Also look on the back of the unit. There might be a momentary push button that is used to change the input acceptance window. If you AC input is marginal then it might be faulting. I have seen APC's cycle on and off line when input voltage is too low/high.
I recently purchased a used APC Smart-UPS 1000 (Model SU1000BX120)
> through ebay. This item was listed as being in "good condition", with
> no batteries included.
>
> Searching APC's web site and other sources, I determined this needed
> an RBC6 battery pack or equivalent. I subsequently purchased two 12
> volt 12 amp hour batteries.
>
> The UPS arrived, the batteries arrived, and APC's documentation is not
> very clear on battery installation. It would appear that the two
> batteries are to be connected in series, thereby providing 24 volts of
> output.
>
> I installed the batteries, plugged in a small load (a 100W lamp),
> plugged in and turned on the UPS. The load gauge showed one LED, the
> battery charge gauge showed one LED, the on-line LED blinked for about
> 5 seconds before going off, and the on-battery LED stayed lit. The
> UPS beeped four times every 30 seconds as it is supposed to when on
> battery power. The lamp did come on. The UPS would not stay powered
> by the utility.
>
> I've read some things that indicate a UPS will do funny things until
> the batteries are charged. So I left the UPS turned off but plugged
> in overnight, as the manual indicates that this will charge the
> battery.
>
> Since the following morning, here is what happens, either with or
> without a load plugged in. Upon pressing the On button, all the LEDs
> flash once, there is a beep, and somewhere inside a switch is thrown
> (I can hear it click). Subsequent presses of the ON button do
> nothing. Pressing and holding the ON button gets one beep after one
> second, and that can be repeated. Pressing and holding the OFF button
> for one second resets the internal switch (I can hear it click), at
> which point I can repeat the action in this paragraph. The lamp
> (load) never even blinks.
>
> On the chance I screwed up the battery connection, I removed one
> battery and tried it again. The only difference is that after the
> LEDs flash and the beep, there is apparently not enough juice to get
> the internal switch to "stick". Repeated pressing of the ON button
> causes the same LED flash and beep.
>
> Any ideas on where to go from here to make this thing work, or have I
> done something to screw it up? Any thoughts/help is appreciated.
> Thanks!
>