Ohm's Laws and Vaping

This article is geared toward those who already understand the basics of sub ohm vaping with mechanical mods. If you are new to this and have never read a safety guide on advanced vaping, please read this before you proceed.

Vaping Sub-Ohm Coils

There is an incredible amount of information available for those who intend to vape at sub ohm resistances. Much is this information is geared toward those who are very new at it, and so many of us who have been vaping this way for a while tend to skim through these quickly, or not read them at all, because we already know these things.

However, there seems to be a lot of misconceptions going around about how to sub ohm safely due to some gaps in our knowledge. As a fellow sub ohm vaper with a bachelors degree in engineering, I wanted to clarify a few things that seem to be frequently misunderstood about how to vape safely.

Please note that my purpose in writing this article is simply to advise vapers and clear up some details. It is not my business if someone wants to push the limits of what is safe – I only wish to make sure you know what you are doing. It is one thing for someone to push these limits knowingly; it is quite another for someone to think they are playing it safe when they are really not.

Minimum Safe Resistance

It seems to be very common for people to seek out batteries specifically for their high current ratings. There is a very high demand for Sony VTC5 batteries primarily because of this, to the point where they tend to be sold out everywhere you look.

However, for the majority of sub ohm vapers, this high of an amp rating is unnecessary, and can even be misleading. In fact, there is a very narrow window between vaping at 20 amps (A) and vaping at 30A, and unless your coil build happens to fall within this tiny window, 30A batteries aren’t going to be much more useful to you.

Let me illustrate my point. We are all familiar with Ohm’s law: voltage equals current times resistance, or V = I*R. With a little algebra we can find out what resistance we have at a certain voltage and current, R = V/I. Using this equation we can find the minimum resistance we can build to given the current rating of the battery.

For all 18650 and 26650 batteries, the maximum voltage when charged is 4.2 volts (V).  Plugging this into the equation along with the max continuous current rating of the battery will give you the absolute minimum resistance you can build to without overstressing your battery.

For a battery rated at 20A continuous, we have R = 4.2V / 20A = 0.21 ohms ().

At 30A, we have R = 4.2V / 30A = 0.14.

The difference between these two is only 0.07! So unless you plan to build your coils to exactly 0.2, that little bit of extra room isn’t going to help you a whole lot. If you always build above 0.2, you will do fine with a 20A battery.

Ohm Meter Inaccuracy

Notice I said 0.2, and not 0.18 or any other resistance. It is not generally safe to build below 0.2, even with a 30A battery. I say this because you have no way of knowing the exact resistance of your coils. Even the most high-tech, expensive ohm meters have some inaccuracy, and most of us are not using those.

Chances are you check your resistance on a digital mod, or using one of those little black boxes made by Tobeco. Some of you may even be using a calibrated multimeter. No matter what instrument you use, you can’t expect it to be more accurate than ± 0.05, and that’s being generous.

There are two major reasons ohm meters are innaccurate. One reason is that the resistance we are trying to measure is extremely tiny. The resistance we use is so low that it is often considered “no resistance” in other engineering applications.

Trying to measure the resistance of our coil builds is like trying to measure the width of something down to hundredths of a millimeter. You can get some really sophisticated calipers to do that, but don’t expect the numbers to be 100% accurate.

The other major reason ohm meters are innaccurate is because our builds are so low that the littlest movement can change the resistance. If you build your coils and check the resistance, then heat them, pinch them, tweak them, and wick them, check it again. Chances are the numbers are going to be different this time, sometimes by a lot. This isn’t because the ohm meter is “wrong” – it’s because you changed the resistance when you moved the coils.

You may also find that your resistance changes a little as you use the coils. Or that the exact same build gives you a different reading on another atomizer, or that tightening the posts a little changes the reading. Again, there is nothing wrong with your ohm meter. Just touching it with your fingers can give you a different reading, because there’s resistance in your fingers.

So if you build a coil to exactly 0.20, you can’t expect that to be spot on. Your ohm meter may be a little off, or you may change the resistance accidentally by moving the coil a little.

A battery rated at 22A allows for a minimum build of 0.19 at max voltage, so you may assume that your 0.20 build is perfectly safe. As I’ve demonstrated, that may not be the case.

If you use 20A or 22A batteries, your best bet is to never build below 0.25, and even that is pushing it. Personally, I don’t build below 0.3 on those batteries.

When using 30A batteries, I believe you can safely build at 0.20, but I wouldn’t try to build below that.

Amp Ratings – Continuous Versus Pulse

Many people say that they can build to very low resistances because they go by the battery’s pulse rating rather than the continuous rating. It is rumored that batteries can handle a much higher current output because of the pulse rating.

This is false.

Continuous amp ratings are the maximum current that the battery can handle discharging over a very long period of time. Pulse ratings, on the other hand, are the maximum amount of current the battery can discharge in a very short period of time.

The problem with pulse ratings is that there is no universally accepted standard for battery companies to test pulse ratings. A pulse is simply defined as a discharge in a brief time window, but there is no way to know exactly how long that pulse lasted in testing. A pulse time can be anywhere from a couple seconds all the way down to a fraction of a second – in theoretical mathematics, the time of a pulse is considered “infinitely” short.

How long do you hold the button down when vaping? Try timing it – I’m willing to bet it’s somewhere between two and ten seconds. Did the battery factory time their pulse testing to accommodate that? Probably not.

Since we have no way of knowing how pulse ratings are determined, it’s safe to assume they don’t apply to vaping. It’s better to stick to the continuous amp rating.

Written by Anthony Cornfield of www.grottovapor.com