Research Of Inconsistencies Of Eliminated LiFePO₄ Power Batteries

LiFePO4 battery is currently the preferentially developed power battery of electrical vehicle in our country. When its capacity drops to below 80%, after one charging, the capacity of the power battery can’t satisfy the requirement of the cruising distance of electric vehicle. With the rapid development of electric vehicle, the necessity for the recycle of the eliminated power battery stands out. If the eliminated battery can be successfully applied to lower battery performance demanded cases, it can effectively extend the battery’s service life and substantially reduce the usage cost of the power battery. Therefore, it will be very helpful to promote the popularity of electric vehicle. However, the recycle of the eliminated batteries is limited by their inconsistencies. The detail aspects of the inconsistency, the sorting problem and the change of the eliminated batteries’ characteristics after series-paralleled are the main problems should be investigated before the recycle. Therefore, the author conducted research on those above problems in this paper.The development status quo of electric vehicle was briefly introduced in the first chapter, especially in the aspect of power battery, working principle and structure components of LiFePO4 battery, as well as the research status of eliminated power battery recycling.The second chapter focused on the study of the main experiment equipment and methods used in this thesis. Based on this, the health state of the eliminated battery was determined by the means of calibrating the battery initial state in the third chapter.The electrochemical properties testing experiment about the capacity, internal resistance and charging-discharging cycle of the eliminated batteries was conducted. Through the contrastive analysis between the eliminated battery and new battery, the degradation of inconsistencies was demonstrated by the data from above. At the same time, the feasibility of power battery recycling was verified by the results.In the fourth chapter, by analyzing the causes of power battery’s inconsistencies, it indicated that the inconsistencies of temperature and operation condition were the main factors contributing to the expansion of the inconsistency of the battery pack. Meanwhile, the main aspects of inconsistencies were investigated thoroughly, which were capacity inconsistency, internal resistance inconsistency and voltage inconsistency. By analyzing these three kinds of inconsistencies under different temperatures and different ratios, it was founded that the inconsistency differences increased when the temperature decreased and the discharging rate increased. At the end of this chapter, based on comprehensive consideration of internal factors including capacity, internal resistance and external factors including temperature and discharging ratio, a multi-step separation method of eliminated battery sorting was presented.In the fifth chapter, by analysis of the change of the properties in different discharging ratio, an investigation was conducted to study the influence of inconsistency in capacity, internal resistance and SOC on the performance of battery pack in series and in parallel. The results showed that the internal resistance inconsistency was the significantly influential factor; if the inconsistency difference was too bigger, it even lead to safety issues. The importance of battery sorting was powerfully confirmed again in the perspective of the battery packing. Meanwhile, this results could provide theory basis for the choice of connection mode in practical application.In the sixth chapter, four kinds of internal resistance testing method including the method of DC impedance, the method of short-circuit current and resistance level, the method of internal resistance tester testing method and the method of AC impedance spectroscopy were analyzed. The testing results showed that the time of test with internal resistance tester was shortest and this method can be applied online. The methods of DC impedance, short-circuit current and internal resistance level had better measurement precision. The methods of short circuit current and internal resistance level had better repeatability when SOC was in the range of 90%-20%. The method of AC impedance spectrum could comprehensively reflect the change of the battery inside parameters.