" I was an electrician for eight years, with three years schooling before that."
Sorry to say that does not really mean much sometimes. I don't mean you though.
I got about a 58 year old life time Electrician with lots of schooling always trying to tell me how I am wasting my time building wind gennies because they simply don't work
HE beleaves that for real!
He may know how to wire up a factory and handle 480V, but he don't know squat about about inverters, magnets, magnet wire, etc...
Anyway, have you looked for a manual for the UPS and see if anything is there about hard starts or cold starts, whatever they called it. Most of the ones I been playing with DO startup off batteries. About 650watts-1000watts for the small home type UPSs.
May need to hold the ON button for a few seconds. May need to hold it till the UPS beeps and let go DURRING the beep. May be the TEST button to use instead of the ON button. Some are trickey to start on batteries, but so far all of mine have.
The 2200 rack mount ups, 1750watts sinewave, I have to hold the Test button till it beeps and let go durring the beep. Let go before the beep or after the beep it does not turn on. Have to let go DURRING the beep! I think I tried to startup about 5 times on batteries before I got it just right the first time. I was staring to think oh crap it ain't gonna do it, but once I got it the first time I now get it everytime easy. Just had to figure the system. I think I only got it right after downloading the manaul for that one and several tries.
A side note on Sinewave UPS started on batteries. I noticed with at least one UPS about 1000watts that the window fan I was testing with ran slower on the UPS when started on the batteries, I pluged UPS into the grid and the fan speeded up to full speed, unplug the grid and fan stayed running at full speed.
I don't know why, but it SEEMS like the fan was running slower like on a modwave inverter to begin with. When I connected the grid and it ran full speed and continued to run full speed after unplugging the grid it made me wonder if there is something there that senses the grid to kick it into proper sinewave operation?
I was testing about 5 UPSs that I had bought as junk for parts cheap (4 worked fine) and only had one set of batteries. I should have went back and further tested that one like with a digital clock and seen if it kept time, but I didn't do that yet.