Study On Thermal Characteristics And Thermal Management System Of Electric Vehicle Power Battery

Electric Vehicle EV（EV）has been recognized by many countries as the ultimate solution to replace traditional fuel vehicles,due to serious oil crisis and environmental pollution all over the world.Chinese government regards the research of electric vehicle technology as an important measure for the upgrading of automobile industry and the sustainable development of environmental economy.As the key component of electric vehicle,power battery system affects the power,safety,range and other key performance of electric vehicle directly.Battery Thermal Management System（BTMS）was design with the functions of monitor and control battery temperature in real time,which is significant for optimizing battery pack performance,improving battery cycle life and preventing Battery thermal runaway.In this study,coiled and laminated type Lithium-ion Battery（LIB）was taken as the research object,Firstly,the thermal characteristics of LIB under different working conditions were experimentally and numerically studied.Secondly,a novel thermal management system based on the thermal characteristics variation of the battery was designed.The main research contents including:（1）Infrared imaging investigation of temperature fluctuation and spatial distribution for a large laminated lithium ion power battery.This study investigates the thermal behaviors of a naturally cooled NCM-type LIB from an experimental and systematic approach.The temperature distribution was acquired for different discharge rates and Depth of Discharge（DOD）by the infrared imaging（IR）technology.Two new factors,the temperature variance and local overheating index,were proposed to assess the temperature fluctuation and distribution.Results showed that the heat generation rate was higher on the cathode side than that on the anode side due to the different resistivity of current collectors.It was found that the temperature variance and local overheating index were capable of holistically exhibiting the temperature non-uniformity both on numerical fluctuation and spatial distribution with varying discharge rates and DOD.With increasing the discharge rate and DOD,temperature distribution showed an increasingly non-uniform trend,especially at the initial and final stage of high-power discharge,the heat accumulation and concentration area increased rapidly.（2）An experimental and numerical examination on the thermal inertia of a cylindrical lithium-ion power battery.The thermal inertia of cells is the basis for battery thermal management design.The results show that the thermal inertia of the battery can greatly affect the thermal behavior of the battery during discharging process.Thus,a battery thermal model was created by COMSOL Multiphysics and infrared imaging technology was applied to investigate the thermal inertia for a Li Fe PO₄（LFP）battery.The temperature distribution and variation of the slack period（after discharge）were studied,including internal temperature,surface temperature and temperature difference.Results showed that the battery radius（R）and discharge rate（C）were the major factors that influenced thermal inertia.In addition,a thermal inertial calculation model was proposed for predicting battery thermal inertia under different operating conditions.（3）Improved thermal performance of large laminated lithium-ion power battery by reciprocating air flow.The present study investigates the thermal behaviors of an air-cooled NCM-type LIB from an experimental and systematic approach.A reciprocating air-flow cooling method was proposed to restrict the temperature fluctuation and homogenize temperature distribution.Results showed that there was a remarkable temperature distribution phenomenon during the discharge process,the temperature distribution was affected by direction of air-flow.Forward air-flow was always recommended at the beginning of the discharge due to the thermal characteristics of the battery.Comprehensive considered on battery temperature limit and cooling effect,the desired initial reversing timing was about 50%DOD at 3C discharge.Different reversing strategies were investigated including isochronous cycle and aperiodic cycle.It was found that the temperature non-uniformity caused by heat accumulation and concentration was mitigated by reciprocating air-flow with optimized reversing strategy.（4）Experiments on heat removal using fins for lithium battery thermal management system.A novel battery thermal management system that equipped the battery pack with fins was proposed and experimentally studied in this paper.The thermal behavior of LIBs with different discharge rates and fin thicknesses was investigated.The results show that under natural convection conditions,the addition of fins restricted the significant increase of the battery pack temperature and improved the uniformity of temperature distribution in the battery pack.Additionally,thicker fins satisfied the temperature requirements at higher discharge rates and greater discharge depths.However,the trade-offs and optimization between the thermal load,weight,and volume increase caused by the addition of fins was also investigated.