Lithium-ion Battery SOC Estimation Method And The Equilibrium Of Series Battery Pack Research

For the recent years, progress has been made on the development of electric vehicles and this has drawn much attention. However, due to the instability of the battery, the estimation of the state of charge (SOC) and the equilibrium of series batteries in the charging state still require further research. Thus, to solve these problems, this thesis does the following research.Firstly, for the lithium-ion battery pack, a power battery model with rated temperature is constructed. It is composed of two models. One is the KiBaM (Kinetic Battery Model) which is based on relations of the change of the battery’s electric charge and the current balance. The other one is the second order battery model whose resistance can be easily influenced by temperature. In this model, controlled voltage source is the electric charge. Besides, current, voltage at the terminal of the battery, SOC, and temperature are effectively combined together. Mathematical model is generally based on experience and there is no direct relation among different phenomena; electrochemistry model is not universal for its strong computation and specified battery input; equivalent circuit model can show the properties of battery, but this is only possible on the basis of current and voltage.Secondly, the state of the lithium-ion battery is classified into two, namely the static state and the charging/discharging state. For the two states, different methods are adopted to estimate the SOC of the battery. The extended Karman Filtering (EKF) is improved for the estimation. Then, it is expanded through first order formula, and the non-linear questions are dealt with by changing them into linear ones. Besides, the equation of the SOC is revised properly with temperature compensation. Current and temperature are the inputs, and voltage is the output. The SOC regarded as state variable in its estimation. The simulation results show that the SOC is mainly influenced by the battery’s voltage, current, and temperature; the estimation of the SOC which is based on the EKF is more accurate.Lastly, the equilibrium of the series battery pack is explored. To begin with, the lack of uniformity of the batteries is measured with the control strategy in the approach of comparing the mean and difference value. Then, this thesis adopts the improved second order bidirectional non-dissipative diffluence equilibrium system in achieving equilibrium of series battery pack. In the simulation experiment, it is achieved successfully. Besides, on the basis of the battery management system (BMS), data collection is accomplished displayed, and software flow pattern of the algorithm for the SOC is presented.