Ever since composite polymeric electrolytes (CPEs) were developed and studied authors publishing their results used to emphasize the role of new materials they used and not the details of the preparation conditions. In this work it is proven that the later can influence markedly the former and shown that the comparison of results obtained for different materials by different authors can be made only in limited number of cases. Herein results of the detailed studies on the composite, PEO-based polymeric electrolytes with surface modified fillers are shown. Extensive studies on the influence of different powders and preparation conditions on the final properties of composite electrolytes are described. The modification of fillers of various type and grain size was studied together with their influence on the functional parameters of electrolytes. Multiple research approaches and analysis methods were utilized in order to obtain as complete image of the materials as possible.