The disclosed embodiments provide a system that manages use of a battery in a portable electronic device. During operation, the system obtains a voltage of the battery and a state-of-charge of the battery and calculates an effective C-rate of the battery using the voltage and the state-of-charge. Next, the system uses the effective C-rate to estimate an inaccessible capacity of the battery. Finally, the system manages use of the battery with the portable electronic device based on the inaccessible capacity.