The specs for this discontinued product state that the five outlets across
the top are backed up by the battery.

These outlets will run until the battery voltage drops to where it can no
longer power the outlets. At that point, those five outlets will all shut
off. KA-CHOOM!

IF the device has a "master" outlet (I don't see it in the specs or the
photos), the "controlled" outlets turn on and off with the master, either
for convenience, or to save energy. (My audio system is wired this way --
the electronic crossovers turn the power amps on and off.) Once the battery
poops out, /everything/ powered by the battery shuts off. There is no
graceful shutdown.

You're confusing a convenience feature with operation of the device as a
source of backup power. The two (apparently) have absolutely nothing to do
with each other.

http://www.apc.com/resource/include/...0bb&tab=models