Research On Low Temperature Heat Transfer Characteristics Of Power Battery Based On Phase Change Materials

Electric vehicles have the advantages of no pollution,less energy consumption,and have a good development prospect.At present,the most suitable battery for electric vehicles is lithium-ion batteries.The electric cars are still in their infancy,because they are limited by battery technology.On the one hand,the cruising range of electric vehicles is insufficient due to the low battery capacity;on the other hand,the operation of lithium-ion batteries as automotive power batteries is seriously affected by temperature.The lithium ion battery may explode when the temperature is too high.When the temperature is too low,the capacity of the lithium ion battery will decrease,the resistance will increase,and the cycle life of the battery will be shortened.While maintaining the temperature of the lithium-ion battery during operation,it is also necessary to study how to increase the operating temperature of the lithium-ion battery at low temperatures and improve the low-temperature performance of the lithium-ion battery.In this paper,the performance of thermal management of power battery thermal management systems based on phase change materials(PCMs)under low temperature conditions is studied through a combination of experiments and simulations.The main research is obtained as follows:(1)The discharge curve of the lithium-ion power battery and the DC internal resistance of the battery were measured by experimental methods.It was found that the voltage and battery capacity of the lithium-ion power battery decreased when the temperature was 0℃ and-20 ℃,And the DC internal resistance of the lithium-ion battery also decreased.This shows that the discharge performance of lithium-ion batteries is greatly affected under low-temperature conditions,and the necessity to improve the operating temperature of lithium-ion batteries under low-temperature conditions.The heat generation mechanism of lithium-ion batteries was discussed.A three-dimensional thermal film type of 18650 lithium-ion batteries was established in ANSYS Fluent.The temperature field distributions of the batteries at 1C,2Cand 3C discharge rates were simulated and compared with experimental results.The change trend of temperature is consistent with the experimental data.(2)Two kinds of composite phase change materials were prepared in the laboratory,namely,phase change materials with expanded graphite and paraffin wax and phase change materials with silica aerogel and paraffin wax.Two kinds of composite phase change materials with different thermal conductivity were tested,including leak test,infrared spectrum analysis,X-ray diffraction(XRD)analysis,thermal conductivity measurement,DSC test,etc.The test results shown that the thermal conductivity of the expanded graphite / paraffin composite phase change material with a mass fraction of 80% was 1.7W /(m · K),and the thermal conductivity of the aerogel / paraffin composite phase change material with a mass fraction of 75% was 0.2 W /(m · K).Both support materials have good encapsulation performance for paraffin.Moreover,infrared spectrum analysis and XRD analysis shown that both materials and paraffin are simple physical mixtures and no new substances were produced.(3)A single-cell thermal management system experimental platform based on the coupling of a composite phase change material and a polyimide heating film was built,and a customized heating film was used to simulate the heating of the battery.An experimental study was performed on the single-cell thermal management system.The thermal management performance of two different composite phase change materials filled in was compared and analyzed.(4)A battery pack thermal management system based on the coupling of a composite phase change material and an electric heating film was established,and a model of the battery pack management system was established.The melting and solidification model of the phase change material was added,and the simulation calculation method was used to study performance of battery thermal management systems at low temperatures.The power of a suitable polyimide electric heating film was selected,and the effects of thermal conductivity and latent heat of phase transition on the performance of the lithium-ion battery thermal management system under low temperature conditions were analyzed.