How do I Tell If It Is a Dead Battery or the Alternator?

by Chris Deziel

You've been out shopping, you hop in the car and turn the key, and nothing happens. It could be because you left the headlights on, which drained the battery, or it could signify a problem with the charging system. When it's operating correctly, the car's alternator provides a steady flow of current to charge the battery when the motor is running. Without this charge, the battery quickly drains, even while the car is in use. Repairing the alternator and replacing the battery are not difficult jobs, but first you have to know which of them to do.

Turn the ignition key.
1

Turn the ignition key. If there is any power left in the battery, the warning lights on the dashboard should come on. If the alternator light, which is sometimes identified as "Charge," doesn't come on, the alternator is probably not working.

An alternator belt.
2

Open the hood and check for a loose or broken alternator belt. It is connected around the engine block and the alternator, and is also hooked up to a cooling fan in some models. If you can lift it more than about 1/4 inch from any of the pulleys with your finger, it is probably too loose too turn the alternator and needs to be tightened or replaced.

A car battery charger.
3

Charge the battery, or hook your car up to another car with jumper cables, and start the car. If the alternator light doesn't go off, then the charging system needs servicing.

Use your headlights to help diagnose the problem.
4

Stop the engine, turn on the headlights, then start the car. If they dim significantly, that is another indication of a charging problem. If the lights get brighter when you rev the engine, the alternator may be partially supplying the battery with power. If there is no change in the headlight intensity when you start the car or rev the engine, the alternator is functioning normally.

Check the charge on your battery.
5

Set a voltmeter to read DC volts and connect the leads to the battery terminals while the engine is running. It should read approximately 14 volts. If it reads less than 12 volts, the alternator is not supplying charge. If the meter reads 14 volts but dips to less than 13 volts when you turn on the heater, rear window defogger, lights and radio at the same time, it may be only supplying a partial charge.

Tips

  • check Be sure the battery leads are tight before diagnosing battery and alternator problems. If they are loose, they are probably causing your charging problems.
  • check If the alternator is broken, you may be able to hear loud clicking sounds coming from the engine while it is running.

Warning

  • close If your alternator light comes on while you are driving, you should head directly to a garage. The car will continue to run normally, but will stop when the battery drains, which won't take long.

Items you will need

About the Author

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.

Photo Credits

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