This paper investigates how the aging of lithium-ion batteries is influenced by the superimposition of an AC waveform on a discharge current. Based on the results of two experiments on LiCoO2 cells, the root mean square (RMS) value of the discharge current is determined to be a significant aging factor, while evidence does not support the importance of either the DC current value, waveform shape or frequency for battery aging. The practical significance of this result lies in the recognition of the importance of passive filtering or other means to reduce the AC component of the battery pack discharge current. Furthermore, a quantitative analysis is provided to numerically determine the aging effect of the RMS current factor. Such quantitative results can provide valuable guidelines for designing features such as battery pack filters using DC bus capacitors. The design of experiments and associated analyses presented in the paper can also be adopted for the investigation of other battery aging factors of interest.