How to chose your Batteries
Welcome, here you can find the way of using your devices choosing the right batteries to vape safe and have the more capacity available for the power range usage you are going to use them on your device.
On an unregulated you need to know the resistance of the coil and the nominal voltage of the battery, this makes sense on an unregulated device because the resistance of the coil is what's going to determine how many Amps you are going to be draining from the battery, however, if you are using a regulated mod, this does not apply, resistance of the coil means absolutely nothing on a regulated mod.
Regulated mods use a DC-DC converter, usually a switched-mode converter so the input and output voltage will be separate (in other words, the battery is separated from the atomizer), just because you have 3.6v input from your battery doesn't mean this is what you will have on the output.
From Ohm's law Power (W) = Voltage (V) x Current (A) so as the voltage drops, this necessarily means less power (watts).
On an unregulated mod, as your battery drains, you have less voltage (and thus less power) hitting the atomizer, this means the vapor production diminishes over time.
On a Regulated mod the regulator will swap voltage (V) for current (A) in order to achieve the power (W) you have your mod set at.
So, because P = I * V. when the voltage drops, the power also must drop then we need to increase the "I" (current) to compensate the battery drain and this will allow you to keep your desired power during the entire battery charge.
The equation for current is I = P / V. That is, Current = Power/Voltage. Voltage will be a fixed parameter on the equation but of course even if you chose to do the math with the nominal voltage of 3.6 Volts that will not apply entirely during the discharge of the battery since most devices are going to drain the batteries down to 3.2, 3.1 or even 3.0 volts.
With the equation I = P / V you can chose an 20Amp battery to vape at 72Watts an a single battery mod but when the battery drains below the nominal voltage you will get the Power decreasing down to 62Watts and on some mods the message "week battery" display'd on the screen.
With that being said, in my personal opinion you can chose a battery based on the nominal voltage but you would get an more reliable and accurate experience choosing a battery based on the minimum voltage as you can check on the table below.
Some Multiple Battery mods will run multiple batteries in series or parallel.
A series configuration will treat the two batteries as one and as consequence the double or the triple of the voltage but still the same Amps. As a consequence of that, having more Volts the device will need to drain less Amps from the batteries.
Parallel configurations do not double the voltage, however, the capacity of the cells (amp hours) is doubled and the available Current (A) also. As consequence of that, with the same voltage, now the device is going to drain more Amps from both batteries.
So, series and parallel devices achieve the same thing but in a different way, series mods are going to do it with more voltage (which decreases the need for more current), while parallel mods are going to do it by doubling the amp hours (increasing the available current).
Example: You have the Wismec Reuleaux RX200, you don't vape more then 90Watts, since the device is a triple 18650 series battery mod, if you chose to use 10A batteries you will have more capacity, for instances if you chose 10A 3100mAh batteries you will end up with 3 x 3100mAh of capacity.
Here you can look for the Watts you vape or the Watts you want to achieve on your device and then pick the right battery, based on the battery configuration on the left column, just look for the Watts you want to use your device.
Here you can search for the better batteries to use. Source