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You do not say what model your charger is, but if you press MODE you should see an option to CYCLE (or similar) each battery individually.
In the CYCLE mode the charger will fully discharge the battery, then fully charge the battery and then fully discharge the battery. It should then display the battery capacity. The charger will them fully charge the battery again ready for use.
For an 18650 LiIon battery, the whole process can take around 14hrs * 4 = 56 hours.