A key to battery life is your charging system. It is best to install a three stage charger in the boat. It consists of a high rate charger, a finishing rate to top it off and a maintenance rate of pulses every few seconds that will not boil the water out. My boat is hooked up all year when it is parked, even winter and the batteries are ready to go after seven years now. There are chargers at your marine store that will maintain three batteries at once.