Calling electrical gurus: accurately measuring parasitic draw

[JohnK]

I have a battery that has gone flat on me a couple of times and I'm trying to determine if I have a parasitic draw problem. Ideally, I'd like to get this sorted before I put the body back on for the final time, as that will make accessing wiring far more complicated. Here's what I've done so far:

I purchased a Klein CL390 (https://www.kleintools.com/catalog/clamp-meters/acdc-digital-clamp-meter-auto-ranging-400-amp#Specifications) digital clamp meter. The meter has two ranges for testing current - 40A and 400A. I put it in 40A mode and set it to DC. When I put it in DC mode, just holding it in my hand standing in the garage, it reads ~200mA current draw (bouncing from 180mA - 220mA, or "0.18 to 0.22" on the display). The instructions mention this, and say:

NOTE: If non-zero values are displayed prior to measuring in DC current mode, a DC zero offset correction is required. With meter in DCcurrentmode,presstheREL/ZERObutton 8 toactivatetheDC current ZERO function. ZERO icon will be present on the display. Subsequent DC current measurements automatically subtract the offsetcorrectionforimprovedaccuracy.PressREL/ZERObutton 8 to return to normal measuring mode.

So I press the REL/Zero button, which zeros the reading and it's now bouncing +/- "0.02", or +/-20mA. I then put the clamp around the positive battery terminal and get a reading that bounces around 0.04 to 0.06 or 40mA to 60mA. I did some testing, turning on seat heaters, interior lights, etc. and those readings all seem normal. Incidentally, the Cobraheat heaters seem to draw about 3.5A each in low heat mode and about 6A each in high heat mode. When I switch everything off, I'm back to a reading of "0.04" to "0.06", or 40mA to 60mA. Of note, when I turn off the master cutoff switch (which powers the RF harness) the parasitic draw number stays the same, which I'm interpreting to mean that the bulk of this parasitic draw is from the coyote PCM and not from things powered by the RF harness.


My questions for those that are experienced with this:
- is this clamp meter even appropriate (accurate enough) to be trying to measure a parasitic draw in the mA range?
- Is Klein's recommendation to do the relative zero correct? I don't want to be inadvertently ignoring what amounts to ~200mA of parasitic draw.
- Does my process and conclusion seem correct - that with everything off I have a parasitic draw of roughly 40mA to 60mA? If so, I think I'm OK and don't have an issue.

I'm open to all comments, suggestions, criticisms of what I've done woefully wrong, etc.

Thanks!

[narly1]

You are on the right track questioning the accurracy of your meter.

I took a look at the data sheet for it online and they don't state anything. Probably because in it's intended use by electricians, they don't care/need to know any better than maybe +/- 0.5 A in 40A.

Think of it this way: On the 40 ampere scale, 1 milliamp (one 1-thousandth of an ampere) represents only 1/40,000 or .0025% of the range. So probably not the best meter choice if you want to accurately measure in the 0 to maybe 500 mA range. (If you had 500 mA of parasitic drain, you'd have significantly noticable battery draining issues IMO).

So given that your clamp on meter is reading ~200 mA it would be safe to disconnect your battery positive cable, connect a digital multimeter in series and read the current draw far more accurately (even the cheapest DMM's are good to at least a couple of amperes).

A word or caution, don't start turning things ON, at the very least you'll blow a fuse in the DMM or at the worst burn out it's current shunt circuit due to current overranging.

If anything you would want to do the reverse: start pulling fuses 1 at a time until you find the culprit (current drawing) circuit. Then you can go looking at the components in that circuit for issues.

Any other questions just ask.

Earl

[Nigel Allen]

G'day John,

Exactly what Earl says above. However you can increase the accuracy / resolution of your clamp meter using the following method (I made a short video to explain it better, hopefully. Gotta admit, being a youtuber is not my not my forte)

https://youtu.be/Zk0FyMJLio0

Try the following:
- Grab a 1 metre length of wire that you can pass the necessary current through.
- Wrap 10 turns through your clamp meter.
- Set it on the lowest DC range (lowest setting)
- Zero your clamp meter. It is important to do this only when there is no NO LOAD connected.
- Remove your negative battery clamp and place this temporary wire between the clamp and the battery post.
- Then follow Earl's guide - start pulling fuses 1 at a time until you find the culprit (current drawing) circuit. Then you can go looking at the components in that circuit for issues. If you still have leakage, then it may be in the wiring or connectors, so you may need to disconnect a few items.

x2 on using a digital meter, set to measure DC current. As you can see from the video, the accuracy is far greater and less likely to lead you astray.


Feel free to reach out if you have any questions.

Cheers,

Nigel

[JohnK]

Thank you both very much! That's very helpful.

[narly1]

Try the following:

G'day Nigel,

Thanks for reminding me about the current transformer trick...I'd forgotten about that. Given that the lowest range on that Klein meter is 40A and that you can buy a cheap DMM that reads down into the milliamp DC range for about $10 here in N America, I kinda just jumped to that reccomendation.

Also what is your thinking re putting the meter on the -ve side ove the battery vs the +ve?

Less chance of an inadvertant short maybe?

Earl

[Nigel Allen]

Hey Earl,

I totally agree with you, the DC current range on a multimeter is the go. Just highlighted the clamp meter idea in case it was the only tool available.

You are right regarding making temporary connections for metering on the negative terminal. Purely less chance of shorts.

You clearly have a good grasp on electrical. Is that your background?

Cheers,

Nigel

[jab351w]

Not much to add as Nigel and Earl have already given some great advice for tracking down any parasitic draw. Will just add that if 40-60 mA turns out to be the real steady-state drain that's not an unreasonable value for a modern PCM. Also note that the value can vary depending upon how recently the engine was shut down, as some systems take time to power down after key off. So it might make sense to try taking a reading after the car has been idle for at least few hours.

[Mastertech5]

I'll just throw my 2 cents in. All said above by these knowledge folks is correct. You want to put an amp meter in series with your positive battery post to the cable and pull fuses as stated. The part I want to add is a normal parasitic draw for a computer controlled vehicle is about 25 milliamps, more or less. Cars with a Body Control Module can take up to 20 minutes to go to sleep before you can get an accurate reading. If your car sits for any length of time, like going on vacation for a month it may draw down to a point where it may not start if your battery is not up to snuff. That's why battery tenders are used for storage purposes.

[narly1]

Hey Earl,


You clearly have a good grasp on electrical. Is that your background?



Nigel

Guilty as charged. I got my ham licence back when I was in high school. After that I did a 3 year electronics course at my local community college.

Started off fixing land mobile radios, then signed on with Varian testing klystons and later millimeter wave microwave R&D with them. Then I jumped into electronics components sales and bounced around between a few different companies. These days I'm working selling measurement sensors and instrumentation products.

So I'm not as hands-on as I used to be, but I still have enough knowledge to be dangerous LOL.

Earl

[JohnK]

Thank you all again for your input. My trusty ~30 year old Fluke 70 series II DMM was a bit too basic and didn't have current measuring capability, so I guess it was time to upgrade to a new Fluke 115 DMM :) I also picked up this (https://www.amazon.com/General-Technologies-Corp-CM100-Current/dp/B001TCWL1E/ref=sr_1_1?crid=1HN72COQW6J3D&keywords=General+Technologies+Corp+GTC+CM100&qid=1670889716&sprefix=general+technologies+corp+gtc+cm100+%2Caps %2C249&sr=8-1&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0) clamp meter by GTC that gets good reviews on Amazon and elsewhere, and claims to have 1mA resolution.

I started off with the easier test first. I zeroed the GTC clamp meter and put it on the positive battery terminal:

https://thefactoryfiveforum.com/attachment.php?attachmentid=176694&d=1670889474

OK, cool. I'm happy with that number. I turned on accessories on and off, and when everything was off again the reading settled back to the same exact number.

Time to validate this with the DMM. I disconnected the negative battery terminal (thank you for the suggestion to test on the - to reduce the possibility of accidentally shorting the +) and then connected the DMM.

https://thefactoryfiveforum.com/attachment.php?attachmentid=176693&d=1670889474

The DMM has a 10A max load so I didn't turn on any heavy draw items like seat heaters but I did "wake up" the accessory circuit with the Digital Guard Dawg keyless ignition in accessory mode. Everything turned on as expected, and when off again the DMM settled on the same number.

So I'm pretty confident that my parasitic draw is ~36mA. I'm calling that good enough, and don't believe I have any further issues I need to track down unless someone feels there are more tests needed?

Thanks agin,
John

Edit: Just for fun, I tested my '67 Mustang with carb'ed 351W. The only things with any parasitic load are the radio memory and the clock in the dash. Parasitic draw on that car measured 9mA. :cool:

[narly1]

You can't go wrong with Fluke DMMs.

They are the gold standard for ths type of measuring instrument.

For most car guys who are of "buy it once" thinking. Fluke is the way do go.

I still have my old 8022A that I got as a salesman's demo back when I was in high school....and now I'm almost ready to retire!


Earl

[JohnK]

Earl, I agree 100% regarding Fluke DMM's. My 70 Series II is still working perfectly after 30 years. It just didn't have the features I needed.

Thanks again for all your help.

-John

[LateApex]

Another anecdote re: parasitic draw. I owned a 1984 BMW 633 CSI, the precursor to the M-series - the "Shark". BMW was one of the first to install a trip computer in the dash. This had all sorts of now obviously useless features, like prediction of when one might run out of fuel. But the parasitic draw in that system was related to the battery to maintain the clock setting in that computer, such that the clock setting would be preserved if for example the battery was disconnected during service. If you didn't drive the vehicle weekly, you would drain the battery.

Others had issues with BMW features, such as rear view mirror defrosters, which would switch on automagically when the temp dropped below freezing. Nothing like a heating coil to draw down a battery.

I have made mistakes like hooking a radar detector to an unswitched 12V source. Only 30 mA required, but if you left the detector on while you parked a car at the airport for a few days, Ta-Da! Need a jump.

There have been other posts on the FFR forum related to parasitics with items like ECU's. I run a Holley TerminatorX, which had been indicted at one time for these sins, but my experience does not reflect any problem. When the switch is off, the ECU is dead. IMO, when the switch (key) is off, there should be no power to any circuit.

Of course, there are pop locks and anti-theft devices and other desired features that conflict with that design point ...

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