Battery degradation will happen in the process of storage, resulting in capacity diminishment and augmentation of inherent resistance. The understanding of the aging mechanism is crucial to predict the state-of-health of lithium-ion batteries (LIBs). In this paper, a pseudo-OCV model of a LIBs is developed to investigate the evolution of internal parameters, and a degradation model which can be used for predicting the calendar life of the battery is developed. The good agreement between the calculated and experimental results is the evidence that the models are able to accurately capture the degradation process of the battery. In addition, we use both incremental capacity and differential voltage analysis to research the aging mechanism of a lithium ion battery. Our results revealed that the capacity fading during the aging process is predominantly caused by the loss of Li-ions and self-discharge. (C) 2017 The Electrochemical Society. All rights reserved.