In this article, a new simulative approach that can determine ageing inhomogeneities in lithium-ion battery systems is outlined. The proposed method is based on a thermal electric ageing model of an entire battery system and the Monte Carlo Method. This method considers temperature inhomogeneities caused by active cooling within the battery system as well as cell spread and ageing spread caused by tolerances in the cell production. Each cell in the battery system model is represented by its own thermal electric cell model, having an initial capacity, initial internal resistance, ageing rate and thermal connection to the cooling system. In the experimental part of this research work, a battery system consisting of 96 lithium-ion cells based on the LiFePO4-technology was tested to investigate the capacity fade during a cycle life test. The results show that discrepancies in cells ageing within the battery system are a consequence of cell-to-cell spread and different loading of the cells, caused by temperature and SoC inhomogeneities within the battery system. Furthermore, the article will evince the gain of accuracy by using a battery system instead of a previous single cell approach for life prediction.