This study proposes a robust SOC estimation scheme for lithium-ion batteries in commercialized electric vehicles: pure electric vehicles and hybrid electric vehicles. Development of a nonlinear-model-based robust SOC estimation method and comprehensive performance comparison with the existing SOC estimation methods under the two kinds of operating scenarios are considered. The features of the results of this study are summarized as follows: a) a robust SOC estimator is designed to deal with the model uncertainty and parameter variation, b) an analysis of asymptotic stability of the proposed robust SOC estimator is provided and c) a comprehensive analysis was performed not only on the different aging levels of battery cells but also on the different operational conditions of the electrified vehicles. The effectiveness of the proposed SOC estimation method was experimentally verified using two 18 650 cylindrical battery cells with different degradation levels. The proposed SOC estimation scheme ensures considerable accuracy under both pure electric vehicle operating mode and hybrid vehicle operating mode even if the battery cell is aged.