Battery & Ohm's Law



So you’ve picked up a safer chemistry battery and know how to avoid the most likely scenarios where a battery could go boom. This isn’t the whole picture though, and the reason for this is that a given battery can only provide so much current. If you ask it for more than it can give, you’re running the very serious risk of suffering a battery failure.


As with many aspects of e-cig safety, all you need to stay safe is a little bit of information and an understanding of how it applies to vaping. Plus, in most cases – aside from when sub-ohm vaping is concerned – it is unlikely to be a problem.


Key Points


  • Ohm’s law says that current = voltage / resistance, and your battery can only provide so much current.
  • The maximum current allowable for a given battery is listed in amps as the maximum continuous discharge rating, or calculated from the “C rating” multiplied by the battery’s capacity.
  • For AW IMR batteries, the 2000 mAh 18650 can provide 10 amps (A), the 1600 mAh 18650 can provide 24 A, the 1100 mAh 18490 can provide 16.5 A and the 700 mAh 18350 can provide 6 A. If you’re looking for maximum amp draw, the Sony US18650VCT4 (or VCT3) offers up to 30 A.
  • For maximum safety, choose a larger battery and keep any coils you build over 0.8 Ω, unless you’re willing to do the math and understand the risk of battery failure (but lower resistances are rarely needed).
  • VV/VW devices have protection built in, so if you’re using a mechanical e-cig mod, the best advice is to fit a safety fuse and ensure you know the batteries limits.

Ohm’s Law for Vaping


Learning about Ohm’s law is essential if you’re getting into e-cig mods, but there are plenty of useful calculators you can use if you’re allergic to math. The rule basically describes the relationship between voltage, resistance and current.


Think of these three quantities like a flow of water down a hosepipe connected to a faucet. The voltage is the raw input energy supply; analogous to how much you’ve opened the faucet to start the water flow, and the current is the flow of the energy; analogous to the speed the water flows down the hosepipe. Resistance is like friction for electronic current, like the bumps and irregularities on the inside of the pipe that slow down the flow of water.


Ditching the analogy, the voltage is the difference between charges at either end of the circuit, the current is the measure of how much electricity is flowing and the resistance (of the conductive material) opposes the flow of electricity.


Current is equal to voltage divided by resistance. Mathematically, it looks like this: I = V / R, where I is current (measured in amps, A), V is voltage (measured in volts, V) and R is resistance (measured in ohms, Ω).


Although resistance converts the electrical energy into heat (warming your atomizer coil and vaporizing your juice), current is the most important factor when it comes to vapor volume. If we set the voltage at 4 V, a resistance of 2 Ω would offer 2 A of current. If the resistance is increased to 3 Ω, the current drops and therefore your coil doesn’t heat up as well (4V / 3 Ω = 1.3333… A).


Of course, the math isn’t always this easy when decimal values of voltage and resistance are involved, but any calculator (even the one on your computer/cell phone by default) takes the mental arithmetic out of it.


E-Cig Mod Battery Limits and C Ratings


With this information alone, you’d be forgiven for thinking that you want to pump the voltage as much as possible and keep the resistance to a minimum. However, you run into issues because any battery can only supply so much current. This is given by the maximum continuous discharge rating of the battery (there is a burst discharge rating too, but this is inapplicable to vaping), which is an amount stated in amps.


Here’s a handy list of the amp limits of the most common vaping batteries, including AW IMR 18650s, 18490 and 18350 batteries. For example, an AW IMR 18650 with a capacity of 1600 mAh (a measure of the number of amps a battery can provide over time before it needs recharging – this will be listed on the body of any battery) can handle 24 A, and an 18350 with 700 mAh can handle 6 A of current.


If – for whatever reason – you can’t get hold of the amp limit for your battery, you can calculate it easily based on the C-rating of the battery. To turn this C-rating into a continuous current draw limit, you simply multiply it by the capacity of the battery and then convert the result (which will be in milliamps, 1/1000 of an amp) into amps.


So for the AW IMR 18650 with a capacity of 1600 mAh and a C rating of 15, the amp limit is given by 1600 mAh × 15 C = 24000 mA. Since 1 mA is 1/1000 A, 24000 mA is 24000/1000 = 24 A. If you’re within this limit, the battery can handle what you’re asking for. Keep in mind, though, that this isn’t a definitive value, just an average, so it’s better to assume the safe limit is a little lower than that stated.


Sub-Ohm Vaping Safety


Now the issue can be investigated pretty clearly. With an IMR 18350 battery with a maximum continuous discharge current of 6 A, it’s unlikely you’ll hit a limit unless you’re using a coil with a resistance of lower than 1 Ω. Using 1.5 Ω at 3.5 V, the amps you’re requesting comes to 3.5 V / 1.5 Ω = 2.3333… A, well within the safe usage limits of even the small battery.


When you drop the resistance to 0.5 Ω, you’re requesting 3.5 V / 0.5 Ω = 7 A from the battery. This is beyond the capability of the battery and therefore is virtually asking for trouble. If you were using a 2000 mAh IMR 18650 battery – with an amp limit of 10 A – you’d still be in the safe zone, which underlines the benefits of a larger battery for anybody interested in sub-ohm vaping.


Using Ohm’s law and the information given on the maximum continuous discharge rating for your battery, you can ensure you stay safe when you’re vaping. The best advice, in terms of safety, is to only use coils with a minimum resistance of around 0.8 to 1 Ω, and if you’re vaping on low resistance, ensure you have a capable battery.


This minimum resistance isn’t a definitive rule, strictly speaking (if you’ve done the math and you know you won’t be overworking your battery), but it reduces the possibility of problems significantly, and you’ll still get amazing performance from your atomizer. If you’re doing the calculations yourself to ensure your battery can cope, be wary of approaching the limit. You’re still affecting the overall lifespan of the battery if you run it close to its maximum output current, so it’s better to leave some wiggle room.


Conclusion – VV/VW E-Cig Mods Are the Safest Option


With the information given here, you can theoretically vape safely on any device, but VV/VW (variable voltage or variable wattage) devices have protective measures against mistakes, so if you ask for too much from your battery, the device will refuse to comply.


For mechanical e-cig mods, the protection isn’t always built in, so it’s all down to you to determine if your battery can provide what you’re asking for. The same goes for an atomizer short; VV/VW mods will let you know something is wrong, mechanical mods without a safety fuse will let you push your battery to breaking point.