Research On State Of Charge Estimation Of Power Lithium-ion Battery Based On Equivalent Circuit Model

In recent years,with the increasing environmental pollution and the reduction of global oil resources,various countries around the world have begun to pay attention to the development of new energy electric vehicles.Electric vehicles have attracted widespread attention because of their energy-saving and low pollution characteristics.Power lithium-ion batteries are regarded as the core component of electric vehicle because of its high safety and high energy utilization efficiency.Their performance has a greater impact on the performance of the vehicle and the cruising range.Battery state of charge?SOC?is one of the important parameters of battery management system,which reflects the available capacity of battery.The accurate estimation of battery SOC can effectively protect the battery and prolong the battery's life,which is of great significance to the balanced management of battery pack and the prediction of cruising range.This paper takes the lithium-ion battery as the research object.The battery model is established and model parameters are identified.The SOC estimation algorithm based on the nonlinear Kalman filter is designed.The main works of this paper are as follows:?1?Firstly,the Li Fe PO₄battery was taken as the research object,and the characteristics,such as voltage,capacity,internal resistance,and other influencing factors of lithium-ion battery were analyzed through the capacity test and open circuit voltage calibration experiment.It is revealed that temperature is the main factor affecting battery characteristics,which lays a foundation for the establishment of the battery model.?2?Through the analysis and comparison of several common battery models,the second-order RC equivalent circuit model is selected as the battery model in this paper.Aiming at the problem that the battery characteristics are affected by the temperature factor,an improved second-order RC equivalent circuit model considering the temperature effect is established.The various rules of open circuit voltage and SOC under different temperatures are studied.The relationship between open circuit voltage,temperature,and SOC is characterized by a hybrid model.In addition,aiming at the problem that the available capacity of the battery is affected by temperature,the capacity correction coefficient is introduced to correct the available capacity of the battery at different temperatures in real-time.The parameters of the battery model were identified by using the experimental design and the least square method.Finally,the battery model was verified under different temperature conditions,and the results showed that the accuracy of the improved model considering the temperature effect was higher than that of the model not considering the temperature effect.?3?The principles of Extended Kalman Filter?EKF?,Unscented Kalman Filter?UKF?,Cubature Kalman Filter?CKF?are researched.Their characteristics are analyzed,and the state space equation and observation equation of SOC estimation based on temperature correction are established.Three filtering algorithms are used to estimate SOC under different experimental conditions.Finally,the estimation accuracy of the three algorithms is compared and verified under the dynamic conditions.The results show that CKF algorithm based on the battery model which is built in this article can obtain better estimation results than EKF and UKF.