Three-Stage Charging (Bulk, Absorption, Float)
This is the charge profile used in the lead-acid family to fill a battery quickly but without damage, made up of a constant current stage and two different constant voltage stages. First most of the capacity is filled at high current, then the voltage is held constant and the current is allowed to fall, and finally the battery is held at a lower voltage.
The logic of the profile is that the current a battery can accept decreases as its state of charge rises. In the first stage the charger delivers all the current it can and the voltage climbs. When the set absorption voltage is reached the charger holds the voltage constant; from then on the battery chemistry determines the current, which falls slowly. When the current drops below a threshold the charger switches to a lower holding voltage.
Typical values for a 12 V lead-acid battery are: absorption 14.4-14.8 V, float 13.4-13.8 V. On gel batteries the absorption value is kept lower. For the exact figure the battery manufacturer's data sheet is what counts.
In an RV this profile is applied by the mains charger, the DC-DC charger and the solar regulator all three at once. All three must be set to the same battery chemistry; differing profiles keep the battery permanently undercharged or permanently overcharged. On LiFePO4 packs the float stage is considered unnecessary by most manufacturers and charging is stopped at the end of absorption.
Context in which this term is used: Guide to automatic RV levelling