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January 03, 2020 3 min read

Understanding Ohm’s law is a fundamental aspect of battery safety when delving into the world of mechanical mods. If you have a good knowledge of battery safety, the next step would be understanding the safety limits of it. There are many online calculators that do the heavy work for you, which is fine if you wish to only apply the results and use an ohm meter. However, if you want to understand the inner mechanisms of it, then keep reading my friend.

At first Ohm’s law may seem like a mystical formula, however it is like any other scientific formula and makes sense once you break it down. This formula is usually depicted in a triangle with a V on the top and I and R at the bottom, respectively.

You remember this formula back in high school physics class, the point of it is to show you the relationship between 3 different electrical forces: Voltage (measured in volts), Current (measured in amps) and Resistance (measured in ohms).

An easy way to remember this triangle, that I used back in high school was the acronym VCR: V stands for voltage and is at the top of the triangle, C stands for current (I) and is on the left of the bottom triangle portion and R stands for resistance.

 

HOW TO USE THE TRIANGLE?

Let’s calculate the current together. Firstly, what is current? In short, it is the flow of electrical charge. Think of it like the water pressure of a hose; the higher the water pressure the greater the current.

If you want to determine the electrical current, cover the I (amps) section of the triangle and you get V/R:

I = V divided by R (I = V/R)

Let’s put this formula to good use. You are using a mechanical mod, with a fully charged battery; this means you theoretically have 4.2V to power your coil. The coils resistance is 0.6 Ω, you know have all the variables needed to calculate correctly. Just plug it in the formula like this:

I = 4.2V divided by 0.6 Ω (4.2/0.6)

I = 7 A

This figure tells you the current draw with a 0.6ohm coil and a fully charged 4.2V battery will be 7 amps. If your battery has a 10-amp limit, then you are well below this safety limit. Note, as the battery drains the voltage decreases and therefore the current. When your battery reaches 3.6V with the same resistance, the current will drop down to 6.2 A.

The maths for this is the same as what we did before, just change the 4.2 to a 3.6 and follow the exact same formula.

 

CALCULATING THE WATTAGE

You are probably now wondering how to calculate the power output of the coil, which is usually measured in wattage (watts). It’s not shown in the triangle, but the formula is simply:

P = V x I

Power = voltage x current

Lets plug in our original numbers

P = 4.2V x 7A

P= 29.4 W

You can now see the relationship between the resistance of the coil, current and wattage. As the resistance increases, current drops and therefore wattage drops.

 

CALCULATING THE RESISTANCE

To calculate the resistance, we apply the same steps as before. Calculating the resistance is as  simple as covering the letter R in the pyramid and you get V over I.

Therefore, the formula would be: R = V/I

R = V ÷ I

Let’s plug our previous real life example figures into the formula

R = 4.2 V ÷ 7A

R = 0.6 Ω

This figure tells you’re the safe lower limit for your 7A battery, if you use a lower resistance you risk it exploding. If you use a 10A battery your resistance drops to 0.47 Ω

R = 4.2 ÷ 10

R = 0.47 Ω

 

CALCULATING THE VOLTAGE

This last part of the equation is not the most useful, however if you wish to calculate the voltage for some reason it is:

V = I x R

 

SUMMED UP

The most important information for vapers and mechanical mod users would be calculating resistance (R = V divided I), current (I = V divided by R) and power (P = V x 1). With these you can figure out the current your coil will draw and the resulting wattage. The pattern we see after observing ohms law is a drop off in power (wattage) and current when you increase the resistance. If you do the opposite and decrease resistance the power and current will increase. Using this allows you to calculate the safe low resistance based on the battery manufactures CDR (continuous discharge rating).

Note: when making these calculations always assume the battery voltage is fully charged, ie. 4.2 for a single battery or 8.4 V for a dual series or parallel batteries.



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