Just wanted to offer; when the batteries in a couple of my APC UPS's went out, I just happened to have a couple of very large deep cycle RV batteries. They have been working fine as substitutes for well over a year. And my power-out run time is measured in hours, not minutes. ;-)
If I had a very lengthy power out and wasn't around to shutdown the computers; I would probably recharge then with my auto battery charger, rather than ask the APC units to do it. Regards, Al