An alternator is an electrical machine. Its function is to charge the battery on a regular basis. However, if the battery is dead, the alternator cannot charge it. It regulates voltage and supplies power used to operate the car. The electricity used by various parts of the car is generated by the alternator; not by the battery which we usually believe. Starters and alternators wear out with time, hence, need replacement. Alternators usually fail when the charging systems have higher electrical demands.