US20060097697A1 - Method and system for cell equalization with switched charging sources - Google Patents

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
• 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
• 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 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 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.
• 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 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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
• 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 block diagram of one exemplary embodiment of a topology of the charging system ofFIG. 2;
• FIG. 4 is a control truth table and operational chart for the topology illustrated inFIG. 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 embodiment of amethod600 for equalizing 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 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.
• Turning now to the figures,FIG. 1 is a block diagram of one exemplary embodiment of adevice100 including asecondary battery130 and acharging system120 to rechargesecondary battery130.Device100, in one embodiment, includespower source110. In an exemplary embodiment,power source110 is a DC power source. In another exemplary embodiment,power source110 is an AC power source. In an aspect of one exemplary embodiment of the invention (e.g., whenpower source100 is a DC power source),power source110 may be a solar panel such thatpower source100 produces a DC signal. In another aspect of the invention (e.g., whenpower source110 is an AC power source),power source110 may be a standard AC outlet along with a transformer, or the like, to provide an appropriate voltage signal for chargingsecondary battery130. The invention contemplates thatpower source110 may be any DC or AC power source known in the art capable of providing charging current to rechargesecondary battery130.
• Device100, in another exemplary embodiment, includes chargingsystem120 electrically connected topower source110. In various aspects of the invention, chargingsystem120 may be suitably configured (as discussed in greater detail below) to charge one or more battery cells (not shown) withinsecondary battery130.
• In one exemplary embodiment,secondary battery130 is a lithium-ion battery. In other embodiments of the invention,secondary battery130 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.
• Device100 includes aload140 which, in an exemplary embodiment, is a device that requires voltage and current. Examples ofload140 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 bysecondary battery130.
• FIG. 2 is a block diagram of an exemplary embodiment of chargingsystem120 ofFIG. 1.Charging system120, in an exemplary embodiment, includescontroller205, which may be any hardware and/or software suitably configured to switch ON and OFF a charging source. As such,controller205 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,controller205 is connected to a charging set of charging sources210 (charging set210). Each charging source in charging set210 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 set210 may each be any charging source known in the art capable of charging one or more battery cells.
• In another exemplary embodiment,controller205 is also connected to a diverting set of charging sources230 (diverting set230). Each charging source in diverting set230 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 set230 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.
• Charging system120, in yet another exemplary embodiment, includes series string of battery cells240 (series string240).Series string240, 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 string240 may be configured to form a secondary battery of any desired voltage.
• Charging system120, in another exemplary embodiment, includes at least one cell monitor250 connected to a respective battery cell andcontroller210.Cell monitor250 may be any hardware and/or software suitably configured to monitor the terminal voltage of one or more battery cells. As such, cell monitor250 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 monitor250 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 monitor250 may be configured to determine if a battery cell, with which cell monitor250 is associated, contains a terminal voltage above or below a pre-determined threshold level. Furthermore,cell monitor250, 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 tocontroller210. 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 chargingsystem120 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 chargingsystem120.
• FIG. 3 is a block diagram of one exemplary embodiment of atopology300 of chargingsystem120. In an exemplary embodiment,topology300 includes apower source301 connected to a charging set of charging sources310 (charging set310). In one exemplary embodiment,power source301 is configured similar topower source110 discussed above, and charging set310 is configured similar to charging set210 discussed above. In accordance with an aspect of one exemplary embodiment of the present invention, charging set310 may include a chargingsource312, a chargingsource314, a chargingsource316, and a chargingsource318, wherein chargingsources312,314,316, and318 are configured similar to the charging sources included in charging set210 discussed above, and each charging source in charging set310 is connected to a different battery cell. Although discussed as including four charging sources (i.e., chargingsources312,314,316, and318), charging set310 may include any number of charging sources and not depart from the spirit and scope of the invention.
• In accordance with an exemplary embodiment of the present invention, the output terminal of chargingsources312,314,316, and318 are each connected to a series string of battery cells340 (series string340), whereinseries string340 is configured similar toseries string240 discussed above. In one exemplary embodiment,series string340 includes abattery cell342, abattery cell344, abattery cell346, and abattery cell348.Battery cells342,344,346, and348, in an exemplary embodiment, are lithium-ion battery cells. In other embodiments,battery cells342,344,346, and348 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 thatbattery cells342,344,346, and348 may be any size battery cell known in the art.
• In one exemplary embodiment oftopology300, the output terminal of chargingsource312 is connected to bothbattery cell342 and a load end of chargingsystem120 via anode322. In another exemplary embodiment, the output terminal of chargingsource314 is connected tobattery cell344 via anode324. The output terminal of chargingsource316, in an exemplary embodiment, is connected tobattery cell346 via anode326. In still another exemplary embodiment, the output terminal of chargingsource318 is connected tobattery cell348 via anode328. As such, the invention contemplates thatnodes322,324,326, and328 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.
• In accordance with another exemplary embodiment of the present invention,topology300 includes a diverting set of charging sources330 (diverting set330) connected toseries string340. In accordance with an aspect of one exemplary embodiment of the present invention, diverting set330 may include a chargingsource334, a chargingsource336, and a chargingsource338, wherein chargingsources334,336, and338 are each configured similar to the charging sources included in diverting set330 discussed above. Although discussed as including three charging sources (i.e., chargingsources334,336, and338), charging set330 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 set330 will include at least one less charging source than chargingset310.
• In an exemplary embodiment oftopology300, an input terminal of chargingsource334 is connected tonode324, and an output terminal of chargingsource334 is connected to a ground end of chargingsystem120. In another exemplary embodiment, an input terminal of chargingsource336 is connected tonode326, and an output terminal of chargingsource336 is connected to the ground end of chargingsystem120. In yet another exemplary embodiment, an input terminal of chargingsource338 is connected tonode328, and an output terminal of chargingsource338 is connected to the ground end of chargingsystem120.
• Charging sources312,314,316,318,334,336, and338 in one exemplary embodiment, are each connected to acontroller305, whereincontroller305 is configured similar tocontroller205 discussed above.Controller305, in an aspect of one exemplary embodiment, may be configured to transmit charging source control signals307 to chargingsources312,314,316,318,334,336, and338 to control the ON/OFF operation of chargingsources312,314,316,318,334,336, and338.
• Topology300, in another exemplary embodiment, includes acell monitor352, acell monitor354, acell monitor356, and acell monitor358, wherein cell monitors352,354,356, and358 are each configured similar to cell monitor250 discussed above. In one embodiment, cell monitors352,354,356, and358 are connected tobattery cells342,344,346, and348, respectively, and are each connected tocontroller305. In an exemplary embodiment, cell monitors352,354,356, and358 are each suitably connected tobattery cells342,344,346, and348 such that cell monitors352,354,356, and358 are each capable of reading the terminal voltage ofbattery cells342,344,346, and348, respectively. In another exemplary embodiment, cell monitors352,354,356, and358 are suitably connected tocontroller305 such that cell monitors352,354,356, and358 are capable of communicating whether their respective battery cells include a terminal voltage above or below the threshold amount tocontroller305.
• FIG. 4 is a control truth table and operational chart fortopology300, as illustrated inFIG. 3. For the illustrated embodiment ofFIG. 3, there are 16 different permutations of the state of charge forbattery cells342,344,346, and348 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,column1 indicates the 16 different possible permutations ofFIG. 3.Columns2,3,4, and5 indicate the state of charge (i.e., fully charged (high) or not fully charged (low)) ofbattery cells342,344,346, and348, respectively.Columns6,7,8, and9 indicate the state of operation (i.e., ON or OFF) of chargingsources312,314,316, and318, respectively.Columns10,11, and12 indicate the state of operation (i.e., ON or OFF) of chargingsources334,336, and338, respectively, andcolumn13 indicates the state of operation of topology300 (i.e., charging system120).
• Inpermutation5, for example,battery cells342,346, and348 are not fully charged and need to be charged, whereasbattery cell344 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, chargingsources312 and316 will be switched ON by controller305 (whereas chargingsources314 and318 will remain switched OFF) to provide charging current tobattery cells342,346, and348. In addition, chargingsource334 is also switched ON to divert charging current from being supplied tobattery cell344. In the case ofbattery cell342, charging current is supplied tobattery cell342 from chargingsource312, wherein charging current is supplied throughnode322 to chargebattery cell342. The charging current is then diverted to ground through chargingsource334 vianode324. As such, chargingsource334, when turned ON, forms an electrical path with lower impedance than the path includingbattery cell344 and diverts charging current away frombattery cell344. In the case ofbattery cells346 and348, charging current is supplied tobattery cells346 and348 from chargingsource316, wherein charging current is supplied throughnode326 to chargebattery cell346, then throughnode328 to chargebattery cell348. In this case, chargingsource318 is not switched ON since charging current supplied from chargingsource316 is capable of also chargingbattery cell348.
• As a note, shouldbattery cell346 become fully charged prior tobattery cell348,controller305 will switch OFF chargingsource316, and switch ON chargingsource318 untilbattery cell348 becomes fully charged (or charged above the threshold amount). Similarly, shouldbattery cell348 become fully charged beforebattery cell346,controller305 will switch ON chargingsource338 to divert charging current from further charging (i.e., overcharging)battery cell348.
• Permutation10 is another example of howtopology300 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 cells344 and346 need to be charged, whereasbattery cells342 and348 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, chargingsources314 and338 are switched ON by controller305 (whereas the remaining charging sources will remain switched OFF) to provide charging current tobattery cells344 and346. In this situation, charging current in supplied tobattery cell344 from chargingsource314 throughnode324. Charging current is also supplied tobattery cell346 from chargingsource314 throughnode326. Since chargingsource338 is also switched ON, the charging current will bypassbattery cell348 since the path including chargingsource338 now has lower impedance than the path includingbattery cell348.
• Again worth noting, shouldbattery cell346 become fully charged prior tobattery cell344,controller305 will switch ON charging source336 (and switch OFF charging source338) to divert charging current from further charging (overcharging)battery cell346. Similarly, shouldbattery cell344 become fully charged beforebattery cell346,controller305 will switch OFF chargingsource314, and switch ON chargingsource316, with chargingsource338 remaining switched ON.
• Permutation15 illustrates the example of when only one battery cell (i.e., battery cell348) requires charging. In this example,controller305 switches ON chargingsource318 such that charging current will flow from chargingsource318 throughnode328 tobattery cell348. As such,battery cells312,314, and316 do not receive charging current since they are fully charged and/or charged above the minimum threshold voltage amount.
• The remaining permutations (i.e., permutations1-4,6-9,11-14, and16) may be analyzed in a manner similar topermutations5,10, and15. Furthermore, the invention contemplates that chargingsystem120 may include any number of battery cells inseries string240, and corresponding charging sources and cell monitors without departing from the spirit and scope of the invention.
• FIG. 5 is a flow diagram of one embodiment of amethod500 for equalizing voltage of a secondary battery being charged. In accordance with an exemplary embodiment of the present invention,method500 initiates by coupling N battery cells (e.g.,battery cells342,344,346,348) in series to form a series string (e.g., series string340) (step505). In one embodiment, the step of coupling N battery cells includes coupling a first battery cell to a load end (step510). In another embodiment, the step of coupling N battery cells includes coupling a N^thbattery cell to a ground end (step515). 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 (step520).
• In accordance with an exemplary embodiment,method500 includes coupling a charging set (e.g., charging set310) of charging sources (e.g., chargingsources312,314,316, and318) to series string340 (step525). In one embodiment, the coupling a charging set step includes coupling at least one charging source in charging set310 to the first battery cell (e.g., battery cell342) at the load end, and coupling each remaining charging source in the charging set to a respective battery cell inseries string340 via a respective cell junction (e.g.,cell junctions324,326, and328).
• Method500, in accordance with another exemplary embodiment, includes coupling a diverting set (e.g., diverting set330) of charging sources (e.g., chargingsources334,336, and338) to series string340 (step530). In one embodiment, the coupling a diverting set step includes coupling each charging source in diverting set330 to each ofcell junctions324,326, and328. As such, various embodiments ofmethod500 includes coupling at least one charging source from charging set310 and coupling at least one charging source from diverting set330 to each cell junction inseries string340.
• 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 toseries string340. 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 toseries string340.
• Method500, in accordance with one exemplary embodiment, includes configuring each charging source in charging set310 to selectively provide charging current to one or more of the N battery cells (step535). In accordance with another embodiment,method500 also includes configuring the charging sources in diverting set330 to selectively divert charging current from one or more of the N battery cells (step540). In yet another embodiment,method500 includes operating each charging source in charging set310 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 (step545). In a further embodiment,method500 includes operating each charging source in diverting set330 in an ON state or an OFF state, wherein when ON, a particular charging source in diverting set330 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 (step550).
• In accordance with yet another exemplary embodiment,method500 includes operating the charging sources in charging set310 and diverting set330 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 (step555). In still another exemplary embodiment,method500 includes coupling one or more cell monitors (e.g., cell monitor250) to each battery cell inseries string340 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 (step560).
• FIG. 6 is a flow diagram of one embodiment of amethod600 for equalizing voltage of a secondary battery being charged. In accordance with one exemplary embodiment,method600 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 (step610). In accordance with another exemplary embodiment,method600 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 (step620).
• 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 (step630). In yet another exemplary embodiment,method600 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 step640).
• 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.”

Claims (18)

1. A charging system for charging a rechargeable battery, comprising:

N battery cells coupled in series forming a series string, wherein said series string comprises:

a first battery cell coupled to a load end,

a N^thbattery cell coupled to a ground end, and

a cell junction between each pair of adjacent battery cells;

a first set of charging sources coupled to said series string, wherein each charging source in said first set comprises an input terminal and an output terminal, wherein an output terminal of a first charging source in said first set is coupled to said load end, and an output terminal of any other charging source is coupled to a respective cell junction occurring thereafter; and

a second set of charging sources coupled to said series string, wherein each charging source in said second set comprise an input terminal and an output terminal, wherein an input terminal of each charging source in said second set is coupled to a respective cell junction, and an output terminal of each charging source in said second set is coupled to said ground end.

2. The charging system ofclaim 1, wherein there are N charging sources in said first set of charging sources.

3. The charging system ofclaim 2, wherein there are (N-1) charging sources in said second set of charging sources.

4. The charging system ofclaim 1, further comprising:

a power source coupled to each input terminal of said charging sources in said first set, wherein each charging source in said first set is configured to provide charging current to at least one of said N battery cells.

5. The charging system ofclaim 4, wherein:

each charging source in said first set of charging sources is configured to operate in a first state and a second state; and when each charging source in said first set is operating in said first state, each charging source provides charging current to a respective cell junction and battery cell where each respective charging source is coupled, and when each charging source in said first set is operating in said second state, each charging source does not provide charging current to said respective cell junction and battery cell where each respective charging source is coupled.

6. The charging system ofclaim 1, wherein:

said ground end is a charging current common return path to each charging source in said second set.

7. The charging system ofclaim 1, wherein:

each charging source in said second set is configured to operate in a first state and a second state; and

when each charging source in said second set is operating in said first state, each charging source in said second set forms a low impedance electrical path between a cell junction where each particular charging source is coupled and said return path, and when each charging source in said second set is operating in said second state, each charging source forms a high impedance electrical path between said cell junction where each particular charging source is coupled and said return path.

8. The charging system ofclaim 7, wherein the charging system is configured such that when each charging source in said second set is operating in said first state, charging current bypasses a battery cell located:

(a) adjacent a cell junction where a particular charging source operating in said first state is coupled, and

(b) between said cell junction where a particular charging source operating in said first state is coupled and said ground end.

9. The charging system ofclaim 1, further comprising:

a plurality of cell monitors, wherein each cell monitor is coupled to a respective battery cell, and configured to measure a terminal voltage in an associated battery cell; and

a controller, wherein said controller is coupled to each of said cell monitors, each charging source in said first set, and each charging source in said second set, and wherein each charging source in said first set is operated by said controller to provide charging current to one or more battery cells containing a terminal voltage below a threshold amount, and each charging source in said second set is operated by said controller to divert charging current from one or more battery cells containing a terminal voltage above said threshold amount.

10. A method for equalizing voltage of a secondary battery being charged, the method comprising the steps of:

coupling N battery cells in series to form a series string, wherein said coupling N battery cells step comprises the steps of:

coupling a first battery cell to a load end,

coupling a N^thbattery cell to a ground end, and

forming a respective cell junction between each adjacent pair of battery cells in said series string;

coupling a first set of charging sources to said series string, wherein said coupling a first set of charging sources step comprises the steps of:

coupling a first charging source of said first set to said first cell at said load end, and

coupling each remaining charging source in said first set to each cell junction formed on said series string; and

coupling a second set of charging sources to said series string, wherein said coupling a second set of charging sources step comprises the step of:

coupling each charging source of said second set to each cell junction formed on said series string.

11. The method ofclaim 10, wherein said coupling a first set of charging sources step comprises the step of:

coupling N charging sources to said series string.

12. The method ofclaim 11, wherein said coupling a second set of charging sources step comprises the step of:

coupling (N-1) charging sources to said series string.

13. The method ofclaim 10, further comprising the steps of:

configuring each charging source in said first set to operate in one of a first state and a second state, wherein said configuring step comprises configuring each charging source in said first set to provide charging current to a respective cell junction and battery cell where said first particular charging source is coupled when operating in said first state, and configuring each charging source in said first step to not provide charging current to said respective cell junction and battery cell where said first particular charging source is coupled when operating in said second state; and

configuring each charging source in said second set to operate in one of a first state and a second state, wherein said configuring step comprises configuring each charging source in said second set to form a low impedance electrical path between a return path and a respective cell junction where each charging source in said second set is coupled when operating in said first state, to form a high impedance electrical path between said return path and said respective cell junction where each charging source in said second set is coupled when operating in said second state.

14. The method ofclaim 10, further comprising the step of:

configuring each charging source in said first set to provide charging current to each battery cell containing a terminal voltage below a threshold amount; and

configuring each charging source in said second set to prevent each battery cell containing a terminal voltage above said threshold amount from receiving charging current.

15. The method ofclaim 10, further comprising the steps of:

coupling a plurality of cell monitors to each battery cell included in said series string, wherein said cell monitors are configured to:

monitor a respective voltage level of each of said N battery cells;

determine which of said N battery cells contains a terminal voltage above said threshold amount; and

determine which of said N battery cells contains a terminal voltage below said threshold amount.

16. The method ofclaim 10, further comprising the steps of:

configuring each charging source in said first set to selectively provide charging current to one or more of said N battery cells; and

configuring each charging source in said second set to selectively divert charging current from one or more of said N battery cells.

17. A method for equalizing voltage of a secondary battery being charged, the method comprising the steps of:

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; and

preventing charging current from being provided to any battery cell in said series string containing a terminal voltage above said pre-determined threshold amount utilizing a charging source, wherein said preventing step comprises 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.

18. The method ofclaim 17, further comprising the step of:

switching ON said charging source to cause said charging current to flow through said first electrical path, and to not flow through said second electrical path.

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