Some PTs regulate the voltage to 3.7V and some don't. It depends on who manufactured the PT. Unless it is stated by the manufacturer (such as Puresmoker), I don't know of any way to tell which is which other than checking with a voltmeter.
As far as the battery packs, I wouldn't get one unless it can supply at least 1.5 Amps output. I have a Kensington and an old APC Moble Power Pack (discontinued, unfortunately) that work great. I have used a couple of different wall-wart usb converters (up to 1000 mA) and an old usb powered hub, but I have been disappointed in their performance. I can only figure it's because they don't put out enough current, so I personally would pass on a battery pack that only outputs 1000 mA (1 Amp).
Another option is to get a PT with an inline 3.7V battery. They will work with most any battery pack since the atomizer is powered by the inline battery, not the usb power. The battery pack will keep the inline battery charged, so you get the best of both worlds. Unless you want the full 5V, of course. If your model doesn't offer a PT with an inline battery, you could always buy a 510 PT and switch out the atomizer connections.
TT33
As far as the battery packs, I wouldn't get one unless it can supply at least 1.5 Amps output. I have a Kensington and an old APC Moble Power Pack (discontinued, unfortunately) that work great. I have used a couple of different wall-wart usb converters (up to 1000 mA) and an old usb powered hub, but I have been disappointed in their performance. I can only figure it's because they don't put out enough current, so I personally would pass on a battery pack that only outputs 1000 mA (1 Amp).
Another option is to get a PT with an inline 3.7V battery. They will work with most any battery pack since the atomizer is powered by the inline battery, not the usb power. The battery pack will keep the inline battery charged, so you get the best of both worlds. Unless you want the full 5V, of course. If your model doesn't offer a PT with an inline battery, you could always buy a 510 PT and switch out the atomizer connections.
TT33