> I thought about adding
> more capacity to this UPS I have here,  but asking them at the
> factory about it they nixed the idea,
> told me it'd burn up the inverter.  They said "some" of
> their models would deal with more battery,  but it'd cost
> me maybe $1000 to find out.
You can connect a big battery to a little UPS and run it for many hours,
but you have to limit your power draw to 1/4 to 1/3 of the rated power.
The UPS can supply full power for 10 to 20 minutes, or 1/4 power almost
indefinitely.  During a power failure, you can also connect a 12V
inverter to your battery if you need more power than 1/4 UPS power.
When the power comes back on you might want to disconnect your big
battery and charge it with a 2 to 8 amp charger.  The UPS will
eventually charge a big battery, but it might take a week or so.
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