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Battery drain test, becareful when using low current clamp meter

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Battery drain test, becareful when using low current clamp meter

Old 04-29-2019, 08:19 AM
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2014 - W212 - E400 ( M276 Dela 30, 3 liter ) RWD not Hybrid
Battery drain test, becareful when using low current clamp meter

Hi All,

Just wanting to share ........

The story goes like this..........
Last year I was reading approx 400 milliamps as what I believe as stand-by current consumption of my car after CAN/SAM are sleeping.
I give it no thought, I assume all the electronics in the car are suckers even when sleeping.
I was using this : https://www.tequipment.net/Fluke381.html

1st reading last year

And late last year I bought a smaller range clamp meter from UNI-T brand, low cost but very accurate. It can read milliamps resolution. I don't choose Fluke brand this time ( I love Fluke ), because someone tested this low cost unit well.
This unit : https://www.eevblog.com/forum/testge...-uni-t-ut210e/

During a discussion last week about batteries, in this forum, someone suggested that 0.4 amps or 400 milliamps is way too much.
So I was googling for : "What is the normal highest drain for a W212 of 2014 with keyless GO ?". There is no specific data, but 100 milliamps is considered as high as it can go , for NORMAL condiition,
as per the attached MB PDF. The only specific data I got on equipment sleeping power consumption is from BenzWorld but no Keyless GO, I also attached.

So I tested again the stand-by current drain, this time with the higher accuracy low amperage unit from UNI-T. 310 milliamps I got and the Fluke got 0.6amps.
Yellow color clamp behind the red UNI-T is the Fluke.

2nd test last week

I then use 310 milliamps as the total drain and potential 210 milliamps of rogue drain.
So I went to test all the fuses for millivolts drop to find approx 210 milliamps unknown drain ( for a 7.5 amps fuse ATOC type, I need to see 2.1 millivolts drop ) and almost all fuses I can test,
except some where I don't have clearance for my probes, about max 5 fuses only. I could not find the supposedly 210 milliamps drain.

I don't like this route, but its a final confirmation for higher accuracy. I even rigged jumper wires before removing negative battery cable as to not have power loss occur to the car computers,
so that I do not need to wait too long to test to wait for CAN/SAM to sleep.

What reading do I get ? Holy crap, its only 5.5 to 6.6 milliamps draw !! Where did I go wrong and got 310 milliamps with current clamp meter?

Using multimeter milliamps function

So what happened actually ?
DC low current measurement under 100 milliamps is a pain if there is any metal within 30cm from the current clamp. The car ferrous metal will generate enough magnetism ( try with a compass if you want to see )
to upset the current clamp reading....which is actually reading magnetic field of the current passing thru a wire. I been using this baby UNI-T low amperage clamp with great accuracy , but on low AC current and
not DC current. AC current alternating magnetic field reading is easier to get less interference in <100 milliamps value and I do not have massive metal like a car engine bay when doing AC current test.

In alternating current low amperage test, if surrounding magnetism DOES effect it and your current clamp is stationary...... it can be seen as jumpy reading.
In a DC current milliamps test, the sensor of the clamp it is reading non changing magnetic field due to a DC current and any metal magnetism close by depending on it is being south or north pole oriented,
can produce the massive 300 milliamps error I am getting.

A digital multimeter milliamps reading to 400 mA or up to 10 amps reading is not using magnetic field, but special high accuracy resistor, where the voltage drop in millivolts across the resistor is measured and translated
into milliamps reading.

Reading a wire carrying a very low DC current even when a distance away from a metal/magnetic field is also not that accurate, due to how "center" can we position the current carrying wire
inside the clamp jaw. See below, that is a 6 to 23 milliamps reading depending how I run the cable inside the jaw and this is already about 30cm away from engine or engine bay metal.

No matter how small a current clam meter jaw is physically, example this one : https://www.picoauto.com/products/cu...-current-clamp
If it is sensitive enough to read such a low <10 milliamps current flow magnetically from a wire, it will be also effected by any metal close by.

So becareful when using low current capable clamp meter for very low DC current drain, other than if you can distance the jaw at least 30cm from any ferrous metal, and be ready for up to 20 milliamps error too
based on my UNI-T.

Attached Files
File Type: pdf
battery drain.pdf (398.6 KB, 4 views)
S-Prihadi is offline  
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Old 04-29-2019, 02:47 PM
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Join Date: Jan 2004
Location: Seattle WA
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99 E320
ha ha, i bought that same chinese clamp meter a couple of months ago (still unused). i also have several
other Fluke devices. they're great when I can grab something within my price range. Fluke HQ is a few
miles from my garage
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