http://www.pprune.org/engineers-tech...-a320-apu.html 
to summarize the old thread, maybe it will work, but probably it won't, and it shouldn't work. If you look at the status page with BAT OFF it says ""APU BATT START NOT AVAIL". But yes, it will have DC power via GPU, and it will spool up, but the ECB is supposed to initiate an auto shutdown, anecdotically this will happen at about 20%, ie before the APU is self-sustaining (if the BAT OFF condition is met); shutdown report in status page "no accel acft batteries not selected".
...and for the following reasons you shouldn't try this:
- IF THERE IS AN APU FIRE, THE APU FIRE-BOTTLES WILL NOT OPERATE
- THERE WILL BE NO DE-OILING OF THE APU DURING SHUT-DOWN. THIS WILL CAUSE
SMOKE IN THE CABIN
- THERE WILL BE NO COOL-DOWN SEQUENCE OF THE APU
- THE IGV WILL NOT CLOSE FULLY. THIS CAN CAUSE PROBLEMS WHEN YOU START THE
APU AGAIN.
(source: AMM)