Research On Low Temperature Constant Voltage Discharge Preheating And Alternating Current Preheating Of Lithium-ion Power Battery

Lithium-ion battery has become the first choice of electric vehicle energy system because of its high power,high energy density,excellent cycle life and storage characteristics.However,in daily use,the performance and service life of lithium-ion battery are greatly affected by temperature,especially in the low-temperature environment,the performance of lithium-ion battery decreases significantly,and even lithium precipitation will occur when charging lithiumion battery in the low-temperature environment,resulting in internal short circuit.Therefore,it is particularly important to preheat and restore the performance of lithium-ion batteries in low temperature environment before use.This paper focuses on two preheating methods of lithiumion battery under low temperature environment: constant voltage discharge preheating and AC preheating.Among them,constant voltage discharge is suitable for power batteries that can discharge at a large rate,while for energy batteries that cannot discharge at a large rate,this paper mainly explores AC preheating.The specific research contents are as follows:(1)Research on influencing factors and efficiency of constant voltage discharge preheating:This paper has carried out a series of experiments on the soft packed lithium-ion battery used to explore the changes of battery low-temperature performance and constant voltage discharge preheating characteristics under different preheating parameters.The results show that the battery performance decreases with the decrease of ambient temperature and decreases with the increase of discharge ratio;The preheating efficiency increases with the decrease of discharge voltage,increases with the increase of ambient temperature and decreases with the decrease of initial state of charge(SOC);The energy consumption ratio decreases with the decrease of discharge voltage,decreases with the increase of ambient temperature,but increases with the decrease of initial SOC.In the process of preheating,external work accounts for a large proportion of energy consumption.After 300 preheating cycles under the selected preheating voltage,it was observed that the battery health state(soh)decreased and the internal resistance of the battery increased,and it was observed from the scanning electron microscope(SEM)that with the decrease of preheating voltage,the electrode material appeared larger cracks and cracks.The minimum preheating SOC of the battery increases with the increase of preheating voltage.After preheating cycle aging,the performance of the battery is reduced to varying degrees under different preheating parameters.(2)Calculation of the optimal preheating voltage for constant voltage discharge preheating:Based on the experimental results,the battery life attenuation model and the preheating time prediction model are established.Using NSGA-Ⅱmulti-objective genetic algorithm,considering the battery life attenuation rate and preheating time,the weight coefficient is 0.5,and the optimal preheating voltage is 2.65 V.(3)Study on battery step by step pulse AC preheating based on terminal voltage avoiding lithium precipitation criterion: the results of EIS experiments at different ambient temperatures show that the electrochemical impedance spectrum of the battery is sensitive to the change of ambient temperature;Based on the EIS experimental results,the appropriate circuit is selected to fit the AC impedance parameters.From the fitting results,it can be seen that the charge to internal resistance of the battery increases significantly and accounts for a large proportion with the decrease of temperature;Based on the criterion of avoiding lithium evolution of terminal voltage,the maximum current boundary under 100 Hz AC is found to be linear with temperature;The battery can be preheated from-20 ℃ to 5 ℃ after 318 s by using the proposed AC preheating step pulse preheating method.