The belt could be slipping at high speed causing the alternator not to charge under load .Or it could be that the alternator is on its way out and that was just a warning. With the car running disconnect the battery negative terminal and see if the car dies if so the alternator is not working if it doesn't die try the same thing with the rpm increased if it dies then the alternator is not working . If it doesn't die then I would look at the battery for a bad cell . Or maybe a direct short on the positive side of the plus cables somewhere.