The gas charged hydraulic accumulators under consideration are used as energy storage devices in automotive hydraulic hybrid systems. The knowledge of the state of charge (SOC) is of utmost importance for an efficient energy management of the hybrid drive-train. Since the direct measurement of the SOC is difficult and costly, an SOC estimator design is presented based on a mathematical model of the gas charged hydraulic accumulator. Two different SOC estimation strategies, a signal-based approach and an Extended Kalman-filter, are presented and compared with each other. The performance of the estimators is validated by means of measurement results of a test driving cycle.