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Ohm's Law... and common vaping questions.

Published by State O' Flux in the blog State's Stuff. Views: 2086

So many questions, so much misinformation - and all it will take to clear up at least 80% of both... is a bit of knowledge.

If you can do basic math... the most useful Ohm's law formulas are:
  • For wattage ("Power") the formula is V² ÷ Ω = P.
  • For amperage ("Current") the formula is V ÷ Ω = C.

This isn't going to be a class in all things Ohm's Law, so much as a few things you need to know about resistance and power... so that both your vape and your safety is well considered. First... lets open up a very easy to use, uncluttered with BS, on-line Ohm's Law calculator, or 'OLC'. Got it? Good. :D

Most common misconceptions I run into are:

"What wattage do I need for X resistance?"
If we are using a regulated APV, then we need a few of it's specifications. The maximum wattage and amperage maximum or limit.
Lets say you want to run up against the "edge of sub-ohm"... but you have a 15 watt limit. Can we go sub-ohm with just 15 watts? Yes... yes you can. :p

Lets put 0.9Ω into the appropriate box of our OLC.

Insert the 15 watt value, the 0.9Ω value... and hit the button. We get 3.7v and 4 amps filling out the other boxes.
Can we use this? Yes we can... because the specs of our APV indicate an amp limit of 5 amps - and we're under than value.
If we were over that value, you could still run your wattage, but at the selected resistance, there would be no additional benefit.
Most VW APVs have lower resistance limiters as well... for example, the Innokin MVP2 has a 1.3Ω/3 amp/11 watt limit. If you insert any two of those numbers into our calculator, you'll see that they line up fairly well with Ohm's Law.
If you try to use a resistance less than Ohm's Law indicates the maximum power of your APV is capable of supporting... there is no benefit.

The above doesn't take into account a value called "heat flux"... which is the coil radiant heat temperature. Heat flux is one of our most desirable tunable elements... second only to net coil surface area for value. You'll learn a lot more about heat flux when you read the two part series on using the Steam Engine coil modeling program.

"What's the smallest battery I can run with X resistance?"
This is the kind of question a new mech mod user will ask, because they don't really know anything about battery ratings or how Ohm's Law applies. Because unregulated mechs are used mostly with low resistance, lets use 0.5Ω as our reference value.
The batteries we have on hand are AW IMR 18350 (6a), 490 (16.5a) and 650 (24a/1600mAh version).

Now... we need to know how much amperage we need for 0.5Ω. Because an 18XXX series battery has the same base voltage regardless of size, we can use that as a constant. I use 3.9v as it's not a full charge, but still above base voltage. Insert 3.9v and 0.5Ω into the OLC. Our required amperage draw is 7.8a.

Because our mech mod is unregulated, we need a margin of safety. I use 10% of the rated battery maximum continuous amperage drain value... so, a battery rated at 15 amps, I would subtract from that, 1.5 amps.

I would seriously suggest that new mech users use 10%~15% margin of safety... until you have a good understanding of what you're doing.

If we look at our battery amp drain ratings, we see that the "smallest" battery we can use is the AW IMR 18490, rated at 16.5 amps... leaving us a substantial margin of safety.

This begs the secondary question... "So, how much lower can I go with that 18490 battery?" Insert your base voltage, and our AW IMR 18490, with a 10% margin of safety subtracted from it's max. amp drain rating. 14.8 amps is the number we're "pushing up" against. Run the numbers, and we can go as low as 0.26Ω.

"How low a resistance can I run with X battery?"
This is a variation of our last question. Lots of folks want to run an 18350 battery to keep their mechs compact. Nothing wrong with that, but there is a resistance limit you can't ignore.
Using our AW IMR 18350 6 amp limit... we can easily calculate our lower resistance limit. With our safety margin, we have 5.4 amps to work with... so, lets insert amps and volts. What did you get? I came up with 0.7Ω.

If we had one if those new Efest "purple" 18350s with a mfg. rating of 10.5 amps... run the numbers again and we come up with 0.4Ω.

"How can I use Ohm's Law to figure out the right voltage for my mechanical mod?"
This question is coming from a place of confusion. Can you control the voltage of a typical unregulated mech mod? Nope... battery voltage is what it is. Even with a Kick, you only control wattage.

I run 0.3Ω dual coils... which Kick should I buy?"
Ahh... the kind of question that makes you go :blink: :closedeyes: :facepalm: To begin, how is it that someone "runs 0.3Ω dual coils", yet apparently has no concept of what a Kick is, or how Ohm's Law works? What are they currently running "0.3Ω dual coils" in - on what APV and why the hell do they think they want a Kick?

Kicks were designed to allow mech mod users the look, feel, style and "simplicity" of a mech... with the safety advantage of a wattage regulated, amperage limited APV. Kinda like turning your $200 Nemesis into a Vamo. :laugh:

All Kicks, from the genuine Evolv Kicks 1 & 2, to the various clones, have lower resistance and maximum amperage limiters... just like any other VW APV.
Toss on your atomizer, crank it up and go. If you didn't do your OL math, installed a atty with too low a resistance... it will either not work, or run your battery down pretty quickly.

What wire is best for X resistance? (Sorry... not everything is about Ohm's Law)
A good question this one... and one encountered often. There are a few ways to go about it. You can read posts, look at YouTube videos, see what others are doing and experiment with those examples.
You can refer to Super_X_Drifter's coil building thread with examples of differing resistance values for a common coil build... or you can figure out the perfect, optimal build yourself using "Steam Engine"... a multifunction vaping calculator that may be the most valuable vaping tool you ever use.

Heat flux (HF) and heat capacity (HC). These are two values available found in Steam Engine... that are far more important than you might first imagine.
You can use HF to determine the optimal wattage value for a given build, or the optimal build for a given wattage.
Although not as useful as HF, pay attention to the HC... if you selected the wrong wire, your HC value will go up, and you'll wonder why it takes so long for your build to heat up and cool down.
More information on these two values... can be found in the Steam Engine Advanced User guide. Yes... yes I have promoted the user guide twice in one article. Steam Engine will be that valuable to your vaping happiness. :thumbs:

What is the momentary or "pulse" rating of a battery?
Batteries may have two ratings. A Maximum Continuous Current Discharge (MCCD) rate... and a pulse rate. Pulse can be anywhere from 1.5 to 3 times that of continuous, and is base on the amperage potential within a short time frame... from as little as one second to as long as 30 seconds.
Pulse rating can be used for an unregulated mech mod to calculate resistance. For example, if a 10a battery has a 20a pulse rate for 10 seconds... then you can calculate resistance for 20a... for 10 seconds. After that, amperage drops rapidly to the continuous rate.
Not all batteries have an advertized pulse rate, and although you might feel comfortable extrapolating your own, based on the continuous rate... you are only guessing.

In the end, I do not recommend using pulse rate values to beginners. Stick with continuous ratings until you have confidence in your knowledge and abilities. Don't go supersonic, until you first learn to fly. :D

Addendum 5-1202015: These fairly recent, unusually high (35, 38 & 40A) 18650 ratings are not in fact MCCD ratings, but rather pulse or "momentary" ratings.
Granted, they are relatively modest pulse ratings within a time limit likely exceeding even a deep draw vapists usual application... but they are still a lie, and not the true Maximum Continuous Current Discharge.
The same applies to the the smaller popular vaping batteries, where the listed amperage is above the norm.

That's it for now. If anyone who reads this has a specific question... feel free to ask and if it's a good one, I'll include it in the above. Same for errors... see any? Let me know.

As always, take it for what it is, do with it what you will. :2cool:
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