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 claims priority to U.S. Provisional Application No. 60/522,816, filed Nov. 11, 2004, which provisional application, in its entirety, is hereby incorporated by reference. 
     
    
     FIELD OF INVENTION  
       [0002]     The invention generally relates to secondary batteries, and more particularly, to cell equalization of such batteries.  
       BACKGROUND OF 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 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.  
         [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 battery 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 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 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 equalization. The equalization 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 a normal cell equalization termination, the string of individual battery cells connected in series generally is considered at the desired state of charge.  
         [0007]     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.  
         [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 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.  
         [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 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  
       [0010]     A system for charging a secondary battery according to various embodiments of the present invention includes N battery cells connected in series forming a series string, wherein the series string includes at least a battery cell connected to a load end, a battery cell connected to a ground end, and a cell junction between each pair of adjacent battery cells. In accordance with an exemplary embodiment, the system includes a charging set of charging sources (charging set) connected to the series string. In one embodiment, the charging set includes a charging source connected to the load end, and a different charging source connected to each cell junction, respectively.  
         [0011]     In accordance with another exemplary embodiment, the system includes a current diverting set of charging sources (diverting set) connected to the series string. In one embodiment, the input terminal of each charging source in the diverting set is connected to a different cell junction, and the output terminal of each charging source is connected to a current return path. In accordance with an aspect of one exemplary embodiment of the present invention, the return path may be a common current return path, on ground. In accordance with another aspect of one exemplary embodiment of the invention, each charging source in the diverting set may include its own current return path.  
         [0012]     The charging system, in accordance with one aspect of an exemplary embodiment, may include N charging sources in the charging set. In accordance with another aspect of one exemplary embodiment, the charging system may include (N-1) charging sources in the diverting set.  
         [0013]     In one exemplary embodiment, the charging system also includes a power source connected to each input terminal of the charging sources in the charging set, with each charging source in the charging set configured to provide charging current to at least one battery cell via an output terminal of each charging source. In accordance with an aspect of one exemplary embodiment, each charging source in the charging set may be configured to operate in a charging state and a non-charging state, and when operating in the charging state, each charging source provides charging current to a respective cell junction and battery cell where each respective charging source is connected; and when operating in the non-charging state, each charging source does not provide charging current to the respective cell junction and battery cell where each respective charging source is connected. In accordance with another aspect of one exemplary embodiment, each charging source in the diverting set may be configured to operate in a diverting state and a non-diverting state, and when each charging source in the diverting set is operating in the first state, each charging source in the diverting set forms a low impedance electrical path between a cell junction where each particular charging source in the diverting set is connected and the current return path; and when operating in the non-diverting state, each charging source forms a high impedance electrical path between the cell junction where each particular charging source is connected and the return path. In accordance with a further aspect of one exemplary embodiment, the charging system is configured to bypass current around a battery cell that is both (i) located adjacent the cell junction where a particular charging source is operating in the first state, and (ii) between the cell junction where the particular charging source operating in the first state is located and the ground end.  
         [0014]     In accordance with another exemplary embodiment, the charging system includes one or more cell monitors. In an aspect of one exemplary embodiment, a cell monitor is connected to each battery cell, and each cell monitor is configured to measure the amount of voltage contained in a battery cell with which the cell monitor is connected.  
         [0015]     The system, in accordance with yet another exemplary embodiment, includes a controller connected to each cell monitor, each charging source in the charging set, and each charging source in the diverting set. In an aspect of one exemplary embodiment, each charging source in the charging set is operated by the controller to provide charging current to one or more battery cells containing a terminal voltage below a threshold amount. In an aspect of another exemplary embodiment, each charging source in the diverting set is operated by the controller to divert charging current from one or more battery cells containing a terminal voltage above the threshold amount.  
         [0016]     A method for equalizing voltage of a secondary battery being charged according to various embodiments of the present invention includes coupling N battery cells in series to form a series string, coupling a charging set of charging sources to the series string, and coupling a diverting set of charging sources to the series string. In one embodiment, the coupling N battery cells step includes coupling a first battery cell to a load end, coupling a Nth battery cell to a ground end, and forming a respective cell junction between each adjacent pair of battery cells in the series string. In another embodiment, the coupling a charging set of charging sources step includes coupling a charging source in the charging set to the first cell and to the load end, and coupling each remaining charging source in the charging set to a different cell junction. In yet another embodiment, the coupling a diverting set of charging sources step includes coupling a different charging source in the diverting set to each respective cell junction. As such, various embodiments of the method include at least coupling one charging source from the charging set and coupling at least one charging source from the diverting set to each cell junction.  
         [0017]     In accordance with an aspect of one exemplary embodiment of the invention, the step of coupling a charging set of charging sources may include coupling N charging sources to the series string. In accordance with another embodiment of one exemplary embodiment, the step of coupling a diverting set of charging sources may include coupling (N-1) charging sources to the series string.  
         [0018]     In an exemplary embodiment, the method also includes the steps of configuring each charging source in the charging set to selectively provide charging current to one or more of the N battery cells, and configuring each charging source in the diverting set to selectively divert charging current from one or more of the N battery cells. In another embodiment, the method includes configuring each charging source in the charging set to operate in a first (e.g., ON) state or a second (e.g., OFF) state, wherein when a particular charging source is operating in the first state, the charging source provides charging current to a respective cell junction and battery cell where the charging source is coupled, and when the charging source is operating in the second state, the charging source does not provide charging current to the respective cell junction and a battery cell; and configuring each charging source in the diverting set to operate in a first state (e.g., ON) or a second (e.g., OFF) state, wherein when operating in the first state, a particular charging source in the diverting set provides a low impedance electrical path between the return path and the cell junction where the charging source is coupled, and when operating in the second state, the charging source provides a high impedance electrical path between the return path and the cell junction.  
         [0019]     In yet another exemplary embodiment, the method includes configuring the charging sources in the charging set to provide charging current to each battery cell containing a terminal voltage below a threshold amount, and configure each charging source the diverting set to not provide charging current to each battery cell containing a terminal voltage above the threshold amount. In still another exemplary embodiment, the method includes coupling a one or more cell monitors to the series string, wherein the monitor(s) is/are configured to monitor a respective voltage level of each of the battery cells, determine which battery cell(s) contain a terminal voltage above the threshold amount, and determining which battery cell(s) contain a terminal voltage below the threshold amount.  
         [0020]     Another method for equalizing voltage of a secondary battery being charged according to various embodiments of the present invention includes providing charging current to at least one battery cell in a series string containing a terminal voltage below a pre-determined threshold amount, and preventing charging current from being provided to any battery cell in the series string containing a terminal voltage above the pre-determined threshold amount utilizing a charging source. In one exemplary embodiment, the method also includes switching ON the charging source to cause charging current to flow through the first electrical path, and to not flow through the second electrical path. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     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:  
         [0022]      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;  
         [0023]      FIG. 2  is a block diagram of an exemplary embodiment of a charging system utilizing cell equalization to charge a secondary battery;  
         [0024]      FIG. 3  is block diagram of one exemplary embodiment of a topology of the charging system of  FIG. 2 ;  
         [0025]      FIG. 4  is a control truth table and operational chart for the topology illustrated in  FIG. 3 ;  
         [0026]      FIG. 5  is a flow diagram of an exemplary embodiment of a method for charging a secondary battery utilizing cell equalization; and  
         [0027]      FIG. 6  is a flow diagram of one embodiment of a method  600  for equalizing voltage of a secondary battery being charged. 
     
    
     DETAILED DESCRIPTION  
       [0028]     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.  
         [0029]     For the sake of brevity, the apparatus and systems (and components of the individual operating components) are described in detail herein. Furthermore, the coupling 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.  
         [0030]     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 an aspect of one 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 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 contemplates 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 .  
         [0031]     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 .  
         [0032]     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 after discharge.  
         [0033]     Device  100  includes a 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 .  
         [0034]      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.  
         [0035]     In one exemplary embodiment, controller  205  is connected to a charging set of charging sources  210  (charging set  210 ). Each charging source in charging set  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, each charging source in charging set  210  may each be any charging source known in the art capable of charging one or more battery cells.  
         [0036]     In another exemplary embodiment, controller  205  is also connected to a diverting set of charging sources  230  (diverting set  230 ). Each charging source in diverting set  230  may be any hardware and/or software suitably configured to provide an electrical path with lower impedance than a path including a battery cell when switched ON, and to provide an electrical path with higher impedance than a path including a battery cell when switched OFF. As such, each charging source in diverting set  230  may each be any charging source known in the art capable of providing a low and/or high impedance electrical path when switched ON and/or OFF, respectively. As used herein, the term “set” means one or more, for example, charging sources.  
         [0037]     Charging system  120 , in yet another exemplary embodiment, includes 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.  
         [0038]     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 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  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 .  
         [0039]      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 set of charging sources  310  (charging set  310 ). In one exemplary embodiment, power source  301  is configured similar to power source  110  discussed above, and charging set  310  is configured similar to charging set  210  discussed above. In accordance with an aspect of one exemplary embodiment of the present invention, charging set  310  may include a charging source  312 , a charging source  314 , a charging source  316 , and a charging source  318 , wherein charging sources  312 ,  314 ,  316 , and  318  are configured similar to the charging sources included in charging set  210  discussed above, and each charging source in charging set  310  is connected to a different battery cell. Although discussed as including four charging sources (i.e., charging sources  312 ,  314 ,  316 , and  318 ), charging set  310  may include any number of charging sources and not depart from the spirit and scope of the invention.  
         [0040]     In accordance with an exemplary embodiment of the present invention, the output terminal of charging sources  312 ,  314 ,  316 , and  318  are each connected to a series string of battery cells  340  (series string  340 ), wherein series string  340  is configured similar to series string  240  discussed above. In one exemplary embodiment, series string  340  includes a battery cell  342 , a battery cell  344 , a battery cell  346 , and a battery cell  348 . 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. In addition, the invention contemplates that battery cells  342 ,  344 ,  346 , and  348  may be any size battery cell known in the art.  
         [0041]     In one exemplary embodiment of topology  300 , the output terminal of charging source  312  is connected to both battery cell  342  and a load end of charging system  120  via a node  322 . In another exemplary embodiment, the output terminal of charging source  314  is connected to battery cell  344  via a node  324 . The output terminal of charging source  316 , in an exemplary embodiment, is connected to battery cell  346  via a node  326 . In still another exemplary embodiment, the output terminal of charging source  318  is connected to battery cell  348  via a node  328 . As such, the invention contemplates that nodes  322 ,  324 ,  326 , and  328  may be any type of node, device, material and/or junction suitably configured to conduct charging current to a battery cell and connect two or more circuit devices.  
         [0042]     In accordance with another exemplary embodiment of the present invention, topology  300  includes a diverting set of charging sources  330  (diverting set  330 ) connected to series string  340 . In accordance with an aspect of one exemplary embodiment of the present invention, diverting set  330  may include a charging source  334 , a charging source  336 , and a charging source  338 , wherein charging sources  334 ,  336 , and  338  are each configured similar to the charging sources included in diverting set  330  discussed above. Although discussed as including three charging sources (i.e., charging sources  334 ,  336 , and  338 ), charging set  330  may include any number of charging sources and not depart from the spirit and scope of the invention. In addition, various embodiments of the invention may be configured such that diverting set  330  will include at least one less charging source than charging set  310 .  
         [0043]     In an exemplary embodiment of topology  300 , an input terminal of charging source  334  is connected to node  324 , and an output terminal of charging source  334  is connected to a ground end of charging system  120 . In another exemplary embodiment, an input terminal of charging source  336  is connected to node  326 , and an output terminal of charging source  336  is connected to the ground end of charging system  120 . In yet another exemplary embodiment, an input terminal of charging source  338  is connected to node  328 , and an output terminal of charging source  338  is connected to the ground end of charging system  120 .  
         [0044]     Charging sources  312 ,  314 ,  316 ,  318 ,  334 ,  336 , and  338  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 an aspect of one exemplary embodiment, may be configured to transmit charging source control signals  307  to charging sources  312 ,  314 ,  316 ,  318 ,  334 ,  336 , and  338  to control the ON/OFF operation of charging sources  312 ,  314 ,  316 ,  318 ,  334 ,  336 , and  338 .  
         [0045]     Topology  300 , in another exemplary embodiment, 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 reading the terminal voltage of 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 whether their respective battery cells include a terminal voltage above or below the threshold amount to controller  305 .  
         [0046]      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  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  indicates 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  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. Columns  10 ,  11 , and  12  indicate the state of operation (i.e., ON or OFF) of charging sources  334 ,  336 , and  338 , respectively, and column  13  indicates the state of operation of topology  300  (i.e., charging system  120 ).  
         [0047]     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 (or at least contains a terminal voltage above a threshold amount) and should not be further charged (i.e., over-charged). In this situation, charging sources  312  and  316  will be switched ON by controller  305  (whereas charging sources  314  and  318  will remain switched OFF) to provide charging current to battery cells  342 ,  346 , and  348 . In addition, charging source  334  is also switched ON to divert charging current from being supplied to battery cell  344 . In the case of battery cell  342 , charging current is supplied to battery cell  342  from charging source  312 , wherein charging current is supplied through node  322  to charge battery cell  342 . The charging current is then diverted to ground through charging source  334  via node  324 . As such, charging source  334 , when turned ON, forms an electrical path with lower impedance than the path including battery cell  344  and diverts charging current away from battery cell  344 . In the case of battery cells  346  and  348 , charging current is supplied to battery cells  346  and  348  from charging source  316 , wherein charging current is supplied through node  326  to charge battery cell  346 , then through node  328  to charge battery cell  348 . In this case, charging source  318  is not switched ON since charging current supplied from charging source  316  is capable of also charging battery cell  348 .  
         [0048]     As a note, should battery cell  346  become fully charged prior to battery cell  348 , controller  305  will switch OFF charging source  316 , and switch ON charging source  318  until battery cell  348  becomes fully charged (or charged above the threshold amount). Similarly, should battery cell  348  become fully charged before battery cell  346 , controller  305  will switch ON charging source  338  to divert charging current from further charging (i.e., overcharging) battery cell  348 .  
         [0049]     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 containing a terminal 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 (or at least contain a terminal voltage above a threshold amount) and should not be further charged (i.e., over-charged). As such, charging sources  314  and  338  are switched ON by controller  305  (whereas the remaining charging sources will remain switched OFF) to provide charging current to battery cells  344  and  346 . In this situation, charging current in supplied to battery cell  344  from charging source  314  through node  324 . Charging current is also supplied to battery cell  346  from charging source  314  through node  326 . Since charging source  338  is also switched ON, the charging current will bypass battery cell  348  since the path including charging source  338  now has lower impedance than the path including battery cell  348 .  
         [0050]     Again worth noting, should battery cell  346  become fully charged prior to battery cell  344 , controller  305  will switch ON charging source  336  (and switch OFF charging source  338 ) to divert charging current from further charging (overcharging) battery cell  346 . Similarly, should battery cell  344  become fully charged before battery cell  346 , controller  305  will switch OFF charging source  314 , and switch ON charging source  316 , with charging source  338  remaining switched ON.  
         [0051]     Permutation  15  illustrates the example of when only one battery cell (i.e., battery cell  348 ) requires charging. In this example, controller  305  switches ON charging source  318  such that charging current will flow from charging source  318  through node  328  to battery cell  348 . 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.  
         [0052]     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.  
         [0053]      FIG. 5  is a flow diagram of one embodiment of a method  500  for equalizing voltage of a secondary battery being charged. In accordance with an exemplary embodiment of the present invention, method  500  initiates by coupling N battery cells (e.g., battery cells  342 ,  344 ,  346 ,  348 ) in series to form a series string (e.g., series string  340 ) (step  505 ). In one embodiment, the step of coupling N battery cells includes coupling a first battery cell to a load end (step  510 ). In another embodiment, the step of coupling N battery cells includes coupling a N th  battery cell to a ground end (step  515 ). In yet another embodiment, the step of coupling N battery cells also includes forming a respective cell junction between each adjacent pair of battery cells in the series string (step  520 ).  
         [0054]     In accordance with an exemplary embodiment, method  500  includes coupling a charging set (e.g., charging set  310 ) of charging sources (e.g., charging sources  312 ,  314 ,  316 , and  318 ) to series string  340  (step  525 ). In one embodiment, the coupling a charging set step includes coupling at least one charging source in charging set  310  to the first battery cell (e.g., battery cell  342 ) at the load end, and coupling each remaining charging source in the charging set to a respective battery cell in series string  340  via a respective cell junction (e.g., cell junctions  324 ,  326 , and  328 ).  
         [0055]     Method  500 , in accordance with another exemplary embodiment, includes coupling a diverting set (e.g., diverting set  330 ) of charging sources (e.g., charging sources  334 ,  336 , and  338 ) to series string  340  (step  530 ). In one embodiment, the coupling a diverting set step includes coupling each charging source in diverting set  330  to each of cell junctions  324 ,  326 , and  328 . As such, various embodiments of method  500  includes coupling at least one charging source from charging set  310  and coupling at least one charging source from diverting set  330  to each cell junction in series string  340 .  
         [0056]     In accordance with an aspect of one exemplary embodiment of the invention, the step of coupling a charging set of charging sources may include coupling N charging sources to series string  340 . In accordance with another exemplary embodiment of one embodiment of the invention, coupling a diverting set of charging sources may include the step of coupling (N-1) charging sources to series string  340 .  
         [0057]     Method  500 , in accordance with one exemplary embodiment, includes configuring each charging source in charging set  310  to selectively provide charging current to one or more of the N battery cells (step  535 ). In accordance with another embodiment, method  500  also includes configuring the charging sources in diverting set  330  to selectively divert charging current from one or more of the N battery cells (step  540 ). In yet another embodiment, method  500  includes operating each charging source in charging set  310  in an ON state or an OFF state, wherein when a particular charging source is ON, the charging source provides charging current to a respective cell junction and battery cell where the charging source is coupled; and when the charging source is OFF, the charging source does not provide charging current to the respective cell junction and battery cell (step  545 ). In a further embodiment, method  500  includes operating each charging source in diverting set  330  in an ON state or an OFF state, wherein when ON, a particular charging source in diverting set  330  provides a low impedance electrical path between the ground end and the cell junction where the charging source is coupled; and when OFF, the charging source provides a high impedance electrical path between the return path and the cell junction (step  550 ).  
         [0058]     In accordance with yet another exemplary embodiment, method  500  includes operating the charging sources in charging set  310  and diverting set  330  to provide charging current to each battery cell containing a terminal voltage below a threshold amount, and to not provide charging current to each battery cell containing a terminal voltage above the threshold amount (step  555 ). In still another exemplary embodiment, method  500  includes coupling one or more cell monitors (e.g., cell monitor  250 ) to each battery cell in series string  340  to monitor the voltage level of each of the battery cells, determine which battery cells contain a terminal voltage above the threshold amount, and determine which battery cells contain a terminal voltage below the threshold amount (step  560 ).  
         [0059]      FIG. 6  is a flow diagram of one embodiment of a method  600  for equalizing voltage of a secondary battery being charged. In accordance with one exemplary embodiment, method  600  initiates with providing charging current to at least one battery cell in a series string containing a terminal voltage below a pre-determined threshold amount, wherein said charging current is provided to charge said at least one battery cell (step  610 ). In accordance with another exemplary embodiment, method  600  includes preventing charging current from being provided to any battery cell in said series string containing a terminal voltage above the pre-determined threshold amount utilizing a charging source (step  620 ).  
         [0060]     In one embodiment, the preventing step includes the step of causing a first electrical path to possess an impedance lower than a second electrical path, wherein said second electrical path includes at least one battery cell containing a terminal voltage above said pre-determined threshold amount (step  630 ). In yet another exemplary embodiment, method  600  includes switching ON the charging source to cause charging current to flow through the first electrical path, and to not flow through the second electrical path step  640 ).  
         [0061]     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.”