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#1871027 - 04/25/10 09:43 PM Parasitic draw test
JHZR2 Offline



Registered: 12/14/02
Posts: 32534
Loc: New Jersey
Hello,

Which way do you test for parasitic draw, ammeter between the negative cable and battery terminal or positive cable and terminal?

I always thought it was on the + side, as all current must originate there, but I figure that based upon resistances, dissipation as heat, etc., not all has to return to the negative.

Yet Ive heard elsewhere to do it on the negative side.

Does it matter? Is one way better than the other?

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#1871042 - 04/25/10 09:54 PM Re: Parasitic draw test [Re: JHZR2]
tom slick Offline


Registered: 05/26/03
Posts: 8386
Loc: Central Coast, Calif.
Do both and see if there IS any difference
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#1871055 - 04/25/10 10:00 PM Re: Parasitic draw test [Re: tom slick]
JHZR2 Offline



Registered: 12/14/02
Posts: 32534
Loc: New Jersey
I will, but it is pouring here for the next few days...

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#1871060 - 04/25/10 10:04 PM Re: Parasitic draw test [Re: JHZR2]
Pablo Offline


Registered: 10/28/02
Posts: 46392
Loc: Duvall WA - Pacific NW USA
I've always done it on the + side. No deep thought really, just figured if juice is leaking.....

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#1871076 - 04/25/10 10:17 PM Re: Parasitic draw test [Re: Pablo]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
It does not matter. The readings will be the same on either side of the battery, so long as you connect the meter with the correct polarity. If you do not the digital meter will indicate a negative reading with a - sign. You could ignore that - sign, as surely the vehicle is not charging the battery when the engine is running.

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#1871083 - 04/25/10 10:30 PM Re: Parasitic draw test [Re: brianl703]
J. A. Rizzo Offline


Registered: 08/31/05
Posts: 477
Loc: Chicago, IL
It's really the negative terminal where the current comes from. Current is moving electrons, and electrons are negatively charged. So the negative terminal is where all the electrons are congregated, waiting to flow over to the positive terminal. I know, it's opposite what you'd think, but in reality, the negative terminal blows and the positive terminal sucks.
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#1871085 - 04/25/10 10:34 PM Re: Parasitic draw test [Re: brianl703]
JimPghPA Offline


Registered: 08/22/09
Posts: 2753
Loc: Pittsburgh,PA U.S.A.
Originally Posted By: brianl703
It does not matter. The readings will be the same on either side of the battery, so long as you connect the meter with the correct polarity. If you do not the digital meter will indicate a negative reading with a - sign. You could ignore that - sign, as surely the vehicle is not charging the battery when the engine is running.


I think instead of "as surely the vehicle is not charging the battery when the engine is running." You wanted to say "as surely the vehicle is not charging the battery when the engine is NOT running."


Edited by JimPghPA (04/25/10 10:34 PM)
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Boy will I be happy when ALL vehicles on public roads are autonomous.


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#1871096 - 04/25/10 10:43 PM Re: Parasitic draw test [Re: JHZR2]
XS650 Offline



Registered: 12/21/03
Posts: 12385
Loc: Northern CA
When you hook your meter between the battery and the cable, it's a good idea to put a jumper in parallel with the meter and leave it connected for a few seconds after you connect the battery. Some cars draw many amps briefly after you connect the power.

Fortunately the fuse in my ammeter protected the meter when I learned that.
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#1871098 - 04/25/10 10:43 PM Re: Parasitic draw test [Re: JimPghPA]
brianl703 Offline


Registered: 05/07/04
Posts: 10487
Loc: Manassas, VA
You are correct, I omitted the "NOT"!

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#1871208 - 04/26/10 12:39 AM Re: Parasitic draw test [Re: brianl703]
Shannow Online   content


Registered: 12/12/02
Posts: 25284
Loc: a prison island
Yep, what comes out must go in, or it doesn't come out in the first place.

Was messing around with exactly this on the E30 the other day.

0.8 Amps for quite some time until everything goes into sleep mode, then less than 20mA when it's asleep.

Apparently 12 min commute isn't enough for a flat battery to recover, so I disconnected the -ve at each end of the trip until it was happy again...that's how I found out my commute is exactly 12 minutes.
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#1871427 - 04/26/10 09:13 AM Re: Parasitic draw test [Re: Shannow]
Vikas Offline


Registered: 07/22/05
Posts: 7322
Loc: NorthEast
Do it on the negative side because that is the one which is removed first

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#1871914 - 04/26/10 03:26 PM Re: Parasitic draw test [Re: Shannow]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Either side should read the same.

But sometimes there is more than one ground .
Putting your meter in series on only one would show a difference from a single '+' lead.

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#1872250 - 04/26/10 08:41 PM Re: Parasitic draw test [Re: mechtech2]
JHZR2 Offline



Registered: 12/14/02
Posts: 32534
Loc: New Jersey
Originally Posted By: mechtech2
Either side should read the same.

But sometimes there is more than one ground .
Putting your meter in series on only one would show a difference from a single '+' lead.


That was kind of my thought. Multiple grounds, different resistances back to the battery terminal, etc. may make some difference in practical terms. Measuring off the positive terminal as the origination of all current means it is all accounted for.

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#1872290 - 04/26/10 09:27 PM Re: Parasitic draw test [Re: JHZR2]
Shannow Online   content


Registered: 12/12/02
Posts: 25284
Loc: a prison island
If there's a single terminal connected to the battery, that's where all the juice collects.

Positive and negative have to match if there's nothing else on the post.
_________________________
Not everything is a conspiracy...
Only the important stuff ontome


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#1872343 - 04/26/10 10:23 PM Re: Parasitic draw test [Re: Shannow]
Rabbler Offline


Registered: 10/19/07
Posts: 501
Loc: Canada
It shouldn't matter but I always do it on the Negative. Draw should be less than 90 ma once all the controllers have timed out. Make sure you disable under hood light if equipped.

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