In some variations, a method of real-time monitoring of battery capacity comprises correlating electrode open-circuit voltage with electrode state of charge for a selected electrode; compiling a look-up table to correlate the electrode open-circuit voltage with the electrode capacity at different values of the active-material capacity; during real-time operation, identifying first and second times at which battery terminal voltages are approximated as battery open-circuit voltages; and calculating battery capacity based on the difference in battery open-circuit voltages at the first and second times, current integration, and the look-up table. No reference electrode is needed, and a complete battery charge/discharge is not necessary to determine the capacity. This technique can therefore be implemented on-board and in real time to provide reliable capacity estimation even as the battery ages. The methods are applicable to various metal-ion secondary battery systems, including lithium-ion batteries, with different material chemistries.