Research On Charge Equalization Of Series-Connected Traction Battery

Electric vehicle has the advantage of low noise and nearly zero emissions, so it is a good alternative to solve the problem of the shortage of energy and the pollution of environment. Traction battery is an important part of electric vehicles, and determines the performance of vehicles to a great extent. Numerous traction batteries should be serially connected to meet the high-power requirement of electric vehicles, and this battery pack is charged by the charger as a whole. The state of each cell in traction battery bank is un-uniformity during charging due to the subtile difference among the cells, thus reduce the lifespan of battery bank. So it is significant to find effective charge-equalization means to make up the difference of each cell during charging, so as to maximize the effectiveness of the whole battery bank. The theoretical and experimental research is oriented to lithium-ion battery in this paper. Because lithium-ion batteries have high power density and high cell voltage, and are prefered by electric vehicles. But the method covered in this paper is also applicable to other second batteries. The main research contents in this paper are as follows:(1) The property of traction battery and the cause of un-uniformity among the batteries are analyzed in detail in this paper, and diverse methods of equalizing charge is spreading out simultaneously. Even if each cell in the battery bank has the same initial performance, the un-uniformity is inevitable after several charging discharging cycles. The equalizing ways must be applied to eliminate the difference. It turns out that equalizing circuit with converter is a outstanding equalizing topology structure throug detailed analysis of present equalizing circuit and balancing control technique. The equalizing method based in converter circuit has the characteristic of high flexibility and efficiency.(2) A charge equalization circuit with inductor symmetrically distributed is proposed after deep research of equalizing circuit and control method. The operating principle and equalizing process are analyzed in detail in this paper. Taking advantage of the relationship between the terminal voltage of battery and the state of charge (SOC) based on the reactive battery circuit model, the duty ratio of switching device in the charge-equalizing circuit can be calculated and the uniformity of SOC in each cell should be implemented during charging-time. It is confirmed by the experiment that the energy among the batteries could be dynamically transferred with this method.(3) The experimental research of equalizing charge system is carried out in the last of this paper. The DSP produced by TI corporation is selected to be the controller. It also designed charging circuit of battery bank, protection circuit, driving circuit in the equalizing circuit module, parameter detection circuit, and so on. The experiment confirmed that the equalizing circuit and control method achieved good result.