I can believe that lead-acid batteries in a thermally-regulated and immobile location would have a longevity advantage. But what magic dance are you doing to get car batteries to last longer than 5 years? I - and my parents, and seemingly everyone else I know - absolutely do seem to have to replace them at just about exactly that interval.

That's not to say they aren't filling their role in a totally reasonable way, I'm just really curious how you're getting more life out of them than the rest of us.