Hello mate,
check out the belt first, look for any nasty chunks that could be taken out of it,
one of my mates had a stone take a big bite out of his v belt and it slipped when the missing bit got to a pulley.
Also check out the tension on the belt.
If you dont notice any problems with the belt then I'd take it to an Auto electrician who can test the current that the alternator is giving out.
Good luck,
Cheers.................