AGM batteries are a question mark, a choice of facility over money-saving.

They require zero maintenance, this is great I must say, and allow one to bury them somewhere not really accessible (my case).

The fact that they do not self discharge is fantastic for a WE or casual cruiser, but almost meaningless for a full time one since self discharge is not an issue: we use them every day.

The drawback of AGM is their number of charge / discharge cycles, meaning their lifespan : about 1,000 cycles. Consider a cycle is day of use, and charge. For a WE cruiser, even using his boat 2 days each WE, that's many years, assuming dock charging during the week. Full time cruising it's about 3 years, assuming that we can do daily full charge cycles, something not really happening : some days no sun, no wind, no wind no sun, and running the engines is always something un-pleasant.

Basically, expect 2 years of nicely working AGM batteries, and the last year of so so service. The will load normally, reach their max power but won't hold their charge much.

As you can read everything and the contrary on how to maintain them - you must equalize / you must never equalize is equally shared - , as they start to never hold their charge, and looking at the price sticker of new ones, you will likely have nothing to lose by doing equalization cycles.

And it works. One equalization cycle will get back some normal behavior for a few months. From my experience, they will take about 4 to 5 equalization cycles, then that will be it, no more gain to get, and it's time to replace them ( I tried so far 3 brands, from premium to first price : same same same…)

For info, I equalize them for 4 hours @ 15.6 V, like lead batteries. I first bring them to full charge using the engines alternators, then start the equalization cycle by programming the solar panel controllers. The wind generator is left free-running up to 15,5V, then shut down, letting the solar panels to finish the job. Better start early on the engine with a sunny forecast and nice breeze…

AGM is one way to go, but… not the only one. If the maintenance is not an issue, because your batteries are accessible and you are an organized and responsible person, then lead batteries will cost less to buy, and last many years longer. They are built to take equalization cycles (these are part of their normal maintenance) and are widely used in most of the demanding applications beside boating. If you go lead batteries, forget the marine suppliers. Go for either golf cart ones (6V, so you need two to make a 12V bank) very economical and rather sturdy. Or forklift ones, even better if the form factor isn't an issue (these are bulky, but have the best ratio power/weight).

Gel Cells batteries are a middle ground between lead and AGM. No maintenance only means no fluid to add, they are sealed. They have more live cycles than AGM but are very sensitive to over-charging (you could kill them rather fast this way), and to partial cycles (meaning not charging them to 100% before starting to really draw on them). They will need flawless charge control, and flawless full charge cycles DAILY. Every cruising boat I know happy with them have a generators, and use it daily for various reasons ( air-con, electric oven, etc…).

New Lithium batteries are now coming to the market. Their pricing could be dissuasive, and the technology is still very new for power hungry applications (as Boeing Dreamliner stories show). There is no doubt that the future is in this direction, but I'd rather wait for the electric car market to develop, break down the technology hurdles and drive the price lower before getting them aboard…