 Electro-chemical impedance spectroscopy is widely used to analyze electro-chemical systems. Most attention is paid to the double-layer capacitance and the charge-transfer resistance as they describe the electro-chemical process on the surface of the electrode. Both values can provide specific information about aging mechanisms, which diminish the surface area. This is of interest when capacity tests are restricted to determine the aging. For lead-acid batteries, for example, this is the case in applications like micro-hybrid vehicles or uninterruptible power supply systems. However, the interpretation of impedance spectra of lead-acid batteries necessitates proper measurements, elaborated verification of measurement validity, and a sufficient model of electro-chemical processes. In this work, impedance spectra, recorded on lead-acid test cells, are processed to identify the ohmic resistance, the double-layer capacitance, and the parameters of the charge-transfer reaction of the negative electrode. This electrode suffers from sulfation, a common aging mechanism in current applications. The aim of the paper is to define a correct processing of impedance spectra for lead-acid batteries, and to depict challenges. Furthermore, possible equivalent electrical circuit models for the negative electrode are evaluated regarding their dependencies on state of charge and current rate. Many of these aspects can be transferred to other electro-chemical systems.