A Unitized Multiwinding Transformer-Based Equalization Method for Series-Connected Battery Strings

Active equalization is an efficient method to improve the performance of series-connected battery strings for the electric vehicles. However, the large-scale applications of the active equalization are restricted by many factors, such as system cost, control complexity, and equalization speed. In this paper, a unitized multiwinding transformer-based equalization method is proposed. The forward converter and flyback converter are realized, simultaneously. The forward converter principle is used to equalize the battery cells in the same subunit, while the flyback converter principle is applied for the equalization of the different subunits. Fewer metal-oxide-semiconductor field-effect transistors and transformer windings are needed for lower control complexity and lower system cost. Furthermore, different subunits of the battery string can be simultaneously equalized, resulting in higher time efficiency. To verify the proposed method, the simulation and experimental workbenches are established. Results indicate that the proposed method can equalize well both for the cells in different subunits and in the same subunit. The urban dynamometer driving schedule tests are further applied and the unbalanced problem is covered in about one hour. The results show that the proposed method has achieved a good performance of equalization in terms of system cost, control complexity, and speed.