Calculating battery current draw for a regulated mod
Calculating the current being drawn from the batteries in a regulated device can be very confusing. You can't do it the same way as you would for a mechanical/unregulated device and there are so many different battery configurations; single, dual parallel, dual series, triple series, etc.
The way I keep it all sorted out is to remember that, in a regulated mod, the coil isn't connected to the battery. The regulator is. To calculate the current being drawn from each battery when using variable-wattage (VW) mode you need to calculate the maximum wattage each battery supplies.
Here's how I do it...
As an example, the RX200 has a maximum wattage rating of 200W. Since it uses three batteries that means each battery supplies 200W / 3 = 67W. For dual parallel or series 150W devices each battery supplies 150W / 2 = 75W. You use this method for series or parallel devices, it doesn't matter.
Once you have the maximum wattage for each battery then you can use the following formula to determine the maximum amount of current that can be drawn from each battery...
Max Amps Per Battery = Max Wattage Per Battery / Minimum Voltage Per Battery
For the RX200 the minimum possible cutoff voltage is 9.0V, which is 3.0V per battery (unless you set the cutoff higher). For most other devices the minimum is 3.2V or 3.1V per battery. Let's use the Sigelei 150W TC device as an example. This device has a minimum battery voltage of 6.4V, which is 3.2V per battery...
Max Amps Per Battery = 75W / 3.2V = 23.4A
So you want a battery that can safely supply 23.4A of current if you're using the mod at its maximum rating of 150W.
I should add that to get as close as possible to calculating the max current being pulled from your batteries you should add an additional 10%. This will account for the inefficiency of the regulator. For example, if your device draws 23.4A then add 2.34A for a total of 25.74A. Not a big difference, but it's there. That changes the equation to...
Max Amps Per Battery = (Max Wattage Per Battery / Minimum Voltage Per Battery) / 0.9
If you know you will not be exceeding a particular wattage that is less than the maximum then you can use that wattage in the equation instead. This often means you're able to use a higher capacity battery like the HG2 or 30Q instead of a high current rated, but lower capacity, battery like the VTC4 or HB6. It's worth doing the math to find out.
This works for series or parallel devices. It does not matter how they are connected as we are already taking that into account when we calculate the max power for each battery.
It takes much longer to explain all this than it does to actually calculate the amount of current being drawn from your batteries. I hope this helps make the very confusing process of determining how much current is being drawn a little bit easier.