Every car today contains a rechargeable 12-volt battery. This battery
powers everything electrical: the engine's control computer, the ignition system, the radio, the headlights, etc. So the battery
is vital to the car's operation.
Like all batteries, a car's 12-volt battery
would eventually go dead if it were not recharged, so your car has a
built-in recharging system. Most cars have an alternator, along with a
voltage regulator, that keeps the battery charged and also provides
electricity to the vehicle when the engine is running. A typical alternator
can put out between 500 and 1,000 watts when necessary.
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 recharging system
should fail. A simple circuit looks at the voltage that the alternator is producing, and turns the battery light on if it is low. The battery light indicates a battery charging problem.
If the battery light comes on and stays on while
you are driving, the most common cause is a broken alternator belt. Total failure of the alternator is another possibility.
The reason why your car can operate normally even though the battery light is on is because your car can run off the energy stored in the battery. Your car will run fine until the battery goes dead. At that point, nothing in your car will work. When the battery light is on, you can still drive your car to the garage -- you will not damage anything. But you want to get to the garage before the battery fails and your car dies.
Here are several useful links to learn more: