In this paper, we introduce an observer-based state of charge (SOC) estimator for batteries. The estimation strategy is based on the battery's state space model. This representation makes stability analysis easier for the proposed estimation technique. Unlike other estimation strategies, only battery terminal voltage and current measurements are required. Simulation results highlight the performance of the proposed estimator in determining the SOC with high accuracy. Robustness to uncertainties such as parameter variations and battery aging effects is also verified.