APC ES 350 – continuous alarm

With the APC ES 350 (there's no SE model) there's two audible alarms:

A continuous low beep: The UPS is now operating in battery mode. This happens if you experience a power outage and the UPS switches to the internal battery. Depending if you are operating at 100 or 200 Watts, you have between 1 and 6 minutes of battery time.

A continuous sound: (and not repetitive beeps) informs you of a power overload. You got a power spike and the UPS switched on to battery mode. The UPS will not resume from this mode until you turn it off and turn it back on. If the spike persists it will again repeat the tone.

The fact your computer turned off immediately means your battery was discharged when this happened. This model takes around 16 hours to recharge its internal battery. If it is continuously connected to a power outlet even when the computer is turned off, this may mean your UPS internal battery is not in good condition.

To test your battery condition:

• Boot your Windows 7 up to the logon screen but don't log on to avoid file corruption.
• Remove the UPS power cable from the wall socket.
• The UPS should start a low beeping sound and your computer should stay on for 1 minute (if your UPS is operating at 100 watts), or 6 minutes (if at 200 watts).

For more information: APC UPS ES 350 Details
Also check your UPS manual for even more detailed information.

I had a MOSFET failure on my APC RS-1000 with the same observations as yours:
Overload light and constant alarm even when nothing was wired on the outputs.

They changed the board (rather the entire unit) and retained the batteries which were still functional.
^{Uh, I had some hard time working this out with the APC Service though. That was not exactly stellar.}