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.
This article is geared toward those who already understand the basics of sub ohm vaping with mechanical mods.
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.
Resistance and Current Limits
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 (Ω).
At 30A, we have R = 4.2V / 30A = 0.14Ω.
The difference between these two is only 0.07Ω! So unless you plan to build your coils to exactly 0.2Ω, that little bit of extra room isn’t going to help you a whole lot. If you always build above 0.2Ω, you will do fine with a 20A battery.
Ohm meter inaccuracy
Notice I said 0.2Ω, and not 0.18 or any other resistance. It is not generally safe to build below 0.2Ω, 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 ± 0.05Ω, 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 0.20Ω, 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 0.19Ω at max voltage, so you may assume that your 0.20Ω 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 0.25Ω, and even that is pushing it. Personally I don’t build below 0.3Ω on those batteries.
When using 30A batteries, I believe you can safely build at 0.20Ω, 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 2 and 10 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.
It is well known at this point that the safest batteries to use are “safe chemistry” batteries. These include IMR and INR batteries, among others. It is no longer recommended to use ICR or any other kind of battery for vaping.
The reason we use safe chemistry batteries is that, unless stressed in a very unusual way (such as being thrown in a fire), they do not explode. When overstressed, these batteries will simply vent noxious gasses.
That said, if you use one of these batteries in a mod that has no venting, it will explode if it starts to vent. That is because the gasses are released at a high pressure, and if the mod has no venting, that pressure will build up inside the mod and explode.
Your best bet is to always check new mods to make sure they have venting built in. If you modify your device in any way, make sure that it still vents.
I know many of you know this, but I felt the need to bring it up due to recent events.
There has been some controversy over Efest battery ratings and whether or not they are accurate.
It seems to me that all of this controversy can be traced back to one battery testing facility in Europe (note: you will need to translate the page). They do quality work and I can’t fault them. However, I cannot verify what they’ve found regarding purple Efest batteries because I haven’t found any other facility testing them and reporting the same results (if any of you know of anyone else who has done the same tests on these batteries, please let me know in the comments).
Also, Efest recently issued a warning that some of the purple Efest batteries being sold are counterfeit. They say that most of these batteries are coming from China, and give detailed instructions for how to tell if they’re counterfeit. They are now putting “anti-fake” labels on their batteries to differentiate them from the counterfeits.
Counterfeit batteries are a very serious issue because they may not even be safe chemistry batteries, so if you suspect your battery is counterfeit it’s best to stop using it. Please note that this is not the same thing as a reputable battery company simply rewrapping someone else’s battery as their own (like Efest rewrapping Sonys), in which case the battery is still guaranteed safe and fully tested.
Buying safe batteries
Due to the proliferation of counterfeits, I highly recommend that you only buy batteries directly from reputable dealers. If you cannot buy directly from the company, buy from a vapor store like Madvapes or a flashlight store (like illumn.com), and know exactly which batteries you are willing to purchase before you even visit the site.
I recommend the following battery brands for sub ohm vaping, in no particular order:
18650, 20A-25A: AW, Efest, Sony, Samsung, LG, Panasonic, Orbtronic, MNKE, EH, Vamped, Subohmcell
18650, 30A or higher: Efest, Sony, LG, Orbtronic, MNKE, Vamped, Subohmcell
Batteries in pairs
There are now many mods available that accommodate two, or more, batteries at once. There is a significant difference in performance capabilities between whether they are connected in series or parallel.
It is important that you always buy the pair of batteries used in the mod together, and always use them together and charge them together. Any difference between them can lead to one battery pumping out more voltage than the other and becoming strained.
Batteries connected in series, or “stacked”, will retain the same current but double the voltage. For instance, if you use two 20A batteries, you will still have a max current discharge of 20A, but you will have a max voltage output of 8.4V. This is helpful if your goal is to maximize power, or wattage (W), without decreasing the resistance of your build.
In fact, this will increase your power by a lot. Power is equal to the voltage times the current, or P = V*I. If you use 20A batteries and build to the lowest resistance possible, your power from one battery at max voltage is 4.2V * 20A = 84W. If you stack two of these batteries, however, your power is now 8.4V * 20A = 168W! Many people who stack batteries find that they prefer building their coils to a higher resistance because of this.
Batteries wired in parallel will retain the voltage but double the current. So now if you use two 20A batteries, you will retain a voltage of 4.2 but have a max continuous current of 40A. This allows you to lower the resistance of your build.
If you pair two 20A batteries, your minimum resistance is now 4.2V / 40A = 0.11Ω. If you pair two 30A batteries, your minimum resistance becomes 4.2V / 60A = 0.07Ω!
That’s about as low as it gets. If you plan to build to extremely low resistances, I highly recommend buying a box mod that runs batteries in parallel.
Charging your batteries
Lastly I will briefly discuss battery charging. Most of you know that it is crucial that you use a good quality battery charger; however, because of issues such as this I feel I must restate the point.
My best recommendations for battery chargers are Nitecore and Efest brands. There are a lot of different chargers out there, but some of them are really not made well, and unfortunately that is one of the easiest ways to start a house fire. (See our review on the new Nitcore Digicharger here)
I must also advise that you never charge your batteries when you are sleeping or not home, that you always take your batteries off the charger as soon as they are fully charged, and that you plug your charger directly into the wall or into a surge protector (i.e. do not use an extension cord no matter how tempting it may be).
Safe Sub-Ohm Vaping
I hope everything I’ve discussed in this article clears up some of the questions you may have. With everything we know about vaping and battery safety, it really is not hard to sub ohm safely. All that takes is understanding how it all works.
As always, if you feel that I’m not right about something, please feel free to comment and let me know. I’m always willing to correct myself if need be.