Is it off warranty already?
Last time I had one go out, the battery was weak when I started it. Then as I drove along the gauge started to drop, normally at about 13V it slowly went down and at 8V I stared to look for a place to park it. As I parked the engine slowed and died. I ended up installing a new alternator in a parking lot, luckily a parts house was within walking distance. Then I had to get a jump to start it, since it had been running on the battery and drained it.
Most parts stores can test the alternator for you (free) to see if it's putting out enough voltage, under load and not.