A system for tracking the capacity of a battery in a portable electronic device is described. While plugged in-to a power adapter, the system measures a first open-circuit voltage for the battery while the battery rests at a first state of charge. Next, the system causes the battery to transition to a second state of charge. During the transition, the system integrates a current through the battery to determine a net change in charge. Next, the system measures a second open-circuit voltage while the battery rests at the second state of charge. Finally, the system estimates a capacity for the battery based on the first and second open-circuit voltages and the net change in charge. This capacity measurement is repeated and the multiple results are fitted to a line. The slope of fitted line is used to estimate how the battery capacity has faded since last measured using traditional techniques.