Car batteries these days are sealed, so you don't really have much opportunity to replace their acid. A regularly-driven car isn't deep-cycling its battery, so I'm not sure what else you'd do to maintain it.
You test it regularly and hook it up to a good desulfator/maintainer when needed. A sealed battery might be acceptable for a car, if the owner has the money to replace it regularly, but not in an off-grid/residential situation.
