If the particular model uses a 12V battery, it's a simple matter to use a car battery. However, you must lengthen the power cables. I have modified several of my APC's in such a manner.
Replacing the regular batteries inside your UPS with external deep cycle batteries can increase your run time tremendously. Depending on the UPS model you can have a decent run time to shut down your equipment. However, if you're looking for a ridiculously long run time you'll need to replace the battery with an external.
I bought a
Smart UPS 1500 without the battery and installed 2x 12v 75ah marine deep cycle batteries. My system runs for days now. It's quite easy to do and if you do a bit of research locally you can actually pick up the UPSs pretty cheap. A lot of times people have them with bad batteries and they're trying to get rid of them. Even a UPS like the
Smart UPS 1400 would work great. The bigger the UPS the more load you can place on it. Both the 1500 and the 1400 have 950W, so most applications are pretty good with that.
If you're thinking of using car batteries make sure you don't store them in doors. There is a whole bunch of safety stuff with those batteries as they're pretty dangerous.
Hope that helps
Joe
