Have a SD2400. One of my two batteries is dead. I can start engine with battery #1. Question is - If I switch to battery #2 after starting will it re-charge via the alternator while the boat is running (like a car does) or do I have to replace the battery or see if it will hold a charge by using a battery charger? Also, exactly how do the 2 batteries get charged via the #1, #2, BOTH or OFF switch? If I have it set to #1 are both batteries charging?