Amp clamp and DC current measurement on multiple conductors?

[TT_Vert]

I have a battery that I am measuring current to. It has 3 conductors. I assume I would have to total the sum of all 3 conductors to the + side of battery to measure total current correct? I also noticed that the sum of those is fairly close to putting the amp clamp around all 3. I have always been told you cannot do that but perhaps that is strictly for AC? Also, any idea why current draw would be different positive terminal to negative terminal?
Thanks
Dave

 

[nadogail]

Sometimes we are told things that seem logical but contradict what we can measure.

I would take the higher of the two readings.

 

[TT_Vert]

They are w/in a few mA. With that said any idea why current on positive side would differ from negative side? It's an RV w/ multiple battery banks and the coach side is seeing about 200mA on the positive side and 300mA on the negative.
Dave

 

[theoldwizard1]

Amp-clamps are not super accurate. I would not expect to be more than +/- 10% of a reading using a precision shunt.

Also, the physics/electronics behind measuring AC current vs DC current is VERY different. Not to many years ago, DC clamp meter were almost non existent. There was a clamp device sold by Tektronix in the 70s-80s. It had a large box to go with the small clamp. $1,000s !

 

[haveissues]

Amp-clamps are not super accurate. I would not expect to be more than +/- 10% of a reading using a precision shunt.

Also, the physics/electronics behind measuring AC current vs DC current is VERY different. Not to many years ago, DC clamp meter were almost non existent. There was a clamp device sold by Tektronix in the 70s-80s. It had a large box to go with the small clamp. $1,000s !

I have found my chepo amp clamp to be significantly more accurate than 10% on DC.

 

[TT_Vert]

I have found my chepo amp clamp to be significantly more accurate than 10% on DC.

Amp-clamps are not super accurate. I would not expect to be more than +/- 10% of a reading using a precision shunt.

Also, the physics/electronics behind measuring AC current vs DC current is VERY different. Not to many years ago, DC clamp meter were almost non existent. There was a clamp device sold by Tektronix in the 70s-80s. It had a large box to go with the small clamp. $1,000s !

I may pull the terminals and use my DVOM inline and compare also. I'm just curious if I could bundle like polarity conductors during this test. My gut would say yes as the meter would be inline just before all 3 of them anyway but I've always been told no.

It's a bit more complicated on the coach battery side as i have 2 6V in series and 2 12V in parallel in that battery bank.

Dave

 

[haveissues]

I may pull the terminals and use my DVOM inline and compare also. I'm just curious if I could bundle like polarity conductors during this test. My gut would say yes as the meter would be inline just before all 3 of them anyway but I've always been told no.

It's a bit more complicated on the coach battery side as i have 2 6V in series and 2 12V in parallel in that battery bank.

Dave

Yes, if you want the total current drawn on the battery you would either have to clamp all the conductors on the positive side or all the conductors on the negative side. Or put your dvm inline the same way.

 

[theoldwizard1]

My gut would say yes as the meter would be inline just before all 3 of them anyway but I've always been told no.

As long as all conductors go to the same source.

It's a bit more complicated on the coach battery side as i have 2 6V in series and 2 12V in parallel in that battery bank.

NOT a good practice. Battery banks should utilize the same size/type of battery for best performance and life.

 

[TT_Vert]

Yes, if you want the total current drawn on the battery you would either have to clamp all the conductors on the positive side or all the conductors on the negative side. Or put your dvm inline the same way.

That's what I think/thought but the EEs tell me it won't work that way.

 

[nadogail]

IMHO, 100 milliamps (.1 amp) should be within the margin of error for a Clamp on Amp meter.

 

[TT_Vert]

As long as all conductors go to the same source.

NOT a good practice. Battery banks should utilize the same size/type of battery for best performance and life.

It actually is, and has been a standard practice in RVs for a long time from what I can tell from seeing/reading in the RV forums I'm part of. Not to say it's a good SOP because it isn't but it seems to be a tried and true method. They're all 12V lead acid batteries but yea the capacities are a bit different obviously. I would never mix an AGM w/ a sealed battery or a LifePo4, etc. w/ anything else. My charge controller has done a decent job so far of keeping these batteries going. Just was doing calcs to determine how much standby I have with my ~185Ah of usable capacity.

The two 6V net a capacity of 225Ah at 12V (Each 6V battery is 225Ah) and the other two in that bank are 75Ah batteries.

 

[haveissues]

If you had 3 conductors and only 3 conductors going to the same terminal on a battery I can't imagine why clamping all together wouldn't measure the total current but it is easy to verify. Turn on a static load and see if the sum of measuring each wire individually adds up to the same reading you get if you clamp all of them together.

 

[mark-NJ]

Unless somehow Kirchoff's first law has been corrupted, your total current readings in & out should....indeed, must...sum out to zero.

 

[TT_Vert]

Unless somehow Kirchoff's first law has been corrupted, your total current readings in & out should....indeed, must...sum out to zero.

Do you mean the negative and positive wires? They certainly are different for some reason. I am ensuring the clamp is zeroed beforehand also.

 

[mark-NJ]

The current you read on the battery neg *must* equal the current on the pos. Kirchoff's first law says so: All current into & out of a "node" equals zero. If that's not what you're measuring, I'd first question your meter.

Think about it: if 10A went "out" of your battery but only 9A came back, where did the "lost amp" go? All of the current is referenced to your battery; how would it "know" to go anywhere else?

 

[American Locomotive]

That's what I think/thought but the EEs tell me it won't work that way.

DC clamp meters use something called a hall-effect sensor to measure the magnetic field strength. The magnetic field strength is proportional to the current going through the wire. Magnetic fields are also additive. So if you have one with two wires, going the same direction with 0.5A going through them, they create the same effective magnetic field strength as 1 wire with 1A going through it.

Not only are magnetic fields additive, but they are direction and subtractive as well. This means they can cancel out. If you bent a single conductor into a "U" shape loop, and clamped over the middle of it - it would read 0 amps. This is because the wire sets up a magnetic field going one direction when it first enters the clamp, and when it turns around, it creates a magnetic field going the opposite direction. The same thing would happen if you clamped on the positive and negative wire to a device - cancels out.

AC clamp meters use a current transformer, but the effect is still the same. That is why you can't just clamp onto a cord and measure the current. It's basically a giant loop, and the hot and neutral wires cancel out because they're going different directions.

Basically, as long as all of the wires are coming from the same source, and going the same direction - you can clamp over all of them and get a summative current measurement.

 

[TT_Vert]

DC clamp meters use something called a hall-effect sensor to measure the magnetic field strength. The magnetic field strength is proportional to the current going through the wire. Magnetic fields are also additive. So if you have one with two wires, going the same direction with 0.5A going through them, they create the same effective magnetic field strength as 1 wire with 1A going through it.

Not only are magnetic fields additive, but they are direction and subtractive as well. This means they can cancel out. If you bent a single conductor into a "U" shape loop, and clamped over the middle of it - it would read 0 amps. This is because the wire sets up a magnetic field going one direction when it first enters the clamp, and when it turns around, it creates a magnetic field going the opposite direction. The same thing would happen if you clamped on the positive and negative wire to a device - cancels out.

AC clamp meters use a current transformer, but the effect is still the same. That is why you can't just clamp onto a cord and measure the current. It's basically a giant loop, and the hot and neutral wires cancel out because they're going different directions.

Basically, as long as all of the wires are coming from the same source, and going the same direction - you can clamp over all of them and get a summative current measurement.

Thank you. Yes that is what I was thinking myself. And I know I can increase the resolution of my meter by looping the wire x amount of times, etc. No idea why my positive and negatives are different. Perhaps because the batteries are series parallel?

Thanks
Dave

 

[American Locomotive]

Thank you. Yes that is what I was thinking myself. And I know I can increase the resolution of my meter by looping the wire x amount of times, etc. No idea why my positive and negatives are different. Perhaps because the batteries are series parallel?

Thanks
Dave

That won't matter. As long as you are clamping around ALL of the negative wires, the measured current should be exactly the same as the positive. Try flipping your clamp meter around and see if the reading better reflects that of the positive wire.

 

[wyliesdiesels]

I have a battery that I am measuring current to. It has 3 conductors. I assume I would have to total the sum of all 3 conductors to the + side of battery to measure total current correct? I also noticed that the sum of those is fairly close to putting the amp clamp around all 3. I have always been told you cannot do that but perhaps that is strictly for AC? Also, any idea why current draw would be different positive terminal to negative terminal?
Thanks
Dave

you cant meter 2 conductors that carry opposite phases as they cancel each other out.

 

[wyliesdiesels]

Amp-clamps are not super accurate. I would not expect to be more than +/- 10% of a reading using a precision shunt.

Also, the physics/electronics behind measuring AC current vs DC current is VERY different. Not to many years ago, DC clamp meter were almost non existent. There was a clamp device sold by Tektronix in the 70s-80s. It had a large box to go with the small clamp. $1,000s !

where do you come up with these things?

my fluke 381 is 2% for AC current with the jaw, 2% for Dc current w/ the jaw and anywhere from .5%-3% with the flexible probe. far far better than the 10% you claim

 

[PCustoms]

The current you read on the battery neg *must* equal the current on the pos. Kirchoff's first law says so: All current into & out of a "node" equals zero. If that's not what you're measuring, I'd first question your meter.

Think about it: if 10A went "out" of your battery but only 9A came back, where did the "lost amp" go? All of the current is referenced to your battery; how would it "know" to go anywhere else?

Isn't the 1A "loss" what is powering the equipment?

 

[Innovate1]

Isn't the 1A "loss" what is powering the equipment?

No. It doesn't work that way with current. Current in a loop remains constant. The total current and the voltage across the load provides voltage x current = watts of power. (for DC. For AC there is power factor that must be included in the calculation). The current in and out of the battery must be equal. Only case I can think of that it wouldn't be is if the case is cracked and leaking on the frame of the vehicle so some current is going in or out of the leaky point to the frame but that's not likely.

You mentioned 3 connections for the battery. Is that ALL the connections or just the positive or negative? If you actually put the clamp around all the connections (both positive and negative) you should get zero. Otherwise you meter isn't giving good readings.

Do you have multiple batteries in series or parallel? There may be other current paths you aren't accounting for.

 

[mark-NJ]

current isn't lost. Again, Kirchoff's law...

 

[American Locomotive]

Isn't the 1A "loss" what is powering the equipment?

Think of it like hydraulics, and currently being roughly equivalent to flow rate. If you have 5 gallons per minute going into a hydraulic motor, 5 gallons per minute must come out.

 

[PCustoms]

Think of it like hydraulics, and currently being roughly equivalent to flow rate. If you have 5 gallons per minute going into a hydraulic motor, 5 gallons per minute must come out.

Yeah, I wasn't awake yet when I questioned it.

The hydraulic analogy is perfect

 

[ycgoat]

I was thinking IR^2 heat losses, but the current in should = current out

 

[Showkey]

IMHO, 100 milliamps (.1 amp) should be within the margin of error for a Clamp on Amp meter.

Problem ……OP was measuring 300ma and 200ma ……….not 300 and 200 AMPS

 

[Innovate1]

I was thinking IR^2 heat losses, but the current in should = current out

That's actually I^2R. IR is the voltage drop across the resistance. That times current (I) again gives watts loss.

 

[RPH]

Get a DC shunt. Install in series and read the mV on your meter. I have them from one amp to one thousand amps. They are handy to have but can be a bear to hook to. These are very accurate when measuring DC current. Price isn’t bad from $20 and up depending on size.

 

[rlitman]

where do you come up with these things?

my fluke 381 is 2% for AC current with the jaw, 2% for Dc current w/ the jaw and anywhere from .5%-3% with the flexible probe. far far better than the 10% you claim

According to Fluke, your AC current via jaw accuracy is 2% +/-5 digits between 10 and 100 Hz. AC current via the flex cable starts at 3.5% at the center of the loop, and rises to 5% at the edge. You left out the 3% starting point on the page above.

2% is pretty typical for a split CT. Closed loop CTs can be had in "revenue grade" at 0.3% accuracy and better, but closed loops are inconvenient for test purposes.

I have found my chepo amp clamp to be significantly more accurate than 10% on DC.

What's a chepo DC amp clamp, and how have you corroborated your findings?

“A man with one watch always knows what time it is. A man with two watches is never sure.”

As part of a PM operation I perform at work, I clamp meter series strings of batteries to watch DC charging current. A contractor performing the maintenance uses his own meter, and I will clamp mine at the same time to be certain things are good. They NEVER agree on the inrush current (the typical difference is in the neighborhood of 25%). To be fair, the meters seem to sample at different rates and the current is not static, and they usually come to within 10% of agreement after the current settles, but I have little faith on what amounts to not much more than a number on a display.

For the record, my contractor uses a Fluke 378 with the same 2% accuracy specs as the 381 above, but in the past he's also had a Klein (I'm not a fan). I'm using an Amprobe ACDC-52NAV, with a stated accuracy of +/-1.5%, and what appears to be a faster sampling rate (I always see higher peak readings).

 

[ycgoat]

Let

That's actually I^2R. IR is the voltage drop across the resistance. That times current (I) again gives watts loss.

sorry my fault

 

[TT_Vert]

Get a DC shunt. Install in series and read the mV on your meter. I have them from one amp to one thousand amps. They are handy to have but can be a bear to hook to. These are very accurate when measuring DC current. Price isn’t bad from $20 and up depending on size.

If this was for my solar system where I always monitor current I would. This is just to get an idea of idle current draw to determine how much time I have before I drain these batteries when not on shore power and RV is idle.

 

[haveissues]

According to Fluke, your AC current via jaw accuracy is 2% +/-5 digits between 10 and 100 Hz. AC current via the flex cable starts at 3.5% at the center of the loop, and rises to 5% at the edge. You left out the 3% starting point on the page above.

2% is pretty typical for a split CT. Closed loop CTs can be had in "revenue grade" at 0.3% accuracy and better, but closed loops are inconvenient for test purposes.


What's a chepo DC amp clamp, and how have you corroborated your findings?

“A man with one watch always knows what time it is. A man with two watches is never sure.”

As part of a PM operation I perform at work, I clamp meter series strings of batteries to watch DC charging current. A contractor performing the maintenance uses his own meter, and I will clamp mine at the same time to be certain things are good. They NEVER agree on the inrush current (the typical difference is in the neighborhood of 25%). To be fair, the meters seem to sample at different rates and the current is not static, and they usually come to within 10% of agreement after the current settles, but I have little faith on what amounts to not much more than a number on a display.

For the record, my contractor uses a Fluke 378 with the same 2% accuracy specs as the 381 above, but in the past he's also had a Klein (I'm not a fan). I'm using an Amprobe ACDC-52NAV, with a stated accuracy of +/-1.5%, and what appears to be a faster sampling rate (I always see higher peak readings).

A cheap uni-T clamp meter. I verified it against a calibrated keithley 6.5 digit dmm and it was close, way better than 10% on dc, even on the mA range.

 

[TT_Vert]

A cheap uni-T clamp meter. I verified it against a calibrated keithley 6.5 digit dmm and it was close, way better than 10% on dc, even on the mA range.

I just did the same, comparing my uni-T to my DVOM inline and they were very close.