Abstract:
A system and method for charging a rechargeable, or secondary, battery including a series string of cells, includes a topology of charging sources that selectively provides charging current to cells that need to be charged, but avoids overcharging cells that are already charged above a predetermined voltage threshold. Based on individual cell voltage measurements, the charging current is controlled in a manner to direct charging current to the battery cell(s) needing charge until these cells are fully charged, and by-passes battery cells that are fully charged or become fully charged.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. application Ser. No. 11/163,667, filed Oct. 26, 2005, which claims priority to U.S. Provisional Application No. 60/522,814, filed Nov. 11, 2004, both of which are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention generally relates to secondary (rechargeable) batteries, and more particularly, to cell equalization of such batteries. 
       BACKGROUND OF THE INVENTION 
       [0003]    Generally, secondary (rechargeable) batteries include a string of individual battery cells connected in series to obtain a higher output voltage level. During charging of secondary batteries, inherent differences in the capacity of the individual battery cells may cause the higher capacity cells to achieve full charge first, and then over-charge while the remaining battery cells continue to charge. Depending on the ability of the cell chemistry to tolerate this over-charge, cell damage may occur. During discharge, a similar problem may be encountered when the lower capacity battery cells reach minimum voltages first and over-discharge. Cell chemistries such as lead-acid and nickel-cadmium may tolerate moderate forms of these conditions, while other cell chemistries, such as silver-zinc and lithium-ion, may be more easily damaged. The probability of damage due to over-charge may be further aggravated by demand for rapid charging systems that require higher currents and cell temperatures. 
         [0004]    For the reasons stated above, charging a series-connected string of individual battery cells normally poses unique monitoring and control difficulties. For example, measuring the voltage of the battery may not necessarily indicate the condition of each individual cell in the battery. If the individual battery cells are, for example, not well balanced, a cell may experience a damaging over-charge condition even though the battery voltage is within acceptable limits. Thus, each battery cell in a string usually is monitored and controlled to insure that each individual cell in the series string does not experience an over-voltage condition during charging. 
         [0005]    When charging, secondary battery cells generally are bulk charged if the cell voltage is above a specified level. Bulk charging continues until any individual cell voltage reaches an upper voltage limit. At the end of bulk charging, one or more battery cells may, however, be only partially charged, and may not have yet reached a 100% state of charge. The partially charged condition is considered adequate for some applications and, thus, the charging process may be terminated prior to each individual cell being 100% charged. Over time, however, the performance of individual cells in the battery may diverge due to each cell being charged to a different level during any one recharge. To minimize divergence, a second step in the charging process often is implemented. 
         [0006]    The second step in the charging process is known as “cell equalization.” Cell equalization generally begins when a battery cell is clamped at an upper voltage limit during charging. The charging current usually decreases because the cell voltage is clamped, and not allowed to increase. To protect against cell failure, safeguards to terminate the charging process prior to cell failure often are employed. Cell charging may be terminated (and cell equalization ended) based on a specified cell charge current level (normal condition), a specified over temperature condition (fault condition), and/or a specified cell charge time out (fault condition). At the end of cell equalization, the string of individual battery cells connected in series generally is considered at a 100% state of charge even though each individual battery cell may not be fully charged. 
         [0007]    In addition to over charging, battery cells may experience damage if the cell temperature falls outside a specific range. Thus, cell temperatures are advantageously kept within a specified temperature range during bulk charging and cell equalization to prevent temperature damage from occurring. 
         [0008]    Another concern for battery cells is over-discharge. Over-discharge often causes serious performance degradation and damage the cell. Over-discharge may occur when any cell voltage drops below a fixed voltage level. To prevent overdischarge, secondary batteries often are equipped with a mechanism that terminates discharge when any cell drops below a fixed voltage level. Sometimes, however, the cell voltage may rise after the discharge is terminated, so hysteresis may be necessary to prevent oscillations. 
         [0009]    Thus, it is generally recognized that recharging a secondary battery having a series-connected string of cells preferably is accomplished in a manner that charges each cell to substantially the same level while avoiding overcharging any of the cells. Thus, there is a need for a cell equalizing charging system that is low-cost, uses simple circuitry, reduces power dissipation during charging, and operates efficiently. 
       SUMMARY OF THE INVENTION 
       [0010]    A system for charging a secondary battery according to various aspects of the present invention comprises a plurality of battery cells connected in a series string, wherein the series string includes a first battery cell at a load end and an n th battery cell at a ground end, and a cell junction located between each respective pair of battery cells. The system, in one embodiment, also includes a plurality of charging sources, wherein a first charging source is electrically coupled to the load end, and a second charging source is electrically coupled to a first cell junction between the first battery cell and a second battery cell located adjacent to the first battery cell. In another embodiment, a charging source is electrically coupled to each cell junction formed every two cells thereafter. In one aspect of an exemplary embodiment of the invention, the system includes (n+2)/2 charging sources, while in another aspect of the invention, there are (n+1)/2 charging sources. 
         [0011]    In one exemplary embodiment, the system includes a plurality of shunt regulators, wherein a respective shunt regulator is connected in parallel across each of the second battery cell to the nth battery cell. In one aspect of an exemplary embodiment of the invention, the system includes (n−1) shunt regulators connected in parallel across (n−1) battery cells. 
         [0012]    In another exemplary embodiment, a charging source is electrically connected to each of the plurality of charging sources to provide charging current to each of the plurality of battery cells via the plurality of charging sources included in the system. In a further embodiment, the system includes a controller connected to each of the plurality of charging sources, wherein the controller includes circuitry to switch on and off each of the plurality of charging sources. In accordance with one aspect of an exemplary embodiment of the invention, the circuitry is configured to allow only one charging source to be switched on at a time. 
         [0013]    In accordance with yet another exemplary embodiment, the system includes a shunt controller coupled to each of the plurality of shunt regulators to switch on and off each of the plurality of shunt regulators. In accordance with one aspect of an exemplary embodiment of the invention, the shunt controller is configured to switch on a shunt regulator if a battery cell with which the shunt regulator is connected across in parallel is fully charged, and switch off the shunt regulator if the battery cell is not fully charged. In still another exemplary embodiment, a controller is connected to each charging source and each shunt regulator, wherein the controller includes circuitry to switch on and off each of the plurality of charging sources, and switch on and off each of the plurality of shunt regulators. 
         [0014]    Furthermore, in accordance with another embodiment, a plurality of cell monitoring circuits is included in the system, wherein at least one cell monitoring circuit is connected to each respective battery cell to monitor an amount of charge within each respective cell monitor, and in communication with the controller. In accordance with one aspect of the invention, the controller switches on only one charging source at a time, and determines if one or more of the plurality of battery cells needs to be charged. In accordance with another aspect of the invention, the controller determines a target battery cell, wherein the target cell is at least one of the plurality of battery cells needing to be charged, and is a battery cell located closer to the load end than any other of the plurality of battery cells that may need to be charged. In accordance with yet another aspect of the invention, the controller switches on a target charging source, wherein the target charging source is located at a cell junction between the target battery cell and the load end, and the target charging source is located at a cell junction farther away from the load end than another charging source located between the target battery cell and the load end. In accordance with still another aspect of the invention, the controller switches on and off each of the plurality of shunt regulators based upon an amount of charge within a battery cell associated with each respective shunt regulator. 
         [0015]    Various exemplary embodiments of the present invention also include a method for equalizing voltage of a secondary battery being charged, the battery comprised of n cells connected in a serial string from a first cell at one end to an nth cell at another end with a respective cell junction being located between each adjacent cell, the method comprising the steps of connecting the plurality of switched charging sources to the serial string, wherein a first switched charging source is electrically coupled to the one end of the serial string, and a second switched charging source is electrically coupled at the cell junction between the first cell and an adjacent second cell, and a respective switched charging source is electrically coupled at the cell junctions occurring every two cells thereafter; connecting a plurality of shunt regulators to the serial string, wherein a respective shunt regulator is connected in parallel across each of the second cells through the nth cell; and operating the switched charging sources and the shunt regulators to selectively provide charging current to one or more of the n cells. In one aspect, the step of connecting a plurality of shunt regulators to the serial string includes the step of connecting (n−1) shunt regulators to the serial string. In another aspect, the step of connecting the plurality of switched charging sources to the serial string includes the step of connecting ((n+2)/2) switched charging sources to the serial string when n is an even number. In yet another aspect, the step of connecting the plurality of switched charging sources to the serial string includes the step of connecting ((n+1)/2) switched charging sources to the serial string when n is an odd number. 
         [0016]    In one exemplary embodiment, the method further comprises the steps of operating each of the switched charging sources in one of a first state and a second state, wherein when a switched charging source is in the first state, the source provides a charging current to the respective cell junction where that switched charging source is electrically connected; and when the switched charging source is in the second state, the source does not provide a charging current to the respective cell junction where that switched charging source is electrically connected; and operating each of the shunt regulators in a first state to bypass charging current around the respective cell across which it is connected and operates as a high-impedance electrical path in a second state. In another exemplary embodiment, the method further comprises the step of operating the shunt regulators and the switched charging sources to provide charging current to each cell having a voltage below a predetermined threshold, and to avoid providing charging current to each cell having a voltage at or above a predetermined threshold. In yet another embodiment, the steps of monitoring a respective voltage level of each of the n cells; and determining which of the n cells is at or above a predetermined voltage threshold are included in the method. 
         [0017]    The invention also includes a second exemplary method for equalizing voltage of a secondary battery. The second exemplary method includes the steps of monitoring an amount of charge contained within a plurality of battery cells utilizing at least one cell monitor to determine if at least one battery cell needs charging; transmitting a signal to begin charging operations from the at least one cell monitor when at least one of the plurality of battery cells needs charging; determining which charging source, of a plurality of charging sources, to utilize to charge said at least one of the plurality of battery cells needing charging; and switching on an appropriate charging source of the plurality of charging sources to charge said at least one of said plurality of battery cells needing charging, wherein the appropriate charging source is determined by its location with respect to at least one battery cell needing charge. In one exemplary embodiment, the method includes switching on at least one shunt regulator coupled in parallel to at least one of the plurality of battery cells, wherein the at least one shunt regulator is a shunt regulator coupled in parallel across a battery cell including a charge amount greater than a threshold amount. In one aspect of the invention, the second method includes the step of switching on at least one shunt regulator occurs prior to said step of switching on an appropriate charging source. 
         [0018]    In another exemplary embodiment, the second method includes the cell monitor continuing to monitor the plurality of battery cells until at least one battery cell receiving charging current is charged to a threshold amount of charge as indicated by the cell voltage, and the controller switching on a shunt regulator coupled in parallel to the battery cell receiving charging current when the battery cell receiving charging current contains the threshold amount of charge as indicated by the cell voltage. These steps may be repeated until each battery cell contains the threshold amount of charge as indicated by the cell voltage. When each battery cell contains the threshold amount of charge as indicated by the cell voltage, the charging source and any shunt regulators that were switched on are switched off by the controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]    A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the drawing Figures, where like reference numbers refer to similar elements throughout the Figures, and: 
           [0020]      FIG. 1  is a block diagram of one exemplary embodiment of a device including a secondary battery, and a charging system to recharge the secondary battery; 
           [0021]      FIG. 2  is a block diagram of an exemplary embodiment of a charging system utilizing cell equalization to charge a secondary battery; 
           [0022]      FIG. 3  is block diagram of one exemplary embodiment of a topology of the charging system of  FIG. 2 ; 
           [0023]      FIG. 4  is a control truth table and operational chart for the topology illustrated in  FIG. 3 ; and 
           [0024]      FIG. 5  is a flow diagram illustrating an exemplary embodiment of a method for charging a secondary battery utilizing cell equalization. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The detailed description of various exemplary embodiments of the invention herein makes reference to the accompanying figures and drawings. While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized in that logical and mechanical changes may be made without departing from the spirit and scope of the invention. Thus, the detailed description herein is presented for purposes of illustration only and not by way of limitation. For example, the steps recited in any of the method or process descriptions may be executed in any order and are not limited to the order presented. 
         [0026]    For the sake of brevity, the apparatus and systems (and components of the individual operating components) are described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative and/or additional functional relationships and/or physical connections may be present in a practical system. 
         [0027]    Turning now to the figures,  FIG. 1  is a block diagram of one exemplary embodiment of a device  100  including a secondary battery  130  and a charging system  120  to recharge secondary battery  130 . Device  100 , in one embodiment, includes power source  110 . In an exemplary embodiment, power source  110  is a DC power source. In another exemplary embodiment, power source  110  is an AC power source. In one aspect of the invention (when power source  100  is a DC power source), power source  110  may be a solar panel such that power source  100  produces a DC signal. In another aspect of the invention (when power source  110  is an AC power source), power source  110  may be a standard AC outlet along with a transformer, or the like, to provide an appropriate voltage signal for charging secondary battery  130 . The invention contemplates, however, that power source  110  may be any DC or AC power source known in the art capable of providing charging current to recharging secondary battery  130 . 
         [0028]    Device  100 , in another exemplary embodiment, includes charging system  120  electrically connected to power source  110 . In various aspects of the invention, charging system  120  may be suitably configured (as discussed in greater detail below) to charge one or more battery cells (not shown) within secondary battery  130 . 
         [0029]    In one exemplary embodiment, secondary battery  130  is a lithium-ion battery. In other embodiments of the invention, secondary battery  130  may be, but is not limited to, a lead-acid battery, a nickel-cadmium battery, a nickel-metal hydride battery, a nickel hydrogen battery, a silver-zinc battery, or any other battery capable of storing a charge and subsequently being recharged. 
         [0030]    Device  100  includes load  140  which, in an exemplary embodiment, is a device that requires voltage and current. Examples of load  140  include, but certainly are not limited to, a personal digital assistant (PDA), a BlackBerry® device, a cellular phone, a pager, a Palm Pilot® device, and/or any other electronic or communication device capable of being supplied power by secondary battery  130 . 
         [0031]      FIG. 2  is a block diagram of an exemplary embodiment of charging system  120  of  FIG. 1 . Charging system  120 , in an exemplary embodiment, includes controller  210 , which may be any hardware and/or software suitably configured to switch on and off charging sources  220  and/or shunt regulators  230 . As such, controller  210  may be any controller known in the art capable of switching on and off charging sources and/or shunt regulators when appropriate to do such. 
         [0032]    In one exemplary embodiment, controller  210  is connected to at least one charging source  220  and at least one shunt regulator  230 . In other embodiments, charging system  120  includes a plurality of controllers (not shown) similar to controller  210 , wherein a controller is connected to each charging source  220  to control the operation (i.e., on/off operation) of their respective charging source  220 . In still other embodiments, charging system  120  includes a plurality of shunt regulator controllers (not shown) similar to controller  210 , wherein a shunt regulator controller is connected to each shunt regulator  230  to control the operation (i.e., on/off operation) of their respective shunt regulator  230 . 
         [0033]    The invention contemplates that charging source  220  may be any hardware and/or software suitably configured to provide charging current to at least one battery cell if switched on (i.e., operating in a charging state), and not provide charging current to a battery cell if switched off (i.e., operating in a non-charging state). As such, charging source  220  may be any charging source known in the art capable of charging one or more battery cells. Likewise, shunt regulator  230  may be any hardware and/or software suitably configured to have a lower resistance than a battery cell connected in parallel if shunt regulator  230  is switched on, and a greater resistance than the battery cell if shunt regulator  230  is switched off. As such, shunt regulator  230  may be any shunt regulator known in the art capable of manipulating the flow of current into and/or away from a battery cell connected in parallel to shunt regulator  230 . 
         [0034]    In another exemplary embodiment, charging system  120  includes series string of battery cells  240  (hereinafter, “series string  240 ”). Series string  240 , in an exemplary embodiment, contains one or more individual battery cells (not shown), wherein each battery cell voltage is dependent on the cell chemistry. As such, series string  240  may be configured to form a secondary battery of any desired voltage. Charging system  120 , in another exemplary embodiment, includes at least one cell monitor  250  connected to a respective battery cell and controller  210 . Cell monitor  250  may be any hardware and/or software suitably configured to monitor the terminal voltage of one or more battery cells. As such, cell monitor  250  may be any cell monitor known in the art capable of detecting the terminal voltage of an individual or plurality of battery cells. In one aspect of the invention, cell monitor  250  may be configured to detect the terminal voltage of a battery cell (with a predetermined amount of error tolerance). In another aspect of the invention, cell monitor  250  may be configured to determine if a battery cell, with which cell monitor  250  is associated, contains a terminal voltage above or below a pre-determined threshold level. Furthermore, cell monitor  250 , in an exemplary embodiment, is configured to communicate the terminal voltage of a battery cell and/or whether the battery cell contains above or below the threshold amount of charge to controller  210 . As used herein, the term “above” with reference to a terminal voltage and/or a threshold amount of voltage means substantially the same as or greater than the amount. In addition, the invention contemplates that charging system  120  may be formed on a printed circuit board (PCB) (not shown) or on any other platform known in the art suitable for forming and/or operating charging system  120 . 
         [0035]      FIG. 3  is a block diagram of one exemplary embodiment of a topology  300  of charging system  120 . In an exemplary embodiment, topology  300  includes a power source (e.g., power source  110 ) electrically connected to charging source  305 , charging source  310 , and charging source  315 , wherein charging sources  305 ,  310 , and  315  are each configured similar to charging source  220  discussed above. In one embodiment, charging source  305  is connected to and provides charging current to battery cell  320  through node  307 . Likewise, charging source  310  is connected to and provides charging current to battery cells  325  and  330  through node  312 . Furthermore, charging source  315  is connected to and provides charging current to battery cell  340  through node  317 . 
         [0036]    Battery cells  320 ,  325 ,  330 , and  335 , in an exemplary embodiment, are lithium-ion battery cells. In other embodiments, battery cells  320 ,  325 ,  330 , and  335  may be, but are not limited to, lead-acid battery cells, nickel-cadmium battery cells, nickel-metal hydride battery cells, nickel hydrogen battery cells, silver-zinc battery cells, or any other type of battery cell capable of storing a charge and subsequently being recharged. In addition, the invention contemplates that battery cells  320 ,  325 ,  330 , and  335  may each be any size battery cell known in the art. 
         [0037]    Charging sources  305 ,  310 , and  315 , in one exemplary embodiment, are each connected to a controller  370  similar to controller  210  discussed above. In another exemplary embodiment, controller  370  is also connected to shunt regulators  350 ,  355 , and  360 , wherein shunt regulators  350 ,  355 , and  360  are each configured similar to shunt regulator  230  discussed above. Controller  370 , in one embodiment, is configured to transmit charging source control signals  374  to charging sources  305 ,  310 , and  315  to control the on/off operation of charging sources  305 ,  310 , and  315 . Similarly, controller  370 , in another embodiment, is configured to transmit shunt regulator control signals  378  to shunt regulators  350 ,  355 , and  360  to control the on/off operation of shunt regulators  350 ,  355 , and  360 . 
         [0038]    In an exemplary embodiment, shunt regulator  350  is coupled in parallel to battery cell  325  such that shunt regulator  350  is coupled to node  312  (i.e., the positive terminal (V+) of battery cell  325 ) and the negative terminal (V−) of battery cell  325 . Furthermore, shunt regulator  355  is connected in parallel to battery cell  330  such that shunt regulator  355  is connected to V+ of battery cell  330 , and to node  317  (i.e., V− of battery cell of  330 ). Moreover, shunt regulator  360  is connected in parallel to battery cell  335  such that shunt regulator  360  is connected to V+ and V− of battery cell  335 . 
         [0039]    Topology  300 , in another exemplary embodiment, includes cell monitor  380 , cell monitor  385 , cell monitor  390 , and cell monitor  395 , each being configured similar to cell monitor  250  discussed above. In one embodiment, cell monitors  380 ,  385 ,  390 , and  395  are connected to battery cells  320 ,  325 ,  330 , and  335 , respectively, and are each connected to controller  370 . In an exemplary embodiment, cell monitors  380 ,  385 ,  390 ,  395  are each suitably connected to cells  320 ,  325 ,  330 , and  335  such that cell monitors  380 ,  385 ,  390 , and  395  are each capable of reading the amount of charge contained within cells  320 ,  325 ,  330  and  335 , respectively. In another exemplary embodiment, cell monitors  380 ,  385 ,  390 , and  395  are suitably connected to controller  370  such that cell monitors  380 ,  385 ,  390 , and  395  are capable of communicating the amount of charge (or whether their respective battery cell includes charge above or below the threshold amount) contained within battery cells  320 ,  325 ,  330 , and  325  to controller  370 . 
         [0040]      FIG. 4  is a control truth table and operational chart for topology  300 , as illustrated in  FIG. 3 . For the illustrated embodiment of  FIG. 3 , there are 16 different permutations of the state of charge for battery cells  320 ,  325 ,  330 , and  335  during a charging operation. Only a few permutations will be described in detail herein, as doing so will make the other states of the control truth table readily apparent. Column  1  reflects the 16 different possible permutations of  FIG. 3 . Columns  2 ,  3 ,  4 , and  5  indicate the state of charge (i.e., fully charged (high) or not fully charged (low)) of battery cells  320 ,  325 ,  330 , and  335 , respectively. Columns  6 ,  7 , and  8  indicate the state of operation (i.e., on or oft) of charging sources  305 ,  310 , and  315 , respectively. Columns  9 ,  10 , and  11  illustrate the state of operation (i.e., on or off) of shunt regulators  350 ,  355 , and  360 , respectively. Column  12  illustrates the state of operation of topology  300  (i.e., charging system  120 ), as illustrated in  FIG. 3 . 
         [0041]    For example, in permutation  5 , battery cells  320 ,  330 , and  340  are not fully charged and need to be charged, whereas battery cell  325  is fully charged and should not be further charged (i.e., over-charged). In this situation, charging source  305  will be switched on by controller  370  since charging source  305  is the charging source electrically closest to battery cell  325 . In other words, the charging source which is: (i) located between a battery cell needing charge that is located closest to the load, and the load, and (ii) located farther away from the load than any other charging source(s) that may be located between the battery cell needing charge that is located closest to the load, and the load. Furthermore, shunt regulators  350  and  360  will also be switched on. As such, current will flow from charging source  305  and charge cell  320 . Also, current will flow through shunt regulator  350  by-passing battery cell  325  since shunt regulator  350  is switched on. Moreover, current will flow through and charge battery cells  330  and  335 . Thus, battery cells  320 ,  330 , and  335  will receive the necessary charging current, but battery cell  325  will not receive charging current. Therefore, shunt regulator  350  allows charging current to effectively by-pass a fully charged battery cell  325  such that battery cell  325  will not become over-charged, and possibly damaged. 
         [0042]    Permutation  10  is another example of how topology  300  provides charging current to battery cells needing to be charged, but yet does not provide charging current to cells fully charged. In this example, battery cells  325  and  330  need to be charged, whereas battery cells  320  and  325  are fully charged, or are at least contain an amount of charge above a threshold amount. As such, charging source  310  is switched on by controller  370  for the same reasons as charging source  305  in the above example. In addition, controller  370  will switch on shunt regulator  360  to prevent battery cell  335  from receiving charging current. Hence, charging current is supplied by charging source  310  to battery cells  325  and  330 , and the charging current flows through shunt regulator  360  to ground to avoid overcharging battery cell  335 . 
         [0043]    Permutation  15  illustrates the example of when only battery cell  335  requires recharging. In this example, controller  370  switches on charging source  315  (for the above reasons) such that charging current will flow from charging source  315  through battery cell  335  to ground. As such, battery cells  320 ,  325 , and  330  do not receive charging current since they are charged above the minimum threshold amount. 
         [0044]    The remaining permutations (i.e., permutations  1 - 4 ,  6 - 9 ,  11 - 14 , and  16 ) may analyzed in a manner similar to permutations  5 ,  10 , and  15 . Furthermore, the invention contemplates that only one of charging sources  305 ,  310 , and  315  will be on at any time. As such, the invention minimizes the amount of charging current that is dissipated during a charging operation. 
         [0045]      FIG. 5  is a flow diagram illustrating an exemplary embodiment of a method  500  for charging a secondary battery utilizing cell equalization. In one exemplary embodiment, method  500  initiates by at least one cell monitor (e.g., cell monitor  250 ) beginning to monitor the amount of charge in at least one battery cell (e.g., battery cell  320 ) to determine if battery cell  320  needs to be charged (step  510 ). When cell monitor  250  determines that battery cell  320  needs to be charged, cell monitor  250 , in one embodiment, transmits a signal to a controller (e.g., controller  270 ) to begin charging operations (step  520 ). In another embodiment, controller  270 , if needed, then switches on at least one shunt regulator (e.g., shunt regulator  230 ) to divert charging current from charging any battery cells  320  that are fully charged or charged above a threshold amount (step  530 ). 
         [0046]    Once any needed shunt regulators  230  are switched on such that charging current will be diverted around any battery cells  320  not needing to be charged (i.e., to prevent over-charging), in one exemplary embodiment, controller  270  will switch on the appropriate charging source (step  540 ). Which controller  270  switches on is determined in the manner discussed above in the examples discussing permutations  5 ,  10 , and  15  of  FIG. 4 . While charging operations are being performed, in one embodiment, cell batteries  320  are monitored by the cell monitor(s)  250  until the threshold voltage is reached in at least one battery cell  320  (step  550 ). 
         [0047]    In one embodiment, once the cell monitor(s)  250  transmits a signal to controller  270  indicating that at least one battery cell  320  has been charged to the threshold charge amount, controller  270  switches on the shunt regulator  230  connected in parallel to that particular cell battery  320  to divert the charging current from further charging the battery cell  320  (step  560 ). In another exemplary embodiment, steps  550  and  560  may be repeated until each battery cell  320  of series string  240  is charged to or above the threshold amount (step  565 ). After each battery cell  320  is charged to or above the threshold amount, controller  270  switches off charging source  305  (step  570 ) and any shunt regulators  230  that are switched on (step  580 ). 
         [0048]    Benefits, advantages and solutions to problems have been described herein with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of the invention. All structural, and functional equivalents to the elements of the above-described exemplary embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, no element described herein is required for the practice of the invention unless expressly described as “essential” or “critical.”