Active Cell Balancing of Li-Ion Batteries for Electric Vehicles

Electric vehicles (EV) use battery banks containing series-parallel combinations of several small units to meet the high power demand of the drive system. In recent years, lithium-ion (Li-ion) batteries are being popularly used in EVs for energy storage instead of conventional lead acid batteries due to the advantages of higher cell voltage, no memory effect, low self-discharge, longer battery life and reduced size and weight. However, Li-ion battery packs require external power balancing circuits for its protection from getting damaged by overcharge and deep discharge. This work proposes an active circuit for charge balancing of Li-ion battery packs. The proposed charge balancing circuit uses a multi-winding flyback converter with n+1 number of active switches to protect a stack of n number of series connected cells. Performance of the proposed system has been verified through computer simulation in MATLAB with a 4 cell battery pack. Simulated results were found in close agreement with the predicted behaviour of the system.