Research On Multi-stage Constant Current Charging Strategy For Lithium Battery Based On Multi-state Joint Estimation Model

In recent years,the new energy vehicle industry has developed rapidly,and the demand for power batteries has increased year by year.The ternary lithium battery has been widely used by major vehicle companies because of its high energy density and high voltage platform.Users are troubled by the traditional constant current and constant voltage charging strategy due to its long charging time,which restricts the development of the new energy vehicle industry.However,a high current charging strategy will cause the temperature to rise rapidly,the battery to age,and even cause the battery to overheat and burn.The charging strategy needs to weigh the charging time and battery life.Therefore,the research on charging strategy for lithium battery is particularly important.In this paper,a SOC(state of charge)-SOH(state of health)-SOT(state of temperature)multi-state joint estimation model for lithium battery is established.Based on the particle swarm optimization(PSO)algorithm,a self-adaptive multi-stage constant current(SMCC)charging strategy is proposed.Then,the key parameters of the model are identified and the battery simulation model is verified by experiments.After that,the simulation model and performance test bench for battery pack with Z-type air-cooling system are built.The simulation results and experimental results are compared and analyzed,and the heat dissipation structure of the battery pack is optimized by genetic algorithm.Finally,the SMCC charging strategy is applied to the optimized battery model to investigate the thermo-electric-aging characteristics of the battery pack under different air-cooling conditions.The main research contents and results are as follows:1)The mathematical model of electric behavior,thermal behavior and life of lithium battery is introduced.The identification process of battery internal resistance,open circuit voltage and electromotive force temperature rise coefficient is deduced in detail.Through the interaction between battery electrical parameters,temperature and life,the coupling relationship between electric model,thermal model and aging model is established.Based on the first-order RC equivalent circuit model,thermal network model and aging model,the SOC-SOH-SOT multi-state joint estimation model of lithium battery is established to simulate the response characteristics of lithium battery..2)A self-adaptive multi-stage constant current(SMCC)strategy is proposed.The PSO algorithm is applied to solve the best charging strategy under different weight coefficients,and the Pareto curve is constructed.Three optimal charging strategies are obtained: the shortest time charging strategy,the minimum aging charging strategy,and the balanced charging strategy,and compared with the traditional constant current and constant voltage strategy.The results show that the shortest time charging strategy is highly consistent with the 2C CCCV strategy.The charging time of the minimum aging charging strategy is 61.7% shorter than the 0.1C CCCV strategy.Compared with the 0.5C CCCV strategy,the balanced charging strategy can reduce the charging time by 44.9% at the expense of 6.4% aging.3)The pulse discharge experiment and the electromotive force temperature rise coefficient experiment are designed.The ohmic internal resistance,polarization internal resistance,polarization capacitance,electromotive force temperature rise coefficient with SOC and temperature are obtained,which are used to improve the multi-state joint estimation model.The thermoelectric model verification experiment and the aging cycle experiment were designed to verify the simulation model.The results show that the root mean square error of the 1C CCCV strategy voltage and temperature curve are 23.4mV and 0.5°C.The root mean square error of the balanced charging strategy voltage curve is 36.3mV,and the relative error of charging time is 3.4%.In the first 10 cycles,the capacity loss of the balanced charging strategy and the 0.5C CCCV strategy is 1.45% and 0.64%,while the charging time was shortened by 43.3%.4)The simulation model for Z-type air-cooling battery pack and the performance test bench are built.The battery temperature curves of the battery pack are compared when the battery pack is discharged with 1C current and the inlet velocity is 6m/s.The results show that the average temperature of the battery pack obtained by simulation and experiment is 34.1 °C and 34.7 °C,and the root mean square error of the simulated and experimental temperature curves for each cell is less than 0.5 °C.5)Based on the genetic algorithm,the heat dissipation of the battery pack is optimized.The temperature of the optimized battery pack drops to 34.5 °C.The average temperature drops to 33.8 °C,and the individual maximum temperature difference drops to 1.8 °C,a decrease of 30.7%.6)Study on the influence of the electro-thermal-aging behavior of the battery pack with different air-cooling conditions when the battery pack is applied with SMCC strategy.The results show that the higher the inlet temperature,the higher the average temperature of the battery pack,and the bigger the aging of the battery.The lower the inlet wind speed,the higher the average temperature of the battery pack.And the aging will be intensified.Especially when the inlet velocity is 0m/s,that is,the forced air-cooling is not considered,the average temperature of the battery pack reaches 57.5 °C,far exceeding the safe temperature of the battery pack.Accordingly,the aging of the battery increases rapidly.