Simplified modeling and characterization of the internal impedance of lithium-ion batteries for automotive applications

This paper presents a fast and simple approach to model and characterize internal impedance of Li-ion battery. This is an important aspect because, measuring the impedance in a wide frequency range, it is possible to investigate on the modifications of the internal electrochemical cell process and evaluating its State of Health (SOH), that it is an important parameter for the automotive applications. The first part of the paper explains the concept of Electrochemical Impedance Spectroscopy (EIS) and how Li-ion batteries can be represented through electrochemical or empirical models. EIS test consists of the excitation of the battery, or cell, with a sinusoidal current signal, and then measuring the cell voltage, while the frequency response of the system is computed. Then the paper describes a procedure to test the cell and extract the parameters necessary for the model, in particular, the dependence between the parameters of the model and the State of Charge (SOC). Finally the paper shows how the internal impedance of a Li-ion battery is a dynamic parameter that depends on different factors and illustrate how the EIS can be used to obtain an impedance model.