My brother had this happen to him on his Dodge Ram. Going through about foot and half of snow on a unplowed seasonal road. Battery light came on, and the belt was squealing big time. About a mile or so on a plowed road it all went back to normal. I suppose there was just too much snow on the belt and it was slipping on the alternator pulley.
1996 Ford Ranger STX 4x4 4.0L
1983 Honda Shadow VT750C