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Name: Andrew
Status: student
Grade: 9-12
Location: OR
Country: USA
Date: Winter 2013-2014

Why does my digital multimeter give me a reading of 27v AC on a car battery when I know it's DC? With the same multimeter I get a 12v DC reading on the DC scale, which I understand but when I am reading the battery on the AC scale should not I be getting a reading of about 12v since the protons or electrons are only going in one direction?

Hi, Andrew

I am an electrical engineer with some experience with this sort of thing.

One question I have is whether the car engine was running when you made this measurement?

Digital multimeters can behave in unpredictable ways when they are subjected to voltages for which they are not specifically intended. A DMM on its AC scale may or may not reject DC voltage (in that case it could possibly read zero.) It is possible that the meter leads may be picking up a lot of noise; this is especially likely if the engine is running because automotive ignition produces many thousands of volts internally and can produce a lot of electrical noise. I would not be surprised if the AC scale was more sensitive to this noise than the DC scale is.

A meter which is "true RMS" would provide different results from one which is not "true RMS". A meter which reads "true RMS AC+DC" will provide different results from one which is "true RMS AC" only. None of what I have said completely explains your 27V reading. My point is that the AC scale on many DMMs is intended only for a "sine wave" AC voltage and that anything else can provide unexpected results.

Do you know that your meter works normally when measuring the AC output of a transformer (such as a small 12VAC output transformer)? This output voltage should be approximately a sine wave so should work reasonably well. This would indicate whether your meter's AC scale is working correctly. Be careful---do not measure high voltages (over 20 - 30 Volts) as they are dangerous.

Best regards, Bob Zwicker

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