Thermal Characteristics And Thermal Management Based On Phase Change Materials Of Lithium Battery

Lithium battery is one of the core of electric vehicles,its characteristics and safety directly affect the performance of electric vehicles.The working performance of lithium battery is greatly affected by the temperature,and at the same time,a large amount of heat is generated during work which makes its own temperature rise.Therefore,mastering the thermal management of lithium battery has become the focus of scholars' research.In the thermal management of lithium battery,phase change material cooling has been paid much attention because of its advantages such as high latent heat,high heat dissipation efficiency and no need of extra pump power.Firstly,the influence of ambient temperature on battery capacity,internal resistance and entropy coeffient was analyzed by setting up the charge-discharge test platform.The heat generation model of battery was established by using Bernardi heat model and the accuracy of the model was verified by experiments.The results show that the actual capacity of the battery is related to the charge-discharge rate and ambient temperature.The actual capacity of the battery at high rate is less than rated capacity.Compared with the high ambient temperature,the capacity of battery is more affected by the low temperature.The ohmic and polarization resistance of battery are affected by ambient temperature and SOC,both ohmic and polarization resistance increase under low temperature and SOC.Entropy coeffient varies with SOC.At the beginning and the end of the discharge,the entropy coeffient is negative.At other stages,the entropy coeffient is positive.Secondly,different mass fraction composite phase change materials were prepared by adding carbon nanotube into paraffin.The thermal conductivity was obviously improved by adding carbon nanotubes.Then,the thermal model for a single cylinder lithium battery coupled with the phase change materials(PCM)was established and used to solve the phase change heat transfer problem by enthalpy way.The influence of PCM thickness,PCM thermal conductivity and air cooling on the temperature rise of lithium battery in the process of charging and discharging was analyzed.The research shows that in the complete charge-discharge process,the accumulation of heat in the previous stage has a great influence on the temperature rise of lithium battery,and the temperature rise is significantly higher than that of a single discharge process.Considering only the amount of PCM in a single discharge process,the temperature rise of lithium battery in the charge-discharge process cannot be effectively controlled.With the increase of PCM thickness,the temperature rises of lithium battery and the liquid fraction of PCM decrease.When the external heat transfer conditions are more sufficient,the temperature rise of lithium battery is more sensitive to the change of PCM thermal conductivity,and improving PCM thermal conductivity is more significant to reduce the temperature rise of lithium battery.Last but not least,according to the numerical simulation,the influence of heat dissipation of battery module based on phase change materials was analyzed.The influence of battery cell spacing,PCM latent heat?PCM conductivity and battery arrangement was explored.The results show that small cell spacing brings difficult to temperature control of the battery module,large cell spacing decreases the battery energy density.Although the maximum temperature and maximum temperature difference of lithium battery module decrease with the increase of thermal conductivity and latent heat of PCM,high thermal conductivity and latent heat of PCM make the cooling effect not obvious.Heat dissipation performance of battery module can not be effectively improved by the staggred arrangement.Under the align arrangement,battery module with arithmetic ratio arrangement is better for the uniformities of temperature.