DYNAMIC EQUALIZATION DURING CHARGING OF SERIAL ENERGY-STORAGE ELEMENTS

A technique is described that equalizes the amount of charge in a serial string of energy-storage cells during charging. The equalization is accomplished using dc-to-dc converters to divert portions of the charging current past selected cells (or groups of cells.) When no converters are operating, the charging current through the string is equal to that of the charging source. As the string charges, one cell eventually reaches a threshold voltage V(A). At threshold, a shunt converter is activated to divert current around the cell, thus maintaining the cell at V(A). The diverted current extracts energy which is placed back into the charging bus, and appears as an additional charging current to the source. This positive feedback of current, with a gain of less than one, increases the current available for charging the string. This recirculated current allows the lowest charged cells, or cells of larger capacity, to be charged at higher rates than available directly from the source. If a cell fails open, the converter across the cell maintains a constant terminal voltage during charging while allowing current to flow in the entire battery string. During discharging, the converters across the remaining cells supply energy to the bus, while the converter across the open cell maintains a constant terminal voltage. The maximum current gain of the system is equal to the number of converters used in the system.