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-   -   Testing for parasitic draining of the battery? (http://forums.tdiclub.com/showthread.php?t=367932)

DBDieseler October 30th, 2012 18:21

Testing for parasitic draining of the battery?
 
I just had a 2 month old battery die on me; from what I was told it was a bad cell. I hope I was unlucky enough to get a bad battery, but I want to make sure something else isn't wrong.

I'm getting ~12V across the terminals while off and ~14V when running, so it appears that the alternator is working. I'm curious if I can check for bad diodes or anything else without removing the alternator from the vehicle...

Any suggestions?

Corsair October 30th, 2012 19:26

With everything off, disconnect the positive (red) wire from the positive terminal of the battery and insert an ammeter (ie. multimeter set on the DC amps function) in between the positive battery post and the big positive (red) wire that was just removed from the positive battery post. (red wire of multimeter to the battery post, black wire of multimeter to the fat red wire recently disconnected from the battery post. Basically, you're inserting the multimeter in "series" with that fat red wire that connects to the battery +. Look for current draw that doesn't go away after about 5 minutes. There are specs, sorry I don't know what they are for VW. Typically, I think the parasitic draw should fall to less than approx. 30 milliAmps (0.03 Amp) within about 5 minutes. You'll probably have to set the meter to a fairly low scale reading to see this. Couple of cautions-

Sometimes it takes a minute or more for various things to "go to sleep" and stop drawing current. I don't know specifically about our Jettas, but I know that's true with, for instance, Dodge Ram trucks.

Keep all doors closed, trunk closed, etc. when doing the parasitic draw test. Opening a door brings on the dome light, which draws at least an amp, and can pop the fuse in your multimeter if it's set to a low current scale reading, trying to look for 20mA draw. How do I know that.....

firehawk618 October 30th, 2012 21:27

Yup follow Corsair's advice. It's THE way to check how much your car is drawing from the battery when turned off and pinpoint if your car is killing the battery or not.

I'd start with the 10A scale if your meter has one. This will work fine and you are much less likely to pop the fuse in your meter.

JSWTDI09 October 30th, 2012 22:47

This is VW's way of doing it.
http://pics.tdiclub.com/data/500/TSB..._04_Matrix.pdf

This is the latest TSB from VW (TSB 27 08 04) plus the fuse matrix from TSB 27 07 11 all wrapped up in one single pdf file.

With a current measuring clamp you can measure the total sleep mode current. If you don't have a current clamp you can measure the voltage drop across every fuse and calculate the current flow to every part of the car. Good Luck.

Have Fun!

Don

firehawk618 October 30th, 2012 22:54

Quote:

Originally Posted by JSWTDI09 (Post 3982327)
This is VW's way of doing it.
http://pics.tdiclub.com/data/500/TSB..._04_Matrix.pdf

This is the latest TSB from VW (TSB 27 08 04) plus the fuse matrix from TSB 27 07 11 all wrapped up in one single pdf file.

With a current measuring clamp you can measure the total sleep mode current. If you don't have a current clamp you can measure the voltage drop across every fuse and calculate the current flow to every part of the car. Good Luck.

Have Fun!

Don


Way too complicated. Just use your meter as stated a few posts up. IF you do find too much draw then proceed with the suggestion from JSWTDI09.



JSWTDI09, good pdf, thanks for sharing.

DBDieseler October 31st, 2012 10:22

I checked the battery using the method Corsair described. I see an initial current spike of ~1A right when I attach the meter (should that happen?), but it drops to 20mA within a few seconds and stays steady.

Thanks for the replies Corsair, Firehawk, and JSW.

microsoot October 31st, 2012 11:45

I think you should read closer to 12.6 volts on the battery fully charged.
Dirty and loose battery and alternator connections not only cause resistance of current out of battery, compromising cranking amp starting but also alternator charging current into battery maintaining electrical potential(voltage).

If you read 12 volts, that's too low.

Start with clean tight battery and alternator connections, as well as battery negative cable to ground connections.

It's a simple , quick, cheap preventive maintenance thing anyways.

It's the only place to start troubleshooting this problem.

good luck

microsoot November 4th, 2012 23:14

how's the battery/?
curious how u made out


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