IR Measurement of an LFP/C Battery: A Different Perspective

Abstract

Energy security and green energy are paving way for entire globe to adopt battery based ESS and EV. Online monitoring of battery parameters namely capacity and impedance are crucial to get better performance and life from the system. Impedance is a complex parameter comprises of ohmic, charge transfer and diffusion components. The effective impedance value differs depending on the measurement methodology and data accusation speed. DC method is generally used in the field due to simplicity and cost effectiveness. The present study adopts DC current interruption method for measuring impedance of 12.8 V/10 Ah, LFP/C battery. The voltage recovery and relaxation segment were used to derive impedance value at various SoC’s. First using electrochemical theory, it was proved that only diffusion is operating after current interruption when data is monitored at 1 s interval. The diffusion resistance changes not only with SoC but also with mode of operation i.e., charge vs. discharge. An empirical model was proposed taking care of three forces namely voltage gradient, repulsive force between lithium atom in graphene layer and Van der Waal’s force between graphene layers. The impact of the diffusion resistance on BMS is highlighted. It is suggested that the study can be used for monitoring ageing and calendaring life of battery, deducing the relationship between raw material and design parameter to performance of the battery.