Beep/click/ computer shuts off

[Anonymous user]

I have an ES-550 on my home computer. Only it and the monitor are plugged into the unit. All indicators are good, Powerchute says battery is at 65% or more usually and no problems.
Recently, when my printer, which is not plugged into the UPS wakes up to come on, the UPS beeps & loudly clicks and the computer immediately shuts off (no power down, just off). This is new, have had this setup for a couple of years without problem. What's causing this? I've tried changing the sensitivity and voltages without resolution. I don't want to waste money on a new battery or unit if it's not the cause. Thank you.

UPDATE: I just realized - I tested the ES-550 by just unplugging it and noted that the it beeps and the computer immediately shuts down. It is not powering the computer. The battery light is NOT red. No bad indicators in Powerchute. Is this battery or a bad UPS?