Most ups support the Carrier Detect (CD) = low battery, and Clear To Send (CTS) = on battery, status signals. These are the easiest to deal with and most portable, however some UPS, especially APC brand, use a newer serial communication method that does not use the CD/CTS lines.
If you have one of these UPS from APC you'll need to buy a special cable from APC called the Native Shutdown Cable, which does not come with your UPS.
The one that comes with will not work for software that wants status via CD/CTS lines.
On the other hand if it is a APC ups you can download and install their software which communicates with the ups serially so there's no problem.
Personally I hate the APC provided software and write my own UPS daemon software that uses CD/CTS statusing methods.
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