Balancing Strategy for a Battery Applied in HEV Based on Bi-directional Flyback Converter and Outlier Detection

Dissipative cell balancing generates heat during its operation. Current techniques do not guarantee optimal balance of battery pack energy, requiring a high-cost Battery Management System (BMS) solution and wasting energy in the form of heat. Mild Hybrid Electric Vehicles uses the combustion engine to recharge the battery. Therefore, this feature requires a BMS balancing system capable of optimizing battery capacity and still be energy efficient. In this way, a non-dissipative balancing system would be interesting, especially if an algorithm works with the former non-dissipative balancing method, which efficiently determines which cells are unbalanced. In this paper, a methodology is proposed to perform non-dissipative balance of lithium-ion cells. This method considers which cells inside a certain range are considered balanced and cells outside this range are considered unbalanced. The range is given by the median of the cells terminal voltage summed with a threshold defined by experimental tests. Due the non-dissipative method presented herein is conceived through Flyback topology, the cells above this range are discharged and their extra energy is employed to charge the lowest charged cells, which were below the range. Simulation results which after the first 5 hours of balancing, the maximum difference does not exceed 1% and the standard deviation 0.5% until the end of the simulation, reducing SOC standard deviation by more than 33 times in one day operation. This result shows the strategy is promising to make a more efficient balancing mechanism for Mild Hybrid Electric Vehicles.