Patent Publication Number: US-2022231537-A1

Title: Conversion device, conversion system, switching device, vehicle including the same, and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage of PCT/JP2019/018790 filed on May 10, 2019, the contents of which are incorporated herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a conversion device, a conversion system, a switching device, a vehicle including the same, and a control method. 
     BACKGROUND 
     Increasing travel ranges and shortening battery charging times are issues in electric automobiles. In light of such issues, both battery capacities and battery voltages are expected to increase (i.e., higher voltages) in the future. 
     Higher battery voltages are expected to improve fast charging output. However, as a battery voltage increases, devices connected to the battery (e.g., a DC/DC converter or the like) must have higher breakdown voltages. JP 2018-85790A, indicated below, proposes a technique for shortening charging time and avoiding an increase in the breakdown voltage of devices by switching the connections of a plurality of batteries in an electric automobile to a series connection when charging and to a parallel connection when traveling. 
     With the technique disclosed in JP 2018-85790A, as described above, the plurality of batteries are put into a parallel connection when the vehicle is traveling in order to avoid increasing the breakdown voltage of the devices connected to the batteries. There is thus a problem in that the vehicle cannot travel while the batteries are in a series-connected state, i.e., a high-voltage state. For the vehicle to travel with the batteries in a series-connected state, it is necessary to use high-breakdown voltage devices which can handle the voltage (high voltage) occurring when the devices connected to the batteries are in a series connection. However, using such high-breakdown voltage devices is problematic in that the devices installed in the electric automobile increase in size. Additionally, in electric automobiles, the motor output is proportional to the system voltage (battery voltage), and thus the configuration disclosed in JP 2018-85790A is problematic in that there is a limit to how much the motor output can be increased. 
     Accordingly, it is an object of the present disclosure to provide a conversion device, a conversion system, a switching device, a vehicle including the same, and a control method that enable driving without providing devices for high voltages when a plurality of batteries are connected in series and output a high voltage. 
     SUMMARY 
     A conversion device according to an aspect of the present disclosure is a conversion device that converts power supplied from a power supply device including a plurality of battery units, and includes a plurality of power conversion units, wherein each of the plurality of power conversion units is connected to the plurality of battery units such that a voltage within a range of a breakdown voltage of the power conversion units is input. 
     A conversion system according to another aspect of the present disclosure includes a power supply device including a plurality of battery units, and the above-described conversion device, the conversion device converting power supplied from the power supply device. 
     A switching device according to another aspect of the present disclosure is a switching device that, in a system including a power supply device having a plurality of battery units and a plurality of power conversion devices that convert power supplied from the plurality of battery units, switches a connection state of the plurality of power conversion devices. Each of the plurality of power conversion units is connected to the plurality of battery units such that a voltage within a range of a breakdown voltage of the power conversion devices is input. The switching device includes a plurality of switches that, by switching, in accordance with a predetermined condition being satisfied, a connection state of the plurality of battery units to one of a series connection state in which the battery units are connected to each other in series and a parallel connection state in which the battery units are connected to each other in parallel, switch a connection state of the plurality of power conversion devices to one of the series connection state and the parallel connection state. 
     A vehicle according to another aspect of the present disclosure includes the above-described conversion system, and a load to which power converted by the conversion system is supplied. 
     A control method according to another aspect of the present disclosure is a control method that, in a system including a power supply device having a plurality of battery units and a plurality of power conversion devices that convert power supplied from the plurality of battery units, controls switching of a connection state of the plurality of power conversion devices. Each of the plurality of power conversion devices is connected to the plurality of battery units such that a voltage within a range of a breakdown voltage of the power conversion devices is input. The control method includes a step of switching, in accordance with a predetermined condition being satisfied, a connection state of the plurality of battery units to one of a series connection state in which the battery units are connected to each other in series and a parallel connection state in which the battery units are connected to each other in parallel, to switch a connection state of the plurality of power conversion devices to one of the series connection state and the parallel connection state. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to drive without providing devices for high voltages when a plurality of batteries are connected in series and output a high voltage. Furthermore, in a vehicle, the motor output can be increased when the vehicle is traveling. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating the configuration of a power conversion system according to an embodiment of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating a vehicle according to an embodiment of the present disclosure. 
         FIG. 3  is a circuit diagram illustrating a detailed example of the configuration of a DC/DC converter. 
         FIG. 4  is a block diagram illustrating a state in which a plurality of battery units illustrated in  FIG. 1  are connected in series. 
         FIG. 5  is a block diagram illustrating a state in which a plurality of battery units illustrated in  FIG. 1  are connected in parallel. 
         FIG. 6  is a block diagram illustrating the configuration of a power conversion system according to a first variation. 
         FIG. 7  is a block diagram illustrating the configuration of a power conversion system according to a second variation. 
         FIG. 8  is a block diagram illustrating the configuration of a power conversion system according to a third variation. 
         FIG. 9  is a block diagram illustrating the configuration of a power conversion system according to a fourth variation. 
         FIG. 10  is a block diagram illustrating a state in which a plurality of battery units illustrated in  FIG. 9  are connected in series. 
         FIG. 11  is a block diagram illustrating a state in which a plurality of battery units illustrated in  FIG. 9  are connected in parallel. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First, the content of embodiments of the present disclosure will be listed and described. The embodiments described hereinafter may be at least partially combined as desired. 
     A conversion device according to a first aspect of the present disclosure is a conversion device that converts power supplied from a power supply device including a plurality of battery units, and includes a plurality of power conversion units, wherein each of the plurality of power conversion units is connected to the plurality of battery units such that a voltage within a range of a breakdown voltage of the power conversion units is input. Through this, it is possible to drive without providing devices for high voltages when a plurality of battery units output a high voltage. In other words, high-voltage output from a power supply device can be handled using a conventional conversion device (e.g., a DC/DC converter). 
     Preferably, a connection state of each of the battery units in the plurality of battery units is switched between a series connection state and a parallel connection state, and the plurality of power conversion units can be switched to one of a series connection state, in which the power conversion units are connected to each other in series, and a parallel connection state, in which the power conversion units are connected to each other in parallel, in accordance with the connection state of each of the battery units in the plurality of battery units. Through this, when the plurality of battery units are connected in series and output a high voltage, the plurality of power conversion units also enter the series connection state, and a voltage obtained by dividing the high voltage is input to each of the power conversion units. Accordingly, a conversion device capable of driving without including devices for high voltages can be achieved with ease. Furthermore, even if one conversion device has failed, this configuration makes it possible for the function of voltage conversion to be maintained by the remaining conversion devices by changing the connection state of the plurality of battery units. This therefore also makes it possible to provide a system having redundancy. 
     More preferably, each of the plurality of battery units has an output rating lower than the breakdown voltage of any of the plurality of power conversion units, and each of the plurality of power conversion units is connected to corresponding ones of the plurality of battery units. This makes it possible to prevent a voltage exceeding the breakdown voltage of the power conversion unit from being input to the power conversion unit. 
     Further preferably, the plurality of power conversion units include a step-down power conversion unit configured to step down and output the power supplied from the battery unit. Through this, the power conversion unit can step down and output the input voltage. 
     Preferably, the conversion device further includes a switching device configured to switch the connection states of the plurality of battery units. Through this, the plurality of battery units can be switched to an appropriate connection state in accordance with the states of the battery units. For example, when the output voltage of the battery units has dropped, the plurality of battery units can be connected in series, which makes it possible to avoid a drop in the output voltage from the power supply device. 
     Preferably, the conversion device further includes a control unit configured to control the switching device. The switching device is configured to switch the connection state of each of the plurality of battery units between the series connection state and the parallel connection state, and switch the connection state of the plurality of power conversion units with respect to the plurality of battery units. When controlling the switching device to switch the connection state of each of the plurality of battery units to the series connection state, the control unit can cause the switching device to perform an operation of switching the connection state of the plurality of power conversion units such that a voltage based on a voltage across both ends of at least one of the battery units among the plurality of battery units is applied to at least one of the plurality of power conversion units. Through this, the voltage across both ends of all the plurality of battery units can be prevented from being applied to any one of the power conversion units when the plurality of battery units are in the series connection state, while making it possible to switch the plurality of battery units between the series connection state and the parallel connection state. In other words, a situation in which the voltage across both ends of all the plurality of battery units put in the series connection state is applied to a power conversion unit while that voltage exceeds the breakdown voltage of the power conversion unit can be prevented. The conversion device can then be caused to operate such that a voltage based on the voltage across both ends of at least one of the battery units is applied to any one of the power conversion units. 
     Preferably, the conversion device further includes a control unit configured to control the switching device. The switching device is configured to switch the connection state of each of the plurality of battery units between the series connection state and the parallel connection state, switch the connection state of each of the plurality of power conversion units between the series connection state and the parallel connection state, and switch the connection state of the plurality of power conversion units with respect to the plurality of battery units. When controlling the switching device to switch the connection state of each of the plurality of battery units to the series connection state, the control unit can cause the switching device to perform an operation of switching the connection state of each of the plurality of power conversion units to the series connection state, and switching the connection state of the plurality of power conversion units such that a voltage based on a voltage across both ends of the plurality of battery units put into the series connection state is applied to both ends of the plurality of power conversion units put into the series connection state. Through this, the voltage across both ends of all the plurality of battery units can be prevented from being applied only to any one of the power conversion units when the plurality of battery units are in the series connection state, while making it possible to switch the plurality of battery units between the series connection state and the parallel connection state. In other words, a situation in which the voltage across both ends of all the plurality of battery units put in the series connection state is applied to a power conversion unit while that voltage exceeds the breakdown voltage of the power conversion unit can be prevented. The conversion device can then be caused to operate such that a voltage obtained by dividing the voltage across both ends of all the plurality of battery units put into the series connection state is applied to each of the plurality of power conversion units. 
     Preferably, when controlling the switching device to switch the connection state of each of the plurality of battery units to the parallel connection state, the control unit can cause the switching device to perform an operation of switching the connection state of the plurality of power conversion units such that a voltage based on a voltage across both ends of the battery units put into the parallel connection state is applied to at least one of the plurality of power conversion units. This makes it possible for at least one of the power conversion units to operate well when the plurality of battery units are in the parallel connection state so as not to exceed the breakdown voltage of the power conversion units. 
     Preferably, when controlling the switching device to switch the connection state of each of the plurality of battery units to the parallel connection state, the control unit can cause the switching device to perform an operation of switching the connection state of the plurality of power conversion units such that a voltage based on a voltage across both ends of the battery units put into the parallel connection state is applied to both ends of the plurality of power conversion units put into the series connection state. Doing so further lowers the voltage applied to each of the power conversion units, which provides more sufficient measures with respect to breakdown voltage. 
     Preferably, the conversion device further includes a control unit configured to control the switching device, and a voltage detection unit configured to detect a voltage. The voltage detection unit detects an output voltage of the plurality of battery units when the plurality of battery units are in the series connection state. The switching device is configured to switch the connection state of each of the plurality of battery units between the series connection state and the parallel connection state, and switch the connection state of the plurality of power conversion units with respect to the plurality of battery units. The control unit can cause the switching device to perform an operation of switching the connection state of each of the plurality of battery units to the series connection state, and switching the connection state of the plurality of power conversion units such that a voltage based on the output voltage of the plurality of battery units put into the series connection state is applied to at least one of the plurality of power conversion units, under a condition that the output voltage detected by the voltage detection unit is less than or equal to a threshold. 
     By doing so, when a voltage based on the output voltage of the plurality of battery units put into the series connection state is applied to any one of the power conversion units and used, a voltage which is too high, and which greatly exceeds the threshold with respect to that power conversion unit, can be prevented from being applied. 
     A conversion system according to a second aspect of the present disclosure includes a power supply device including a plurality of battery units, and the above-described conversion device, the conversion device being configured to convert power supplied from the power supply device. Through this, it is possible to drive without providing devices for high voltages when a plurality of battery units are connected in series and output a high voltage. 
     More preferably, the conversion system further includes an inverter being configured to be supplied with the power from the power supply device, and a motor being configured to be supplied with power via the inverter. Through this, when a vehicle in which the conversion system is installed is traveling at a high speed, the plurality of battery units can be connected in series and supply the high voltage necessary for high-speed rotation of the motor. 
     A switching device according to a third aspect of the present disclosure is a switching device that, in a system including a power supply device having a plurality of battery units and a plurality of power conversion devices that convert power supplied from the plurality of battery units, switches a connection state of the plurality of power conversion devices. Each of the plurality of power conversion devices is connected to the plurality of battery units such that a voltage within a range of a breakdown voltage of the power conversion devices is input. The switching device includes a plurality of switches that, by switching, in accordance with a predetermined condition being satisfied, a connection state of the plurality of battery units to one of a series connection state in which the battery units are connected to each other in series and a parallel connection state in which the battery units are connected to each other in parallel, switch a connection state of the plurality of power conversion devices to one of the series connection state and the parallel connection state. Through this, it is possible to drive without providing devices for high voltages when a plurality of battery units are connected in series and output a high voltage. 
     Preferably, the switching device is installed, along with the system, in a vehicle, and the predetermined condition includes a condition pertaining to a travel condition. Through this, for example, when the vehicle is traveling at a high speed, the plurality of battery units can be connected in series and supply the high voltage necessary for high-speed rotation of the motor. 
     More preferably, at least one of the plurality of switches includes a semiconductor relay. This makes it possible to realize a switching device that has a long lifespan, can switch with high responsiveness, and does not act as a noise source. 
     A vehicle according to a fourth aspect of the present disclosure includes the above-described conversion system, and a load to which power converted by the conversion system is supplied. Through this, it is possible to drive without providing devices for high voltages when a plurality of battery units are connected in series and output a high voltage. If another device directly connected to the batteries (an air conditioners or the like) is connected to one of the plurality of battery units, when the plurality of battery units are connected in series, no high voltage is applied to the device, and there is therefore no need to make the device having high-voltage specifications. 
     A control method according to a fifth aspect of the present disclosure is a control method that, in a system including a power supply device having a plurality of battery units and a plurality of power conversion devices that convert power supplied from the plurality of battery units, controls switching of a connection state of the plurality of power conversion devices. Each of the plurality of power conversion devices is connected to the plurality of battery units such that a voltage within a range of a breakdown voltage of the power conversion devices is input. The control method includes a step of switching, in accordance with a predetermined condition being satisfied, a connection state of the plurality of battery units to one of a series connection state in which the battery units are connected to each other in series and a parallel connection state in which the battery units are connected to each other in parallel, to switch a connection state of the plurality of power conversion devices to one of the series connection state and the parallel connection state. Through this, it is possible to drive without providing devices for high voltages when a plurality of battery units are connected in series and output a high voltage. 
     In the following embodiments, identical reference numerals are assigned to identical components. The names and functions thereof are also the same. Accordingly, detailed descriptions thereof will not be repeated. Embodiment 
     Referring to  FIG. 1 , a power conversion system  100  according to an embodiment of the present disclosure includes a battery unit  102 , a battery unit  104 , a conversion device  105 , a first DC/DC converter  106 , a second DC/DC converter  108 , a low-voltage battery  110 , a load  112 , an inverter  114 , a motor  116 , an electrical device  118 , an in-vehicle charger  120 , switches  200  to  208 , and switch sections  210  to  214 . 
     The battery unit  102  and the battery unit  104  are units constituted by storage batteries that can be charged and discharged. The battery unit  102  and the battery unit  104  constitute a high-voltage battery section  124 , which is an example of a power supply device. The battery unit  102  and the battery unit  104  are, for example, battery units having 400 V specifications (rated at 400 V for charging voltage and output voltage) and are connected to a switching device  125  constituted by the switches  200  to  204 . The switches  200  to  204  are semiconductor relays, for example. Semiconductor relays have long lifespans, can be switched with good responsiveness, do not generate high-frequency noise during switching and therefore do not act as noise source, and are therefore preferred as switches. Note, however, that the switches  200  to  204  may be electromagnetic relays. One terminal (a terminal of the same polarity (positive)) of each of the battery unit  102  and the battery unit  104  is connected to the other via the switch  200 . Another terminal (a terminal of the polarity opposite to the one terminal (negative)) of each of the battery unit  102  and the battery unit  104  is connected to the other via the switch  204 . Furthermore, the other terminal of the battery unit  102  and the one terminal of the battery unit  104  (terminals of different polarities) are connected to each other via the switch  202 . Note that the high-voltage battery section  124  may be configured to include the switching device  125 . Additionally, each of the battery unit  102  and the battery unit  104  is not limited to a plurality of batteries in a unit, but may be a single ordinary battery. 
     The conversion device  105  is constituted by at least the first DC/DC converter  106  and the second DC/DC converter  108 . The first DC/DC converter  106  and the second DC/DC converter  108  are step-down DC/DC converters that convert a high voltage supplied from the high-voltage battery section  124  to a low voltage (e.g., 12 V). The first DC/DC converter  106  and the second DC/DC converter  108  supply the converted voltage to the low-voltage battery  110 . The first DC/DC converter  106  and the second DC/DC converter  108  are input with a voltage within the range of the converters&#39; respective breakdown voltages. Note that even if the switches  200  to  204  are switched as described later, the first DC/DC converter  106  and the second DC/DC converter  108  are input with a voltage within the range of the converters&#39; respective breakdown voltages. The first DC/DC converter  106  and the second DC/DC converter  108 , for example, have an input voltage specification of 400 V. A wire  130  and a wire  132  connected to an input terminal of the first DC/DC converter  106  are connected to one terminal of the battery unit  102  and the other terminal of the battery unit  102 , respectively. A wire  134  and a wire  136  connected to an input terminal of the second DC/DC converter  108  are connected to one terminal of the battery unit  104  and the other terminal of the battery unit  104 , respectively. 
     Output terminals of the first DC/DC converter  106  and the second DC/DC converter  108  are connected in parallel, and are connected to an input terminal of the low-voltage battery  110 . An output terminal of the low-voltage battery  110  is connected to the load  112 . The low-voltage battery  110  is charged by voltage input from the first DC/DC converter  106  and the second DC/DC converter  108 , and supplies power to the load  112 . 
     Input terminals of the inverter  114  are connected to the wire  130  and the wire  136  via the switches of the switch section  210 . An output terminal of the inverter  114  is connected to an input terminal of the motor  116 . The motor  116  is an electrical drive device such as a main engine system motor or the like. The inverter  114  supplies power for driving the motor  116  to the motor  116 . Note that the inverter  114  may include, for example, a step-up DC/DC converter, which steps up the input voltage to generate a voltage suitable for driving the motor  116 . 
     The electrical device  118  is an air conditioner, a heater, or the like. The electrical device  118  is connected to the wires  130  to  136  via the four switches of the switch section  212 . Of output terminals A to D of the four switches of the switch section  212  (terminals connected to the input terminals of the electrical device  118 ), the output terminal A of the switch connected to the wire  130  and the output terminal C of the switch connected to the wire  134  are connected to each other, and the output terminal B of the switch connected to the wire  132  and the output terminal D of the switch connected to the wire  136  are connected to each other. By having the electrical device  118  connected to the wires  130  to  136  via the switch section  212  in this manner, power is supplied from any single battery unit even if the connection states of the battery unit  102  and the battery unit  104  switch between a series connection and a parallel connection, as will be described later. 
     The in-vehicle charger  120  is a device used to charge the battery unit  102  and battery unit  104  from commercial power supplied to a home, for example. The in-vehicle charger  120  may include a charging device for wireless power transmission. The in-vehicle charger  120  is connected to the one terminal of the battery unit  102  and the other terminal of the battery unit  104  via the switches of the switch section  214 . During charging by the in-vehicle charger  120  or wireless charging, the series-parallel connections of the battery units may be switched according to the breakdown voltages of the devices such as the DC/DC converters. 
     The switch  206  and the switch  208  are switches that are turned on when the battery unit  102  and battery unit  104  are charged by power supplied from a quick-charging device such as a charging stand or the like. The one terminal of the battery unit  102  is connected to one power line of the quick-charging device via the switch  206 . The other terminal of the battery unit  104  is connected to another power line of the quick-charging device via the switch  208 . 
     A switch control unit  122  is connected to the switches  200  to  208  and the switches in the switch sections  210  to  214 , and controls the turning on and off of those switches.  FIG. 1  does not illustrate wires connecting the switch control unit  122  to the switches. 
     Referring to  FIG. 2 , the power conversion system  100  is installed in a vehicle such as a PHEV (Plug-in Hybrid Electric Vehicle) or an EV (Electric Vehicle). The power conversion system  100  charges the high-voltage battery section  124  and the low-voltage battery  110  with AC power supplied from an external AC power source. The power conversion system  100  supplies power from the high-voltage battery section  124  and the low-voltage battery  110  to the motor  116 , an auxiliary system load  126 , and the like while the vehicle is traveling. The auxiliary system load  126  is an auxiliary device necessary for operating the engine, the motor, and the like, and mainly includes a cell motor, an alternator, a radiator cooling fan, or the like. The auxiliary system load  126  may include the load  112  (lighting, a wiper drive unit, a navigation device, or the like) and the electrical device  118  (an air conditioner, a heater, or the like) as well. 
     Referring to  FIG. 3 , the first DC/DC converter  106  corresponds to an example of a power conversion unit and a power conversion device, and includes a capacitor  300 , a DC/AC converter  302 , a transformer  304 , and a rectifier  306 . The second DC/DC converter  108  also corresponds to an example of a power conversion unit and a power conversion device, and has the same configuration as the first DC/DC converter  106 . The DC/AC converter  302  includes switch elements  320 ,  322 ,  324  and  326  that constitute a full bridge circuit. An input terminal of the DC/AC converter  302  is connected to both terminals of the capacitor  300 . An output terminal of the DC/AC converter  302  is connected to both terminals of a primary-side winding of the transformer  304 . The DC/AC converter  302  converts DC voltage input from the capacitor  300  side into AC voltage and outputs the AC voltage to the primary-side winding of the transformer  304 . 
     The rectifier  306  includes a switch element  340  and a switch element  342 , an inductor  344 , and a capacitor  346 . An input side of the rectifier  306  is connected to both terminals of a secondary-side winding of the transformer  304 . The secondary-side winding of the transformer  304  is a center-tapped coil. As a result, the rectifier  306  rectifies and smooths the AC voltage generated in the secondary-side winding of the transformer  304 , and outputs the voltage as DC voltage. Accordingly, the first DC/DC converter  106  converts the high DC voltage input from the capacitor  300  side into a low DC voltage and supplies that voltage to the low-voltage battery  110 . 
     Each switch element is constituted by, for example, a FET (Field Effect Transistor) having a freewheeling diode. For the purpose of protection from surge current and the like, the switch elements and freewheeling diodes are connected in parallel so that forward bias directions thereof are opposite from each other. The switch elements may be semiconductor devices aside from FETs, such as GaN-HEMTs (High Electron Mobility Transistors), for example. 
     Functions of the power conversion system  100  will be described with reference to  FIGS. 4 and 5 . Referring to  FIG. 4 , consider a case where, for example, a quick-charging device supplies a voltage exceeding the voltage specifications of both the battery unit  102  and the battery unit  104  (e.g., 800 V), and charging is performed. In this case, the switch  202 , the switch  206 , and the switch  208  are turned on under the control of the switch control unit  122 . The switch  200  and the switch  204 , as well as the switch sections  210  to  214 , are off. In  FIG. 4 , the lines to which power is supplied are indicated by bold lines. The battery unit  102  and the battery unit  104  are connected in series as a result. The first DC/DC converter  106  and the second DC/DC converter  108  are also connected in series. A connection node of the battery unit  102  and battery unit  104  connected in series is connected to a connection node of the first DC/DC converter  106  and the second DC/DC converter  108  connected in series. Accordingly, an 800 V charging voltage can be supplied from the quick-charging device to charge the battery unit  102  and battery unit  104  having 400 V specifications, and the low-voltage battery  110  can be charged with the output voltage from the first DC/DC converter  106  and the second DC/DC converter  108  having 400 V specifications. 
     Referring to  FIG. 5 , consider a case where, for example, the quick-charging device supplies a voltage suited to the voltage specifications of both the battery unit  102  and the battery unit  104  (e.g., 400 V), and charging is performed. In this case, the switch  200  and the switch  204 , and the switch  206  and the switch  208 , are turned on under the control of the switch control unit  122 . The switch  202 , as well as the switch sections  210  to  214 , are off. In  FIG. 5 , the lines to which power is supplied are indicated by bold lines. The battery unit  102  and the battery unit  104  are connected in parallel as a result. The first DC/DC converter  106  and the second DC/DC converter  108  are also connected in parallel. Accordingly, a 400 V charging voltage can be supplied from the quick-charging device to charge the battery unit  102  and battery unit  104  having 400 V specifications, and the low-voltage battery  110  can be charged with the output voltage from the first DC/DC converter  106  and the second DC/DC converter  108  having 400 V specifications. 
     In this manner, the connection state of each battery unit in the plurality of battery units  102  and  104  can be switched to either a series connection state in which the battery units are connected to each other in series, or a parallel connection state in which the battery units are connected to each other in parallel. Furthermore, the first DC/DC converter  106  and the second DC/DC converter  108  can be switched to either a series connection state or a parallel connection state according to the connection state of the battery units  102  and  104 . It is therefore possible to prevent voltage exceeding the respective breakdown voltages of the first DC/DC converter  106  and the second DC/DC converter  108  from being input thereto. 
     The battery unit  102  and the battery unit  104  may be configured to be connected in series as illustrated in  FIG. 4  (with the first DC/DC converter  106  and the second DC/DC converter  108  also being connected in series) not only during charging, but also when the vehicle in which the battery units are installed is traveling. During travel, the switch  206  and the switch  208  are turned off, and the switches of the switch section  210  are turned on, under the control of the switch control unit  122 . Although the output voltage of each of the battery unit  102  and the battery unit  104  is 400 V, the inverter  114  is supplied with 800 V, which is the voltage across the two terminals, of the series-connected battery unit  102  and the battery unit  104 , that are not connected to each other (called a “series-connected voltage” hereinafter). In this case, the inverter  114  generates power to drive the motor  116  directly from the input 800 V, without going through the internal step-up DC/DC converter. In other words, a high voltage necessary for rotating the motor  116  at high speeds during high-speed travel can be supplied. 
     As described above, the connection node of the battery unit  102  and the battery unit  104  connected in series is connected to the connection node of the first DC/DC converter  106  and the second DC/DC converter  108  connected in series. Accordingly, the first DC/DC converter  106  can convert the 400 V supplied from the battery unit  102  to a low voltage, the second DC/DC converter  108  can convert the 400 V supplied from the battery unit  104  to a low voltage, and the low voltages can be supplied to the low-voltage battery  110 . By turning on the switches of the switch section  212  as appropriate, the input terminals of the electrical device  118  can be supplied with the voltage across both terminals of the battery unit  102  (e.g., 400 V) or the voltage across both terminals of the battery unit  104  (e.g., 400 V). In other words, in a state where the high voltage (800 V) for the motor  116  is supplied from the high-voltage battery section  124 , the first DC/DC converter  106  and the second DC/DC converter  108 , as well as the electrical device  118 , having the conventional specification (400 V) can be used as-is, and there is no need to put the first DC/DC converter  106  and the second DC/DC converter  108 , as well as the electrical device  118 , into the high voltage specification. Conventionally, depending on the motor output, a vehicle will also need to be provided with a step-up converter. However, by using the power conversion system  100 , the high voltage for the motor  116  can be supplied from the high-voltage battery section  124 , and there is therefore no need to provide a step-up converter. 
     Note that the battery unit  102  and the battery unit  104  may be configured to switch to a series connection in accordance with travel conditions of the vehicle in which those units are installed (vehicle speed, road speed limit, traffic jam conditions, and so on). For example, the battery unit  102  and the battery unit  104  may be configured to be connected in parallel when the vehicle in which those units are installed begins traveling, and switched to a series connection when the speed of the vehicle exceeds a predetermined speed. Additionally, the battery unit  102  and the battery unit  104  may be connected in parallel as illustrated in  FIG. 5  when the vehicle in which those units are installed is traveling (with the first DC/DC converter  106  and the second DC/DC converter  108  also being connected in parallel). At this time, the inverter  114  is supplied with an output voltage of 400 V from each of the battery unit  102  and the battery unit  104 . In this case, when the vehicle is traveling at a high speed, the inverter  114  steps the 400 V being input up to 800 V, via the internal step-up DC/DC converter, to generate the power for driving the motor  116 . 
     Additionally, the first DC/DC converter  106  can convert the 400 V supplied from the battery unit  102  to a low voltage, the second DC/DC converter  108  can convert the 400 V supplied from the battery unit  104  to a low voltage, and the low voltages can be supplied to the low-voltage battery  110 . By turning on the switches of the switch section  212  as appropriate, the input terminals of the electrical device  118  can be supplied with at least one of the voltage across both terminals of the battery unit  102  (e.g., 400 V) and the voltage across both terminals of the battery unit  104  (e.g., 400 V). 
     Thus in the configuration described thus far, the conversion device  105  includes the switching device  125  and the switch control unit  122  that controls the switching device  125 . The switch control unit  122  corresponds to an example of a control unit. The switching device  125  is configured to switch the connection state of each of the plurality of battery units  102  and  104  between a series connection state and a parallel connection state, and switch the connection states of the first DC/DC converter  106  and the second DC/DC converter  108  (a plurality of power conversion units) with respect to the plurality of battery units  102  and  104 . When the switch control unit  122  (the control unit) controls the switching device  125  to switch the connection state of each of the plurality of battery units  102  and  104  to the series connection state as illustrated in  FIG. 4 , the switching device  125  can be caused to operate so that the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  is switched such that a voltage based on the voltage across both ends of at least one of the battery units in the plurality of battery units  102  and  104  is applied to each of the first DC/DC converter  106  and the second DC/DC converter  108  (the plurality of power conversion units). Specifically, the switch control unit  122  controls the switching device  125  so that the voltage based on the voltage across both ends of the one battery unit  102  is applied to the first DC/DC converter  106  and the voltage based on the voltage across both ends of the one battery unit  104  is applied to the second DC/DC converter  108 . 
     Note that the breakdown voltage of each of the first DC/DC converter  106  and the second DC/DC converter  108  is a predetermined guaranteed operating voltage and is a predetermined fixed value. The voltage across both ends of both the battery units  102  and  104  when fully charged is lower than the breakdown voltage of each of the first DC/DC converter  106  and the second DC/DC converter  108 . On the other hand, the voltage across both ends of all of the plurality of battery units  102  and  104  in a series connection state when each of the plurality of battery units  102  and  104  is fully charged is higher than the breakdown voltage of each of the first DC/DC converter  106  and the second DC/DC converter  108 . 
     Furthermore, when the switch control unit  122  (the control unit) controls the switching device  125  to switch the connection state of each of the plurality of battery units  102  and  104  to the parallel connection state as illustrated in  FIG. 5 , the switching device  125  can be caused to operate so that the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  is switched such that a voltage based on the voltage across both ends of the battery units  102  and  104  in the parallel connection state is applied to each of the first DC/DC converter  106  and the second DC/DC converter  108  (the plurality of power conversion units). This makes it possible for the first DC/DC converter  106  and the second DC/DC converter  108  to operate well when the plurality of battery units  102  and  104  are in the parallel connection state so as not to exceed the breakdown voltage of the power conversion units. 
     First Variation 
     In the configuration illustrated in  FIG. 1 , additional switches may be provided. For example, the configuration illustrated in  FIG. 1  may be modified as illustrated in  FIG. 6 .  FIG. 6  illustrates a power conversion system  150  according to a first variation, and is achieved by adding switches  220  to  226  to the configuration illustrated in  FIG. 1 , and changing the wire connecting the wire  130  and the switches of the switch section  210  to a wire  152 , which connects the one terminal of the battery unit  102  to the switches of the switch section  210 . Because the rest of the configuration is the same as in  FIG. 1 , descriptions thereof will not be repeated, and will focus mainly on the differences. 
     A conversion device  155 , which constitutes part of the conversion system  150 , includes at least the first DC/DC converter  106  and the second DC/DC converter  108 , the switching device  125 , and the switch control unit  122 . 
     The switch  220  is connected between the wire  130  and the switch  200 , the switch  222  is connected between the wire  132  and one terminal of the switch  202 , the switch  224  is connected between the wire  134  and another terminal of the switch  202 , and the switch  226  is connected between the wire  136  and the switch  204 . Accordingly, after the switches  200  to  204  are controlled to turn on and off by the switch control unit  122  and the battery unit  102  and the battery unit  104  have been put into a series connection (see  FIG. 4 ) or a parallel connection (see  FIG. 5 ), having the switch control unit  122  turn the switches  220  to  224  on sets the first DC/DC converter  106  and the second DC/DC converter  108  to an appropriate connection state according to the connection state of the battery unit  102  and the battery unit  104 , as described above. Accordingly, an appropriate voltage (e.g., 400 V) is supplied to each of the first DC/DC converter  106  and the second DC/DC converter  108 . When the switches of the switch section  210  are turned on by the switch control unit  122 , the inverter  114  is supplied with a voltage based on the connection state of the battery unit  102  and the battery unit  104  (e.g., 800 V or 400 V). By having the switch control unit  122  turn the switches of the switch section  212  on and off as appropriate, the voltage of either one of the battery unit  102  and the battery unit  104  (e.g., 400 V), or the voltage from the battery unit  102  and the battery unit  104  connected in parallel (e.g., 400 V) is supplied to the electrical device  118 . 
     Second Variation 
     The configuration illustrated in  FIG. 1  may be modified as illustrated in  FIG. 7 .  FIG. 7  illustrates a power conversion system  160  according to a second variation, and is achieved by adding the switches  220  to  226  to the configuration illustrated in  FIG. 1 , and changing the wire connecting the wire  130  and the switches of the switch section  210  to a wire  162 , which connects the wire  134  to the switches of the switch section  210 . Because the rest of the configuration is the same as in  FIG. 1 , descriptions thereof will not be repeated, and will focus mainly on the differences. 
     A conversion device  165 , which constitutes part of the conversion system  160 , includes at least the first DC/DC converter  106  and the second DC/DC converter  108 , the switching device  125 , and the switch control unit  122  (the same as the switch control unit  122  in  FIG. 1 ; not shown in  FIG. 7 ). 
     The switches  220  to  226  are connected to the wires  130  to  136  in the same way as in  FIG. 6 . Accordingly, in the same manner as in  FIG. 6 , by having the switch control unit  122  control the switches  200  to  204  to turn on and off, and turning the switches  220  to  224  on, the first DC/DC converter  106  and the second DC/DC converter  108  are set to an appropriate connection state according to the connection state of the battery unit  102  and the battery unit  104 . Accordingly, an appropriate voltage (e.g., 400 V) is supplied to each of the first DC/DC converter  106  and the second DC/DC converter  108 . By having the switch control unit  122  turn the switches of the switch section  212  on and off as appropriate, the voltage of either one of the battery unit  102  and the battery unit  104  (e.g., 400 V), or the voltage from the battery unit  102  and the battery unit  104  connected in parallel (e.g., 400 V) is supplied to the electrical device  118 . On the other hand, unlike in  FIG. 6 , when the switches of the switch section  210  are turned on by the switch control unit  122 , the voltage from the battery unit  104  (e.g., 400 V), or the voltage from the battery unit  102  and the battery unit  104  connected in parallel (e.g., 400 V), is supplied to the inverter  114 , according to the connection state of the battery unit  102  and the battery unit  104 . 
     Third Variation 
     The configuration illustrated in  FIG. 1  may be modified as illustrated in  FIG. 8 .  FIG. 8  illustrates a power conversion system  170  according to a third variation, and is achieved by adding switches  240  to  244  to the configuration illustrated in  FIG. 1 , and changing the connection relationship of the wire  132  and the wire  134 . Because the rest of the configuration, including items not illustrated in  FIG. 8 , is the same as in  FIG. 1 , descriptions thereof will not be repeated, and will focus mainly on the differences. 
     A conversion device  175 , which constitutes part of the conversion system  170 , includes at least the first DC/DC converter  106  and the second DC/DC converter  108 , switching devices  125  and  127 , and the switch control unit  122  (the same as the switch control unit  122  in  FIG. 1 ; not shown in  FIG. 8 ). 
     The switches  240  to  244  constitute the switching device  127 , which is for switching the connection state of the first DC/DC converter  106  and the second DC/DC converter  108 . One terminal (a terminal to which one voltage level (e.g., high voltage) is input) of each of the first DC/DC converter  106  and the second DC/DC converter  108  (corresponding to the wire  130  and the wire  134 ) is connected via the switch  244 . Another terminal (a terminal to which a voltage level different from the one voltage level (e.g., low voltage) is input) of each of the first DC/DC converter  106  and the second DC/DC converter  108  (corresponding to the wire  132  and the wire  136 ) is connected via the switch  240 . Another terminal of the first DC/DC converter  106  and the one terminal of the second DC/DC converter  108  (terminals to which mutually-different voltage levels are input) (corresponding to the wire  132  and the wire  134 ) are connected via the switch  242 . Note that the one terminal of the second DC/DC converter  108  (corresponding to the wire  134 ) is connected to the one terminal of the battery unit  104 . 
     By employing the configuration illustrated in  FIG. 8 , the connection state of the battery unit  102  and the battery unit  104 , and the connection state of the first DC/DC converter  106  and the second DC/DC converter  108 , can be changed independently. In other words, the connection state of the battery unit  102  and the battery unit  104  can be set to a series connection state by turning on the switch  202  with the switch  200  and the switch  204  turned off, and can be set to the parallel connection state by turning on the switch  200  and the switch  204  with the switch  202  turned off, as described above. The connection state of the first DC/DC converter  106  and the second DC/DC converter  108  can be set to a series connection state by turning on the switch  242  with the switch  240  and the switch  244  turned off, and can be set to the parallel connection state by turning on the switch  240  and the switch  244  with the switch  242  turned off. In other words, the first DC/DC converter  106  and the second DC/DC converter  108  can be put into the series connection state as well as the parallel connection state when the battery unit  102  and the battery unit  104  are connected in series. The first DC/DC converter  106  and the second DC/DC converter  108  can also be put into the series connection state and the parallel connection state when the battery unit  102  and the battery unit  104  are connected in parallel. 
     In this configuration, the switch control unit  122  (the control unit) controls the switching devices  125  and  127 . The switching devices  125  and  127  are configured to switch the connection state of each of the plurality of battery units  102  and  104  between the series connection state and the parallel connection state, and to switch the connection state of each of the first DC/DC converter  106  and the second DC/DC converter  108  (the plurality of power conversion units) between the series connection state and the parallel connection state. The switching devices  125  and  127  are furthermore configured to switch the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  with respect to the plurality of battery units  102  and  104 . 
     When the switch control unit  122  (the control unit) controls the switching devices  125  and  127  to switch the connection state of each of the plurality of battery units  102  and  104  to the series connection state, the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  can also be switched to the series connection state. In this case, the switch control unit  122  (the control unit) can cause the switching devices  125  and  127  to operate so that the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  is switched such that a voltage based on the voltage across both ends of the plurality of battery units  102  and  104  in the series connection state is applied to both ends of the first DC/DC converter  106  and the second DC/DC converter  108  that have been put into the series connection state. In this case, a voltage obtained by dividing the voltage across both ends of all of the plurality of battery units  102  and  104  in the series connection state is applied to each of the first DC/DC converter  106  and the second DC/DC converter  108 . Note that even when each of the plurality of battery units  102  and  104  is fully charged, the voltage (divided voltage) applied to each of the first DC/DC converter  106  and the second DC/DC converter  108  is adjusted to be no greater than the breakdown voltage of each of the first DC/DC converter  106  and the second DC/DC converter  108 . 
     Furthermore, when the switch control unit  122  (the control unit) controls the switching devices  125  and  127  to switch the connection state of each of the plurality of battery units  102  and  104  to the parallel connection state, the switching devices  125  and  127  can be caused to operate so that the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  (the first DC/DC converter  106  and the second DC/DC converter  108  connected in parallel) is switched such that a voltage based on the voltage across both ends of the plurality of battery units  102  and  104  in the parallel connection state is applied to each of the first DC/DC converter  106  and the second DC/DC converter  108  (the plurality of power conversion units). 
     Furthermore, when the switch control unit  122  (the control unit) controls the switching devices  125  and  127  to switch the connection state of each of the plurality of battery units  102  and  104  to the parallel connection state, the switching devices  125  and  127  can be caused to operate so that the connection state of the first DC/DC converter  106  and the second DC/DC converter  108  (the first DC/DC converter  106  and the second DC/DC converter  108  connected in series) is switched such that a voltage based on the voltage across both ends of the plurality of battery units  102  and  104  in the parallel connection state is applied to both ends of the first DC/DC converter  106  and the second DC/DC converter  108  (the plurality of power conversion units) in the series connection state. 
     Therefore, the connection state of the battery unit  102  and the battery unit  104 , as well as the connection states of the first DC/DC converter  106  and the second DC/DC converter  108 , can be changed according to the states (e.g., the output voltages) of the battery unit  102  and the battery unit  104 . For example, when the series-connected voltage drops while the battery unit  102  and the battery unit  104  are connected in series, the first DC/DC converter  106  and the second DC/DC converter  108  may be switched from a series connection to a parallel connection. Additionally, when the output voltage has dropped while the battery unit  102  and the battery unit  104  are connected in parallel, the battery unit  102  and the battery unit  104  may be switched from a parallel connection to a series connection. This makes it possible to avoid a drop in the output voltage from the high-voltage battery section  124 . The series-connected voltage from the battery unit  102  and the battery unit  104  may be calculated by monitoring the output voltage of each of the battery unit  102  and the battery unit  104  and adding those output voltages together, or by detecting the overall output voltage when the battery unit  102  and the battery unit  104  are in the series connection state. 
     Specifically, the following configuration can be employed. 
     In the example in  FIG. 8 , a voltage detection unit  260  is configured to be capable of detecting the output voltage of each of the battery unit  102  and the battery unit  104 . Furthermore, the voltage detection unit  260  is configured to be capable of calculating “an output voltage when the battery unit  102  and the battery unit  104  are in the series connection state” (the overall voltage across both ends of the units in a series connection when the battery unit  102  and the battery unit  104  are in the series connection state) by adding together the respective output voltages of the battery unit  102  and the battery unit  104 . 
     The switch control unit  122  (the control unit) monitors whether or not the stated output voltage detected by the voltage detection unit  260  has become less than or equal to a threshold, and when the output voltage has become less than or equal to the threshold, control the switching devices  125  and  127  such that the first DC/DC converter  106  and the second DC/DC converter  108  are connected in parallel with respect to both ends of the battery unit  102  and the battery unit  104  in the series connection state (both ends of all the units in a series connection) while keeping the connection state of the battery unit  102  and the battery unit  104  in the series connection state. In this case, a voltage based on the stated output voltage of the battery unit  102  and the battery unit  104  in the series connection state (the overall voltage across both ends of the units in a series connection) is applied to each of the first DC/DC converter  106  and the second DC/DC converter  108 . Because the stated threshold is set to a lower value than the respective breakdown voltages of the first DC/DC converter  106  and the second DC/DC converter  108 , the breakdown voltages will not be exceeded even if the voltages across both ends of the battery units  102  and  104  connected in series are applied to the respective converters. 
     Fourth Variation 
     Although the foregoing has described a case where there are the same number of battery units as there are DC/DC converters, the configuration is not limited thereto. The number of battery units and the number of DC/DC converters may be different. For example, the configuration illustrated in  FIG. 1  may be modified as illustrated in  FIG. 9 . 
     Referring to  FIG. 9 , a power conversion system  180  according to a fourth variation is achieved by adding a battery unit  182  and switches  240  to  250  to the configuration illustrated in  FIG. 1 , and changing the connection relationship of the wire  132  and the wire  134 . The switches  200  to  204 , which constitute the switching device  125  in  FIG. 1 , are included in a high-voltage battery section  184  illustrated in  FIG. 9 . Because the rest of the configuration, including items not illustrated in  FIG. 9 , is the same as in  FIG. 1 , descriptions thereof will not be repeated, and will focus mainly on the differences. 
     A conversion device  185 , which constitutes part of the conversion system  180 , includes at least the first DC/DC converter  106  and the second DC/DC converter  108 , switching devices  125  and  129 , and the switch control unit  122  (the same as the switch control unit  122  in  FIG. 1 ; not shown in  FIG. 9 ). 
     Like the battery unit  102  and the battery unit  104 , the battery unit  182  is a unit constituted by a storage battery that can be charged and discharged. The battery unit  102 , the battery unit  104 , and the battery unit  182  are connected by the switches  200  to  204  and the switches  246  to  250 , and constitute the high-voltage battery section  184 , which is an example of a power supply device. Note that in the example in  FIG. 9 , the switching device  125  corresponds to the part of the high-voltage battery section  184  excluding the battery units  102 ,  104 , and  182 . One terminal (a terminal of the same polarity (positive)) of each of the battery unit  102  and the battery unit  104  is connected to the other via the switch  200 . Another terminal (a terminal of the polarity opposite to the one terminal (negative)) of each of the battery unit  102  and the battery unit  104  is connected to the other via the switch  204 . The other terminal of the battery unit  102  and one terminal of the battery unit  182  (terminals of different polarities) are connected to each other via the switch  248 . The one terminal of the battery unit  182  and the one terminal of the battery unit  104  (terminals of the same polarity (positive)) are connected to each other via the switch  246 . Another terminal of the battery unit  182  and another terminal of the battery unit  104  (terminals of the same polarity (negative)) are connected to each other via the switch  250 . The other terminal of the battery unit  182  and the one terminal of the battery unit  104  (terminals of different polarities) are connected to each other via the switch  202 . 
     The switches  240  to  244 , which constitute the switching device  129 , are connected to the first DC/DC converter  106  and the second DC/DC converter  108  in the same way as in the third variation (see  FIG. 8 ). As with the third variation, by employing the configuration illustrated in  FIG. 9 , the connection state of the plurality of battery units (the battery unit  102 , the battery unit  104 , and the battery unit  182 ), and the connection state of the first DC/DC converter  106  and the second DC/DC converter  108 , can be changed independently. In other words, the first DC/DC converter  106  and the second DC/DC converter  108  can be connected in series, as well as in parallel, with the battery unit  102 , the battery unit  104 , and the battery unit  182  connected in series. The first DC/DC converter  106  and the second DC/DC converter  108  can also be connected in series, as well as in parallel, with the battery unit  102 , the battery unit  104 , and the battery unit  182  connected in parallel. 
       FIGS. 10 and 11  illustrate an example. Referring to  FIG. 10 , the switch  202 , the switch  242 , and the switch  248  are turned on under the control of the switch control unit  122  (see  FIG. 1 ). All other switches are kept off. The battery unit  102 , the battery unit  104 , and the battery unit  182  are connected in series as a result. The first DC/DC converter  106  and the second DC/DC converter  108  are also connected in series. Accordingly, the voltage supplied from both terminals that are not connected to each other in the series connection of the battery unit  102 , the battery unit  104 , and the battery unit  182  (the series-connected voltage) can be shared by the first DC/DC converter  106  and the second DC/DC converter  108  connected in series, and the voltage input to each of the first DC/DC converter  106  and the second DC/DC converter  108  is lower than the series-connected voltage. 
     Referring to  FIG. 11 , the switch  200 , the switch  204 , the switch  240 , the switch  244 , the switch  246 , and the switch  250  are turned on under the control of the switch control unit  122  (see  FIG. 1 ). All other switches are kept off. The battery unit  102 , the battery unit  104 , and the battery unit  182  are connected in parallel as a result. The first DC/DC converter  106  and the second DC/DC converter  108  are also connected in parallel. Accordingly, the voltage supplied from each of the battery unit  102 , the battery unit  104 , and the battery unit  182  connected in parallel (e.g., 400 V) is supplied to each of the first DC/DC converter  106  and the second DC/DC converter  108  connected in parallel. 
     Accordingly, the connection state of the battery unit  102 , the battery unit  104 , and the battery unit  182 , as well as the connection state of the first DC/DC converter  106  and the second DC/DC converter  108 , can be changed in accordance with the states of the battery unit  102 , the battery unit  104 , and the battery unit  182  (e.g., the output voltages). For example, when the series-connected voltage has dropped in a state where the battery unit  102 , the battery unit  104 , and the battery unit  182  are connected in series (see  FIG. 10 ), the first DC/DC converter  106  and the second DC/DC converter  108  may be switched from a series connection to a parallel connection. To switch the first DC/DC converter  106  and the second DC/DC converter  108  from a series connection to a parallel connection, the switch  242  is switched from on to off, and the switch  240  and switch  244  are switched from off to on (see  FIG. 11 ). Additionally, when the output voltage drops while the battery unit  102 , the battery unit  104 , and the battery unit  182  are connected in parallel, the battery unit  102 , the battery unit  104 , and the battery unit  182  may be switched from a parallel connection to a series connection. The voltage in the series connection of the battery unit  102 , the battery unit  104 , and the battery unit  182  may be calculated by monitoring the output voltage of each of the battery unit  102 , the battery unit  104 , and the battery unit  182  and adding the output voltages together, or by detecting the overall output voltage when in a series connection state as illustrated in  FIG. 10 . 
     Even in the example illustrated in  FIG. 9 , the voltage detection unit  260  can be configured to be capable of detecting the output voltage of each of the battery units  102 ,  104 , and  182 . Furthermore, the voltage detection unit  260  may be configured to be capable of calculating the “output voltage when the battery units  102 ,  104 , and  182  are in the series connection state” (the voltage across both ends of all the units in a series connection when the battery units  102 ,  104 , and  182  are in the series connection state) by adding together the output voltages of the battery units  102 ,  104 , and  182 . 
     The switch control unit  122  (the control unit) monitors whether or not the stated output voltage detected by the voltage detection unit  260  has become less than or equal to a threshold, and when the output voltage has become less than or equal to the threshold, controls the switching devices  125  and  127  such that the first DC/DC converter  106  and the second DC/DC converter  108  are connected in parallel with respect to both ends of the battery units  102 ,  104 , and  182  in the series connection state (both ends of all the units in a series connection) while keeping the connection state of the battery units  102 ,  104 , and  182  in the series connection state. In this case, a voltage based on the stated output voltage of the battery units  102 ,  104 , and  182  in the series connection state (the overall voltage across both ends of the units in a series connection) is applied to each of the first DC/DC converter  106  and the second DC/DC converter  108 . Even in this example, because the stated threshold is set to a lower value than the respective breakdown voltages of the first DC/DC converter  106  and the second DC/DC converter  108 , the breakdown voltages will not be exceeded even if the voltages across both ends of the battery units  102  and  104  connected in series are applied to the respective converters. 
     Although the foregoing has described a case where there are three battery units and two DC/DC converters, the configuration is not limited thereto. There may be four or more battery units, and three or more DC/DC converters. Providing switches that connect the terminals of the plurality of battery units to each other makes it possible to switch the connection state of the plurality of battery units between the series connection state and the parallel connection state. The same applies to the plurality of DC/DC converters. In other words, the connection state of the plurality of battery units and the connection state of the plurality of DC/DC converters can be switched between the series connection state and the parallel connection state independently. 
     Although the foregoing has described the circuits illustrated in  FIG. 3  as specific examples of the first DC/DC converter  106  and the second DC/DC converter  108 , the configuration is not limited thereto. The first DC/DC converter  106  and the second DC/DC converter  108  may be publicly-known DC/DC converters. Additionally, although the foregoing has described each battery unit, as well as the first DC/DC converter  106  and the second DC/DC converter  108 , as having 400 V specifications, with a charging voltage of 800 V or 400 V being supplied from the quick-charging device, the configuration is not limited thereto. Each battery unit, as well as the first DC/DC converter  106  and the second DC/DC converter  108 , may be specifications aside from 400 V. Furthermore, a charging voltage aside from 800 V or 400 V may be supplied from the quick-charging device. 
     Although the foregoing has described the power conversion system has being installed in a vehicle, the configuration is not limited thereto. The power conversion system may be used in applications aside from vehicles. 
     While the present invention has been described through descriptions of embodiments, the foregoing embodiments are examples, and the present invention is not intended to be limited only to the foregoing embodiments. The scope of the present invention is defined by the respective patent claims in light of the detailed description of the invention, and includes all modifications made within a meaning and scope equivalent to the wording given therein.