Experimental analysis of commercial LiFePO4 battery life span used in electric vehicle under extremely cold and hot thermal conditions

摘要

To widely commercialize electric vehicles more efforts for their life improvement seem extremely inevitable. Thermal conditions can have profound and nonlinear effects on the degradation rate of an electric vehicle battery pack as well as its performance and safety level. In the current study, both cycle life and calendar life of a commercial LiFePO4 cell are investigated experimentally by means of capacity fading and resistance increment evaluation for 4 different thermal conditions from extremely cold condition of - 20 degrees C-which is not well studied in the literature-till hot condition of 55 degrees C. The calendar life tests show that the best condition for storing cells is at 5 degrees C and 50% SOC and the cycle life tests demonstrate that the best operating temperature is 25 degrees C based on the dynamic stress test discharge/charge profile (a test profile for electric vehicles). It is also found that the capacity fading and resistance increment at a high temperature such as 50 degrees C are destructively significant. The presented curves in this paper can also serve as an aging data source for further work on battery lifetime modeling and diagnostics. The role of temperature on the degradation level is also discovered via scanning electron microscopy.