Capacity Measurements of Li-Ion Batteries using AC Impedance Spectroscopy

This work presents AC impedance measurements (EIS) on a Li-Tec 40 Ah Li-Ion battery cell, including measurements in a climate chamber at different temperatures. The objective of this work is to provide a preliminary basis for estimating the potential of using EIS as a diagnostic tool for battery capacity measurements. In order to estimate the feasibility a Li-Ion battery cell is characterized by several EIS measurement, including measurements in a controlled temperature environment. From the measurements it can be seen that the impedance spectrum indeed changes as a function of State-of-Charge (SOC). However, measurements also show that the same spectrum is also strongly temperature dependent. Is is however concluded that EIS can potentially be used as a capacity diagnostic tool if a non-isothermal battery model based on EIS input data can be developed. The climate chamber measurements also features temperature data of the battery temperature compared to surrounding temperature, this data shows that a battery voltage drop will invoke a battery temperature increase. As the impedance measurements presented in this work are carried out on flexible low-cost Labview platform using conventional data acquisition equipment, suggestions have been presented on how EIS, as a diagnostic tool, preferably can be embedded in the battery management system.