| Lithium-ion power battery has become the ideal power source for electric vehicles today because of high specific energy and specific power.However,due to the complex working characteristics and sensitive characteristic to the abuse condition,the charging performance of electric vehicle is greatly affected by charging method and charging environment.In the process of charging,because of the limitation of battery charging performance,charging time,charging efficiency and charging capacity and other charging requirements are contradictory which cannot reach the best.Therefore,in order to improve comprehensive charging performance of batteries,it is necessary to adjust the charging strategy according to the specific charging demand.At the same time,the charging characteristics are sensitive to the change of environmental temperature,especially in low temperature environment.lithium-ion electrochemical reaction rate is reduce and the number of lithium-ion decreases,which results in the increase of the internal resistance of charging,charging capacity decreases,and even lithium dendrite are produced which is easy to cause internal short circuit,so electric vehicle safety is threatened.In the north of China,the low temperature in winter lasts long and electric vehicles are difficult to charge,which seriously hinders the popularization of electric vehicles.Therefore,exploring and studying the scientific problems of charging characteristics,breaking through and grasping the key technologies of charging optimization control,optimizing the charging performance and enhancing the adaptability at low temperature are the key to solve the problem of popularization and promotion of electric vehicles in China.At present,electric vehicles have low comprehensive energy efficiency,environmental adaptability and low safety technical problems.This paper focuses on the research of scientific problems such as internal charging resistance,charging capacity and physical performance and evolution law of acceptable charging current under different charging mode and ambient temperature,and overcoming the key technologies of process modeling of lithium-ion power battery,multi objective optimization of charge control strategy and charging control combined with low temperature and heating,which would solve the multi-objective optimization problems with the objectives of charging time,capacity of lithium-ion and charging efficiency,realize fast and safe charging of lithium-ion power battery in a low temperature environment,improve the comprehensive charging performance and environmental adaptability of electric vehicles,provide the key technical support for promoting the development of electric vehicle industry.In order to achieve this purpose,firstly the external characteristics and internal process of lithium-ion batteries charging from the perspective of theory and practice were studied,then aiming at the hysteresis of open circuit voltage in lithium-ion batteries,the hysteresis voltage model was established and the equivalent circuit model is improved;according to the multi-objective problems of electric vehicle charging,a multi-objective optimization algorithm was used for charging control strategy optimization to enhance the comprehensive performance of charging;finally,A joint charging and heating control system is set up for the problem of low temperature charging and developed the optimization control strategy,to speed up the battery charging speed at low temperature,low temperature charging safety.The specific research contents are as follows:Theoretical study on charge characteristics of lithium-ion batteries.Based on the in-depth analysis on the working principle of lithium-ion batteries,the electrochemical thermal coupling model was built,internal process of charging conditions of lithium-ion batteries was studied,the kinetic process of lithium-ion embedded in and out in different charging modes was analyzed from the microscopic point of view,the distribution of the solid and liquid lithium ions in the battery and the distribution of the electric potential in the battery are analyzed in detail.Experimental study on charge characteristics of lithium-ion batteries.Based on the battery charging experiment data under different environmental temperature and charging methods,the influence of ambient temperature and charging mode on the charge resistance,charging capacity and acceptable charging current from a macro point of view was analyzed;the safe charging current of batteries at different ambient temperatures was obtained by the experiments of charging and discharging cycle under low temperature environment.On the basis of theoretical research and experimental testing,a hysteresis voltage modeling method based on recursive Preisach algorithm is proposed in view of the delay of open circuit voltage,and the parameters of the model are identified by the parameter identification experiment.On this basis,the effectiveness and robustness of the algorithm are analyzed.An equivalent circuit model based on the improved hysteresis voltage model is built,and the validity of the model is verified experimentally.In order to improve the integrated energy efficiency of charging,a multi-objective optimization charge control strategy is proposed in view of the optimal charging control strategy of electric vehicles.Considering multi-stage constant current charging as the basic control strategy,the charging time,charging capacity and charging efficiency as the optimization goal,based on multi-objective particle swarm optimization algorithm for multi-stage charging current,multi-stage charging current was optimized.The influence of the number of charging stage and the cut-off voltage on the charging effect was deeply studied and the superiority of the proposed control strategy was proved by comparison experiment.Aiming at improving the adaptability of charging environment and the low temperature charging problem of electric vehicles,a low temperature charging control strategy based on adaptive fuzzy control algorithm is proposed.Taking battery acceptable charging current as a constraint condition,adjust the charging and heating power according to the temperature of the battery and the temperature rise rate,and improve the low temperature charging speed and ensuring the safety of low temperature charging.The charging-heating joint control experiment was carried out by building a charging-heating hardware platform.Compared with the existing low temperature charging method,the validity and reliability of the control strategy are verified.The research results provide a new idea for the development of electric vehicle charging control strategy,which can also be applied to the field of grid energy storage,to further expand the application scope of lithium-ion power battery. |
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