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(16) Explain it for the dumb noob: Ohm's Law calculations for a Mechanical Mod.

Published by Baditude in the blog Baditude's blog. Views: 32087

Here is possibly a simpler explanation of Ohm's Law as it applies to vaping.
Ohm's Law Explained for Vapors

Now, back to your coil question. NO, THAT BUILD IS NOT SAFE FOR THE BATTERY YOU ARE USING.
List of Batteries and Amp Limits By using this list in the link, you'll find your battery has a 20 amp continuous discharge rate (CDR).

The two most important things to know when rebuilding coils is to know the amp limit of the battery you have and to know the measured resistance of your coil. This is where Ohm's Law comes into play. If you come away with anything after reading this article, this is it.

When you push the button on a mech mod, you complete a DC circuit. The battery in your mod doesn't know or doesn't care what the resistance of the coil wire is. All the battery knows is the circuit is complete. And that it has to obey Ohms Law.

Ohms Law says that the amount of amps will be equal to the voltage divided by the resistance. The more resistance you have with the coil, the higher the ohms will be and the less amps from the battery are needed. The lower the resistance of the coil, the lower the ohm will be and higher amps will be required to fire the coil.

So a freshly charged battery at 4.2 Volts needs to provide 14 Amps if the Coil Resistance is 0.3 Ohms.

Amps = Volts / Ohms => Amps = 4.2 / .3 => Amps = 14

It doesn't matter if the battery can safely do this. It's what the battery has to do.

So if you ask a battery to provide more amps than it is capable of, it is going to get hot trying to power the coil. Just like a thin extension cord gets hot when you draw too many amps thru it.

But a battery isn't an extension cord. It is a cylinder full of chemical compounds. And these chemical compounds can rapidly react, breakdown and even burn/explode. And when they do, they release gases. This is called "thermal runaway", or venting.

It's these vented gases that can build up in your mod causing it to explode. Thus, mechanical mods must have adequate vent holes to allow the escape of gas.

IMR batteries use what is called "safer chemistry". In that they do not (should not) catch fire or release large amounts of gases when they Vent. So they are MUCH safer to use than regular rechargeable batteries (ICR or Li-Ion).

But they cost more. So people don't use them unless they need them. Vapers who use mech mods need to use IMR (or INR ) batteries so if there is a problem, and the battery fails, it shouldn't cause a mech mod to explode. The IMR battery will just basically stop working or release some gas. Just make sure the mod has adequate vent holes.

If you're going to use a mech mod, you have to be able to calculate how many amps you are going to ask your battery to safely provide. Then you have to look at what the "continuous" amp rating is for the battery.

So if you battery is rated at 20 amps continuous discharge, you should not build anything that requires the battery to provide more than about 18 Amps. You should ALWAYS leave a little "headroom" or safety margin.

In review, when you build your coil and fire it on your mod, it will draw a specific amount of current (amps) from the battery. That current must not be more than the total amps in continuous discharge rate of the battery, or very bad things could happen.

[​IMG] [​IMG] [​IMG]
hand trauma from vented battery and mod explosion

Never fire a coil without first confirming the ohm resistance on an ohm reader or multimeter. You can't just rely on a coil wrapping calculator or somebody's recommendations, there's too much chance for human error. The smallest error can be catastrophic. Even seasoned veterans always check the resistance of their coils on a meter to make sure they are safe.

1) To find out what current (amps) battery you'll need for a regulated mod, see Calculating battery current draw for a regulated mod

2) To find out what current (amps) the coil will pull from the battery in a mechanical mod, you use an Ohms Law Calculator.


You have the resistance of the coil (what you measured with your ohm meter) and the voltage (always use 4.2 volts of a fully charged battery), so type those figures into the calculator and then click calculate. The current is the amps that coil will draw from the battery. Now compare the coil amp draw to your battery's amp limit. Are they compatible? Not so hard, right?

The below calculations demonstrate that the lower you go in ohms the higher the amp requirement becomes. See how your 0.19 ohm coil will draw OVER 20 amps from your battery. You are also putting a lot of faith into a cheap ohm reader in being precisely accurate to the tenth/hundreth of an ohm. Always tend to err on the side of safety when you make your builds by allowing some safety head room.

1.0 ohm = 4.2 amp draw
0.9 ohm = 4.6 amp draw
0.8 ohm = 5.2 amp draw
0.7 ohms = 6 amp draw
0.6 ohms = 7 amp draw
0.5 ohms = 8.4 amp draw
0.4 ohms = 10.5 amp draw
0.3 ohms = 14.0 amp draw
0.2 ohms = 21.0 amp draw
0.1 ohms = 42.0 amp draw
0.0 ohms = dead short = battery goes into thermal runaway​

Everyone is free to set their own parameters, and I can only say what mine are.

I try to never exceed 50% of the CDR (continuous discharge rating) of a fully charged battery (4.2v). So with a 20A batteries, that would be 10A. The above Ohm's Law Calculator tells me that a .4 ohm build is as low as I would want to use.

The reason that I place a 50% limit is because as a battery ages the mAh of the battery degrades, as the mAh degrades so does the batteries c rating (amp limit). So down the road, your 20A battery may only be a 10A battery.

Sorry for the graphic photos above, but I believe its important to get the point across that you should not mess with Ohm's Law when it comes to batteries. The above pics are an extreme example. The batteries we have can be quite safe if you use the correct batteries and do not abuse them beyond their recommended amp limit. Most battery incidents result from user error or wrong calculations, or ignoring safe battery practices.

A battery venting in thermal runaway will release extremely hot gas, toxic chemicals, and possibly flames. Once this chemical reaction begins, there is no stopping it. The gas can build up inside a mod, and if there is inadequate venting the mod becomes a little pipe bomb.

[​IMG] What's left of an exploded mechanical mod after a vented battery

Man Severely Injured After E-Cigarette Explodes

I personally don't believe anyone should build lower than 0.2 ohms over their battery's maximum continuous discharge rate. This gives a tiny bit of head room should your post screws become loose which can change the coil resistance, and also accounts for some error in your Ohm reader. Periodically recheck your build's resistance to insure it doesn't unknowingly fall below your target resistance. Also know there are two amp ratings: Continuous and pulse (burst) discharge rating. I prefer using the continuous discharge rating over the pulse discharge rating. Pulse ratings are always higher than the continuous, and are not as reliable as the continuous rating.
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