What is this battery gauge on my dashboard trying to tell me?

Dear Car Talk

Dear Car Talk | Sep 01, 1997

Dear Tom and Ray:

On the dashboard of my 1990 Plymouth Voyager is a dial with a picture of a
battery. The numbers show a range from 8 to 18. What is this gauge, and what is
it trying to tell me? -- James

RAY: It's your voltmeter, James. And it's trying to tell you that they had
extra space on the Voyager dashboard and that some supplier was having a
special on voltmeters in 1989.

TOM: It's supposed to tell you how many volts of electricity your car's
charging system is producing.

RAY: Under normal conditions, it should read between 12 and 14 volts.

TOM: The electricity to start the car comes from the battery. But once the
engine is running, the belt-driven alternator takes over. It not only produces
electricity for the ignition, lights and other accessories, but it also
produces enough electricity to recharge the battery -- so you'll be able to
start the car again next time.

RAY: And a voltage regulator -- another piece of the charging system -- tells
the alternator how much charging the battery needs, and therefore, how much
electricity to produce.

TOM: When the battery is low, the alternator will produce a little more than 14

RAY: When the battery is charged up, only about 12 volts will be produced.

TOM: If your voltmeter occasionally reads less than 12 volts, that's probably
nothing to be alarmed about. If you have the lights on, the windshield wipers
going and the air conditioner on, and you're sitting at idle, a temporary drop
below 12 may just mean that you're using a lot more electricity than the
alternator can produce at that low speed.

RAY: But if the gauge ever stays above 14 1/2 or drops below 12 for a long
period of time, then you either have a problem with your charging system, or
you have a problem with that 89-cent voltmeter they slapped in your car. And
only an accurate test at your mechanic's shop can tell you for sure.

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