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.
So considering the possibility that it is there to help the main batts to be charged for take off then how does the aircraft prioritise drawing current for APU start from the mains versus APU batteries? They are all switched on for starts. I've not read anything about this in FCOM1 or elsewhere.
(thanks to all, keep it coming)