If the alternator dies, the battery doesn't get a charged and rebuild its power storage from any drain it had to cover. When the car is running, the charging system (alternator, voltage regulator) should recharge the battery and provide sufficient electrical power to the vehicle. The battery serves as a backup.
Will a bad alternator cause a car to die while driving?
When there is something wrong with the alternator, the car runs off of the battery. The battery can only run for so long before it's completely drained. Malfunctioning ignition switch. This loss of power causes the engine of the car to die while driving.
How do I bypass the alternator to start my car?
Why does my car keep dying while driving?
A car can die while driving but then will often restart if there are problems with the battery or alternator, a lack of fuel, a broken timing belt, problems with the fuel delivery system or if the engine over heats.