from what I recall reading on the forums, PT's vapes at 5v when you use an AC usb adapter only with 5V and dedicated 2 amp I think it was.. when you use it on your puter, it does not.. Think if you do a search (at bottom of page), you should get lots of info about it..
From personal experience, PT on puter is better than batteries but PT on my AC usb adapter is even better but, only with cartos like the cool cartos..