Hi, I'm new to the site, but felt compelled to reply on the dead marine battery that needed to be charged up. Your battery is more than likely okey-dokey but remember that a deep-cycle/marine battery needs to be fully charged before you take it out on the water, and that a slow "trickle" charge is what you need to bring your battery up to a full charge. This might take up to three days to fully charge your deep-cycle battery. You can charge multiple batteries by hooking them together, positive to positive, & negative to negative. This will keep your battery'(s) charging in a 12 volt pattern, called a parallel circuit.
A second method of hooking batteries together is called a series circuit. This method means you hook our batteries together positive to negative, negative to positive. In this form of hooking batteries together increases the voltage of the total sum of voltage times the amount of batteries wired together. An example of this is 12v + 12v = 24volts. Adding another battery in this manner would increase the voltage to 36 volts, & adding one more would increase the voltage to 48 volts.
A four battery, 24 volt system requires that a series & parallel hook-up is being utilized. A combination of both systems.
Goodluck with your battery, it probably just needs a good slow charge...