Most modern battery chargers have safeties built in to shut off when a battery is fully charged, but it is still possible to overcharge a deep-cycle battery. When the battery overheats, the water in the sulfuric acid solution within its case will boil and escape, leaving dry lead plates behind, which sulfate and lose their capacity. If the overcharging continues or the water isn’t replaced, the battery can be a complete loss.   Overcharging often is the result of the user not choosing the correct setting on their battery charger. While rare, a charger or battery maintainer’s failsafe can malfunction, so it’s always best to keep an eye on your charging status and not rely too heavily on the charger handling it for you.