Temperature Characteristics Of Traction Battery And Its Influence On SOC Estimation For Electric Vehicle

In China,the new energy vehicle has a rapid development,but there are still many deficiencies in the key technologies,especially in the field of the battery system.The battery management system(BMS)would lead to many issues when the ambient temperature changes,such as the charging jump of battery,the instability of battery's endurance and the abnormal change of SOC,and so on,which results from the lack knowledge of the temperature characteristics of battery and the deficient designing of the estimation algorithm.Considering the apparent influence of ambient temperature on traction battery,the ternary-material battery was adopted for research in this thesis,and the work is as follows:The battery capability was analyzed by some tests in terms of battery capacity,discharge characteristic,internal resistance,voltage and polarization in different ambient temperature to determine the high and the low characteristics of battery.Besides the single battery,the batteries in parallel-connection for meeting the requirements of capacity were also researched to enhance the understanding of the temperature characteristics of battery and the inconsistency between singles.Based on the characteristics of battery,the estimation model of SOC was designed using the algorithm of Square-root of Unscented Kalman Fitler(SR-UKF),which has good performance in SOC estimating.Considering the effect of the dimension of model variable on estimation,the estimation models with the structures of one-dimension and two-dimension variable are built.Except for the room temperature,the SOC estimation was also researched in terms of the model traceblity and modification under higher or lower temperature.The results show that:(1)The ternary-material battery is more sensitive to low temperature than high temperature,especially lower than 5?.The voltage drops faster during the later period of discharging process at higher temperature(?45?),which is considered to be attributed to the polarization internal resistance increased in the later discharge process.The influence of temperature on battery is mainly because of the ohmic internal resistance,following the battery polarization.(2)The battery performance can be improved under low temperature when the batteries are connected in parallel.The inconsistency of internal resistance should be considered as the first criterion for measuring the inconsistency between singles.The condition of temperature should be introduced to screen battery because of the instability of battery inconsistency under low temperature.(3)The estimation model based on SR-UKF algorithm has good performance,and the maximum error of SOC estimation is within 3%.The model with one dimension of variable has good performance in calculation speed,estimation precision and stability,comparing with the model with two-dimension variable,and the dimension of estimation model should be reduced during the estimation model design.(4)The estimation model based on the equivalent circuit model(ECM)with one-RC off-line has poor accuracy when temperature decreases,which can be overcomed by developing the model with an offset term caused by the model accuracy and environment.The estimation model under room temperature has good performance in higher temperature without any model parameter modification,and model has acceptable estimation performance with ohmic resistance modification and the offset of polarization when temperature decreasing,which can simplify model parameter modification.