These were very nice units for their time. I used a Fortress LI-720
at home (on ~70 Ah of external batteries) to support computer and
telecom equipment for about five years, before converting most loads
to native DC, and it never gave me any trouble.

They do produce a true sinewave, like an APC Smart-UPS, and the
circuit topology is quite similar between both, with multi-tap
buck/boost transformers, etc.

The Fortress takes better care of its batteries than does an APC,
though, keeping them at a proper float voltage most of the time
(following bulk recharge), with occasional, higher-voltage
"equalizing" cycling to keep sulfation at bay.

Also, you can talk to them through a plain-ASCII command prompt on the
serial port, with almost every parameter, including charge voltages
being adjustable.

See

http://www.networkupstools.org/protocols/fortress/page6.html

http://www.networkupstools.org/protocols/fortress/page5.html

(and so on back to 'page1.html'). There were several varieties of
Fortress sold, and parameters can vary a bit.

Note that most of the settings marked Change Not Allowed *can* be
adjusted, but you need to supply the "Factory" password to do so.

This is 18473 for every Best Power UPS I've worked with (including
the Fortress, FerrUPS, and others). Enter

pw 18473

and its command prompt should change to "Fact =>".

Yup, that's a big drawback to nearly all UPSes of this type, with the
more sophisticated "line interactive" models (Fortress, SmartUPS)
being worse than the simple & cheap ones.

My 720VA Fortress also drew about 22W from the grid whenever it was
plugged in and "on", even with no load and fully charged batteries.