Hey there! I'm an IT guy and found this forum through searching on replacing UPS batteries with deep cycle 12v marine batteries. The reason I was looking is I happen to have 4 spare Compaq rack mount UPSs with dead batteries taking up space in my basement. I was thinking of putting in a backup sump pump in case of a power outage, and then thought "wait a minute, a UPS will back up the existing electric sump pump". While I know regular UPS batteries are really just meant for systems to have a few minutes for a graceful shutdown, I want to set this up so it will be able to run a sump pump on and off for several hours after a power outage. The average time to drain the sump is about 30 seconds. It's a 6.5amp 115v pump. The batteries that the UPS is supposed to use are these. http://www.apc.com/shop/us/en/products/APC-Replacement-Battery-Cartridge-23/P-RBC23 How many 12v Deep Cycle marine batteries would I have to use to guarantee 48 hours of service assuming the pump runs once every 30 minutes for 30 seconds during that time? From what I just read, if anyone can figure this out you folks can. I don't just want the answer, so if you can provide the math you use to get it I'd LOVE that for future projects. Thanks in advance!