When I talked to Rob today, I asked what was wrong with the alternator that failed. He said that nothing was actually found wrong with it, but it appears there is some intermittent fault or adjustment needed in the control unit. During the flight today, the second alternator dropped off line again, about 15 minutes out from Christmas Is., but if it was cycled and the main alternator switched off, the second alternator did come back on line and sustain the full load.
This will be investigated further after it gets to Australia.