[2001 ZTS]

I've only had experience with one alternator failure. It wasn't a gradual thing instead it just went tits up all of a sudden. Based on that I'll guess your battery is going.

EDIT: It wasn't on the Focus that the alternator failed by the way, it was a ten year old car with 250,000km. That in it itself is another reason not to suspect the alternator, it is not the sort of thing that "should" fail on such "young" cars like your Focus.