Brett, With setting set to high beformance, will porably still get error. A desktop with an UPS attacted will still detect a "low Battery condition" - But as you indicated a very simple check.
Unksol. Concur as most probable cause. But add Check battery in UPS. If it Is running low, that is when UPS does its Self check/test and finds that the battery is "dieing" it is probably telling the software that it can not support a loss of power. This would be the same as a Laptop, using the brick with a Nearly dead laptop battery. Power outage = Instant power off. Which would be BAD.
To check the UPS run time, If you have a lamp with a 100 Watt bulb, see what the run time is when you pull the ups power cord. Any high load will do (100 W to 500 Watt), but Don't recommend tring using the Computer up and running.