Abstract: A reliable technique of predicting the available reserve time remaining to a selected end voltage of a discharging battery is based on an adaptive state-of-charge algorithm that is active in real time to respond to changing conditions of a battery system. The adaptive state-of-charge algorithm is based on measured discharge characteristics of the battery whose reserve time is to be predicted which have been reduced to two parameters plotted as a single curve with a linear and an exponential region. These discharge characteristics are combined with dynamic parameters of the battery system which are monitored and processed in real time to provide a continuous evaluation and reevaluation of the reserve time remaining under changing conditions. As the discharge proceeds there is a continual improvement in the prediction of reserve time available.