Abstract: The present invention relates to an implementation of a battery State of Charge (SOC) estimator for any battery type. It addresses the problems associated with the existing implementations such as high error uncertainty, limited application (i.e. only one battery type) and susceptibility to temperature changes. Embodiments of the present invention use Kalman filter or Extended Kalman filter for a battery model that has SOC as an internal system state. Having an SOC internal state allows the invention to provide an uncertainty associated with its SOC estimation. One embodiment does not take battery temperature as a parameter in its SOC estimation. Another embodiment uses battery temperature as a parameter to adjust its SOC estimation to keep the accuracy of the SOC estimation from being affected by changing temperature. One embodiment allows different modeling parameters during battery operation to accommodate highly dynamic batteries used in Hybrid Electric Vehicles and Electric Vehicles.