Creating Equivalent-Circuit Models

The EIS software includes a powerful nonlinear least-squares curve-fitting tool. Normally you use this tool to fit equivalent-circuit models to impedance spectra. However, it can be used to fit any model that can be expressed in a standard mathematical form.

The EIS system is supplied with equivalent-circuit models for a few of the more common electrochemical situations. However, these do not approach the enormous number of models required to describe real-world electrochemical cells.

Because of this, the EIS software also includes a graphical model editor. This editor is used to modify the standard EIS models or to create completely new equivalent-circuit models. Models are built up using resistors, capacitors, and inductors, all of which are familiar to electrical engineers. You can also add less-common components to your design: Warburg and Gerischer impedances which describe diffusion, and Constant Phase Elements which describe non-ideal capacitors.

This section discusses the routine use of the model editor in creating and editing equivalent-circuit models. This discussion assumes that you have not altered the model editor in any way, and that you only use the supplied circuit elements in your design. A simple example—creating a new Randles Cell model—is used to illustrate the use of the model editor.