The alternator converts mechanical energy into electrical energy that charges the battery and supplies power for the electrical components of the vehicle. It is driven by the crankshaft pulley via a belt. There is also a voltage regulator, either on or in the alternator, that prevents the alternator from producing too much voltage. While the vehicle is running the alternator turns, producing enough energy to run all the electrical components and have enough to charge the battery. When the vehicle is at an idle, and there is a lot of electrical demand, the alternator cannot keep up and electrical power is drawn from the battery.
The most common cause of failure of the alternator is bad brushes. Once the brushes within the alternator go bad, then it cannot convert energy, preventing the electrical components on the vehicle from working and the car from starting. Also, if the regulator is located in the alternator and it goes bad, then there will be no electrical connection between the battery and the alternator. In either situation, the alternator will need to be replaced.
Most manufacturer factory warranties will cover the alternator under the bumper-to-bumper warranty. For extended coverage, an extended auto warranty is an option to extend coverage of the alternator beyond the expiration of the factory warranty.