Shoot me down but I don't remember APU being required for 330 ETOPS. Might have changed but...
From memory, APU is not required but the a/c can be dispatched with an engine generator inop providing the APU is servicable.
I think.
Maybe this is also one of the reasons for a seperate APU battery.
I had a look at the 330 MEL and you can't dispatch with a main Batt inop.
I guess you might be able to swap with the APU Batt if they are the same but I don't know if they are and I doubt Airbus would have installed an APU batt just for spare parts.
Batteries are the same. As are the battery chargers which tend to go tits up more often in my experience.
You are quite right, airliners do not lug batteries around as spare parts, but when you consider all the other advantages, increased redundancy and ability to dispatch away from main base must add to the pros and overcome some of the cons. After all, many airlines fly spare main and nose wheels along with a parts/tools kit in the bulk hold just in case.
