AGM batteries of the same physical size and terminal type/layout are a direct drop-in replacement for flooded call lead acid batteries. As Mike says, they ARE lead acid batteries, but I will add that they are far more difficult for a boater to diagnose early signs of failure. Like Mike, I can't see why they resist sulphation any better than flooded cell batteries, but they normally have a longer/larger cyclic life. This means that ignoring sulphation for a moment and as long as you have a satisfactory recharging regime they will last longer than typical flat plate flooded cell batteries. Now sulphation. This is caused by leaving the batteries in a partially charged state so solar helps a lot, especially on holiday boats. There is/was one vendor who stated their AGMs needed a higher charging voltage than many alternators of the time could produce and recommended the use of an alternator to battery charger, but they got very threatening when I tried to question them on this - chemistry suggests otherwise. I feel it was because their AGM batteries would not gas at a slightly higher charging voltage and thus help with effective charging to reduce sulphation. There have also been an AGM chemistry/design the needed a lower than normal charging voltage, but I have not seen anything about them for years. If you have effective battery monitoring equipment, know its limitations, how to set it up and use it, and ensure the batteries spend more time fully charged than discharged then AGMs should give you a slightly longer life, but that is more difficult on a live-aboard boat. Take care, do not be seduced by marketing claims, the battery market seems full of them.
