Free...Right...and you get less than half the life cycles and have to fill 'em with water AND under heavy amp load they last about 1/2 the time to discharge as an AGM even though the 20 hr rating for amp hours is the same.
And here's what Interstate (your company) says about THEIR AGM's....
Lasts up to 2 times
Can sit unused 3 times
Over 15 times
more resistant to vibration.
and can mount in almost any position.
It provides constant performance quality
keeping your battery running at the same level even as it's being discharged.
So...your u2200 flooded may indeed be the MOST economical and best choice for you depending on how you use your boat...BUT AGM's ARE better batteries in several ways AND may be more economical if you use your boat heavily.
Here is a cost comparison vs. days aboard per year (assumes one cycle per day) using several popular brands:
Note that the ZERO line represents the base cost effectiveness of LIFELINE AGM's in the 8D format. Anything above the zero line represents a % increase in lifetime costs...below the line represents better cost effectiveness.
The background and inputs to this graph are QUITE complex and are available on the link I gave in post #9 above. The formula is driven by spreadsheet inputs on everything from battery cost to engine run time and alternator capacity AND you can download the speadsheet and put in YOUR OWN BOAT's parameters and any manufacturer's battery specs and costs to determine what is most cost effective FOR YOU.
Example...even if you live aboard full time...wet cells may be more cost effective than AGM's if you have a large bank and a small alternator.
We owe Mr. VonWentzel a debt of gratitude for his incredible work on this spreadsheet!