I have had a problem with this with a number of batteries and asked a question about it a few years ago. It was a pretty good discussion. https://www.instructables.com/answers/Can-these-bat... I bought one of those desulfater units. It didn't really work. One of my battery chargers even has a special cycle for desulfating. I use it be truthfully don't know if it does anything. The batteries in battery backup units will get sulfated if they are not charged and left for a few months. I had a customer who had a brand new one, never opened, and he had it for a couple of years on a shelf. When he finally got around to plugging it in the battery was bad because it had sulfated. If you keep a battery charged it will not form much in the way of sulfate. If you leave it sit with only a partial charge or no charge it will sulfate to the point of being no good. I have not had very good results in trying to bring them back. I have been able to add water to "sealed" batteries like those in the APC units and get them working again but other than that not much success.