Implementation of active cell balancing circuit for battery management system in electric vehicles

In electric vehicles (EVs), a battery management system (BMS) is used to prevent the harmful effect of intercellular imbalance in the battery pack consisting of many cells, to improve the pack capacity and to keep the cells in a safe operating range. Battery balancing system (BBS) is designed to eliminate imbalances between cells and keep the SOC or terminal voltage of each cell equal. The performance of BBS is expressed by the available capacity and balancing speed at the end of balancing. The increase in balancing speed and efficiency is largely dependent on the balancing control strategy. In this thesis, control strategies are proposed for two different active balancing topologies. In the first of the proposed designs, an optimal balancing control strategy for a single flyback converter- based cell-to-pack balancing topology is developed to minimize balancing time and maximize the balancing efficiency. While the shortening of the balancing time provides a safer battery use, the usable capacity of the battery increases with the increase in efficiency. This means longer ranges for electric vehicles. In the second of the designs, a novel balancing control strategy for a multiple flyback converter-based cell-to-pack-to-cell balancing topology is developed to minimize balancing time and maximize the balancing efficiency. With the proposed control strategy, the switching losses and the charging losses in the balancing circuit are minimized by ensuring the use of the power circuit with minimum time.