How do I measure internal resistance?

[hildicat]

I've got a multi-meter. How do I use it to measure the internal resistance of my mech mods? Would this process be the same for a tube mod like the Natural and a box mod like a Reo Grand?

 

[StaircaseWit]

I've got a multi-meter. How do I use it to measure the internal resistance of my mech mods? Would this process be the same for a tube mod like the Natural and a box mod like a Reo Grand?

I recently did research into this for measuring the voltage drop (loss of voltage from unloaded battery voltage to loaded atomizer voltage delivery) of a mechanical mod and I couldn't find a good consensus on how exactly to achieve the most accurate numbers.

You ask about internal resistance, which is generally quite low (0.05ohm or possibly lower). I think what you're really looking for is the "voltage drop" from the battery to the atomizer coil, the net effect of the circuit containing the mod and coil.

First read this: Voltage Drop - Not Where You Think | atmizoo vaping modware

What they're saying is this: the *battery itself* exhibits a voltage drop when there's a loaded circuit (an atomizer). So you have to start with that knowledge, that the battery isn't a perfect voltage source. It doesn't deliver, say, 4.1V consistently when under load. It might deliver something as low as 3.9V, or lower for a stressed/old/bad battery.

The other very important bit of information: you can only compare two devices (mods) by using the same battery and same coil. Comparisons made by two people using different batteries and coils are basically meaningless.

Here's about the best we can do without getting too complicated:

1. Measure the battery's unloaded voltage by placing the probes on positive and negative of the battery for a voltage reading. This should be 4.1-4.2 for a fully-charged 3.7V (AW IMR or similar) battery.

2. Put the battery in your mod with an atomizer coil mounted. Measure the voltage at the positive and negative terminals of the atomizer while firing the device.

Step 2 will give you a voltage reading you can subtract from the unloaded voltage you got in step 1. This is the net "voltage drop" of the battery, mod and coil combined circuit.

Just remember, the battery isn't necessarily delivering the voltage you measured in step 1, because when it's in a mod with a coil, it's under load. The process is the same for any mod that doesn't use a circuit board. Things like a Provari or Zmax or Vamo can't be measured using this method.

Your numbers are only meaningful if you can test another mod using the same exact battery and coil.

 

[hildicat]

Wow! Thank you for such a detailed explanation. Your efforts are greatly appreciated.

 

[Lhartman89]

To measure the IR of the mod, I put one probe on the top of the mod, just the tube not the postive post on the 510 and the other on the button contact or the negative spring. If you want to find the watts your device is putting out, use the link posted above and scroll down to a post made by MANU, 06/02/2013, 1:27 PM. He goes in detail on how to get all this info.

 

[StaircaseWit]

Lhartman is correct, and I meant to point out MANU's comment which is absolutely brilliant. Thanks for pointing that out.

The internal resistance number (Rm in MANU's comment) for the mod is typically so low as to not have much effect on the voltage drop calculation, in the 0.05ohm range or thereabouts.

 

[shortee611]

I just did all that math from that post. And my measured V drop was less than the math showed it to be. I've done a bit of searching and the internal resistance of a brand new AW IMR 18650 2000 mah is around .002 Ohms not .04 ohms. .002 ohms works out almost perfectly to my measured voltage drop.