Power smoothing (PS) techniques are employed to mitigate the network problems caused by the variable output power of non-dispatchable renewable energy sources (RESs). Keystone of these techniques is the use of a battery energy storage system (BESS) acting as an intermediate energy buffer to smooth the RES output power. Scope of this paper is to investigate the impact of the most well-established PS techniques on the long-term performance of BESSs focusing on capacity degradation. To this end, an accurate BESS aging model based on the rainflow cycle-counting algorithm is integrated to the examined PS techniques to estimate the cycle aging of the BESS. Furthermore, a parametric analysis with high-resolution RES measurements is performed to identify the most critical parameters that affect the PS capability and the BESS capacity degradation.