[mike31154]

What CM125 said. When converting UPS to use larger external batteries, you need to be aware of the charging characteristics & capacity of the UPS circuitry. The switching circuit/relay & fusing of the whole shebang also needs to be looked at & modified. If you're not familiar with how that functions, you could be setting yourself up for a disaster. Personally I'm not a fan of trying to do this with something that's designed for a smaller capacity battery. My Dad had a small UPS for his computer years ago. When it stopped working I disassembled it & found the battery bulging at the sides. Could have blown & caused a fire or corrosive damage. I don't think UPS units available for home computer back up are particularly well built. Be careful.