I have a 650VA APC UPS (Back-UPS Pro 650)
that has worn batteries (occasionally, it starts to
beep and the red battery light comes ON) .
The UPS is there to ride through brownouts and
outages lasting usually less than 10 seconds (have
quite a few of those) so if the batteries are less
than perfect, I don't care as long as it can last for
10 - 20 seconds.

So, how does the UPS's circuitry determines if
the battery is going bad? Is it simply a voltage
measurement, or something more sophisticated?
How difficult would be to modify the UPS to
lower the treshold for a bad battery?

Alternatively, how would the unit work if I were
to connect a marine or car battery instead of
the manufacturer supplied battery?

TIA