Your car's alternator supplies power to keep the battery charged and run all the electric accessories. If there's a problem in the charging system while you're driving, the alternator light should begin to glow. When the light comes on, it's a good idea to pull over and check the tension on the alternator belt. You should not attempt to drive with a loose or broken alternator belt since most alternator belts also turn the water pump. Driving with a disabled water pump can do serious damage to your car's engine. If the alternator belt feels tight and looks okay, than go ahead and drive to the nearest service station. Even if your alternator has failed completely, the battery should supply enough power to run your car for around an hour with a full charge. Remember that cold weather may drop that time to around fifteen minutes. If you suspect your car may have alternator problems, contact a qualified mechanic in your area.