Design And Analysis Of Composite Thermal Structure Of Lithium-ion Battery Pack For Vehicle

Lithium-ion batteries are widely used in hybrid or electric vehicles,and the temperature has a great influence on them.Too high or too low temperature are easy to cause system performance degradation,life shortening or even thermal runaway.In order to ensure that the batteries work in a proper temperature range,it is particularly important to design and analyze the reasonable thermal structure.In this paper,the battery pack was taken as the research object.The theoretical analysis,experimental verification and numerical calculation were used to analyze the battery and design the thermal structure of the battery pack.The main research contents and conclusions are as follows:(1)Based on the analysis of the basic structure,working principle and heat generation mechanism of lithium-ion battery,the influence of temperature on the performance parameters of lithium-ion battery was experimentally studied,including the influence of temperature on battery capacity,discharge voltage and internal resistance,as well as the temperature characteristics of battery under different discharge conditions,which laid the foundation for the establishment of the model.(2)The physical and mathematical models of cell heat transfer were established,and the differences between the models of constant or time dependent heat generation under different discharge rate were studied.The results showed that the difference of temperature rise between the two models was less.When the batteries were discharged at 0.5C,1C,1.5C and 2C at 25?,the maximum temperature difference between the two models was less than 0.01 ?,and the maximum temperature difference is less than 0.21? at the end of discharge;Compared with the experiment,the maximum temperature difference between the two models was less than 0.57 ?;When the batteries were discharged at 1C,during the whole discharge process,the temperature difference between the two models and the experimental results was not more than 0.4 ?.The rationality of constant internal resistance calculation was confirmed.(3)In this paper,the heat transfer model of the battery module was constructed under the conditions of air gap,heat conducting aluminum sheet and close fit.The temperature distribution of the battery under 1C power was studied.The results showed that the use of heat-conducting aluminum sheet could effectively improve the temperature uniformity of the battery pack and reduce the maximum temperature.When designing the battery module,it was recommended to use heat-conducting aluminum sheet to improve the temperature uniformity between the batteries.(4)Based on the actual process and the structure of the battery pack,the heating / cooling integrated heat exchange plate was designed.The influence of the structural parameters such as the length,thickness and pipe distance of the channel of the heat exchange plate on its heat exchange performance is analyzed.After orthogonal test and analysis,the heat exchange plate with thickness of 6mm,length of 3mm and pipe distance of 22.5mm was selected.(5)Combining the heat exchange plate with the battery pack,two schemes were designed,which were the combination of conduction silica gel and the combination of the main and auxiliary heat exchange plate.The use of the heat conduction silica gel could meet the performance requirements of the system,while the use of the auxiliary heat exchange plate could obtain better heat exchange performance than conduction silica gel one.When the batteries were discharged at 1C,1.5C and 2C,the maximum temperature of using the auxiliary heat exchange plate was 0.55?,1.09?,1.62? lower than that of the conduction silica gel,and the maximum temperature difference is 0.33?,0.65? and 0.97?.The scheme can be selected according to the proportion between the cost and performance.(6)The main/auxiliary heat exchange plate model was used to analyze the cooling and heating characteristics of the system,and the temperature changed in the coooling and heating process were analyzed.The results showed that: the use of the same heat exchange medium temperature as the ambient temperature could avoid the large temperature difference peak in the discharge process,the length of heating time under the heating condition was related to the heat exchange medium flow,and the improvement of the heat exchange medium temperature had little effect on it.When the ambient temperature was-10? and the heat exchanger temperature was 25?,heat exchange medium with 2g/s mass flow could make the minimum temperature and maximum temperature difference of the system reach a stable state in 20 minutes.(7)The temperature value of the fluid flowing into the heat exchange plate was often regarded as a constant value in the process of battery discharge,which was bound to cause some deviation.When the discharge rate was 1C at 25?,the maximum temperature of linear and exponential model was 3.34? and 2.54? higher than that of constant one.when the battery was heated at-10? with fluid temperature at 25?,the minimum temperature of linear and exponential model was 4.06? and 4.36? lower than that of constant one;The comparison of numerical calculation showed that the assumption of constant fluid temperature would overestimate the influence of the system on the maximum temperature of the module.When cooling,the assumption of constant fluid temperature would underestimate the influence of the system on the maximum temperature difference of the module.