I did some pretty extensive research about a year ago on this and my findings were that if money is no object, Lithium Ion will be the battery of choice. However, the primary use is on performance race boats where weight is a factor. AGM is great battery for a little longer life expectancy and less maintenance. However, I believe that where the decision needs to be made is how the boat is maintained and used. If the intent was to keep it regularly on a quality trickle charger ALL THE TIME, a flooded lead acid battery will do great and could give you 3-4 years. They are easy on the wallet. However, the problem lies when a flooded lead acid battery sits for long periods of time without maintenance (charging). It is at this point that an AGM or Lithium will really leap ahead of a flooded lead acid battery. I see it similar to a car battery that is typically a flooded acid battery which is constantly kept topped off (commuter/daily driver car). Ever let a car sit for months at a time? The battery will typically deteriorate rather quickly. However, in the marine world we are really talking about two types of batteries....a starting battery (which needs constant charging to keep it topped off) and a marine deep cycle ("house") which can go to full depth of discharge (drain) much more regularly without damaging the battery. Speaking of depth of discharge, you will see 50% "usable battery" with a flooded lead acid, 80% with AGM about approximately 80-95% with a lithium Ion. On a side note, I have changed to AGM batteries every time in my vehicles (for the past 15yrs) and find that I sell the vehicle before the AGM battery dies on me.
