[rotorwrench]

An AGM or absorbed glass mat cell type battery is a bit different than the full wet cell type but not by a lot. The electrolyte is absorbed into the fiberglass mat between the plates. The mat will hold electrolyte against the plates and resolves some of the evaporation problems that happen with a wet cell. This is why they can go with no water replacement that is required for wet cells and it makes them service free.

The plates can still build sulfates so it's important to keep the battery working as much as possible to prevent that. Many folks use the battery tenders that cycle usage when the car is setting a lot. The round cell designs like the Optima has more room for plates the way it is rolled up but this battery is more expensive due to the expense of manufacture and the extra cell material that goes in there (near pure lead).