Why does the UPS still stay in battery model and keep emitting a beep sound when the utility power is in place?

This situation might happen in several situation:

A. The user forgot to connect the Power plug to the AC mains or there is a improper connection between the UPS and the AC mains so that the UPS can not sense the normal input power and stay in the battery back up mode. Solution in this situation is easy, check from the AC outlet to the Power plug connection to the UPS body, and make sure every connection is firm and secure.

B. The input voltage range goes below or higher than the UPS input range, for example, UPS-650D has a input range 165V-170Vac, if the input voltage is lower than 165V or higher than 170V, then even if the utility power is in place, the UPS shall also work at battery back up mode, because the input power condition is dangerous to the appliances. In this case, we can’t do anything but to wait until the utility power resumes back to the UPS regulation stage.

C. The UPS has a broken fuse, what you have to do is to replace the fuse with a same rated one.