I have a "mid-century technology" ferroresonant charger aboard. It provides a float charge that is just right for maintaining batteries - I am using mine with Optima batteries.
The problem is that it can never fully charge the batteries at float voltage, because they need higher to absorb all they can. A modern charger has three levels, for bulk charge, float, and maintenance. I get bulk charge from the alternators, float from the charger, and maintenance with a solar cell, 85 watts, that puts out a constant 2.5A up to perhaps 17 volts. This tops the batteries up and maintains some charge even when there is no ac. The batteries will handle the 2.5A indefinitely.
Good modern chargers are great but expensive.
I recommend the Optima blue top without reservation. My house batteries are going on their eighth year with no problems. I replaced the starter batteries after six, although they still started the boat they had lost about 30% on a load test.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
