Are there any fuses on the board?
Most UPS have a fuse for AC input, another for the output and another
for battery, check all them.
The battery one might look like the fuses found in cars. Otherwise you
will have to troubleshoot and see where the current gets or doesn't
get. Keep an eye that if the battery fuse is gone there might be
something else wrong like shorted transistors, etc. I would try to
supply with a 12V current limited source after replacing the fuse.

ian field wrote:
When I looked on their website for the user manual the current offering of
that unit has a choice of serial or USB, mine only has serial so I'm
guessing it isn't all that new.

Just in case I plugged a domestic fan in as a load, it didn't even twitch,
the outputs are a row of flush IEC plugs - I thought it possible that these
might have embedded switches to disable it if nothings plugged in, but this
turned out not to be the case.

There's no LEDs lit on the front panel, no feed through power, no bleeps and
no sign of life from the inverter, although as I rested my hand on the
chassis I'm sure I could feel it humming.