Battery Safety Part 2 ~ Doing the Math

Math? Really?

Last week we touched a little on the dangers of overdrawing current from your batteries and the events that might occur if you do. This week we will get into how to avoid doing so, with the awesome power of math. Not all of us excelled in it in school, but knowing the math is one of the most important things you can know about rebuildables. Besides, many of us have learned more about science through vaping than we ever did in school.

Down to Business
The formula, which will be explained below, to measure the amperage draw of your build is V / R = I where:

V = Voltage, measure in volts
R = Resistance of your atomizer, in ohms (Ω)
I = Current, measured in amps

Voltage is the voltage coming from the battery. On a fresh charge, battery voltage is 4.2 volts with a small tolerance one way or the other. There is a small amount of voltage drop that factors into the equation (especially in mechanical devices), but without testing that particular resistance and device combination, it’s nearly impossible to know exactly how much drop there is, so for safety we always assume you’re getting the full battery voltage to the atomizer.

Divide the battery voltage by the resistance of the atomizer (which you should have measured on an ohm meter beforehand). This will give you the amperage, or current draw. As an example, suppose you've built a coil measuring 0.4Ω that you’d like to use on your device. Plugging values into the equation you should have 4.2V / 0.4Ω = 10.5 amps. This is well within the limits of most of the IMR batteries that we carry.

Lower and Lower
Remember, going to lower resistances will cause the amperage draw to increase exponentially. Using the same formula, we find that a 0.2Ω coil will draw 21 amps, where battery ratings start becoming an issue for safety, and a 0.1Ω coil will draw 42 amps, which only a very few batteries available can handle safely.

Need a Cheat Sheet?
Everyone does. Even I do occasionally, either because I know the numbers will be close to my limitations if I'm trying mental math, or because I have a case of the lazies and want to make life a little easier.

There are calculators abound on the internet, but the one I go to myself and recommend for others is a nifty little site called Steam Engine. There's a link to it here. I like this one because it calculates almost anything you'd want to know about in vaping, from Ohm's Law to DIY e-liquid and some I think are exclusive to Steam Engine like expectations of battery runtime and sweet spots in resistance for specific devices based on their specs.