An alternator is simply an electric generator that recharges the battery and powers the electrical system when the engine is operating. When an alternator dies, the vehicle draws electricity from the battery. When the battery doesn't get recharged by the alternator, the headlights grow dimmer. If the alternator fails, the car can only operate until the battery dies, which might be 5 minutes or 30 minutes.

The most common reason for the failure of an alternator is bad diodes. The rectifier assembly converts the alternator's AC output to DC with the help of these diodes. If a diode gets leaked, it can drain out the current from the battery through the alternator. 

Signs of a bad alternator are:

• Strange Noises
• Dim or flickering headlights
• Car stalls
• Battery Dies
• Other electrical failures