The Use of Equalizing Converters for Serial Charging of Long Battery Strings - IEEE '94
The use of serial battery cells for primary, secondary and auxiliary power is common in electric vehicles, space power and uninterruptable power supply (UPS) systems. One difficulty with charging serial cells is maintaining equal charge in all cells regardless of the charging conditions and initial condition of the cells. Continual unequal charging and discharging eventually causes certain cells to degrade rapidly and limits the energy capacity and lifetime of the string. To minimize unequal charging, battery manufacturers use extensive measures to match cells. Though the cells are guaranteed to match initially, aging and unpredictable charging and discharging leave longterm matching speculative. Described is a technique that uses dc-dc converters to shunt energy from cells (or groups of cells) while the entire string of cells is being charged. The amount of charging of each cell can be controlled by varying the amount of diverted energy. The technique does not exclude the use of different capacity cells within the string nor exclude the use of different types of energy storage elements.