Abstract:
A power supply apparatus is provided that has a plurality of secondary batteries connected in series, outputs a composite voltage of all of the secondary batteries, and outputs an output of a part of the secondary batteries as a partial voltage. The power supply apparatus includes a detecting unit that detects states of the secondary batteries, a changing unit that changes an order of series connection of the plurality of secondary batteries based on a detection result of the detecting unit in such a manner that the partial voltage is outputted from a secondary battery that is in a relatively good state.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of International Patent Application No. PCT/JP2012/065898 filed Jun. 21, 2012, which claims the benefit of Japanese Patent Application No. 2011-145149, filed Jun. 30, 2011, the full contents of all of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to a power supply apparatus and a power supply method. 
         [0004]    2. Background Art 
         [0005]    There are many large-sized vehicles such as trucks or buses that have two power supplies a 24V-power supply that mainly drives a starter motor and a 12V-power supply that drives in-vehicle accessories. As a method of supplying power from the two power supplies, it is common to connect 12V-secondary batteries in series, and to obtain 24V as their total voltage and obtain 12V from one of the secondary batteries. 
         [0006]    As has been described above, when the two secondary batteries are connected in series in the aforementioned manner, there may be a case in which the discharging of a secondary battery on a 12V-side progresses faster as compared to the other secondary battery, and a difference in the charging rate may occur between the two secondary batteries. In such a case, the life of one of the secondary batteries may shorten, or as the discharging progresses, it may not be able to drive the starter motor. 
         [0007]    Accordingly, in order to prevent such a situation, in the related art, there are a technique of connecting a resistance element to a secondary battery on a high-voltage side and discharging electricity to balance them (Japanese Laid-Open Patent Publication No. 2007-267454), a technique of transferring an electric power from a secondary battery of a high voltage to a secondary battery of a low voltage using a convertor or a transformer (Japanese Laid-Open Patent Publication Nos. 2010-93980 and 2000-60019), and a technique of equalizing a charged state by bypassing a secondary battery that has come to a full-charge state during the charging (Japanese Laid-Open Patent Publication No. 1998-14002). 
         [0008]    According to the technique described in Japanese Laid-Open Patent Publication No. 2007-267454, since an electric power is converted into heat by a resistor, there is a disadvantage that efficiency is low due to an occurrence of a loss of an electric power. According to the technique described in Japanese Laid-Open Patent Publication No. 2010-93980, since an electric power is transferred between the secondary batteries, there is a disadvantage that time is required for the transfer. According to the technique described in Japanese Laid-Open Patent Publication No. 2000-60019, since an electric power is transferred using a transformer, there is a disadvantage that the transfer is not appropriately performed due to a winding ratio of the coil and that time is required for the transfer. Further, according to the technique described in Japanese Laid-Open Patent Publication No. 1998-14002, since it is a technique of managing the charging, there is a disadvantage that the discharging cannot be managed. 
       SUMMARY 
       [0009]    Accordingly, it is an object of the present disclosure to provide a power supply apparatus and a power supply method that can perform charge/discharge management of a plurality of secondary batteries efficiently in a short time. 
         [0010]    In order to achieve the above object, a power supply apparatus that has a plurality of secondary batteries connected in series, outputs a composite voltage of all of the secondary batteries, and outputs an output of a part of the secondary batteries as a partial voltage is provided that includes a detecting unit that detects states of the secondary batteries, and a changing unit that changes an order of series connection of the plurality of secondary batteries based on a detection result of the detecting unit in such a manner that the partial voltage is outputted from a secondary battery that is in a relatively good state. 
         [0011]    With such a structure, discharge/charge management of a plurality of secondary batteries can be performed efficiently in a short time. Further, by changing an order of series connection of the plurality of secondary batteries in such a manner that the partial voltage is outputted from a secondary battery that is in a relatively good state, the load can be prevented from being concentrated on one of the secondary batteries, and thus the life of the battery can be lengthened. 
         [0012]    According to another configuration, in addition to the aforementioned configuration, the power supply apparatus has two secondary batteries, the power supply apparatus outputs a composite voltage of terminal voltages of the two secondary batteries and a terminal voltage of a secondary battery connected to a ground side as a partial voltage, the detecting unit detects states of the two secondary batteries, and the changing unit changes an order of the series connection in such a manner that a secondary battery that is in a relatively good state, which has been detected by the detecting unit, is connected to a ground side. 
         [0013]    According to such a configuration, charge/discharge management of two secondary batteries can be performed efficiently in a short time. 
         [0014]    According to another configuration, in addition to the aforementioned configuration, the changing unit has four switches each having a single common terminal, a first selective terminal and a second selective terminal, a positive pole and a negative pole of each of the two secondary batteries are connected to the respective common terminals of the four switches, respective first selective terminals of the four switches are connected with one another, the respective first selective terminals being a terminal that outputs the partial voltage, respective second selective terminals of two switches connected to the positive pole through the respective common terminals are connected with each other, the respective second selective terminals being a terminal that outputs the composite voltage, respective second selective terminals of the two switches connected to the negative pole through the respective common terminals are connected with each other, the respective second selective terminals being a terminal that is connected to a ground, and the order of series connection is changed by controlling a connecting state of the four switches. 
         [0015]    According to such configuration, charge/discharge management of two secondary batteries can be performed efficiently with a simple configuration. 
         [0016]    According to another configuration, in addition to the aforementioned configuration, the detecting unit is connected between a terminal connected to the ground and the ground. 
         [0017]    With such a configuration, by positively detecting a current flowing to the two secondary batteries, the states of the secondary batteries can be detected accurately. 
         [0018]    According to another configuration, in addition to the aforementioned configuration, a discharge unit that makes the secondary battery discharge when detecting the states of the secondary batteries is connected between the terminal that outputs the partial voltage and the ground. 
         [0019]    With such a configuration, by causing the secondary battery to discharge and by detecting the voltage and current during the discharge, the states of the secondary batteries can be detected accurately. 
         [0020]    According to another configuration, in addition to the aforementioned configuration, the two secondary batteries are connected in parallel by the four switches and the partial voltage is supplied to a load. 
         [0021]    With such a configuration, when outputting a partial voltage, an unbalance in the states can be prevented by connecting the secondary batteries in parallel. 
         [0022]    According to another configuration, in addition to the aforementioned configuration, in detecting the states of the secondary battery by the detecting unit, the state of one of the secondary batteries is detected by the detecting unit and a terminal voltage of the other secondary battery is supplied to the load as a partial voltage. 
         [0023]    With such configuration, since the states of the secondary batteries can be detected while an electric current is not flowing to the load, the states of the secondary batteries can be detected even more accurately. 
         [0024]    According to another configuration, in addition to the aforementioned configuration, the power supply apparatus has three secondary batteries, the power supply apparatus outputs a composite voltage of terminal voltages of the three secondary batteries and a terminal voltage of one of the secondary batteries connected to a ground side as a partial voltage, the detecting unit detects the states of the three secondary batteries, and the changing unit changes an order of series connection in such a manner that a secondary battery having a relatively good state, which has been detected by the detecting unit, is connected to a ground side. 
         [0025]    According to such a configuration, charge/discharge management of the three secondary batteries can be performed efficiently in a short time. 
         [0026]    According to another configuration, in addition to the aforementioned configuration, the power supply apparatus has four secondary batteries, the power supply apparatus outputs a composite voltage of terminal voltages of the four secondary batteries and outputs a terminal voltage of one or two of the secondary batteries, which is connected to the ground side, as a partial voltage, the detecting unit detects the states of the four secondary batteries, and the changing unit changes an order of the series connection in such a manner that a secondary battery having a relatively good state, which has been detected by the detecting unit, is connected to a ground side. 
         [0027]    With such a configuration, charge/discharge management of the four secondary batteries can be performed efficiently in a short time. 
         [0028]    According to another configuration, in addition to the aforementioned configuration, in the power supply apparatus, a single detecting unit that detects the states of the secondary batteries is provided. 
         [0029]    With such a configuration, the states of a plurality of secondary batteries can be detected with a single state detecting unit by changing an order of series connection of the plurality of secondary batteries. Thereby, since a detecting unit need not be arranged for each individual secondary battery, the cost of the apparatus as a whole can be reduced. 
         [0030]    According to another configuration, in addition to the aforementioned configuration, a power supplying method that has a plurality of secondary batteries connected in series, outputs a composite voltage of all of the secondary batteries, and outputs an output of a part of the secondary batteries as a partial voltage is provided that includes detecting states of the secondary batteries, and changing an order of series connection of the plurality of secondary batteries in such a manner that the partial voltage is outputted from a secondary battery in a relatively good state, based on a detection result of the detecting unit. 
         [0031]    With such a method, charge/discharge management of a plurality of secondary batteries can be performed efficiently in a short time. Also, by changing an order of series connection of the plurality of secondary batteries in such a manner that the partial voltage is outputted from a secondary battery in a relatively good state, the load can be prevented from being concentrated on a single secondary battery, and thus the life of the battery can be lengthened. 
         [0032]    Further, according to another configuration, in addition to the aforementioned configuration, in the power supplying method, a changing switch is switched only when a current detected in the detecting is less than or equal to a predetermined current. 
         [0033]    With such a method, it is possible to prevent deterioration of contacts of a switch due to the switching during a large current flow having a current value greater than or equal to a predetermined current value. 
         [0034]    According to the present disclosure, a power supply apparatus and a power supply method that can perform a discharge and charge management of a plurality of secondary batteries efficiently in a short time can be provided. 
         [0035]    Further, according to the present disclosure, by changing an order of series connection of the plurality of secondary batteries in such a manner that the partial voltage is outputted from a secondary battery that is in a relatively good state, the load can be prevented from being concentrated on one of the secondary batteries, and thus the life of the battery can be lengthened. 
         [0036]    Further, the states of a plurality of secondary batteries can be detected by a single state detecting unit by changing the order of series connection of the plurality of secondary batteries. Thereby, since a detecting unit need not be provided for each individual secondary battery, the cost of the entire apparatus can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0037]      FIG. 1  is a diagram showing an exemplary configuration of a power supply apparatus according to an embodiment of the present disclosure. 
           [0038]      FIG. 2  is a flow chart for explaining a process flow executed in the embodiment shown in  FIG. 1 . 
           [0039]      FIGS. 3A and 3B  are diagrams showing a change in a connected state in the embodiment shown in  FIG. 1 . 
           [0040]      FIGS. 4A and 4B  are diagrams showing a change in a connected state at the time of measurement in the embodiment shown in  FIG. 1 . 
           [0041]      FIG. 5  is a flow chart for explaining a process flow executed in an embodiment shown in  FIG. 1 . 
           [0042]      FIG. 6  is a diagram showing a connected state when the flow chart shown in  FIG. 5  is executed. 
           [0043]      FIG. 7  is a diagram for explaining another embodiment of the present disclosure. 
           [0044]      FIG. 8  is a diagram for explaining yet another embodiment of the present disclosure. 
           [0045]      FIG. 9  is a diagram for explaining still another embodiment of the present disclosure. 
           [0046]      FIG. 10  is a diagram for explaining still another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    Further features of the present disclosure will become apparent from the following detailed description of exemplary embodiments with reference to the accompanying drawings. 
       (A) Description of a Configuration of the Embodiment 
       [0048]      FIG. 1  is a diagram illustrating an exemplary configuration of a power supply apparatus according to an embodiment of the present disclosure. As shown in  FIG. 1 , a power supply apparatus  10  includes secondary batteries  11  and  12 , switches  13  to  16 , a state detecting unit  17  and a control unit  18  as its main constituent elements, and a discharging circuit  19 , a 12V-load  20 , a 24V-load  21 , a starter motor  22  and an alternator  23  are connected externally. 
         [0049]    Each of the secondary batteries  11  and  12  is constituted by, for example, a secondary battery such as a lead-acid battery, a nickel-cadmium battery, a nickel metal hydride battery, a lithium-ion battery, and, for example, generates and outputs a direct current electric power of 12V. 
         [0050]    The switches  13  to  16  are constituted by, for example, electromagnetic relays. The switch  13  has a common terminal connected to a positive pole of the secondary battery  11 , a selective terminal connected to a connection point A, and another selective terminal connected to a connection point B. The switch  13  is controlled by the control unit  18  and connects the positive pole of the secondary battery  11  either to the connection point A or to the connection point B. The switch  14  has a common terminal connected to a negative pole of the secondary battery  11 , a selective terminal connected to the connection point B, and another selective terminal connected to a connection point C. The switch  14  is controlled by the control unit  18  and connects the negative pole of the secondary battery  11  either to the connection point B or to the connection point C. 
         [0051]    The switch  15  has a common terminal connected to a positive pole of the secondary battery  12 , a selective terminal connected to the connection point A, and another selective terminal connected to the connection point B. The switch  15  is controlled by the control unit  18  and connects the positive pole of the secondary battery  12  either to the connection point A or to the connection point B. The switch  16  has a common terminal connected to a negative pole of the secondary battery  12 , a selective terminal connected to the connection point B, and another selective terminal connected to the connection point C. The switch  16  is controlled by the control unit  18  and connects the negative pole of the secondary battery  12  either to the connection point B or to the connection point C. 
         [0052]    The state detecting unit  17  detects states of the secondary batteries  11  and  12  and notifies the control unit  18 . More specifically, the state detecting unit  17  detects a current, a voltage and a temperature of the secondary batteries  11  and  12 , and notifies the control unit  18 . 
         [0053]    The control unit  18  is constituted by, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), or the like, and the switches  13  to  16  and the discharging circuit  19 , etc., are controlled based on the states of the secondary batteries  11  and  12  supplied from the state detecting unit  17 . Note that in  FIG. 1 , each broken line connected from the control unit  18  to each part indicates a control line. 
         [0054]    In response to the control of the control unit  18 , the discharging circuit  19  periodically discharges electricity from the secondary battery connected to the connection point B. By detecting a current, a voltage, and an internal resistance at this instant, a charging rate SOC (State of Charge) or a degradation state SOH (State of Health) are measured. 
         [0055]    The 12V-load  20  is, for example, a car audio equipment, a car navigation device, a horn, etc., that is operated by a voltage of 12V applied to the connection point B. The 24V-load  21  is, for example, an ABS (Anti Brake System), a TCU (Transmission Control Unit), etc., that operates with a voltage of 24V applied to the connection point A. 
         [0056]    The starter motor  22  is constituted by a direct-current motor and starts up an engine, not shown, by supplying a direct current power of 24V. The alternator  23  is rotationally driven by an engine, not shown, and produces a direct current power of 24V and charges the secondary batteries  11  and  12 . 
       (B) Description of Operation according to the Embodiment 
       [0057]    An operation of the present embodiment will now be described.  FIG. 2  is a flow chart for explaining a process flow executed in the present embodiment. When the process shown in  FIG. 2  is started, the following steps are performed. 
         [0058]    In step S 10 , the control unit  18  determines whether or not an ignition key, not shown, has been operated to bring an ignition switch into an ignition ON state. If it is determined to be in an ignition ON state (step S 10 : Yes), the process proceeds to step S 11 , and if not (step S 10 : No), the process is terminated. Specifically, in a case where a driver has brought an ignition switch into an ignition ON state to start up the engine of the vehicle, the process proceeds to step S 11 . 
         [0059]    In step S 11 , the control unit  18  controls the switches  13  to  16  to select one of the secondary batteries  11  and  12  as an object to be measured.  FIG. 3A  shows a state in which, by the switches  13  to  16 , the secondary battery  12  has been selected as an object to be measured, and  FIG. 3B  shows a state in which the secondary battery  11  has been selected as an object to be measured. In the state shown in  FIG. 3A , since each of the switches  13  and  14  has selected a selective terminal on an upper side in the diagram and each of the switches  15  and  16  has selected a selective terminal on a lower side in the diagram, the positive pole of the secondary battery  12  is connected to the discharging circuit  19  and the negative pole is grounded via the state detecting unit  17 . In the state shown  FIG. 3B , since each of the switches  13  and  14  has selected a selective terminal on a lower side in the diagram and each of the switches  15  and  16  has selected a selective terminal on an upper side in the diagram, the positive pole of the secondary battery  11  is connected to the discharging circuit  19  and the negative pole is grounded via the state detecting unit  17 . 
         [0060]    In step S 12 , the control unit  18  measures a charging rate SOC 1  and a state of degradation SOH 1  of the secondary battery selected in step S 11 . Specifically, the control unit  18  controls the discharging circuit  19  to obtain a current value and a voltage value from the state detecting unit  17  for a case where the selected secondary battery is discharged with a constant current, and retrieves an SOC corresponding to the obtained current value and the obtained voltage value from, for example, a table, and defines the retrieved SOC as SOC 1 . At this time, correction by temperature obtained from the state detecting unit  17  and correction by degradation based on SOH 1  described below are performed. Further, the control unit  18  also controls the discharging circuit  19 , causes the selected secondary battery to be discharged at a predetermined frequency, obtains an internal resistance from changes in voltage and current at that time and obtains SOH 1  from the obtained internal resistance. The frequency of the discharge used for measurement can be, for example, chosen appropriately in the range of several tens of Hz to several kHz.  FIG. 4A  shows a state in which the secondary battery  12  is measured and  FIG. 4B  shows a state in which the secondary battery  11  is measured. In  FIG. 4A , the positive pole of the secondary battery  12  is connected to the discharging circuit  19  and the negative pole is grounded via the state detecting unit  17 . In  FIG. 4B , the positive pole of the secondary battery  11  is connected to the discharging circuit  19 , and the negative pole is grounded via the state detecting unit  17 . In such a state, SOC 1 , SOH 1 , SOC 2  and SOH 2  of the secondary batteries  11  and  12  are measured. 
         [0061]    In step S 13 , based on information from the state detecting unit  17 , the control unit  18  determines whether or not the current flowing from the secondary battery selected in step S 11  to the ground has a current value lower than a predetermined current value. If it is lower than the predetermined current value, the process proceeds to step S 14  (step S 13 : Yes) and if not, a similar process is repeated (step S 13 : No). 
         [0062]    In step S 14 , the control unit  18  controls the switches  13  to  16 , and selects the other one of the secondary batteries  11  and  12 , which was not selected in step S 11 , as an object to be measured. For example, in a case where the secondary battery  11  has been selected in step S 11  as an object to be measured (in the case of  FIG. 3B ), the secondary battery  12  is selected in step S 14  as an object to be measured (brought to  FIG. 3A ). The connections are switched over after having determined whether or not it is less than the predetermined current value in step S 13 . One of the reasons for this is to prevent the apparatus from being overloaded by a high voltage which may be generated by self-induction due to the switching, which may be caused when the switches  13  to  16  are switched over while a big current having a current value greater than or equal to a predetermined current value is flowing and in a case where a 12V-load  20  or a 24V-load  21  includes an inductive load. Another reason is to prevent the degradation of the contacts of the switches  13  to  16  caused by the switching while a relatively large current is flowing. The predetermined current value can be appropriately set by an allowable current value or the like of the switches. 
         [0063]    In step S 15 , the control unit  18  measures SOC 2  and SOH 2  of the other secondary battery selected in step S 14 . The measuring process is similar to that of step S 12 . 
         [0064]    In step S 16 , SOC 1  of one of the secondary batteries measured in step S 12  is compared with SOC 2  of the other of the secondary batteries measured in step S 15 . If SOC 1 &gt;SOC 2  (step S 16 : Yes), the process proceeds to step S 17 , and if not (step S 16 : No), the process proceeds to step S 18 . For example, in a case where SOC 1 &gt;SOC 2  is satisfied, where SOC 1  is the charging rate of the secondary battery  11  and SOC 2  is a charging rate of the secondary battery  12 , the process proceeds to step S 17 . 
         [0065]    In step S 17 , the control unit  18  controls the switches  13  to  16 , and sets one of the secondary batteries to a low-voltage side. More specifically, in a case where the secondary battery  12  is selected as one of the secondary batteries, SOC 1  represents a measured value for the secondary battery  12  and SOC 2  represents a measured value for the secondary battery  11 . When SOC 1 &gt;SOC 2  is satisfied, i.e., when the secondary battery  12  has a higher charging rate than that of the secondary battery  11 , the secondary battery  12  is set to a low-voltage side, and comes to a connected state shown in  FIG. 3A . As a result, the 24V-load  21  and the starter motor  22  are supplied with an electric power from both of the secondary batteries  11  and  12 , and the 12V-load  20  is supplied with an electric power from the secondary battery  12  having a higher charging rate. 
         [0066]    In step S 18 , the control unit  18  controls the switches  13  to  16 , and the other secondary battery is set at the low-voltage side. More specifically, when the secondary battery  12  is selected as one of the secondary batteries, SOC 1  becomes a measured value of the secondary battery  12  and SOC 2  becomes a measured value of the secondary battery  11 . In a case where SOC 1 &lt;SOC 2  is satisfied, i.e., when the secondary battery  11  has a charging rate higher than that of the secondary battery  12 , the secondary battery  11  is set at the low-voltage side, and comes to a connected state shown in  FIG. 3B . As a result, the 24V-load  21  is supplied with an electric power from both the secondary batteries  11  and  12 , and the 12V-load  20  is supplied with an electric power from the secondary battery  11  having a higher charging rate. 
         [0067]    In the case of SOC 1 =SOC 2 , for example, it is possible to select one of them at random or compare SOH 1  with SOH 2 , and to set the secondary battery in which the degradation state has not progressed (one with a greater SOH value) to the low-voltage side. 
         [0068]    In step S 19 , the control unit  18  determines whether or not SOH 1  of one of the secondary batteries is less than a predetermined threshold Th. If SOH 1  is less than the threshold Th (step S 19 : Yes), the process proceeds to step S 20 , and if not (step S 19 : No), the process proceeds to step S 21 . Specifically, if SOH 1  is less than a predetermined threshold Th as compared to SOH of a new secondary battery, the process proceeds to step S 20 , and if not, the process proceeds to step S 21 . The predetermined threshold can be appropriately set depending on a property or the like of the secondary battery to be used, and when a lead battery is used, it can be set in the range of, for example, 30 to 60%. 
         [0069]    In step S 20 , the control unit  18  presents a message for encouraging the replacement of one of the secondary batteries. Specifically, in step S 19 , when it is determined that SOH 1  of one of the secondary batteries is less than 50% of the new product, for example, a message for encouraging the replacement of one of the secondary batteries is presented on a display unit, not shown. 
         [0070]    In step S 21 , the control unit  18  determines whether or not SOH 2  of the other secondary battery is less than the predetermined threshold Th. If it is less than the threshold Th, the process proceeds to step S 22  (step S 21 : Yes), and if not (step S 21 : No), the process terminates. Details of this process are similar to the case of step S 19 . 
         [0071]    In step S 22 , the control unit  18  presents a message for encouraging the replacement of the other secondary battery. Details of this process are similar to the case of step S 20 . 
         [0072]    According to the aforementioned process, when a driver has operated an ignition key to start up the engine of the vehicle and an ignition switch is brought into an ignition ON state, the secondary batteries  11  and  12  are sequentially selected and the respective SOC and SOH are measured. Then, since the secondary battery having a greater SOC is selected and connected to the low-voltage side, the 12V-load  20  is supplied with an electric power from the secondary battery having a greater SOC. Thereby, an unbalance between the charging rates of the secondary batteries is corrected. When the SOHs of the secondary batteries  11  and  12  have become less than the predetermined threshold Th, a message for encouraging the replacement is presented. 
         [0073]    Referring now to  FIG. 5 , a case in which the secondary batteries  11  and  12  are connected in parallel and an electric power is supplied to the 12V-load  20  will be described. When a flow chart shown in  FIG. 5  is started, the following steps are performed. 
         [0074]    In step S 30 , the control unit  18  determines whether or not an ignition key, not shown, has been operated and an ignition switch has come to an ignition ON state. If is determined to be in an ignition ON state (step S 30 : Yes), the process proceeds to step S 32 , and if not (step S 30 : No), the process proceeds to step S 31 . 
         [0075]    In step S 31 , the control unit  18  determines whether or not an ignition key, not shown, has been operated and it has come to an ACC (Accessory) ON state. If it is determined to be in an ON state (step S 31 : Yes), the process proceeds to step S 32 , and if not (step S 31 : No), the process terminates. 
         [0076]    In step S 32 , the control unit  18  determines whether or not the engine is stopped. If the engine is stopped (step S 32 : Yes), the process proceeds to step S 33 , and if not (step S 32 : No), the process is terminated. For example, when it is brought to an ignition ON state or an ACC ON state while the engine is being stopped (when power is supplied to the 12V-load  20 ), the process proceeds to step S 33 , and if not (e.g., when the engine is started up), the process is terminated. 
         [0077]    In step S 33 , the control unit  18  controls the switches  13  to  16 , and as shown in  FIG. 6 , the secondary batteries  11  and  12  are brought to a state where they are connected in parallel. That is, in the example of  FIG. 6 , all of the switches  13  to  16  are in a state where they are connected to the lower selective terminals and the positive poles of the secondary batteries  11  and  12  are both connected to the connection point B, and, the negative poles are both connected to the connection point C. 
         [0078]    According to the aforementioned process, since an electric power is supplied to the 12V-load  20  with the secondary batteries  11  and  12  being connected in parallel, in a state where the engine is stopped and the charging is not performed, it is possible to prevent a decrease in a charging rate of one of the secondary batteries only. 
         [0079]    As has been described above, in the present embodiment, an order of series connection of the secondary batteries  11  and  12  is changed by the switches  13  to  16  depending on the state of the secondary batteries  11  and  12 . Therefore, an unbalance between the charging rates can be corrected by setting the secondary battery having a high charging rate to the low-voltage side and supplying an electric power to the 12V-load  20 . In this manner, since concentration of a load on a single secondary battery can be avoided, the life of the battery can be lengthened. 
         [0080]    Further, in the present embodiment, since SOC and SOH are measured while selecting the secondary batteries  11  and  12  one at a time using the switches  13  to  16 , the measurement can be performed accurately. In this manner, since the states of the two secondary batteries  11  and  12  can be detected with a single state detecting unit  17  and thus it is not necessary to provide a state detecting unit for each of the secondary batteries, a cost reduction for the overall apparatus can be achieved. 
         [0081]    Further, in the present embodiment, since the connections are changed by the switches, an unbalance between the charging rates of the secondary batteries can be corrected while reducing a power loss. 
         [0082]    Further, in the present embodiment, SOH of the secondary batteries  11  and  12  are measured and when they are less than the predetermined threshold Th, a message for encouraging the replacement is presented. Accordingly, it is possible to know the time for replacement of the secondary batteries  11  and  12 . 
         [0083]    Further, in the present embodiment, in a state where the engine is stopped and the alternator  23  is not in operation, when it is brought to an ignition ON state or an ACC ON state, the secondary batteries  11  and  12  are brought into parallel connection and the 12V-load  20  is supplied with an electric power. Accordingly, it is possible to prevent only one of the secondary batteries from being discharged which may lead to an occurrence in an unbalance in the charging rate. 
       (C) Variant Embodiments 
       [0084]    Each of the above embodiments is described by way of example and various variant embodiments other than the embodiments described above exist. For example, in the aforementioned embodiment, an example in which the secondary batteries  11  and  12  have a terminal voltage of 12V was described by way of example, but may also be of other voltages. 
         [0085]    In the flow chart shown in  FIG. 2 , the state of the secondary batteries  11  and  12  are determined based on the charging rates SOC 1  and SOC 2 , but the state of the secondary batteries  11  and  12  may also be determined based on the degradation states SOH 1  and SOH 2 . Specifically, SOH 1  and SOH 2  may be compared and the secondary battery having a greater value may be connected to the low-voltage side. Alternatively, it is possible to perform the determination based on both of SOC 1 , SOC 2  and SOH 1 , SOH 2 . Specifically, it can be determined based on magnitudes of products SOC 1 ×SOH 1  and SOC 2 ×SOH 2 . 
         [0086]    In the aforementioned embodiments, the charging rate SOC and the degradation state SOH are detected based on the changes in current and voltage at the time of discharge by the discharging circuit  19 . However, the charging rate SOC and the degradation state SOH may be detected in accordance with a method other than this. 
         [0087]    In the aforementioned embodiment, the flow chart shown in  FIG. 2  is performed in such a manner that the process from step S 11  onwards are performed in a case where the ignition key is brought to an ignition ON state, but the process from step S 11  onwards may also be performed in a case where the engine is stopped. Alternatively, instead of immediately after the stoppage of the engine, the processes from step S 11  onwards may be performed after a predetermined period of time has passed since the engine has been stopped and the secondary batteries  11  and  12  have come to a stable state. Also, SOC of the secondary battery on the ground side may be detected while running or idling, and in a case where the SOC has become a predetermined threshold or less, the order of series connection of the two secondary batteries may be changed at a predetermined timing (e.g., after the engine has stopped or when the discharging and charging current has come to a predetermined threshold or below). Further, the timing of changing the order of series connection is not limited to after the stoppage of the engine, and the order may also be changed in a case where the charge/discharge current has become a predetermined threshold or below during the running or idling. Further, SOC or SOH that has been measured during the running, during the stoppage or during the idling may be presented to the user by being displayed on a display unit disposed on a front panel, etc. 
         [0088]    Further, in the aforementioned embodiment, an electromagnetic relay is used as switch, but, for example, a semiconductor switch such as an FET (Field Effect Transistor) or an IGBT (Isolated Gate Bipolar Transistor) may also be used. 
         [0089]    Further, in the aforementioned embodiment, the discharging circuit  19  and the 12V-load  20  are connected in parallel with the secondary battery to be measured. However, the discharging circuit  19  and the 12V-load  20  may be connected to another secondary battery.  FIG. 7  is a diagram showing an embodiment in which the discharging circuit  19  and the 12V-load  20  are connected to another secondary battery. In the example of  FIG. 7 , as compared to the case of  FIG. 1 , the switches  14  and  16  are replaced with switches  34  and  36 , and switches  37  and  38  are newly added. Here, the switch  34  has a single common terminal and three selective terminals. The common terminal is connected to the negative pole of the secondary battery  11 , an upper selective terminal is connected to the connection point B, a middle selective terminal is connected to the connection point C, and a lower selective terminal is grounded. Similarly, the switch  36  also has a single common terminal and three selective terminals. The common terminal is connected to the negative pole of the secondary battery  12 , an upper selective terminal is connected to the connection point B, a middle selective terminal is connected to the connection point C, and a lower selective terminal is grounded. The switch  37  has two selective terminals and a single common terminal. The switch  37  selects one of the selective terminals of either the switch  13  or  15  and connects it to the 24V-load  21 , the starter motor  22  and the alternator  23 . The switch  38  has two selective terminals and a single common terminal. The selective terminals are connected to the selective terminals of the switches  13  and  15 , respectively, and the common terminal is connected to the discharging circuit  19 . 
         [0090]    An operation of the embodiment of  FIG. 7  will now be described. In the state shown in  FIG. 7 , the switch  13  has selected a lower selective terminal, the switch  15  has selected an upper selective terminal, the switch  34  has selected a lower selective terminal, the switch  36  has selected a middle selective terminal, and the switches  37  and  38  have selected left selective terminals. In this case, the positive pole of the secondary battery  12  is connected to the discharging circuit  19  via the switches  15  and  38 , and the negative pole is grounded via the state detecting unit  17 . On the other hand, the positive pole of secondary battery  11  is connected to the 12V-load  20  via the switch  13 , and the negative pole is grounded via the switch  34 . 
         [0091]    When the secondary battery  11  is selected as an object to be measured, it is to be brought to a state where the switch  13  has selected an upper selective terminal, the switch  15  has selected a lower selective terminal, the switch  34  has selected a middle selective terminal, the switch  36  selects a lower selective terminal, and the switches  37  and  38  have selected right selective terminals. 
         [0092]    When the measurement is terminated, and, for example, when the secondary battery  11  is set to the low-voltage side, it is to be brought to a state where the switch  13  has selected a lower selective terminal, the switch  15  has selected an upper selective terminal, the switch  34  has selected a lower selective terminal, and switch  36  has selected an upper selective terminal, and the switches  37  and  38  have selected right selective terminals. On the other hand, in a case where the secondary battery  12  is set to the low-voltage side, it is to be brought to a state where the switch  13  has selected the upper selective terminal, the switch  15  has selected the lower selective terminal, the switch  34  has selected the upper a selective terminal, the switch  36  has selected the lower selective terminal, and the switches  37  and  38  have selected the left selective terminal. 
         [0093]    When it is brought to a state where the switches  13  and  15  have selected the lower selective terminals and the switches  34  and  36  have selected the lower selective terminals, the secondary batteries  11  and  12  may be connected in parallel and an electric power may be supplied to 12V-load  20  as shown in  FIG. 6 . 
         [0094]    As has been described above, in the embodiment shown in  FIG. 7 , since it is possible to connect only one of the secondary batteries to the discharging circuit  19 , an influence of the 12V-load  20  is excluded and measurement can be performed accurately. 
         [0095]      FIG. 8  is a diagram showing still another embodiment. In the example of this  FIG. 8 , as compared to the case of  FIG. 1 , switches  37 ,  38 ,  40  and  41  are newly added. Other configuration is similar to the case of  FIG. 1 . The switch  37  has two selective terminals and a single common terminal, selects one of the selective terminals of either the switch  13  or  15 , and connects to the 24V-load  21 , the starter motor  22  and the alternator  23  that are connected to the common terminal. The switch  38  has two selective terminals and a single common terminal. The selective terminals are connected to the selective terminals of the switches  13  and  15 , respectively, and the common terminal is connected to the discharging circuit  19 . The switch  40  has two selective terminals and a single common terminal. The selective terminals are connected to the lower selective terminal of the switches  14  and  16 , respectively, and the common terminal is connected to the state detecting unit  17 . The switch  41  has two selective terminals and a single common terminal. The selective terminals are connected to the lower selective terminals of the switches  14  and  16 , respectively, and the common terminal is grounded. 
         [0096]    In the example shown in  FIG. 8 , the switches  13  and  14  have selected the lower selective terminals, the switch  15  has selected the upper selective terminal, the switch  16  has selected the lower selective terminal, the switches  37  and  38  have selected the left selective terminals, and the switches  40  and  41  have selected the lower selective terminal. In such a connected state, the secondary battery  11  is connected to the 12V-load  20 . Also, since the positive pole of the secondary battery  12  is connected to the discharging circuit  19  and the negative pole is connected to the state detecting unit  17 , the secondary battery  12  can be measured independently. 
         [0097]    On the other hand, in a state where the switch  13  has selected the upper selective terminal, the switch  14  has selected a lower selective terminal, the switches  15  and  16  have selected a lower selective terminal, the switches  37  and  38  have selected the right selective terminal, and the switches  40  and  41  have selected the upper selective terminal, the secondary battery  12  is connected to the 12V-load  20 . Since the positive pole of the secondary battery  11  is connected to the discharging circuit  19  and the negative pole is connected to the state detecting unit  17 , the secondary battery  11  can be measured independently. 
         [0098]    When the measurement is terminated and the secondary battery  11  is to be set to the low-voltage side, the switches  13  and  14  are set to select the lower selective terminals, the switches  15  and  16  are set to select the upper selective terminals, the switches  37  and  38  are set to select the right selective terminals, and the switches  40  and  41  are set to select the lower selective terminals. On the other hand, when the secondary battery  12  is to be set to the low-voltage side, the switches  13  and  14  are set to select the upper selective terminals, the switches  15  and  16  are set to select the lower selective terminals, the switches  37  and  38  are set to select the left selective terminals, and the switches  40  and  41  are set to select the upper selective terminals. 
         [0099]    As has been described above, in the embodiment shown in  FIG. 8 , since it is possible to connect only one of the secondary batteries to the discharging circuit  19 , an influence of the 12V-load  20  can be excluded and measurement can be performed accurately. 
         [0100]    In each of the aforementioned embodiments, a case in which two secondary batteries  11  and  12  are connected in series has been taken as an example, but three or more secondary batteries may be connected in series.  FIG. 9  shows an embodiment for a case in which three secondary batteries  11 ,  12  and  50  are connected in series. In an example shown in this diagram, as compared to the case of  FIG. 1 , a secondary battery  50  and switches  51  to  55  are added, the 24V-load  21  is replaced with a 36V-load  56 , and the alternator  23  is replaced with an alternator  23 A. 
         [0101]    Here, regarding the switch  51 , a common terminal is connected to a connection point A, one of the selective terminals is connected to one of the selective terminals of other switches  52  to  54 , and the other selective terminal is connected to the other selective terminal of the switch  53 . Regarding the switch  52 , a common terminal is connected to a connection point C, one of the selective terminals is connected to one of the selective terminals of other switches  51 ,  53  and  54 , and the other selective terminal is connected to the other selective terminal of the switch  54 . Regarding the switch  53 , a common terminal is connected to a positive pole of secondary battery  50 , one of the selective terminals is connected to one of the selective terminals of the other switches  51 ,  52  and  54 , and the other selective terminal is connected to the other selective terminal of the switch  51 . Regarding the switch  54 , a common terminal is connected to a negative pole of the secondary battery  50 , one of the selective terminals is connected to one of the selective terminals of the other switches  51 ,  52 , and  53 , and the other selective terminal is connected to the other selective terminals of the switch  52 . Regarding the switch  55 , a common terminal is connected to the 12V-load  20 , one of the selective terminals is connected to a connection point E, and the other of the selective terminals is connected to the connection point B. 
         [0102]    The 36V-load  56  is a load whereto a voltage of 36V is supplied. The alternator  23 A outputs a voltage of 36V, and charges the secondary batteries  11 ,  12  and  50 . 
         [0103]    An operation of an embodiment shown in  FIG. 9  will now be described. In the embodiment shown in  FIG. 9 , the operation of the switches  13  to  16  is similar to the case of  FIG. 1 , and an order of series connection of the secondary batteries  11  and  12  is changed by a connecting state of these switches  13  to  16 . On the other hand, the switches  51  to  54  change an order of the series connection of the secondary batteries  11  and  12  and the secondary battery  50 . Specifically, as shown in  FIG. 9 , when the switches  51  and  52  have selected upper selective terminals, the switches  53  and  54  have selected lower selective terminals, and the switch  55  has selected an upper selective terminal, the secondary battery  50  is grounded via the state detecting unit  17  and the secondary batteries  11  and  12  are connected in series in this order to a positive side of the secondary battery  50 . At this time, the 12V-load  20  is supplied with an electric power from the secondary battery  50  and the 36V-load  56  is supplied with an electric power from the secondary batteries  11 ,  12  and  50 . In such a state, similarly to the case of  FIG. 1 , the order of connection of the secondary batteries  11  and  12  can be changed by changing a connecting state of the switches  13  to  16 . 
         [0104]    On the other hand, when the switches  51  and  52  have selected lower selective terminals, the switches  53  and  54  have selected upper selective terminals, and the switch  55  has selected a lower selective terminal, the secondary battery  12  is grounded and the secondary batteries  11  and  50  are, in this order, connected to the positive side of the secondary battery  12 . At this time, the 12V-load  20  is supplied with an electric power from the secondary battery  12  and the 36V-load  56  is supplied with an electric power from the secondary batteries  11 ,  12  and  50 . In such a state, similarly to the case of  FIG. 1 , the order of connection of the secondary batteries  11  and  12  can be changed by changing the connecting state of the switches  13  to  16 . 
         [0105]    As has been described above, in the embodiment shown in  FIG. 9 , since the three secondary batteries  11 ,  12  and  50  can be connected in series and the order of series connection can be changed, the charge/discharge control of three secondary batteries can be performed easily. 
         [0106]    Referring now to  FIG. 10 , a case in which four secondary batteries  11 ,  12 ,  61  and  62  are connected in series will be described. In the embodiment shown in  FIG. 10 , as compared to  FIG. 1 , secondary batteries  61  and  62 , switches  63  to  71 , and a 48V-load  72  are added, and the alternator  23  is replaced with an alternator  23 B. 
         [0107]    The secondary batteries  61  and  62  and the switches  63  to  66  are connected similarly to a manner in which the secondary batteries  11  and  12  and the switches  13  to  16  are connected. Regarding the switches  67  and  69 , common terminals are connected to the connection points A and A′, respectively. One of the selective terminals of the switches is connected with one of the selective terminals of the other switch and also connected to one of the selective terminals of the switches  68  and  70 . The other selective terminal thereof is connected with the other selective terminal of the other switch and is also connected to the 48V-load  72  and the alternator  23 B. Regarding the switches  68  and  70 , common terminals are connected to the connection point C and C′, respectively. One of the selective terminals thereof is connected with the one of the selective terminals of the other switch and also connected to one of the selective terminals of the switches  67  and  69 . The other selective terminal thereof is connected with the other selective terminal of the other switch and also connected to the state detecting unit  17 . Regarding the switch  71 , a common terminal is connected to the 12V-load  20 . One of the selective terminals is connected to the connection point B′, and the other selective terminal is connected to the connection point B. 
         [0108]    An operation of the embodiment of  FIG. 10  will now be described. In the embodiment of  FIG. 10 , by changing the connection of switches  67  to  70 , it is possible to select which of the secondary batteries  11  and  12  or the secondary batteries  61  and  62  are to be on the ground side. Specifically, as shown in  FIG. 10 , when the switches  67  and  68  have selected the upper selective terminals, the switches  69  and  70  have selected lower selective terminals, and the switch  71  has selected an upper selective terminal, the secondary batteries  61  and  62  are connected to the ground side. At this time, the 12V-load  20  is supplied with an electric power from the secondary battery  61 , the 24V-load  21  is supplied with an electric power from the secondary batteries  61  and  62  connected in series, and the 48V-load  72  is supplied with an electric power from the secondary batteries  61 ,  62 ,  11  and  12  connected in series. On the other hand, when the switches  67  and  68  have selected the lower selective terminals, the switches  69  and  70  have selected the upper selective terminals, and the switch  71  has selected a lower selective terminal, the secondary batteries  11  and  12  are connected to the ground side. At this time, the 12V-load  20  is supplied with an electric power from the secondary battery  12 , the 24V-load  21  is supplied with an electric power from the secondary batteries  11  and  12  connected in series, and the 48V-load  72  is supplied with an electric power from the secondary batteries  61 ,  62 ,  11  and  12  connected in series. 
         [0109]    As has been described above, in the embodiment shown in  FIG. 10 , since the four secondary batteries  11 ,  12 ,  61  and  62  can be connected in series and the order of series connection can be changed, the charge/discharge management of the four secondary batteries can be performed in a facilitated manner.