The battery light comes on when there is a problem in the battery charging system. Because your car is so dependent on the battery all cars have a battery light on the dashboard that is designed to warn you if the charging system fails. A simple circuit reads the voltage that the alternator is producing and turns the battery light on if itâ€™s too low. If the battery light stays on while youâ€™re driving, the most common problem is a broken, damaged or loose belt. Total failure of the alternator another possibility.