Overcharging flooded batteries is undesirable. Among other things, it requires that water be added more frequently, which is a nuisance. AGM batteries are sealed, however, and if they are overcharged to the point that they vent hydrogen/water, they cannot be refilled and their life span will be shortened.
In terms of the aforementioned charge profile, particularly when using the combination of an alternator or generator and a shore charger, the challenge comes with charging the final 10 to 15 percent. Because the battery’s internal resistance increases with its SOC, pushing these final amps back into the battery is a slow process — and because of this, many batteries never achieve the all-important 100 percent SOC goal. Understandably, engines and generators are often shut down before the charge is complete, which in turn can lead to sulfation. This deficit can be offset or reversed by periodically carrying out a full charge by using dockside power, making a long run under power, or using an adequately sized solar array.