I had a UPS lying around and decided to use it. I plugged it in to my
computer and two monitors.
What specific monitors?
Generally speaking, you should not plug two monitors into a
300VA UPS, that is too high a load and may exceed the
current potential. That is CRTs though, you might be able
to manage it for awhile with LCDs, but even then, it would
be best to use a far larger UPS or only one monitor if the
UPS is working right (and/or, after you rectify the problem
and it is then working right).
I switched my computer on, and then after
some time (so it could charge)
We may need more specifics, not generalizations... like,
what was "Some time" and how long had the UPS been lying
around and how old is the battery? While lying around, was
it plugged in or did the battery drain for that period?
i turned the mains off and the computer
shut down immdeiatly (isn'nt the power supply meant to last 20 minutes
or something.
.... or something
Generally you do not want to make it last till the battery
is drained, that is what is hardest on batteries. The best
use of an UPS is to allow a graceful, but almost immediate,
shutdown. You didn't imply otherwise though and I realize
what you mean that it should have still ran longer, so it
would tend to be one of the aforementioned factors that you
either have a battery going bad or the total current was too
high and you could retry with fewer parts connected.
If it won't run for at least 5 minutes with the system and
one LCD, or a budget grade system (not a well endowed
workstation, gaming or server system) plus a moderate sized
CRT, I'd replace the battery (though now, you can measure
the battery voltage with a multimeter and observe any
indicator LEDs on the front of the UPS). If your powered
device needs exceed what I just outlined, you might be
better off putting the money into a larger UPS instead of a
new battery for that one- or get the battery anyway and you
can always use it for some other powered equipment or only
some of the stuff you have currently attached.