Methods and apparatus are provided for estimating the remaining state-of-charge of a non-rechargeable power source, such as a back-up battery for an electronic module normally connected to a primary power source. A processor within the electronic module implements a prediction algorithm, in order to monitor the temperature of the back-up attery, and to calculate various current draws of the back-up battery. The processor adjusts the nominal self-discharge rate of the back-up battery in accordance with the measured temperatures, and calculates an adjusted self-discharge quantity. The processor also accumulates current draw data and reduces the initial state-of-charge value of the back-up battery by the temperature-adjusted self-discharge quantity and the cumulative current draw quantities to determine a remaining state-of-charge of the back-up battery. At a predetermined state-of-charge threshold level, an indicator can be activated to advise a user to replace the back-up battery.