Car Battery and AC Voltage

[strnjss]

I don't know if this is the proper forum for this, but in testing car batteries, I have come across something kind of strange.

It seems when I test a 12 volt DC car battery with a multimeter on the AC scale, I will get around 30v AC steady reading. I tested several different batteries, and each outputs around the same 30v. They all still test ~12.5 on the DC scale as they are supposed to.

What is going on here? How can a DC battery be putting out AC voltage? Is it a false reading? A multimeter issue? Or is there some real AC voltage going on here?

 

[wbrian63]

I think it's a meter thing. If you test A/C voltage on the D/C scale, it will show low #'s when the line is live.

My multi-meter defaults to DC scale, and I have to remember to set it to AC each time.

There may be a technological reason why this is - other smarter people probably know the "real" reason.

 

[rwhite692]

When your meter is set to measure AC it is trying to read an RMS value.

http://www.kpsec.freeuk.com/acdc.htm

 

[porcupine73]

Maybe you're seeing 30mV AC? Otherwise maybe try a different meter. Because 30VAC across car battery terminals isn't right. Now if the engine is idling and you're reading AC on the battery terminals, it can indicate one or more failed diodes in the alternator rectifier. Most modern alts are three phase, so if one or two of the diodes fails it can leave you reading some AC volts.

 

[larry_g]

It’s a meter thing. Why do people do the wrong thing and then need an explanation of the results. Have you checked the resistance or the amps yet? If not don’t do it, If you did what color smoke did you get?

Lg
No neat sig line

 

[a.pengue]

It’s a meter thing. Why do people do the wrong thing and then need an explanation of the results. Have you checked the resistance or the amps yet? If not don’t do it, If you did what color smoke did you get?

Lg
No neat sig line

LOL probably the best answer.
but on a serious note, the values you will get on AC are not the right ones. They are just numbers. Like when a car run on open loop and the pcm just disregards anything the O2 sensor is giving as an input..

 

[MattT]

When your meter is set to measure AC it is trying to read an RMS value.

http://www.kpsec.freeuk.com/acdc.htm

Probably a averaging meter that's averaged out +12.5 Vdc to 30 Vac

Just checked a battery with my trms Fluke and it only read a couple mV on ac.

 

[strnjss]

Maybe you're seeing 30mV AC? Otherwise maybe try a different meter. Because 30VAC across car battery terminals isn't right. Now if the engine is idling and you're reading AC on the battery terminals, it can indicate one or more failed diodes in the alternator rectifier. Most modern alts are three phase, so if one or two of the diodes fails it can leave you reading some AC volts.

Yes, that's actually what I was trying to determine. I had the car running, and read 30v AC, which would indicate bad diodes. But to be sure, I tested it with the car off, and still got the 30v reading. I tested another car too to be sure, and it's battery read the same.

So I guess my meter isn't capable of determining the state of my alternator diodes

It’s a meter thing. Why do people do the wrong thing and then need an explanation of the results. Have you checked the resistance or the amps yet? If not don’t do it, If you did what color smoke did you get?

Lg
No neat sig line

I didn't do the wrong thing. Checking for AC voltage is a proper procedure for determining if the alternator's diodes are screwed.

So I guess my meter is just not capable of doing this accurately.

and if my meter was capable of actually reading up to 1000 amps, I would check the amps. But I can barely read mA with mine.

 

[MattT]

I didn't do the wrong thing. Checking for AC voltage is a proper procedure for determining if the alternator's diodes are screwed.

How exactly? What readings should good and bad rectifiers give on the AC range?

 

[strnjss]

How exactly? What readings should good and bad rectifiers give on the AC range?

As I understand it, you shouldn't read any more than a few mA of AC power if any. A fried diode would cause more AC current to the battery, screwing things up. Supposedly, you can test for this by checking the AC power across the terminals.

You should read very little if any if the diodes are good, but if they have gone bad, there will be AC voltage across the battery terminals. Basically, it's letting unconverted power through.

 

[ddawg16]

I understand exactly what your doing....

What brand of meter? If it's a cereal box brand....and not an HP or Fluke....I would say that it's the meter. At a min, I would short the leads together to see what it reads in AC mode...should read 0.

BTW....even with bad diodes, I wouldn't expect to see a lot of ac at the battery. It is just a low impedance that it should be almost all pure DC. I would check it at the back terminal of the alt...there will be enough resistance in the wire to show the AC...

But at the end of the day....I would use an oscope....if you have a laptop laying around....you can get some pretty decent USB scopes that use your laptop....and if your car is a newer OBI or OBII....use it as an OBD scanner...

 

[strnjss]

I understand exactly what your doing....

What brand of meter? If it's a cereal box brand....and not an HP or Fluke....I would say that it's the meter. At a min, I would short the leads together to see what it reads in AC mode...should read 0.

BTW....even with bad diodes, I wouldn't expect to see a lot of ac at the battery. It is just a low impedance that it should be almost all pure DC. I would check it at the back terminal of the alt...there will be enough resistance in the wire to show the AC...

But at the end of the day....I would use an oscope....if you have a laptop laying around....you can get some pretty decent USB scopes that use your laptop....and if your car is a newer OBI or OBII....use it as an OBD scanner...

Thanks, it's a General Electric meter I got for like $20 years ago. It's been great except for this issue.

I actually just tested a 1.5v AA battery just now to see what it would do. It measured 1.59v DC like it should, but when I switched it to the AC scale, it actually measured 2.9v AC?!

So apparently it must be my meter. I'll have to invest in a lower end Fluke or Extech meter I guess....

Yes I do actually have a USB OBD II scanner. It's awesome! I've found so many issues from the sensor data! Saved me hundreds so far.

I've never heard of a USB oscilloscope though, that sounds pretty cool actually. I'd need to learn how to use an oscilloscope first though. I'll definitely look into that!

 

[MattT]

So apparently it must be my meter. I'll have to invest in a lower end Fluke or Extech meter I guess....

I wouldn't guarantee a low end fluke or extech being any different because they're also averaging on the AC range. With a higher end true RMS meter you might be able to diagnose bad diodes but for that kind of money you'd be better off getting some kind of 'scope if your primary use is automotive. Like ddawg said test the alt output terminal not across the battery.

 

[SiGmA_X]

One of my friends referred me to a USB oscilloscope a while back, and in my limited use with it, its been great. Not to mention its dirt cheap, and small, and I always have a laptop with me. Side note, I couldn't get the driver to work on my x64 Win7 laptop. Works perfect on my 32bit XP laptop tho, which was its intended laptop anyway.

http://hobbylab.us/

 

[lametec]

You don't live under some high voltage power lines, do you?