To improve the accuracy of the initial state-of-charge (SOCo) estimation, an enhanced SOC0 model-based ampere-hour (Ah) model of a LiFePO4 battery is presented for application in a battery energy storage system. It varies as a function of the open circuit voltage (OCV), rates, and relaxation effect. Here, the Thevenin equivalent circuit model is selected to model the LiFePO4 battery, and its mathematic equations are deduced to determine the inherent trade-off between the time constant and main parameter of the battery, which can address the dilemma of the conventional SOC0 method (OCV model) as the relaxation effect and how long the battery takes to reach a steady state. To illustrate advantages of the enhanced estimation model, a comparison analysis is performed on the enhanced model with the former model under the dynamic stress test. Furthermore, the experimental results indicate that the correction of SOC0 can significantly decrease the estimation error. The enhanced SOC0 model is expected to help determine operational parameters for batteries of variable power sources.