With some chargers like the Xantrex (not the best but the most prolific I think) you can switch from 3 stage to 2 stage charging. After a full charge is reached the charger sleeps until the voltage drops to 12.5 and it then comes on to fully charge before sleeping again. It refreshes the batteries each 21 days automatically in this mode. This is better for the batteries than a constant float and takes care of any self discharge issues.
Living aboard in Victoria Harbour