Holistic battery system models are a key tools for the design and optimization of stationary battery systems. The objective of this contribution is the development of a Modelica-based system model which can be used for the investigation of operational optimization strategies and predictive maintenance approaches. The model includes a 0D-battery cell model based on an electrical and thermal equivalent circuit. The model has been verified via comparison to a validated Python-based model. A particular focus of the project is the model scaling, which describes the extension of the battery cell model up to system size representing a variable number of cells structured in modules and racks. Therefore a statistical approach for electrical scaling, which calculates the effects of resistance and capacity deviation is presented. It offers lower computational effort and simplified results analysis while retaining a comparable information content. In addition, the thermal influences of scaling are also calculated by an integrated thermal equivalent circuit model, which is expandable for a variable number of cells and a arbitrary two-dimensional geometry. Further work concerning integration of additional system components and validation has to be carried out during consecutive works.