5v on a protank ii is pretty high. Sounds like one great hit followed by a very dry/burnt hit. But, as it's been mentioned, it all depends on the resistance of the coils you're using. I keep mine at 4.2v on a 2.5o coil, but that's just my preference. I wouldn't go any higher than 4.4v, and anything under 2.0o I wouldn't go higher than 3.8v