v70 Alternator problems

I have just bought a 1997 v70 with 227000km - in excellent condition. I left the parking lights on for 11/2 hours and the battery went flat. Mechanic checked battery and said it is nearly dead, but the alternator is only charging at 12.7volts, and needs something done about it. The previous (and only other) owner said he had had the alternator replaced twice. Should we be looking somewhere else for problems. My 240 did 320000km before the alternator needed replacing, not that I would swap this for a

Thanks Jeff