Research On Electric Vehicle. Lithium-ion Power Battery Pack Equalization Methods

Electric vehicle is a long-tem solution to the future traffic, but power battery is limiting its development now. Although battery pack technology works well in dealing with the problem that the capacitor, voltage and power of cell can't meet the needs of the vehicle, another problem of non-uniformity of internal resistance, capactor, and voltage of cell is introduced.Cell uniformity will decrease the performance, life and safety of battey pack. It is a feasible way to improve cell uniformity that uses cell equalizer. According energy dissipation or not, cell equalization methods can be divided into two categories, active equalization and passive equalization. Passive equalization generally realizes the cell equalization by dischare resistor, while active equalization by the means of energy storing device, such as capactor, inductor, transformer and DC-DC circuits.The subject research on the equalization methods of Lithium-ion power cell belongs to the 02 nation major science and technology project. Firstly, based on the analysis of the causes of cell uniform and working principle of each cell equalization method, 8 kinds of typical cell equalization method was simulated using Simulink-PSpice co-simulation. The results proved that Centralized inductor cell equalizain and DC-DC cell equalization are best cell equalization method with the advantages of high efficience, high speed, small size, and easy to be integrated.Secondly, an innovative dual-inductor DC-DC convert circuit without isolation transformer was proposed, which can ahieve electrical isolation by the sequentially turning on/off of two MOSFET arrays A and B. Equalization process of the circuit includes four step: step 1, tun on MOSFET array A, cell begans to charge the inductor L1; step 2, turn off MOSFET array B, L1 begans to charge capacitor C; step 3, turn on MOSFET array B, L1 and C began to charge inductor L2; step 4, turn off MOSFET B, L2 will charge the battery module. Further more, a standard design design flow was proposed, based on the simualiton research on the relative of circuit performance and circuit parameters.Thirdly, the proposed dual-inductor DC-DC converter circuit was applied to the cell equalization. This cell equalization method can be called dual-inductor active cell equalization method. And relevant discharging /charging equalization strategies were designed.Finally, two test that dual-inductor DC-DC convetor performance test and system equalization test were carried. The results shown that the two-inductor active equalization method was approach to the effience of 80% with high equalization speed. No transient overvoltage or overcurrent pulse was found when MOSFET switching. Two-inductor active equalization method has a very practical value for its advantages of small size, high equalization current and easy to be interged.