Parasitic draw test

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JHZR2

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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?
 
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.
 
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.
 
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."
 
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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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
The exact same amount of current will be flowing so it does not matter which side you take your measurements from. The only reason the negative side is usually recommended is because it is the uninsulated side of the circuit. If the battery was disconnected from the negative side and you accidentally dropped a wrench between the fender and the negative terminal, nothing would really happen. The same amount of current would be flowing through it (usually less than .050 amps) as if the negative clamp was tightened on the terminal. However, if you had the positive disconnected and you dropped a wrench between the fender (or any other ground) and the positive post, you would now have a complete circuit with very little resistance. Which means a lot of amps will be flowing. The current flowing through the wrench would heat it nearly red hot. Your instinct would be to grab the wrench, not knowing it was blazing hot. You would burn your hands like a barbecue grill and have to wear bandages on them for several days. Trust me on that one.
 
:eek:t: when I was doing a vehicle maintenance course at tech, our teacher showed us a great scar around his wrist from bringing his spring banded watch between starter terminal and chassis.
 
Originally Posted By: J. A. Rizzo
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.


Law of physics said that you have to have a loop to have current, and if the amount of electron flow isn't the same, it is going somewhere else and come back. Everyone knows that electron comes out of the negative and go back into the positive, but they have to all go back to form a circuit, so the current measurement is the same as long as you connect the meter in serial, doesn't matter positive or negative.
 
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