The battery light is misnamed: It doesn't go on when the battery is low. The battery light will go on only if the alternator is not charging the battery.
Do not panic - you have some time. You don't have to pull over right away. But don't ignore this light! You have anywhere from a half-hour to a day or even longer, depending on how much charge is left in the battery.
Turn off all unnecessary electrical accessories, such as the radio, heater, defroster headlights and windshield wipers.
Avoid using the horn, turn signals, flashers, dome light and power windows.
Avoid turning the engine off and on. Starting your car uses more of the battery's charge than anything else.
Drive to your auto repair shop if you can. Ask your mechanic to do an alternator output test to check if the alternator is charging.
Take the following steps if you can't visit the mechanic right away.
Turn the engine off and open the hood.
Look for whitish/bluish powdery corrosion on the battery cable ends. Clean with a wire brush if it's in evidence.
Make sure the battery cable ends are tight. You shouldn't be able to turn them at all.
Check the negative battery cable end. Make sure it's tight and secure at both ends.
Check the alternator belt (see "Check Your Engine Belts," under Related eHows). A loose belt will cause the alternator to undercharge and trip the battery light.
Make sure all the connections at the back of the alternator are secure.
