It sounds like your alternator. My car had the light come on while driving to work. I drove to lunch and it was off. I took it to a good Indy, and they said it was charging like crazy. They took it for a spin and got the light to come on. It turns out it was working only sporadically. Things like that never get better, only worse. It was a touch over $500 for the alternator, belt and one pulley. These cars use a lot of juice, so once it craps out for good it'll strand you.
I believe all of the alternators for our cars are rebuilt these days, even the shiny one from the dealer.