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Lithium ion (Li-ion) batteries exhibit high power and energy densities, as well as high-cycle-lifetime capabilities. Li-ion cells based on LiFePO4 cathodes are promising alternatives to predominant metal oxide technologies, particularly for applications with high power and cycle lifetime requirements. This paper includes investigations on high-power LiFePO4 cells' cycle lifetime. Therefore, 3000 full charge-discharge cycles are applied to the cells with high constant current rates. Through reference testing, increasing battery impedance and a fading capacity are identified as a consequence of battery aging. Capacity fade proceeds in a way that cells' open-circuit voltages (OCVs) change only in the range of high state-of-charge (SOC). A model-based method that can detect the decreasingly utilizable capacity is outlined. The proposed method considers the changing OCV characteristics and enables the determination of capacity loss without complete charge and discharge being necessary during battery operation. By the evaluation of capacity loss information, an accelerated battery aging or even possible battery damage caused by overcharge can be avoided during battery charging scenarios.