Patent Publication Number: US-8541980-B2

Title: System and method for cell balancing and charging

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation in part of U.S. patent application Ser. No. 12/650,775, filed Dec. 31, 2009, entitled SYSTEM AND METHOD FOR CELL BALANCING AND CHARGING, which claims benefit from U.S. Provisional Patent Application No. 61/180,618, filed May 22, 2009, entitled SYSTEM FOR CELL BALANCING AND CHARGING and U.S. Provisional Patent Application No. 61/244,643, filed Sep. 22, 2009, entitled SYSTEM FOR CELL BALANCING AND CHARGING, each of which is incorporated herein by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying Drawings in which: 
         FIG. 1  is a block diagram illustrating the connection of a cell balancing circuit with a series connection of battery cells; 
         FIG. 2  illustrates voltage differences between two cells as a function of the percent of state of charge of the cells; 
         FIG. 3  illustrates a schematic diagram of a circuit for charging and balancing of cells; 
         FIG. 4  illustrates the battery charging cycle during transition; 
         FIG. 5  illustrates the battery discharging cycle during transition; 
         FIG. 6  illustrates an alternative embodiment of  FIG. 3 ; 
         FIG. 7  illustrates yet another embodiment of the circuit of  FIG. 3 ; 
         FIG. 8  illustrates yet a further alternative embodiment of the circuit of  FIG. 3 ; 
         FIG. 9  illustrates a further embodiment of the battery charging and balancing circuit; 
         FIG. 10  illustrates a nested configuration of the charging and balancing circuit; 
         FIG. 11  is a block diagram of an alternative embodiment of the circuit of  FIG. 3  wherein the polarities are reversed on some of the secondary winding portions; 
         FIG. 12  illustrates an alternative embodiment including the plurality of series connected transformer portions enabling a stacked configuration that is scalable; 
         FIG. 13  illustrates yet a further embodiment of a system for reducing the number of transformer secondaries for cell balancing and charging; 
         FIG. 14  illustrates an alternative embodiment of  FIG. 13  wherein diodes are used instead of switches; and 
         FIG. 15  is a flow diagram describing the operation of the embodiment of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein like reference numbers are used herein to designate like elements throughout, the various views and embodiments of a system and method for cell balancing and charging are illustrated and described, and other possible embodiments are described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated and/or simplified in places for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations based on the following examples of possible embodiments. 
     Cell balancing and charging systems provide the ability to charge a series connection of battery cells using a single source. Systems using multiple lithium ion or super capacitor cells require balancing of the individual cells in order to maximize the energy available from the batteries and to prolong the life of the system. Resistive balancing systems for charging cells dissipate excess charge as heat are one common solution but these types of systems waste energy. Energy transfer systems which are based on a “nearest neighbor” inductive or capacitive energy transfer reduce the amount of wasted energy but are complex and generally provide less than satisfactory results when transferring charge over a distance of several cells. Thus, there is a need for a cell balancing and charging system that solves the dual problems of balancing the state of charge of cells within a stack of battery cells without dissipating the energy in an associated resistor and further providing efficient transfer of charge to any cell in the stack without a distance penalty. The common way of balancing cells within a multi cell battery is by discharging the highest cell through a pass element or alternatively by passing the charge from a pass element to an adjacent cell. 
     Referring now to the drawings, and more particularly to  FIG. 1 , there is illustrated a configuration of a cell balancing circuit  102  which is connected with a series connection of battery cells  104 . The charge level on a particular battery cell  104  may be moved from one cell to another in order to balance the charge load across each of the cells  104 . The cell balancing circuit  102  is responsible for carrying out this cell balancing/charging functionality. Various types of systems, as discussed herein above, exist for transferring the charge from one cell within a cell stack to an adjacent cell. However, these systems are overly complex and expensive and suffer from poor efficiency when transferring charge over several cells such as from one end of the cell stack to the other. 
     Referring now to  FIG. 2 , there is illustrated the voltage differences between two cells as a function of the percent state of charge. When batteries of different impedances or voltages are connected in series, the state of charge of the entire pack is limited. At a low state of charge percentage the voltage deviation is very high and can approach 500 millivolts deviation. The voltage deviation significantly decreases and approaches zero as the state of charge approaches 20%. Thus, during the charging cycle, the battery including a higher charge voltage may end up overcharged and damaged, or alternatively, a battery including a lower charge level may end up undercharged in order to protect the higher charge battery. In either case, the battery&#39;s cells will not reach their maximum charge voltage. During discharge, the lower charge battery may pull the total capacity of the series connection to a low level and prevent the taking of maximum charge from the system. 
     Referring now to  FIG. 3 , there is illustrated a first embodiment of a circuit for providing charging and load balancing of a series connection of battery cells  302 . The series connection of battery cells  302  are connected between node  304  and node  306 . A charging voltage is supplied to the battery cells  302  via a voltage source  308  provided between nodes  304  and  306 . Node  306  comprises the ground node while node  304  comprises the input voltage node. A high-side switching transistor  310  (MOSFET) has its source/drain path connected between node  304  and node  312 . A low-side switching transistor  314  (MOSFET) has its drain/source path connected between node  312  and the ground node  306 . 
     A resonant tank circuit consisting of inductor  316  and capacitor  320  is connected between node  312  and node  322 . The inductor  316  is connected between node  312  and node  318 . The capacitor  320  is connected in series with the inductor  316  between node  318  and node  322 . A primary side  324  of a transformer  325  is connected to node  322  and to the ground node  306 . The secondary side of the transformer  325  includes a number of secondary portions  326 , each of which are connected across the terminals of an associated battery cell  302 . The polarity of adjacent secondary side portions  326  of the transformer are reversed from each other. A switching MOSFET  328  has its drain/source path connected between the secondary portion  326  of the transformer  325  and the negative terminal of the associated battery cell  302 . The switch  328  would receive control signals from a control circuit (not shown) which also controls switching transistors  310  and  314 . 
     During the charging cycle, the system of  FIG. 3  is based upon a resonant converter for every switching cycle, and the amount of energy that is put into the resonant tank by the voltage source  308  is then transferred to the secondary side portions  326 . The lowest charged voltage cells will then take most of the energy transmitted to the secondary side  326  from the resonant tank and the highest charged voltage cells the least. Thus, the charge is transferred to the second portion  326  in proportion to the charge on the associated battery cells. In order to add more protection and control, the switch  328  is added in series with each secondary portion  326  to increase or decrease the overall impedance of the battery cell  302 . This allows selective charging of the battery cells such as might be required when a cell is to be charged to a higher voltage than other cells. Thus, the cells are balanced during charging. 
     As can be seen in  FIG. 4 , the lowest voltage cells are taking all of the energy provided by the resonant tank while the higher voltage battery cells are sitting idle until the lower battery cells catch up in charge value with the higher value tanks. Thus, waveform  402  represents the charging battery voltage of the lower charge battery cell while waveform  404  represents the higher voltage battery. 
     During the discharge cycle, the input to the primary side  324  of the transformer  325  will comprise the total series voltages of all of the battery cells  302 . The energy is circulating from all of the battery cells  302  back to the lowest charged cells.  FIG. 5  illustrates the ampere hour taking every cycle from every cell is the same while the energy put back into the system is higher for the lower voltage batteries. Thus, waveform  502  represents the highest voltage battery cell, waveform  504  represents the next highest voltage battery cell while waveform  506  represents the lowest voltage battery cell. 
     The main difference between previous solutions and the implementation described herein above with respect to  FIG. 3 , is that the energy is taken from the entire stack of battery cells  302  and then redistributed back based on the battery cell that needs more energy than the other battery cells. This scheme permits very simple systems which automatically distribute charge without the need for a sophisticated control mechanism. A more sophisticated implementation is possible in which balancing may be performed using complex algorithms in a manner that maintains optimal performance with a variety of systems over the entire system life. The system may be equally implemented as a charger, balancer or both. 
     Referring now to  FIG. 6 , there is illustrated an alternative implementation of the circuit of  FIG. 3  wherein the MOSFET switches  328  between the transformer secondaries  326  and the battery cells  302  are replaced by diodes  602 . In another implementation illustrated in  FIG. 7 , the switches feeding the tank may be removed and the tank input grounded. In this system the switches between the transformer secondaries and the cells are replaced by a suitable arrangement of switches and conducting elements. Energy is passed to and from the tank circuit by selective use of the secondary side switches. E.g. the secondary side in  FIG. 2  becomes both primary and secondary depending on the configuration of the switch elements. Alternatively, as illustrated in  FIG. 8 , the lower drive MOSFET  314  may be replaced by a diode  802 . In an alternative control scheme, the currents through the transformer primary  324  may be sensed to determine a current limit providing an on time termination point for the circuit and a switch termination timing to determine when to turn off the switching transistors  310  and  314 . 
     Referring now to  FIG. 9 , there is illustrated a further embodiment of the charging/balancing circuit of  FIG. 3 . The series connection of battery cells  902  are connected between node  904  and node  906 . A charging voltage is supplied to the battery cells  902  via a voltage source  908  provided between nodes  904  and  906 . Node  906  comprises the ground node while node  904  comprises the input voltage node. A high-side switch  910  is connected between node  904  and node  912 . A low-side switch  914  is connected between node  912  and the ground node  906 . A resonant tank circuit consisting of inductor  916  and capacitor  920  is connected between node  912  and node  922 . The inductor  916  is connected between node  912  and node  918 . The capacitor  920  is connected in series with the inductor  916  between node  918  and node  922 . 
     A primary side  924  of a transformer  925  is connected to node  922  and to the ground node  906 . The secondary side of the transformer  925  includes a number of secondary portions  926 , each of which are connected across the terminals of the associated battery cell  902 . A switch  928  is connected between the secondary portion  926  of the secondary side of the transformer  925  and the negative terminal of the associated battery cell  902 . The switch  928  would receive control signals from a control circuit (not shown) which also controls switches  915  and  914 . In addition to the switch  928  connected between the transformer secondary portion  926  and the battery cell  902 , a capacitor  930  is connected in parallel with the switch  928 . In this scheme, current may be directed to individual cells  902  through the selective use of the secondary side switches  928  allowing programmable charge balancing or charge redirection to deliberately produce an unbalanced condition. 
     Referring now also to  FIG. 10 , there is illustrated a nested balancing system. Nested arrangements are possible in which each of the battery cells are replaced by the balancing circuit  1002  as described previously with respect to  FIG. 3  and a series of battery cells  1004 . The circuit of  FIG. 10  comprises a series connection of battery cells  1004  are connected between node  1005  and node  1006 . A charging voltage is supplied to the battery cells  1004  via a voltage source  1008  provided between nodes  1005  and  1006 . Node  1006  comprises the ground node while node  1005  comprises the input voltage node. A high-side switch  1016  is connected between node  1005  and node  1012 . A low-side switch  1014  is connected between node  1012  and the ground node  1006 . 
     A resonant tank circuit consisting of inductor  1013  and capacitor  1021  is connected between node  1012  and node  1022 . The inductor  1013  is connected between node  1012  and node  1018 . The capacitor  1021  is connected in series with the inductor  1013  between node  1020  and node  1022 . A primary side  1024  of a transformer  1025  is connected to node  1022  and to the ground node  1006 . The secondary side of the transformer  1025  includes a number of secondary portions  1026 , each of which are connected across the terminals of the associated battery cell stack  1004 . A switch  1028  is connected between the secondary portion  1026  of the secondary side  1026  of the transformer  1025  and the negative terminal of the associated battery cell stack  1004 . The switch  1028  would receive control signals from a circuit which also controls switches  1016  and  1014 . 
     As mentioned previously, rather than a single cell, a series of cells  1004  are connected across each of the secondary portions  1026  of the secondary side of the transformer. Connected across these cells  1004  is the balancing circuit described previously with respect to  FIG. 3 . Thus, the battery cells  1004  would comprise the source  308  and the balancing circuit  1002  would connect with the source at nodes  304  and  306 . Thus, each stack of cells  1004  includes its own balancing system  1002  such that nested balancing systems may be produced which optimizes the complexity/performance trade off. 
     In an alternative embodiment of the circuit of  FIG. 10 , the switches  1016  and  1014  feeding the resonant tank may be removed and the tank input grounded. In this implementation, the switches  1028  between the transformer secondaries  1026  and the cell stacks  1004  are replaced by a suitable arrangement of switches and conducting elements. Energy is passed to and from the resonant tank circuit by the selective use of the secondary side switches  1028 . Thus, the secondary side becomes both the primary and secondary depending on the configuration of the switching elements. 
     In yet a further embodiment illustrated in  FIG. 11 , the circuitry is configured in substantially the same manner as that described with respect to  FIG. 3 . However, the polarities on the secondary side portions  326  are altered such that some (ideally half) of the secondary windings have one polarity and the remainder of the secondary windings have the opposite polarity. The actual sequence between the reversed polarities within the secondary windings is not important. The benefit that this configuration provides is that charge may be transferred on both half cycles of the transformer. The first half cycle feeds the secondaries with one polarity and the second half cycle feeds those with the opposite polarity. 
     Referring now to  FIG. 12 , there is illustrated a further embodiment that comprises a stacked configuration including additional transformer  1233  placed in series with the first transformer  1225 . The series connection of battery cells  1202  are connected between node  1204  and node  1206 . A charging voltage is supplied to the battery cells  1202  via a voltage source  1208  provided between nodes  1204  and  1206 . Node  1206  comprises the ground node while node  1204  comprises the input voltage node. A high-side switch  1210  is connected between node  1204  and node  1212 . A low-side switch  1214  is connected between node  1212  and the ground node  1206 . A resonant tank circuit consisting of inductor  1216  and capacitor  1220  is connected between node  1212  and node  1222 . The inductor  1216  is connected between node  1212  and node  1218 . The capacitor  1220  is connected in series with the inductor  1216  between node  1218  and node  1222 . 
     A primary side  1224  of a first transformer  1225  is connected to node  1222  and to the ground node  1206 . The secondary side of the transformer  1225  includes a number of secondary portions  1226 , each of which are connected across the terminals of the associated battery cell  1202 . A switch  1228  is connected between the secondary portion of the secondary side  1226  of the transformer  1225  and the negative terminal of the associated battery cell  1202 . The switch  1228  would receive control signals from a control circuit (not shown) which also controls switches  1215  and  1214 . In addition to the switch  1228  connected between the transformer secondary portion  1226  and the battery cell  1202 , a capacitor  1230  is connected in parallel with the switch  1228 . In this scheme, current may be directed to individual cells  1202  through the selective use of the secondary side switches  1228  allowing programmable charge balancing or charge redirection to deliberately produce an unbalanced condition. 
     In the second transformer  1223  of the stacked configuration, a primary side  1235  of the transformer  1223  is connected in series with the primary side  1224  of the first transformer  1225 . Additionally, a further series of transformer secondaries  1236  are connected across additional battery cells  1202  in series with the transformer secondary portion  1226  of transformer  1225 . As in the first portion of the circuit, a switch  1228  would receive control signals from a control circuit (not shown). In addition to the switch  1228  connected between the transformer secondary portion  1236  and the battery cell  1202 , a capacitor  1230  is connected in parallel with the switch  1228 . The stacked configuration is completely scalable. As many sections as needed may be added in series. Thus, rather than the two illustrated in  FIG. 12 , any number may be further added. A single pair of switches  1215  and  1214  and a single tank circuit consisting of inductor  1216  and capacitor  1220  then feed the series connected transformer windings. 
     Referring now to  FIG. 13 , there is illustrated yet a further embodiment of a charge/balancing circuit comprises an improved configuration for reducing the number of transformer secondaries is illustrated. A series connection of battery cells  1302  are connected between node  1304  and node  1306 . A charging voltage is supplied to the battery cells  1302  via a voltage source  1308  provided between nodes  1304  and  1306 . Node  1306  comprises a ground node while node  1304  comprises the input voltage node. A high side switch  1310  is connected between node  1304  and node  1312 . A low side switch  1314  is connected between node  1312  and the ground node  1306 . A resonant tank consisting of inductor  1316  and capacitor  1320  is connected between node  1312  and node  1322 . The inductor  1316  is connected between node  1312  and node  1318 . The capacitor  1320  is connected in series with the inductor  1316  between node  1318  and node  1322 . 
     A primary side  1324  of a transformer  1325  is connected to node  1322  and to the ground node  1306 . The secondary side of the transformer  1325  includes a number of portions  1326 . Each secondary portion  1326  is associated with two separate battery cells  1302  via an associated set of switches  1328  and  1330 . A switch  1328  is connected between a first portion of the secondary side  1326  of the transformer  1325  and a first terminal of an associated battery  1302 . The switch  1328  is similarly associated for each secondary portion of the secondary side  1326  within the circuit. Similarly, the switch  1330  is connected between the secondary portion of the secondary side  1326  of the transformer  1325  and a second terminal of a second cell. The switch  1330  is associated with each of the secondary portions of the secondary side  1326 . The polarities of the secondary side  1326  applied to the battery cells  1302  are reversed during each half cycle application. 
     Each portion of the transformer secondary  1326  is connected via switches  1328  and  1330  to two adjacent cells  1302 . The switches  1328  and  1330  enable a charging current to be transferred to each of the two connected cells  1302  during opposite half cycles of current in the primary winding  1324 . For example, the secondary winding  1326  is connected via switch  1328  to cells  1302   a  and  1302   c  during one half of the duty cycle, and the transformer secondary  1326  is connected to a second group of cells  1302   b  and  1302   d  via switches  1330  during the other half of the duty cycle. The sequence will then repeat during each half cycle so that both groups of cells are charged by the actions of the secondary winding  1326 . 
     Referring now to  FIG. 14 , there is illustrated yet a further embodiment of a charge/balancing circuit wherein an alternative configuration for reducing the number of transformer secondaries is illustrated. The series connection of battery cells  1402  are connected between node  1404  and node  1406 . A charging voltage is supplied to the battery cells  1402  via a voltage source  1408  provided between nodes  1404  and  1406 . Node  1406  comprises a ground node while node  1404  comprises the input voltage node. A high side switch  1410  is connected between node  1404  and node  1412 . A low side switch  1414  is connected between node  1412  and the ground node  1406 . A resonant tank consisting of inductor  1416  and capacitor  1420  is connected between node  1412  and node  1422 . The inductor  1416  is connected between node  1412  and node  1418 . The capacitor  1420  is connected in series with the inductor  1416  between node  1418  and node  1422 . 
     A primary side  1424  of a transformer  1425  is connected to node  1422  and to the ground node  1406 . The secondary side of the transformer  1425  includes a number of portions  1426 . Each secondary portion  1426  is associated with two separate battery cells  1402  via an associated set of diodes  1428  and  1430 . A diode  1428  is connected between a first portion of the secondary side  1426  of the transformer  1425  and a first terminal of an associated battery  1402 . The diode  1428  is similarly associated for each secondary portion of the secondary side  1426  within the circuit. Similarly, the diode  1430  is connected between the secondary portion of the secondary side  1426  of the transformer  1425  and a second terminal of a second cell. The diode  1430  is associated with each of the secondary portions of the secondary side  1426 . The polarities of the secondary transformer  1426  applied to the battery cells  1402  are reversed during each half cycle application. 
     Each portion of the transformer secondary  1426  is connected via diodes  1428  and  1430  to two adjacent cells  1402 . The diodes  1428  and  1430  enable a charging current to be transferred to each of the two connected cells  1402  during opposite half cycles of current in the primary winding  1424 . For example, the secondary winding  1426  is connected via diode  1428  to cells  1402   a  and  1402   c  during one half of the duty cycle, and the transformer secondary  1426  is connected to a second group of cells  1402   b  and  1402   d  via diodes  1430  during the other half of the duty cycle. The sequence will then repeat during each half cycle so that both groups of cells are charged by the actions of the secondary winding  1426 . In the implementation of  FIG. 14 , the switches  1328  and  1330  of the embodiment of  FIG. 13  have been replaced by diodes  1428  and  1430 . In additional configurations, the diodes or switches may be replaced by a combination of diodes and switches to permit simple control of the system. 
     Referring now to  FIG. 15 , there is illustrated a flow diagram describing the operation of the circuit of  FIG. 13 . Once the operation of the circuit is initiated at step  1502 , the first group of switches  1328  is closed while the second group of switches  1330  is open. When this occurs, the first group of batteries is then charged at step  1506 . Inquiry step  1508  determines if the first half cycle of the current in the primary winding  1324  has been completed. If not, control passes back to step  1506 . If inquiry step  1508  determines that the half cycle is completed, the second group of switches  1330  is closed while the first group of switches  1328  is opened at step  1510 . This initiates the charging of the second group of batteries at step  1512 . Inquiry step  1514  then determines whether the last half cycle of the current in the primary winding  1326  has been completed and if not, continues charging the battery at step  1512 . Once the half cycle is completed, control passes back to step  1504  and the first group of switches are closed while the second group of switches are open to begin charging of the first group of batteries. The cycle continues to repeat as described herein above. 
     Thus, the main difference between previous solutions and the present disclosure is that the energy is taken from the entire cell stack and redistributed based upon the cells that need more energy than the other. The scheme permits very simple systems which automatically charge without the need of a sophisticated control mechanism. More sophisticated implementations are possible in which the balancing may be performed using complex algorithms in a manner that maintains the optimal performance with a variety of systems and over the entire system life. Additionally, the above method and systems may be utilized to reduce the number of transformer secondaries within the system by a factor of 2. This will greatly reduce the component costs within the system and provide significant cost savings. 
     It will be appreciated by those skilled in the art having the benefit of this disclosure that this system and method for cell balancing and charging provides an improved manner of charging/balancing a stack of battery cells. It should be understood that the drawings and detailed description herein are to be regarded in an illustrative rather than a restrictive manner, and are not intended to be limiting to the particular forms and examples disclosed. On the contrary, included are any further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments apparent to those of ordinary skill in the art, without departing from the spirit and scope hereof, as defined by the following claims. Thus, it is intended that the following claims be interpreted to embrace all such further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments.