A method and apparatus for measuring the state-of-charge of a battery is provided, which includes a metering system to communicate the state-of-charge to an operator of a device powered by the battery. A digital electronic model is provided to compute the instantaneous charge in a storage battery that tracks the actual terminal voltage of the battery. In the digital model, the battery model is expressed as a computer program. The electronic model derives from the measurement of the actual terminal voltage of an operating battery an internal voltage corresponding to the open circuit voltage of the battery. This internal voltage, representative of the state-of-charge of the battery, is communicated to an operator through a display.