There is no direct technical possibility to determine the capacity of a battery by some measuring instrument. The capacity of a battery can only be estimated by observing its charge and discharge behavior, measuring a series of values like current, voltage and time.
For this reason, the capacity values and remaining run times displayed by macOS and Battery Monitor are estimates based on previously made observations, recorded by the charge processor inside the battery unit. The capacity reading and the resulting runtime are as exact as possible if the charge processor could observe a complete discharge/charge cycle a short time before. The processor “learns” how the battery is performing and readjusts its computations based on this behavior. This operation is called “calibrating a battery”.
Apple recommends to calibrate the battery in regular intervals, usually once in a month.
Note: Portable Apple computers with a non-removable battery (battery unit not replaceable by the end user) do not require manual calibration. These batteries come pre-calibrated and calibrate themselves automatically during normal operation. The steps outlined below are not necessary for these computer series.
To perform a manual calibration, do the following:
Further information about this procedure might also be found in the reference manual of macOS which can be opened via the Help menu of the Finder.
Note that calibrating has no influence whatsoever on the battery cells themselves. The lifetime of the battery unit will not be prolonged by calibration. Only the charge processor in the battery unit will enhance the quality of its computations and estimates.