Modeling Study of Commercial NCA Cylindrical Battery Cells’ Impedance Behavior Under Different Operation Conditions

Abstract

The classic 18650 and 21700 format cylindrical Li-ion batteries have been widely used today to power electric vehicles (EVs). Their wound jelly-roll structure (in a form of an Archimedean spiral) can provide a relatively higher energy density in the limited EV given space comparing with the battery cells using stacked electrodes. As the rapid development and deployment of EV technology require high performance and reliable Li-ion batteries, it is critical to understand the mechanism and process behind battery cell behaviors under various driving conditions. In this study, a series of impedance experiments is designed to explore the impact of battery state of charge (SOC), temperature, C-rates and other operating conditions. Then, an impedance mathematical model is developed to explore the potential mechanisms behind experimental observations. Our obtained results show that the impedance response of cylindrical NCA battery cells varies with the state of charge in both charge and discharge operation, and the SOC dependent impedance existed in different operating temperatures and C-rates. Through modeling analysis, results indicate the operating temperature and charge-discharge rate could significantly influence the impedance response of the battery cell but could not directly cause the variation of the impedance response at different SOC conditions. Besides, modeling results also show that the contact resistance variation, resulted from the expansion and contraction of cell electrodes during charge and discharge, could lead to a SOC dependent impedance behavior of the commercial cell and influence battery output performance.