Development of charge equalization circuit

The individual cells within a battery string differ due to manufacturing variations, temperature gradients, and aging effects. If the string is treated as a two-terminal device while being charged and discharged, the individual cells will have different states of charge (SOC). Some cells will be chronically overcharged, undercharged, or over discharged. Over many cycles, this tends toward capacity decrease for the cell and the pack. Sophisticated battery users are aware that SOC balance is necessary, particularly for valve-regulated lead-acid (VRLA) battery. The paper impact is to enhance the uniformity of the battery cells and hence to improve the life of the battery through active charge equalization process. A charge equalization circuit to equalize three lead acid batteries is developed and presented in this paper. To compensate for the differences in the capacities, and the variation in the rate of charging/discharging among different batteries in a stack, an active charge equalization topology has been developed to maintain all the batteries in the stack at the same voltage level. The implemented architecture monitors the voltage across each battery in the stack and then identifies the battery with lowest SOC in the stack. The main objective of this paper is to obtain a first-hand experience with the design of a battery equalization circuit.