There is a new generation of PV's on the market now: vv ego style with usb charging. While not a true usb pass through device since they run off the battery, they will satisfy all but the most rabid chain vaper. These are true 3 to 6 volt devices with voltage increments of .1 v. Match them up with a carto tank or RBA tank, and your only limitation is how much juice your tank holds. When you are done driving, just unplug the cable and use them as you would an ego twist style with the advantage of a display-for around $30. Tanks seem to be a natural for these devices. No fiddling with dripping and swapping batteries while driving (dripping and driving is very high up on the list of really dumb things to do). The mini vv usb passthrough would not be a bad device for $15 or so, but it is an ergonomic disaster. It is too small to hold any type of tank system comfortably, and it can get really hot since there is not enough metal to act as a decent heat sink. The cable can work it's way out of the protective sheath until you are left with some bare wire (can be solved with some glue). I epoxied mine into an old e power tube, but have basically abandoned it in favor of the vv ego style usb models. There seems to be a supply problem right now, but that probably has something to do with the Chinese New Year. Last time I looked, Hossiervapes has some in stock.