(16) Explain it for the dumb noob: Ohm's Law calculations.
by, 08-17-2014 at 03:03 PM (3792 Views)
Ohm's Law for Dummies
Now, back to your coil question. NO, THAT BUILD IS NOT SAFE FOR THE BATTERY YOU ARE USING.
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 resistance of your coil. This is where Ohm's Law comes into play.
List of Batteries and Amp Limits Your battery has a 20 amp continuous discharge rate.
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.
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.
To find out what current the coil will pull from the battery, 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. Not so hard, right?
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
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.
What's left of an exploded mechanical mod after a vented battery
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.