Pulse Polarization for Li-Ion Battery under Constant State-of-Charge: Part II. Modeling of Individual Voltage Losses and SOC Prediction

Abstract

A Li-Ion battery, as most batteries, is an unsteady state system. In order to generate polarization curves under constant state of charge (SOC), an experimentally tedious task, we used a pulse discharge method where the initial SOC was adjusted such that at the end of the pulse and during the relaxation the SOC is pre-determined and constant. In this paper the pulse discharge data were simulated using Newman's model, allowing the identification of individual overpotentials under various discharge times, C-rates, and SOC. The agreement between the model and experimental results allowed the construction of pulse polarization curves (PPC) and the identification of individual overvoltages: charge transfer kinetics, ionic mass transport, and solid-state diffusion. The pulse polarization curves converged by the 240 seconds pulse discharge, forming a quasi steady-state polarization curve for the battery, which can be used to determine the SOC of a battery by measuring its steady voltage and current density. In electric cars this method can resolve the driver's range anxiety associated in accurately determining the SOC, especially at low SOC.