Research On Charging Control Strategy Of Lithium-ion Power Batteries

At present,social issues such as energy and environment promote the accelerated development of the electric vehicle(EV)industry at home and abroad.As an important choice for the energy storage system of EV,lithium-ion power batteries are increasingly widely used.However,the high and low temperature environmental adaptability of lithium-ion power batteries,especially the charging speed,battery temperature rise and life attenuation during high and low temperature charging,seriously restrict the further promotion of electric vehicles in high latitude and cold regions of our country.Therefore,it is necessary to carry out in-depth research on the charging strategy of lithium-ion power batteries for vehicles,and formulate faster charging strategies based on safe charging,which are more environment adaptable and can also meet the diversified charging needs of users.This research is crucial for promoting the highquality development of the industry,and has important theoretical research significance and engineering application value.Aiming at the problems of simple charging control objectives,poor environmental adaptability and low-temperature charging energy efficiency in the current electric vehicle charging strategy,this paper studies the charging of lithium-ion power batteries at different temperatures and different rates from macro and micro perspectives.The discharge characteristics are theoretically and experimentally studied,and different types of lithium-ion power battery charge and discharge test data sets and internal mechanism simulation models are established;on this basis,a compound heating method based on simultaneous internal and external heating are proposed The optimized current control strategy is used to meet the heating demand during the lowtemperature charging process of lithium-ion power batteries.At the same time,a multiobjective charging strategy based on variable weight coefficients is proposed for different ambient temperatures and different user charging needs,in order to improve the environmental temperature adaptability of charging,battery state adaptability and user demand adaptability.Finally,in view of the low energy efficiency of lowtemperature charging of electric vehicles,based on the compound heating method and multi-objective charging strategy,a low-temperature power adaptive distribution charging control strategy for lithium-ion power batteries is proposed to improve the battery state adaptation and energy efficiency of low-temperature charging.The specific research contents of the paper are as follows:(1)Test and analyze two representative lithium-ion batteries according to the laboratory battery charging and discharging equipment,and obtain the charging rate characteristics,temperature characteristics and internal resistance characteristics of lithium batteries under different conditions.At the same time,the temperature rise characteristics of discharging and charging,the capacity decay characteristics and the curve of acceptable charging current with temperature were obtained through the test at low temperature,which laid a research foundation for the formulation of the following charging strategies.(2)In order to deeply understand the relationship between the internal characteristics and external characteristics of the charging process,the paper uses an electrochemical model to simulate the lithium-ion migration characteristics during the charging process of lithium-ion batteries from a microscopic point of view,and the heat obtained by the electrochemical model is input into the 3D model.The temperature distribution is then modeled,which is fed back to the electrochemical model to form an electrochemical-thermal coupled model.Through this model,the solid-liquid phase lithium-ion concentration distribution,internal and external temperature distribution cloud map of the battery during charging at different rates are obtained,which provides a reference for the establishment of the following distributed equivalent thermal circuit model.(3)In view of the low preheating efficiency of lithium-ion power batteries at low temperatures and the inability of traditional lumped parameter models to reflect temperature inhomogeneity,a distributed equivalent thermal circuit model is established to study the rapid compound heating method.According to the simulation results,the effects of different heating methods,environment contact thermal resistance and heating currents on the heating efficiency are analyzed,the the accuracy and robustness of the proposed Kalman-based internal temperature estimation method are verified.After seclecting heating method 4 as a suitable heating scheme,the simulation results were verified by experiments,which proved the effectiveness of the compound heating method.According to further comparative analysis,this heating method is not only much faster than the external heating method of the typical battery module referenced in this article,but also reduces the heating time by 29.1% and energy consumption by 25.9% compared with the purely internal self-heating method.(4)Aiming at the problem that users have differentiated charging requirements under different use conditions,and the resulting multi-objective balance and optimization problems of charging targets such as charging time,charging capacity and battery temperature,etc.,a variable weighting coefficient-based multi-objective charging method is designed.Firstly,the Pareto solution set of charging time,charging capacity and temperature rise is obtained through the multi-objective particle swarm algorithm,and then the weighting coefficients of the three objectives are changed according to the needs of users for fast charging and ordinary charging,and finally the weight of charging is further optimized based on the ambient temperature.Optimizing the weight coefficient ratio can realize intelligent charging strategy adjustment based on user needs and battery temperature,and ensure that the charging temperature range is within the optimal range to ensure safety and efficiency.(5)Aiming at optimizing and improving the preheating efficiency and charging performance of the power batteries under low temperature conditions,this paper proposes a low-temperature charging-heating combined control strategy based on adaptive power distribution.Firstly,according to the temperature-acceptable charging current curve and adaptive fuzzy control,the charging power and heating power are distributed once to improve the energy efficiency of the system power;then the heating power is distributed twice for the purpose of reducing the temperature difference in the heating process.Through the adaptive distribution of the heating power,the temperature uniformity of the module is improved,thereby increasing the charging speed.Compared with the traditional two-heater external and four-heater power average distribution methods,the power adaptive distribution method proposed in this paper can reduce the charging time by 15.8% and 3.1%,and the system energy consumption by 16.8% and 2.6%,respectively.