| The material deterioration and performance degradation of the electric vehicle power battery during operation will have a serious impact on the use performance of the electric vehicle and the life of the battery system.Analyzing the decay speed of the power battery under various cycle conditions and studying the heat generation characteristics during the whole life cycle are of great significance to the efficient utilization of the battery system and the optimization of thermal management measures.To this end,this article takes the direct cooling and heat management system as the background,and studies the changes in the capacity,internal resistance,and heat generation characteristics of the power battery during the decay process.The thermal management measures of the direct cooling system under different cycles of the cycle are optimized,which provides a basis for the improvement of the temperature drop effect and performance optimization of the direct cooling system.The research work is carried out by establishing nickel-cobalt-manganese ternary lithium ion(Li Nix Coy Mn1-x-y O2,NCM)power battery electrochemical model,heat generation and heat transfer model,empirical aging model and thermal management system model.The decay characteristics of the battery’s electrical parameters,thermal characteristics and internal resistance capacity during the aging process of the battery charge and discharge cycle conditions are used as the battery characteristic quantities,and the corresponding experimental measurement data is used as the standard to ensure the rationality of the simulation model.Simulate the experimental conditions of the direct cooling heat management system with the simulation model of the compressor,condenser,throttling device,and direct cooling evaporator in the thermal management system.The average temperature of the battery,the pressure difference between the inlet and outlet of the direct cooling plate and the inlet pressure of the compressor are selected as the characteristic parameters of the direct cooling system to verify the cooling effect of the system.Aiming at the problem of performance degradation during power battery cycling.Taking the daily driving conditions of urban taxis as the analysis background,simulating the long-period charging and discharging cycle process of the battery and setting threshold control temperature management measures based on the direct cooling and heating management system,comparing and analyzing the battery life between cities with different congestion levels difference.Aiming at the aging stresses of thermal management temperature and charging rate,a comparison of multiple sets of simulation working conditions is designed.The aging laws of battery capacity,internal resistance and discharge characteristics under the influence of different factors are explored.The calculated results provide a data basis for the study of the heat generation law of the aging process later.In order to explore the heat production law of the battery under different decay degrees,a simulation analysis is carried out,and the battery under different cycle periods is taken for a complete discharge process,and the change of the total heat production and the average heat production power are explored.The results show that as the number of cycles increases,battery degradation increases,heat generation power increases,discharge time shortens,and total heat generation shows a trend of first decreasing and then increasing.On the basis of this research,the influence of different urban working conditions,different thermal management temperatures and different charging rates on the heat generation characteristics of the battery after cyclic aging was explored.The results show that after the same cycle time and other conditions remain unchanged,the higher the thermal management temperature,the higher the heat generation power of the aging battery,and the higher the temperature range,the greater the heat production increase caused by the unit temperature increase;The higher the charging rate,the higher the heat generation power after aging.When the charging rate is less than 1.5C,the relationship between the charging rate and the increase in heat generation is linear.When the charging rate is greater than 1.5C,the increase in heat generation is significantly improved.In view of the change law of direct cooling and heating management measures with the deepening of battery aging,firstly,three time nodes are set at the beginning of the cycle,the middle of the cycle,and the end of the cycle,and the batteries at different time nodes are simulated with different rates of constant load discharge.Through the adjustment of the compressor speed,the control law of battery pack temperature stability under different decay degrees is explored,and the results show that,when the battery decay degree is not high,the battery heat generation change is small,and the speed change range is also small,but at the end of aging,especially in the case of high-rate discharge,the compressor needs to adjust the speed of the speed range..Under the same discharge rate,the direct cooling and heating of the battery is managed,and the compressor speed adjustment follows the decay process of the battery.The trend is slow and fast.This is basically the same as the growth law of the battery pack’s thermal load during the whole life cycle.In addition,in the early stage of the power battery cycle,because the internal resistance changes very little and the increase in heat production is small,the initial compressor speed can meet the cooling demand,and the compressor speed in the early stage does not need to be adjusted.The lower the rate of the discharge process,the later the adjustment start year of the compressor.Under the same discharge rate,the longer the cycle time,the greater the difference between the compressor speed change rate and the thermal load change rate.This is because as the battery thermal load increases,the increase in cooling capacity will change with the speed adjustment.Gradually slow down,in order to get the expected cooling capacity,the speed range that needs to be adjusted is larger.Through the optimization method of adjusting the compressor speed,the control effect of the battery pack temperature under different cycles is guaranteed.However,in the process of increasing the speed,although the compressor provides more cooling capacity,it increases the power consumption of the system.Make the COP lower.Aiming at the energy consumption optimization of thermal management measures in the whole life cycle of the thermal management system,the speed of the condenser fan is changed under different cycle periods,and the change of the heat exchange effect of the direct cooling system is studied.With the increase of thermal load and the increase of compressor speed,the optimization effect of condenser wind speed adjustment on the system COP will be less and less,and at a higher discharge rate,the condenser wind speed in the middle and late stages of the cycle will have an adjustment threshold..Under the same discharge rate,the longer the cycle time,the smaller the condenser wind speed gear corresponding to this adjustment threshold.In the early stage of the cycle,the condenser fan speed can be appropriately increased to obtain a higher COP as much as possible.However,there is a threshold for the adjustment of the wind speed in the later stage of the cycle,and the fan speed can be appropriately reduced to save energy and increase efficiency. |
