Patent Publication Number: US-2022224147-A1

Title: In-vehicle backup power supply device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage of PCT/JP2020/018760 filed on May 11, 2020, which claims priority of Japanese Patent Application No. JP 2019-098237 filed on May 27, 2019, the contents of which are incorporated herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an in-vehicle backup power supply device. 
     BACKGROUND 
     If a failed state leading to a stop of supply of power from a main power supply occurs in an in-vehicle power supply system, power is no longer supplied to a load, and the load can be no longer electrically operated. However, there is a strong demand for some loads to operate continuously, and thus a system which includes a separate dedicated backup power supply that is different from the main power supply is known as a configuration that meets such a demand. JP 2018-13136A and JP 2018-62253A disclose examples of this kind of power supply system. 
     However, if a dedicated backup power supply is provided only in order to perform a backup operation at the time of a failure, then this leads to an increase in size and weight of the hardware. 
     In view of this, the present disclosure suggests a technique with which a backup operation can be performed at the time of a failure without using a dedicated backup power supply. 
     SUMMARY 
     An in-vehicle backup power supply device according to the present disclosure is an in-vehicle backup power supply device in an in-vehicle power supply system, the in-vehicle backup power supply device including: a battery unit in which a plurality of unit batteries are connected in series; a discharge circuit configured to perform a first discharge operation for supplying power to a conductive path on a load side based on a charge accumulated in the battery unit; a balance circuit configured to perform a cell balancing operation on the battery unit; and a control unit configured to control the balance circuit, wherein the balance circuit is configured to perform a second discharge operation for supplying power to the conductive path on the load side based on a charge accumulated in a plurality of power storage elements, and the control unit performs a first control for causing the balance circuit to perform the cell balancing operation and a second control for causing the balance circuit to perform the second discharge operation, and if a failure occurs in which the first discharge operation is not normally performed, performs the second control. 
     Advantageous Effects of Invention 
     According to the present disclosure, a backup operation can be performed with a simple configuration without providing a dedicated configuration for backing up a battery unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram schematically showing an in-vehicle backup power supply device according to a first embodiment, in which switch elements are in a non-connecting state. 
         FIG. 2  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the first embodiment, in which the switch elements are performing a first operation. 
         FIG. 3  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the first embodiment, in which the switch elements are performing a second operation. 
         FIG. 4  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the first embodiment, in which a balance circuit is performing a second discharge operation. 
         FIG. 5  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the first embodiment, in which an end element electrode portion is electrically connected to an end electrode portion of a unit battery that can no longer discharge normally, and an inter-element electrode portion that is adjacent to the end element electrode portion is electrically connected to an inter-battery electrode portion that is adjacent to the end electrode portion. 
         FIG. 6  is a circuit diagram schematically showing an in-vehicle backup power supply device according to a second embodiment, in which the switch elements are in a non-connecting state. 
         FIG. 7  is a circuit diagram schematically showing an in-vehicle backup power supply device according to a third embodiment, in which first switch elements and second switch elements are in the non-connecting state. 
         FIG. 8  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the third embodiment, in which a first discharge operation is performed. 
         FIG. 9  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the third embodiment, in which switch units are performing an alternative operation. 
         FIG. 10  is a circuit diagram schematically showing an in-vehicle backup power supply device according to a fourth embodiment, in which the first switch elements and the second switch elements are in the non-connecting state. 
         FIG. 11  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the fourth embodiment, in which the switch units are performing an alternative operation. 
         FIG. 12  is a circuit diagram schematically showing an in-vehicle backup power supply device according to a fifth embodiment, in which the first switch elements, the second switch elements, and power storage unit switch units are in the non-connecting state. 
         FIG. 13  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the fifth embodiment, in which the power storage unit switch units are performing a third operation. 
         FIG. 14  is a circuit diagram schematically showing the in-vehicle backup power supply device according to the fifth embodiment, in which the power storage unit switch units are performing a fourth operation. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First, embodiments of the present disclosure will be listed and described. 
     An in-vehicle backup power supply device according to the present disclosure is an in-vehicle backup power supply device in an in-vehicle power supply system, the in-vehicle backup power supply device including a battery unit in which a plurality of unit batteries are connected in series, a discharge circuit configured to perform a first discharge operation for supplying power to a conductive path on a load side based on a charge accumulated in the battery unit. An in-vehicle backup power supply device according to the present disclosure includes a balance circuit configured to perform a cell balancing operation on the battery unit, and a control unit configured to control the balance circuit. The balance circuit is configured to perform a second discharge operation for supplying power to the conductive path on the load side based on a charge accumulated in a plurality of power storage elements. The control unit performs a first control for causing the balance circuit to perform the cell balancing operation and a second control for causing the balance circuit to perform the second discharge operation, and if a failure occurs in which the first discharge operation is not normally performed, performs the second control. In this manner, with the in-vehicle backup power supply device according to the present disclosure, it is possible to perform a backup operation with a simple structure without providing a dedicated configuration for backing up the battery unit. 
     In the in-vehicle backup power supply device according to the present disclosure a plurality of the balance circuits, a configuration is also possible in which the battery unit includes a plurality of unit battery groups, the plurality of balance circuits correspond to the plurality of unit battery groups, and the control unit operates each of the balance circuits independently. 
     With this configuration, if one balance circuit can no longer operate, the operation of another balance circuit can be continued, and thus it is possible to perform the backup operation more reliably. 
     In the in-vehicle backup power supply device according to the present disclosure, a configuration is also possible in which the discharge circuit includes a converter for stepping up or down a voltage that is input and outputting the resultant voltage, and when performing the second control, the control unit operates the converter such that the converter steps up or down an input voltage based on the power supplied from the power storage element, and applies an output voltage to the conductive path on the load side. 
     With this configuration, it is possible to supply power of a desired magnitude to the conductive path on the load side, based on the power supplied from the power storage element. In particular, if the power from the power storage element is stepped up by the converter, it is possible to effectively use the power accumulated in the power storage element. 
     In the in-vehicle backup power supply device according to the present disclosure, a configuration is also possible in which the discharge circuit includes a converter configured to step up or down a voltage that is input and outputting the resultant voltage, and when performing the first control, the control unit operates the converter such that the converter steps up or down an input voltage based on the power supplied from the power storage element and supplies the power to the battery unit side. 
     With this configuration, if the balance circuit performs the cell balancing operation, since the power supplied from the power storage unit is stepped up by the converter, it is possible to suppress a case in which a current flowing between the power storage unit and the battery unit decreases when the cell balancing operation has progressed to some extent. In this manner, it is possible to positively cause a current to flow from the power storage unit toward the battery unit, and shorten the time required for the balancing operation. 
     In the in-vehicle backup power supply device according to the present disclosure, a configuration is also possible in which the discharge circuit includes a converter configured to step up or down and output a voltage that is input, and when performing the first control, the control unit operates the converter to step up or down an input voltage based on the power supplied from the power storage element and supply the power to the battery unit side, and when performing the second control, the control unit operates the converter to step up or down an input voltage based on the power supplied from the power storage element and supply the power to the conductive path on the load side. 
     With this configuration, if the balance circuit performs the second discharge operation, it is possible to supply the power of a desired magnitude, based on the power supplied from the power storage unit to the conductive path on the load side. In particular, if the input voltage based on the power supplied from the power storage unit is stepped up by the converter, it is possible to effectively use the power accumulated in the power storage unit. Also, when performing the cell balancing operation, by the input voltage based on the power supplied from the power storage unit being stepped up by the converter, a decrease of a current flowing between the power storage unit and the unit batteries is suppressed when the cell balancing operation has progressed to some extent. Also, exchange of current can be positively performed between the power storage unit and the unit batteries. In this manner, it is possible to shorten the time required for the balancing operation. 
     In the in-vehicle backup power supply device according to the present disclosure, the battery unit includes end electrode portions and inter-battery electrode portions between the unit batteries. The balance circuit includes the power storage unit in which the plurality of power storage elements are connected in series, and a switch unit provided with a plurality of switch elements. The power storage unit includes an end element electrode portion and an inter-element electrode portion between the power storage elements. The switch elements respectively correspond to the unit batteries. The inter-element electrode portion or the end element electrode portion that corresponds to each unit battery is electrically connected to the high potential electrode or the low potential electrode of the unit battery to which each switch element corresponds. When performing the first control, the control unit operates the switch elements respectively corresponding to the unit batteries such that the high potential electrode and the low potential electrode of each unit battery are alternately and electrically connected to the inter-element electrode portion or the end element electrode portion that corresponds to the unit battery. When performing the second control, the control unit can operate the switch elements such that at least two of the inter-battery electrode portions or the end electrode portions are respectively and electrically connected to the inter-element electrode portion or the end element electrode portion. 
     With this configuration, at least the two of the inter-battery electrode portions or the end electrode portions are respectively and electrically connected to the inter-element electrode portion or the end element electrode portion, and thus it is possible to backup not only the battery unit but also the individual unit batteries. 
     In the in-vehicle backup power supply device according to the present disclosure, the balance circuit includes a power storage unit formed by one or more power storage elements and a switch unit provided with a plurality of the switch elements. The switch elements respectively correspond to the unit batteries. One end element electrode portion of the power storage unit is electrically connected to the high potential electrode of the unit battery to which the switch element corresponds, and the other end element electrode portion of the power storage unit is electrically connected to the low potential electrode of the unit battery to which the switch element corresponds. When performing the first control, with respect to one unit battery, the control unit electrically connects the high potential electrode to one end element electrode portion of the power storage unit. Also, the control unit controls the plurality of switch elements to perform an operation for electrically connecting the low potential electrode to the other end element electrode portion of the power storage unit, alternately on each of the plurality of unit batteries. When performing the second control, the control unit can operate the plurality of switch elements such that the high and low potential electrodes of the unit batteries and the end element electrode portions are not connected to each other. 
     With this configuration, since the cell balancing operation can be performed using one power storage element, it is possible to reduce the size of the power supply device itself. 
     In the in-vehicle backup power supply device according to the present disclosure, the balance circuit includes a power storage unit formed by one or more power storage element and a switch unit provided with a plurality of switch elements. The switch elements respectively correspond to the unit batteries. One end element electrode portion of the power storage unit is electrically connected to the high potential electrode of the unit battery to which the switch element corresponds to via the converter, and the other end element electrode portion of the power storage unit is electrically connected to the low potential electrode of the unit battery to which the switch element corresponds to via the converter. When performing the first control, with respect to one unit battery, the control unit electrically connects the high potential electrode to the one end element electrode portion of the power storage unit via the converter. Also, the control unit controls the plurality of switch elements to perform the operation for electrically connecting the other end element electrode portion of the power storage unit to the low potential electrode via the converter, alternately on each of the plurality of unit batteries. Then, the control unit operates the converters to supply power to whichever of the unit batteries or the power storage units has the lower voltage thereacross. When performing the second control, the control unit can operate the plurality of switch elements such that the high and low potential electrodes of the unit battery and the end element electrode portion are not connected to each other. 
     With this configuration, at the time of cell balancing operation, since the unit batteries are alternately connected to the converters through the switch units, it is possible to make one converter handle the plurality of unit batteries, and simplify the configuration of the power supply device. 
     In the in-vehicle backup power supply device according to the present disclosure, the balance circuit includes a power storage unit formed by one or more power storage elements and a switch unit provided with the plurality of switch elements. The switch elements respectively correspond to the unit batteries. The high and low potential electrodes of the unit battery to which each switch element corresponds are electrically connected to the converter via the conductive path on the battery unit side. A power storage unit switch unit is provided, which collectively switches electrical connection of the two end element electrode portions of the power storage unit to either the conductive path on the battery unit side or the conductive path on the load side. When performing the first control, the control unit operates the power storage unit switch unit to collectively switch electrical connection of the two end element electrode portions to the conductive path on the load side, and, with respect to one unit battery, electrically connects the high potential electrode to one end element electrode portion of the power storage unit via the converter. Also, the control unit operates the plurality of switch elements to perform the operation for electrically connecting the low potential electrode to the other end element electrode portion of the power storage unit via the converter, alternately on each of the plurality of unit batteries. Then, the control unit operates the converters to supply power to whichever of the unit batteries or the power storage units has the lower voltage thereacross. When performing the second control, the control unit controls the power storage unit switch unit such that the two end element electrode portions are collectively and electrically connected to the conductive path on the battery unit side. The control unit can operate the plurality of switch elements such that the high and low potential electrodes of the unit batteries and the conductive paths on the battery unit side are not connected to each other. 
     With this configuration, since the power storage unit switch unit collectively switches electrical connection of the two end element electrode portions to the conductive path on the battery unit side or the conductive path on the load side, it is possible to prevent a case in which the power storage unit is connected to both the conductive path on the battery unit side and the conductive path on the load side. In this manner, it is possible to suppress a case in which a malfunction occurs in the converter. 
     First Embodiment 
     Configuration of Power Supply Device 
     An in-vehicle backup power supply device  1  according to a first embodiment (hereinafter also referred to as “power supply device  1 ”) is used as power supply for outputting the power for driving an electromotive device (e.g., motor) in vehicles such as hybrid cars or electric cars (EV (Electric Vehicle)). As shown in  FIG. 1 , the power supply device  1  includes a battery unit  10 , a discharging circuit  11 , a balance circuit  70 , and a control unit  12 . In the battery unit  10 , a plurality of unit batteries  10 A are electrically connected in series. Batteries such as lithium-ion batteries are used for the unit batteries  10 A. The battery unit  10  includes inter-battery electrode portions  10 B and end electrode portions  10 C. The inter-battery electrode portions  10 B are portions at which adjacent unit batteries  10 A are electrically connected in series. The end electrode portions  10 C are the high potential electrode of the unit battery  10 A located at the highest potential in the battery unit  10  and the low potential electrode of the unit battery  10 A located at the lowest potential in the battery unit  10 . 
     Each of the end electrode portions  10 C of the battery unit  10  is electrically connected to a power generation device  50  mounted in the vehicle, and the battery unit  10  can be charged by the power generation device  50 . The power generation device  50  is configured as a known in-vehicle power generator, and can generate power through rotation of the rotational axis of an engine (not shown). When the power generation device  50  operates, power generated by the power generation device  50  is rectified, and then supplied to the battery unit  10  as DC power. 
     The discharge circuit  11  includes a plurality of converters  11 A. The converters  11 A are, for example, configured as known bi-directional step up/down DC-DC converters provided with semiconductor switching elements and inductors, and the operation is controlled by the control unit  12 . The converters  11 A step up or down a voltage that is input and output the resultant voltage. The converters  11 A are electrically connected to the battery unit  10  via a first circuit unit  30  that is a conductive path on the battery unit side. The first circuit unit  30  constitutes a power path between the discharge circuit  11  and the battery unit  10 . The first circuit unit  30  includes a first conductive path  30 A and a second conductive path  30 B. The converters  11 A are electrically connected to the highest potential electrode in the battery unit  10  via the first conductive path  30 A. The converters  11 A are electrically connected to the lowest potential electrode in the battery unit  10  via the second conductive path  30 B. The potential difference between the first conductive path  30 A and the second conductive path  30 B is input to each converter  11 A as an input voltage. 
     A first load  51  and a second load  52  are electrically connected to the respective converters  11 A via third conductive paths  31 A that are conductive paths on the load side. Although the first load  51  and the second load  52  have similar functions, this is merely a typical example, and there is no limitation to this configuration. Also, a ground path G that is the conductive path on the load side is electrically connected to each converter  11 A. 
     The first load  51  is, for example, an electric power steering system in which an electrical component such as a motor operates by receiving power supply from the battery unit  10  via the converter  11 A. The second load  52  is an electric power steering system having a configuration and function similar to the first load  51 . When there is an abnormality in the first load  51 , the second load  52  operates instead of the first load  51 , and thus the function of the first load  51  can be maintained even if there is an abnormality in the first load  51 . 
     The one converter  11 A to which the first load  51  is electrically connected can perform a discharge operation in which, when a first condition is satisfied, the control unit  12  steps up or down the potential difference between the first conductive path  30 A and the second conductive path  30 B as the input voltage and apply the output voltage to the third conductive path  31 A, for example. That the first condition is satisfied may mean that an ignition switch (not shown) provided in the vehicle is switched from off to on, for example. 
     Also, when the first load  51  can no longer operate normally, the control unit  12  causes the other converter  11 A to which the second load  52  is electrically connected to execute the discharge operation, and supply power to the second load  52  via the third conductive path  31 A. The control unit  12  can obtain the voltage value from a detection unit that detects the voltage value and current value of the third conductive path  31 A that is connected to the first load  51 , and based on this voltage value, determine whether the first load  51  can no longer operate normally. 
     The balance circuit  70  includes the power storage unit  71  and the switch unit  72 . The power storage unit  71  is formed by a plurality of power storage elements  71 A that can temporarily store power being electrically connected in series. Electric double layer capacitors or the like are used for the power storage elements  71 A, for example. The power storage unit  71  includes inter-element electrode portions  71 B and end element electrode portions  71 C. The inter-element electrode portions  71 B are portions at which the adjacent power storage elements  71 A are electrically connected in series. The end element electrode portions  71 C are the high potential electrode of the power storage element  71 A located at the highest potential in the power storage unit  71  and the low potential electrode of the power storage element  71 A located at the lowest potential in the power storage unit  71 . 
     The switch unit  72  is provided with a plurality of switch elements  72 A. MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or the like are used for the switch elements  72 A, for example. One inter-element electrode portion  71 B or end element electrode portion  71 C is electrically connected to each switch element  72 A. The switch elements  72 A respectively correspond to the unit batteries  10 A. The switch elements  72 A can connect the inter-element electrode portion  71 B or end element electrode portion  71 C connected thereto to either the high potential electrode or the low potential electrode of the unit battery  10 A that corresponds to it, by being controlled by the control unit  12 . The switch elements  72 A can also bring the switch elements  72 A into a state in which the switch elements  72 A do not connect any of the inter-element electrode portion  71 B or end element electrode portion  71 C connected thereto to the high and low potential electrodes of the unit battery  10 A that correspond thereto (hereinafter also referred to as “non-connecting state”), by being controlled by the control unit  12 . 
     The control unit  12  is constituted mainly by a microcomputer, for example, and includes a computation device such as a CPU (Central Processing Unit), a memory such as a ROM (Read Only Memory) or a RAM (Random Access Memory), an A/D converter and the like. The control unit  12  can monitor the potential difference between the two ends of the unit batteries  10 A of the battery unit  10 , and the connecting state of the unit batteries  10 A to the inter-battery electrode portions  10 B and the end electrode portions  10 C. The control unit  12  can monitor the potential difference between the two ends of the power storage elements  71 A of the power storage unit  71 . 
     Next, the operation of the power supply device  1  will be described. 
     First Discharge Operation 
     When the ignition switch is switched from off to on in the vehicle in which the power supply device  1  is mounted, power is supplied from the battery unit  10  to the converters  11 A of the discharge circuit  11  via the first circuit unit  30 . In the discharge circuit  11 , the control unit  12  starts the operation of the one converter  11 A which is electrically connected to the first load  51 , and the operation of the other converter  11 A which is electrically connected to the second load  52  is maintained in a stopped state. In this manner, the one converter  11 A performs the first discharge operation for supplying power to the third conductive path  31 A that is the conductive path on the load side. 
     If the first load  51  can no longer operate normally, the control unit  12  stops the operation of the one converter  11 A, and starts the operation of the other converter  11 A to which the second load  52  is electrically connected. In this manner, the other converter  11 A performs the first discharge operation for supplying power to the third conductive path  31 A. Specifically, the control unit  12  determines whether the first load  51  can no longer operate normally, based on the voltage value from the detection unit that detects the voltage value and current value of the third conductive path  31 A that is connected to first load  51 . If the control unit  12  determines that the first load  51  can no longer operate normally, the control unit  12  stops the operation of the one converter  11 A and starts the operation of the other converter  11 A. In this manner, the power is supplied from the other converter  11 A to the second load  52 . 
     Active Cell Balancing operation 
     The amount of power accumulated in the unit batteries  10 A of the battery unit  10  depends on the temperature and deterioration state of the unit batteries  10 A, and thus varies among the unit batteries  10 A. To eliminate this variation, the balance circuit  70  executes an active cell balancing operation (hereinafter also referred to as “cell balancing operation”). The control unit performs a first control to cause the balance circuit  70  to perform the cell balancing operation. 
     The first control is a control performed by the control unit  12  for causing the plurality of switch elements  72 A to alternately repeat a first operation and a second operation. The first operation is an operation (see  FIG. 2 ) in which the control unit  12  electrically connects the high potential electrodes of the unit batteries  10 A to which the switch elements  72 A correspond, to the inter-element electrode portion  71 B or the end element electrode portion  71 C that are electrically connected to those switch elements  72 A. Also, the second operation is an operation (see  FIG. 3 ) in which the control unit  12  electrically connects the inter-element electrode portions  71 B and the end element electrode portion  71 C that are electrically connected to those switch elements  72 A to the low potential electrodes. The length of the time period of the first operation and the length of the time period of the second operation may be the same or different as necessary. Between the first operation and the second operation, a fixed non-conduction time in the non-connecting state (see  FIG. 1 ) may be provided, in which neither the inter-element electrode portion  71 B nor the end element electrode portion  71 C is electrically connected to any of the inter-battery electrode portions  10 B and the end electrode portions  10 C. The length of the non-conduction time can be set as necessary. 
     When the switch elements  72 A of the switch unit  72  perform the first operation, the power storage elements  71 A of the power storage unit  71  are respectively connected in parallel to the unit batteries  10 A, except for the lowest potential unit battery  10 A of the battery unit  10  (see  FIG. 2 ). The movement of the charge between the unit batteries  10 A and the power storage elements  71 A depends on the voltage at the unit batteries  10 A and the power storage elements  71 A. Specifically, when looking at just one unit battery  10 A and one power storage element  71 A that is connected to this unit battery  10 A in parallel, if the voltage of this unit battery  10 A is higher than the voltage of this power storage element  71 A, the charge moves from the unit battery  10 A to the power storage element  71 A to charge the power storage element  71 A. On contrary, if the voltage of the power storage element  71 A is higher than the voltage of the unit battery  10 A, the charge moves from the power storage element  71 A to the unit battery  10 A to charge the unit battery  10 A. This holds true also for the other unit batteries  10 A and the other power storage elements  71 A connected to the respective unit batteries  10 A in parallel. 
     When the switch elements  72 A of the switch unit  72  perform the second operation, the power storage elements  71 A of the power storage unit  71  are respectively connected in parallel to the unit batteries  10 A except the highest potential unit battery  10 A of the battery unit  10  (see  FIG. 3 ). The movement of the charge between the unit batteries  10 A and the power storage elements  71 A varies depending on the magnitude of the voltages of the unit batteries  10 A and the power storage elements  71 A. Specifically, when looking at just one unit battery  10 A and one power storage element  71 A that is connected to this unit battery  10 A in parallel, if the voltage of the unit battery  10 A is higher than the voltage of the power storage element  71 A, the charge moves from the unit battery  10 A to the power storage element  71 A to charge the power storage element  71 A. On contrary, if the voltage of the power storage element  71 A is higher than the voltage of the unit battery  10 A, the charge moves from the power storage element  71 A to the unit battery  10 A to charge the unit battery  10 A. This holds true also for the other unit batteries  10 A and the other power storage elements  71 A connected in parallel to the respective unit batteries  10 A. In this manner, in the power supply device  1 , through the first control performed by the control unit  12 , the switch elements  72 A alternately repeat the first and second operations, and execute the cell balancing operation. 
     For example, when the control unit  12  determines that the difference between the potential differences of the two ends of the unit batteries  10 A is less than or equal to a predetermined value (in other words, the potential differences between the two ends of the unit batteries  10 A are similar), the device  1  shifts to the state shown in  FIG. 4  to charge the power storage unit  71 . After that, the balance circuit  70  ends the cell balancing operation. When the cell balancing operation ends, the switch elements  72 A enter the non-connecting state (see  FIG. 1 ). In this manner, the power storage elements  71 A are maintained in a state in which power is accumulated. 
     Second Discharge Operation 
     If there is a failure (hereinafter also referred to as a “failed state”) in which the first discharge operation cannot be performed normally, the control unit  12  performs the second control for causing the balance circuit  70  to perform the second discharge operation. A failed state may be given e.g. by an electrical connection between the adjacent unit batteries  10 A of the battery unit  10  entering an open state, the unit batteries  10 A no longer being capable of discharging normally, and the like. 
     In the failed state, as shown in  FIG. 4 , in the switch element  72 A (hereinafter also referred to as “high potential switch element  72 A”) that is connected to the highest potential unit battery  10 A in parallel, the control unit  12  electrically connects the end electrode portion  10 C and the end element electrode portion  71 C to each other. Also, in the switch element  72 A (hereinafter also referred to as “low potential switch element  72 A”) that is connected in parallel to the unit battery  10 A on the lowest potential side, the control unit  12  electrically connects the end electrode portion  10 C and the end element electrode portion  71 C to each other. Also, the control unit  12  brings the switch elements  72 A other than the high potential switch element  72 A and the low potential switch element  72 A into the non-connecting state. Thus, the charge accumulated in the power storage unit  71  is supplied to the discharge circuit  11  via the first circuit unit  30 . 
     Also, as shown in  FIG. 5 , if the highest potential unit battery  10 A can no longer discharge normally, the control unit  12  electrically connects the end electrode portion  10 C and the end element electrode portion  71 C to each other. Also, the control unit  12  may operate the switch element  72 A to electrically connect the inter-battery electrode portion  10 B that is adjacent to the end electrode portion  10 C to the inter-element electrode portion  71 B that is adjacent to the end element electrode portion  71 C. In this manner, since the power supply device  1  can form a circuit that passes through the power storage element  71 A while bypassing the unit battery  10 A that can no longer discharge normally, it is possible to backup the individual unit batteries  10 A. 
     Next, the effects of this configuration will be illustrated. 
     The in-vehicle backup power supply device  1  of the present disclosure is included in the in-vehicle power supply system provided with the battery unit  10  in which the plurality of unit batteries  10 A are connected in series, and the discharge circuit  11  that performs the first discharge operation for supplying power to the third conductive path  31 A based on the charge accumulated in the battery unit  10 . The in-vehicle backup power supply device  1  of the present disclosure is provided with the balance circuit  70  for performing the cell balancing operation on the battery unit  10 , and the control unit  12  for controlling the balance circuit  70 . The balance circuit  70  is configured to perform the second discharge operation for supplying power to the third conductive path  31 A based on the charge accumulated in the plurality of power storage elements  71 A. The control unit  12  performs the first control for causing the balance circuit  70  to perform the cell balancing operation and the second control for causing the balance circuit  70  to perform the second discharge operation. The control unit  12  performs the second control when there is a failure in which the first discharge operation is not normally performed. In this manner, the in-vehicle backup power supply device  1  of the present disclosure can perform the backup operation with a simple structure without providing a dedicated configuration for backing up the battery unit  10 . 
     In the in-vehicle backup power supply device  1  of the present disclosure, the battery unit  10  includes the end electrode portions  10 C and the inter-battery electrode portions  10 B between the unit batteries  10 A. The balance circuit  70  includes the power storage unit  71  in which the plurality of power storage elements  71 A are connected in series, and the switch unit  72  provided with the plurality of switch elements  72 A. The power storage unit  71  includes the end element electrode portions  71 C and the inter-element electrode portions  71 B between the power storage elements  71 A, and the switch elements  72 A respectively correspond to the unit batteries  10 A. The high potential electrode or the low potential electrode of the unit battery  10 A to which each switch element  72 A corresponds is electrically connected to the inter-element electrode portion  71 B or the end element electrode portion  71 C which corresponds to each unit battery  10 A. When performing the first control, the control unit  12  operates the switch elements  72 A that corresponds to the unit batteries  10 A such that the inter-element electrode portion  71 B or the end element electrode portion  71 C that corresponds to the unit battery  10 A is alternately and electrically connected to the high and low potential electrodes of the unit battery  10 A. When performing the second control, the control unit  12  operates the switch elements  72 A such that at least two of the inter-battery electrode portions  10 B or the end electrode portions  10 C are electrically connected to the inter-element electrode portions  71 B or the end element electrode portions  71 C, respectively. 
     With this configuration, at least two of the inter-battery electrode portions  10 B or the end electrode portions  10 C are electrically connected to the inter-element electrode portions  71 B or the end element electrode portions  71 C, respectively. For this reason, it is possible to backup not only the battery unit  10  but also the individual unit batteries  10 A. 
     The discharge circuit  11  of the in-vehicle backup power supply device  1  of the present disclosure includes the converters  11 A for stepping up or down the voltage that is input and outputting the resultant voltage. When performing the second control, the control unit  12  operates the converters  11 A to step up or down the input voltage based on the power supplied from the power storage element  71 A and supply the power to the third conductive path  31 A. 
     With this configuration, it is possible to supply the power of the desired magnitude to the third conductive path  31 A based on the power supplied from the power storage elements  71 A. Specifically, when stepping up the power supplied from the power storage elements  71 A through the converters  11 A, the power accumulated in the power storage elements  71 A can be effectively used. 
     Second Embodiment 
     Next, an in-vehicle backup power supply device  2  (hereinafter also referred to as “power supply device  2 ”) according to a second embodiment will be described with reference to  FIG. 6 . The power supply device  2  is different from the power supply device  1  of the first embodiment in that a plurality of the balance circuits  170  are included, the balance circuits  170  respectively correspond to unit battery groups  110 A and  110 B formed by dividing the battery unit  110 , the configuration of a third conductive path  131 A, the configuration of a first circuit unit  130 , and the like. The constituent elements that are same as the first embodiment are given the same reference numerals, and description of their structures, operations, and effects will be omitted. 
     Configuration of Power Supply Device 
     The battery unit  110  is formed by a plurality of unit batteries  10 A being connected in series. The battery unit  110  includes the plurality of unit battery groups  110 A and  110 B. 
     The balance circuits  170  are provided respectively corresponding to the unit battery groups  110 A and  110 B. The configuration of the electrical connection between the unit battery groups  110 A and  110 B and the balance circuits  170  is the same as the first embodiment. Each balance circuit  170  includes a power storage unit  171  and a switch unit  172 . The power storage unit  171  is different from the power storage unit  71  of the first embodiment only in the number of power storage elements  71 A and the number of inter-element electrode portions  71 B. The switch unit  172  is different from the switch unit  72  of the first embodiment only in the number of switch elements  72 A. 
     The battery groups  110 A and  110 B of the battery unit  110  are electrically connected to the converters  11 A via the first circuit units  130 , respectively. Each first circuit unit  130  constitutes a power path between the discharge circuit  11  and the battery unit  110 . The first circuit unit  130  is provided with a first conductive path  130 A and a second conductive path  130 B. The one converter  11 A is electrically connected to the highest potential electrode in the unit battery group  110 A of the battery unit  110  via the first conductive path  130 A. This converter  11 A is electrically connected to the lowest potential electrode in the unit battery group  110 A of the battery unit  110  via the second conductive path  130 B. The other converter  11 A is electrically connected to the highest potential electrode in the unit battery group  110 B of the battery unit  110  via the first conductive path  130 A. This converter  11 A is electrically connected to the lowest potential electrode in the unit battery group  110 B of the battery unit  110  via the second conductive path  130 B. The potential difference between the first conductive path  130 A and the second conductive path  130 B is input to the converters  11 A as an input voltage. 
     The third conductive paths  131 A that are the conductive paths on the load side are electrically connected to the respective converters  11 A. The first load  51  and the second load  52  are electrically connected to the converters  11 A via the third conductive paths  131 A. Each third conductive path  131 A includes a first load switch  131 B and a second load switch  131 C. MOSFETs or the like are used for the first load switch  131 B and the second load switch  131 C. The first load switches  131 B are controlled by the control unit  12  and switch the current flow between the converters  11 A and the first load  51  to the open state or the closed state. The second load switches  131 C are controlled by the control unit  12  and switch the current flow between the converter  11 A and the second load  52  to the open state or the closed state. Also, the ground path G that is the conductive path on the load side is electrically connected to each of the converters  11 A. 
     Next, the operation of the power supply device  2  will be described. 
     First Discharge Operation 
     In the vehicle on which the power supply device  2  is mounted, when the ignition switch is switched from off to on, for example, power is supplied to the converters  11 A of the discharge circuit  11  via the first circuit units  130  from the battery unit  110 . The control unit  12  starts the operation of the converters  11 A in the discharge circuit  11 . Also, the first load switch  131 B of each of the third conductive paths  131 A is closed by the control unit  12 , and the second load switch  131 C is opened by the control unit  12  (not shown). In this manner, power is supplied from the converters  11 A to the first load  51 . 
     If the control unit  12  determines that the first load  51  can no longer normally operate, the control unit  12  turns the first load switch  131 B from the closed state to the open state, and turns the second load switch  131 C from the open state to the closed state (not shown). In this manner, power is supplied from the converters  11 A to the second load  52 . 
     If the control unit  12  determines that either of the balance circuits  170  can no longer operate normally based on the potential difference at the two ends of the power storage elements  71 A of the power storage unit  71  or the like, the control unit  12  stops the operation of the converter  11 A that corresponds to the balance circuit  170  that cannot operate normally. Also, the control unit  12  opens the first load switch  131 B and the second load switch  131 C of the third conductive path  131 A that is electrically connected to this converter  11 A. At this time, the control unit  12  continues the operation of the converter  11 A to which the other balance circuit  170  is electrically connected. Also, the control unit  12  keeps the closed state of the first load switch  131 B of the third conductive path  131 A and the open state of the second load switch  131 C that are electrically connected to this converter  11 A. In this manner, even if either of the balance circuits  170  can no longer operate normally, power supply to the first load  51  can be maintained from the converter  11 A to which the other balance circuit  170  is electrically connected. 
     Note that the control unit  12  may make the following determination. First, the control unit  12  determines whether either the unit battery group  110 A or  110 B of the battery unit  110  can no longer operate normally, based on the potential difference at both ends of the unit batteries  10 A of the battery units  110  and the connecting state of the unit batteries  10 A to the inter-battery electrode portions  10 B and the end electrode portions  10 C. Next, even if it is determined that either the unit battery group  110 A or  110 B can no longer operate normally, the control unit  12  may also stop the operation of the converter  11 A that corresponds to the unit battery group  110 A or  110 B which cannot operate normally. 
     Active Cell Balancing Operation 
     The cell balancing operations of the balance circuits  170  of the unit battery groups  110 A and  110 B are similar to that of the balance circuit  70  of the first embodiment. The control unit  12  performs the first control to cause the balance circuits  170  to perform the cell balancing operation. For example, when the control unit  12  determines that the potential differences of the two ends of the unit batteries  10 A are less than or equal to a predetermined value (in other words, the potential differences between the two ends of the unit batteries  10 A become similar), the balance circuits  170  end the cell balancing operation. When the cell balancing operation ends, the switch elements  72 A enter the non-connecting state (see  FIG. 6 ). In this manner, the power storage elements  71 A are maintained in a state in which power is accumulated. 
     Second Discharge Operation 
     When power is supplied to the third conductive paths  131 A from either of the converters  11 A, the power supply device  2  can keep up the first discharge operation by keeping up power supply to the first load  51  from this converter  11 A. In the power supply device  2 , when there is a failure in both unit battery groups  110 A and  110 B of the battery unit  10 , the control unit  12  performs the second control to cause the balance circuits  170  to perform the second discharge operation. The second discharge operation performed by the balance circuits  170  of the unit battery groups  110 A and  110 B is similar to that of the balance circuit  70  of the first embodiment. 
     Next, the effects of this configuration will be illustrated. 
     The in-vehicle backup power supply device  2  of the present disclosure includes the plurality of balance circuits  170 . The battery unit  110  includes the plurality of unit battery groups  110 A and  110 B. The plurality of unit battery groups  110 A and  110 B respectively corresponds to the plurality of balance circuits  170 . The control unit  12  operates each of the balance circuits  170  independently. 
     With this configuration, even if one of the balance circuits  170  cannot operate normally, the operation of the other balance circuit  170  can be continued, and thus the backup operation can be more reliably performed. 
     Third Embodiment 
     Next, an in-vehicle backup power supply device  3  (hereinafter also referred to as “power supply device  3 ”) according to the third embodiment will be described with reference to  FIGS. 7 to 9 . The power supply device  3  is different from the second embodiment in the configuration of balance circuits  270 , for example. The constituent elements that are the same as that of the second embodiment are given the same reference numerals, and description of their structures, operations, and effects will be omitted. 
     Configuration of Power Supply Device 
     The power supply device  3  is provided with a plurality of balance circuits  270 . Each balance circuit  270  includes a switch unit  272  and a power storage unit  271 . Each switch unit  272  includes a plurality of switch pairs  272 A. Each switch pair  272 A includes a first switch element  272 B and a second switch element  272 C. MOSFETs or the like are used for the first switch elements  272 B and the second switch elements  272 C. The power storage units  271  are different from the power storage units  171  of the second embodiment only in that the inter-element electrode portions  71 B are not connected to the switch units  272 . 
     The switch pairs  272 A respectively correspond to the unit batteries  10 A. Specifically, the first switch elements  272 B of the switch pairs  272 A are respectively and electrically connected to the high potential electrodes of the unit batteries  10 A, and the second switch elements  272 C of the switch pairs  272 A are respectively and electrically connected to their low potential electrodes. In each of the unit battery groups  110 A and  110 B, the high potential electrodes of the unit batteries  10 A are electrically connected to the first conductive path  130 A of the first circuit unit  130  and the one end element electrode portion  71 C of the power storage unit  271  via the first switch elements  272 B. In each of the unit battery groups  110 A and  110 B, the low potential electrodes of the unit batteries  10 A are electrically connected to the second conductive path  130 B of the first circuit unit  130  and the other end element electrode portion  71 C of the power storage unit  271  via the second switch elements  272 C. 
     The control unit  12  is configured to monitor the potential difference at the two ends of the unit batteries  10 A of the battery unit  110 , the connecting state of the unit batteries  10 A to the inter-battery electrode portions  10 B and the end electrode portions  10 C. The control unit  12  can monitor the potential difference between the two ends of the power storage elements  71 A of the power storage unit  271 . 
     Next, the operation of the power supply device  3  will be described. 
     First Discharge Operation 
     In the vehicle on which the power supply device  3  is mounted, when the ignition switch is switched from off to on, for example, the first switch elements  272 B connected to the high potential electrodes of the unit battery groups  110 A and  110 B of the battery unit  110  and the second switch elements  272 C connected to the low potential electrodes are closed. Also, the other first switch elements  272 B and the second switch elements  272 C are opened (see  FIG. 8 ). Accordingly, power is supplied from the battery unit  110  to the converters  11 A in the discharge circuit  11  via the first circuit units  130 . In the discharge circuit  11 , the control unit  12  starts the operation of the converters  11 A. Also, the first load switches  131 B of each of the third conductive paths  131 A are closed by the control unit  12 , and the second load switches  131 C are opened by the control unit  12 . In this manner, power is supplied from the converters  11 A to the first load  51 . 
     Active Cell Balancing Operation 
     The control unit  12  performs the first control to cause the balance circuits  270  to perform the cell balancing operation. The control unit  12  operates each switch unit  272  to perform the operation of selectively closing the one switch pair  272 A in the switch unit  272  of the balance circuit  270 , and opening the other switch pairs  272 A, alternately on all the switch pairs  272 A (hereinafter also referred to as “alternative operation”) (see  FIG. 9 ). The switch pair  272 A being closed means that both the first switch element  272 B and the second switch element  272 C in the switch pair  272 A are closed. The switch pair  272 A being open means that both the first switch element  272 B and the second switch element  272 C in the switch pair  272 A are open. The lengths of time for the alternative operations of the switch pairs  272 A may be the same, or different as necessary. Also, at the time of alternative operation, when switching the closed state of the switch pair  272 A that is currently closed to the next switch pair  272 A, a fixed non-conduction time in which all the switch pairs  272 A are opened is provided. The length of the non-conduction time can be set as necessary. 
     When the alternative operation is performed in the switch unit  272  of each balance circuit  270 , one unit battery  10 A and power storage unit  271  are connected in parallel (see  FIG. 9 ). The movement of the charge between the unit batteries  10 A and the power storage unit  271  depends on the voltage at the unit batteries  10 A and the power storage unit  271 . Specifically, when the voltage of the unit battery  10 A is higher than the voltage of the power storage unit  271 , the charge moves from the unit battery  10 A to the power storage unit  271  to charge the power storage unit  271 . On the other hand, if the voltage of the power storage unit  271  is higher than the voltage of the unit battery  10 A, the charge moves from the power storage unit  271  to the unit battery  10 A to charge the unit battery  10 A. This holds true also for the other unit batteries  10 A and the power storage units  271  when the other switch pairs  272 A are closed in the alternative operation. For example, when the control unit  12  determines that the differences of the potential differences of the two ends of the unit batteries  10 A are less than or equal to a predetermined value (in other words, the potential differences between the two ends of the unit batteries  10 A became the same), the balance circuits  270  end the cell balancing operation. 
     Second Discharge Operation 
     In the failed state, the control unit  12  causes the balance circuits  270  to perform the second discharge operation. In the failed state, the control unit  12  opens all the switch pairs  272 A in the switch units  272  (see  FIG. 7 ). In this manner, only the charge accumulated in the power storage units  271  is supplied to the converters  11 A via the first circuit unit  30 . Due to the control unit  12  performing the second control, the converters  11 A perform the second discharge operation for stepping up or down the input voltage based on the power from the power storage elements  71 A to supply power to the third conductive path  131 A. 
     Next, the effects of this configuration will be illustrated. 
     The discharge circuit  11  of the in-vehicle backup power supply device  3  of the present disclosure includes the converters  11 A for stepping up or down the voltage that is input and output the resultant voltage. When performing the second control, the control unit  12  can operate the converters  11 A to step up or down the input voltage based on the power supplied from the power storage elements  71 A and supply power to the third conductive paths  131 A. 
     With this configuration, the power of the desired magnitude can be supplied to the third conductive paths  131 A based on the power supplied from the power storage elements  71 A. Specifically, when stepping up the power supplied from the power storage elements  71 A through the converters  11 A, the power stored in the power storage elements  71 A can be effectively used. 
     In the in-vehicle backup power supply device  3  of the present disclosure, the balance circuit  270  includes a power storage unit  271  formed by a plurality of power storage elements  71 A and the switch unit  272  including the plurality of switch pairs  272 A. The switch pairs  272 A respectively correspond to the unit batteries  10 A. One end element electrode portion  71 C of the power storage unit  271  is electrically connected to the high potential electrode of the unit battery  10 A to which the switch pair  272 A corresponds. Also, the other end element electrode portion  71 C of the power storage unit  271  is electrically connected to the low potential electrode of the unit battery  10 A to which the switch pair  272 A corresponds. When performing the second control, the control unit  12  electrically connects the one end element electrode portion  71 C of the power storage unit  271  to the high potential electrode with respect to one unit battery  10 A. Also, the control unit  12  operates the plurality of switch pairs  272 A to perform the operation in which the other end element electrode portion  71 C of the power storage unit  271  is electrically connected to the low potential electrode, alternately on each of the plurality of unit batteries  10 A. When performing the second discharge control, the control unit  12  operates the plurality of switch pairs  272 A such that the high and low potential electrodes of the unit batteries  10 A and the end element electrode portions  71 C are not connected to each other. 
     With this configuration, since the cell balancing operation can be performed using one power storage element  71 A, the power supply device  3  can be made smaller. 
     Fourth Embodiment 
     Next, an in-vehicle backup power supply device  4  (hereinafter also referred to as “power supply device  4 ”) according to a fourth embodiment will be described with reference to  FIGS. 10 and 11 . The power supply device  4  is different from the third embodiment in the location to which the two end element electrode portions  71 C of the power storage unit  271  are connected. The constituent elements that are the same as that of the third embodiment are given the same reference numerals and description of their structure, operation, and effects will be omitted. 
     Configuration of Power Supply Device 
     Each balance circuit  370  includes a switch unit  272  and a power storage unit  271 . The configurations of the switch unit  272  and the power storage unit  271  are the same as those of the third embodiment. One end element electrode portion  71 C of the power storage unit  271  is electrically connected to the discharge circuit  11  side of the third conductive path  31 A. The other end element electrode portion  71 C of the power storage unit  271  is electrically connected to the ground path G that is the conductive path on the load side. The converters  11 A are also electrically connected to the ground path G. The control unit  12  is configured to monitor the potential difference between the two ends of the unit batteries  10 A of the battery unit  110 , and the connection of the unit batteries  10 A to the inter-battery electrode portions  10 B and the end electrode portions  10 C. The control unit  12  is configured to monitor the potential difference between the two ends of the power storage elements  71 A of the power storage unit  271  and the like. 
     Next, the operations of the power supply device  4  will be described. 
     First Discharge Operation 
     The first discharge operation of the power supply device  4  is similar to that of the third embodiment. 
     Active Cell Balancing Operation 
     The control unit  12  performs the first control to cause the balance circuits  370  to perform the cell balancing operation. When the alternative operation is performed in the switch units  272  of the balance circuits  370 , one unit battery  10 A and one power storage unit  271  are connected in parallel via the converter  11 A (see  FIG. 11 ). 
     If the control unit  12  determines that the voltage of the unit batteries  10 A is higher than the voltage of the power storage units  271 , the charge moves from the unit batteries  10 A to the power storage units  271  via the converters  11 A to charge the power storage units  271 . At this time, the converters  11 A perform the step up operation based on the voltage of the unit batteries  10 A to supply power to the power storage units  271 , and thus the charge moves to the power storage units  271  at an early stage. 
     On the other hand, if the control unit  12  determines that the voltage of the power storage units  271  is higher than the voltage of the unit batteries  10 A, the charge moves from the power storage units  271  to the unit batteries  10 A via the converters  11 A to charge the unit batteries  10 A. At this time, the converters  11 A perform the step up operation based on the voltage of the power storage units  271  to supply power to the unit batteries  10 A, and thus the charge moves to the unit batteries  10 A at an early stage. For example, when the control unit  12  determines that the difference between the potential difference between the two ends of the unit batteries  10 A is less than or equal to a predetermined value (in other words, the potential differences between the two ends of the unit batteries  10 A are similar), the balance circuits  370  end the cell balancing operation. 
     Second Discharge Operation 
     In the failed state, the control unit  12  performs the second discharge operation to cause the balance circuits  370  to perform the second discharge operation. In the failed state, as shown in  FIG. 10 , the control unit  12  opens all the switch pairs  272 A of the switch units  272 . In this manner, only the charge stored in the power storage units  271  is supplied to the first load  51  via the third conductive paths  31 A. 
     Next, the effects of this configuration will be illustrated. 
     The discharge circuit  11  of the in-vehicle backup power supply device  4  of the present disclosure includes the converters  11 A for stepping up or down the voltage that is input and outputting the resultant voltage. When performing the first control, the control unit  12  operates the converters  11 A to step up or down the input voltage based on the power supplied from the power storage elements  71 A and supply power to the battery unit  110 . 
     With this configuration, when the balance circuits  370  perform the cell balancing operation, since the input voltage based on the power supplied from the power storage units  271  is stepped up by the converters  11 A, it is possible to suppress a decrease of a current flowing between the power storage units  271  and the battery unit  110  when the cell balancing operation has progressed to some extent. Accordingly, it is possible to positively allow a current to flow from the power storage units  271  to the battery unit  110 , and the time required for performing the balancing operation can be further shortened. 
     In the in-vehicle backup power supply device  4  according to the present disclosure, each balance circuit  370  includes a power storage unit  271  formed by one or more power storage elements  71 A, and a switch unit  272  including a plurality of switch pairs  272 A. The switch pairs  272 A respectively correspond to the unit batteries  10 A. One end element electrode portion  71 C of the power storage unit  271  is electrically connected via the converter  11 A to the high potential electrodes of the unit batteries  10 A to which the switch pairs  272 A correspond. Also, the other end element electrode portion  71 C of the power storage unit  271  is electrically connected via the converter  11 A to the low potential electrodes of the unit batteries  10 A to which the switch pairs  272 A correspond. When performing the first control, with respect to one unit battery  10 A, the control unit  12  electrically connects the one end element electrode portion  71 C of the power storage unit  271  to the high potential electrode via the converter  11 A. Also, the control unit  12  operates the plurality of switch pairs  272 A to perform the operation for electrically connecting the other end element electrode portion  71 C of the power storage unit  271  to the low potential electrode via the converter  11 A, alternately on each of the plurality of unit batteries  10 A. The control unit  12  operates the converters  11 A so as to supply power to whichever of the unit batteries  10 A or the power storage units  271  has the lower voltage thereacross. When performing the second control, the control unit  12  can operate the plurality of switch pairs  272 A such that the high and low potential electrodes of the unit batteries  10 A and the end element electrode portions  71 C are not connected to each other. 
     With this configuration, when the cell balancing operation is performed, the unit batteries  10 A are alternatively connected to the converters  11 A through the switch units  272 . For this reason, one converter  11 A can correspond to a plurality of unit batteries  10 A, and the configuration of the power supply device  4  can be simplified. 
     Fifth Embodiment 
     Next, an in-vehicle backup power supply device  5  according to the fifth embodiment (hereinafter also referred to as “power supply device  5 ”) will be described with reference to  FIGS. 12 to 14 . The power supply device  5  is different from that of the third and fourth embodiments in that the locations to which the two end element electrode portions  71 C of the power storage unit  271  are connected are collectively changed through power storage unit switch units  273 . The constituent elements that are the same as those of the third and fourth embodiment are given the same reference numerals, and description of their structures, operations, and effects will be omitted. 
     Configuration of Power Supply Device 
     The balance circuits  470  each include the switch unit  272  and the power storage unit  271 . The configurations of the switch unit  272  and the power storage unit  271  are similar to the third and fourth embodiments. The two end element electrode portions  71 C of the power storage unit  271  are electrically connected to the respective power storage unit switch units  273 . MOSFETs or the like are used for the power storage unit switch units  273 . The operations of the power storage unit switch units  273  can be controlled by the control unit  12 . 
     Specifically, the power storage unit switch units  273  can perform third and fourth operations. The third operation is an operation performed by the control unit  12  for electrically connecting the one end element electrode portion  71 C to the first conductive path  130 A of the first circuit unit  130 , and the other end element electrode portion  71 C to the second conductive path  130 B of the first circuit unit  130  (see  FIG. 13 ). The fourth operation is an operation for electrically connecting the one end element electrode portion  71 C to the third conductive path  31 A and the other end element electrode portion  71 C to the ground path G (see  FIG. 14 ). In other words, the power storage unit switch units  273  collectively switch the electric connection of the two end element electrode portions  71 C of the power storage unit  271  to either the first circuit unit  130 , or the third conductive path  131 A and the ground path G. 
     Also, by the control unit  12 , each power storage unit switch unit  273  can also bring the two end element electrodes  71 C into a state in which they are electrically connected to neither the first circuit unit  130 , the third conductive path  31 A, nor the ground path G (hereinafter also referred to as “non-connecting state”) (see  FIG. 12 ). 
     Next, the operation of the power supply device  5  will be described. 
     First Discharge Operation 
     The first discharge operation performed by the power supply device  5  is similar to that of the third embodiment. 
     Active Cell Balancing Operation 
     When the balance circuits  470  perform the cell balancing operation, the control unit  12  causes the power storage unit switch units  273  to perform the fourth operation (see  FIG. 14 ). The control unit  12  performs the first control for causing the balance circuits  470  to perform the cell balancing operation. When the control unit  12  controls the switch unit  272  of each balance circuit  470  to operate the alternative operation, one of the unit batteries  10 A and the power storage unit  271  are connected in parallel via the converters  11 A (not shown). If the voltage of the unit battery  10 A is higher than the voltage of the power storage unit  271 , the charge moves from the unit battery  10 A to the power storage unit  271  via the converter  11 A to charge the power storage unit  271 . At this time, due to the converter  11 A performing the step up operation, the charge can move to the power storage unit  271  at an early stage. 
     On the other hand, if the voltage of the power storage unit  271  is higher than the voltage of the unit battery  10 A, the charge moves from the power storage unit  271  to the unit battery  10 A via the converter  11  to charge the unit battery  10 A. At this time, due to the converter  11 A performing the step-up operation, the charge can move to the unit battery  10 A at the early stage. 
     For example, when the control unit  12  determines that the potential differences of the two ends of the unit batteries  10 A are less than or equal to a predetermined value (in other words, the potential differences between the two ends of the unit batteries  10 A become similar), the balance circuits  470  end the cell balancing operation. 
     When the cell balancing operation ends, the power storage unit switch units  273  of the balance circuits  470  enter the non-connecting state (see  FIG. 12 ). In this manner, the power storage units  271  are maintained in a state in which the power is accumulated. 
     Second Discharge Operation 
     In the failed state, the control unit  12  causes the power storage unit switch unit  273  to perform the third operation (see  FIG. 13 ). Then, the control unit  12  opens all the switch pairs  272 A of the switch units  272 . In this manner, only the charge stored in the power storage units  271  is supplied to the converters  11 A via the first circuit unit  30 . By the control unit  12  performing the second control, the converters  11 A perform the operation for stepping up or down the input voltage based on the power supplied from the power storage elements  71 A to supply the power to the third conductive paths  131 A. 
     Next, the effects of this configuration will be illustrated. 
     The discharge circuit  11  of the in-vehicle backup power supply device  5  of the present disclosure includes the converters  11 A for stepping up or down the voltage that is input and outputting the resultant voltage. When performing the first control, the control unit  12  operates the converters  11 A to step up or down the input voltage based on the power supplied from the power storage elements  71 A and supply the power to the battery unit  110 . When performing the second control, the control unit  12  operates the converters  11 A to step up or down the input voltage based on the power supplied from the power storage elements  71 A and supply the power to the third conductive paths  131 A. 
     With this configuration, when the balance circuits  470  performing the second discharge operation, the power of the desired magnitude can be supplied to the third conductive paths  131 A based on the voltage of the power storage units  271 . Specifically, when stepping up the voltage of the power storage units  271  via the converter  11 A, the power stored in the power storage units  271  can be effectively used. Then, when performing the cell balancing operation, due to the power supplied from the power storage units  271  being stepped up by the converters  11 A, it is possible to suppress a decrease of the current flowing between the power storage units  271  and the unit batteries  10 A when the cell balancing operation has progressed to some extent. Accordingly, the current can be caused to flow between the power storage units  271  and the unit batteries  10 A positively. In this manner, the time required for the balancing operation can be further shortened. 
     In the in-vehicle backup power supply device  5  according to the present disclosure, each balance circuit  470  includes the power storage unit  271  formed by one or more power storage elements  71 A and the switch unit  272  provided with the plurality of switch pairs  272 A. The switch pairs  272 A respectively correspond to the unit batteries  10 A. The high potential electrode and the low potential electrode of the unit battery  10 A to which each switch pair  272 A corresponds are electrically connected to the converters  11 A via the first circuit unit  130 . The power storage unit switch units  273  for collectively switching the electrical connection of the two end element electrode portions  71 C of the power storage unit  271  to either the first circuit unit  130  or the third conductive path  131 A is provided. When performing the first control, the control unit  12  operates the power storage unit switch units  273  such that the two end element electrode portions  71 C are collectively and electrically connected to the third conductive paths  131 A. Also, with respect to one unit battery  10 A, the control unit  12  electrically connects the one end element electrode portion  71 C of the power storage unit  271  to the high potential electrode via the converter  11 A. Also, the control unit  12  operates the plurality of switch pairs  272 A to perform the operation for electrically connecting the other end element electrode portion  71 C of the power storage unit  271  to the low potential electrode via the converter  11 A, alternately on each of the plurality of unit batteries  10 A. Then, the control unit  12  operates the converter  11 A to supply the power to whichever of the unit batteries  10 A or the power storage units  271  has the lower voltage thereacross. When performing the second control, the control unit  12  operates each power storage unit switch unit  273  such that the two end element electrode portions  71 C are collectively and electrically connected to the first circuit unit  130 . In addition, the control unit  12  operates the plurality of switch pairs  272 A such that the high and low potential electrodes of the unit batteries  10 A and the first circuit unit  130  are not connected to each other. 
     With this configuration, since the power storage unit switch units  273  collectively switches electrical connection of the two end element electrode portions  71 C to the first circuit unit  130  or the third conductive path  131 A, it is possible to suppress a case in which the power storage unit  271  is connected to both the first circuit unit  130  and the third conductive path  131 A. In this manner, it is possible to suppress a case in which a malfunction occurs in the converters  11 A. 
     Other Embodiment 
     This configuration is not limited to the embodiment described using the above description and the drawings, and for example, the following embodiments are also encompassed within the technical scope of the present invention. 
     Although the first and second embodiments illustrated the configuration in which one power storage element  71 A corresponds to one unit battery  10 A, it is also possible that a plurality of power storage elements connected in series or parallel correspond to one unit battery. 
     Although the control unit  12  is mainly constituted by a microcomputer in the first embodiment, the control unit  12  may also be realized by a plurality of hardware circuits other than a microcomputer. 
     Although the number of unit batteries  10 A in each of the unit battery groups  110 A and  110 B of the battery unit  110  is three in the second to fifth embodiments, the number of unit batteries may also be two or four or more. Also, the numbers of the unit batteries of the unit battery groups need not necessarily be the same. 
     The embodiments disclosed herein should be construed to be exemplary in all aspects, and not be restrictive. The present invention is not limited to the embodiments disclosed herein, but defined in the claims, and intended to include all modifications within the meaning and the scope equivalent thereof.