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Old 07-17-2010, 11:24 PM   #1
RobertGary1
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Default Proper alternator voltage?

I'm trying to determine if I have a bad battery or alterator. Occasionally I will get a really weak engine start and when I apply the breaks the panel lights dim (something I've traditionally seen with a bad regulator).

I put my charge system tester on the battery and it indicates 13.2v while running. It showed over 12v at shutdown and indicates 200amp in the load test. I checked AllData and it says anything over 13v is good on the alternator. Haynes says 14-15 minimum. In my experience with aircraft, boats, etc I've always expected 14 volts from an alternator. So is 13.2v really ok???

-Robert

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Old 07-17-2010, 11:49 PM   #2
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Default Re: Proper alternator voltage?

13 volts sounds a bit low. It should be ~12.6V off and at least close to 14V if not a bit above when running. If you go to advance or similar FLAPS they can test your alternator, battery and starter, for free. The test isn't perfect or thorough, but it wouldn't be a bad idea.

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Old 07-18-2010, 07:41 AM   #3
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Default Re: Proper alternator voltage?

Quote:
Originally Posted by PlasticCarsRock View Post
13 volts sounds a bit low. It should be ~12.6V off and at least close to 14V if not a bit above when running. If you go to advance or similar FLAPS they can test your alternator, battery and starter, for free. The test isn't perfect or thorough, but it wouldn't be a bad idea.
Autozone did test it and said that 13.2v is ok for the alternator and that my issue was the battery. I just want to ensure that is correct before I pay California battery disposal fees on a new battery.

-Robert

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Old 07-18-2010, 09:17 AM   #4
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Default Re: Proper alternator voltage?

If the battery is good then a properly functioning alternator should produce 14.2 to 14.7vDC at the battery terminals with the engine at 1500RPM.

With the car off for 8 to 10 hours the battery terminal voltage should be 12.6 minimum and 12.8 is normal in summer weather. If the battery terminal voltage does read 12.6 or above start the engine and let it idle with all accessories shut off. After the car warms up to normal temperature and the idle is down to normal check the terminal voltage at idle, should be 13.2 minimum. Now increase RPM to 1500 and the terminal voltage should increase above 13.2v. After 30 seconds or so max it should read between 14.2 and 14.7. Batteries and alternators are properly tested individually by battery load tests and alternator load tests. The quick in-car testers can provide inconclusive results as the interpretation of the info provided relies on the skill of the operator.

Plastic cars Rock is correct and the ALLData is questionable as the test conditions are not specified. Haynes and Autozone, are as usual flat incorrect.

The above voltage readings are obtained with a DVM with ambient temperatures in the high 60s low 70s and the measurement point is the battery terminal bolt heads. Readings from these points do not check/test the condition of the terminal connections and that could also be your problem. Any out of specification reading requires further testing and does not immediately condemn the alternator or battery.

Age of battery and alternator figure into this also as well as location.

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Old 07-18-2010, 06:43 PM   #5
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Default Re: Proper alternator voltage?

Have another battery to substitute the suspect one? When the engine is started either the alternator will output the same voltage, possibly indicating a faulty alternator, or voltage will jump up to 14 volts+ to indicate a faulty battery. You'll eliminate at least one variable by using another battery.

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Old 07-18-2010, 09:53 PM   #6
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Default Re: Proper alternator voltage?

Hmm, I just finished preping a new Gill aircraft battery (aircraft batteries don't come ready-to-use like car batteries). I guess I could try that. It's a 12 volt battery.

-Robert


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Originally Posted by fdryer View Post
Have another battery to substitute the suspect one? When the engine is started either the alternator will output the same voltage, possibly indicating a faulty alternator, or voltage will jump up to 14 volts+ to indicate a faulty battery. You'll eliminate at least one variable by using another battery.

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Old 07-18-2010, 10:17 PM   #7
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Default Re: Proper alternator voltage?

You probably know that sometimes we have to think outside the box.

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