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 it’s too low. If the battery light stays on while you’re driving, the most common problem is a broken, damaged or loose belt. Total failure of the alternator another possibility.