Research On Thermal Management Of Power Lithium-ion Battery Based On Phase Change Materials

As new energy electric vehicles can effectively cope with the energy crisis and environmental pollution,countries around the world are vigorously developing new energy electric vehicles.Lithium-ion batteries are used as power batteries for vehicles because of their high specific energy,high cell voltage,long cycle life and no memory effect.But they usually generate a large amount of heat during charging and discharging.In order to extend the service life of lithium-ion batteries,ensure the safety and power of vehicle batteries,it is extremely urgent to carry out research on thermal management of lithium-ion battery pack.Since phase change material has unique advantages in controlling the maximum temperature and equilibrium temperature difference of battery pack,this paper studies the thermal management of battery pack based on phase change material.The main research contents are as follows:(1)From the perspective of the heat generation mechanism,the heat generated by the lithium-ion battery during operation is mainly composed of two parts:reversible heat and irreversible heat.Herein,the Bernardi battery heat generation rate model was used to express these two parts of heat in a mathematical model.At the same time,a Multi-rate Hybrid Pulse Power Characterization(Multi-rate HPPC)internal resistance test method was proposed for the internal resistance,which is one of the key parameters of the Bernardi battery heat generation rate model.The Multi-rate HPPC internal resistance test method enabled accurate and efficient testing of internal resistance.(2)Study on multi-factor dynamic internal resistance model:The proposed Multi-rate HPPC method was used to test the internal resistance of the battery under the influence of multiple factors.By analyzing the relationship between temperature,State of Charge(SOC)and charge-discharge rate and internal resistance,the binary polynomial fitting theory based on least squares method was combined with cubic spline interpolation method to construct multi-factor dynamic internal resistance model.The effectiveness of the proposed multi-factor dynamic internal resistance model was verified from the charging and discharging process by changing the temperature,SOC and charge-discharge rate.The experimental results demonstrated that the multi-factor dynamic internal resistance model had high estimation accuracy.(3)Study on dynamic heat generation rate model of battery:Firstly,based on the lumped thermal properties of the battery and the energy conservation equation in the finite element analysis theory,the thermal effect model of the battery was constructed.Then,based on the Bernardi battery heat generation rate model,the multi-factor dynamic internal resistance model and the dynamic temperature entropy coefficient model were used to construct the dynamic heat generation rate model of the battery by introducing the SOC estimation method.The UDF program of the battery dynamic heat generation rate model under various charging and discharging conditions was written in C++language.The temperature characteristics and temperature field distribution of the battery under various charging and discharging conditions were simulated by Ansys Fluent.Finally,the experiment was carried out to test the temperature variation of the battery under the same working conditions,and the validity of the battery thermal effect model and the battery dynamic heat generation rate model was verified.(4)Experimental test and analysis of a novel composite phase change material with natural oil and graphene mixed:the phase change temperature,latent heat of phase change,specific heat capacity and thermal conductivity were studied.The phase change temperature of the composite phase change material is 43℃,the latent heat of phase change is as high as 204 Jg-1,which is 15.65%higher than that of composite phase change material composed of expanded graphite and paraffin at 20%of expanded graphite,the specific heat capacity is dynamically changed,which is greatly affected by temperature.The specific heat capacity at 43℃ is 188.6 Jkg-1K-1,the thermal conductivity at density 870 kgm-3 is greater than other densities.It lays the foundation for the subsequent application of thermal management to lithium-ion battery pack.(5)Battery pack thermal management design and optimization:According to the above research contents(1)～(4),a reasonable thermal management of power lithium-ion battery pack based on phase change material was designed.Under the same working conditions,compared with the temperature characteristics and temperature field distribution of the battery pack under natural air-cooling conditions,the results showed that the thermal management of the power lithium-ion battery pack based on phase change material is superior,it could achieve the goal of battery pack thermal management.Meanwhile,the thermal management of the battery pack had a significant effect on the temperature difference between the different surface positions of the battery and the temperature difference between the different batteries.The batteries in the battery pack could be basically operated in the same temperature platform,which is important to maintain consistency between the individual batteries in the battery pack and to extend the cycle life of the battery.Through the comparison of experiments and simulations,it was verified that the thermal management of the battery pack had good thermal insulation performance.For the high temperature environment,the thermal management of the battery pack was optimized,and the thermal management of the active and passive battery pack based on phase change material and with heat dissipation structure was proposed.The experimental results showed that the thermal management of the active and passive battery pack could effectively control the temperature of the battery pack in a high temperature environment.