They cost twice as much, but are designed to last 3 times as long.

 

So I've read... from the marketing department's copy. Exactly what "last" means I suppose we'll never know.

The fact is they will not last anywhere near as long if you are cycling them. That's why almost all the AGM's sold are "dual purpose" type, a compromise between starting and deep-cycle.

But an Optima yellow-top in group size 27 for example has a reserve capacity (@25A rate) of 140 minutes or 66Ah at the 20 hour rate, whereas a deep-cycle battery (plain old flooded) in this size has 200 minutes RC, and 115Ah (C/20). It's true that it will not give you the high CCA figures, but exactly how many amps does a Rover V8 starter require? 800? 850?

 

I had a Mazda Miata that had the factory AGM battery in it ten years later. That's 3 times the 3 years a regular "flooded" battery seems to last where I live.

 

It's not as simple as AGM = lasts longer, flooded = lasts shorter.

Most flooded automotive batteries are starting batteries. Most AGM batteries are dual-purpose.

If you cycle your battery, then:

Dual-purpose battery = lasts longer
Starting battery = lasts shorter

If you see it this way, then you realize it's not AGM or any kind of VRLA flavor that makes the battery last longer. Your experience with flooded batteries isn't a valid comparison because it was with starting batteries, not deep-cycle. If you cycled them, then it's no wonder they only lasted 3 years. But an AGM dual-purpose will not last as long in cycle-life (measured in cycles not years) as a flooded deep-cycle.

 

Thanks for the technical explanation. Very helpful !