the plates in a battery can sulfate over in a day or so of not charging. sulfated plates prevent charging and theres really no way to fix it once its bad. Buy yourself an onboard charger (bank numbers depend on the amount of batteries you have). 5 amps per bank is plenty, and leave it plugged in all the time. it will turn off when the voltage gets high enough. I'm on my 5th season on my set of batteries, and they are still strong. A fully charged battery will read approx 12.82 vdc. anything less, and it needs put on the charger.
you can put a battery anywhere, since they have a plastic case now, they dont discharge through the ground. Also, if you keep the onboard charger plugged in all the time, you wont need to remove them from the boat in the winter, because the charger will keep them warm enough to prevent freezing.