Study On SOC Estimation Method Of Lithium-ion Battery For Plug-in Hybrid Electric Vehicle

Energy shortage and environmental pollution are the most serious challenges to the development of modern automobile,plug-in hybrid electric vehicle(PHEV)is regarded as one of the most promising new energy vehicles for its low fuel consumption,low emissions and strong electric endurance.As one of the power sources of PHEV,the state of charge(SOC)is one of the most important parameters of the battery management system,and is also the basis of the vehicle control strategy.Accurate SOC value plays an important role in improving the safety and service life of the battery,also important in improving the power and economy of the vehicle.This paper takes the polymer ternary lithium-ion battery with soft package produced by a domestic company as the research object,mainly studied the characteristics,the equivalent circuit model,the parameter identification and the SOC estimation of the battery.The main work is as follows:(1)After analyzing the characteristics and working principle of the ternary lithium-ion battery,the open circuit voltage characteristic experiment,the rate discharge experiment,the temperature capacity characteristic experiment,the cycle life experiment and the self discharge characteristic experiment of the battery were carried out based on the battery experimental platform.We obtained the influence factors of charge and discharge rate,temperature and the number of cycles on the battery capacity based on the experimental data,then improved the traditional SOC definition based on the influence factors.(2)A second-order RC equivalent circuit model was established based on the battery characteristics,then the least square method was used to fit the voltage rebound characteristics under different SOC values,identified the battery model parameters.Based on the battery characteristics,a second-order RC equivalent circuit model was established.The battery model parameters are identified by using the least squares method to fit the voltage rebound characteristics under different SOC values.(3)Combined the square root unscented kalman filter(SRUKF)with the improved SOC definition and the discrete battery equivalent circuit model to estimate the SOC value,which solves the numerical instability of the unscented kalman filter during the operation.The paroposed estimation method is verified under the condition of inaccurate SOC initial value and capacity fading caused by aging,the results show that the SRUKF correction algorithm can eliminate the interference quickly,the estimation accuracy is high and the robustness is good.(4)Built the vehicle dynamic model in Simulink based on PHEV vehicle parameters,and the SOC estimation method is embedded into the battery model.Simulated in multiple NEDC conditions,the results showed that SOC changes in the reasonable range,which meet the vehicle control strategy,the SRUKF correction algorithm is suitable for the battery SOC estimation of the PHEV.