Equivalent circuit modelling is the most common technique to analyze EIS data . Thereby, the battery is modelled as an electrical circuit consisting of different classical and non-classical passive circuit elements. Equivalent circuit elements may be related to particular processes in a battery. Resistors and capacitors are the main conventional elements used to characterize electrochemical systems. Resistors are suitable for characterizing not only electronic, but also ionic conductivites, while capacitors describe charge retention or double layers. Diffusive processes behave differently, with a response typically at low frequencies down to µHz . These processes can often be modelled by so-called Warburg elements . A fundamental limitation of ECM fitting is the non-uniqueness of the model to reproduce a certain impedance spectrum, even if data is available at arbitrary sensitivity .