Abstract:
A power supply unit includes: a plurality of secondary batteries connected in series; limiting resistors connected to individual electrodes of the plurality of secondary batteries; a voltage detection circuit detecting inter-terminal voltages of each of the plurality of secondary batteries based on potentials obtained from the plurality of secondary batteries through the limiting resistors; a control circuit specifying one of the secondary batteries to be discharged based on the inter-terminal voltages of the plurality of secondary batteries detected by the voltage detection circuit and sending a discharge instruction; and a discharge circuit allowing the specified secondary battery to discharge through the limiting resistors in response to the instruction from the control circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a power supply unit including a secondary battery and specifically, relates to a technique to enhance safety of a power supply unit including a secondary battery when the power supply unit is at fault. 
         [0003]    2. Description of the Related Art 
         [0004]    In a secondary battery, a remaining charge estimation for estimating the charge of the secondary battery and charge/discharge control for preventing excessive charge and discharge are performed using measured battery voltage of the secondary battery. Moreover, in a power supply unit including a plurality of secondary batteries connected in series, when amounts of charges stored in the secondary batteries become imbalanced, some of the secondary batteries reach cutoff voltage earlier than the others at the time of charging/discharging. The total charge/discharge capacity of the power supply unit is therefore reduced. Accordingly, to equalize the battery voltages of the secondary batteries, balance control is carried out, which discharges the secondary batteries with higher voltage to equalize the amounts of charges of the secondary batteries. 
         [0005]    As such a power supply unit including secondary batteries, Patent Literature 1 discloses a power supply unit in which secondary batteries are configured to be used for as long time as possible by detecting a deterioration causing a small short circuit repeatedly before positive and negative electrodes are completely short circuited.  FIG. 1  is a circuit diagram showing voltage detection circuits  9  and discharge circuits  10  used in this conventional power supply unit. 
         [0006]    The voltage detection circuits  9  of the conventional power supply unit individually detect voltages between positive and negative terminals of the respective secondary batteries C. Specifically, each of the voltage detection circuits  9  detects voltage of the positive terminal of the corresponding secondary battery C through a detection resistor  40  and a buffer circuit  41 . Herein, the voltage of the negative terminal of the secondary battery C positioned at the most negative side among the plurality of secondary batteries C connected in series is the earth potential. Each operational amplifier  42  outputs, to a not-shown controller, a potential difference between voltage of the positive terminal of the corresponding secondary battery C and voltage of the positive terminal of the secondary battery C adjacent to the same on the positive terminal side. 
         [0007]    The discharge circuit  10  is composed of a discharge path  45  connected between the positive and negative terminals of each secondary battery C in parallel thereto. The discharge path  45  is composed of a switching transistor  43  and discharge resistors  44 . The switching transistor  43  is connected to the discharge resistors  44  in series. The switching transistor  43  is turned on/off through a photocoupler  47  by a control signal sent from the not-shown controller. Such a discharge path  45  is provided for each secondary battery C. The not-shown controller selectively gives the control signal to any of the discharge paths  45 , thus selectively turn the discharge path  45  on or off.
   Patent Publication 1: Japanese Patent Application Laid-open Publication No. 2003-9405.   
 
         [0009]    However, in the aforementioned conventional power supply unit, the electrically conducting paths directly lead from the positive and negative electrodes of each secondary battery to the corresponding voltage detection circuit  9  and discharge circuit  10 . Accordingly, if insulation failure occurs in these electrically conducting paths, the positive and negative electrodes of the secondary battery C are short-circuited. 
         [0010]    The secondary batteries C generally use electrolyte liquid. The electrolyte liquid sometimes leaks to short the electrically conducting paths, thus forming a short circuit. In the thus-formed short circuit, there is no element controlling current other than only internal resistance of the secondary battery C and resistance of the electrically conducting path of the short circuit. Accordingly, there is a possibility that large current may flow through the short circuit and cause burnout of the secondary battery C. 
         [0011]    Moreover, in the discharge circuit  10 , a single failure such as a short circuit of any discharge resistor  44  causes a short circuit of the secondary battery  10 , and this can cause burnout of the discharge circuit  10 . Especially in a battery having large capacity or fast charge performance, the secondary batteries have low internal resistance, and there is a problem of safety. 
       SUMMARY OF THE INVENTION 
       [0012]    An object of the present invention is to provide a power supply unit including secondary batteries having such excellent safety that accidents due to a short circuit can be prevented. 
         [0013]    To solve the aforementioned problems, a power supply unit according to a first aspect of the present invention includes: a plurality of secondary batteries connected in series; limiting resistors connected to individual electrodes of the plurality of secondary batteries; a voltage detection circuit detecting inter-terminal voltage between terminals of each of the plurality of secondary batteries based on potentials obtained from the plurality of secondary batteries through the limiting resistors; a control circuit specifying one of the secondary batteries to be discharged based on the inter-terminal voltages of the plurality of secondary batteries detected by the voltage detection circuit and sending an instruction to allow the specified secondary battery to discharge; and a discharge circuit allowing the specified secondary battery to discharge through the limiting resistors in response to the instruction received from the control circuit. 
         [0014]    Herein, preferably, the discharge circuit includes a discharge resistor and a discharge switch which are connected in series between the limiting resistors connected to individual electrodes of the plurality of secondary batteries. In this case, the discharge switch is turned on or off in response to the instruction from the control circuit. With such a configuration, even if a short circuit occurs in the electrically conducting paths or discharge circuit because of insulation failure, a defective component of the discharge circuit, or the like, the limiting resistors limit the short-circuit current. Accordingly, the power supply unit and secondary batteries can avoid accidents due to short-circuit current such as firing or smoking. 
         [0015]    Moreover, the power supply unit according to the first aspect of the present invention may further include: a disconnection determination voltage holding circuit which, upon receiving the instruction from the control circuit, calculates a certain value times the detected voltage detected by the voltage detection circuit when the discharge switch of the discharge circuit is off and holds the calculated value as disconnection determination voltage; and a comparator which compares the inter-terminal voltage detected by the voltage detection circuit with the respective disconnection determination voltage held by the disconnection determination voltage holding circuit or a disconnection determination voltage previously set and determines that there is a disconnection in electrically conducting paths connected to the secondary battery corresponding to the inter-terminal voltage which is smaller than the compared disconnection determination voltage. 
         [0016]    With such a configuration, when there is a disconnection of the electrically connecting paths between the secondary batteries and the voltage detection circuit or an open defect of components, the detected voltage of any one of the secondary batteries is reduced to lower than the disconnection determination voltage. Accordingly, it is possible to reliably detect a fault such as a disconnection of the electrically connecting paths or an open defect of components. 
         [0017]    Furthermore, the power supply unit according to the first aspect of the present invention may further include: a detected voltage compensation circuit which, upon receiving the instruction from the control circuit, calculates backward each of the battery voltages of the plurality of secondary batteries as each of compensated detected voltages based on the detected voltages detected by the voltage detection circuit, resistance values of the limiting resistors, and resistance values of the discharge resistors. In this case, the control circuit executes at least one of estimation of remaining charge, charge/discharge control, and balance control of the secondary batteries based on the compensated detected voltages calculated by the detected voltage compensation circuit. 
         [0018]    Still furthermore, the power supply unit according to the first aspect of the present invention may further include: a detected voltage compensation circuit which, upon receiving the instruction from the control circuit, calculates backward battery voltage of the secondary battery corresponding to the discharge switch of the discharge circuit turned on and battery voltage of the secondary battery which is adjacent to the secondary battery corresponding to the discharge switch turned on as each of compensated detected voltages based on detected voltages detected by the voltage detection circuit of the secondary battery corresponding to the discharge switch turned on and the adjacent secondary battery, resistance values of the limiting resistors, and resistance values of the discharge resistors. In this case, the control circuit executes at least one of estimation of remaining charges, charge/discharge control, and balance control of the secondary batteries based on the compensated detected voltages calculated by the detected voltage compensation circuit and detected voltages of the secondary batteries other than the secondary battery corresponding to the discharge switch turned on and the adjacent battery which are detected by the voltage detection circuit. 
         [0019]    With such detection voltage compensation circuits, the battery voltages are accurately detected even when any one of the discharge switches of the discharge circuit is on. Accordingly, the control circuit does not perform a wrong remaining charge estimation, wrong charge/discharge control, or the like. The wrong charge/discharge control of the control circuit may cause excessive charge of the secondary batteries or the like. However, the provision of the detected voltage correction circuit can prevent the secondary batteries from deteriorating or generating abnormal heat because of excessive charge or the like. 
         [0020]    Still furthermore, the control circuit may activate the discharge circuit during a period except a period necessary for the voltage detection circuit to detect the inter-terminal voltages of the secondary batteries. 
         [0021]    With such an operation of the control circuit, the control circuit can accurately detect the battery voltages even when activating the discharge circuit for the balance control. This prevents the control circuit from performing a wrong remaining charge estimation or charge/discharge control. 
         [0022]    As described above, according to the first aspect of the present invention, it is possible to provide a power supply unit having such excellent safety that accidents due to a short circuit can be prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a circuit diagram showing voltage detection circuits and discharge circuits used in a conventional power supply unit. 
           [0024]      FIG. 2  is a block diagram showing a constitution of a main portion of a power supply unit according to Embodiment 1 of the present invention. 
           [0025]      FIG. 3  is a circuit diagram showing a discharge circuit used in the power supply unit according to Embodiment 1 of the present invention. 
           [0026]      FIG. 4  is a block diagram schematically showing a constitution of a power supply unit according to Embodiment 2 of the present invention. 
           [0027]      FIG. 5  is a block diagram schematically showing a constitution of a power supply unit according to Embodiment 3 of the present invention. 
           [0028]      FIG. 6  is a block diagram schematically showing a constitution of a power supply unit according to Embodiment 4 of the present invention. 
           [0029]      FIG. 7  is a timing chart for explaining an operation of the power supply unit according to Embodiment 5 of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    Hereinafter, a description is given of embodiments of the present invention with reference to the drawings. In the following description, constituent elements same or similar to those of the conventional power unit explained in Description of the Related Art are given same reference numerals as those used in Description of the Related Art. 
       Embodiment 1 
       [0031]      FIG. 2  is a block diagram showing a constitution of a main portion of a power supply unit according to Embodiment 1 of the present invention. The power supply unit includes a plurality of secondary batteries C 1  to C 3 , limiting resistors  5   a  to  5   d,  a voltage detection circuit  9 , a discharge circuit  10 , a control circuit  17 , a current detection resistor  39   a,  and a current detection circuit  39 .  FIG. 2  shows an example of the power supply unit provided with the three secondary batteries C 1  to C 3 , but the number of secondary batteries can be arbitrarily determined. 
         [0032]    The secondary batteries C 1  to C 3  are connected in series. One end of the limiting resistor  5   a  is connected to a negative electrode of the secondary battery C 1 , and the other end thereof is connected to the voltage detection circuit  9  and the discharge circuit  10  through electrically conducting paths  11 . The limiting resistor  5   a  limits current flowing between the negative electrode of the secondary battery C 1  and the voltage detection circuit  9  or the discharge circuit  10 . 
         [0033]    One end of the limiting resistor  5   b  is commonly connected to a positive electrode of the secondary battery C 1  and a negative electrode of the secondary battery C 2 , and the other end thereof is connected to the voltage detection circuit  9  and the discharge circuit  10  through electrically conducting paths  11 . The limiting resistor  5   b  limits current flowing between the connection point of the positive electrode of the secondary battery C 1  and the negative electrode of the secondary battery C 2  and the voltage detection circuit  9  or discharge circuit  10 . 
         [0034]    One end of the limiting resistor  5   c  is commonly connected to a positive electrode of the secondary battery C 2  and a negative electrode of the secondary battery C 3 , and the other end thereof is connected to the voltage detection circuit  9  and discharge circuit  10  through electrically conducting paths  11 . The limiting resistor  5   c  limits current flowing between the connection point of the positive electrode of the secondary battery C 2  and the negative electrode of the secondary battery C 3  and the voltage detection circuit  9  or discharge circuit  10 . 
         [0035]    One end of the limiting resistor  5   d  is connected to a positive electrode of the secondary battery C 3 , and the other end thereof is connected to the voltage detection circuit  9  and discharge circuit  10  through electrically conducting paths  11 . The limiting resistor  5   d  limits current flowing between the positive electrode of the secondary battery C 3  and the voltage detection circuit  9  or discharge circuit  10 . 
         [0036]    The voltage detection circuit  9  detects a magnitude of voltage between the terminals of the secondary battery C 1  based on potentials obtained through the limiting resistors  5   a  and  5   b  and sends the detected magnitude to the control circuit  17  as a detected voltage V D1 . Moreover, the voltage detection circuit  9  detects a magnitude of voltage between the terminals of the secondary battery C 2  based on potentials obtained through the limiting resistors  5   b  and  5   c  and sends the detected magnitude to the control circuit  17  as a detected voltage V D2 . Still moreover, the voltage detection circuit  9  further detects a magnitude of voltage between the terminals of the secondary battery C 3  based on potentials obtained through the limiting resistors  5   c  and  5   d  and sends the detected magnitude to the control circuit  17  as a detected voltage V D3 . 
         [0037]    The current detection resistor  39   a  is connected to the secondary batteries C 1  to C 3  in series. When current flows through the secondary batteries C 1  to C 3 , the current detection resistor  39   a  produces a voltage proportional to the current between the both ends thereof. Moreover, the current detection circuit  39  is connected to the both ends of the current detection resistor  39   a.  The voltage produced at the current detection resistor  39   a  is measured by the current detection circuit  39 . The connection point of the negative electrode of the secondary battery C 1  and current detection resistor  39   a  is connected to the ground potential. 
         [0038]    The current detection circuit  39  detects a magnitude of the current flowing through the secondary batteries C 1  to C 3  based on the voltage between the both ends of the current detection resistor  39   a  and sends the same to the control circuit  17  as a detected current. 
         [0039]    The control circuit  17  executes at least one of a remaining charge estimation, charge/discharge control, and balance control of the secondary batteries C 1  to C 3  using the detected voltages V D1  to V D3  of the secondary batteries C 1  to C 3  sent from the voltage detection circuit  9 , the detected current sent from the current detection circuit  39 , temperature of the secondary batteries C 1  to C 3  detected by a not-shown temperature sensor, and the like. For example, the control circuit  17  sends a discharge switch ON instruction to the discharge circuit  10  to control discharge of the secondary batteries C 1  to C 3 . Moreover, the control circuit  17  sends detected or estimated information to the outside of the power supply unit. 
         [0040]    The discharge circuit  10  controls discharge of the secondary batteries C 1  to C 3  in response to the discharge switch ON instruction from the control circuit  17 . As shown in  FIG. 3 , the discharge circuit  10  includes discharge resistors  44   a  to  44   c  and discharge switches S 1  to S 3 . Each of the discharge switches S 1  to S 3  is composed of a semiconductor switch such as for example, an FET or a transistor and is turned on or off in response to the discharge switch ON instruction from the control circuit  17 . 
         [0041]    A discharge part composed of the discharge resistor  44   a  and discharge switch S 1  controls discharge of the secondary battery C 1 . Specifically, upon receiving the discharge switch ON instruction from the control circuit  17 , the discharge switch S 1  is turned on. Current then flows through a path: the positive electrode of the secondary battery C 1 →the limiting resistor  5   b →the discharge resistor  44   a →the discharge switch S→the limiting resistor  5   a →the negative electrode of the secondary battery C 1 . The secondary battery C 1  is thus discharged. 
         [0042]    In a similar manner, a discharge part composed of the discharge resistor  44   b  and discharge switch S 2  controls discharge of the secondary battery C 2 . Specifically, upon receiving the discharge switch ON instruction from the control circuit  17 , the discharge switch S 2  is turned on. Current then flows through a path: the positive electrode of the secondary battery C 2 →the limiting resistor  5   c →the discharge resistor  44   b →the discharge switch S 2 →the limiting resistor  5   b →the negative electrode of the secondary battery C 2 . The secondary battery C 2  is thus discharged. 
         [0043]    In a similar manner, a discharge part composed of the discharge resistor  44   c  and discharge switch S 3  controls discharge of the secondary battery C 3 . Specifically, upon receiving the discharge switch ON instruction from the control circuit  17 , the discharge switch S 3  is turned on. Current then flows through a path: the positive electrode of the secondary battery C 3 →the limiting resistor  5   d →the discharge resistor  44   c →the discharge switch S 3 →the limiting resistor  5   c →the negative electrode of the secondary battery C 3 . The secondary battery C 3  is thus discharged. 
         [0044]    Furthermore, the control circuit  17  sends the discharge switch ON instruction to the discharge switch corresponding to the secondary battery outputting a voltage higher than the other secondary batteries. The secondary battery corresponding to the discharge switch which receives the discharge switch ON instruction therefore discharges to balance the voltages of the plurality of secondary batteries C 1  to C 3 . 
         [0045]    In the power supply unit configured as described above, when there is a short circuit in the electrically conducting paths  11  or discharge circuit  10  because of insulation failure or the like, current flows through the limiting resistors  5   a  to  5   d.  Accordingly, the current flowing through the secondary batteries C 1  to C 3  is limited. 
         [0046]    As described above, in the power supply unit according to Embodiment 1 of the present invention, even if there is a short circuit in the conducting paths  11  or discharge circuit  10  because of insulation failure, a defect of the discharge resistors  44   a  to  44   c,  and the like, the short-circuit current is limited by the limiting resistors  5   a  to  5   d.  The power supply unit and secondary batteries can avoid accidents caused by the short-circuit current such as firing or smoking. 
       Embodiment 2 
       [0047]    A power supply unit of Embodiment 2 shown in  FIG. 4  further includes a disconnection determination voltage holding circuit  12  and a comparator  13  in addition to the power supply unit of Embodiment 1. 
         [0048]    Upon receiving the discharge switch ON instruction from the control circuit  17 , the disconnection determination voltage holding circuit  12  holds, as disconnection determination voltages, voltages equal to k times (k&lt;1) the detected voltages detected by the voltage detection circuit  9  while the discharge switches S 1  to S 3  are off. The disconnection determination voltages held in the disconnection determination voltage holding circuit  12  are sent to the comparator  13 . 
         [0049]    The comparator  13  compares the disconnection determination voltages held by the disconnection determination voltage holding circuit  12  and the respective detected voltages V D1 , V D2 , and V D3  detected by the voltage detection circuit  9 . When the detected voltage V D1 , V D2 , or V D3  is smaller than the respective disconnection determination voltages, the comparator  13  determines that there is a disconnection in the electrically conducting paths  11  connected to the secondary battery C 1 , C 2 , or C 3  corresponding to the detected voltage V D1 , V D2 , or V D3  which is smaller than the corresponding disconnection determination voltage. The result of the determination by the comparator  13  is sent to the outside. 
         [0050]    In the power supply unit according to Embodiment 2 configured as described above, the control circuit  17  turns on/off the discharge switches S 1  to S 3  of the discharge circuit  10  at proper time intervals. In this case, if the discharge switches S 1  to S 3  are turned on while the battery charges of the secondary batteries C 1  to C 3  are balanced, the charges of the secondary batteries C 1  to C 3  are unnecessarily discharged. Accordingly, the time in which the discharge switches S 1  to S 3  are on is made short enough not to greatly affect the charges stored in the secondary batteries C 1  to C 3 . 
         [0051]    When it is determined by the comparator  13  in such a state that there is a disconnection, the secondary batteries C 1  to C 3  cannot be safely used. Accordingly, the comparator  13  notifies a user of the failure of the power supply unit or informs an upper system of the failure of the power supply unit, thus securing the safety of the power supply unit. 
         [0052]    Herein, consideration is given to the case where the discharge switch S 2  of the discharge circuit  10  corresponding to the secondary battery C 2  is turned on. In this case, the detected voltage V D2  detected by the voltage detection circuit  9  is expressed by the following equation, 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    herein, R 1  indicates the resistance value of the limiting resistors  5   b  and  5   c;  R 2 , the resistance value of the discharge resistor  44   b;  and V C2 , the battery voltage of the secondary battery C 2 . If the conduction resistances of the discharge switches S 1  to S 3  are included in R 2 , the detected voltage V D2  can be calculated more accurately. 
         [0053]    Equation (1) above calculates the detected voltage V D2  detected when the discharge switch S 2  is turned on, and the detected voltages V D1  and V D2  detected when the other discharge switch S 1  or S 3  is turned on can be obtained in a similar manner. Moreover, k is set to a value smaller than R 2 /(2R 1 +R 2 ) based on equation (1). 
         [0054]    Moreover, when the discharge switch S 2  is turned on, the detected voltages V D1  and V D3  of the discharge switches S 1  and S 3  adjacent thereto on both sides are expressed by the following equations, 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    herein, V C1  and V C3  indicate the battery voltages of the secondary batteries C 1  and C 3 , respectively. 
         [0055]    As shown in equations (1) to (3), when any one of the discharge switches S 1  to S 3  is turned on, the detected voltages detected by the voltage detection circuit  9  have values different from the battery voltages of the respective secondary batteries C 1  to C 3 . Accordingly, the disconnection determination voltage holding circuit  12  generates the disconnection determination voltage using the detected voltages detected during a period when the discharge switches S 1  to S 3  are off. 
         [0056]    As described above, the power supply unit of Embodiment 2 of the present invention has the following effect. Specifically, in the power supply unit of Embodiment 1 shown in  FIG. 2 , when there is a disconnection of the electrically conducting paths  11  between the secondary batteries C 1  to C 3  and the voltage detection circuit  9  or an open defect of components, the voltages of the secondary batteries C 1  to C 3  are divided by the input impedances of the voltage detection circuit  9 . 
         [0057]    The detected voltages V D1  to V D3  detected in this case are sometimes different from the battery voltages V C1  to V C3  of the secondary batteries C 1  to C 3 . If the battery voltages V C1  to V C3  of the secondary batteries C 1  to C 3  cannot be accurately detected, there is a possibility that fault of the battery voltages V C1  to V C3  is not noticed. Under ordinary circumstances, it is necessary to detect the abnormal voltage and limit the charge/discharge current supplied to the secondary batteries C 1  to C 3 . However, without such a measure, the secondary batteries C 1  to C 3  will deteriorate or generate abnormal heat, and at worse, will fire or smoke. 
         [0058]    On the other hand, in the power supply unit of Embodiment 2 of the present invention, when there is a disconnection of the electrically conducting paths  11  between the secondary batteries C 1  to C 3  and the voltage detection circuit  9  or an open defect of components, the detected voltages V D1  to V D3  of the secondary batteries C 1  to C 3  are reduced to lower than the disconnection determination voltage. It is therefore possible to reliably detect a failure such as a disconnection of the electrically conducting paths or an open defect of components. 
       Embodiment 3 
       [0059]    In a power supply unit of Embodiment 3 of the present invention shown in  FIG. 5 , a fixed value smaller than the result of multiplying a minimum value of the voltage of the secondary batteries C 1  to C 3  varying in a normal range of use by R 2 /(2R 1 +R 2 ) is used as a disconnection determination voltage instead of the disconnection determination voltages held by the disconnection determination voltage holding circuit  12  of the power supply unit of Embodiment 2. 
         [0060]    An operation of the power supply unit according to Embodiment 3 is the same as that of the power supply unit according to Embodiment 2 above except that the disconnection determination voltage takes a fixed value. The power supply unit according to Embodiment 3 can provide a similar effect to that of the power supply unit according to Embodiment 2. 
       Embodiment 4 
       [0061]      FIG. 6  is a block diagram schematically showing a configuration of the power supply unit according to Embodiment 4 of the present invention. This power supply unit further includes a detected voltage compensation circuit  14  in addition to the power supply unit according to Embodiment 1. 
         [0062]    The detected voltage compensation circuit  14  receives the discharge switch ON instruction sent from the control circuit  17  and calculates battery voltages of the secondary batteries based on the detected voltages detected by the voltage detection circuit  9  using a method described below. The detected voltage compensation circuit  14  sends the calculated voltages to the control circuit  17  as compensated detected voltages. The control circuit  17  executes a remaining charge estimation, charge/discharge control, or balance control by using the compensated detected voltages sent from the detected voltage compensation circuit  14  instead of the detected voltages sent from the voltage detection circuit  9 . 
         [0063]    Next, a description is given of the method of calculating the compensated detected voltages. When the discharge switch S 2  of the discharge circuit  10  is turned on by the control circuit  17 , the detected voltages V D1  to V D3  shown in above equations (1) to (3) are detected by the voltage detection circuit  9 . The detected voltages V D2  to V D3  are different from the respective battery voltages V C1  to V C3  of the secondary batteries C 1  to C 3 . Accordingly, if the control circuit  17  estimates the remaining charges or performs charge/discharge control to prevent excessive charge or discharge by directly using the detected voltages V C1  to V C3 , wrong estimation of remaining charges or wrong charge/discharge control is performed. 
         [0064]    When the discharge switch S 2  is turned on, the battery voltages V C1  to V C3  of the secondary batteries C 1  to C 3  are calculated by the following equations based on the detected voltages V D1  to V D3  detected by the voltage detection circuit  9 . The following equations are calculated backward from equations (1) to (3), 
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                              
                             2 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0065]    When the discharge switch S 1  is turned on, the battery voltages V C1  to V C3  of the secondary batteries C 1  to C 3  are calculated by the following equations in a similar manner based on the detected voltages V D1  to V D3  detected by the voltage detection circuit  9 , 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       
                         C 
                          
                         
                             
                         
                          
                         1 
                       
                     
                     = 
                     
                       
                         
                           
                             2 
                              
                             
                               R 
                               1 
                             
                           
                           + 
                           
                             R 
                             2 
                           
                         
                         
                           R 
                           2 
                         
                       
                        
                       
                         V 
                         
                           D 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       V 
                       
                         C 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     = 
                     
                       
                         V 
                         
                           D 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       - 
                       
                         
                           
                             R 
                             1 
                           
                           
                             R 
                             2 
                           
                         
                          
                         
                           V 
                           
                             D 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     
                       C 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   = 
                   
                     
                       V 
                       
                         D 
                          
                         
                             
                         
                          
                         3 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0066]    Also when the discharge switch S 3  is turned on, the battery voltages V C1  to V C3  of the secondary batteries C 1  to C 3  can be calculated in a similar manner based on the detected voltages V D1  to V D3  detected by the voltage detection circuit  9 . 
         [0067]    As shown in equations (5) and (7), when one of the discharge switches is turned on, the battery voltage of the secondary battery corresponding to the discharge switch turned on is calculated by multiplying the detected voltage of the secondary battery by (2R 1 +R 2 )/R 2 . 
         [0068]    Moreover, as shown in equations (4), (6), and (8), when one of the discharge switches is turned on, battery voltage of each of the secondary batteries adjacent to the secondary battery corresponding to the discharge switch turned on can be calculated by subtracting R 1 /R 2  times the detected voltage of the secondary battery corresponding to the discharge switch turned on from the detected voltage of the same adjacent secondary battery. 
         [0069]    Furthermore, as shown in equation (9), the battery voltage of the secondary battery which corresponds to any one of the discharge switches turned off and is not adjacent to the secondary battery corresponding to the discharge switch turned on is equal to the detected voltage of the same. 
         [0070]    As described above, according to the power supply unit of Embodiment 4 of the present invention, in addition to the effect of the power supply unit of Embodiment 1, the battery voltages can be accurately detected even when any one of the discharge switches is on. Accordingly, the power supply unit of Embodiment 4 does not perform wrong estimation of remaining charges or wrong charge/discharge control. The wrong charge/discharge control may cause excessive charge or the like, but with the power supply unit according to Embodiment 4, it is possible to prevent the secondary batteries from deteriorating or generating abnormal heat. 
       Embodiment 5 
       [0071]    The configuration of the power supply unit of Embodiment 5 of the present invention is the same as that of the power supply unit according to Embodiment 1, and only operations thereof are different from each other. Hereinafter, a description is given of mainly the points different from Embodiment 1 with reference to the timing chart shown in  FIG. 7 . 
         [0072]    When the voltage detection circuit  9  detects the voltages of the secondary batteries at time intervals of voltage measurement period Tw, the measurement requires voltage measurement period Tm equal to the sum of stabilization period Ts taken for the detected voltages V D1  to V D2  to be stabilized and transform period Tc to obtain the voltages. 
         [0073]    In the power supply unit of Embodiment 5, when the voltages of the secondary batteries become imbalanced, the discharge switches S 1  to S 3  are turned on during measurement idle period Tr, which is equal to a result of subtracting the voltage measurement Tm from the voltage measurement period Tw. In other words, ON period Td of the discharge switches S 1  to S 3  is included within the measurement idle period Tr and is set equal to or shorter than the measurement idle period Tr. The battery voltages of the secondary batteries can be therefore accurately detected while the operation of balancing the voltages of the secondary batteries is performed. 
         [0074]    As described above, according to the power supply unit of Embodiment 5 of the present invention, in addition to the effect of the power supply unit of Embodiment 1, the battery voltages can be accurately detected even while the discharge switches are activated for balance control. Accordingly, the power supply unit of Embodiment 5 does not perform wrong estimation of remaining charges or wrong charge/discharge control. The wrong charge/discharge control may cause excessive charge or the like. However, the power supply unit of Embodiment 5 can prevent the secondary batteries from deteriorating or generating abnormal heat.