If you know someone in the PC repair industry, see if you can get them to save old UPS batteries. These are usually replaced every year or two and are still good for what you want to do. That way its no great loss if you kill them with over or under charging. I had a 17AH gell cell from a large UPS in my ride on mower for a couple of years before it gave up. The battery was free, so every time it started the mower was a bonus.

Glenn