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Method and system for cell equalization with switched charging sourcesMethod and system for cell equalization with switched charging sources description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060097697, Method and system for cell equalization with switched charging sources. Brief Patent Description - Full Patent Description - Patent Application Claims
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. Continue reading about Method and system for cell equalization with switched charging sources... 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