DC internal resistance during charge: Analysis and study on LiFePO4 batteries

DC internal resistance (IR) is considered one of the most important parameters of a battery, as it is used to evaluate the battery's power performance, energy efficiency, aging mechanisms or equivalent circuit modeling. In electric vehicle (EV) applications, the IR during charge gives also essential information related with regenerative braking and dynamic charge efficiency. In this work, we tested four lithium iron phosphate batteries (LFP) ranging from 16 Ah to 100 Ah, suitable for its use in EVs. We carried out the analysis using three different IR methods, and performed the tests at three charging rates (nominal, mid and high) through several states of charge (SOC). In this paper, we study the IR dependency with battery's capacity, SOC and the charging rate; also, the convenience of using a certain IR method is analyzed. Furthermore, the main results are put into context for practical EV applications, to enhance the design of battery management systems (BMS) in relation with the system's energy efficiency.