On the dynamic behavior of an aged Lithium-iron phosphate battery

To date, the ageing of a lithium-ion battery is mainly detected by the reduction in capacity compared to the rated value and by the increase in internal impedance. These effects are considered in the electrical equivalent circuit through the representation of suitable empirical laws for the evaluation of the state of charge and that of the internal impedance. However, less attention has been paid to the analysis of alternate dynamic behavior of the battery voltage to a step variation in its current. This paper deals with the dynamic behavior of aged Lithium Phosphate-iron battery and introduces a novel dynamic ageing index. That is, to evaluate the dynamic voltage response following a discharge pulse at different states of charge and under different temperatures. A voltage undershoot was identified following a discharge step which highlights an inductive behavior at certain states of charge and temperatures. This phenomenon does not appear to be present for all states of charge and temperatures. This is likely stems from changes in internal resistance, polarization, and capacitance. The equivalent circuit has been modified by including a parasitic inductance in parallel with the polarization capacitance. The proposed model has been experimentally validated through a comprehensive set of tests in a climatic chamber.