Study On The Lithium Ion Battery Thermal Management System For Electric Vehicles

Due to its high energy density and long cycle life,lithium ion power battery is widely used in the energy storage system of electric vehicles.However,lithium ion power battery has strict requirements on operating temperature: the electrical performance of the battery will decline if working at too low operating temperature.And the capacity attenuation of the battery will become too fast if working at too high operating temperature,which may shorten the service life of the battery.Moreover,When the battery temperature exceeds the high temperature limit,the battery thermal runaway will be occurred and endangering the safety of users.Therefore,it is of great significance to analyze and optimize the thermal management system of lithium ion power battery.In this paper,the lithium ion power battery was chosen as the research object,the battery heat generation,forced air cooling system and the system control strategy have been researched,respectively.The purpose of this paper is to optimize the battery thermal management system,provide a comfortable working temperature environment,extend the service life for the battery and improve the safety of the battery system.In addition,it is necessary to reduce the energy consumption of each auxiliary device as much as possible to extend the limited range of electric vehicles.The specific research of this paper is shown as follows:(1)The influence of temperature on the electrical characteristics of the battery and the characteristics of runaway heat of the battery were analyzed through the experimental study of the battery under different temperature conditions,and established a transient thermogenesis model of lithium ion power battery.The effects of different SOC,temperature,discharge rate and continuous discharge time on the equivalent internal resistance of the battery were analyzed by experiment.The experimental results show that the equivalent internal resistance of the battery fluctuate with the change of SOC.With the battery temperature increases,the temperature decreases gradually.When the discharge rate is lower than 0.5C,the equivalent internal resistance decrease with the increase of discharge rate.And when the discharge rate is greater than 0.5C,the equivalent internal resistance of battery is no longer changes with the increase of discharge rate significantly.Extending the duration of continuous discharge time can increasing the battery’s equivalent internal resistance,and when the duration of discharge time is less than 100 s,the relationship between the battery’s equivalent internal resistance and the duration of continuous discharge will be different under the influence of SOC.Finally,based on the experimental results,a transient thermogenesis model was established to estimate the thermogenesis of the battery under different conditions.(2)The 4P8 S air-cooled module of 18650 lithium ion power battery was designed,and the forced air cooling system with three different battery arrangement structures,i.e.,aligned,staggered and cross battery module,were studied.The cooling effect of the system with different heat generate rate was analyzed through changing the discharge rate of the battery.The cooling capacity,system energy consumption and battery temperature distribution of the battery module under different cooling wind speeds were analyzed through changing the cooling wind speed.The experiment results shown that,With the increase of discharge rate,the temperature rise of the battery increase linearly.Increasing the speed of cooling wind can reduce the temperature rise of the battery effectively,but the battery temperature uniformity reduce at the same time.Meanwhile,with the increase of the speed of cooling wind,the improve trends of the cooling effect of the battery system become weaken gradually.However,increasing the speed of cooling wind may lead to the energy consumption of the fan increase exponentially.As for the temperature distribution characteristics of air-cooled battery module,the minimum temperature of the aligned structure module appears in the inlet place of cooling air,while the staggered and cross-arranged structure appears in the second column along the cooling air flow direction.Along the flow direction of the cooling wind,the temperature difference between the two adjacent batteries gradually decreases.The maximum temperature points of the three batteries are located at the outlet of the cooling air.Compared with the other two kinds of arrangement structures,the battery module with aligned arrangement structure has the best heat dissipation performance and temperature uniformity.(3)A Simulink simulation model was established for the active forced air cooling system,considering the battery heat generation,air cooling heat exchange,energy consumption of fan and refrigeration system as well as battery capacity attenuation.The model parameters were determined by the experiments of battery module.The system control strategies of different temperature threshold,different power ratio between cooling fan and refrigeration system,and battery temperature pre-adjustment control strategies are analyzed.The result indict that at 308 K ambient temperature,the battery temperature control at about 313 K has smaller cooling energy consumption and higher cooling effect.The larger temperature threshold range possesses higher maximum temperature and smaller temperature difference of the module.Increasing the input power of the fan or the refrigeration system can reduce the module temperature and the battery capacity attenuation effectively.However,the inconsistency between module temperature difference and battery degradation will increase with the input power of the refrigeration system,but decrease with the input power of the fan.