To support a long-term sustainable electric future, there is ongoing development of electric vehicles, and thus, a serious problem is rising regarding recycling and the echelon application of batteries retired from electric vehicles. Herein, we propose to reuse such retired vehicle batteries as large-scale storage, based on which the operational reliability and the economy of composite generation and transmission systems are analyzed. In details, Gaussian functions were fitted according to data obtained using a holistic aging model. The proposed aging method simulated the capacity degradation of the retired batteries, with a low state of health, under various operating conditions. A universal generating function was adopted for probabilistic modeling of the capacity for battery modules consisting of hundreds of cells. The feasibility and effectiveness of using retired batteries in composite power systems were verified. The test results show that a significant investment can be saved, and system reliability improved, compared with systems without retired batteries. Finally, a method for scheduling the second retirement of retired batteries was established from the perspective of asset management.