Same thing happened to me a couple of months ago. My alternator failed slowly, but surely when I drove from Utah to Florida. By Atlanta, I was in big trouble, but made sure I pulled over frequently and drove at low rpm's in town to charge my battery. I had my alternator replaced with one that whines, but it charges the battery really well so I didn't get a non-whining model. People ask me if I have a turbo! 4000 miles later, I'm in the Bay Area and the thing still whines but charges the battery.
