Using RC charger to measure wire resistance

[kilou]

Hi,

I'd like to measure the resistance of a AWG5 wire over a distance of 50cm. I know that I may calculate a theoretical value by considering the ohm per km for such cable (about 1.1 Ohm/km) but I'd like something a bit more precise by sending a constant DC current through the wire a measure the voltage drop.

I have an iCharger 3010B which can e.g. charge Nimh batteries with a constant current (say 10A). Do you think it would be possible to just plug the positive and negative leads of the charger to each end of the wire, start the Nimh charging process i.e. sending 10A through the wire and measure the voltage drop across the wire (using a multimeter) to then calculate the wire resistance? I guess the voltage drop should be close to 5mV. Any risk doing so for the RC charger since this basically involves shorting the 2 charging leads with the wire but with current limited to 10A? Thanks!

 

[DogDipstick]

Nimh has no such thing as an IR value, that I know of, reported by an iCharger. I know my powerlabs do not need any sort of balance lead with the nimh setting.

You propose doing the math for a DCIR reading.

Icharger will most probably error out. If it sees a short ( that a length of wire would appear as to it.... ) on the nimh setting.... ... My chargers also have a " Hot wire constant current functions"... Where you could do this easily. I am pretty sure the Icharger has a CC hot wire foam cutter too.... that puts out a set amperage.... that can be measured for Vdrop.


Unless you have a ballast resistance, calculated to take the power, and dissipate, the known value and calculate the rest through math. (Trick the charger thingking a battery is there.. ( it will probally look for voltage on its lines before it begins charge)( i know my chargers will NOT try to charge a dead battery, but report an error) Using a lithium profile. Then.... Yeah, mebbe it will work. Tell you the AC IR.



It is should be 0.3133mOh per foot.

I just take this written value ( from the AWG chart) whenever I want to calculate. I dont virtual IR measure ( DCIR). I just assume the charts are right when tiny measurements are taken. Or get an ACIR reading... The error in my measurement scheme might be greater than the actual value.

Yeah try the " HOT WIRE " constant current function. With a set current, a DC multimeter should be able to pick it up accurately (VRdrop).

 

[kilou]

My chargers also have a " Hot wire constant current functions"... Where you could do this easily. I am pretty sure the Icharger has a CC hot wire foam cutter too.... that puts out a set amperage.... that can be measured for Vdrop.

Yes my Icharger 3010B also has a hot wire foam cutter option and actually I thought about using it initially. The charger's documentation mentions that you need to set both voltage and max amperage for this mode and I (stupidly) thought I'd need the actual current to be fixed rather than the max current... However since the resistance of the wire is so small, I guess that the charger would immediately output the max current through it so that actual=max current, which should work indeed!

Thanks for pointing that out! :thumb:

 

[kilou]

The purpose of measuring the resistance of the AWG5 cable is to then use that cable as a shunt resistor to measure very large DC currents (e.g 500-1000A over few milliseconds) in a DIY battery spotwelder. The sense wires that will probe the voltage drop across the AWG5 cable will likely have to run along that main AWG5 cable before reaching an analog to digital converter connected to an Arduino board. However I'm now thinking that when the welding current will flow through the AWG5 cable, it will induce a current in the sense wires, which would likely mess up the voltage drop measurement... Is that correct? I know that I can somehow limit inductance by twisting pairs of wires together (either the power leads and/or the sense wires or both) but I'm not sure this would be sufficient. Do you know if using a coaxial cable for the sense wires would allow to have a propoer voltage drop reading that is not polluted by the inductance from the power leads during the welding process?

 

[liveforphysics]

If you have dirty high spike and ripple DC it would couple noise onto your sense leads. If your supply makes clean DC, then you only make a big steady EM field which induced noise into your sense leads only on its rising fall current transitions from off to on, but not while running.

Make sure your sense taps aren't touching your current drive to the wire. When I'm measuring wire, I give myself a few inches of extra wire length at my current injection point vs where I sense inboard away from it at a known distance. As current is the same throughout the circuit, it only matters to measure the wire length span between sense leads.

 

[kilou]

Thanks for your input! The power supply is going to be a car battery but I don't know if it this will provide "clean" DC as the battery is going to be basically shorted (using a heavy duty contactor solenoid) to make the spotwelds. The current spike should only last about 20 milliseconds or so (controlled by Arduino).

The problem is that I have to put the sense wires on each end of the negative power lead (which will act as the shunt). So the sense wire close to the negative electrode will have to be routed along the power lead back to the arduino. I don't see how different I could do that. The thing is the the positive and negative power leads will run parallel to each other so the magnetic field should cancel out. I was thinking to sandwich the sense wires between the two power leads for that specific reason. Would that not be an option?