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Author Topic: Alternator ripple voltage test gives 30V AC  (Read 332 times)

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grifter

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Alternator ripple voltage test gives 30V AC
« on: 10 July 2019, 23:34:16 »

Had a bit of trouble with battery the other day, had to get jump start, been ok since. Decided to test charging and battery standing voltage. Seem ok 12.5v and charging at about 14v. I then checked AC voltage and was getting 30v. Seems like the alternator is goosed perhaps, as is my understanding the output should be pure DC as the AC is rectified. Anyone done a similar test and what did they find. Model is 2.6 facelift.
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LC0112G

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Re: Alternator ripple voltage test gives 30V AC
« Reply #1 on: 11 July 2019, 00:48:41 »

How/Where were you measuring the AC voltage - Across the battery?

If so, you can't do that and get meaningful results. The AC reading (if it claims to be showing RMS) will typically be 1.4142135623730950488016887242097 (ish) times the true DC voltage, depending how good/accurate the voltmeter is. So you should see 20 ish volts AC if the battery is charging at 14V. Or if the Voltmeter is claiming to show peak voltage, it'll show 28V if the battery is charging at 14V.

You'll never see a real AC voltage across a DC battery unless the battery is FUBAR or smoke is coming out of something.
« Last Edit: 11 July 2019, 00:52:05 by LC0112G »
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grifter

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Re: Alternator ripple voltage test gives 30V AC
« Reply #2 on: 11 July 2019, 08:12:31 »

Yep across battery. Going by that I might be ok then. Strange though, I get 0 volts AC on my cavalier, and have always got 0 Volts on any other car I check, will also check meter is blocking DC by doing a reading with engine off on AC reading. Will keep an eye on the DC output as well. Might give it another look and do a load test today. Cheers
« Last Edit: 11 July 2019, 08:14:46 by grifter »
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grifter

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Re: Alternator ripple voltage test gives 30V AC
« Reply #3 on: 11 July 2019, 19:17:49 »

Tried on battery without running and got 26v AC so guess it's not that.
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