The battery is the central component of every electric vehicle – it determines range, operational reliability and cost efficiency. Unlike in passenger cars, batteries in buses and commercial vehicles face high stress: high charging power, many cycles and extreme climatic conditions. Therefore, precise sizing is crucial. We determine energy demand based on real driving profiles, rotations and route conditions, and design charging and battery scenarios accordingly. At the same time, we consider the choice of cell chemistry (LTO, LFP, NMC) and its impact on cycles, lifetime and costs. Using established simulation models, we forecast ageing (SOH, SOR) over a full schedule year, enabling a realistic assessment of battery life. Already at this stage, second-life use is considered – e.g. as stationary storage. The result is a battery system that maximises operational safety, minimises cost risks and optimally supports vehicle lifetime.