Abstract:
The embodiments of the present disclosure disclose a battery system. The battery system at least comprises a battery pack, an inductor and two sets of switch branches. The battery system controls the inductor to store and release energy, so as to transfer energy between the battery pack and a battery cell or between two battery cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of CN application No. 201110042752.0, filed on Feb. 21, 2011, and incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to battery systems, and more particularly but not exclusively to battery systems with balance function. 
       BACKGROUND 
       [0003]    A battery pack usually comprises several battery cells connected in series. In the battery pack, a cell imbalance may occur due to the differences in the characteristics of the battery cells, such as the charge states, cell capacities, temperature characteristics, etc. This imbalance will shorten the battery life and reduce the capacity of the entire battery pack. 
         [0004]      FIG. 1  schematically shows a prior battery system  10  with a passive balance circuit. As illustrated in  FIG. 1 , bypass resistors and bypass FETs (field effect transistor) are connected to the corresponding battery cells in parallel. When the voltage across one battery cell is higher than that of the rest battery cells, the battery cell with higher voltage is discharged through the corresponding bypass resistors and FET. The battery system  10  of  FIG. 1  can only adjust the battery cell having a higher voltage and its efficiency is low. 
         [0005]      FIG. 2  schematically shows a prior battery system  20  with an active balance circuit. As illustrated in  FIG. 2 , the battery system  20  comprises capacitors coupled between every two adjacent battery cells. The capacitor stores and releases energy to balance the corresponding two adjacent battery cells. The battery system  20  of  FIG. 2  can only balance two adjacent battery cells. Furthermore, the efficiency of the battery system  20  is low since a lot of energy is wasted during the charge of the capacitors. 
         [0006]      FIG. 3  schematically shows another prior battery system  30  with an active balance circuit. As illustrated in  FIG. 3 , the battery system  30  comprises a transformer and energy can be transferred from the battery pack to an individual battery cell through the transformer. However, the size and the cost of the battery system are increased because of the transformer. 
         [0007]      FIG. 4  schematically shows still another prior battery system  40  with an active balance circuit. As illustrated in  FIG. 4 , the battery system  40  comprises several inductors and the battery cell system  40  can work as a buck-boost converter to transfer energy between two adjacent battery cells. The battery system  40  of  FIG. 4  can only balance two adjacent battery cells and its efficiency is limited. 
       SUMMARY 
       [0008]    The present invention is directed to a battery system comprising a battery pack, a first set of N switch branches, a second set of N switch branches, an inductor, a first switch, a second switch, a third switch and a fourth switch. The battery pack comprises N battery cells connected in series, and each of the battery cells has an anode and a cathode. Each switch branch has a first terminal and a second terminal. The first terminals of the first set of N switch branches are coupled together to form a first common node, and the second terminals of the first set of N switch branches are coupled to the anode of the N battery cells respectively. The first terminals of the second set of N switch branches are coupled to the cathode of the N battery cells respectively, and the first terminals of the second set of N switch branches are coupled together to form a second common node. The first switch having a first terminal and a second terminal, wherein the first terminal is coupled to the anode of the battery pack, the second terminal is coupled to the first terminal of the inductor. The second switch having a first terminal and a second terminal, wherein the first terminal is coupled to the first common node, the second terminal is coupled to the second terminal of the inductor. The third switch having a first terminal and a second terminal, wherein the first terminal is coupled to the first terminal of the inductor, the second terminal is coupled to the second common node. The fourth switch having a first terminal and a second terminal, wherein the first terminal is coupled to the second terminal of the inductor, the second terminal is coupled to the cathode of the battery pack. 
         [0009]    The present invention is also directed to a battery system comprising a battery pack, a first set of N+1 switch branches, a second set of N+1 switch branches and an inductor. The battery pack having an anode and a cathode, wherein the battery pack comprises N battery cells connected in series, and each of the battery cells has an anode and a cathode. Each switch branch has a first terminal and a second terminal. The first terminals of the first set of N+1 switch branches are coupled together to form a first common node. The second terminals of the first N switch branches of the first set of N+1 switch branches are coupled to the anode of the N battery cells respectively, and the second terminal of the last switch branch of the first set of N+1 switch branches is coupled to the cathode of the last battery cell. The first terminals of the first N switch branches of the second set of N+1 switch branches are coupled to the anode of the N battery cells respectively, and the first terminal of the last switch branch of the second set of N+1 switch branches is coupled to the cathode of the last battery cell. The second terminals of the second set of N+1 switch branches are coupled together to form a second common node. The inductor has a first terminal and a second terminal, wherein the first terminal of the inductor is coupled to the first common node, and the second terminal of the inductor is coupled to the second common node. 
         [0010]    The present invention is further directed to a battery system with stackable connection comprising M battery balance units and M−1 pairs of diodes. Each pair of the diodes comprises a first diode and a second diode, and wherein the cathode of the first diode is coupled to the anode of the battery pack of the corresponding battery balance unit, and the anode of the first diode is coupled to the second terminal of the inductor of the next battery balance unit, the cathode of the second diode is coupled to the first terminal of the inductor of the corresponding battery balance unit, and the anode of the second diode is coupled to the cathode of the battery pack of the next battery balance unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  schematically shows a prior battery system  10  with a passive balance circuit. 
           [0012]      FIG. 2  schematically shows a prior battery system  20  with an active balance circuit. 
           [0013]      FIG. 3  schematically shows another prior battery system  30  with an active balance circuit. 
           [0014]      FIG. 4  schematically shows still another prior battery system  40  with an active balance circuit. 
           [0015]      FIG. 5  schematically shows a battery system  50  in accordance with an embodiment of the present disclosure. 
           [0016]      FIG. 6   a  shows waveforms of the battery system  50  of  FIG. 5  when energy is transferred from a battery cell to battery pack. 
           [0017]      FIG. 6   b  and  FIG. 6   c  show the operation of the battery system  50  of  FIG. 5  when energy is transferred from a battery cell to the battery pack. 
           [0018]      FIG. 7   a  shows waveforms of the battery system  50  of  FIG. 5  when energy is transferred from the battery pack to a battery cell. 
           [0019]      FIG. 7   b  and  FIG. 7   c  show the operation of the battery system  50  of  FIG. 5  when energy is transferred from the battery pack to a battery cell. 
           [0020]      FIG. 8  schematically shows a battery system with N battery cells in accordance with an embodiment of the present disclosure. 
           [0021]      FIG. 9  schematically shows an improved battery system  90  in accordance with another embodiment of the present disclosure. 
           [0022]      FIG. 10  schematically shows an exemplary battery system  100  in accordance with an embodiment of the present disclosure. 
           [0023]      FIG. 11  schematically shows an improved exemplary battery system  110  in accordance with an embodiment of the present disclosure. 
           [0024]      FIG. 12   a  shows waveforms of the battery system  110  of  FIG. 11  when energy is transferred from the battery pack to a battery cell. 
           [0025]      FIG. 12   b  and  FIG. 12   c  show the operation of the battery system  110  of  FIG. 11  when energy is transferred from the battery pack to a battery cell. 
           [0026]      FIG. 13   a  shows waveforms of the battery system  110  of  FIG. 11  when energy is transferred from a battery cell to the battery pack. 
           [0027]      FIG. 13   b  shows the operation of the battery system  110  of  FIG. 11  when energy is transferred from a battery cell to the battery pack. 
           [0028]      FIG. 14   a  shows waveforms of the battery system  110  of  FIG. 11  when energy is transferred between two battery cells. 
           [0029]      FIG. 14   b  shows the operation of the battery system  110  of  FIG. 11  when energy is transferred between two battery cells. 
           [0030]      FIG. 15  schematically shows a battery system  150  in accordance with an embodiment of the present disclosure. 
           [0031]      FIG. 16  schematically shows an improved battery system  160  in accordance with an embodiment of the present disclosure. 
           [0032]      FIG. 17   a  and  FIG. 17   b  show the operation of the improved battery system  160  of  FIG. 16 . 
           [0033]      FIG. 18  schematically shows a battery system  180  with stackable connection in accordance with an embodiment of the present disclosure. 
           [0034]      FIG. 19  schematically shows an improved battery system  190  with stackable connection in accordance with an embodiment of the present disclosure. 
           [0035]      FIG. 20   a˜d  show the operation of the improved battery system  190  of  FIG. 19  when energy is transferred between two battery packs. 
           [0036]      FIG. 21  schematically shows an improved battery system  210  with stackable connection in accordance with another embodiment of the present disclosure. 
           [0037]      FIG. 22   a˜d  show the operation of the improved battery system  210  of  FIG. 21  when energy is transferred between two battery systems. 
       
    
    
       [0038]    The use of the same reference label in different drawings indicates the same or like components. 
       DETAILED DESCRIPTION 
       [0039]    In the present disclosure, numerous specific details are provided, such as examples of circuits, components, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
         [0040]      FIG. 5  schematically shows a battery system  50  in accordance with an embodiment of the present disclosure. In the example of  FIG. 5 , the battery system  50  comprises an inductor L 1 , switches M 1 ˜M 4 , a first set of switch branches  5011 , a battery pack  5012  and a second set of switch branches  5013 . The first set of switch branches  5011  comprises switch branches S 1 ˜S 4 , the battery pack  5012  comprises battery cells C 1 ˜C 4  connected in series, and the second set of switch branches  5013  comprises switch branches S 5 ˜S 8 . Each of the switch branches S 1 ˜ 58  has a first terminal and a second terminal. The first terminals of the switch branches S 1 ˜S 4  are connected together to form a first common node. The second terminals of the switch branches S 1 ˜ 54  are respectively coupled to the anodes of the battery cells C 1 ˜C 4 . The first terminals of the switch branches S 5 ˜S 8  are respectively coupled to the cathodes of the battery cells C 1 ˜C 4  and the second terminals are connected together to form a second common node. The inductor L 1  has a first terminal and a second terminal. The switch M 1  is coupled between the first terminal of the inductor L 1  and the anode of the battery pack  5012 . The switch M 2  is coupled between the second terminal of the inductor L 1  and the first common node. The switch M 3  is coupled between the first terminal of the inductor L 1  and the second common node. The switch M 4  is coupled between the second terminal of the inductor L 1  and the cathode of the battery pack  5012 . 
         [0041]    In one embodiment, the battery system  50  further comprises diodes D(M 1 )˜D(M 4 ) which are respectively coupled to the switches M 1 ˜M 4  in parallel. The cathode of the diode D(M 1 ) is coupled to the anode of the battery pack  5012 , and the anode of the diode D(M 1 ) is coupled to the first terminal of the inductor L 1 . The cathode of the diode D(M 2 ) is coupled to the first common node, and the anode of the diode D(M 2 ) is coupled to the second terminal of the inductor L 1 . The cathode of the diode D(M 3 ) is coupled to the first terminal of the inductor L 1 , and the anode of the diode D(M 3 ) is coupled to the second common node. The cathode of the diode D(M 4 ) is coupled to the second terminal of the inductor L 1 , and the anode of the diode D(M 4 ) is coupled to the cathode of the battery pack  5012 . In one embodiment, the diodes D(M 1 )˜D(M 4 ) are the body diodes of the switches M 1 ˜M 4 . 
         [0042]    The operation of the battery system  50  is now explained with reference to  FIG. 6˜FIG .  7 . 
         [0000]    When Energy is Transferred from a Battery Cell to the Battery Pack 
         [0043]    The operation of the battery system  50  when energy is transferred from a battery cell to the battery pack  5012  is now explained with reference to  FIG. 6   a ˜ FIG. 6   c . The battery system  50  transfers energy from a battery cell to the battery pack  5012  when the voltage of the battery cell is higher than the voltages of the rest battery cells. Take the battery cell C 2  for example, when the voltage of C 2  is higher than the voltages of the battery cells C 1 , C 3  and C 4 , the switch branches S 2  and S 6  are turned on (i.e., closed) and the switch branches S 1 , S 3 ˜S 5 , S 7  and S 8  are turned off (i.e., opened) to select the battery cell C 2 . Meanwhile, the switches M 1  and M 4  are turned off, and the switches M 2  and M 3  are turned on and off synchronously with a constant frequency and a constant duty cycle. 
         [0044]    When the switches M 2  and M 3  are turned on, the operation of the battery system  50  is illustrated in  FIG. 6   b . Current flows from the anode of the battery cell C 2 , through the switch M 2 , the inductor L 1  and the switch M 3 , and into the cathode of the battery cell C 2 . Thereby, energy is stored in the inductor L 1 . As shown in  FIG. 6   a , the voltage V L1  of the inductor L 1  is equal to the voltage V C2  of the battery cell C 2 , i.e., V L1 =V C2 . The inductor current i L1  of the inductor L 1  begins to increase and can be represented by the expression 
         [0000]    
       
         
           
             
               
                 di 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 D 
                 · 
                 T 
               
             
             = 
             
               
                 V 
                 
                   C 
                    
                   
                       
                   
                    
                   2 
                 
               
               
                 L 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0000]    , wherein T represents the switching period of the switches M 2  and M 3 , and D is the duty cycle. 
         [0045]    When the switches M 2  and M 3  are turned off, the operation of the battery system  50  is illustrated in  FIG. 6   c . The inductor L 1  begins to release energy with current flowing through the diode D(M 1 ), the battery pack  5012  and the diode D(M 4 ). As shown in  FIG. 6   a , the voltage V L1  of the inductor L 1  and the voltage of the battery pack  5012  have a relationship expressed by V L1 =−V PACK . The inductor current i L1  begins to decrease and can be represented by the expression 
         [0000]    
       
         
           
             
               
                 di 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 
                   ( 
                   
                     1 
                     - 
                     D 
                   
                   ) 
                 
                 · 
                 T 
               
             
             = 
             
               
                 
                   V 
                   PACK 
                 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
         [0000]    When Energy is Transferred from the Battery Pack to a Battery Cell 
         [0046]    The operation of the battery system  50  when energy is transferred from the battery pack  5012  to a battery cell is now explained with reference to  FIG. 7   a ˜ FIG. 7   c . The battery system  50  transfers energy from the battery pack  5012  to a battery cell when the voltage of the battery cell is lower than the voltages of the rest battery cells. Take the battery cell C 2  for example, when the voltage of C 2  is lower than the voltages of the battery cells C 1 , C 3  and C 4 , the switch branches S 2  and S 6  are turned on and the switch branches S 1 , S 3 ˜S 5 , S 7  and S 8  are turned off to select the battery cell C 2 . Meanwhile, the switches M 2  and M 3  are turned off, and the switches M 1  and M 4  are turned on and off synchronously with a constant frequency and a constant duty cycle. 
         [0047]    When the switches M 1  and M 4  are turned on, the operation of the battery system  50  is illustrated in  FIG. 7   b . Current flows from the anode of the battery pack  5012 , through the switch M 1 , the inductor L 1  and the switch M 4 , and into the cathode of the battery pack  5012 . Thereby, energy is stored in the inductor L 1 . As shown in  FIG. 7   a , the voltage V L1  of the inductor L 1  is equal to the voltage V PACK  of the battery pack  5012 , i.e., V L1 =V PACK . The inductor current i L1  of the inductor L 1  begins to increase and can be represented by the expression 
         [0000]    
       
         
           
             
               
                 di 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 D 
                 · 
                 T 
               
             
             = 
             
               
                 
                   V 
                   PACK 
                 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
         [0048]    When the switches M 1  and M 4  are turned off, the operation of the battery system  50  is illustrated in  FIG. 7   c . The inductor L 1  begins to release energy with current flowing through the diode D(M 2 ), the battery cell C 2  and the diode D(M 3 ). As shown in  FIG. 7   a , the voltage V L1  of the inductor L 1  and the voltage of the battery pack  5012  have a relationship expressed by V L1 =−V PACK . The inductor current i L1  begins to decrease and can be represented by the expression 
         [0000]    
       
         
           
             
               
                 di 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 
                   ( 
                   
                     1 
                     - 
                     D 
                   
                   ) 
                 
                 · 
                 T 
               
             
             = 
             
               
                 
                   V 
                   PACK 
                 
                 
                   L 
                    
                   
                       
                   
                    
                   1 
                 
               
               . 
             
           
         
       
     
         [0049]    As described above, the battery system  50  in accordance with an embodiment of the present disclosure can be employed to transfer energy between the battery pack and a battery cell. It is more effective compared with a prior battery system. 
         [0050]    In the examples of  FIG. 5˜FIG .  7 , the operation of the battery system  50  is explained with the voltage of the battery cell C 2  being higher or lower than that of the rest battery cells. As can be appreciated, the battery system  50  works in a similar way when the voltage of any one of the battery cells C 1 ˜C 4  is higher or lower than that of the rest battery cells. 
         [0051]    In the examples of  FIG. 5˜FIG .  7 , the battery pack comprises 4 battery cells. However, persons of ordinary skill in the art can appreciate that in another embodiment, the battery pack may comprise N battery cells, where N is an integer greater than 2. 
         [0052]      FIG. 8  schematically shows a battery system with N battery cells in accordance with an embodiment of the present disclosure. In the embodiment of  FIG. 8 , the first set of switch branches  8011  comprises N switch branches S( 2 ), S( 4 ), . . . , S(2N−2), S(2N), the second set of switch branches  8013  comprises N switch branches S( 1 ), S( 3 ), . . . , S(2N−3), S(2N−1). Each of the switch branches S 1 ˜S(2N) has a first terminal and a second terminal. The first terminals of the switch branches S( 2 ), S( 4 ), . . . , S(2N) are coupled together to form a first common node. The second terminals of the switch branches S( 2 ), S( 4 ), . . . , S(2N) are respectively coupled to the anode of the battery cells C 1 ˜C(N). The first terminals of the switch branches S( 1 ), S( 3 ), . . . , S(2N−3), S(2N−1) are respectively coupled to the cathode of the battery cells C 1 ˜C(N) and the second terminals are coupled together to form a second common node. In one embodiment, each of the switch branches S 1 ˜S(2N) may comprise a MOSFET and a diode serially connected to the MOSFET, or comprise a transistor or two MOSFETs connected back to back. 
         [0053]      FIG. 9  schematically shows an improved battery system  90  in accordance with an embodiment of the present disclosure. The battery system  90  comprises a first set of switch branches  9011 , a battery pack  9012 , a second set of switch branches  9013  and an inductor L 1 . Further, the first set of switch branches  9011  comprises switch branches S 1 ˜S 7 , the battery pack  9012  has an anode and a cathode, and the battery pack  9012  comprises battery cells C 1 ˜C 6  connected in series, and the second set of switch branches  9013  comprises switch branches S 8 ˜S 14 . Each of the switch branches S 1 ˜S 14  has a first terminal and a second terminal. The first terminals of the switch branches S 1 ˜S 7  are connected together to form a first common node. The second terminals of the switch branches S 1 ˜S 6  are respectively coupled to the anodes of the battery cells C 1 ˜C 6 , the second terminal of the switch branch S 7  is coupled to the cathode of the battery cell C 6 . The first terminals of the switch branches S 8 ˜S 13  are respectively coupled to the anodes of the battery cells C 1 ˜C 6 , the first terminal of the switch branch S 14  is coupled to the cathode of the battery cell C 6 , and the second terminals of the switch branches S 8 ˜S 14  are connected together to form a second common node. The inductor L 1  has a first terminal and a second terminal. The first terminal of the inductor L 1  is coupled to the first common node, and the second terminal of the inductor L 1  is coupled to the second common node. 
         [0054]      FIG. 10  schematically shows an exemplary battery system  100  in accordance with an embodiment of the present disclosure. In the exemplary battery system  100  of  FIG. 10 , each of the switch branches S 1 ˜S 14  comprises a MOSFET and a diode connected in series. Referring to  FIG. 10 , the switch branch S 1  comprises a MOSFET M 1  and a diode D 1 . The MOSFET M 1  has a drain terminal, a source terminal and a gate terminal, and the diode D 1  has an anode and a cathode. The anode of the diode D 1  is coupled to the source terminal of the MOSFET M 1 . The cathode of the diode D 1  is configured as the first terminal of the switch branch S 1 . The drain terminal of the MOSFET M 1  is configured as the second terminal of the switch branch S 1 . The switch branches S 2 ˜S 7  are configured in the same way as the switch branch S 1  is. The switch branch S 8  comprises a MOSFET M 8  and a diode D 8 . The MOSFET M 8  has a drain terminal, a source terminal and a gate terminal, and the diode D 8  has an anode and a cathode. The cathode of the diode D 8  is coupled to the drain terminal of the MOSFET M 8 . The anode of the diode D 8  is configured as the second terminal of the switch branch S 8 . The source terminal of the MOSFET M 8  is configured as the first terminal of the switch branch S 8 . The switch branches S 9 ˜S 14  are configured in the same way as the switch branch S 8  is. 
         [0055]      FIG. 11  schematically shows an improved exemplary battery system  110  in accordance with an embodiment of the present disclosure. Compared to the battery system  100  of  FIG. 10 , the diodes D 1  and D 14 , and the MOSFETs M 7  and M 8  are removed in the battery system  110  of  FIG. 11 . Since fewer components are used in the battery system  110 , the size and cost of the system are both reduced and the efficiency is improved. Persons of ordinary skill in the art can appreciate that, in effect, battery systems  90 ,  100  and  110  operate in the similar way. The operation of these battery systems will be explained with reference to battery system  110  of  FIG. 11 . 
         [0000]    When Energy is Transferred from the Battery Pack to a Battery Cell 
         [0056]    The operation of the battery system  110  when energy is transferred from the battery pack to a battery cell is now explained with reference to  FIG. 12   a ˜ FIG. 12   c . The battery system  110  transfers energy from the battery pack to a battery cell when the voltage of the battery cell is lower than that of the rest battery cells. Take the battery cell C 3  for example, when the voltage of C 3  is lower than the voltages of the rest battery cells, the MOSFETs M 4  and M 10  are turned on, the MOSFETs M 1  and M 14  are turned on and off synchronously with a constant frequency and a constant duty cycle, and the rest MOSFETs are turned off. 
         [0057]    When the MOSFETs M 1  and M 14  are turned on, the operation of the battery system  110  is illustrated in  FIG. 12   b . Current flows from the anode of the battery pack  1112 , through the MOSFET M 1 , the inductor L 1  and the MOSFET M 14 , and into the cathode of the battery pack  1112 . Thereby, energy is stored in the inductor L 1 . The voltage V L1  of the inductor L 1  is equal to the voltage V PACK  of the battery pack  1112 , i.e., V L1 =V PACK . The inductor current i L1  of the inductor L 1  begins to increase. 
         [0058]    When the MOSFETs M 1  and M 14  are turned off, the operation of the battery system  110  is illustrated in  FIG. 12   c . The inductor L 1  begins to release energy, with current flowing through the diode D 10 , the MOSFET M 10 , the battery cell C 3 , the MOSFET M 4  and the diode D 4 . The voltage V L1  of the inductor L 1  and the voltage of the battery cell C 3  have a relationship expressed by V L1 =−V C3 . 
         [0000]    When Energy is Transferred from a Battery Cell to the Battery Pack 
         [0059]    The operation of the battery system  110  when energy is transferred from a battery cell to the battery pack is now explained with reference to  FIG. 13   a ˜ FIG. 13   b . The battery system  110  transfers energy from a battery cell to the battery pack when the voltage of the battery cell is higher than that of the rest battery cells. Take the battery cell C 2  for example, when the voltage of C 2  is higher than the voltages of the rest battery cells, the MOSFETs M 2  and M 10  are turned on and off synchronously with a constant frequency and a constant duty cycle, and the rest MOSFETs are turned off. When the MOSFETs M 2  and M 10  are turned on, the operation of the battery system  110  is illustrated in  FIG. 13   a . Current flows from the anode of the battery cell C 2 , through the MOSFET M 2 , the diode D 2 , the inductor L 1 , the diode D 10  and the MOSFET M 10 , and into the. Thereby, energy is stored in the inductor L 1 . The voltage V L1  of the inductor L 1  equals to the voltage V C2  of the battery cell C 2 , i.e., V L1 =V C2 . The inductor current i L1  of the inductor L 1  begins to increase. When the MOSFETs M 2  and M 10  are turned off, the operation of the battery system  110  is illustrated in  FIG. 13   b . The inductor L 1  begins to release energy, with current flowing through the diode D 8 , the battery pack  1112  and the diode D 7 . The voltage V L1  of the inductor L 1  and the voltage of the battery pack  1112  have a relationship expressed by V L1 =−V PACK . 
         [0000]    When Energy is Transferred from a Battery Cell to Another Battery Cell 
         [0060]    The operation of the battery system  110  when energy is transferred from a battery cell to another battery cell is now explained with reference to  FIG. 14   a ˜ FIG. 14   b . The battery system  110  transfers energy from a battery cell to another battery cell when the voltage of one battery cell is higher than that of the rest battery cells and the voltage of another battery cell is lower than that of the rest battery cells. Take the battery cells C 2  and C 5  for example, when the voltage of C 2  is higher than the voltages of the rest battery cells and the voltage of C 5  is lower than the voltages of the rest battery cells, the MOSFETs M 2  and M 10  are turned on first, and the rest MOSFETs are turned off to choose the battery cell C 2 . When the MOSFET M 2  and M 10  are turned on, the operation of the battery system  110  is illustrated in  FIG. 14   a . Current flows out from the anode of the battery cell C 2 , through the MOSFET M 2 , the diode D 2 , the inductor L 1 , the diode D 10  and the MOSFET M 10 . Thereby, energy is stored in the inductor L 1 . The voltage V L1  of the inductor L 1  equals to the voltage V C2  of the battery cell C 2 , i.e., V L1 =V C2 . The inductor current i L1  of the inductor L 1  begins to increase. 
         [0061]    The MOSFETs M 2  and M 10  are turned on until the voltage of C 2  is equal to the voltages of the rest battery cells. Then, as illustrated in  FIG. 14   b , the MOSFETs M 2  and M 10  are turned off, the MOSFETs M 6  and M 12  are turned on, and the rest MOSFETs are turned off. The inductor L 1  begins to release energy, with current flowing through the diode D 12 , the MOSFET M 12 , the battery cell C 5 , the MOSFET M 6  and the diode D 6 . The voltage V L1  of the inductor L 1  and the voltage of the battery cell C 5  have a relationship expressed by V L1 =−V C5 . 
         [0062]    Complementary charge is to charge each battery cell to a common full voltage in the charge stage, i.e., to charge each battery cell to a balance state in the charge stage. 
         [0063]      FIG. 15  schematically shows a battery system  150  in accordance with an embodiment of the present disclosure. Compared to the battery system  100  of  FIG. 10 , a voltage source V C  and a MOSFET M C  are added in the battery system  150  of  FIG. 15 . Referring to  FIG. 15 , the voltage source V C  and the MOSFET M C  are connected in series, and they are connected with the inductor L 1  in parallel. In detail, the voltage source V C  has an anode and a cathode, and the MOSFET M C  has a source terminal, a drain terminal and a gate terminal. The anode of the voltage source V C  is coupled to the first terminal of the inductor L 1 . The cathode of the voltage source V C  is coupled to the source terminal of the MOSFET M C . And the drain terminal of the MOSFET M C  is coupled to the second terminal of the inductor L 1 . In the example of  FIG. 15 , the voltage source V C  is employed to supply power to the battery system  150 . The MOSFET M C  is configured to control the voltage source V C  to charge each battery cell. Switch branches are configured to select the battery cell to be charged complementarily. 
         [0064]    In the example of  FIG. 15 , a MOSFET M C  is used to control the voltage source V C  to charge the battery cells. However, persons of ordinary skill in the art can appreciate that other forms of switch can also be used to realize the function. 
         [0065]      FIG. 16  schematically shows an improved battery system  160  in accordance with an embodiment of the present disclosure. Compared to the battery system  150  of  FIG. 15 , the diodes D 1  and D 14 , and the MOSFETs M 1 , M 7 , M 8  and M 14  are removed in the battery system  160  of  FIG. 16 . Since fewer components are used in the battery system  160 , the size and cost of the system are both reduced and the efficiency is improved. Persons of ordinary skill in the art can appreciate that, in effect, the battery systems  150  and  160  operate in the similar way. The operation of these battery systems will be explained with reference to the battery system  160  of  FIG. 16 . 
         [0066]    The operation of the battery system  160  is now explained with reference to  FIG. 17   a  and  FIG. 17   b . The battery system  160  charges a battery cell in the charge stage when the battery cell is not charged to a common full voltage. Take the battery cell C 1  for example, when the voltage of C 1  is lower than the common full voltage, the MOSFET M 2  is turned on, the MOSFET M C  is turned on and off with a constant frequency and a constant duty cycle, and the rest MOSFETs are turned off. 
         [0067]    When the MOSFET M C  is turned on, the operation of the battery system  160  is illustrated in  FIG. 17   a . Current flows from the voltage source V C , through the inductor L 1  and the MOSFET M C . Thereby, energy is stored in the inductor L 1 . When the MOSFET M C  is turned off, the operation of the battery system  160  is illustrated in  FIG. 17   b . The inductor L 1  begins to release energy, with current flowing through the diode D 8 , the battery cell C 1 , the MOSFET M 2  and the diode D 2 . 
         [0068]    The same explanation applies to other battery cells which need to be charged to the common full voltage. The complementary charge is completed when all the battery cells in the battery pack are charged to the common full voltage in the charge stage. 
         [0069]    In some applications, the battery pack may comprise a great number of battery cells (such as 100 battery cells). In these cases, the balance speed of the battery system is limited since there is only one inductor in the battery system. Besides, the MOSFETs and diodes used in the battery system are required to have a high rated voltage, which increases the cost of the system. For example, when the battery pack comprises 24 battery cells and the full voltage of each battery cell is 3.8V, the rated voltage of each MOSFET and diode is (24−1)*3.8=87.4V. 
         [0070]    To solve the problem mentioned above, the present disclosure provides an improved battery system with stackable connection.  FIG. 18  schematically shows a battery system  180  with stackable connection in accordance with an embodiment of the present disclosure. In the example of  FIG. 18 , the battery system  180  comprises 3 battery systems  100  (labeled as “P 1 ”, “P 2 ” and “P 3 ” respectively in  FIG. 18 , hereinafter referred to as battery balance unit) of  FIG. 10  and diodes D(A 1 )˜D(A 4 ). Each of the diodes D(A 1 )˜D(A 4 ) has an anode and a cathode. The anode of the diode D(A 1 ) is coupled to the second terminal of the inductor L 2  in the battery balance unit P 2 , and the cathode of the diode D(A 1 ) is coupled to the anode of the battery pack in the battery balance unit P 1 . The diode D(A 1 ) is used to transfer energy from the battery balance unit P 2  to P 1 . The anode of the diode D(A 2 ) is coupled to the cathode of the battery pack in the battery balance unit P 2 , and the cathode of the diode D(A 2 ) is coupled to the first terminal of the inductor L 1  in the battery balance unit P 1 . The diode D(A 2 ) to transfer energy from the battery balance unit P 1  to P 2 . The anode of the diode D(A 3 ) is coupled to the cathode of the battery pack in the battery balance unit P 3 , and the cathode of the diode D(A 3 ) is coupled to the first terminal of the inductor L 2  in the battery balance unit P 2 . The diode D(A 3 ) is used to transfer energy from the battery balance unit P 2  to P 3 . The anode of the diode D(A 4 ) is coupled to the second terminal of the inductor L 3  in the battery balance unit P 3 , and the cathode of the diode D(A 4 ) is coupled to the anode of the battery pack in the battery balance unit P 2 . The diode D(A 4 ) is used to transfer energy from the battery balance unit P 3  to P 2 . 
         [0071]      FIG. 19  schematically shows an improved battery system  190  with stackable connection in accordance with an embodiment of the present disclosure. Compared to the battery system  180  of  FIG. 18 , the MOSFETs M 1 -(N+2) and M 3 -(N+1) and diodes D 1 - 1 , D 1 -(2N+2), D 2 - 1 , D 2 -(2N+2), D 3 - 1  and D 3 -(2N+2) are removed in the battery system  190  of  FIG. 19 . Since fewer components are used in the battery system  190 , the size and cost of the system are both reduced and the efficiency is improved. Persons of ordinary skill in the art can appreciate that, in effect, battery systems  180  and  190  operate in the similar way. The operation of these battery systems will be explained with reference to battery system  190  of  FIG. 19 , when energy is transferred from the battery balance unit P 2  to the battery balance units P 1  and P 3 . 
         [0072]    When energy is transferred from the battery balance unit P 2  to P 1 , the MOSFET M 2 - 1  is turned on, the MOSFET M 2 -(2N+2) is turned on and off with a constant frequency and a constant duty cycle, and other MOSFETs are turned off. When the MOSFET M 2 -(2N+2) is turned on, the operation of the battery system  190  is illustrated in  FIG. 20   a . Current flows from the anode of the battery pack in the battery balance unit P 2 , through the MOSFET M 2 - 1 , the inductor L 2 , and the MOSFET M 2 - 2 (N+2), and into the cathode of the battery pack in the battery balance unit P 2 . Thereby, energy is stored in the inductor L 2 . Then, as illustrated in  FIG. 20   b , the MOSFET M 2 -(2N+2) is turned off. The inductor L 2  begins to release energy, with current flowing through the diode D(A 1 ), the battery pack in the battery balance unit P 1  and the MOSFET M 2 - 1 . 
         [0073]    When energy is transferred from the battery balance unit P 2  to P 3 , the MOSFET M 2 -(2N+2) is turned on, the MOSFET M 2 - 1  is turned on and off with a constant frequency and a constant duty cycle, and other MOSFETs are turned off. When the MOSFET M 2 - 1  is turned on, the operation of the battery system  190  is illustrated in  FIG. 20   c . Current flows from the anode of the battery pack in the battery balance unit P 2 , through the MOSFET M 2 - 1 , the inductor L 2 , and the MOSFET M 2 - 2 (N+2), and into the cathode of the battery pack in the battery balance unit P 2 . Thereby, energy is stored in the inductor L 2 . Then, as illustrated in  FIG. 20   d , the MOSFET M 2 - 1  is turned off. The inductor L 2  begins to release energy, with current flowing through the MOSFET M 2 -(2N+2), the battery pack in the battery balance unit P 3  and the diode D(A 3 ). The battery system  190  of  FIG. 19  solves the problems when the battery pack comprises a great number of battery cells, but it still has a high requirement for the rated voltages of MOSFETs. For example, when energy is transferred from the battery balance unit P 2  to P 1 , the diode D(A 1 ) is turned on, the voltage of node B (see  FIG. 20   b ) is equal to the voltage V PACK1+  at the anode of the battery pack in the battery balance unit P 1 . Node B is connected to the anodes of the diodes D 2 -(N+2), D 2 -(N+3), . . . , D 2 -(2N+1), D 2 -(2N+2), and the voltage V PACK1+  is higher than the voltage of the cathodes of the diodes D 2 -(N+2), D 2 -(N+3), . . . , D 2 -(2N+1), D 2 -(2N+2). Thereby, the diodes D 2 -(N+2), D 2 -(N+3), . . . , D 2 -(2N+1), D 2 -(2N+2) are turned on, and there is a high voltage stress across the MOSFETs M 2 -(N+2), M 2 -(N+3), . . . , M 2 -(2N+1), M 2 -(2N+2). The same explanation applies when energy is transferred from the battery balance unit P 2  to P 3 . 
         [0074]      FIG. 21  schematically shows an improved battery system  210  with stackable connection in accordance with another embodiment of the present disclosure. Compared to the battery system  190  of  FIG. 19 , the rated voltages of the diodes in the battery system  210  of  FIG. 21  are relatively high and the rated voltages of the MOSFETs are relatively low. The cost of the battery system  210  is reduced since the diodes is much cheaper than the MOSFETs with the same rated voltage. Compared with the battery system  180  of  FIG. 18 , the diodes D 1 -(N+1), D 1 -(N+2), D 2 -(N+1), D 2 -(N+2), D 3 -(N+1) and D 3 -(N+2) are removed in the battery system  210 . In the battery system  210  of  FIG. 21 , the MOSFETs M 1 -(N+1) and M 1 -(N+2) are used to release the energy of the battery pack in the battery balance unit P 1 , the MOSFETs M 2 -(N+1) and M 2 -(N+2) are used to release the energy of the battery pack in the battery balance unit P 2 , and the MOSFETs M 3 -(N+1) and M 3 -(N+2) are used to release the energy of the battery pack in the battery balance unit P 3 . The diode D(A 1 ) is used to transfer energy from the battery balance unit P 2  to the battery balance unit P 1 , the diode D(A 2 ) is used to transfer energy from the battery balance unit P 3  to the battery balance unit P 2 , and the diode D(A 3 ) is used to transfer energy from the battery balance unit P 2  to the battery balance unit P 3 . 
         [0075]    When energy is transferred from the battery balance unit P 2  to P 1 , the MOSFET M 2 -(N+2) is turned on, the MOSFET M 2 -(N+1) is turned on and off with a constant frequency and a constant duty cycle, and other MOSFETs are turned off. When the MOSFET M 2 -(N+1) is turned on, the operation of the battery system  210  is illustrated in  FIG. 22   a . Current flows from the anode of the battery pack in the battery balance unit P 2 , through the MOSFET M 2 -(N+2), the inductor L 2 , and the MOSFET M 2 -(N+1), and back into the cathode of the battery pack in the battery balance unit P 2 . Thereby, energy is stored in the inductor L 2 . Then, as illustrated in  FIG. 22   b , the MOSFET M 2 -(N+1) is turned off. The inductor L 2  begins to release energy, with current flowing through the diode D(A 1 ), the battery pack in the battery balance unit P 1  and the MOSFET M 2 -(N+2). 
         [0076]    When energy is transferred from the battery balance unit P 2  to P 3 , the MOSFET M 2 -(N+1) is turned on, the MOSFET M 2 -(N+2) is turned on and off with a constant frequency and a constant duty cycle, and other MOSFETs are turned off. When the MOSFET M 2 -(N+2) is turned on, the operation of the battery system  210  is illustrated in  FIG. 22   c . Current flows from the anode of the battery pack in the battery balance unit P 2 , through the MOSFET M 2 -(N+2), the inductor L 2 , and the MOSFET M 2 -(N+1), and into the cathode of the battery pack in the battery balance unit P 2 . Thereby, energy is stored in the inductor L 2 . Then, as illustrated in  FIG. 22   d , the MOSFET M 2 -(N+2) is turned off. The inductor L 2  begins to release energy, with current flowing through the MOSFET M 2 -(N+1), the battery pack in the battery balance unit P 3  and the diode D(A 4 ). 
         [0077]    When energy is transferred from the battery balance unit P 2  to P 1 , the voltage of node A (see  FIG. 22   b ) equals to the voltage V PACK1+  of the anode of the battery pack in the battery balance unit P 1 . Node A is connected to the cathodes of diodes D 2 - 1 , D 2 - 2 , . . . , D 2 -(N), D 2 -(N+1), and the voltage V PACK1+  is higher than the voltages of the anodes of diodes D 2 - 1 , D 2 - 2 , . . . , D 2 -(N), D 2 -(N+1). Thereby, there is a high voltage stress across the diodes D 2 - 1 , D 2 - 2 , . . . , D 2 -(N), D 2 -(N+1). The same explanation applies when energy is transferred from the battery balance unit P 2  to P 3 . As can be seen from above description, the diodes of the battery system  190  resist a relatively high voltage stress, and the rated voltages of MOSFETs can be relatively low expect the MOSFETs M 3 -(N+2), M 2 -(N+1), M 2 -(N+2) and M 3 -(N+1). 
         [0078]    While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.