Battery draw on a Variable Wattage Mod

[jasonculp]

I just wanted to post this so everyone could visually see what is going on with your Variable Wattage Mod in relation to battery drain.

With a Mechanical Mod as our battery drains, the Amps go down.  With a Variable Wattage Mod as our battery drains the the Amps go up.

We can't calculate battery draw based on the ohms of the coil in our atomizer like we do in a mechanical mod, because of the way the board works inside of the device.

In a mechanical mod we use ohm's law to calculate the draw on the battery.  For example 4.2Volts / .353Ohms = 11.9Amps or 4.2Volts² /.353Ohms = 50Watts.

In a single battery Variable Wattage Mod we have to calculate both sides of the board separately.  The only thing that is constant is the power (watts).

The board also uses power during the voltage conversion, so we normally assume it is 90% efficient. 

The way to calculate this (still using Ohm's law) is: 50Watts / 3.2Volts (the lowest voltage you want to run your battery) X 1.1 (adding 10% for a 90% efficient board) = 17.2Amps

*For a dual battery mod in series you would use 6.2Volts

Here is a chart to help visualize it:

 

Here is a link that will do the math for you.  Steam Engine - Battery Drain

 

Edited September 26, 2015 by jasonculp

[Rixter]

This information seriously highlights the need for good high-amp batteries to power variable wattage mods in combination with sub-Ω tanks. Kudos jasonculp.

[jasonculp]

This information seriously highlights the need for good high-amp batteries to power variable wattage mods in combination with sub-Ω tanks. Kudos jasonculp.

Thank you Rixter.  I have struggled with how to post this without being too nerdy.  There is so much more to it, and people think you are nuts when you tell them that the ohms of the atomizer have nothing to do with the amount of draw on the battery with a Variable Wattage device.  The device does not care if it is a .1 ohm or a 1.8 ohm coil.  It only cares how many Watts you are asking it to put out. Just to show the math:

Attomizer Side:

50 watts at .1 ohms is 2.24 volts and 22.4 amps

50 watts at 1.8 ohms is 9.5 volts and 5.3 amps

Since the battery (alone) can not supply 2.24 volts or 9.5 volts it is having to either boost or buck the voltage.  The only thing we have that is constant is the amount of power the board is asking for. So we have to use Ohm's law to see how much current it takes to get that much power from the battery.

Battery Side:

50 watts / 3.2 Volts X 1.1 to add 10% for the efficiency of the board = 17.2 amps

 

This calculation works on Variable Voltage mods too.  You just have to figure out how many Watts you are asking for on the given atomizer.

 

It even works on the Ego Style batteries.  They just have a 3.7 volt (nominal) battery in them. So we have a board doing all of the boost/buck for us.

1.8 ohms and 4.8 volts = 12.8 watts on the atomizer side (volts² / ohms)

12.8 watts / 3.2 (I really don't know how low the board will let it go) X 1.1 = 4.4 amps drawn from the battery.

Sorry for going all nerdy on this, but think we need to understand this so we choose the correct battery for our mods.  There is so much bad information out there and we need to keep this hobby as safe as possible. 

Here is a good rule of thumb for a single 18650 variable wattage mod:

Use a 20A continuous battery for under 58 watts

Use a 25A continuous battery for under 70 watts

Use a 30A continuous battery for under 85 watts

Here is examples for a dual 18650:

Use a 20A continuous battery for under 115 watts

Use a 25A continuous battery for under 145 watts

Use a 30A continuous battery for under 175 watts

Over 200 watts, use at your own risk.  There are LiPo packs, and triple battery mods that can do this safely.

This is why I stick with OEM cells, LG, Samsung, Sony, ect.  Using them beyond the mfg's CDR rating is possible in short bursts.  The Samsung 18650 25R is actually rated to 45A for 5 seconds, and even higher for shorter periods, but this causes temps to raise, and high temps cause degradation of the battery.

I personally choose not to go beyond the CDR.

 

 

Edited September 26, 2015 by jasonculp