Master Guide to Vaping Part 8 – Ohm’s Law and Battery Safety
Battery safety and Ohm’s Law go hand-in-hand. You can’t learn one without the other, so this guide will require some rudimentary math skills. Overall, many of the concepts you’re about to learn are simple, and the safe practices are mostly common sense once you have a basic understanding. No doubt, you’ve heard of stories about batteries exploding. After reading this guide you’ll have a good idea as to how you can prevent personal injury, property damage, and possibly ending up on the news.
The majority of safety concerns pertain to removable batteries. Mods with internal batteries are inherently safer, despite their drawbacks. So the first thing we need to do is explore the batteries themselves.
The most common battery used in vaping is the 18650 battery. The number “18650” refers to its size; “18” denotes that it is 18mm in diameter, “65” means it has a height of 65mm, and the “0” simply means it’s round or cylindrical. So a 26650 battery is 26mm in diameter and 65mm tall, a 20500 battery is 20mm in diameter and 50mm tall, and so on and so forth.
18650 batteries use lithium ion chemistry, as do the majority of rechargeable batteries. Different batteries may use slightly different chemistries to increase capacity or current rating, but they all use some type of lithium ion. The most common used in vaping is the IMR battery, or LiMn. This stands for lithium manganese, but with vaping becoming more popular and devices becoming more demanding, many manufacturers have built upon IMR technology and created proprietary blends. However, the chemistry isn’t as important as the specifications, which we’ll address shortly. The only chemistry that should stand out is ICR, or LiCo which uses lithium cobalt. These are the original batteries used in flashlights when mods were first being made, and therefore found their way into early vaping devices. Back then, devices required less power and atomizers were built with higher resistances, so ICR technology was fine. Today, however, you want to avoid ICR batteries at all costs. They offer the highest battery capacity, but also produce the most violent reaction when they fail.
Most all lithium ion batteries are rated for 3.7V, but this is an average. Fully charged, they will output 4.2V, and below 3.2V they may become damaged and never work again. You may stumble upon 4.35V lithium ion batteries when shopping around, but these are not suited to vaping. The higher voltage will result in absolutely no advantage, and may even damage your device.
The next important number is the capacity, measured either in milliamp-hours (mAh) or watt-hours (wH). In vaping, mAh is the more common number to see, but it has no absolute value. This is to say, it’s impossible to gauge how long the battery will last with that information alone. In order to get a sense of how long any given battery will last, you would need to have used a battery previously, and also guesstimate based on your atomizer’s resistance and the voltage or wattage you’re using. A good rule of thumb is that for every 100mAh, you can expect 1 hour of constant use with a 2.4Ω coil at 3.7V. Constant use means simply using your device normally for an hour, not holding the fire button down for an hour. Of course, this also depends on the length of your draw and how often you take a drag, so you can already see how mAh requires some previous vaping experience in order for you to get an idea how long a particular battery will last for you.
Watt-hours are much more straightforward, but rarely used. 1 Wh means that a battery will last 1 hour when used at 1W, and that’s 1 total hour of power, not the above-mentioned “constant use.” By using an online calculator, you can convert mAh to Wh easily. As an example, a 3000mAh lithium ion battery equates to 11.1Wh. This means that you’ll get 1 hour of vape time at 11.1W. From there, you can estimate your battery life. Let’s say you vape at 30W. Since you’ll get 1 hour of vape time at 11.1W, and 30W is approximately three times that number, you can expect to get about 1/3 of an hour of vape time at 30W, which is about 20 minutes. If each drag you take is 5 seconds, that means for every minute of vape time you take 12 drags. 12 drags multiplied by 20 minutes means you can take 120 drags with a 3000mAh battery when the device is set to 30W and drags are 5 seconds long. As you can see, this is a much more precise way of gauging battery life. All you need to know is the mAh rating and voltage of your battery (or watt-hours, if that information is available), and what wattage you’ll be vaping at. But what about mods that use more than one battery?
First, it’s important to know the difference between batteries in series and batteries in parallel. Batteries in series are sometimes referred to as “stacked,” so imagine two batteries stacked on top of one another. In this configuration, the natural voltage of the total circuit is doubled if there are two batteries in the circuit. The mAh rating and maximum current remain the same. In parallel, multiple batteries are oriented with the same polarity. With two batteries, the mAh rating and maximum current are doubled, but the voltage remains the same as a single battery; in this case, 3.7V.
If you’re calculating watt-hours for a multi-battery mod where the batteries are in series, the mAh rating you’ll input into the calculator will be the mAh rating of a single battery, and the voltage will be 3.7V multiplied by the number of batteries. For example, if you’re using three 3000mAh batteries in series in a Reuleaux RX200, you would input 3000mAh and 11.1V into the calculator and let it convert that to watt-hours. Alternatively, if you’re using the same three batteries in parallel, you would input 9000mAh and 3.7V. Coincidentally, you will get 33.3Wh with both calculations. In a regulated mod, battery life is the same when using the same batteries regardless of whether they are in series or in parallel.
If that doesn’t make sense to you, don’t worry. It doesn’t necessarily pertain to battery safety. However, if you’re interested in delving deeper into the world of batteries or want to more accurately predict battery life, it’s useful information.
The most important number when it comes to battery safety is the maximum discharge rate. This information is usually available right on the battery wrapper or from the store or website you bought the battery from. Batteries designed for vaping usually go out of their way to make this information as prominent as possible, and if you can’t find this information, play it safe and don’t use that battery for vaping. Current is the amount of electricity in a circuit, and drawing too much from a battery is dangerous. The battery is liable to heat up or even explode if its current rating is exceeded.
Modern mods generally will have some sort of documentation telling you what the current rating of your battery should be, so all you need to do is find a battery that meets or exceeds this number. However, the recommended current rating on a mod is generally based on the maximum wattage and minimum resistance that it can handle. The documentation might recommend 30A batteries, and the mod is capable of firing at 200W down to a resistance of 0.1Ω. If you plan on vaping at 50W with a 0.3Ω resistance, a 20A battery with a 3000mAh capacity will still be safe.
Knowing all that, we can now talk about Ohm’s Law. Ohm’s Law applies mostly to unregulated and mechanical mods. Because there is no board between the battery and the coil, there is nothing to protect you if you exceed the current rating. Ohm’s Law states that voltage is equivalent to the current multiplied by the resistance, or:
V=IR, where V=Voltage, I=Current, and R=Resistance
With some basic algebra, it also states:
In most cases, you’ll be trying to find the safest resistance you can use based on what battery you have. V should always be 4.2V. Even though the battery is rated for 3.7V, it increased to 4.2V on a full charge. As for the current, I, that information should be known. If it’s not, again, find a new battery to use where you know the maximum continuous discharge rate. 20A is a common value, so let’s use that. Since we’re trying to find resistance, R, we’re going to use R=V/I. We know the voltage and current, so plug those numbers in:
Easy! The minimum resistance you can safely use with a 20A battery in an unregulated mod is 0.21Ω. You can even avoid doing the math altogether by using an online calculator. Just plug in the numbers that are known, voltage and current rating, and it will calculate the minimum safe resistance for you.
You might be wondering where power, or wattage, comes in. To find wattage, you need to know the voltage and the resistance:
Again, plug in the numbers and get the result. Alternatively, use an online calculator.
Most any mod nowadays is adjusted in wattage, so you won’t really need to do this calculation since wattage will always be known. However, when it comes to figuring out how much current you’re pulling from your batteries in a regulated mod, the above formula becomes relevant.
In a regulated mod, the wattage is split between how many batteries there are. For example, if your mod is using two batteries and you’re vaping at 80W, each battery is carrying the strain of 40W. As a rule of thumb, a single battery works best at 50W or less. Higher than 50W isn’t necessarily dangerous, but battery life will be short, and performance will suffer, especially if you’re vaping sub-ohm. But how can we determine how much current is being pulled from each battery. More math!
To find the current, we need to know the resistance and the voltage. The resistance is easy to find. It will be printed on the coil you are using, or you can test a coil you’ve built yourself on an ohm meter. Voltage is a little tricky. Since we know P and R and are looking for V, we need to use algebra to make the P=(V*V)/R formula work for us:
In English, you’re going to multiply the wattage by the resistance, then take the square root of that number. The result will be your voltage. Since we want to find the current of each of the two batteries, we’re going to use 40W, which is 80W split between the two batteries:
Even though each battery is not actually getting 2.9V in practice, we’re using this number to get the actual current value of each battery. Now that we have values for V and R, we can figure out I:
There you have it! In a regulated mod set to 80W where a 0.21Ω atomizer is being used, you’re getting 13.8A, well below the 20A rating of your battery. And you thought middle school algebra was useless!
Rest assured, you won’t need to do any of this math on a regular basis. Once you find batteries and mods that you know are safe with a given resistance and how you like to vape, it will all become second nature. Additionally, many mods will simply tell you most of this information on the display, but if you’re planning on giving unregulated or mechanical mods a shot, knowing Ohm’s Law and how to determine if your setup is safe to vaping is absolutely necessary.
When finding a battery to use for vaping, you may also see a maximum pulse discharging rate. It’s highly recommended that you ignore this number. While the continuous rate will be safe when the device is activated for long periods, pulse rate is only safe when quick pulses are used. The problem is that there is no standard for how long these pulses are. Some of these pulse ratings may have been based on a pulse of less than one seconds, while others may have been based on five-second pulses. There’s really no way to know, and anything over the continuous rating will cause batteries to heat up which can lead to failure. The only question is how fast they will heat up.
Another number you may see is the C-rating. This number can be used to find the continuous discharge rate, but it isn’t in itself the actual discharge rate. First, you need to convert milliamp-hours (mAh) into amp-hours (Ah), which is really easy. Just move the decimal place to the left by three spaces. For that 3000mAh battery we were previously talking about: 3000mAh=3Ah. That’s it. Once you have that number, multiply it by the C-rating. So if a battery reads “7C” and “3000mAh,” you would convert 3000mAh to 3Ah and multiply it by 7 to get a result of 21A. That is your maximum continuous discharge rate.
When it comes to basic battery safety, you also want to make sure that the wrap on the battery is 100% intact, with no tears or rips. You probably already know that the top of the battery is the positive side, and the bottom is the negative side. What you may not know is that the entire area underneath the wrapper is also negative. If the wrapper is torn and the metal underneath touches something it’s not supposed to, you can end up with the device auto-firing, or the battery venting or hard-shorting. Venting occurs when the battery is punctured, either by something physical or by excessive heat. The gasses inside start to spew out and cause even more heat. Avoid this at all costs. A hard-short occurs when there is no resistance when a circuit is made between the positive and negative sides of a battery. To avoid this, NEVER carry loose batteries. Keys, coins, or other metal objects can shift in a pocket, purse, or backpack and actually create a circuit if they touch the battery in just the right way. When traveling with batteries, always make sure to keep them stored in a designated plastic or fabric battery case.
When a battery vents, ideally you need to let it run its course in an isolated, secure space while someone is prepared to put out a chemical fire with a specific type of extinguisher. Afterwards, the battery must be disposed of properly according to hazardous material standards. Realistically, very few people are prepared for this. The most pragmatic solution is not ideal, but in only requires a glass of tap water. If your battery starts to vent, you should remove the battery as quickly as possible. The longer you wait, the hotter it will get so make sure to not burn yourself and use gloves if you have to. Then, take the battery and submerge it in a glass of tap water or salt water. Make sure the glass is actual glass or ceramic; nothing plastic or paper. Once the battery is submerged, wait for the venting to run its course. The water is actually shorting out the battery by creating a circuit between the positive and negative poles. At the same time, the water is absorbing the heat and keeping the battery relatively safe.
After the battery is done venting, you’re going to have dirty water in a glass, and this is why this solution isn’t perfect. This dirty water is toxic to humans, animals, and the environment. Pouring it down the drain or flushing it down the toilet is damaging to animals and the environment, while keeping it around poses a risk to you and any other living thing in your home. You want to avoid coming into direct contact with the water, and if you’re trying to mitigate as much damage as possible, look to see if any facility exists near you that can properly dispose of the liquid. If you have children or pets, or if you’re simply not comfortable having it in your home, the best solution is to flush it or pour it down a drain. It’s unfortunate, but it will be up to you to weigh the risks of each solution and make an informed decision. The good news is that this whole situation is actually very rare, and there’s a good chance you won’t ever have to deal with it. However, you should know what to do if it does occur.
That should be a good introduction to Ohm’s Law and battery safety. If any of the above information is confusing, I encourage you to watch our video on the subject.
In summary, make sure to follow our guidelines:
Don’t let this information discourage you. Battery failures are very rare in vaping, and virtually non-existent when following safe practices. Plus, this information isn’t exclusive to vaping. The concepts and safety tips above apply to anything that uses lithium ion batteries, including mobile devices, tablets, laptops, etc. Remember, vaping is about reducing certain risks, not eliminating them completely. Nothing is 100% safe, but you can mitigate the maximum amount of risk by arming yourself with knowledge and being safe and smart when vaping.