A Kalman filter is developed from a model which characterizes the float service life of a battery into two phases. Once the latter phase of the float service life, that time when the capacity begins to decrease rapidly, has been detected the Kalman filter is started. Outputs of the filter are a smoothed version of the battery capacity and the projected capacity at specified time intervals in the future. It is this project ahead step that is used to estimate the remaining float service life of the battery.