# 12VDC, Scope and Shunt.

Discussion in 'The Projects Forum' started by Martin Koen, Mar 11, 2014.

1. ### Martin Koen Thread Starter New Member

Mar 11, 2014
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0
Hello All.

This is a wonderful community and I enjoy the opportunity to contribute.

Primary objective: Measure starting current drawn from automotive battery in automotive application in order to inform design and development of 12VDC batteries and related systems.

Secondary objective: Measure current draw on 12VDC battery that triggers Battery Management System (BMS) to shut off (close) draw from battery, in order to assess batteries for technical specifications and monitor stock to meet specifications.

Equipment at hand.
Some knowledge of electronics, physics and chemistry.
Large assortment of varying styles and capacities of batteries.
Hantek DSO1000 Series Handheld Scopemeter
'Heavy Duty' Shunt 200A, 50mV
Globe automotive, 55/60W 12VDC
Wire, etc.

Unfortunately I have little experience using a scope, even less using a shunt. I wanted to create a circuit where I measure the current spike connecting the globe to the battery, a somewhat miniature of my two objectives.

My diagram of miniature.

Shunt specs
Max. current 200A
Voltage at max. current 50mV
Shunt resistance 0.25mOhm
Accuracy 0.5%

Measuring mV across shunt I get 11.4 mV (by across shunt I mean -ve probe to battery side little screw, +ve probe to little screw globe side, with switch closed, globe lit.) Probes used for multimeter on scope.

Measuring Ohm's across shunt I get 0.1 Ohm's with switch open.
Measuring Ohm's across shunt I get 4.9 Ohm's with switch closed.

Ohm's Law states Current = Volts / Resistance
Current = 0.0114 / 4.8
Current = 0.0024

Is this correct? How can I confirm my findings?

I have read that I need to measure the difference across the shunt, by connecting one channel to the battery side of the shunt and one channel to the load side of the shunt. Each channel has the sharp measuring probe and the alligator clip (ground). Apparently then I can subtract one channel from the other in order to more accurately measure the voltage difference across the shunt.

How do I connect these channels in terms of probes and grounds, either side of the shunt to measure mV?

How do I then effectively calculate current? How can I cross reference my findings to ensure accuracy?

Thank you in advance for your contributions on my miniature and objectives on my project.

2. ### inwo Well-Known Member

Nov 7, 2013
2,433
315
Measurements don't seem right.

Are you sure it's not 1.4mV?

=5.6 amps which is close to expected 5amp reading. (at least closer than 50 amps)

Resistance checks aren't accurate or needed. .25milliohm not easily measued.
You have the shunt specs. ie. .00025ohm
And the amp/mV of 200/50=4

For accuracy there may be a better way.
For any measurements I've used, one channel across shunt as you've done, does it.
I doubt any cross check that you do will be as accurate as the shunt that you are using.

Someone more experienced in accuracy may correct me on figuring in the probe impedance.
Seems splitting hairs for your purposes.

I'm assuming this is not testing in a vehicle. Or thread may be in jeopardy!

Last edited: Mar 11, 2014
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3. ### #12 Expert

Nov 30, 2010
16,313
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No, 200 amps wouldn't run a 6 cylinder starter motor. It isn't a car unless it's a small one.

Measuring across the shunt? No, you're measuring the resistance where the leads plug into your meter. Pay no attention to the man behind the curtain.

You don't even need 2 channels. If you do that, you get the error of both channels plus the error in your ability to read the display. One side of the shunt IS ground for this measurement. One channel, 2 wires, probe and common. Put them across the shunt. Measure voltage.

and, where is this, "switch"? Can you make a drawing for us?
Are you trying to read ohms across the shunt with the globe powered?

It's the old, "Everything is simple...right after you know how".
We know how. Just keep helping us get you organized.

Last edited: Mar 11, 2014
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4. ### Martin Koen Thread Starter New Member

Mar 11, 2014
7
0
Wow.

Thank you inwo

Thank you #12

This is my circuit diagram. Link in initial post worked as well as my initial measurements.

You were right in your assumptions of my fault inwo.
Measurement was out by one significant figure. On taking measurement again it was 1.2mV. That was taken with a normal multimeter so I would not confuse the issue with too much technology. Possibly the 'starting current' was closer to 11.4mV, I will work towards that result now.

So the current draw on the globe was 4.8 Amps.

I think its important to mention the shunt is a 'precision resistor'. I guess I was hooked up on the idea that the resistance would change as the current and voltage changed. Not sure where I got that idea. Using the actual variable, the voltage, I am more confident in the results.

Yes #12 I measured the resistance with the globe powered. In fact I measured it with and without the globe powered and calculated the difference. Now that I am aware of the 'man behind the curtain' I can ignore his existence with a little more comfort.

The 200A shunt is used in my miniature only. I need to gain experience in taking these measurements and creating these circuits so that I can build up toward accomplishing the primary objective. If I purchased a much larger shunt that I would use in my final build, it would be to cumbersome to measure my small voltage difference across this range. Also this 200A shunt will suit perfectly on my second battery as an AH monitor in my dual battery system in my 4WD, once it becomes superfluous.

Thank you both again for your 3rd person perspectives, they have directed me in the right path, indicated obvious flaws and introduced new considerations in to my ways of thinking.

Next I will be capturing the voltage jump on start (inrush? or starting current?) of the miniature circuit.

Incidently my switch is an alliagtor clip with wire twisted around the battery handle, the clip has a battery terminal cover slipped over the top which I use to push the clip on to the terminal. It gives a nice, unique jump in voltage each time its closed. I am trying to mimic the relay switching to start the motor from my primary objective.

5. ### inwo Well-Known Member

Nov 7, 2013
2,433
315
Looks like you're on the right track. Not as complicated as it first seemed.

I wouldn't worry to much about overloading the shunt with starting current. At least not for 1 or 2 seconds with a cool down cycle.

500 amps is 62.5 watts only.
Do you have a picture of actual shunt size? To estimate dissipation.

If it isn't hot the accuracy is not being effected much. It all depends on how close you want to get.

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6. ### #12 Expert

Nov 30, 2010
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Repeat after me: Never measure resistance with power on.
The voltage in the circuit plays havoc with an ohm meter.

Your drawing shows an error. Cut the wire that is in parallel with the shunt. I know it's called a shunt, but that doesn't mean shunt for the wire, it means shunt for the meter.

Be careful not to burn your finger pressing on the, "switch".

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7. ### #12 Expert

Nov 30, 2010
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You said a dirty word.

We are not allowed to discuss automotive modifications on this site.
As far as I'm concerned, this thread is closed.

8. ### inwo Well-Known Member

Nov 7, 2013
2,433
315
I could be wrong, but I think he mention using his left over parts in a vehicle.

His testing is about "order to inform design and development of 12VDC batteries and related systems." From first post.

I may be naive!

9. ### #12 Expert

Nov 30, 2010
16,313
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Me too. I think we won't get a personal note from the teacher because this is miniature scale experimentation, but as soon as he says, "car" I have to back up so I don't get my knuckles rapped.

10. ### Martin Koen Thread Starter New Member

Mar 11, 2014
7
0
First off:
Never measure resistance with power on.

hahaa!
Yes my project is to create the ability to collect data for the creation of battery management systems within the 12V DC environment. I am, from here on in, told and warned of how my project may straddle the fence containing discussion within this forum.

#12
I did create an error in my drawing. I was not aware of how to represent a shunt in a wiring diagram. I will recreate my diagram to show the shunt on the wire with the meter/scope in parallel. Thank you for pointing this out.

Also I have already burned my fingers on this test rig, when I shorted the battery with my scope grounds. They became very hot very quickly and melted off the insulation. I removed them as quick as possible as the alligator clip instantly welded onto the terminal at the short. Not considering the ramifications I was concerned with the scope as it is not the cheapest piece of equipment available. I now have 2 white lines on my forefinger and thumb, a lesson that loose wires and exposed terminals are not the best way to create circuitry. Hence the battery cover to insulate my pretty little piggy's from the big bad current man...

I remade my probe grounds and used heat shrink to double insulate the leads. I also used larger alligator clips and surrounded them with heat shrink to the teeth. I was not surprised that the probes needed recalibration.

inwo
You are again correct. Not as difficult as the unknowing mind can create. I will capture an image of the shunt and the test rig to show you guys. It isn't pretty and it will contain 2 components from those things we do not talk about but it is effective in that it is helping me lightly tread the path toward my final goal.

Thank you both again for your assistance, I would not have come so far with out it.

Many thanks.

11. ### Martin Koen Thread Starter New Member

Mar 11, 2014
7
0
This is the test rig;

Here is the shunt:

More detail of the shunt:

12. ### Martin Koen Thread Starter New Member

Mar 11, 2014
7
0
You can see above the blue terminal cover acting as a switch with the alligator clip underneath.

I also put the 60W 12V globe behind the box in the upper left corner, in case the filament became a little too excited and tried to exit its glass housing. I'm sure something like this would only occur in my imagination.

I have since created an input and output lead for the shunt from 00B&S or 00 gauge wire and some 8MM terminal eyelets. On one side I attached the female negative terminal of a 12V battery and on the other side I attached the post of a negative terminal, as the shunt will be connected in series on the negative post of a 12V battery. More pics of the shunt in battle dress ready to attack 'primary objective' tomorrow.

Last edited: Mar 12, 2014
13. ### Martin Koen Thread Starter New Member

Mar 11, 2014
7
0
Here is my amended circuit diagram.

14. ### #12 Expert

Nov 30, 2010
16,313
6,817
That will work. Trust the shunt. It is more accurate than your meter.
That is, until you wreck it with an overload.
It's guaranteed to survive 200 amps, and even a short burst that is a lot higher. They are really pretty tough, but you can destroy it if you try.
The paint is there to protect it against corrosion (which will change its accuracy) and to give warning that it is melting. If the smoke escapes, you made a mistake, and you don't have equipment good enough to re-calibrate it. Try to keep your abuse time down to a few seconds and let it cool off until you can't feel any warmth with your fingers.

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15. ### Martin Koen Thread Starter New Member

Mar 11, 2014
7
0
Thanks for the tips.

I used the shunt in line on the earth cable of a starter for a small internal combustion engine. The shunt measured a wave with an upper limit (flat lining) at 40mV which I anticipated as being the maximum difference across the shunt, although its stated as being 50mV, in my calculations do I interpret this upper limit as being 160 Amps or as 200 Amps? I fear I have done something wrong, however I have ordered a 1000 Amp shunt in order to capture the current draw in its entirety.

I also used the shunt to measure current drawn from a Lithium Ferro Phosphate battery, 12V 200AH. The BMS was cutting out unexpectedly and needed to be assessed. I received excellent (consistent) results and calculated them using the 4 Amps per millivolt ratio as advised above. No response from manufacturers yet so no news is good news!

16. ### inwo Well-Known Member

Nov 7, 2013
2,433
315
There is no maximum difference across the shunt.

Current peaked at 160 amps. Which seems reasonable.

Over 200 amps, or more than short tests, will heat the shunt and affect accuracy.

What is BMS?

Last edited: Apr 10, 2014