Vaping safety starts with the vaper having a clear understanding of the battery limits. If you haven’t learned this yet, or want to review this knowledge, you can refer to The Vape Club’s previous article here .
If you feel confident about safety knowledge your battery, the next step is to use a calculator to make sure that your coil is within the safe limits of the battery, and furthermore, that there is still enough headroom to allow you to change the coil to create the desired experience. There are many Ohm's Law calculators online, sites like Steam Engine will do the math for you.
If you're happy with these, and want to continue to ignore what the numbers mean, that's fine. As long as you know how to apply the results and use an ohm meter, you'll be on your way to a safe, healthy vaping life. But...
...If you want to continue learning about how these calculations work, read on.
There is nothing mysterious or magical about Ohm’s Law. It is just some mathematical formulas arranged in a triangle, and anyone can learn and use these formulas with a regular calculator. No need for an “Ohm’s Law calculator”, or Steam Engine.
In fact, I'm sure you learned Ohm's Law in your first or second electronics class. It's that simple. The goal here is to see the math behind Ohm's Law, and hopefully understand the interrelationships between the various elements in a basic electronic circuit that's involved in vaping.
Triangle correlation
In this triangle, you will see the three main elements in most common circuit boards represented by the letters V, I, and R. I will read this triangle as “V equals I times R” with the “times” being multiplied. The hardest part of this triangle is remembering what these letters stand for, which is very simple:
V = voltage (battery voltage)
I = current intensity (number of Amps drawn by the coil)
R = resistance (coil resistance in ohms)
So how do we use this Ohm’s Law triangle? Simply put, it visually describes the relationship between voltage, current, and resistance. In the examples below, we’ll learn how to apply this triangle and formula to build a coil that targets the current and wattage you need.
Calculate current intensity
If you want to specify the current going through the coil, the formula is:
I = V ÷ R (or I = V/R)
Why this formula? Look at the triangle and you will see that to calculate the current (I) you have to divide the voltage (V) by the resistance (R).
Let’s apply this formula to a real-world example. If you’re using a mech mod, with a fully charged battery that supplies 4.2V to the coil. If the coil has a resistance of 0.5Ω, you have enough data to calculate the current in amps:
I = 4.2V ÷ 0.5Ω (or 4.2/0.5)
We will have the result:
I = 8.4A
As you can see, the current drawn by the 0.5Ω coil with a 4.2V battery is 8.4A. If your battery limit is 10A, you are still in the safe zone. And remember, as the battery drains, the current will also decrease. For example, if the voltage drops to 3.7V, with the same data, the current will drop to 7.4A.
How to calculate power
The next thing you want to know for sure is the power that will be generated by the coil. This calculation is not shown in the triangle, but the formula is simple. You just multiply the current in the circuit by the voltage.
Recipe
P = V x I
Also according to the above example, we will have the following formula:
P = 4.2V x 8.4A
Result:
P = 35.3W
So, a 0.5Ω coil with a 4.2V battery will draw 8.4A and produce 35.3W. As you can see, as the coil resistance increases, the current and power will decrease.
How to calculate resistance
The second Ohm’s Law formula we can use is to calculate resistance. Let’s say you have a battery with a current limit of 10A and you want to calculate the lowest coil resistance you can safely use without exceeding the CDR of the battery.
To calculate resistance, you will use the following formula:
R = V ÷ I
Since you know the CDR limit of a 10A battery, you will take the 9A number for your calculation, leaving about 1A. With the battery voltage assuming 4.2V for a single battery mod. So we have the following calculation:
R = 4.2V ÷ 9A
Infer
R = 0.47Ω
According to the above results, the lowest resistance limit of a 10A battery is 0.47Ω, any lower, and you will exceed the current limit of the battery, and... boom. Of course, if you have a 25A battery, the resistance limit will go down to 0.17Ω.
R = 4.2V ÷ 25A R = 0.17Ω
R = 0.17Ω
What do these values mean?
There are really only three formulas for calculating current (I = V/R), power (P = V x I), and resistance (R = V/I). These formulas will help you predict how much current the coil will draw and the corresponding power. As you increase the resistance, the current and power will decrease. If the resistance decreases, the current and power will increase. The resistance formula allows you to calculate a safe resistance that matches the CDR of the battery.
This information will help you stay within the safe range of your battery and adjust the wattage to your coil to achieve your goals. Of course, there are some factors like coil ramp time and coil temperature that are determined by the wire. Ohm’s Law won’t account for these, so sites like Steam Engine can help.
And the last and most important tip: Always use the voltage rating in the formula as the value of a full battery (4.2V) for single or parallel battery mods, or 8.4V for series battery mods. Some people will argue that the coil will never use that voltage level because the voltage will often drop in the mod, but using the battery rating at full charge is always the safest.
