Just a few basics.

Flooded batteries can self-discharge by 3 to 4 percent a month. Storing batteries in a discharged state accelerates their decomposition. A short maintenance charger is a good idea on the starting battery since it is put up relatively charged, as part of a dual-bank set-up.

However, deep cycle batteries that are deeply discharged when the stereo is played at rest, deserve greater capacity charging in order to properly desulphate and condition. For this you would want a dual-bank multi-stage AC shore charger that has a charging capacity equal to or greater than ten percent of the total batteries' amp/hour rating. A smart dual bank charger that distributes its capacity on demand to the bank in need is also an advantage over a charger that limits the current to each bank.

A dual-bank charger has the advantage of aggressively restoring the stereo bank without overcharging the starting battery.

Extra battery capacity is a benefit because if you continually exhaust your stereo bank to empty (10.5 volts for example) you'll get fewer cycles than if you partially deplete the batteries.

Full restoration should take place immediately upon storage rather than waiting just prior to the next outing.

With big stereos and play at rest you dont want to be solely dependent on your alternator. Get the best AC charger you can justify. Do what the tournament fisherman do as they've been dialed into battery technology and maintenance forever. Imagine a 40 amp trolling motor fighting a head wind for 8 hours. Those guys have to maximize their batteries because when their batteries are dead they're finished fishing.

David
Earmark Marine