It all has to do with Ohm's Law and the interrelationship of voltage (volts), resistance (ohms), power (watts), and current (amps). It
can get complicated I suppose, but it
doesn't have to be ... and you don't have to have any background in electronics to understand it.
I don't.
batteries have
voltage and atomizers have
resistance, but what we vapers really care about is
power (i.e., the intensity of the
vape, measured in
watts). And we also need to be concerned about
current (i.e., the
amps, which can burn out atomizers).
The
power formula is: Watts = Volts X Volts / Ohms. So a 2.2 ohm 510 cartomizer on a 6V Prodigy V3.1 yields 16 watts: too intense for most of us. [6-8 watts typically is considered the "sweet spot."] But of greater concern here is the current.
The
current formula is: Amps = Volts / Ohms. So that same setup yields 2.7 amps: atty frying level. [Around 1.5 amps is friendly to atomizers.]
"HV" atomizers (actually high-resistance) are designed for use on high-voltage mods. 4.5 ohms is typical. On a 6V Prodigy, these would yield: 8 watts (yummy) and 1.3 amps (comfy for the atty).
High resistance atomizers don't really bring down 7.4V (or 6V) to 5V, but to a power level (watts) consistent with what a regular resistance atomizer on a 5V mod would yield. I don't know whether the Prodigy should be used with two 3.7V
batteries. If you do, a
5.2 ohm HV atomizer should be used: 10.5 watts (too high for me, but good for some I guess) and 1.4 amps.
For more on this, check out my
What are these HV and LR attys? article on VapingGuides.