The alternator converts mechanical energy into electrical energy in order to keep the battery fully charged and power the vehicle's electrical systems. It produces an alternating current, which is changed to usable direct current via the rectifier diodes within the alternator. Then a regulator, either in or on the alternator, keeps the alternator from producing too much voltage. The voltage regulator can be replaced separately from the alternator. An alternator replacement is needed when the alternator quits converting energy, usually caused by the brushes within the alternator going bad, and the vehicle will not start.
When replacing the alternator, the technician disconnects the battery and removes the alternator drive belt. Then they remove the alternator and install the new one. Finally, the technician reinstalls and tightens the alternator belt and reconnects the battery.
Most manufacturers cover an alternator replacement under the bumper-to-bumper section of the factory warranty. There are extended auto warranties available that will extend coverage to the alternator beyond the manufacturer's factory warranty expiration. When looking for extended coverage, be sure to look at what level of coverage will cover the cost of an alternator replacement.