A novel measuring method for the battery state is presented to supplement the usual methods on the basis of electrical measurements and computational battery models. Measurement setup and procedure for observing the change in the electrode material during the storage and removal of lithium ions are presented in detail. The ion intercalation of the electrochemical process can be made visible by means of a test cell with a transparent window and continuous image recording and processing. The change in reflectance and color on the surface of a lithium iron phosphate electrode was found to closely correlate with the charge state calculated from cell current. Furthermore, we discovered that an electrochromic marker introduced into the electrode material significantly enhances the optical effect. Observing the electrode allows for the analysis of the ion intercalation within the electrode. The spatially resolved information could be used to determine the parameters of the reaction kinetics and material characteristics. Image recording on optical test cells was used for comparative materials testing in various geometries. The aim is to develop a sensor that can be used in commercial battery cells. For this purpose, preliminary experiments with optical fibers have been conducted. The optical fibers are to be prepared so that the transmitted light interacts with the electrode material, and inserted into the cell structure. This fiber sensor could provide a complementary battery management function to calculate the state-of-charge and as a redundant safety level to avoid critical battery conditions.