BACKGROUND 
       [0001]    A lithium-ion battery typically includes an anode, cathode and electrolyte. Lithium ions move from the anode to the cathode during discharge and from the cathode to the anode during charge. 
         [0002]    Graphite may be used for the anode. A layered oxide (lithium cobalt oxide), polyanion (lithium iron phosphate) or spinel (lithium manganese oxide) may be used for the cathode. Other materials may also be used. Depending on the choice of material for the anode, cathode and electrolyte, the voltage, capacity and life of the lithium-ion battery may change. 
         [0003]    Lithium-ion batteries may be electrically connected in series to form a battery pack for an automotive vehicle. Power from such a battery pack may be used to generate motive power, via an electric machine, to move the vehicle. This use of the battery pack may result in charge imbalances among the batteries. 
       SUMMARY 
       [0004]    A vehicle power system may include a plurality of series connected power storage units configured to supply power to move a vehicle, and a transformer including a plurality of primary windings and a secondary winding. Each of the primary windings may be configured to be selectively electrically connected with a respective one of a set of the power storage units. The secondary winding may be electrically connected with a different set of the power storage units. 
         [0005]    A vehicle power system may include a plurality of charge balancing modules, each including a plurality of series connected power storage units configured to supply power to move a vehicle, and at least one transformer including a plurality of primary coils and a secondary coil. Each of the primary coils may be configured to be selectively electrically connected with at least one of the power storage units via at least one switch. The secondary coil may be electrically connected with power storage units of another of the charge balancing modules. Each of the charge balancing modules may further include circuitry configured to selectively activate the at least one switch. 
         [0006]    A vehicle power system may include a plurality of series connected power storage units configured to supply power to move a vehicle, and at least one flyback switch mode converter having a plurality of primary windings and a single secondary winding. Each of the primary windings may be configured to be selectively electrically connected with a respective one of a set of the power storage units. The secondary winding may be electrically connected with a different set of the power storage units. 
         [0007]    While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of an embodiment of an automotive vehicle. 
           [0009]      FIG. 2  is a block diagram of an embodiment of the traction battery pack of  FIG. 1 . 
           [0010]      FIG. 3  is a schematic diagram of the traction battery pack of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Referring now to  FIG. 1 , an embodiment of an automotive vehicle  10  may include a traction battery pack  12 , electric machine  14  and tire/wheel assemblies  16 . As known in the art, electrical energy stored in the battery pack  12  may be converted to mechanical energy by the electric machine  14  to move the tire/wheel assemblies  16  (and thus the vehicle  10 ); mechanical energy generated during braking events may be converted to electrical energy by the electric machine  14  and stored in the battery pack  12 . Of course, the vehicle  10  may also include an engine and/or fuel cell stack, as well as other related powertrain components (not shown) adapted to move the tire/wheel assemblies  16  as known in the art. 
         [0012]    Referring now to  FIG. 2 , an embodiment of the battery pack  12  includes a plurality of cell modules  18   n  ( 18   a ,  18   b ,  18   c ). Each of the cell modules  18   n  includes, inter alia, a plurality of power storage units, (e.g., lithium-ion batteries, etc.), electrically connected in series. (The cell modules  18   n  are also electrically connected in series.) Other cell/module arrangements, however, are also possible. As explained below, a particular power storage unit of one of the cell modules  18   n  (such as the cell module  18   a ) may be electrically connected with power storage units of another of the cell modules  18   n  (such as the cell module  18   b ) such that energy from the power storage unit of the cell module  18   a  may be transferred to the power storage units of the cell module  18   b  to effectuate cell balancing. 
         [0013]    Referring now to  FIG. 3 , an embodiment of the cell module  18   a  includes a plurality of power storage units  20   n  ( 20   a - 20   h ). In the embodiment of  FIG. 3 , the power storage units  20   n  are lithium-ion batteries. Any suitable power storage units (e.g., capacitors, nickel metal hydride batteries, etc.) in any suitable quantities, however, may be used. As discussed above, the power storage units  20   n  are electrically connected in series. 
         [0014]    The cell module  18   a  may also include transformers  22   a ,  22   b . The transformer  22   a  includes a plurality of primary windings  26   a - 26   d , a core  28   a , and secondary winding  30   a . Likewise, the transformer  22   b  includes a plurality of primary windings  26   e - 26   h , a core  28   b , and secondary winding  30   b . A primary winding is associated with each of the power storage units. (That is, the number of power storage units  20   n  matches the number of primary windings  26   n .) In other embodiments, however, these numbers need not match. For example, every two power storage units may be associated with a primary winding, etc. As discussed below, the transformer  22   a  may be electrically connected with the power storage units  20   a - 20   d ; the transformer  22   b  may be electrically connected with the power storage units  20   e - 20   h.    
         [0015]    The cell module  18   a  may further include a plurality of electrical switches  32   n  ( 32   a - 32   d ),  34   n  ( 34   a - 34   d ),  36   n  ( 36   a - 36   d ),  38   n  ( 38   a - 38   d ). The electrical switches  32   n ,  34   n  may electrically connect the power storage units  20   a - 20   d  with the transformer  22   a . The electrical switches  36   n ,  38   n  may electrically connect the power storage units  20   e - 20   h  with the transformer  22   b . In the embodiment of  FIG. 3 , the electrical switches  32   n ,  34   n  are p-type MOSFETS and the electrical switches  36   n ,  38   n  are n-type MOSFETS. Of course, any suitable electrical switching type/arrangement may be used. 
         [0016]    The secondary windings  30   a ,  30   b  may be electrically connected with power storage units of other cell modules  18   n . Similarly, the power storage units  20   n  may be electrically connected with a secondary winding of another cell module  18   n , etc. In the embodiment of  FIG. 3 , the secondary winding  30   a  is electrically connected with power storage units of the cell module  18   b , and the secondary winding  30   b  is electrically connected with power storage units of the cell module  18   c . In other embodiments, the secondary windings  30   a ,  30   b  may be electrically connected with power storage units of all the cell modules  18   n ; the secondary windings  30   a ,  30   b  may be electrically connected with power storage units of the same cell module  18   n , etc. 
         [0017]    Diodes  40   a ,  40   b  and capacitors  42   a ,  44   b  may be associated with the electrical connections between the secondary windings  30   a ,  30   b  and the cell modules  18   b ,  18   c . As known in the art, the diode  40   a  prevents current from flowing through the secondary winding  30   a  while current is ramping up in any of the primary windings  26   a - 26   d ; the diode  40   b  prevents current from flowing through the secondary winding  30   b  while current is ramping up in any of the primary windings  26   e - 26   h . As also known in the art, the capacitors  42   a ,  42   b  smooth the current output by the secondary windings  30   a ,  30   b  respectively. 
         [0018]    To electrically connect the power storage unit  20   a  with the primary winding  26   a , the switches  32   a ,  34   a  may be activated. Current will flow (clockwise as illustrated) from the positive terminal of the power storage unit  20   a , through the primary winding  26   a  (thus building an electromagnetic field), and to the negative terminal of the power storage unit  20   a . As discussed above, while current is ramping up in the primary winding  26   a , the diode  40   a  will prevent current flow through the secondary winding  30   a  (given the dot convention of the primary winding  26   a  and secondary winding  30   a ). Once the switches  32   a ,  34   a  are deactivated, current will flow (clockwise as illustrated) from the secondary winding  30   a  (due to the collapse of the electromagnetic field built up by the primary winding  26   a ) to power storage units of the cell module  18   b . (Of course, the turns ratio of the primary winding  26   a  to the secondary winding  30   a  may be selected such that an appropriate voltage is output to the cell module  18   b .) The other power storage units  20   b - 20   h  may be electrically connected with their associated transformer  22   a ,  22   b  by similar operation. 
         [0019]    As apparent to those of ordinary skill, the transformers  22   a ,  22   b , and associated components, each form a flyback switch mode converter with multiple primary windings and a single secondary winding. The cell module  18   a  of  FIG. 3  has two such flyback switch mode converters. In other embodiments, any suitable number of such flyback switch mode converters may be used. For example, a cell module may include twenty four ( 24 ) power storage units and three (3) switch mode converters of the type described herein. Each switch mode converter may be arranged to be electrically connected with eight (8) of the power storage units, etc. 
         [0020]    The cell module  18   a  may further include a balance control circuit  44 . The control circuit  44  facilitates the activation/deactivation of the switches  32   n ,  34   n ,  36   n ,  38   n  based on received state of charge information (determined in any suitable manner) regarding the power storage units  20   n . In the embodiment of  FIG. 3 , the control circuit  44  is electrically connected with gates of the switches  32   n ,  34   n ,  36   n ,  38   n : the control circuit  44  has an electrical connection with gates of the switches  32   a ,  34   a , the control circuit has an electrical connection with gates of the switches  36   a ,  38   a , etc. The control circuit  44  also has its reference electrically connected with the mid-point of the power storage units  20   n.    
         [0021]    Each of the cell modules  18   n  need not have its own control circuit  44 . In some embodiments, a single balance control circuit may be arranged to facilitate the activation/deactivation of switches of some/all of the cell modules  18   n . Other arrangements are also possible. 
         [0022]    To activate the switches  32   a ,  34   a , the control circuit  44  pulls the gate voltages of the switches  32   a ,  34   a  negative (with respect to their source leads) via connection to the reference of the control circuit  44 . Switches  32   b - 32   d ,  34   b - 34   d  may be activated in a similar manner. To activate the switches  36   a ,  38   a , the control circuit  44  drives the gate voltages of the switches  36   a ,  38   a  positive (with respect to their source leads) via connection to the reference of the control circuit  44 . Switches  36   b - 36   d ,  38   b - 38   d  may be activated in a similar manner. 
         [0023]    If any of the power storage units  20   n  has a state of charge greater than a desired threshold, its additional energy may be passed to power storage units of other cell modules  18   n . For example, if the power storage unit  20   g  has a state of charge greater than the desired threshold (as indicated by state of charge information received by the control circuit  44 ), the control circuit  44  may drive gates of the switches  36   c ,  38   c  positive (activating the switches  36   c ,  38   c ). Energy stored by the power storage unit  20   g  may thus be transferred (via the transformer  22   b ) to power storage units of the cell module  18   c  until the state of charge of the power storage unit  20   g  achieves the desired range. 
         [0024]    If any of the power storage units  20   n  has a state of charge less than a desired threshold, energy from the other power storage units  20   n  may be passed to power storage units of other cell modules  18   n . For example, if the power storage unit  20   d  has a state of charge less than the desired threshold, the control circuit  44  may utilize the switches  32   a ,  34   a  to transfer power from the power storage unit  20   a  via the transformer  22   a ; the control circuit  44  may utilize the switches  32   b ,  34   b  to transfer power from the power storage unit  20   b  via the transformer  22   a , etc. This power transfer may continue until the states of charge of the power storage units  20   a - 20   h  are approximately equal. Energy may then be driven into the power storage units  20   a - 20   h  (from other cell modules  18   n ) to raise their states of charge to a desired level. Other control schemes are also possible. 
         [0025]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
A vehicle power system may include a plurality of series connected power storage units configured to supply power to move a vehicle, and at least one flyback switch mode converter having a plurality of primary windings and a single secondary winding. Each of the primary windings may be configured to be selectively electrically connected with a respective one of a set of the power storage units. The secondary winding may be electrically connected with a different set of the power storage units.