Experimental validation of an efficient charge equalization system for Lithium-ion batteries

The experimental validation of a Battery Management System (BMS) provided with an innovative high-efficiency active balancing circuit is described in this paper. Charge equalization among the series connected battery cells is achieved with a cell to cell balancing topology. The balancing circuit consists of a switch matrix for the individual access of each cell and a bidirectional DC-DC converter to transfer a controlled amount of charge from a cell to a supercapacitor and back to another cell. The balancing algorithm and the experimental setup are also described. The experiments show the functionality of the balancing circuit that restores a fully balanced battery consisting of eleven 40 Ah Lithium-ion cells. For instance, if one of the cells is 17.7 % imbalanced, battery equalization lasts around 25 h and only costs 1 % of the battery energy. It allows the recovery of the full battery charge, otherwise limited to 82.3 %, with an energy saving factor of 6, if compared with passive balancing. The efficiency of the process, calculated by measuring the energy lost during balancing, is higher than 75 %.