This article introduces a new Electrochemical-Polarization System (EPS) Model to improve lithium-ion battery models for autonomous electric vehicles (AEVs). The model incorporates an additional RC network to capture the relaxation effect in these batteries. The Nernst model is used to express the open-circuit voltage as a function of State of Charge (SoC), eliminating the need for time-consuming tests. Model parameters are estimated using the least squares method with data from a hybrid power pulse characteristic test. Experimental results validate the accurate simulation of battery behavior over time. Implementing this model and parameter determination approach eliminates the need for laborious tests traditionally used for parameter calibration.