Turn a small home UPS into a giant UPS (video)

Having a UPS is fairly common these days. However, one gripe I’ve always had and that you might too is the limited capacity you get on them. Even nicer $200+ ones might only give you a few minutes on a powerful PC, and for large capacity you could easily spend $1000 or more. Neither option was good for me. So I decided to make a better one myself.
To start with, I purchased 3 “Ultra” brand UPSs about a year ago. They are completely silent, small, and functional. Cheap, yes, but functional. However, with my machine on one of them, it only lasts around 3 minutes before powering off. This might be enough to shut it down, but if the power is out for only 15 minutes I’d rather just ride it through. Inside the UPS were 2 small sealed lead acid batteries, like you find in most UPS units, 12V each, in series. They are the same type of battery you find in cars, trucks and boats – just smaller. Using that logic, I took some common hardware and rebuilt this UPS. I did a small bit of research to determine the proper wire size given the load. The UPS used is an Ultra 1000VA. (warning, a small bit of profanity is in the video)…

The tools involved were simple. I had 20 feet of 10 Gauge wire, two ring terminals and several Male/Female disconnects. I needed wire strippers and wire crimpers for that. The UPS itself only required a screwdriver to take apart. I used a nice Dremel to bore a hole in the plastic, though realistically you could do that with a knife. For the batteries, I purchased battery boxes. It was a simple matter to remove the stock batteries, run and terminate the wire, then put the new batteries in place.
There are downsides to doing it with these batteries. Space, of course, and safety. These are standard

lead-acid deep cycle batteries, meaning that they can and do release gas when discharging. For that reason, I have these batteries situated outside. To do this safely indoors, you need a well-ventilated room OR you need to use sealed batteries.
I am going to do this with the other two UPS units, too. Next time, however, several things will change. I am going to use sealed batteries, slightly more expensive but completely safe to use indoors. I will also use shorter cable lengths. I will remove the buzzer inside that makes that awful beep, and I will install a slow 80MM or perhaps 120MM fan inside, quiet but enough to bring some air over the unit in case sustained operation heats it up too much.
All in all, I spent about $300, including the tools, to make a UPS with an ~80AH capacity.