In this paper, the nonlinear extended state observer (NESO) is proposed to estimate the state-of-charge (SOC) for the Lithium-ion batteries (LIBs). The NESO is designed based on the simple first-order equivalent circuit model (ECM) to estimate the total disturbance of the battery model and compensate them as the new extended state, which can achieve the active disturbance rejection to estimate the battery state accurately. Furthermore, the observability and stability of the NESO is proven, and estimation performance of the observer is verified under different ambient temperatures, testing driving and different noise levels. The SOC estimation results indicate that the NESO can obtain accurate estimation values and strong robustness against different disturbances, even though all validation conditions use constant battery value of the first-order ECM. In addition, compared with other common estimation methods, the NESO exhibits a faster convergence time and more accurate estimation results.