Reading Voltage Through The Main Current Wires

[rg12]

If measuring voltage without a separate set of sense wires that go from the measuring device straight to the battery, the voltage being read will be lower than the pack voltage since there is a voltage drop from the wires resistance.

My question is if there is another factor other than this that reading the voltage where current is flowing further distorts the voltage reading?

I'm asking that because I have a small load tester that shows the same voltage as my multimeter connected straight to the pack's terminals.
When 10A current is applied through 20cm 14AWG wires the voltage in the multimeter is about 3.6V while the load tester shows about 3.0V which doesn't make sense since such short and thick wires for 10A can't produce a 0.6V drop.

Also, when calculating resistance in the wires, should I consider a 20cm long wires as double since it goes from negative wire through the positive wire?

 

[DogDipstick]

What does it calculate to? Calculation would show you which " measuring tool" was more accurate...

The resistance. I doubt you can even measure it accurately. Easier to calculate it mathematically and use that reliably. Thats how my engioor taught me to do everything. Calculate it. We even make shunts by hand and would never be able to measure the resistance: we just calculate it ...but... It is tough if you are not a *perfect mathematician* . Make a hypothesis: then test that hypothesis against a known formula for accuracy...

Apparently one, or the other, measurings tool you are using is wrong. Possibly both are wrong.


And, also: Yes. 40cm (if it is two, 20cm wires, ea. 20cm... 20+20=40... or 0.4M... that are making a current path) .. calculate it for 40cm... IF it is a pair or 20cm wires, one pos ( red) and one neg ( blk)... each a path of 20cm... that is two 20cm individual runs... so 40cm total current path...

Also, when calculating resistance in the wires, should I consider a 20cm long wires as double since it goes from negative wire through the positive wire?

40 cm of 14g should not drop much (voltage) at all. Next to none. 0.003 ohms.

 

[john61ct]

If measuring voltage without a separate set of sense wires that go from the measuring device straight to the battery, the voltage being read will be lower than the pack voltage since there is a voltage drop from the wires resistance.

No.

If there is no current flowing you are measuring VoC.

In a 3 or 4-wire setup, the power leads have lower resistance than the sense leads.

The point of separate sense leads is to avoid the interference from current flowing so you get a more accurate voltage measure.

Of course putting your instrument directly on the pack posts is best of all.

And yes, if (trying to) measure wire resistance, the full circuit length is taken into account.

You need a precise and accurate DMM, ideally calibrated, in order to measure actual voltage drop if you have sized your wires properly for the peak currents they will carry.

Best to shoot for well under 2% even over long distances, but that is not always practical.

 

[rg12]

If measuring voltage without a separate set of sense wires that go from the measuring device straight to the battery, the voltage being read will be lower than the pack voltage since there is a voltage drop from the wires resistance.

No.

If there is no current flowing you are measuring VoC.

In a 3 or 4-wire setup, the power leads have lower resistance than the sense leads.

The point of separate sense leads is to avoid the interference from current flowing so you get a more accurate voltage measure.

Of course putting your instrument directly on the pack posts is best of all.

And yes, if (trying to) measure wire resistance, the full circuit length is taken into account.

You need a precise and accurate DMM, ideally calibrated, in order to measure actual voltage drop if you have sized your wires properly for the peak currents they will carry.

Best to shoot for well under 2% even over long distances, but that is not always practical.

That interference is what my question is all about.
I want to know if aside from the voltage drop created by the resistance of the wires if the fact that current flows there distorts the reading in addition to the resistance?
Because the voltage drop can't be 0.6V over such gauge at 40cm of wire.
Both sides read exactly the same voltage until current is passed and the load tested drops extra 0.6V which is so off.

 

[john61ct]

Yes I answered that.

Interference from current flow IS the only factor.

Higher the current, the more useless the voltage feedback.

Which is why all good charge sources use separate sensor wires.

 

[rg12]

Yes I answered that.

Interference from current flow IS the only factor.

Higher the current, the more useless the voltage feedback.

Which is why all good charge sources use separate sensor wires.

Ahh now it's all clear, and I thought that the only interference is the voltage drop across the wires...

 

[john61ct]

That too, but the thinner the gauge the more drop for a given current flow.

I guess one could call the "real" voltage drop, the other is an error of measurement.

 

[rg12]

That too, but the thinner the gauge the more drop for a given current flow.

I guess one could call the "real" voltage drop, the other is an error of measurement.

Got it, thanks mate