The calendar aging of commercial 18650 lithium-ion batteries with lithium nickel manganese cobalt oxide cathode and graphite anode is studied by regular electrochemical characterization of batteries stored at defined conditions. The cell capacity is found to decrease linearly with time and shows a faster decrease at higher storage temperatures. From current pulse tests, it is determined that both higher temperature and higher state of charge (SOC) cause accelerated resistance increase with storage time. Changes in different battery parameters during storage are also quantified by analyzing electrochemical impedance spectroscopy (EIS) spectra. The cell degradation causes a gradual increase of the ohmic and the total polarization resistance with storage duration, where the latter one is found to be the main contributor to the increased cell impedance. An increase in the mean relaxation time constant and changes in the porous structure for the electrode processes are observed from EIS analysis. Resistance for this cell chemistry is found to be current independent by comparing the cell resistance calculated from the current pulse method after 1s and from the EIS analysis at 1 Hz. Furthermore, it is seen that the additional charge throughput due to the periodic electrochemical characterization induces significant cell degradation effects.