Winter Battery Storage
When wet-cell batteries are allowed to discharge, the electrolyte becomes pure water, which will freeze and ruin the battery. If you can, take the battery off your boat and store it somewhere dry and cool (but not freezing). You may have heard that a battery will discharge if left on a concrete floor, but modern batteries make this precaution unnecessary.
Wash and thoroughly dry the top of the battery to reduce the potential for self-discharge. Don't leave stored batteries connected to a portable charger. Unless the charger turns off completely — few do — the batteries will suffer damage. However, stored batteries should be brought to full charge once a month, so post a reminder for yourself. If the batteries will be stored aboard because they're too heavy for convenient removal, they must be maintained in a full-charge condition all winter. This requires a charger with a "float" stage and power connection. In lieu of an unattended power connection, a solar panel might be employed to counter self-discharge