A320 APU Gen loses power with altitude
 

"A third AC generator driven directly by the APU is rated at the same power as each engine driven generator and can replace either or both engine generators at any time. However, the maximum power output of the generator decreases with altitude. "

Why?

Its only a small gas turbine and the reduced air density to maintain a load.

From the FCOM

https://cimg6.ibsrv.net/gimg/pprune....9a7edfda17.png

So it is not a generator caused problem but a power unit problem.

Not a problem, just physics...gas turbine needs air. The gas turbine powers the generator in the APU.

Also, am I reading the graph correctly.. You can't restart the APU from the battery, above FL 250.. ?

No you are not reading correctly. This limit is only for APU start in ELECTRICAL EMERGENCY configuration.

While this is not an APU turbine, this is an example of the turbine power curve with altitude.

https://cimg2.ibsrv.net/gimg/pprune....eb6ae3242c.jpg

APU Battery Start
 

Perhaps you misunderstood his question. My Operator lists "Max FL for APU Battery Start" as FL250. -Both CEO & NEO. -note that's a battery start; which is what he was asking I believe. APU Max start envelope is upto the Service Ceiling, though it's no good for Bleed Air until FL225...

The engine driven generators have an integrated constant speed drive to keep them running at the same speed while the engine speed varies, as the APU is either ON or OFF its generator doesn't need this feature. Dispatch with an engine generator U/S is allowable with the APU running for the entire flight. ETOPS requires a diversion in the event of an IDG failing and being unable to get the APU generator online.

The A320 is much more dependent on electrics and hydraulics then previous generations of conventional aircraft and a higher level of redundancy is called for, two generators offline and I would be looking at suitable enroute alternates rather than continuing a long flight. The emergency electrical configuration is a last resort serious emergency requiring a MAYDAY call, not to be thought of as an additional back up.

Does the APU ever start without battery?

I'm flabbergasted at the question on an APU battery start, the mention of battery starts itself means no other generator involved, the start is off the ships battery alone and each attempted start is going to drain the battery.

This is pretty basic stuff and any pilot who has been operating a descent turbo prop or jet should know this.

I'm flabbergasted at the question on an APU battery start, the mention of battery starts itself means no other generator involved, the start is off the ships battery alone and each attempted start is going to drain the battery.

This is pretty basic stuff and any pilot who has been operating a descent turbo prop or jet should know this.

Is the question being asked by a pilot or just an interested party?

Yep.
So if the APU runs with constant RPM how does the geny lose power with altitude?

If the gen is 'on load' then it needs a finite amount of power to drive it at a constant rpm, if you increase altitude you will get to the point where the required demand load on the gen is too much for the reducing available power from the gas turbine. The APU will begin to slow down and once the frequency drops off below its limit, EG 398Hz, then the APU gen breaker will trip leaving you on battery power only until the RAT comes on line.