Absorption Stage
The second stage, in which the charge voltage is held constant and the current is left to fall by itself. In lead-acid the last part of the capacity is restored in this stage, and when the current falls to a set threshold the charge moves on to the float stage. Its duration is typically on the order of a few hours.
In this stage the charger behaves like a voltage source. As the battery fills, the internal electrochemical counter voltage rises, the difference between them shrinks and the current falls exponentially. The device considers the battery full when the current drops to a small percentage of the capacity.
The typical absorption voltage is 14.4-14.8 V for a 12 V lead-acid battery and lower for gel. The duration is critical: if it is kept too short the battery is chronically undercharged and sulphates, and if it is kept longer than necessary water loss and plate corrosion increase. Good chargers set this duration by looking at how deeply the battery was discharged during the bulk stage.
In an RV the absorption stage is usually the one that gets skipped. If you drive the vehicle for two hours and then stop, the battery appears full but in fact has not been fully charged. This behaviour reduces the usable capacity over time and causes the voltage to sag more on loads with high instantaneous current, such as levelling jacks. Completing a full charge cycle a few times a month prevents this build-up.
The context in which this term is used: RV automatic levelling guide