Abstract: Based on the rain flow algorithm, a modeling method to predictive the state of health（SOH） of the LiCoO₂ lithium-ion battery was proposed in terms of the complex driving conditions of the electric vehicles（EV）. First, a real-time rain flow algorithm was developed to count the cycling numbers according to different depth of discharge（DOD）. Based on the enveloped area surrounding by the equivalent charging/discharging currents and the state of charge（SOC）, a degradation factor via integrating the enveloped area was proposed, and a way to describe the battery SOH was given by accumulating the cycling numbers of different degradation factors. Finally, the performance of the proposed battery SOH model was evaluated through experiments, which are able to on-line estimate the battery SOH even under the cases of arbitrary charging/discharging currents, DOD and cycle numbers.