Fuzzy Control for Battery Equalization Based on State of Charge

Battery equalization, aiming at keeping the state of charge of inside cells in the same level, is of great importance to maximize the capacity of whole battery pack and keep cells away from overcharge and overdischarge damage. In this paper, based on the analysis of bi-directional Cuk converter, we have proposed a fuzzy controller to adaptively tune the equalizing current. The inputs of fuzzy controller are selected as the difference in state of charge, the average of state of charge and the total internal resistance. The overall performance of the proposed equalizer is evaluated by multi-indexes such as equalizing speed, efficiency and cell protection. Simulations are conducted based on a well established 6Ah Li-ion battery provided in Advisor. The results under various initial conditions show that the proposed equalizer has the ability to balance the equalizing speed and efficiency. Any pair of cells with difference in state of charge less than 0.3 can be equalized within one hour and with the energy efficiency around 0.95.