The life of any battery is a function of its duty cycle, the quality of its construction, and the maintenance it received. Some flooded cells like the CS series from Rolls/Surrette will last longer than AGMs under ideal conditions.
However, the many benefits of AGMs (little self-discharge, high efficiency, no spill, low chance of fumes, little maintenance, etc.) have made me convert over.
Particularly if you use your main engine to recharge your banks and bulk charge each day, will the higher charge acceptance and efficiency of AGMs end up making them a cheaper option than flooded cells. More on that on my web-site: http://www.vonwentzel.net
I go through the benefits of AGMs and also have a bulk-charging battery model available that you can customize to compare batteries based on your specifications.
Do make sure before you abandon the wet cell batteries that your charger is compatible with the AGM batteries. AGM batteries have a recommended charging voltage and procedure. If you deviate from this recommendation you''ll get no better life or performance from the AGM batteries than you would from the "wet ones"! You might want to add another battery to your bank as the AGM batteries have lower AMP hour capacities as compared to wet cells. Check this out very carefully.
The amp hour capacity of AGMs per unit volume is usually the same if not higher than premium flooded cells. Only the cheapest of flooded cells will have higher Ah capacities (due to very thin Pb plates) but will have correspondingly shorter lifespans.
AGMs are a premium energy storage system. As such, they deserve a charge system that is worthy of them. I''d make sure the system on board is capable of dealing with AGMs and that the charging voltages are set correctly. These voltages do not differ very much from flooded cells (unlike Gel) and thus even a "flooded" setting on a regulator is unlikely to damage an AGM.
The primary difference is the bulk stage where AGMs can be recharged up to 18x faster than flooded cells. Not too many people have that kind of charging capability on board... Anyway, see my website mentioned above for sizing and other considerations.
I added a bank of 2 series 31 AGM batteries as a house bank with a flooded cell series 27 for the engine starter. I used a combiner with a high voltage sensor to charge the banks in unision unless the flooded cell draws at a higher rate, ( above 14.2 volts ) then the sensor disconnects the AGMs until the flooded cell charge rate drops and the combiner ties them together again. I used a Yandina combiner.
Definitely have to check your charger, as AGM batteries like to be charged at a lower peak voltage than wet cell batteries. The are more susceptible to serious damage from overcharging or charging at too high a voltage.
AGM batteries are known in the industry as VRLA (valve regulated lead acid). These batteries have a stringent charge requirement to prevent a condition known as thermal runaway at high temperatures and maintain full charge at low temperatures. Without the proper charger these batteries have been known to explode at high temperatures (such as a boat in the sun w/o ventilation). Admittedly, these explosions have happened as far as I know only in industrial settings. I have never heard of one happening on a boat, but there is always a first time. I have yet to see a marine charger with temperature compensation (lowering the float or charging voltage at high temperature and raising it at low temperature). They may be out there, if so, those of you who have sealed batteries should invest in one.
As an electrical engineer who has specialized in DC power, i.e. battery charging, I have flooded (or wet) cells on my boat. They can take much more abuse; can tolerate a deeper discharge, more discharge cycles, higher & lower temperatures then sealed batteries. Wet cells vent the hydrogen that sealed batteries store, under pressure, within the cells. That does not mean that sealed batteries don’t vent, that’s what the valve is for, to vent pressure, i.e. hydrogen that builds up during use/abuse. A good wet cell should outlast a AGM cell considerably. Just make sure the water in the cells is always topped off.
Always vent the battery compartment, weather you are using wet or VRLA batteries, to the outside to prevent hydrogen gas build-up. Hydrogen is very volatile (explosive) but fortunately is lighter then air.
Wet cells on a boat are a pain to maintain and most (not the real expensive ones) wear out relatively quickly in the heavy duty cycling of live aboard use. This is anecdotal based on my own experience and that of many fellow cruisers I've chatted with.Whether this is due to poor maintenance or deep discharging or simply design factors is not something I know enough about to comment on...but I can say that despite my attempts at good maintenance and proper cycling, I have been disappointed in wet cells (East Penn Mfg. & West Marine 8D's).
On the other hand, I am well pleased with the trouble free nature of my present AGM bank which also seems much more robust after over a year in everyday use. No real worries about charging with these compared to wet cells...it is the Gel Cells that require less than 14.1V bulk charging.