Don't let the "maintenance-free" battery in your car lull you into complacency about the battery in your boat. Neglecting a marine battery is certain to shorten its life.
Get the most from your boat's batteries by following these guidelines:
Keep idle batteries fully charged. Leaving a battery even partially discharged leads to sulfation and loss of capacity.
Check the water level regularly. Top up as necessary to keep the level about 1/4" above the plates.
Use only distilled water for topping up; trace minerals and/or chlorine in tap water really do shorten battery life.
Fill cells after charging. If you fill them before, expansion during charging will pump electrolyte out on top of the battery, causing a corrosive mess and reducing the acid level inside the battery.
Keep the top of the case clean and dry. Dampness, dirt, and acid on the battery case can create a circuit between the terminals that will drain the battery.
Keep terminals and cable clamps corrosion-free. Use a wire brush to remove corrosion. Coat both terminal and clamp with petroleum jelly (not grease) to prevent future corrosion.