When fully charged, most 18650
batteries have a charge of 4.2 volts. In a perfect situation, all 4.2 Volts should go across your coil when you fire the
mod, but due to internal resistances of the mech and the internal resistance of the battery, the actual voltage across your
coils will be lower, say 3.7V. The voltage across your coil determines the how powerful your vape is, so big voltage drops are looked down upon because all that power is wasted
So basically, there will be a drop in voltage between what your battery's charge actually is compared to what is actually going across your coil.