Research On Lifespan Factors And Test Methods Of Traction Lithium-ion Batteries

The lifespan of traction lithium-ion batteries and battery packs is one of the key constraints for electric vehicles and battery storage systems. Based on battery degradation mechanism and battery management, this dissertation focuses on factors affecting battery-pack’s life and battery test methods. The specific contents and conclusions are as followings:1. Taking the lithium manganese oxide batteries that used in the Olympic Games as demonstration, the actual battery operating environment and various factors affecting the cycling performance are analyzed. The equivalent stress testing method in actual operating environment is selected, which is designed by coupling BJDST typical current profile, different temperatures and different depths of discharge. The parameters of battery life model are obtained by fitting the degradation data, and then a simplified life model is proposed by using linear cumulative method and verified in comparison with cycling data of four seasons. The model reduces the time cost of battery life test and battery life prediction.2. Considering lithium iron phosphate battery as research object, the model parameters of single cell are analyzed through equivalent circuit model, EIS impedance spectroscopy and dQ/dV peak curve respectively, and cycling test of single cell is carried out under different temperatures to make a detailed analysis of the single cell’s internal resistance, polarization, diffusion coefficient, charge transfer impedance and the degradation mechanism of dQIdV peak curves. Through analyzing the measured loss of lithium and degradation of active material, it comes to a conclusion that the temperature conditions coupled with the aging cycle will cause different paths of cell degradation.3. A parallel battery test and simulation platform is established. The causes of unbalanced current are analyzed based on the model of parallel batteries. The current imbalance level is studied, which caused by internal resistance, polarization and capacity differences. Meanwhile, the degradation mechanism of each single cell is analyzed in-depth. The performance decrease and lifespan factors of parallel batteries are obtained. In addition, the equivalent range between parallel batteries and single cells is also described. The evalution indicators of parallel performance are further proposed, which provide the basis of extending cycle life of parallel batteries.4. On account of the actual operating characteristics of series and parallel battery pack, a simulation platform of series-parallel grouping batteries is built. The online parameter identification of series branch is proposed, and dynamic assessment of the imbalanced currents among branches is stated simultaneously. The impact analysis on cycle life of battery pack with different connection types is also analyzed. Finally, the degradation path analysis of single cell and parallel battery is integrated into the lifespan research of series-parallel battery pack, and a novel operating and management method to improve pack’s cycling performance is proposed comprehensively.