ANG, Wanleng (1 Toyota-cho,Toyota-shi, Aichi-ken, 471-8571, JP)
| CLAIMS 1. An electric power control apparatus for a vehicle, comprising: a plurality of first switches that are provided between an electric device and positive electrodes of a plurality of electricity storage devices, respectively, and that are switched between a conductive state and a non-conductive state, wherein the electric device is provided with a capacitor, and wherein the plurality of electricity storage devices are configured to connect to the capacitor in parallel; a plurality of second switches that are provided between the electric device and negative electrodes of the plurality of electricity storage devices, respectively, and that are switched between the conductive state and the non-conductive state; and a controller that executes one of a first control and a second control by controlling the plurality of first switches and the plurality of second switches if voltage difference between the plurality of electricity storage devices is greater than a predetermined value, wherein: the first control is a control of maintaining the conductive state of the plurality of first switches and causing the plurality of second switches to be in the conductive state alternately with each other; and the second control is a control of maintaining the conductive state of the plurality of second switches and causing the plurality of first switches to be in the conductive state alternately with each other. 2. The electric power control apparatus according to claim 1, wherein the controller moves electric energy from one of the plurality of electricity storage devices that is relatively high in voltage to another one of the plurality of electricity storage devices that is relatively low in voltage, via the capacitor, by executing the one of the first control and the second control. 3. The electric power control apparatus according to according to claim 1 or 2, wherein the controller alternately executes the first control and the second control with a predetermined period. 4. The electric power control apparatus according to claim 3, wherein when control is changed from one of the first control and the second control to another one of the first control and the second control, the controller causes both the plurality of first switches and the plurality of second switches to be in the non-conductive state for a predetermined period after the one of the first control and the second control stops, and executes the another one of the first control and the second control after the predetermined period elapses. 5. The electric power control apparatus according to any one of claims 1 to 4, wherein: the vehicle is configured so that the plurality of electricity storage devices are charged using an external electric power supply; the electric device includes a charging circuit that converts electric power of the external electric power supply into electric power that is to be charged into the plurality of electricity storage devices; and the capacitor is provided between a positive electrode line and a negative electrode line that are provided for supplying the electric power converted by the charging circuit to the plurality of electricity storage devices. 6. The electric power control apparatus according to claim 5, wherein the controller executes the one of the first control and the second control while the charging circuit is in a stopped state. 7. The electric power control apparatus according to any one of claims 1 to 4, wherein: the vehicle is configured to move by power of an electric motor; the electric device includes an electric power conversion circuit that converts electric power of the plurality of electricity storage devices into electric power that drives the electric motor; and the capacitor is provided between a positive electrode line and a negative electrode line that are provided for supplying the electric power of the plurality of electricity storage devices to the electric power conversion circuit. 8. The electric power control apparatus according to claim 7, wherein the controller executes the one of the first control and the second control while the electric power conversion circuit is in a stopped state. 9. An electric power control method for a vehicle that includes: an electric device provided with a capacitor; a plurality of electricity storage devices configured to connect to the capacitor in parallel; a plurality of first switches that are provided between the electric device and positive electrodes of the plurality of electricity storage devices, respectively, and that are switched between a conductive state and a non-conductive state; and a plurality of second switches that are provided between the electric device and negative electrodes of the plurality of electricity storage devices, respectively, and that are switched between the conductive state and the non-conductive state, the electric power control method comprising executing one of a first control and a second control by controlling the plurality of first switches and the plurality of second switches if voltage difference between the plurality of electricity storage devices is greater than a predetermined value, wherein: the first control is a control of maintaining the conductive state of the plurality of first switches and causing the plurality of second switches to be in the conductive state alternately with each other; and the second control is a control of maintaining the conductive state of the plurality of second switches and causing the plurality of first switches to be in the conductive state alternately with each other. |
CONTROL METHOD FOR VEHICLE
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The invention relates to an electric power control apparatus and an electric power control method for a vehicle that is equipped with a plurality of electricity storage devices that are configured to connect to an electric device in parallel.
2. Description of the Related Art
[0002] In recent years, electric vehicles that move by using an electric motor that is driven by electric power stored in an electricity storage device have been commercialized as environmentally friendly motor vehicles. Generally, the electric vehicle has, as an electricity storage device, a battery pack that is configured to connect a plurality of battery cells in series. Therefore, if the battery cells vary in internal resistance, one of the battery cells may be charged with more than the maximum voltage when the battery cells in the battery pack are charged.
[0003] With regard to this problem, Japanese Patent Application Publication No. 2009-232659 (JP-A-2009-232659) describes a technology in which the charging start voltage of each of the battery cells of a battery (battery pack) is set to the lower value the greater the internal resistance of the battery cell, and in which each battery cell is discharged until its charging start voltage is reached. According to the technology disclosed in Japanese Patent Application Publication No. 2009-232659 (JP-A-2009-232659), even if the battery cells vary in internal resistance, it is possible to restrain any one of the battery cells from exceeding its maximum voltage.
[0004] Incidentally, in a vehicle that moves by power from an electric motor, in order to increase the distance to empty, that is, the distance that the vehicle can travel before being charged, it is necessary to increase the capacity of the electricity storage device of the vehicle. In order to increase the capacity of the electricity storage device, it is conceivable to use many battery packs that are connected in parallel. However, in the case where a plurality of battery packs are connected in parallel, if there are large voltage differences between the battery packs, it sometimes happens that excessively large current flows from a high- voltage battery pack toward a low-voltage battery pack. This may result in degradation of battery cells or breakage of a switch (a relay or the like) or a wire harness provided between battery packs.
SUMMARY OF THE INVENTION
[0005] The invention provides an electric power control apparatus and an electric power control method that are capable of equalizing the voltages of a plurality of electricity storage devices that are arranged in a vehicle so as to connect to an electric circuit in parallel.
[0006] A first aspect of the invention is an electric power control apparatus for a vehicle An electric power control apparatus for a vehicle, including: a plurality of first switches that are provided between an electric device and positive electrodes of a plurality of electricity storage devices, respectively, and that are switched between a conductive state and a non-conductive state, wherein the electric device is provided with a capacitor, and wherein the plurality of electricity storage devices are configured to connect to the capacitor in parallel; a plurality of second switches that are provided between the electric device and negative electrodes of the plurality of electricity storage devices, respectively, and that are switched between the conductive state and the non-conductive state; and a controller that executes one of a first control and a second control by controlling the plurality of first switches and the plurality of second switches if voltage difference between the plurality of electricity storage devices is greater than a predetermined value, wherein: the first control is a control of maintaining the conductive state of the plurality of first switches and causing the plurality of second switches to be in the conductive state alternately with each other; and the second control is a control of maintaining the conductive state of the plurality of second switches and causing the plurality of first switches to be in the conductive state alternately with each other.
[0007] The controller may move electric energy from one of the plurality of electricity storage devices that is relatively high in voltage to another one of the plurality of electricity storage devices that is relatively low in voltage, via the capacitor, by executing the one of the first control and the second control. The controller may alternately execute the first control and the second control with a predetermined period. When control is changed from one of the first control and the second control to another one of the first control and the second control, the controller may cause both the plurality of first switches and the plurality of second switches to be in the non-conductive state for a predetermined period after the one of the first control and the second control stops, and may execute the another one of the first control and the second control after the predetermined period elapses.
[0008] The vehicle may be configured so that the plurality of electricity storage devices are charged using an external electric power supply, and the electric device may include a charging circuit that converts electric power of the external electric power supply into electric power that is to be charged into the plurality of electricity storage devices; and the capacitor may be provided between a positive electrode line and a negative electrode line that are provided for supplying the electric power converted by the charging circuit to the plurality of electricity storage devices.
[0009] The controller may execute the one of the first control and the second control while the charging circuit is in a stopped state.
[0010] The vehicle may be configured to move by power of an electric motor, and the electric device may include an electric power conversion circuit that converts electric power of the plurality of electricity storage devices into electric power that drives the electric motor, and the capacitor may be provided between a positive electrode line and a negative electrode line that are provided for supplying the electric power of the plurality of electricity storage devices to the electric power conversion circuit.
[0011] The controller may execute the one of the first control and the second control while the electric power conversion circuit is in a stopped state.
[0012] A second aspect of the invention is an electric power control method for a vehicle that includes: an electric device provided with a capacitor; a plurality of electricity storage devices constructed so as to connect to the capacitor in parallel; a plurality of first switches that are provided between the electric device and positive electrodes of the plurality of electricity storage devices, respectively, and that are switched between a conductive state and a non-conductive state; and a plurality of second switches that are provided between the electric device and negative electrodes of the plurality of electricity storage devices, respectively, and that are switched between the conductive state and the non-conductive state, the electric power control method including executing one of a first control and a second control by controlling the plurality of first switches and the plurality of second switches if voltage difference between the plurality of electricity storage devices is greater than a predetermined value, wherein: the first control is a control of maintaining the conductive state of the plurality of first switches and causing the plurality of second switches to be in the conductive state alternately with each other; and the second control is a control of maintaining the conductive state of the plurality of second switches and causing the plurality of first switches to be in the conductive state alternately with each other.
[0013] According to the foregoing constructions, in a vehicle equipped with a plurality of electricity storage devices that are configured to connect to an electric device (the capacitor) in parallel, it is possible to equalize the voltages of the electricity storage devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
FIG. 1 is an overall block diagram of a vehicle;
FIG. 2 is a functional block diagram of an ECU;
FIG. 3 is a first flowchart showing a processing procedure of the ECU;
FIG. 4 is a second flowchart showing a processing procedure of the ECU;
FIG 5 is a diagram showing manners of change of variables n, m and X and charging relays Rl and R2; and
FIG. 6 is a third flowchart showing a processing procedure of the ECU.
DETAILED DESCRIPTION OF EMBODIMENTS
[0015] Hereinafter, an embodiment of the invention will be described in detail with reference to the drawings. The same or comparable portions in the drawings are denoted by the same reference characters, and descriptions thereof will not be repeated.
[0016] FIG. 1 is an overall block diagram of a vehicle 100 that is equipped with an electric power control apparatus in accordance with a first embodiment of the invention.
[0017] As shown in FIG. 1, the vehicle 100 includes an electric power supply device constructed of a first electric power supply 110 and a second electric power supply 11 OA, a power control unit (PCU) 120 and a motor- generator (MG) 130 that are drive devices (electric loads), a power transmission gear 140, driving wheels 150, and an electronic control unit (ECU) 300 that functions as a controller. [0018] Although the vehicle 100 shown in FIG. 1 is provided with one motor-generator, the number of the motor-generators provided in a vehicle is not limited so, that is, a plurality of motor-generators may be provided. Besides, as a motive power source, an engine (not shown) may also be provided, besides the motor-generator. That is, the invention is applicable generally to electric vehicles that include hybrid motor vehicles that produce drive force using an engine and a motor-generator as well as electric motor vehicles and fuel cell electric motor vehicles that are not equipped with an engine.
[0019] The first electric power supply 110 includes a battery B l, a system main relay SMR1 (hereinafter, sometimes referred to simply as "SMR1"), and a charging relay R 1.
[0020] The battery Bl is electrically chargeable and dischargeable. The battery Bl is constructed of secondary batteries, such as lithium-ion batteries, nickel metal hydride batteries, lead storage batteries, or electricity storage elements such as electric double layer capacitors or the like.
[0021] The battery B l is connected to the PCU 120 via a positive electrode line PL1 and a negative electrode line NLl. The battery B l supplies the PCU 120 with electric power for generating the drive force for the vehicle 100. Besides, the battery B l stores electric power that is generated by the MG 130. The battery B l outputs a voltage of, for example, about 200 volts.
[0022] The battery B l is, for example, a battery pack constructed of a plurality of battery cells connected in series. A service plug SPl is connected between the battery cells in series. The service plug SPl functions as a safety switch for forcing disconnection of the circuit at the time of maintenance or the like. For example, the service plug SPl is configured to open the contact point when an external casing (not shown) of the battery B 1 is opened.
[0023] The system main relay SMR1 includes relays SMR1P, SMR1R and SMRIN, and a resistor RE1. One end of the relay SMR1P and one end of the relay SMRIN are connected to a positive electrode and a negative electrode, respectively, of the battery B l. The other end of the relay SMR1P and the other end of the relay SMRIN are connected to a positive electrode line PL1 and a negative electrode line NLl, respectively, that interconnect the first electric power supply 110 and the PCU 120. Besides, the relay SMR1R, together with the resistor RE1 connected to the relay SMR1R in series, is connected to the relay SMR1P in parallel. The relays SMR1P, SMR1R and SMR1N are controlled by a control signal SE1 from the ECU 300 independently of each other, so that each of the relays SMR1P, SMR1R and SMR1N is switched between an on-state (conductive state) and an off-state (non-conductive state), to thereby switch between connection and disconnection between the battery B l and the PCU 120.
[0024] Incidentally, the resistor RE1 functions as a current decreasing resistor for reducing the inrush current at the time of turning on the system main relay SMR1. The inrush current is an electric current that suddenly flows to charge a capacitor C 1. To turn on the system main relay SMR1, the relays SMR1N and SMR1R are firstly turned on. After the capacitor CI is charged with low current, the relay SMR1P is turned on and the relay SMR1R is turned off.
[0025] The charging relay Rl includes relays RIP and RIN. One end of the relay RIP and one end of the relay RIN are connected to a positive electrode and a negative electrode, respectively, of the battery Bl The other end of the relay RIP and the other end of the relay RIN are connected to a positive electrode line PL2 and a negative electrode line NL2, respectively, that interconnect the first electric power supply 110 and a charging device 200. The relays RIP and RIN of the charging relay Rl are controlled by a control signal SE2 from the ECU 300 independently of each other, so that each of the relays RIP and RIN is switched between the on-state and the off-state, to thereby switch between connection and disconnection between the battery B 1 and the charging device 200.
[0026] The charging relay Rl is turned on at the time of external charging, that is, when the battery B l is charged with electric power from an external electric power supply 500. Incidentally, although the charging relay Rl may be provided outside the first electric power supply 110, it is preferable to provide the charging relay Rl within the first electric power supply 110 in order to avoid application of voltage from the positive electrode line PL1 and the negative electrode line NL1 to a charging pathway outside the first electric power supply 110 during the traveling of the vehicle 100, and the like.
[0027] The second electric power supply 110A is connected to the PCU 120 in parallel with the first electric power supply 110. Besides, the second electric power supply 11 OA is also connected to the charging device 200 in parallel with the first electric power supply 110. The second electric power supply 11 OA includes a battery B2, a system main relay SMR2 (hereinafter, sometimes referred to simply as "SMR2"), and a charging relay R2. The system main relay SMR2 includes relays SMR2P, SMR2R and SMR2N, and a resistor RE2. The charging relay R2 includes relays R2P and R2N. The constructions and functions of the battery B2, the relays SMR2P, SMR2R and SMR2N, the resistor RE2 and the charging relay R2 are the same as those of the battery Bl, the relays SMR1P, SMR1R and SMR1N, the resistor RE1 and the charging relay Rl, respectively. The relays SMR2P, SMR2R and SMR2N are controlled by a control signal SE3 from the ECU 300 independently of each other, so that each of the relays SMR2P, SMR2R and SMR2N is switched between the on-state and the off-state, to thereby switch between connection and disconnection between the battery B2 and the PCU 120. The relays R2P and R2N of the charging relay R2 are controlled by a control signal SE4 from the ECU 300 independently of each other, so that each of the relays R2P and R2N is switched between the on-state and the off-state to thereby switch between connection and disconnection between the battery B2 and the charging device 200.
[0028] The PCU 120 includes a converter 121, an inverter 122, and capacitors
CI and C2.
[0029] The converter 121 is connected to the positive electrode line PL1 and the negative electrode line NLl, and to a positive electrode line HPL and a negative electrode line NLl. The converter 121 is controlled by a control signal PWC from the ECU 300, and performs voltage conversion between the voltage between the positive electrode line PL1 and the negative electrode line NLl and the voltage between the positive electrode line HPL and the negative electrode line NLl.
[0030] The inverter 122 is connected to the converter 121 via the positive electrode line HPL and the negative electrode line NLl. The inverter 122 is controlled by a control signal PWI from the ECU 300, and converts direct-current electric power supplied from the converter 121 into alternating-current electric power for driving the MG 130. Besides, the inverter 122 converts alternating-current electric power generated by the MG 130 into direct-current electric power with which the batteries B 1 and B2 can be charged.
[0031] The capacitor CI is connected between the positive electrode line PL1 and the negative electrode line NLl, and reduces fluctuations of the voltage between the positive electrode line PL1 and the negative electrode line NLl. The capacitor C2 is connected between the positive electrode line HPL and the negative electrode line NLl, and reduces fluctuations of the voltage between the positive electrode line HPL and the negative electrode line NL1.
[0032] The MG 130 is an alternating-current rotary electric machine, for example, a permanent magnet type synchronous electric motor that is equipped with a rotor in which permanent magnets are embedded.
[0033] The output torque of the MG 130 is transmitted to the driving wheels
150 via the power transmission gear 140 constructed of a speed reducer and a power splitting mechanism, so that the vehicle 100 is moved. During the regenerative braking of the vehicle 100, the MG 130 is able to generate electricity from the turning force of the driving wheels 150. The electric power generated by the MG 130 is converted into charging electric power for the electric power supply devices (the batteries B l and B2) by the PCU 120.
[0034] Furthermore, the vehicle 100 is equipped with voltage sensors 112 and 112 A. The voltage sensor 112 detects the voltage V 1 of the battery B 1. The voltage sensor 112A detects the voltage V2 of the battery B2. These sensors output the detection results to the ECU 300.
[0035] The ECU 300 includes a central processing unit (CPU) and a memory (neither of which is shown in FIG 1), and generates control signals for controlling various appliances, on the basis of information stored in the memory and signals from various sensors. The ECU 300 performs controls of the vehicle 100 and the various appliances by outputting to the appliances the control signals that the ECU 300 generates. These controls may be processed by dedicated hardware devices (electronic circuits) instead of software processes.
[0036] Incidentally, although FIG. 1 shows the ECU 300 as a single unit, the ECU 300 may also be divided into two or more units according to functions or control objects.
[0037] Furthermore, the vehicle 100 includes a charging device 200 and an inlet 220 as constructions for charging the batteries Bl and B2 with electric power from outside the vehicle 100.
[0038] The inlet 220 is provided on a body of the vehicle 100 so as to receive alternating-current electric power from the external electric power supply 500. A charging connector 410 of a charging cable 400 is connected to the inlet 220. By connecting a plug 420 of the charging cable 400 to an electric outlet 510 of the external electric power supply 500 (e.g., a commercial electric power supply or the like), alternating-current electric power is transmitted from the external electric power supply 500 to the vehicle 100 via an electric wire portion 430 of the charging cable 400. Besides, an intermediate portion of the electric wire portion 430 of the charging cable 400 is provided with a cut-off device 440 for switching between the supply of electric power from the external electric power supply 500 to the vehicle 100 and the cut-off of the supply.
[0039] The charging device 200 includes a charging circuit 210 and capacitors C3 and C4. The charging circuit 210 is connected to the inlet 220 via the positive electrode lines ACL1 and ACL2. The charging circuit 210 is connected to the charging relay Rl via the positive electrode line PL2 and the negative electrode line NL2. The charging circuit 210 is also connected to the charging relay R2 via a positive electrode line that branches from the positive electrode line PL2 and a negative electrode line that branches from the negative electrode line NL2.
[0040] The charging circuit 210 is controlled by a control signal PWE from the ECU 300, and converts the alternating-current electric power supplied from the inlet 220 into direct-current electric power with which the batteries B l and B2 can be charged.
[0041] The capacitor C3 is connected between the positive electrode line PL2 and the negative electrode line NL2, and reduces fluctuations of the voltage between the positive electrode line PL2 and the negative electrode line NL2. Incidentally, because the capacitor C3 is not used for ordinary travel of the vehicle 100, the electric storage capacity of the capacitor C3 may be smaller than that of the capacitor CI. The capacitor C4 is connected between the positive electrode lines ACL1 and ACL2.
[0042] By the way, there sometimes occurs a voltage difference AV between the voltage VI of the battery B l and the voltage V2 of the battery B2, when the batteries B 1 and B2 have been left unused for a long time, or when one of the batteries B 1 and B2 is replaced with a new one. If the voltage difference AV is greater than a permissible value when the batteries B l and B2 are interconnected in parallel, short-circuit between the batteries B 1 and B2 occurs.
[0043] Therefore, the ECU 200 determines whether or not the voltage difference AV is greater than a predetermined value (permissible value). Then, if the voltage difference AV is greater than a predetermined value, the ECU 200 executes a control of equalizing the voltage VI of the battery B l and the voltage V2 of the battery B2 by controlling the charging relays Rl and R2 (hereinafter, referred to as "equalizing control"). [0044] FIG. 2 is a functional block diagram of a portion that relates to the equalizing control of the ECU 300. The functional blocks shown in FIG. 2 may be realized by hardware processes performed by electronic circuits or the like, or may also be realized by software processes performed by execution of programs or the like.
[0045] The ECU 300 includes a setting portion 310 and a control portion 320.
The setting portion 310 sets the variables n, m and X according to predetermined rules (described later), and outputs to the control portion 320 the variables n, m and X as parameters for controlling the charging relays Rl and R2. The following description will be made on the assumption that each of the variables n, m and X is an integer equal to or greater than zero. A concrete technique for setting the variables m and X will be described later with reference to FIGS. 3 and 4.
[0046] The control portion 320 generates control signals SE2 and SE4 so as to equalize the voltage VI and the voltage V2 by using the variables m and X and voltages VI and V2, and outputs the control signals SE2 and SE4 to the charging relays Rl and R2. A concrete control technique for the charging relays Rl and R2 will be described later.
[0047] FIG. 3 is a flowchart showing a process procedure that the ECU performs to realize the function of the setting portion 310 (of setting the variables m and X). The process flow shown below is repeatedly executed with a predetermined period. Besides, each of the steps (hereinafter, abbreviated as "S") in the process flow described below may be realized by hardware processes or may also be realized by software processes as mentioned above.
[0048] In S 10, the ECU 300 determines whether or not the variable n is equal to an upper limit value K. If the variable n is equal to the upper limit value (YES in S 10), the ECU 300 resets the variable n to an initial value of "0" in S 13, and increases the variable m by "1" in S 14.
[0049] On the other hand, if the variable n is not equal to the upper limit value K (NO in S 10), the ECU 300 increases the variable n by "1" in S l l, and maintains the value of the variable m in S 12.
[0050] After the process of S 12 or S 14, the ECU 300 increases the variable X by "1" in S I 5. Incidentally, the latest values of the variables n, m and X are stored in the memory provided in the ECU 300.
[0051] FIG. 4 is a flowchart showing a processing procedure that the ECU 300 executes so as to realize the function of the control portion 320. Incidentally, the process flow shown in FIG. 4 is executed with a predetermined period after a predetermined start condition is satisfied. The start condition may be, for example, a condition that the external charging has been completed, or a condition that there is a demand for start of a drive system after a long time of stop of the drive system. In either case, there is no need to drive the charging circuit 210, the converter 121 or the inverter 122. In the following description, it is assumed that the charging circuit 210, the converter 121 and the inverter 122 are stopped when the process flow shown in FIG. 4 is executed.
[0052] In S20, the ECU 300 determines whether or not a state in which an absolute value |V1-V2| of a difference between the voltage VI and the voltage V2 is greater than a predetermined value V0 has continued for at least a preset time. If the state of |V1-V2| > V0 has continued for at least the preset time (YES in S20), the process proceeds to S21. Otherwise (NO in S20), this process ends.
[0053] In S21, the ECU 300 determines whether or not the variable m has increased. If the variable m has increased (YES in S21), the process proceeds to S22. If the variable m has not increased (NO in S21), the process proceeds to S23.
[0054] In S22, the ECU 300 turns off all the relays RIP, R1N, R2P and R2N for a predetermined period of time. The process of S22 is a process for providing a time during which both batteries B l and B2 are temporarily disconnected from the charging device 200. Due to the process of S22, it is possible to restrain the occurrence of short circuit between the batteries B l and B2 when the switching between the first control and the second control is carried out by a subsequent process of S23 to S31.
[0055] In S23, the ECU 300 determines whether or not the variable m is an odd number. If the variable m is an odd number (YES in S23), the process proceeds to S24.
[0056] In S24, the ECU 300 turns on the positive electrode-side relays RIP and R2P of the charging relays Rl and R2.
[0057] In S25, the ECU 300 determines whether or not the variable X is an odd number. If the variable X is an odd number (YES in S25), the process proceeds to S26. If the variable X is not an odd number (NO in S25), the process proceeds to S27.
[0058] In S26, the ECU 300 turns on the negative electrode-side relay R1N of the charging relay Rl, and turns off the negative electrode-side relay R2N of the charging relay R2. In this state, the battery B 1 is connected to the capacitor C3 of the charging device 200.
[0059] In S27, the ECU 300 turns off the relay RIN, and turns on the relay R2N. In this state, the battery B2 is connected to the capacitor C3 of the charging device 200.
[0060] On other hand, if the variable m is not an odd number (NO in S23), the process proceeds to S28.
[0061] In S28, the ECU 300 turns on the negative electrode-side relays RIN and R2N of the charging relays Rl and R2.
[0062] In S29, the ECU 300 determines whether or not the variable X is an odd number. If the variable X is an odd number (YES in S29), the process proceeds to S30. If the variable X is not an odd number (NO in S29), the process proceeds to S31.
[0063] In S30, the ECU 300 turns on the positive electrode-side relay RIP of the charging replay Rl, and turns off the positive electrode-side relay R2P of the charging relay R2. In this state, the battery B 1 is connected to the capacitor C3 of the charging device 200.
[0064] In S31, the ECU 300 turns off the positive electrode-side relay RIP of the charging relay Rl, and turns on the positive electrode-side relay R2P of the charging relay R2. In this state, the battery B2 is connected to the capacitor C3 of the charging device 200.
[0065] FIG. 5 is a diagram showing manners of change of the variables n, m and X and the charging relays Rl and R2 during execution of the equalizing control. Incidentally, FIG. 5 shows an example in which the upper limit value K is "5".
[0066] As shown in FIG. 5, the variable n increases by "1" at every predetermined period, and is returned to "0" after the variable n reaches the upper limit value of "5". The variable m increases by "1" every time the variable n reaches the upper limit value of "5". The variable X increases by "1" with the same period as the variable n. However, the variable X is not provided with an upper limit value.
[0067] In the case where the variable m is an odd number (m=l, 3, ...), the
ECU 300 alternately turns on the negative electrode-side relays RIN and R2N with the period of increase of the variable X (turns on and off the relays RIN and R2N complementarily with each other) while keeping the positive electrode-side relays RIP and R2P on. Hereinafter, this control will be referred to as "first control". [0068] In the case where the variable m is not an odd number (m=0, 2, 4, ...), the ECU 300, conversely to the first control, alternately turns on the positive electrode-side relays RIP and R2P with the period of increase of the variable X while keeping the negative relays RIN and R2N on. Hereinafter, this control will be referred to as "second control".
[0069] By executing the first control or the second control, the battery B 1 and the battery B2 are alternately connected to the capacitor C3 of the charging device 200 with the period of increase of the variable X. This makes it possible to move electric energy from a battery that is higher in voltage to a battery that is lower in voltage, via the capacitor C3, without connecting the batteries B l and B2 in parallel. For example, if the voltage VI is higher than the voltage V2, electric energy from the battery B l is temporarily stored in the capacitor C3 when the battery Bl is connected to the capacitor C3, and the electric power stored in the capacitor C3 is supplied to the battery B2 when the capacitor C3 is connected to the battery B2. This makes it possible to equalize the voltage VI and the voltage V2 while restraining the short circuit between the batteries B l and B2. At this time, it is not necessary to drive the charging circuit 210, the converter 121, or the inverter 122. The voltage VI and the voltage V2 can be equalized by a relatively simple control of controlling the relays as described above.
[0070] Furthermore, during execution of the first control or the second control, one of the pair of the positive electrode-side relays RIP and R2P and the pair of the negative electrode-side relays RIN and R2N is turned on and off while the other pair is kept on. Therefore, the number of switch on/off actions of the relays is reduced in comparison with the case where the two pairs of relays are turned on and off. Furthermore, the two pairs are alternately selected to be kept on, with the period of increase of the variable m. Therefore, the frequencies of action of the relays are equalized further than in the case where the pair that is kept on is not changed but fixed. Thus, since the relays are controlled in such a manner as to reduce the numbers of actions of the relays and equalize the frequencies of action of the relays, decline of the durability of each relay can be restrained.
[0071] Incidentally, at the time of switch between the first control and the second control, both the positive electrode-side relays RIP and R2P and the negative electrode-side relays RIN and R2N are turned off, and therefore both batteries B l and B2 are temporarily disconnected from the charging device 200. Therefore, the short circuit between the batteries B 1 and B2 is restrained.
[0072] As described above, the ECU 300 in accordance with the embodiment switches between the first control of turning on the negative electrode-side relays R1N and R2N alternately with each other while maintaining the on-state of the positive electrode-side relays RIP and R2P, and the second control of turning on the positive electrode-side relays RIP and R2P alternately with each other while maintaining the on-state of the negative electrode-side relays R1N and R2N, with a predetermined period (i.e., every time the variable m increases by "1"). Due to this, electric energy from a battery whose voltage is relatively high can be moved to a battery whose voltage is relatively low via the capacitor C3, so that it is possible to equalize the voltage VI and the voltage V2 while restraining the short circuit between the batteries B 1 and B2. Furthermore, since the number of actions of the relays are reduced and the frequencies of action of the relays are equalized, the durability of each relay can be improved.
[0073] Incidentally, the number of the batteries connected in parallel may also be three or more. In such a case, it is appropriate to turn on the three or more relays at one of the electrode sides alternately with one another while maintaining the on-state of the three or more relays at the other electrode side. Incidentally, the term "to turn on the three or more relays (at one of the electrode sides) alternately with one another" means that one of the three or more relays is turned on (the other relays are off) at a time and the relay that is turned on is periodically changed to the other one of the three or more relays in rotation.
[0074] [MODIFICATIONS] Although in the foregoing embodiment, energy is moved between the batteries B 1 and B2 via the capacitor C3 of the charging device 200, the movement of energy between the batteries B 1 and B2 may also be performed via the capacitor CI of the PCU 120. In this case, it suffices that the control objects are changed from the relays RIP, R2P, R1N and R2N in the process shown in FIG. 4 to the relays SMR1P, SMR2P, SMR1N and SMR2N, respectively.
[0075] FIG. 6 shows a flowchart showing a processing procedure performed by the ECU 300 in accordance with a modification. Incidentally, of the steps shown in FIG. 6, the steps denoted by the same reference characters as those in FIG. 4 will not be redundantly described since descriptions thereof have been given above.
[0076] In S22a, the ECU 300 turns off all the relays SMR1P, SMR2P, SMR1N and SMR2N for a predetermined time. [0077] In S24a, the ECU 300 turns on the positive electrode-side relays SMRIP and SMR2P of the system main relays SMRl and SMR2.
[0078] In S26a, the ECU 300 turns on the negative electrode-side relay SMR1N of the system main relay SMRl, and turns off the negative electrode-side relay SMR2N of the system main relay SMR2. In this state, the battery Bl is connected to the capacitor CI of the PCU 120.
[0079] In S27a, the ECU 300 turns off the relay SMR1N, and turns on the relay SMR2N. In this state, the battery B2 is connected to the capacitor CI of the PCU 120.
[0080] In S28a, the ECU 300 turns on the negative electrode-side relays
SMR1N and SMR2N of the system main relays SMRl and SMR2.
[0081] In S30a, the ECU 300 turns on the positive electrode-side relay SMRIP of the system main relay SMRl, and turns off the positive electrode-side relay SMR2P of the system main relay SMR2. In this state, the battery B l is connected to the capacitor C 1 of the PCU 120.
[0082] In S3 la, the ECU 300 turns off the relay SMRIP of the system main relay SMRl, and turns on the positive electrode-side relay SMR2P of the system main relay SMR2. In this state, the battery B2 is connected to the capacitor CI of the PCU 120.
[0083] In this manner, electric energy can be moved from the battery whose voltage is relatively high to the battery whose voltage is relatively low, via the capacitor CI. Besides, the electricity storage capacity of the capacitor CI is larger than that of the capacitor C3. Therefore, in comparison with the case where the capacitor C3 is used, the current that flows between the capacitor CI and the batteries B 1 and B2 is larger, and the moving speed of the electric energy is greater, so that the voltage V 1 and the voltage V2 can be equalized earlier in the case where the capacitor CI is used.
[0084] The foregoing embodiment and modifications are illustrative in all respects, and not restrictive at all. The scope of the invention is defined not by the foregoing description, but by the appended claims for patent, and is intended to cover all the changes and modifications within the meaning and scope equivalent to the claims for patent.
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