My first thought was also alternator. I had the same thing happen to me a couple years ago. The worst part was that I bought TWO bad alternators from auto zone. You can about imagine how confused I was. The guy "in the zone" kept arguing with me the first time until he tested it. Then he gave me ANOTHER junk one. The worst part was when I brought that one back (driving my car), he had the nerve to tell me I probably paid too much for the one I bought somewhere else.