<quote>I have an older APC 800RT backup system for my computers, and I've modified it so I can use a couple of older car batteries for the backup power source. Those terminals are what I used to connect them up.</quote>
Might want to do a test and see if it overheats when run for a while. A lot of UPSes are designed for 20 minutes of run time, and then they get too hot. Particularly the inexpensive ones you see at the bigbox stores, because they're very price sensitive, so they don't spend any more on heatsinks and/or fans and/or overtemperature shutdown circuit parts than they have to.
Already taken care of. I've installed two fans in series that will come on on power switchover to keep the circuitry cool. The 800RT is a robust backup--you can even start up your system on battery from it. Not that I make a habit of doing that--I was just curious about it and tried it one day.