Design of an energy-saving charge equalization system

Batteries with a higher state of charge in a series-connected battery string have a higher charging power and reach full charge before the overall battery terminal voltage attains its nominal voltage. If a constant charging current is ongoing, the batteries are charged into gassing and are prone to a dramatic reduction in life and potential damage. To equalize the state of charge for a series-connected battery string, an energy-saving charge equalization strategy is presented. Unlike traditional strategies that redistribute the charge energy according to the dynamic battery voltage, the proposed strategy inserts a reasonable rest period between equalization cycles so that the battery open circuit voltage, which serves as a relative state of charge indicator to the charge equalization control, is detectable. The rest period is also advantageous in that it converts the battery charge into chemical energy, and so minimizes undesired by-products, such as gas and heat. The proposed equalization strategy is realized by a peripheral interface controller (PIC) based experimental setup in which a buck-boost converter is used as the charging current redistributor and a push-pull converter is mandatory in order to fully charge the battery string. The experimental results confirm the performance of the proposed strategy.