Abstract:
The present invention relates to a method for more efficiently performing a balancing operation for a plurality of battery cells of which charges are not equal, in a battery cell balancing circuit using an LC series resonant circuit. The method may include calculating balancing charge for all battery cells of which the charges are not equal, selecting the strongest battery cell storing the highest charge and the weakest battery cell corresponding to the strongest battery cell, among the entire battery cells, and performing a series of balancing operations.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a balancing technology of a battery cell module, and more particularly, a battery cell balancing method which is capable of balancing a plurality of battery cells of which the stored charges are not equal, in a battery cell balancing circuit using LC series resonance circuit. 
         [0003]    2. Related Art 
         [0004]    In general, when a voltage across a battery cell exceeds a predetermined value, the battery cell is likely to explode, and when the voltage falls below a predetermined value, the battery cell is likely to receive permanent damage. Since a hybrid electric vehicle or notebook computer requires a power supply with a relatively large capacity, a battery cell module (battery pack) including a plurality of battery cells connected in series is used to supply power. However, when such a battery cell module is used, a voltage imbalance may occur due to a difference in performance among the battery cells. 
         [0005]    When one battery cell within the battery cell module reaches the upper limit voltage before the other battery cells while the battery cell module is charged, the battery cell module cannot be charged any more. Thus, the charging operation must be ended in a state where the other battery cells are not sufficiently charged. In this case, the charge capacity of the battery cell module may not reach the rated charge capacity. 
         [0006]    Furthermore, when one battery cell within the battery cell module reaches the lower limit voltage before the other battery cells while the battery cell module is discharged, the battery cell module cannot be used any more. Thus, the use time of the battery cell module is reduced as much. 
         [0007]    Therefore, when the battery cell module is charged or discharged, the electrical energy of a battery cell having relatively high electrical energy may be supplied to another battery having relatively low electrical energy, in order to improve the use time of the battery cell module. Such an operation is referred to as battery cell balancing. 
         [0008]      FIG. 1  is a circuit diagram of a conventional battery cell balancing circuit using parallel resistors. As illustrated in  FIG. 1 , the conventional battery cell balancing circuit includes a battery module  11  including battery cells CELL 1  to CELL 4  connected in series, resistors R 11  to R 14  connected in series, and switches SW 11  to SW 15  configured to selectively connect the battery cells CELL to CELL 4  of the battery module  11  to the resistors R 11  to R 14 . 
         [0009]    Referring to  FIG. 1 , when the charge voltage of an arbitrary battery cell among the battery cells CELL 1  to CELL 4  within the battery module  11  reaches the upper limit voltage before the charge voltages of the other battery cells while the battery module  11  is charged, the corresponding switch among the switches SW 11  to SW 15  is turned on to discharge the battery cell through the corresponding resistor among the resistors R 11  to R 14 . 
         [0010]      FIG. 2  is a circuit diagram of a conventional battery cell balancing circuit using capacitors. As illustrated in  FIG. 2 , the conventional battery cell balancing circuit includes a battery module  21  including battery cells CELL 1  to CELL 4  connected in series, capacitors C 21  to C 23  connected in series, and switches SW 11  to SW 15  configured to selectively connect the capacitors C 21  to C 23  to the battery cells CELL to CELL 4 . 
         [0011]    Referring to  FIG. 2 , the battery cell balancing circuit using the capacitors has two kinds of connection states. In a first connection state, one terminal of the capacitor C 21 , a connection terminal between the capacitors C 21  and C 22 , a connection terminal between the capacitors C 22  and C 23 , and the other terminal of the capacitor C 23  are connected to one terminals (anode terminals) of the battery cells CELL 1  to CELL 4 , respectively, as illustrated in  FIG. 2 . In a second connection state, the one terminal of the capacitor C 21 , the connection terminal between the capacitors C 21  and C 22 , the connection terminal between the capacitors C 22  and C 23 , and the other terminal of the capacitor C 23  are connected to the other terminals (cathode terminals) of the battery cells CELL 1  to CELL 4 , respectively. 
         [0012]      FIG. 3  is a circuit diagram of a conventional battery cell balancing circuit using a flyback structure. As illustrated in  FIG. 3 , the conventional battery cell balancing circuit includes a battery module  31  including battery cells CELL 1  to CELL 4  connected in series, a flyback converter  32 , and switches SW 31  to SW 35  configured to selectively connect a plurality of secondary coils of the flyback converter  32  to the battery cells CELL to CELL 4 , respectively. 
         [0013]    The battery cell balancing circuit of  FIG. 3  is a battery cell balancing circuit using a flyback structure which is one of switch mode power supplies (SMPS). The battery cell balancing circuit can transfer electrical energy to the battery cells CELL 1  to CELL 4  connected in series within the battery module  31  using the switches SW 31  to SW 34 , and transfer electrical energy between both terminals of the battery module  31 . 
         [0014]    However, the conventional battery cell balancing circuit is configured to repeat an operation of recovering charge from the battery cell storing the highest charge and supplying the recovered charge to the battery cell storing the lowest charge. 
         [0015]    Thus, when there exist a plurality of battery cells of which the stored charges are not equal, the conventional battery cell balancing circuit has difficulties in completing balancing all of the battery cells, and requires a large amount of balancing time, which makes it possible to degrade the efficiency of the balancing operation. 
       SUMMARY 
       [0016]    Various embodiments are directed to a battery cell balancing method which calculates balancing charge for all battery cells of which the charges are not equal in a battery cell balancing circuit using LC series resonance, and performs a series of balancing operations using the balancing charge, thereby improving balancing efficiency. 
         [0017]    In an embodiment, a battery cell balancing method may include: (a) comparing a standard deviation for charges of the entire battery cells to a threshold, and proceeding to the next step or an idle mode according to the comparison result; (b) calculating balancing charge which serves as a reference value when a balancing operation is performed on the entire battery cells, based on the charges of the entire battery cells and charge transfer efficiency of a balancing circuit; (c) calculating a balancing time of a strong battery cell to supply charge and a balancing time of a weak battery cell to receive charge according to a comparison result between a difference between the charge of the strong battery cell storing the highest charge and the balancing charge and a difference between the charge of the weak battery cell storing the lowest charge and the balancing charge, among the charges of the entire battery cells, and performing a balancing operation; and (d) calculating the charge of the strong battery cell and the charge of the weak battery cell, sorting the entire battery cells based on the charges, and returning to the step (c) or (a) according to whether the charge of the strong battery cell and the charge of the weak battery cell among the charges of the sorted battery cells are equal to each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a circuit diagram of a conventional battery cell balancing circuit using parallel resistors. 
           [0019]      FIG. 2  is a circuit diagram of a conventional battery cell balancing circuit using capacitors. 
           [0020]      FIG. 3  is a circuit diagram of a conventional battery cell balancing circuit using a flyback structure. 
           [0021]      FIG. 4  is a signal flowchart of a battery cell balancing method according to an embodiment of the present invention. 
           [0022]      FIG. 5  is a circuit diagram of a battery cell balancing circuit using LC series resonance, to which the battery cell balancing method according to the embodiment of the present invention is applied. 
           [0023]      FIG. 6  is a circuit diagram of another battery cell balancing circuit using LC series resonance, to which the battery cell balancing method according to the embodiment of the present invention is applied. 
           [0024]      FIG. 7  illustrates an example in which battery cells included in a battery cell module are sorted in order of the charges stored therein. 
           [0025]      FIG. 8A  is a diagram illustrating an example in which charge of an arbitrary weak battery cell is closer to balancing charge than charge of an arbitrary strong battery cell. 
           [0026]      FIG. 8B  is a diagram illustrating an example in which the charge of the strong battery cell is closer to the balancing charge than the charge of the weak battery cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Exemplary embodiments will be described below in more detail with reference to the accompanying drawings. The disclosure may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the disclosure. 
         [0028]      FIG. 4  is a signal flowchart of a battery cell balancing method according to an embodiment of the present invention. As illustrated in  FIG. 4 , the battery cell balancing method includes a balancing determination process ST 1  to ST 3 , a balancing charge calculation process ST 4  to ST 7 , a balancing process ST 8  to ST 12 , and a balancing termination determination process ST 13  to ST 15 . 
         [0029]      FIG. 5  is a circuit diagram of a battery cell balancing circuit using LC series resonance, to which the battery cell balancing method according to the embodiment of the present invention is applied. As illustrated in  FIG. 5 , the battery cell balancing circuit includes a battery cell module unit  510  and a series resonant circuit unit  520 . The battery cell module unit  510  includes a battery cell module  511 , a first switch unit  512 , and a second switch unit  513 , and the series resonant circuit unit  520  includes a series resonant circuit  521  and a third switch unit  522 . 
         [0030]    Charge stored in an arbitrary battery cell among the battery cells CELL 1  to CELL 4  of the battery cell module  511  is temporary stored in a capacitor Cs of the series resonant circuit  521  through the switch units  512 ,  513 , and  522 , and then supplied to an arbitrary battery cell among the battery cells CELL 1  to CELL 4  through the switch units, in order to charge the battery cell. 
         [0031]    For example, when the battery cell CELL 3  among the battery cell CELL 1  to CELL 4  has the highest charge value and the battery cell CELL 1  has the smallest charge value, a surplus of charge in the charge stored in the battery cell CELL 3 , based on balancing charge, is temporarily stored in the capacitor Cs of the series resonant circuit  521 , and then supplied to the battery cell CELL 1 . This process will be described as follows. The balancing charge indicates the average charge of the battery cells CELL 1  to CELL 4 . 
         [0032]    First, among switches SW 1  to SW 5  of the first switch unit  512 , the switch SW 3  is turned on, and the other switches maintain the turn-off state. At this time, among switches SW 6  to SW 10  of the second switch unit  513 , the switch SW 9  is turned on, and the other switches maintain the turn-off state. Furthermore, among switches SW 11  to SW 14  of the third switch unit  522 , the switches SW 11  and SW 12  are turned on, and the other switches maintain the turn-off state. Thus, one terminal of the battery cell CELL 3  of the battery cell module  511  is connected to a third common node N 3  corresponding to one terminal of the series resonant circuit  521  through the switches SW 3  and SW 11 , and a fourth common node N 4  corresponding to the other terminal of the series resonant circuit  521  is connected to the other terminal of the battery cell CELL 3  of the battery cell module  511  through the switches SW 12  and SW 9 . 
         [0033]    Therefore, the charge stored in the battery cell CELL 3  is recovered through the switches SW 3  and SW 11 , and stored in the capacitor Cs of the series resonant circuit  521 . 
         [0034]    Then, among the switches SW 1  to SW 5  of the first switch unit  512 , the switch SW 2  is turned on, and the other switches maintain the turn-off state. At this time, among switches SW 6  to SW 10  of the second switch unit  513 , the switch SW 6  is turned on, and the other switches maintain the turn-off state. Furthermore, among switches SW 11  to SW 14  of the third switch unit  522 , the switches SW 13  and SW 14  are turned on, and the other switches maintain the turn-off state. Thus, the third common node N 3  corresponding to the one terminal of the series resonant circuit  521  is connected to one terminal of the battery cell CELL 1  of the battery cell module  111  through the switches SW 14  and SW 6 , and the fourth common node N 4  corresponding to the other terminal of the series resonant circuit  521  is connected to the other terminal of the battery cell CELL 1  of the battery cell module  111  through the switches SW 13  and SW 2 . 
         [0035]    Thus, the charge which is temporarily stored in the capacitor Cs of the series resonant circuit  521  is supplied to the battery cell CELL 1  of the battery cell module  511  through the switches SW 14  and SW 6 , in order to charge the battery cell CELL 1 . 
         [0036]      FIG. 6  is a circuit diagram of another battery cell balancing circuit using LC series resonance, to which the battery cell balancing method according to the embodiment of the present invention is applied. As illustrated in  FIG. 6 , the battery cell balancing circuit is different from the battery cell balancing circuit of  FIG. 5  in that the number of switch paths is reduced to the half because the battery cell balancing circuit can be configured to apply a current in both directions of first and second switch units  612  and  613 . 
         [0037]    For example, a path through which a surplus of charge in the stored charge of a battery cell CELL 3 , based on the average charge, is recovered may include one terminal of the battery cell CELL 3 , a switch SW 2 , a first common node N 1 , a switch SW 6 , a series resonant circuit  621 , a switch SW 7 , a second common node N 2 , a switch SW 5 , and the other terminal of the battery cell CELL 3 . 
         [0038]    For another example, a path through which charge recovered and stored in a capacitor Cs is supplied to a battery cell CELL 4  may include the other terminal of the battery cell CELL 4 , a switch SW 3 , the first common node N 1 , a switch SW 8 , the series resonant circuit  621 , a switch SW 9 , the second common node N 2 , the switch SW 5 , and one terminal of the battery cell CELL 4 . 
         [0039]    Hereafter, referring to  FIGS. 7 and 8 , the battery cell balancing method according to the embodiment of the present invention will be described in detail. In the present embodiment, the case in which the battery cell balancing method is applied to a battery cell module including 12 battery cells connected in series will be taken as an example for description. 
         [0040]    First, charge Q k  stored in a k-th battery cell is calculated by multiplying an SOC (State Of Charge) of the k-th battery cell by a k-th capacity C among the capacities of the batteries, which are sorted in descending order. In this way, the charges of the entire battery cells are calculated at step ST 1 . 
         [0041]      FIG. 7  illustrates an example in which the battery cells included in the battery cell module, for example, 12 battery cells B 1  to B 12  are sorted in order of the charges stored therein. Referring to  FIG. 7 , the charge Q 1  of the battery cell B 1  is the largest, and the charge Q 12  of the battery cell B 12  is the smallest. The number of battery cells is not limited to 12, but may be set to m (natural number). 
         [0042]    The standard deviation σ(Q k ) of the charges Q 1  to Q 12  of the battery cells B 1  to B 12  is calculated and compared to a predetermined threshold σ th  at step ST 2 . When the standard deviation σ(Q k ) is determined to be equal to or less than the threshold σ th , the battery cell module is considered to be balanced, and stays in the idle mode. However, when the standard deviation σ(Q k ) is determined to exceed the threshold σ th , the procedure proceeds to the next step, at step ST 3 . 
         [0043]    When an imbalance occurs among the charges Q 1  to Q 12  of the battery cells B 1  to B 12  as illustrated in  FIG. 7 , the charges of strong battery cells B 1  to B 6  which store a larger amount of charge than the average charge may be transferred to weak battery cells B 7  to B 12  which store a smaller amount of charge than the average charge, at a charge transfer efficiency of η e . The charge transfer efficiency may be expressed as Equation 1 below. The charge transfer efficiency indicates the charge transfer efficiency of the balancing circuit. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             1 
                           
                           n 
                         
                          
                         
                             
                         
                          
                         
                           ( 
                           
                             
                               Q 
                               k 
                             
                             - 
                             
                               Q 
                               b 
                             
                           
                           ) 
                         
                       
                       ) 
                     
                     × 
                     
                       η 
                       e 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         
                           n 
                           + 
                           1 
                         
                       
                       12 
                     
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         
                           Q 
                           b 
                         
                         - 
                         
                           Q 
                           k 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0044]    Equation 1 is used to calculate the balancing charge Q b  corresponding to the average charge which serves as a reference value when a balancing operation is performed for the entire battery cells B 1  to B 12 . At this time, when the charges Q k  of the battery cells B 1  to B 12  and the charge transfer efficiency are known, the balancing charge Q b  may be calculated as Equation 2 below, at steps ST 4  to ST 7 . 
         [0000]    
       
         
           
             
               
                 
                   
                     Q 
                     b 
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             1 
                           
                           n 
                         
                          
                         
                             
                         
                          
                         
                           
                             Q 
                             k 
                           
                           × 
                           
                             η 
                             e 
                           
                         
                       
                       + 
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             
                               n 
                               + 
                               1 
                             
                           
                           12 
                         
                          
                         
                             
                         
                          
                         
                           Q 
                           k 
                         
                       
                     
                     
                       12 
                       - 
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               η 
                               e 
                             
                           
                           ) 
                         
                         × 
                         n 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0045]    That is, the balancing charge Q b  for each of the battery cells B 1  to B 12  is calculated while n is increased by one from 1, and compared to charges Q n  and Q n+1 . At this time, when the balancing charge Q b  satisfies the condition in which the balancing charge Q b  is smaller than the charge Q n  and larger than the charge Q n+1 , the calculation operation for the balancing charge Q b  is ended, and the calculated balancing charge Q b  is set to the balancing charge which is to be acquired. For example, when the balancing charge Q b  for each of the battery cells B 1  to B 12  is calculated under the condition in which the charges Q 1  to Q 12  of the battery cells B 1  to B 12  are set as illustrated in  FIG. 7 , the balancing charge Q b  migrates in the direction from Q 1  to Q 12  whenever n is increased by one. When the balancing charge Q b  is positioned between Q 6  and Q 7 , the calculation operation for the balancing charge Q b  is ended, and the calculated balancing charge Q b  is set to the balancing charge which is to be acquired. 
         [0046]    Then, a strong battery cell is selected to perform a strong balancing mode, or a weak battery cell is selected to perform a weak balancing mode. 
         [0047]    For example, when the charge Q 12  of the weak battery cell (for example, B 12 ) is closer to the balancing charge Q b  than the charge Q 1  of the strong battery cell (for example, B 1 ) as illustrated in  FIG. 8A , the weak balancing mode is selected to perform a balancing operation only until the charge Q 12  of the weak battery cell B 12  reaches the balancing charge Q b . 
         [0048]    For another example, when the charge Q 1  of the strong battery cell B 1  is closer to the balancing charge Q b  than the charge Q 12  of the weak battery cell B 12  as illustrated in  FIG. 8B , the strong balancing mode is selected to perform a balancing operation only until the charge Q 1  of the weak battery cell B 1  reaches the balancing charge Q b . 
         [0049]    When the charges of the battery cells B 1  to B 12  are sorted in descending order as illustrated in  FIG. 7 , it may indicate that the battery cell B 1  stores the highest charge Q 1  and the battery cell B 12  stores the lowest charge Q 12 . 
         [0050]    Thus, when η e (Q 1 -Q b ) obtained by multiplying (Q 1 -Q b ) by the charge transfer efficiency is larger than (Q b -Q 12 ) under the condition where (Q 1 -Q b ) is larger than (Q b -Q 12 ), it corresponds to the case in which the charge Q 12  of the battery cell B 12  is closer to the balancing charge Q b  than the charge Q 1  of the battery cell B 1  as illustrated in  FIG. 8A . Furthermore, when η e (Q 1 -Q b ) is smaller than (Q b -Q 12 ), it corresponds to the case in which the charge Q 1  of the strong battery cell B 1  is closer to the balancing charge Q b  than the charge Q 12  of the weak battery cell B 12  as illustrated in  FIG. 8B . 
         [0051]    According to the comparison result between η e  (Q 1 -Q b ) and (Q b -Q 12 ), a first balancing time t B1  during which the balancing operation is to be performed is calculated through Equation 3 below, and a second balancing time t B2  is calculated through Equation 4 below, at steps ST 10  and ST 11 . 
         [0052]    That is, the balancing time is calculated by dividing the total charge to be transferred by an average current to be transferred or received. When the charge Q 12  of the battery cell B 12  is closer to the balancing charge Q b  than the charge Q 1  of the battery cell B 1  according to the comparison result between η e  (Q 1 -Q b ) and (Q b -Q 12 ), the first balancing time t B1  is calculated through Equation 3 below. 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     
                       B 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   = 
                   
                     
                       
                         Q 
                         b 
                       
                       - 
                       
                         Q 
                         12 
                       
                     
                     
                       
                         η 
                         e 
                       
                        
                       
                         i 
                         
                           S 
                           . 
                           avg 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
         [0053]    Furthermore, when the charge Q 1  of the battery cell B 1  is closer to the balancing charge Q b  than the charge Q 12  of the battery cell B 12  according to the comparison result between η e (Q 1 -Q b ) and (Q b -Q 12 ), the second balancing time t B2  is calculated through Equation 4 below. 
         [0000]    
       
         
           
             
               
                 
                   
                     t 
                     
                       B 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       
                         Q 
                         1 
                       
                       - 
                       
                         Q 
                         b 
                       
                     
                     
                       i 
                       
                         S 
                         . 
                         avg 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
         [0054]    In Equation 3, i S.avg  represents the average balancing current of the strong battery cells, and the current i S.avg  is transferred to a weak battery cell at the charge transfer efficiency of η e . Thus, the current received by the weak battery cell becomes η e i S.avg . 
         [0055]    Then, based on the balancing times t B1  and t B2 , a strong battery cell (for example, B 1 ) and a corresponding weak battery cell (for example, B 12 ) are selected to perform a balancing operation, at step ST 12 . 
         [0056]    At this time, as described with reference to  FIGS. 5 and 6 , the switches are selectively operated to recover (store) the charge stored in the battery cell B 1  into the capacitor of the series resonant circuit during the time t B1  or t B2 , and the recovered (stored) charge is distributed (supplied) to the battery cell B 12  during the time t B1  or t B2 . 
         [0057]    Then, the changed charges Q′ 1  and Q′ 12  of the strong battery cell B 1  and the weak battery cell B 12 , on which the balancing operation was performed, are calculated through Equation 5 below. 
         [0000]    
       
      
       Q 
       1 
       ′=Q 
       1 
       −i 
       S.avg 
       ×t 
       B  
      
     
         [0000]        Q   12   ′=Q   12 +η e   i   S.avg   ×t   B   [Equation 5]
 
         [0058]    In Equation 5, Q 1  represents the previous charge of the strong battery cell, Q′ 1  represents the current charge of the strong battery cell, i S.avg  represents the balancing current, t B  represents a balancing time, Q 12  represents the previous charge of the weak battery cell, Q′ 12  represents the current charge of the weak battery cell, and η e  represents the charge transfer efficiency. 
         [0059]    Then, the updated battery cells are sorted based on the charges, and the strong charge Q s  of the strong battery cell and the weak charge Q w  of the weak battery cell are compared. When the strong charge Q s  of the strong battery cell and the weak charge Q w  of the weak battery cell are determined to be different from each other, the procedure proceeds to the balancing process ST 8  to ST 14 , at steps ST 14  and ST 15 . 
         [0060]    For example, when the battery cells are sorted in order of the charge values as illustrated in  FIG. 7  before the first balancing operation is performed and the strong battery cell B 1  and the weak battery cell B 12  are selected to perform a balancing operation during the balancing process ST 8  to ST 14 , the charge of the weak battery cell B 12  reaches the balancing charge Q b , and the charge value of the strong battery cell B 1  becomes smaller than the charge value of the strong battery cell B 2 . 
         [0061]    Thus, when the second balancing operation is performed, the charge of the strong battery cell B 2  is set to the strong charge Q s , and the charge of the weak battery cell B 11  is set to the weak charge Q w . Then, the balancing process ST 8  to ST 14  is performed. The balancing process is repeated until the strong charge Q s  of the strong battery cell becomes equal to the weak charge Q w  of the weak battery cell. 
         [0062]    When it is determined at step ST 15  that the strong charge Q s  is equal to the weak charge Q w , the standard deviation σ(Q k ) of the battery cells is calculated and compared to the threshold σ th . Then, when the standard deviation σ(Q k ) is equal to or less than the threshold σ th  according to the comparison result, the procedure proceeds to the idle mode, and when the standard deviation σ(Q k ) exceeds the threshold σ th , the procedure proceeds to step ST 4  to perform the above-described process. 
         [0063]    Thus, after the balancing charge Q b  is calculated, N balancing operations may be performed for imbalance among N battery cells using the balancing charge Q b , such that the charges of all the battery cells can accurately reach the condition of balance within a short time. 
         [0064]    According to the embodiment of the present invention, when a balancing operation is performed in the battery cell balancing circuit using LC series resonance, the battery cell balancing method calculates the balancing charge for all battery cells of which charges are not equal to each other, and performs the series of balancing operations using the balancing charge, thereby reducing the balancing time and improving the balancing efficiency. 
         [0065]    While various embodiments have been described above, it will be understood to those skilled in the art that the embodiments described are by way of example only. Accordingly, the disclosure described herein should not be limited based on the described embodiments.