In the past I have written articles on batteries and charging . I will give you the short dissertation.
Very simly put: a fully charged battery after settling while not being charged has 12.6 volts at the terminals, and discharged is about 12.0. They very worst thing you can do to a battery is to put a load on it and run it dead, and leave it that way. The battery will sulfate. Some lead actually gets consumed and there is NO way to put it back externally.
Charging: a modern, 3 stage (or smart) charger starts out at 14.5 volts and crams the maximum current for a few minutes, then shift to 14.2 or so for the bulk charge, then it senses about 90-95% charge and shifts to about 13.2 or so which will fininish to 100% and can be left on the battery forever without boiling.
Buy a decent digital volt-ohm-ammeter. These can be had for $20-30. You can clip it onto the battery and see the above voltages happening. So in the Ranger tugs, with the charging relays, when starting nthe engine, you will see the voltage jump tp about 14.3 or so when they kick in. If they only come to, say, 13.5 that means the battery in question is partly discharged, but the system is working. On my previous boat I bought a regular 2" boat battery gauge, and a 3 position switch (I miss radio shack) so I can watch either battery bank or the selected one. Its in the cabin so i can watch the charger while at a marina or anchored. Present boat has a gauge and its set up on the selected battery.
I have my boat on a trailer in my yard. It's connected to shore (house?) power while here all teh time. I get 6-7 years out of my batteries.
Caveat: the above voltages are for a flooded electrolyte battery. AGM, Gel, lithium and other batteries have slightly different voltages, as well as the ambient temperature causes a difference. however, the charging works the same as above.
Hope y'all got a charge out of this. (pun intended)
