Re: Lifeline Batteries
Clarity 36 Bill,
Little doubt about it, flooded golf cart batteries are the most economical and -- given the treatment most receive on cruising boats -- are likely to last longer (more cycles) than AGMs.
As mitiempo said, if you plan to go ahead with AGMs be very certain to examine your boat's ability to get them up to near 100% full charge at least once every 2-3 weeks. Otherwise, they will sulfate, lose capacity, and have a shortened useful lifetime.
AGMs have other advantages, of course. They can take an incredible amount of charging (more than 115% of their AH rating when discharged to 50% SOC)....for a few minutes at least. They continue to accept high amperage for 30 minutes to over two hours, depending on the size of your alternator and/or battery charger.
This means that IF you have sufficient charging capacity aboard, you can put more AH back into the AGM batteries faster than you can with flooded or gelled batteries. This will reduce charging costs, especially if you're using your engine/alternator.
AGMs may be a good choice for some discriminating and battery-savvy sailors aboard well-equipped boats, but for many sailors flooded batteries may be a better choice.
BTW, Calder's note re: same size batteries applied to batteries in the same bank only, i.e., the house battery bank in your case. No problem installing a separate smaller starting battery. I'd use an EchoCharge to maintain the start battery, thus avoiding any need to worry about switching.