Patent Publication Number: US-2022231619-A1

Title: Power converter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application No. PCT/JP2020/031038 filed Aug. 17, 2020 which designated the U.S. and claims priority to Japanese Patent Application No. 2019-183117 filed with the Japan Patent Office on Oct. 3, 2019, the contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a power converter. 
     Related Art 
     Conventionally, a voltage equalization device is known that equalizes terminal voltages between battery cells forming an assembled battery. Specifically, this voltage equalization device includes two switching elements, one for each of two adjacent battery cells, and a reactor. The switching elements and the reactor form a buck-boost converter. This buck-boost converter operates to transfer energy between the battery cells so as to equalize the terminal voltages between the battery cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is schematic diagram of a power converter according to a first embodiment; 
         FIG. 2  is a flowchart of process steps performed by a control unit; 
         FIGS. 3A-3B  are schematic diagrams of an equivalent circuit; 
         FIG. 4  is a block diagram illustrating a process performed by the control unit when a vehicle is stationary; 
         FIG. 5  is an illustration of a method of setting a command current; 
         FIG. 6  is a block diagram illustrating a process performed by the control unit when the rotating electric machine is being driven; 
         FIG. 7  is a timing chart illustrating a transition of each modulation rate during power transfer from a first rechargeable battery to a second rechargeable battery; 
         FIG. 8  is a timing chart illustrating a transition of each modulation rate during power transfer from the second rechargeable battery to the first rechargeable battery; 
         FIGS. 9A-9D  are a timing chart illustrating a transition of each phase current or the like during power transfer from the first rechargeable battery to the second rechargeable battery; 
         FIGS. 10A-10D  are a timing chart illustrating a transition of each phase current or the like during power transfer from the second rechargeable battery to the first rechargeable battery; 
         FIG. 11  is a schematic diagram of a power converter according to a second embodiment; 
         FIG. 12  is a schematic diagram of a power converter according to a third embodiment; 
         FIG. 13  is a schematic diagram of a power converter according to a fourth embodiment; 
         FIG. 14  is a schematic diagram of a power converter according to a fifth embodiment; 
         FIG. 15  is a flowchart of process steps performed by a control unit; 
         FIGS. 16A-16D  are a timing chart illustrating a transition of each phase current or the like during charging from an external charger; and 
         FIG. 17  is a schematic diagram of a power converter according to another embodiment. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     The above known voltage equalization device, as described in JP 2013-247690 A, needs dedicated switching elements and the reactor to transfer energy between the battery cells. Thus, there is a concern that the voltage equalization device may increase in size. 
     In view of the above, it is desired to have a power converter that can be downsized. 
     One aspect of the present disclosure provides a power converter includes a rotating electric machine including windings, and an inverter including a series connection of an upper arm switch and a lower-arm switch for each phase. The power converter further includes a connection path electrically connecting a negative side of a first rechargeable battery, a positive side of a second rechargeable battery electrically connected in series with the first rechargeable battery, and a neutral point of the windings, and a control unit configured to perform switching control of the upper-arm switch and the lower-arm switch for each phase in order to transfer energy between the first and second rechargeable batteries by conducting current between the first and second rechargeable batteries via the inverter, the windings, and the connection path. 
     In the present disclosure, the negative side of the first rechargeable battery and the positive side of the second rechargeable battery are electrically connected to the neutral point of the windings by the connection path. Therefore, switching control of the upper and lower arm switches allows current to flow between the first and second rechargeable batteries via the inverter, the windings, and the connection path, which enables energy transfer between the first and second rechargeable batteries. 
     According to the present disclosure described above, energy can be transferred between the first and second rechargeable batteries by sharing the windings and inverter of the rotating electric machine. This makes it possible to reduce the size of the power converter. 
     First Embodiment 
     A power converter according to a first embodiment of the present disclosure will now be described with reference to the accompanying drawings. The power converter of the present embodiment is mounted to, for example, an electric vehicle (EV) or a hybrid vehicle (HV). 
     As illustrated in  FIG. 1 , the power converter  10  includes an inverter  30  and a rotating electrical machine  40 . The rotating electrical machine  40  is a three-phase synchronous machine having U-, V-, and W-phase windings  41 U,  41 V,  41 W star-connected as stator windings. The U-, V-, and W-phase windings  41 U,  41 V,  41 W are 120 degrees in electrical angle out of phase. The rotating electrical machine  40  is, for example, a permanent magnet synchronous machine. In the present embodiment, the rotating electrical machine  40  is a vehicle-mounted prime mover that serves as a driving power source of the vehicle. 
     The inverter  30  includes a series connection of an upper-arm switch QUH and a lower-arm switch QUL of the U-phase, an upper-arm switch QVH and a lower-arm switch QVL of the V-phase, and an upper-arm switch QWH and a lower-arm switch QWL of the W-phase. In the present embodiment, each of the switches QUH, QVH, QWH, QUL, QVL, and QWL is a voltage-controlled semiconductor switching element, such as an insulated gate bipolar transistor (IGBT). Therefore, the high-side terminal of each of the switches QUH, QVH, QWH, QUL, QVL, and QWL is the collector. The low-side terminal of each of the switches QUH, QVH, QWH, QUL, QVL, and QWL is the emitter. Each of the switches QUH, QVH, QWH, QUL, QVL, and QWL is provided with a freewheel diode connected in anti-parallel. 
     The emitter of the U-phase upper-arm switch QUH and the collector of the U-phase lower-arm switch QUL are connected to a first end of the U-phase winding  41 U through a U-phase conductive member  32 U, such as a bus bar. The emitter of the V-phase upper-arm switch QVH and the collector of the V-phase lower-arm switch QVL are connected to a first end of the V-phase winding  41 V through a V-phase conductive member  32 V, such as a bus bar. The emitter of the W-phase upper-arm switch QWH and the collector of the W-phase lower-arm switch QWL are connected to a first end of the W-phase winding  41 W through a W-phase conductive member  32 W, such as a bus bar. Second ends of the U-, V-, and W-phase windings  41 U,  41 V,  41 W are connected to each other at a neutral point O. This means that the phase windings  41 U,  41 V,  41 W are configured to have the same inductance. 
     The collector of each of the upper-arm switches QUH, QVH, and QWH is connected to the positive terminal of the assembled battery  20  by a positive bus Lp such as a bus bar. The emitter of each of the lower-arm switches QUL, QVL, QWL is connected to the negative terminal of the assembled battery  20  by a negative bus Lp such as a bus bar. 
     The power converter  10  includes a capacitor  31  that connects the positive bus Lp and the negative bus Ln. The capacitor  31  may be included in the inverter  30  or provided outside the inverter  30 . 
     The assembled battery  20  is configured as a series connection of battery cells as a single battery, with a terminal voltage of several hundred volts, for example. In the present embodiment, the terminal voltages (e.g., rated voltage) of respective ones of the battery cells forming the assembled battery  20  are set to the same as each other. For example, each battery cell may be a secondary battery, such as a lithium-ion battery. The assembled battery  20  may be provided outside the power converter  10 . 
     In the present embodiment, among the battery cells forming the assembled battery  20 , a series connection of a plurality of battery cells on the high side forms a first rechargeable battery  21 , and a series connection of a plurality of battery cells on the low side forms a second rechargeable battery  22 . That is, the assembled battery  20  is divided into two blocks. In the present embodiment, the number of battery cells forming the first rechargeable battery  21  and the number of battery cells forming the second rechargeable battery  22  are equal to each other. Therefore, the terminal voltage (e.g., rated voltage) of the first rechargeable battery  21  and the terminal voltage (e.g., rated voltage) of the second rechargeable battery  22  are equal to each other. 
     In the assembled battery  20 , the negative terminal of the first rechargeable battery  21  and the positive terminal of the second rechargeable battery  22  are connected to an intermediate terminal B. 
     The power converter  10  includes a monitoring unit  50 . The monitoring unit  50  monitors the terminal voltage, the state of charge (SOC), the state of health (SOH), the temperature, and the like, of each of the battery cells forming the assembled battery. 
     The power converter  10  includes a connection path  60  and a connection switch  61 . The connection path  60  electrically connects the intermediate terminal B of the assembled battery  20  to the neutral point O. The connection switch  61  is provided on the connection path  60 . In the present embodiment, a relay is used as the connection switch  61 . When the connection switch  61  is turned on, the intermediate terminal B and the neutral point O are electrically connected. When the connection switch  61  is turned off, the intermediate terminal B and the neutral point O are electrically disconnected. 
     The power converter  10  includes a current sensor  62  and a phase current sensor  63 . The current sensor  62  detects a current flowing through the connection path  60 . The phase current sensor  63  detects at least two of the three phase currents. The phase current sensor  63  detects, for example, currents flowing through at least two of the conducting members  32 U- 32 W. Detected values from each current sensor  62 ,  63  are transmitted to the control unit  70  (corresponding to a control unit) included in the power converter  10 . 
     The control unit  70  is mainly configured as a microcomputer and performs switching control for controlling switching of the switching elements forming the inverter  3  to feedback-control a controlled variable to its command value. The controlled variable is, for example, torque. In each phase, the upper-arm switch and the lower-arm switch are turned on alternately. 
     The control unit  70  turns on and off the connection switch  61  and is communicable with the monitoring unit  50 . The control unit  70  performs various control functions by executing programs stored in a storage device provided in the control unit  70 . The various control functions may be implemented by electronic circuits as hardware, or by both hardware and software. 
     Equalization control performed by the control unit  70  will now be described.  FIG. 2  is a flowchart illustrating process steps of an equalization control process. This equalization control process is repeatedly performed by the control unit  70 , for example, every predefined control cycle. 
     At step S 10 , the control unit  70  determines whether there is an equalization request for equalizing the terminal voltages of the first rechargeable battery  21  and the second rechargeable battery  22 . In the present embodiment, if the control unit  70  determines that an absolute value of a difference between the terminal voltage VBH of the first rechargeable battery  21  and the terminal voltage VBL of the second rechargeable battery  22  exceeds a predefined value ΔV, the control unit  70  determines that there is the equalization request for equalizing the terminal voltages of the first rechargeable battery  21  and the second rechargeable battery  22 . The terminal voltage VBH of the first rechargeable battery  21  and the terminal voltage VBL of the second rechargeable battery  22  may be acquired from the monitoring unit  50 . 
     If at step S 10  the control unit  70  determines that there is no equalization request, the process flow proceeds to step S 11 , where the control unit  70  determines whether there is a drive request for driving the rotating electric machine  40 . In the present embodiment, this drive request includes a request for driving the vehicle by rotationally driving the rotating electrical machine  40 . 
     If at step S 11  the control unit  70  determines that there is no drive request, the process flow proceeds to step S 12 . At step S 12 , the control unit  70  sets the operating mode of the rotating electrical machine  40  to a standby mode. Setting the operating mode to the standby mode allows each of the switches of the inverter  30 , QUH to QWL, to be turned off. Then, at step S 13 , the control unit  70  turns off the connection switch  61 . This electrically disconnects the intermediate terminal B from the neutral point O. 
     If at step S 11  the control unit  70  determines that there is the drive request, the process flow proceeds to step S 14 . At step S 14 , the control unit  70  sets the operating mode of the rotating electric machine to a drive mode. Then, at step S 16 , the control unit  70  turns on the connection switch  61 . This electrically connects the intermediate terminal B and the neutral point O via the connection path  60 . Then, at step S 16 , the control unit  70  performs switching control of each of the switches of the inverter  30 , QUH to QWL, to rotationally drive the rotating electrical machine  40 . This causes drive wheels of the vehicle to rotate, thereby enabling movement of the vehicle. 
     If at step S 10  the control unit  70  determines that there is the equalization request, the process flow proceeds to step S 17 . At step S 17 , the control unit  70  sets the operating mode of the rotating electric machine to an equalization control mode. Then, at step S 18 , the control unit  70  turns on the connection switch  61 . 
     At step S 19 , the control unit  70  performs the equalization control to equalize the terminal voltages of the first rechargeable battery  21  and the second rechargeable battery  22 . This equalization control will now be described. 
       FIG. 3A  illustrates an equivalent circuit of the power converter  10  used in equalization control. In  FIG. 3A , each of the phase windings  41 U- 41 W is denoted as a winding  41 , each of the upper-arm switches QUH, QVH, and QWH is denoted as an upper-arm switch QH, and each of the upper-arm diodes DUH, DVH, and DWH is denoted as an upper-arm diode DH. Each of the lower-arm switches QUL, QVL, and QWL is denoted as a lower-arm switch QL, and each of the lower-arm diodes DUL, DVL, and DWL is denoted as a lower-arm diode DL. 
     The equivalent circuit in  FIG. 3A  may be illustrated as an equivalent circuit in  FIG. 3B . The circuit in  FIG. 3B  is a buck-boost chopper circuit that can bidirectionally transfer power between the first and second rechargeable batteries  21  and  22 . In  FIG. 3B , IBH represents current flowing through the first rechargeable battery  21 , and IBL represents current flowing through the second rechargeable battery  22 . When charging current flows through each of the first and second rechargeable batteries  21 ,  22 , IBH and IBL take a negative value. When discharging current flows through each of the first and second rechargeable batteries  21 ,  22 , IBH and IBL take a positive value. VR represents a terminal voltage across the winding  41 , and IR represents current flowing through the neutral point O. When current flows through the neutral point O in a positive direction from the winding  41  to the intermediate terminal B, IR takes a negative value. When current flows through the neutral point O in a negative direction from the intermediate terminal B to the winding  41 , IR takes a positive value. 
     Referring to  FIG. 3B , when the upper-arm switch QH is turned on, the terminal voltage VR of the winding  41  becomes “VBH”. When the lower-arm switch QL is turned on, the terminal voltage VR of the winding  41  becomes “VBL”. That is, turning on the upper-arm switch QH can cause a positive excitation current to flow through the winding  41 . Turning on the lower-arm switch QL can cause a negative excitation current to flow through the winding  41 . 
       FIG. 4  illustrates a block diagram of equalization control.  FIG. 4  is a control block of equalization control that is performed while the vehicle is stationary before the rotating electrical machine  40  is driven. 
     The control unit  70  includes an equalization control unit  90 . The equalization control unit  90  includes a command value setting unit  91 , a neutral-point deviation calculation unit  92 , a neutral-point control unit  93 , and U-, V- and W-phase superposition units  94 U to  94 W. 
     The command value setting unit  91  sets a neutral-point command current IM*. Specifically, the command value setting unit  91  subtracts the terminal voltage VBL of the second rechargeable battery  22  from the terminal voltage VBH of the first rechargeable battery  21  to thereby calculate a judgment voltage Vj (=VBH−VBL). If the calculated judgment voltage Vj is positive, the command value setting unit  91  sets the neutral-point command current IM* to a positive value. Specifically, as illustrated in  FIG. 5 , the higher the judgment voltage Vj is, the larger the neutral-point command current IM* is set. 
     If the calculated judgment voltage Vj is negative, the command value setting unit  91  sets the neutral-point command current IM* to a negative value. Specifically, as illustrated in  FIG. 5 , the larger the absolute value of the judgment voltage Vj, the larger the absolute value of the neutral-point command current IM* is set. 
     The neutral-point deviation calculation unit  92  subtracts a neutral-point current IMr that is a current detected by the current sensor  62  from the neutral-point command current IM* to thereby calculate a neutral-point current deviation ΔIM. In the present embodiment, the neutral-point command current IM* is a direct-current (DC) signal. 
     The neutral-point control unit  93  calculates an offset correction amount CF as a manipulated variable for feedback-controlling the calculated neutral-point current deviation ΔIM to zero. In the present embodiment, proportional-integral control is used for this feedback control. The feedback control is not limited to proportional-integral control, but may be, for example, proportional-integral-derivative control. 
     The U-phase superposition unit  94 U adds the offset correction amount CF to a U-phase command voltage Vu to calculate a U-phase final command voltage “Vu+CF”. The V-phase superposition unit  94 V adds the offset correction amount CF to a V-phase command voltage Vv to calculate a V-phase final command voltage “Vv+CF”. The W-phase superposition unit  94 W adds the offset correction amount CF to a W-phase command voltage Vw to calculate a W-phase final command voltage “Vw+CF”. In the process illustrated in  FIG. 4 , as the vehicle is stationary, each of the U-, V-, and W-phase command voltages Vu, Vv, and Vw is zero. Therefore, each of the U-, V-, and W-phase final command voltages is the offset correction amount CF. 
     The control unit  70  includes a U-, V-, and W-phase modulation units  95 U,  95 V,  95 W. The U-phase modulation unit  95 U divides the U-phase final command voltage by a power-source voltage Vdc to calculate a U-phase modulation ratio Mu. The power-source voltage Vdc is a sum of the terminal voltage VBH of the first rechargeable battery  21  and the terminal voltage VBL of the second rechargeable battery  22  acquired from the monitoring unit  50 . The V-phase modulation unit  95 V divides the V-phase final command voltage by the power-source voltage Vdc to calculate a V-phase modulation ratio Mv. The W-phase modulation unit  95 W divides the W-phase final command voltage by the power-source voltage Vdc to calculate a W-phase modulation ratio Mw. 
     Based on each of the calculated modulation ratios Mu, Mv, and Mw, the control unit  70  performs switching control of the U-, V-, and W-phase switches QUH to QWL. Specifically, for example, the control unit  70  may perform switching control of the U-, V-, and W-phase switches QUH to QWL by PWM control based on comparison in magnitude between a carrier signal (e.g., triangular wave signals) and each of the modulation ratios Mu, Mv, and Mw. 
     Equalization control can be performed not only when the vehicle is stationary, but also when the vehicle is moving by driving the rotating electrical machine  40 .  FIG. 6  illustrates a control block of equalization control performed when the vehicle is moving by driving the rotating electrical machine  40 . For illustration purposes, in  FIG. 6 , the same structural elements as in  FIG. 4  share the same reference numerals. 
     In the control unit  70 , a d-axis deviation calculation unit  100   d  subtracts a d-axis current Idr from a d-axis command current Id* to calculate a d-axis current deviation Δ. A q-axis deviation calculation unit  100   q  subtracts a q-axis current Iqr from a q-axis command current Iq* to calculate a q-axis current deviation Δ. The d-axis command current Id* and the q-axis command current Iq* are set based on a command torque for the rotating electrical machine  40 . The d-axis current Idr and the q-axis current Iqr are set based on detected values from the phase current sensor  63  and electrical angles of the rotating electrical machine  40 . The electrical angle may be a detected value from a rotation angle sensor such as a resolver, or may be an estimated value estimated in position sensorless control. 
     A d-axis control unit  101   d  calculates a d-axis voltage Vd as a manipulated variable to feedback-control the calculated d-axis current deviation ΔId to zero. A q-axis control unit  101   q  calculates a q-axis voltage Vq as a manipulated variable to feedback-control the calculated q-axis current deviation ΔIq to zero. In the present embodiment, proportional-integral control is used as the feedback control of each of the control units  101   d ,  101   q . The feedback control is not limited to proportional-integral control, but may be, for example, proportional-integral-derivative control. 
     A three-phase conversion unit  102  calculates U-, V-, and W-phase command voltages Vu to Vw in the 3-phase fixed coordinate system based on the d-axis voltage Vd, the q-axis voltage Vq, and the above electrical angles. The respective phase command voltages Vu to Vw are signals (specifically, sinusoidal signals) shifted in phase from each other by an electrical angle of 120 degrees. 
     U-, V- and W-phase superposition units  94 U to  94 W adds the offset correction amount CF to the U-, V- and W-phase command voltages calculated by the three-phase conversion unit  102 . The U-, V-, and W-phase final command voltages are thereby calculated. 
       FIG. 7  illustrates a transition of each of the phase modulation rates Mu to Mw in a case where the neutral-point command current IM* is positive. In this case, current is supplied from the first rechargeable battery  21  to the second rechargeable battery  22 , and the terminal voltages of the respective rechargeable batteries  21 ,  22  are equalized. 
       FIG. 8  illustrates a transition of each of the phase modulation rates Mu to Mw in a case where the neutral-point command current IM* is negative. In this case, current is supplied from the second rechargeable battery  22  to the first rechargeable battery  21 , and the terminal voltages of the respective rechargeable batteries  21 ,  22  are equalized. 
       FIG. 9  illustrates each waveform in a case where the neutral-point command current IM* is set to a positive value.  FIG. 9A  illustrates a transition of each of the phase currents Iu, Iv, and Iw.  FIG. 9B  illustrates a transition of the neutral-point current IMr.  FIG. 9C  illustrates a transition of the current IBH flowing through the first rechargeable battery  21 .  FIG. 9D  illustrates a transition of the current IBL flowing through the second rechargeable battery  22 .  FIG. 10  illustrates each waveform in a case where the neutral-point command current IM* is set to a negative value.  FIGS. 10A-10D  correspond to  FIGS. 9A-9D , respectively. As illustrated in  FIGS. 9B and 10B , DC current flows through the connection path  60 . 
     The present embodiment described in detail above can provide the following advantages. 
     The intermediate terminal B and the neutral point O are electrically connected by the connection path  60 . Therefore, when it is determined that there is the equalization request, switching control of the switches QUH to QWL is performed to conduct electrical current between the first rechargeable battery  21  and the second rechargeable battery  22  via the inverter  30 , the windings  41 U- 41 W and the connection path  60 , which enables equalization of the terminal voltages of the first and second rechargeable batteries  21  and  22 . This allows the terminal voltages of the first and second rechargeable batteries  21  and  22  to be equalized using the existing windings  41 U- 41 W and inverter  30 . Therefore, there is no need to add a dedicated reactor for equalization, which allows the power converter  10  to be downsized. 
     When it is determined that there is the equalization request, the connection switch  61  provided along the connection path  60  is turned on. When it is determined that there is no equalization request, the connection switch  61  is turned off. This can inhibit current from flowing between the neutral point O and the intermediate terminal B when there is no equalization request. 
     When it is determined that there is the equalization request while the rotating electrical machine  40  is being driven, switching control of each of the switches QUH to QWL is performed to equalize the terminal voltages of the first rechargeable battery  21  and the second rechargeable battery  22  while driving the rotating electrical machine  40 . This allows both drive control of the rotating electrical machine  40  and equalization control to be performed. 
     In equalization control, switching control of the U-, V-, and W-phase upper-arm switches QUH, QVH, and QWH is synchronized, and switching control of the U-, V-, and W-phase lower-arm switches QUL, QVL, and QWL is synchronized. This allows the phase windings  41 U,  41 V,  41 W to be regarded as an equivalent circuit in which the windings are connected in parallel. Therefore, the inductance of the windings during equalization control can be reduced. This can increase an amount of change in current flowing through the neutral point in one switching cycle of each of the switches QUH to QWL, which allows equalization control to be performed with a large current, for example, when the vehicle is stationary. 
     Second Embodiment 
     A second embodiment will now be described with reference to the accompanying drawings, focusing on differences from the first embodiment. In the present embodiment, as illustrated in  FIG. 11 , the second rechargeable battery  22  is connected in parallel with an electric compressor  110  and a DC-DC converter  111 . For illustration purposes, in  FIG. 11 , the same structural elements as in  FIG. 1  share the same reference numerals. 
     The electric compressor  110  is provided for cabin air conditioning and is driven to circulate refrigerant in the refrigeration cycle. The DC-DC converter  111  is driven to buck or step down the output voltage of the second rechargeable battery  22  and supply it to a low-voltage rechargeable battery  120 . The low-voltage rechargeable battery  120  is, for example, a lead-acid battery with a rated voltage of 12 V. 
     In the present embodiment, in response to the control unit  70  determining that at least one of the electric compressor  110  and the DC-DC converter  111  is being driven, the control unit  70  determines that there is the equalization request. In response to determining that there is the equalization request, the control unit  70  performs switching control of the respective switches QUH to QWL to conduct current from the first rechargeable battery  21  to the second rechargeable battery  22  via the inverter  30  and the connection path  60 , thereby equalizing the terminal voltages of the first and second rechargeable batteries  21  and  22 . 
     According to the above described embodiment, even in cases where electric power is taken out of the second rechargeable battery  22  by driving at least one of the electric compressor  110  and the DC-DC converter  111 , it is possible to suppress significant variations in the state of charge (SOC) of each of the rechargeable batteries  21  and  22 . 
     Third Embodiment 
     A third embodiment will now be described with reference to the accompanying drawings, focusing on differences from the second embodiment. In the present embodiment, as illustrated in  FIG. 12 , the first rechargeable battery  21  is connected in parallel with the electric compressor  110 , and the second rechargeable battery  22  is connected in parallel with the DC-DC converter  111 . For illustration purposes, in  FIG. 12 , the same structural elements as in  FIG. 11  share the same reference numerals. 
     In response to the control unit  70  determining that at least one of the electric compressor  110  and the DC-DC converter  111  is being driven, the control unit  70  determines that there is the equalization request. In response to determining that there is the equalization request, the control unit  70  performs switching control of the respective switches QUH to QWL to conduct current between the first rechargeable battery  21  and the second rechargeable battery  22  via the inverter  30  and the connection path  60  to equalize the terminal voltages of the first and second rechargeable batteries  21  and  22 . 
     For example, in response to the control unit  70  determining that electric power taken out of the first rechargeable battery  21  by driving the electric compressor  110  is greater than electric power taken out of the second rechargeable battery  22  by driving the DC-DC converter  111 , the control unit  70  performs switching control of the respective switches QUH to QWL to conduct current from the second rechargeable battery  22  to the first rechargeable battery  21  via the inverter  30  and the connection path  60 . In response to the control unit  70  determining that electric power taken out of the second rechargeable battery  22  by driving the DC-DC converter  111  is greater than electric power taken out of the first rechargeable battery  21  by driving the electric compressor  110 , the control unit  70  performs switching control of the respective switches QUH to QWL to conduct current from the first rechargeable battery  21  to the second rechargeable battery  22  via the inverter  30  and the connection path  60 . 
     According to the above described embodiment, even in cases where electric power taken by the electric compressor  110  and the DC-DC converter  111  respectively out of the rechargeable batteries  21  and  22 , the drive timings or the operation rates of the electric compressor  110  and the DC-DC converter  111  are significantly different, it is possible to suppress significant variations in the state of charge (SOC) of each of the rechargeable batteries  21  and  22 . 
     Fourth Embodiment 
     A fourth embodiment will now be described with reference to the accompanying drawings, focusing on differences from the second embodiment. In the present embodiment, as illustrated in  FIG. 13 , the first rechargeable battery  21  is connected in parallel with the electric compressor  110  and the DC-DC converter  111 . For illustration purposes, in  FIG. 13 , the same structural elements as in  FIG. 11  share the same reference numerals. 
     The present embodiment of the present disclosure can provide advantages similar to those of the second embodiment. 
     Fifth Embodiment 
     A fifth embodiment will now be described with reference to  FIG. 14 , focusing on differences from the first embodiment. For illustration purposes, in  FIG. 14 , the same structural elements as in  FIG. 1  share the same reference numerals. 
     In the present embodiment, the rated voltage of each of the first and second rechargeable batteries  21  and  22  is 400 V. Therefore, the rated voltage of the assembled battery  20  is 800 V. 
     The second rechargeable battery  22  (corresponding to a “subject battery”) is connectable to a first charger  121  provided outside the vehicle. The series connection of the first rechargeable battery  21  and the second rechargeable battery  22  is connectable to a second charger  122  outside the vehicle. The charging voltage of the second charger  122  is higher than that of the first charger  121 . The first charger  121  supports fast charging, and the second charger  122  supports ultra-fast charging. 
     The intermediate terminal B is connectable to the positive side of the first charger  121  via the first switch SW 1 . The negative side of the second rechargeable battery  22  is connectable to the negative side of each of the first charger  121  and the second charger  122  via the switch SW 2 . The positive side of the first rechargeable battery  21  is connectable to the positive side of the second charger  122  via the third switch SW 3 . In the present embodiment, the first to third switches SW 1  to SW 3  are turned on or off by the control unit  70 . 
     Process steps of the equalization control process according to the present embodiment will now be described with reference to  FIG. 15 . This process is repeatedly performed by the control unit  70 , for example, every predefined control cycle. 
     At step S 30 , the control unit  70  determines whether there is a request for fast-charging of the second rechargeable battery  22  with the first charger  121 . 
     If the answer is YES at step S 30 , the control unit  70  determines that there is the equalization request. Then, the process flow proceeds to step S 31 . At step S 31 , the control unit  70  turns on the first and second switches SW 1  and SW 2  and turns off the third switch SW 3 . The control unit  70  further turns on the connection switch  61 . 
     At step S 32 , the control unit  70  performs switching control of the U-, V-, and W-phase switches QUH to QWL to conduct current from the second rechargeable battery  22  to the first rechargeable battery  21  via the inverter  30  and the connection path  60 . Even in the absence of the second charger  122 , this allows the assembled battery  20  to be properly charged by the first charger  121  while equalizing the terminal voltages of the first rechargeable battery  21  and the second rechargeable battery  22 .  FIG. 16  illustrates a transition of each waveform during the process step S 32 .  FIGS. 16A-16D  respectively correspond to  FIGS. 9A-9D  described above. 
     If the answer is NO at step S 30 , the process flow proceeds to step S 33 . At step S 33 , the control unit  70  determines whether there is a request for ultra-fast charging of the assembled battery  20  with the second charger  122 . 
     If the answer is YES at step S 32 , the process flow proceeds to step S 34 . At step S 34 , the control unit  70  turns on the second and third switches SW 2  and SW 3  and turns off the first switch SW 1 . The control unit  70  further turns on the connection switch  61 . This allows the assembled battery  20  to be charged by the second charger  122 . 
     If the answer is NO at step S 33 , the control unit  70  may turn off the first to third switches SW 1  to SW 3  and the connection switch  61 . 
     According to the embodiment described above, in a system that supports ultra-rapid charging at 800 V, performing equalization control allows the assembled battery  20  to be fast-charged at 400 V. 
     In the present embodiment, for example, the second rechargeable battery  22  may be connected in parallel with the electric compressor  110  and the DC-DC converter  111 . In this case, in a system that supports ultra-fast charging at 800 V, the high-voltage electrical load may be used. That is, in the system that supports ultra-fast charging at 800 V, the input voltage of the high-voltage electrical load can be halved. 
     Other Embodiments 
     The above embodiments may be modified and implemented as follows. 
     (1) In the configuration illustrated in  FIG. 14  of the fifth embodiment, the first charger  121  may be used to charge not the second rechargeable battery  22  but the first rechargeable battery  21 . 
     (2) The rotating electrical machine and the inverter are not limited to three-phase rotating electrical machine and inverter, but may be, for example, five-phase or seven-phase rotating electrical machine and inverter.  FIG. 17  illustrates a five-phase power converter. For illustration purposes, in  FIG. 17 , the same structural elements as in  FIG. 1  share the same reference numerals. 
     In  FIG. 17 , the inverter  30  further includes X-phase upper and lower-arm switches QXH, QXL and diodes DXH, DXL, and Y-phase upper- and lower-arm switches QYH, QYL and diodes DYH, DYL. In addition, the rotating electrical machine  40  further includes an X-phase winding  41 X and a Y-phase winding  41 Y. The power converter  10  further includes an X-phase conductive member  32 X and a Y-phase conductive member  32 Y. 
     (3) The installation position of the current sensor that detects the current flowing through the neutral point O is not limited to the position illustrated in  FIG. 1 . For example, the current sensor may be provided along each of the conductive members  32 U,  32 V,  32 W. In such a configuration, during equalization control, the neutral-point current IMr may be a sum of currents detected by the respective current sensors provided along the conductive members  32 U,  32 V,  32 W. 
     (4) In equalization control, the control unit  70  may not synchronize switching control of the U-, V-, and W-phase upper-arm switches QUH, QVH, and QWH. In equalization control, the control unit  70  may not synchronize switching control of the U-, V-, and W-phase lower-arm switches QUL, QVL, and QWL. 
     (5) The connection switches  61  is not limited to a relay. As the connection switches  61 , for example, a pair of N-channel MOSFETs with their sources electrically connected or IGBTs with their sources electrically connected may be used. 
     (6) The connection switch  61  is not indispensable. In a configuration where the connection switch  61  is absent, the intermediate terminal B and the neutral point O are always electrically connected. 
     (7) The upper and lower-arm switches that form the inverter are not limited to IGBTs, but may be N-channel MOSFETs. 
     (8) The first and second rechargeable batteries may not form an assembled battery. 
     (9) In each of the above embodiments, energy is transferred between the first rechargeable battery  21  and the second rechargeable battery  22  in order equalize the terminal voltages of the first and second rechargeable batteries  21  and  22 . Alternatively, energy may be transferred between the first and second rechargeable batteries  21  and  22  regardless of whether to equalize the terminal voltages of the first and second rechargeable batteries  21  and  22 . 
     In this case, for example, in a modification to the first embodiment, instead of making a determination as to whether there is a request for equalizing the terminal voltages of the first and second rechargeable batteries  21  and  22 , a determination may be made as to whether there is a request for energy transfer from one of the first and second rechargeable batteries  21  and  22  to the other. In response to determining that there is a request for energy transfer, the command value setting unit  91  may calculate a target value of energy to be transferred from one of the first and second rechargeable batteries  21  and  22  to the other. Based on the calculated target value of energy, the command value setting unit  91  may set the neutral-point command current IM*. Specifically, for example, when transferring energy from the first rechargeable batteries  21  to the second rechargeable battery  22 , the command value setting unit  91  may calculate a positive target value of energy such that the larger the positive target value of energy, the larger the neutral-point command current IM* is set. When transferring energy from the second rechargeable batteries  22  to the first rechargeable battery  21 , the command value setting unit  91  may calculate a negative target value of energy such that the larger the absolute value of the negative target value of energy, the larger the absolute value of the neutral-point command current IM* is set. 
     (10) In the above-described embodiments and modifications, the control unit  70  and its method described in the present disclosure may be implemented by a dedicated computer including a processor and a memory programmed to execute one or more functions embodied by computer programs. Alternatively, the control unit  70  and its method described in the present disclosure may be implemented by a dedicated computer including a processor formed of one or more dedicated hardware logic circuits, or may be implemented by one or more dedicated computers including a combination of a processor and a memory programmed to execute one or more functions and a processor formed of one or more dedicated hardware logic circuits. The computer programs may be stored, as instructions to be executed by a computer, in a non-transitory, tangible computer-readable storage medium. 
     Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the aforementioned embodiments and configurations. The present disclosure includes various variations and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms further including only one element, or more or less than that, are within the scope and spirit of the present disclosure.