used. A secondary battery, such as a nickel-metal hydride battery and a lithium ion battery, may be used as each single cell. Instead of the secondary battery, an electric double layer capacitor may be used.

[0034] The battery pack 10 is connected to a load 20 via the positive electrode line PL and the negative electrode line NL. The load 20 operates upon reception of electric power from the battery pack 10. When the load 20 generates electric power, the battery pack 10 is allowed to be charged with the generated electric power.

[0035] A system main relay SMR-B is provided in the positive electrode line PL. The system main relay SMR-B may be regarded as a first relay recited in the appended claims. A system main relay SMR-G is provided in the negative electrode line NL. The system main relay SMR-G may be regarded as a second relay recited in the appended claims. When the system main relays SMR-B, SMR-G are in an on state (energized state), the battery pack 10 is connected to the load 20. When the system main relays SMR-B, SMR-G are in an off state (non-energized state), connection of the battery pack 10 with the load 20 is interrupted.

[0036] The battery pack 10 may be, for example, mounted on a vehicle. Here, a motor generator may be used as the load 20. The motor generator generates kinetic energy for causing the vehicle to travel upon reception of electric power from the battery pack 10. The kinetic energy is transmitted to wheels. During braking of the vehicle, the motor generator generates electric power, and the electric power is supplied to the battery pack 10.

[0037] Next, a circuit (drive circuit) that drives the system main relays SMR-B, SMR-G will be described with reference to FIG. 2.

[0038] The system main relay SMR-B has a movable contact MC I and a fixed contact FC l . The fixed contact FCl is connected to the positive electrode line PL. When the movable contact MC I contacts the fixed contact FC l , the system main relay SMR-B enters the on state. When the movable contact MC I moves away from the fixed contact FC l , the system main relay SMR-B enters the off state.

[0039] The system main relay SMR-G has a movable contact MC2 and a fixed contact FC2. The fixed contact FC2 is connected to the negative electrode line NL. When the movable contact MC2 contacts the fixed contact FC2, the system main relay SMR-G enters the on state. When the movable contact MC2 moves away from the fixed contact FC2, the system main relay SMR-G enters the off state.

[0040] A drive circuit 30 for driving the system main relays SMR-B, SMR-G includes a coil 31 and switch elements SW l . SW2. By passing current through the coil 31 , electromagnetic force is generated. The movable contacts MC I , MC2 are moved by using the electromagnetic force.

[0041] A known structure is used for each of the system main relays SMR-B, SMR-G where appropriate. Therefore, the detailed description of these structures is omitted. In the present embodiment, the movable contacts MC I . MC2 are mechanically connected, and the movable contact MC I moves together with the movable contact MC2. That is, the movable contacts MC I , MC2 are mechanically interlocked with each other. Therefore, the common coil 31 is used for the system main relays SMR-B, SMR-G. That is, by passing current through the coil 31 , the movable contacts MC I , MC2 move together, and the movable contacts MC I , MC2 respectively contact the fixed contacts FC 1 , FC2. When no current is caused to flow through the coil 3.1 , the movable contacts MC I , MC2 are respectively moved away from the fixed contacts FC 1 , FC2 by an urging mechanism. A spring, or the like, is used as the urging mechanism.

[0042] One end of the coil 31 is grounded. The other end of the coil 31 is electrically connected to a power supply 32 via a power supply line SL. Thus, electric power from the power supply 32 is supplied to the coil 31. The battery pack 10 or a power supply different from the battery pack 10 is used as the power supply 32. When the electromotive voltage of the battery pack 10 is excessively high, the voltage value of the battery pack 10 is stepped down, and the stepped-down electric power is supplied to the coil 31.

[0043] The switch elements SWl , SW2 are provided in the power supply line SL. The switch elements SWl , SW2 are electrically connected in series with each other. Upon reception of a control signal from a controller 40, the energization state of each of the switch elements SWl , SW2 switches between an on state (energized state) and an off state (non-energized state).

[0044] A so-called mechanical switch or a semiconductor switch (for example, transistor) may be used as each of the switch elements SWl , SW2. Each mechanical switch has a movable contact and a fixed contact. When the movable contact contacts a corresponding one of the fixed contacts, a corresponding one of the switch elements (mechanical switches) SWl , SW2 enters the on state. When the movable contact moves away from a corresponding one of the fixed contacts, a corresponding one of the switch elements (mechanical switches) SW l . SW2 enters the off state. The semiconductor switch is switched between the on state and the off state in response to the migration state of ions.

[0045] One end of the switch element (which corresponds to a first switch element according to the invention) SWl is connected to the power supply 32, and the other end of the switch element SWl is connected to one end of the switch element (which corresponds to a second switch element according to the invention) SW2. The other end of the switch element SW2 is connected to the coil 31.

[0046] A voltage sensor 33 is connected to a connection point at which the switch element SWl and the switch element SW2 are connected to each other. The voltage sensor 33 may be regarded as a first voltage sensor recited in the appended claims. The voltage sensor 33 detects a voltage value V I between the connection point of the switch elements SWl , SW2 and a ground. A voltage sensor 34 is connected to a connection point at which the switch element SW2 and the coil 31 are connected to each other. The voltage sensor 34 may be regarded as a second voltage sensor recited in the appended claims. The voltage sensor 34 detects a voltage value V2 between the connection point of the switch element SW2 and coil 31 and the ground. Output signals of the voltage sensors 33, 34 are input to the controller 40.

[0047] In the present embodiment, the common coil 31 is used for the system main relays SMR-B, SMR-G. Therefore, it is possible to reduce the electric power consumption of the coil 31. Specifically, in comparison with the case where the coil 31 is provided one by one for the system main relays SMR-B, SMR-G, the electric power consumption of the coil 31 is reduced. The reason is as follows. The configuration that uses the single coil 31 allows the overall size of the coil 31 to be reduced more easily than a configuration that uses the two coils 31. Accordingly, electric power consumed by the configuration that uses the single coil 31 tends to be lower than electric power consumed by the configuration that uses the two coils 3 1 .

[0048] Here, it is assumed that a circuit that supplies electric power to the coil 3 1 has a failure. If the circuit that supplies electric power to the coil 3 1 has a failure, there is a concern that current continues flowing through the coil 31 . When current continues flowing through the coil 31 , the system main relays SMR-B, SMR-G remain in the on state. Therefore, the battery pack 10 remains connected to the load 20. If the battery pack 10 remains connected to the load 20, for example, electric power from the load 20 continues being supplied to the battery pack 10. As a result, there is a concern that the battery pack 10 becomes an overcharged state.

[0049] In a configuration that only one of the switch elements SW l , SW2 is provided, current can continue flowing through the coil 3 1 because of a failure of the only one switch element. The above-described failure of the switch element is a stuck on failure. The stuck on failure is a state where the switch element remains in the on state although control for setting the switch element to the off state is executed.

[0050] In the present embodiment, the switch elements SW l , SW2 are connected in series with each other in the power supply line SL. By providing the switch elements SW l , SW2 in series with each other in this way, even when one of the switch elements has a stuck on failure, it is possible to set the other one of the switch elements to the off state. Thus, it is possible to interrupt connection of the battery pack 10 with the load 20, so it is possible to prevent current from continuing flowing through the coil 31 . In the present embodiment, the two switch elements SW l , SW2 are provided; however, the number of the switch elements may be two or more. By connecting the two or more switch elements in series with each other, even when part of the switch elements has a stuck on failure, it is possible to set the remaining switch elements to the off state.

[0051] The invention may be applied to not only the battery system shown in FIG.

1 but also a battery system shown in FIG 3. In the battery system shown in FIG. 3, a system main relay S R-P and a resistive element R are electrically connected in parallel with the system main relay SMR-G. Here, the system main relay SMR-P and the resistive element R are electrically connected in series with each other. The structure of the system main relay SMR-P may be a similar structure to that of each of the system main relays SMR-B. SMR-G.

[0052] The resistive element R is used to inhibit flow of inrush current. Inrush current flows at the time when the battery pack 10 is connected to the load 20. When the battery pack 10 is connected to the load 20, the system main relay SMR-P is switched from an off state to an on state before the system main relay SMR-G is switched from the off state to the on state. Thus, it is possible to cause current to flow through the resistive element R, so it is possible to inhibit flow of inrush current. After current is caused to flow through the resistive element R in this way, the system main relay SMR-G just needs to be switched from the off state to the on state. When the system main relay SMR-G is set to the on state, the system main relay SMR-P just needs to be switched from the on state to the off state.

[0053] The battery system shown in FIG. 3 is able to cause the system main relays SMR-B, SMR-G to operate together with the use of the single coil 31. The battery system is able to cause the system main relays SMR-B, SMR-P to operate together. When the system main relays SMR-B, SMR-P are caused to operate together, the system main relay SMR-P may be regarded as the second relay recited in the appended claims.

[0054] Here, the system main relays that operate together just need to be system main relays that allow the battery system to be charged or discharged by connecting the battery pack 10 to the load 20. That is, (at least two) system main relays provided respectively in the positive electrode line PL and the negative electrode line NL just need to operate together.

[0055] Next, the process of determining whether at least one of the switch elements SWl , SW2 has a stuck on failure will be described with reference to the flowchart shown in FIG. 4. The process shown in FIG. 4 is executed by the controller 40. When the process shown in FIG. 4 is started, the switch elements SWl , SW2 are in the off state. For example, the process shown in FIG. 4 may be executed when the battery pack 10 is not connected to the load 20.

[0056] In step S I 01 , the controller 40 outputs a control signal for switching the switch element SW 1 from the off state to the on state. An off control signal from the controller 40 is input to the switch element SW2.

[0057] In step S I 02, the controller 40 detects the voltage value V2 on the basis of the output signal of the voltage sensor 34, and determines whether the voltage value V2 is lower than a voltage value Vb. The voltage value Vb is the voltage value of the power supply 32. The controller 40 just needs to detect the voltage value Vb before the controller 40 compares the voltage value Vb with the voltage value V2. The timing at which the voltage value Vb is detected may be set as needed under this condition. For example, when the process shown in FIG. 4 is started, the controller 40 may detect the voltage value Vb in advance.

[0058] When the voltage value V2 is lower than the voltage value Vb, the controller 40 executes the process of step S I 03. When the voltage value V2 is higher than or equal to the voltage value Vb, the controller 40 executes the process of step S I 04. When the switch element SW2 is in the off state, the voltage value V2 becomes substantially 0 [V], and the voltage value V2 is lower than the voltage value Vb. Substantially 0 [V] means that a detection error of the voltage sensor 34 is included. When the voltage value V2 is lower than the voltage value Vb, it is understood that the switch element SW2 is operating in accordance with the control signal from the controller 40. Therefore, the controller 40 determines in step S I 03 that the switch element SW2 is normal.

[0059] On the other hand, when the switch element SW2 has a stuck on failure, electric power from the power supply 32 is supplied to the coil 31. and the voltage value V2 is substantially equal to the voltage value Vb. Substantially equal means that a detection error of the voltage sensor 34 and an error at the time of detecting the voltage value Vb are included. As described above, the voltage value V2 at the time when the switch element SW2 is normal differs from the voltage value V2 at the time when the switch element SW2 has a stuck on failure.

[0060] When the voltage value V2 is higher than or equal to the voltage value Vb, it is understood that the switch element SW2 is in the on state although the off control signal is output to the switch element SW2. Therefore, the controller 40 determines in step S I 04 that the switch element SW2 has a stuck on failure. After the process of step S I 04, the controller 40 ends the process shown in FIG. 4.

[0061] In step S I 05, the controller 40 outputs the control signal for switching the switch element SW l from the on state to the off state. Here, the off control signal remains output to the switch element SW2. In step S 106, the controller 40 detects the voltage value V I on the basis of the output signal of the voltage sensor 33, and determines whether the voltage value V I is lower than the voltage value Vb. As described above, the voltage value Vb is the voltage value of the power supply 32.

[0062] When the voltage value V I is lower than the voltage value Vb, the controller 40 executes the process of step S I 07. When the voltage value V I is higher than or equal to the voltage value Vb, the controller 40 executes the process of step S I 08. In the process of step S 105, the control signals for setting the switch elements SW l , SW2 in the off state are output. Therefore, when the switch element SW l is normal, the voltage value V I becomes substantially 0 [V], and the voltage value V I becomes lower than the voltage value Vb. Substantially 0 [V] means that a detection error of the voltage sensor 33 is included. In step S I 07, the controller 40 determines that the switch element SW l is normal.

[0063] When the switch element SW l has a stuck on failure, the voltage value V I is substantially equal to the voltage value Vb. Substantially equal means that a detection error of the voltage sensor 33 and an error at the time of detecting the voltage value Vb are included. Because the voltage value V I is higher than or equal to the voltage value Vb, the controller 40 determines in step S I 08 that the switch element SW l has a stuck on failure. As described above, the voltage value V I at the time when the switch element SWl is normal differs from the voltage value V I at the time when the switch element SW l has a stuck on failure. After the processes of step S I 07 and step S I 08, the controller 40 ends the process shown in FIG. 4.

[0064] FIG. 5 is a timing chart at the time when the process shown in FIG. 4 is executed. FIG. 5 shows the behaviors of the switch elements SW l , SW2 and system main relays SMR-B, SMR-G when the switch elements SW l , SW2 are normal.

[0065] As shown in FIG. 5, while control for setting the switch element SW l to the on state is being executed, it is detemiined whether the switch element SW2 has a stuck on failure. That is. it is checked whether the switch element SW2 is in the energized state while control for setting the switch element SW2 to the off state is being executed. Thus, it is detemiined whether the switch element SW2 has a stuck on failure.

[0066] After control for switching the switch element SW l from the on state to the off state is , executed, it is detemiined whether the switch element SW l has a stuck on failure. That is, it is checked whether the switch element SW l is in the energized state while control for setting the switch element SW l to the off state is being executed. Thus, it is determined whether the switch element SW l has a stuck on failure.

[0067] As shown in FIG. 5, when the switch elements SW l , SW2 are normal, the system main relays SMR-B, SMR-G remain in the off state. That is, when it is detemiined whether any one of the switch elements SW l , SW2 has a stuck on failure in the case where the switch elements SW l , SW2 are normal, the system main relays SMR-B, SMR-G do not switch from the off state to the on state.

[0068] Even when the switch elements SW l , SW2 are normal, but when the system main relays SMR-B, SMR-G are switched between the off state and the on state in order to detennine whether any one of the switch elements SW l , SW2 has a stuck on failure, degradation of the system main relays SMR-B, SMR-G advances. That is. because the number of times the movable contacts MC I , MC2 are operated increases, there is a concern that abrasion of the movable contacts MC I , MC2 or abrasion of the fixed contacts FC 1 , FC2 advances.

[0069] In the present embodiment, as described above, the system main relays SMR-B, SMR-G remain in the off state at the time of determining whether any one of the switch elements SW l , SW2 has a stuck on failure, so it is possible to suppress degradation of the system main relays SMR-B, SMR-G.

[0070] In the present embodiment, it is detemiined whether any one of the switch elements SW l , SW2 has a stuck on failure in the sequence shown in FIG. 4; however, determination as to whether any one of the switch elements SW l , SW2 has a stuck on failure is not limited to this sequence. Specifically, the processes of step S I 01 to step S I 04 may be executed after the processes of step S I 05 to step S I 07 are executed. That is, it is possible to determine whether the switch element SW2 has a stuck on failure after it is determined whether the switch element SW l has a stuck on failure.

[0071 ] When it has been identified that any one of the switch elements SW l , SW2 has a stuck on failure in the processes of step S I 04 and step S I 08, a user, or the like, may be warned. A known method may be employed as needed as the warning to the user, or the like.

[0072] For example, a sound and an indication may be used as means for warning.

Specifically, by generating a sound, it is possible to cause the user, or the like, to recognize that at least one of the switch elements SW l , SW2 has a failure. By showing predetermined infomiation on a display, it is possible to cause the user, and the like, to recognize that at least one of the switch elements SW l , SW2 has a failure. Here, the user, or the like, does not need to recognize specific details of a failure, and the user, or the like, just needs to recognize that a failure is occurring.

[0Q73] On the other hand, when it is determined that both the switch elements SW l . SW2 have a stuck on failure, the system main relays SMR-B, SMR-G remain in the on state. In this case, in addition to the above-described wanning, for example, it is possible to execute the process of inhibiting an overcharged state of the battery pack 10 by limiting the input of the battery pack 10. This process may be executed by the controller 40.

[0074] Next, another process of determining whether any one of the switch elements SW l , SW2 has a stuck on failure will be described with reference to the flowchart shown in FIG. 6. The process shown in FIG. 6 is executed by the controller 40. When the process shown in FIG. 6 is started, the switch elements SW l , SW2 are in the off state.

[0075] In step S201 , the controller 40 outputs the control signal for switching the switch element SW2 from the off state to the on state. The off control signal remains output to the switch element SW 1 . in step S202, the controller 40 detects the voltage value V I on the basis of the output signal of the voltage sensor 33, and determines whether the voltage value V I is lower than the voltage value Vb (the voltage value of the power supply 32).

[0076] In the process of step S201. the switch element SW 1 is controlled to the off state. Therefore, when the switch element SW 1 operates in accordance with this control; the voltage value V I is substantially 0 [V]. That is, the voltage value V I is lower than the voltage value Vb. Therefore, the controller 40 determines in step S203 that the switch element SW 1 is normal.

[0077] On the other hand, when the switch element SW 1 has a stuck on failure, the voltage value V I is substantially equal to the voltage value Vb. That is, the voltage value V I is higher than or equal to the voltage value Vb. Therefore, the controller 40 determines in step S204 that the switch element SW 1 has a stuck on failure.

[0078] In step S205, the controller 40 outputs the control signal for switching the switch element SW 1 from the off state to the on state, and outputs the control signal for switching the switch element SW2 from the on state to the off state. In step S206, the controller 40 detects the voltage value V2 on the basis of the output signal of the voltage sensor 34, and determines whether the voltage value V2 is lower than the voltage value Vb (the voltage value of the power supply 32).

[0079] When the switch elements SW 1 , SW2 operate in accordance with the process of step S205, the voltage value V2 is substantially 0 [V]. That is, the voltage value V2 is lower than the voltage value Vb. Therefore, the controller 40 determines in step S207 that the switch element SW2 is normal.

[0080] On the other hand, when the switch element SW2 has a stuck on failure although control for setting the switch element SW2 to the off state is executed, electric power is supplied to the coil 3 1 because the switch element SW 1 is in the on state. Thus, the voltage value V2 is substantially equal to the voltage value Vb. Because the voltage value V2 is higher than or equal to the voltage value Vb, the controller 40 determines in step S208 that the switch element SW2 has a stuck on failure. After the processes of step S204, step S207 and step S208 are executed, the controller 40 ends the process shown in FIG. 6.

[0081] FIG. 7 is a timing chart at the time of executing the process shown in FIG. 6. FIG. 7 shows the behaviors of the switch elements SWl , SW2 and system main relays SMR-B, SMR-G when the switch elements S W 1 , S W2 are normal.

[0082] As shown in FIG. 7, while control for setting the switch element SW2 to the on state is being executed, it is determined whether the switch element SW l has a stuck on failure. That is, it is checked whether the switch element SWl is in the energized state while control for setting the switch element SWl to the off state is being executed. Thus, it is determined whether the switch element SWl has a stuck on failure.

[0083] While control for setting the switch element SWl to the on state is being executed, it is determined whether the switch element SW2 has a stuck on failure. That is, it is checked whether the switch element SW2 is in the energized state while control for setting the switch element SW2 to the off state is being executed. Thus, it is determined whether the switch element SW2 has a stuck on failure.

[0084] At the time of executing the process shown in FIG. 6 as well, when the switch elements SWl , SW2 are normal, the system main relays SMR-B, SMR-G remain in the off state. Therefore, as in the case of the process shown in FIG. 4, it is possible to reduce the number of times the system main relays SMR-B, SMR-G operate, so it is possible to suppress degradation of the system main relays SMR-B, SMR-G.

[0085] In the process shown in FIG. 6, it is determined whether any one of the switch elements SWl , SW2 has a stuck on failure on the basis of the output signals of the voltage sensors 33, 34; however, determination as to whether any one of the switch elements SWl , SW2 has a stuck on failure is not limited to this configuration. Specifically, as shown in FIG. 8, the voltage sensors 33, 34 may be omitted, and a current sensor 35 may be arranged in a current path between the switch element SW2 and the coil 31. The output signal of the current sensor 35 is input to the controller 40, and the controller 40 is able to detect a current value I.

[0086] The current sensor 35 is used to determine whether current is flowing through the coil 31. Therefore, a location at which the current sensor 35 is provided is not limited to the location shown in FIG. 8. Specifically, the current sensor 35 may be provided in the power supply line SL at the side that grounds the coil 31.

[0087] In the configuration shown in FIG. 8, when control for setting one of the switch elements SWl , SW2 to the on state and setting the other one of the switch elements SWl , SW2 to the off state is executed, no current flows through the coil 31 when the switch elements SWl , SW2 are normal. At this time, the current value I that is detected by the current sensor 35 is substantially 0 [A]. Substantially 0 [A] means that a detection error of the current sensor 35 is included.

[0088] When control for setting one of the switch elements SW l , SW2 to the on state and setting the other one of the switch elements SWl , SW2 to the off state is executed, current flows through the coil 31 when the other one of the switch elements has a stuck on failure. At this time, the current value I is equal to the current value that is supplied from the power supply 32 to the coil 31.

[0089] In the configuration shown in FIG. 8, when it is determined whether any one of the switch elements SWl , SW2 has a stuck on failure, the process shown in FIG. 9 is executed. The process shown in FIG. 9 corresponds to the process shown in FIG. 6, and like step numbers are used for the same processes as the processes shown in FIG. 6.

[0090] In the process shown in FIG. 9, the processes of step S209 and step S210 are executed instead of the processes of step S202 and step S206 shown in FIG. 6. In step S209 and step S210, the controller 40 determines whether current is flowing through the coil 31 on the basis of the output signal of the current sensor 35. By comparing the current value I that is detected by the current sensor 35 with a threshold, it is possible to determine whether current is flowing through the coil 31.

[0091] Specifically, when the current value I is smaller than the threshold, it may be determined that no current is flowing through the coil 31. When the current value I is larger than or equal to the threshold, it may be determined that current is flowing through the coil 31. Here, the threshold may be set to 0 [A] or may be set to a value other than 0 [A] in consideration of a detection error of the current sensor 35. [0092] When it is determined in the process of step S209 that no current is flowing through the coil 3 1 , the controller 40 determines in step S203 that the switch element SWl is normal. When it is determined in the process of step S209 that current is flowing through the coil 3 1 , the controller 40 determines in step S204 that the switch element SW l has a stuck on failure. In this way, the output signal of the current sensor 35 changes between when the switch element SW l is normal and when the switch element SW l has a stuck on failure.

[0093] When it is determined in the process of step S210 that no current is flowing through the coil 3 1 , the controller 40 determines in step S207 that the switch element SW2 is normal. When it is determined in the process of step S210 that current is flowing through the coil 3 1 , the controller 40 determines in step S208 that the switch element SW2 has a stuck on failure. In this way, the output signal of the current sensor 35 changes between when the switch element SW2 is normal and when the switch element SW2 has a stuck on failure.

[0094] In the processes shown in FIG. 6 and FIG. 9, the processes from step S205 are executed after the processes of step S201 to step S203 are executed; however, the sequence of the processes is not limited to this configuration. Specifically, the processes of step S201 to step S204 may be executed after the processes of step S205 to step S207 are executed.

[0095] As is apparent from comparison between FIG. 5 and FIG. 7, in the process shown in FIG. 6, the switch elements SW l , SW2 need to be switched between the on state and the off state. In contrast, in the process shown in FIG. 4, only the switch element SW l needs to be switched between the on state and the off state. Therefore, with the process shown in FIG. 4. it is possible to reduce the number of times the switch elements SW l , SW2 are caused to operate as compared to the process shown in FIG. 6.

[0096] By reducing the number of times the switch elements SW l , SW2 operate, it is possible to reduce a time required to determine whether any one of the switch elements SW l , SW2 has a failure. It is assumed that the switch elements SW l . SW2 are mechanical switches. In this case, by reducing the number of times the switch elements SW 1 , SW2 operate, it is possible to suppress degradation of the switch elements SW 1 , SW2. As described above, in the mechanical switch, the movable contact contacts the fixed contact, so abrasion of the movable contact and the fixed contact advances when the number of times the mechanical switch operates increases.

[0097] In the present embodiment, it is determined whether each of the two switch elements SW 1 , SW2 has a stuck on failure; however, the invention is not limited to this configuration. That is. even when three or more switch elements are used, the invention is applicable. A control signal for setting any one of the switch elements to an off state should be output to the any one of the switch elements, and it should be determined whether the any one of the switch elements is in an on state. In order to determine whether the any one of the switch elements is in the on state, a detected result of a voltage sensor or current sensor may be used as in the case of the present embodiment.