Quantitative diagnosis of the soft short circuit for LiFePO4 battery packs between voltage plateaus

Abstract

An accurate diagnostic method on the soft short circuit (SSC) for the Li-ion batteries before it evolves to a critical safety issue is recognized as one of the most important functions in battery management systems. Because the SOC (state of charge)-OCV (open circuit voltage) curve of Lithium Iron Phosphate (LiFePO4 or LFP) batteries is flat, there are few diagnostic algorithms that focus on LFP. Therefore, this paper proposes a quantitative SSC diagnosis method for LFP battery packs within a narrow voltage window. The proposed method firstly find the median voltage between the two voltage plateaus during the constant current charging for all the cells in the battery pack. It takes the cell with the highest voltage as the reference cell, and calculates the time differences and electric quantity differences (EQDs) when the other batteries reach the median voltage during every charge. The leakage current and SSC resistance of the SSC cell can be calculated from the increase in EQD after each charge. A series of experiments with external resistors show that this method can identify the SSC resistances of 510 Ω and is suitable for various applications, especially when they are seldom fully charged or discharged.