bestups was designed to monitor Best Power UPS hardware like the Fortress, Fortress Telecom, and Patriot Pro. It also recognizes and supports SOLA units such as the 520 and 620.
Other UPS hardware using the Phoenixtec protocol should also work, but they will generate a warning since their battery information is not known.
The battery percentage is derived from the voltage data that the UPS returns, since the UPS doesn't return that value directly. On some hardware, the charge will remain at 100% for a long time and then drops quickly shortly before the battery runs out. You can confirm from the BATTVOLT readings that this is a problem with the UPS and not this driver.
Similarly, the float from the charger in some models forces the battery charge percentage back up to 100% immedately after the UPS goes back on-line, so you can't tell when it is really recharged.
The core driver:
The NUT (Network UPS Tools) home page: http://www.exploits.org/nut/