The coil on the mini protank 2 ships at 2.5 ohm I think?
That is NOT going to be ideal on a 3.7v or < battery.
Yup just checked mine - it's reading 2.6 ohms on the itaste VV 3. When I run the protank on the ego (vision spinner) I crank that thing up to 4.6-4.8v. Unless you replace the coil with a lower resistance one, I do not believe you will get the performance you "could" on that protank.
Anything I could find online says 3.7v (fixed). If; and bear with me, I'm new. That means it pushes out a little more when it comes off the charger, and then drops as it dies... you will never put enough power to the PT 2.5 ohm coil to break 7 watts. Even if it comes off the charger pushing out 4.2v.
If fixed means that it's regulated at 3.7v until it literally turns off. I would want 1.8 ohm coils. That puts you around 8 watts the whole time and is about where I run my protank now and it's a good vape. Not too hot, not to weak, good balanced performance.
So if someone can clarify whether your voltage changes - or it is regulated. You could pick an appropriate (ohm) resistance coil for your non VV battery and still get all the performance you can out of the PT with that battery!