What Happens When an Alternator Is Dead?

by Paul Novak
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New car petrol engines image by Christopher Dodge from Fotolia.com

A common misconception of automotive electrical systems is that the battery supplies the electrical power for the vehicle. In reality the battery is primarily the source of power for starting the vehicle, and the alternator provides energy for running the electrical systems and keeping the battery fully charged. Because of this, the alternator has a direct effect on all the electrical components of the vehicle. Several things happen when an alternator dies.

Dimming Headlights

When an alternator dies and is no longer supplying electrical power, the electrical system of the vehicle draws power from the battery. As the battery is no longer being recharged by the alternator, the battery loses its charge and the headlights grow dimmer as less power is provided by the battery.

Slow Accessories

All of the electrical accessories such as power seats and power windows rely on the alternator for the proper amount of power to operate properly. When an alternator dies, the battery is unable to provide the proper amount of electrical energy. This causes the windows and other accessories to operate slowly or not at all.

Battery Drain

As an alternator dies, it can no longer keep the battery in a fully charged state. As a vehicle is driven with a dead alternator, all the electrical power for operation of the vehicle is then drawn from the battery, which is eventually completely drained of its electrical charge.


Ignition systems, fuel systems and on-board computers all rely on electrical energy from the alternator to operate correctly. When an alternator fails, these systems no longer receive the correct amount of electrical energy. This can cause these systems to operate erratically or not at all, resulting in stalling and poor running of the engine.

Hard Cranking

A failed alternator will be unable to replace any charge that is lost by the battery during operation of the vehicle. When the vehicle is shut off, and later attempted to be restarted, the weakened battery will not have sufficient power to properly turn over the engine. The result is a slowly rotating starter that gradually stops turning altogether when the key is turned.

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