I have a pretty large stereo bank (450Ah) and in order to protect and hopefully prolong not only the batteries but also the alternator, I leave my boat plugged into the charger anytime the boat is not in the water. I have a perko switch wired through an automatic charging relay so sometimes if we run the stereo really hard, the voltage will drop to the point where the ACR will not stay closed so the alternator does not recharge the stereo bank. Usually, if needed, I'll bypass the ACR and let the alternator (100A) apply a bit of a charge to them.
I bought a nice Xantrex Truecharge 2 40A charger that is wired onboard the boat and plugged into a GFI outlet. This charger is a 2 (bulk, absorption), 3 (bulk, absorption, float) or even 4 stage (bulk, absorption, float and equalization) charger. I use it with their battery temperature sensor and I always open the battery compartments to make sure they have plenty of airflow. I also have a fan that stays on in the garage, blowing air into the boat.
I do have the ability to remotely turn off the outlet that the charger is plugged into if I wanted to but unless it is going to be there for awhile, unused, I just see any reason to turn it off. I feel much more comfortable leaving it plugged in while we are not there and usually unplug it once we get there. If something is going to happen, I am glad for it to happen while no one is home.
Here's a quote from a review of the charger:
"I’m often asked if it’s permissible to leave the RV plugged into shore power during lengthy periods of non-use. I usually warn against such a practice primarily because it’s likely the battery bank will be over-charged by a typically mediocre converter/charger at some point and literally boil the electrolyte out of the batteries. Over-charging a flooded, wet cell battery (lead-acid and lead-calcium) produces dangerous gasses containing hydrogen and oxygen. I’ve personally witnessed exploding batteries in the shop during an improper charging cycle; it ain’t pretty (see my sidebar note below).
With the Truecharge2, the over-charging concern mentioned above is totally eliminated. Under normal circumstance (the absence of 12-volt DC anomalies), it is now possible to leave the RV plugged into shore power continuously, even without adult supervision. Here’s why, once the battery bank enters the Float stage and no 12-volt device is energized, it will stay in float for seven days or until the voltage falls below 12.5 volts for fifteen minutes. Assuming nothing is draining amperage out of the battery bank, after the seventh day, the Truecharge2 again begins its charging algorhythm anew, just to keep the batteries refreshed. Because of the integral design principles, the proper charging sequence remains in effect for as long as the Truecharge2 is powered by the 120-volt AC electrical supply."
Here's the link to the review: