Abstract:
A system and method for charging a rechargeable, or secondary, battery including a series string of battery cells, a topology of charging sources that selectively provides charging current to battery cells that need to be charged, but avoids overcharging battery 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 bypasses battery cells that are fully charged or become fully charged.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/522,815 filed Nov. 11, 2004, which provisional application, in its entirety, is hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The invention generally relates to secondary batteries, and more particularly, to cell equalization of such batteries. 
     BACKGROUND OF INVENTION 
     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 battery 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. 
     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 battery cell in the series string does not experience an over-voltage condition during charging. 
     When charging, secondary battery cells generally are bulk charged if the battery 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 battery cell being charged to a different level during any one recharge. To minimize divergence, a second step in the charging process often is implemented. 
     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 usually 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 battery cell may not be fully charged. 
     In addition to overcharging, 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. 
     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 over-discharge, 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. 
     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 battery 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 INVENTION 
     A system for charging a secondary battery according to various embodiments of the present invention includes N battery cells connected in a series string, wherein the series string includes a first battery cell located at a load end and a N th  battery cell located at a ground end. In one embodiment, two or more charging sources are connected to the series string, wherein each charging source is connected in parallel to a respective battery cell. 
     In accordance with one exemplary embodiment of the present invention, each charging source is connected to a central charging source. In another embodiment, each charging source is configured to provide charging current to each respective battery cell via a positive path, and provide a charging current return path via a negative path. In yet another embodiment, each charging source is configured to operate in a first (e.g., charging) state and a second (e.g., non-charging) state, wherein when operating in the first state, each charging source provides charging current to a respective battery cell, and when operating in the second state, does not provide charging current to the battery cell. 
     In one exemplary embodiment, the charging system includes a controller in communication with each charging source. In accordance with an aspect of one exemplary embodiment, the controller selectively controls the operation of each charging source, such that each charging source is operating in the first state or the second state. 
     In accordance with another exemplary embodiment, the charging system includes one or more cell monitors connected to the series string wherein each cell monitor is configured to measure the voltage of a battery cell connected to each respective cell monitor. The charging system, in accordance with yet another exemplary embodiment, includes a controller connected to each cell monitor and connected to each charging source, wherein each charging source is controlled by the controller to (i) provide charging current to their respective battery cells when the battery cell contains an amount of voltage below a threshold amount, and (ii) not provide charging current to the battery cell when the battery cell contains an amount of voltage above the threshold amount. 
     A method for equalizing voltage of secondary battery being charged according to various embodiments of the present invention includes the steps of connecting N battery cells in series to form a series string, wherein the connecting step includes connecting one battery cell to a load end, connecting a N th  cell to a ground end, and connecting two or more charging sources to the series string, wherein each charging source is connected in parallel to a respective battery cell. In accordance with one exemplary embodiment of the present invention, the method includes configuring the charging sources to selectively provide charging current to one or more of the N battery cells. In accordance with an aspect of one exemplary embodiment of the present invention, the step of configuring the charging sources includes configuring a particular charging source to operate in a first (e.g., charging) state to provide charging current to a respective battery cell, and configuring the particular charging source to operate in a second (e.g., non-charging) state to not provide charging current to the battery cell. 
     In accordance with another exemplary embodiment, the method includes connecting each charging source to a power source. In accordance with yet another exemplary embodiment, the method includes configuring the charging sources to provide charging current to each cell containing an amount of voltage below a threshold amount, and to not provide charging current to each battery cell containing an amount of voltage above the threshold amount. The method, in accordance with still another exemplary embodiment, includes connecting one or more cell monitors to the series string, wherein each cell monitor is connected to a respective battery cell, configured to monitor the voltage level in the battery cell(s), and determine which battery cell(s) is/are above and/or below the threshold amount. 
     Another method for equalizing voltage of a secondary battery being charged according to various embodiments of the present invention includes connecting two or more battery cells in series to form a series string, connecting in parallel across each battery cell an associated charging source, charging a particular battery cell with the associated charging source when an amount of voltage in the particular battery cell is below a threshold level, and not charging any battery cell(s) including an amount of voltage above the threshold level. In accordance with an aspect of one exemplary embodiment, the step of charging a particular cell includes switching ON the a respective charging source to charge the associated battery cell, and switching OFF the isolated charging source to avoid charging (or overcharging) its associated battery cell. 
     The method, in accordance with another exemplary embodiment of the invention, includes monitoring a respective voltage level in each of the battery cells, and independently operating each of the charging sources in an ON state or an OFF state based on the voltage level of an associated battery cell. In accordance with an aspect of one exemplary embodiment of the present invention, charging a particular cell may include the step of providing charging current to the particular battery cell via the associated charging source. In accordance with another aspect of one exemplary embodiment of the present invention, charging a particular cell may include returning the charging current to the associated charging source via a charging current return path (e.g., a ground end). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         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; 
         FIG. 2  is a block diagram of an exemplary embodiment of a charging system utilizing cell equalization to charge a secondary battery; 
         FIG. 3  is a block diagram of one exemplary embodiment of a topology of the charging system of  FIG. 2 ; 
         FIG. 4  is a control truth table and operational chart for the topology illustrated in  FIG. 3 ; 
         FIG. 5  is a flow diagram of an exemplary embodiment of a method for charging a secondary battery utilizing cell equalization; and 
         FIG. 6  is a flow diagram of one exemplary embodiment of a method for equalizing the voltage of a secondary battery being charged. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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. 
     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 exemplary embodiment, includes a power source  110 , which may be a DC power source or an AC power source. In one aspect of an exemplary embodiment of the invention (e.g., 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 one exemplary embodiment of the invention (e.g., 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 does contemplate, however, that power source  110  may be any DC or AC power source known in the art capable of providing charging current to recharge secondary battery  130 . 
     Device  100 , in another exemplary embodiment, includes charging system  120  connected to power source  110 . In various exemplary embodiments 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 . 
     In one exemplary embodiment, secondary battery  130  is a lithium-ion battery including one or more battery cells. 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 including one or more battery cells capable of storing a charge and subsequently being recharged after discharge. 
     Device  100  includes load  140  connected to secondary battery  130 , wherein device  100 , in an exemplary embodiment, is a device that requires voltage and current. Examples of load  140  may include, but are certainly 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 . 
       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  205 , which may be any hardware and/or software suitably configured to switch ON and OFF a charging source. As such, controller  205  may be any controller known in the art capable of switching ON and OFF charging sources when appropriate to do such. 
     In one exemplary embodiment, controller  205  is connected to charging source(s)  210 . Charging source  210  may be any hardware and/or software suitably configured to provide charging current to at least one battery cell when switched ON (i.e., operating in a charging state), and not provide charging current to a battery cell when switched OFF (i.e., operating in a non-charging state). As such, charging source  210  may be any charging source known in the art capable of charging one or more battery cells. 
     In another exemplary embodiment, charging system  120  includes a series string of battery cells  240  (series string  240 ). Series string  240 , in an exemplary embodiment, includes 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  205 . 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 pre-determined 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  205 . 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 . 
       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  301  connected to a charging source  312 , a charging source  314 , a charging source  316 , and a charging source  318 . In an exemplary embodiment, power source  301  is configured similar to power source  110  discussed above, and charging sources  312 ,  314 ,  316 , and  318  are each configured similar to charging source  210  discussed above. 
     In one exemplary embodiment of the present invention, charging source  312  is connected to battery cell  342  via positive path  322  and negative path  332 . In accordance with an aspect of one exemplary embodiment of the present invention, charging source  312  is coupled in parallel to battery cell  342 , wherein charging source  312  is coupled to the positive terminal (V+) and negative terminal (V−) of battery cell  342  via positive path  322  and negative path  332 , respectively. 
     In another exemplary embodiment, charging source  314  is connected to battery cell  344  via positive path  324  and negative path  334 . In accordance with another aspect of one exemplary embodiment of the present invention, charging source  314  is coupled in parallel to battery cell  344 , wherein charging source  314  is coupled to V+ and V− of battery cell  342  via positive path  324  and negative path  334 , respectively. 
     Charging source  316 , in an exemplary embodiment, is connected to battery cell  346  via positive path  326  and negative path  336 . In accordance with yet another aspect of one exemplary embodiment of the present invention, charging source  316  is coupled in parallel to battery cell  346 , wherein charging source  316  is coupled to V+ and V− of battery cell  346  via positive path  326  and negative path  336 , respectively. 
     In still another exemplary embodiment, charging source  318  is connected to battery cell  348  via positive path  328  and negative path  338 . In accordance with an aspect of one exemplary embodiment of the present invention, charging source  318  is coupled in parallel to battery cell  348 , wherein charging source  318  is coupled to V+ and V− of battery cell  348  via positive path  328  and negative path  338 , respectively. 
     Battery cells  342 ,  344 ,  346 , and  348 , in an exemplary embodiment, are lithium-ion battery cells. In other embodiments, battery cells  342 ,  344 ,  346 , and  348  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 battery cells capable of storing a charge and subsequently being recharged after discharge. In addition, the invention contemplates that battery cells  342 ,  344 ,  346 , and  348  may each be any size battery cell known in the art. 
     Positive paths  322 ,  324 ,  326 , and  328  may be any hardware and/or device suitably configured to conduct charging current. As such, positive paths  322 ,  324 ,  326 , and  328  may be formed of any material known in the art capable of conducting charging current supplied from a charging source to a battery cell to charge the battery cell. Negative paths  332 ,  334 ,  336 , and  338  may also be any hardware and/or device suitably configured to conduct charging current. As such negative paths  332 ,  334 ,  336 , and  338  may be formed of any material known in the art capable of conducting and/or returning charging current from a battery cell to a charging source. 
     Charging sources  312 ,  314 ,  316 , and  318 , in one exemplary embodiment, are each connected to a controller  305 , wherein controller  305  is configured similar to controller  205  discussed above. Controller  305 , in one exemplary embodiment, is configured to transmit charging source control signals  307  to charging sources  3312 ,  314 ,  316 , and  318  to control the ON/OFF operation of charging sources  312 ,  314 ,  316 , and  318 . 
     Topology  300 , in one exemplary embodiment, also includes a cell monitor  352 , a cell monitor  354 , a cell monitor  356 , and a cell monitor  358 , wherein cell monitors  352 ,  354 ,  356 , and  358  are each configured similar to cell monitor  250  discussed above. In one embodiment, cell monitors  352 ,  354 ,  356 , and  358  are connected to battery cells  342 ,  344 ,  346 , and  348 , respectively, and are each connected to controller  305 . In an exemplary embodiment, cell monitors  352 ,  354 ,  356 , and  358  are each suitably connected to battery cells  342 ,  344 ,  346 , and  348  such that cell monitors  352 ,  354 ,  356 , and  358  are each capable of determining the amount of charge contained within battery cells  342 ,  344 ,  346 , and  348 , respectively. In another exemplary embodiment, cell monitors  352 ,  354 ,  356 , and  358  are suitably connected to controller  305  such that cell monitors  352 ,  354 ,  356 , and  358  are capable of communicating the amount of charge (or whether their respective battery cell includes an amount of charge above or below a threshold amount) contained within battery cells  342 ,  344 ,  346 , and  348 , respectively, to controller  305 . 
       FIG. 4  is a control truth table and operational chart for topology  300 , as illustrated in  FIG. 3 . For the exemplary embodiment illustrated in  FIG. 3 , there are 16 different permutations representing the state of charge of battery cells  342 ,  344 ,  346 , and  348  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. In the control truth table, column 1 reflects the 16 different possible permutations of the exemplary embodiment 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  342 ,  344 ,  346 , and  348 , respectively. Columns 6, 7, 8, and 9 indicate the state of operation (i.e., ON or OFF) of charging sources  312 ,  314 ,  316 , and  318 , respectively, and column 10 indicates the state of operation of topology  300  (i.e., charging system  120 ). 
     In permutation 5, for example, battery cells  342 ,  346 , and  348  are not fully charged and need to be charged, whereas battery cell  344  is fully charged (i.e., the cell voltage is above a threshold amount) and should not be further charged (i.e., over-charged). In this situation, charging sources  312 ,  316 , and  318  will be switched ON by controller  305  (whereas charging source  314  will remain switched OFF) to provide charging current to battery cells  342 ,  346 , and  348 , respectively. In the case of battery cell  342 , charging current is supplied to battery cell  342  from charging source  312 , wherein the charging current is supplied through positive path  322  to charge battery cell  342 , then returns to charging source  312  via negative path  332  so as not to supply charging current to battery cells  344 ,  346 , and  348 . Similarly, in the case of battery cell  346 , charging current in supplied to battery cell  346  from charging source  316 , wherein the charging current is supplied through positive path  326  to charge battery cell  346 , then returns to charging source  316  via negative path  336  so as not to supply charging current to battery cells  342 ,  344 , and  348 . Furthermore, in the case of battery cell  348 , charging current is supplied to battery cell  348  from charging source  318 , wherein the charging current is supplied through positive path  328  to charge battery cell  348 , then returns to charging source  318  via negative path  338  so as not to supply charging current to battery cells  342 ,  344 , and  346 . 
     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 battery cells fully charged or have a cell voltage above a threshold amount. In this example, battery cells  344  and  346  need to be charged, whereas battery cells  342  and  348  are fully charged (i.e., a cell voltage above a threshold amount) and should not be further charged (i.e., over-charged). As such, charging sources  314  and  316  are switched ON by controller  305  (whereas charging sources  312  and  318  will remain switched OFF) to provide charging current to battery cells  344  and  346 , respectively. In the case of battery cell  344 , charging current is supplied to battery cell  344  from charging source  314 , wherein the charging current is supplied through positive path  324  to charge battery cell  344 , then returns to charging source  314  via negative path  334  so as not to supply charging current to battery cells  342 ,  346 , and  348 . Likewise, in the case of battery cell  346 , charging current is supplied to battery cell  346  from charging source  316 , wherein the charging current is supplied through positive path  326  to charge battery cell  346 , then returns to charging source  316  via negative path  336  so as not to supply charging current to battery cells  342 ,  344 , and  348 . 
     Permutation 15 illustrates the example of when only one battery cell (i.e., battery cell  318 ) requires charging. In this example, controller  305  switches ON charging source  318  such that charging current will flow from charging source  318  via positive path  328  to battery cell  348 , and return to charging source  318  via negative path  338 . As such, battery cells  312 ,  314 , and  316  do not receive charging current since they are fully charged and/or charged above the minimum threshold voltage amount. 
     The remaining permutations (i.e., permutations 1-4, 6-9, 11-14, and 16) may be analyzed in a manner similar to permutations 5, 10, and 15. Furthermore, the invention contemplates that charging system  120  may include any number of battery cells in series string  240 , and corresponding charging sources and cell monitors without departing from the spirit and scope of the invention. In addition, negative path  338  may be omitted since charging current leaving battery cell  348  will not charge any other battery cell, but will instead, flow to ground. 
       FIG. 5  is a flow diagram of an exemplary embodiment of a method  500  for charging a secondary battery utilizing cell equalization. In accordance with an exemplary embodiment, method  500  initiates by coupling N battery cells (e.g., battery cells  342 ,  244 ,  246 , and  348 ) in series to form a series string (e.g., series string  240 ) on a platform (step  510 ). In one exemplary embodiment, the step of coupling N battery cells includes coupling a different battery cell to a load end (step  520 ), and coupling a battery cell to a ground end (step  530 ). 
     In accordance with an exemplary embodiment, method  500  also includes coupling a plurality of charging sources (e.g., charging sources  312 ,  214 ,  316 , and  318 ) in parallel to the N battery cells (step  540 ). In accordance with an aspect of one exemplary embodiment of the present invention, coupling the plurality of charging sources in parallel includes coupling each charging source to a respective battery cell via a positive path (e.g., positive path  322 ) and a negative path (e.g., negative path  332 ). 
     Method  500 , in accordance with an exemplary embodiment, includes configuring each charging source to selectively provide charging current to a single battery cell in series string  240  (step  550 ). In accordance with an aspect of one exemplary embodiment of the present invention, configuring each charging source may include configuring each charging source to provide charging current to each battery cell containing an amount of voltage below a threshold amount, and not provide charging current to each cell containing an amount of voltage above the threshold amount. In accordance with another aspect of one exemplary embodiment of the present invention, configuring each charging source may include configuring each charging source to operate in a charging state to provide charging current to a respective battery cell, and configuring each charging source to operate in a non-charging state to not provide charging current to the battery cell. 
     In another exemplary embodiment, method  500  includes coupling each charging source to a power source (e.g., power source  110 ) to provide power to each charging source (step  560 ). In yet another embodiment, method  500  includes coupling a cell monitor to each battery cell to monitor the voltage level of each of battery cell (step  570 ). Method  500 , in still another embodiment, includes configuring the cell monitors to determine which battery cell(s) contain an amount of voltage above and/or below the threshold amount (step  580 ). 
       FIG. 6  is a flow diagram of another exemplary embodiment of a method  600  for equalizing the voltage of a secondary battery being charged including coupling two or more battery cells (e.g., battery cells  342 ,  344 ,  346 . and  348 ) in series to form a series string (e.g., series string  240 ) (step  610 ). In one exemplary embodiment, method  600  includes coupling a charging source (e.g., charging source  312 ) in parallel across each battery cell (step  620 ). Method  600 , in another exemplary embodiment, includes charging any battery cell(s) containing an amount of voltage below a threshold level (step  630 ), and not charging any battery cell(s) containing an amount of voltage above the threshold level (step  640 ). In accordance with another aspect of the present invention, charging a battery cell may include providing charging current to a particular battery cell via a charging source coupled in parallel to the battery cell. In accordance with another aspect of one exemplary embodiment of the present invention, charging a battery cell may include switching ON a charging source to charge a particular battery cell needing to be charged, and switching OFF the charging source to stop charging the battery cell when it contains a voltage level above the threshold amount. 
     In one exemplary embodiment, method  600  also includes monitoring the voltage level of battery cell in the series string (step  650 ). In another embodiment, method  600  includes operating each charging source independently in an ON state or in an OFF state based on the voltage level of a battery cell connected to the charging source (step  660 ). 
     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.”