USBs run at 5 volts. If there is some kind of inline battery (aka. a little black box), I think it's a 3.7 volt model. I'd recommend a the 5 volt since it's a great way to try out higher voltage for a cheap price. Around $15. Did that answer your question?
BTW There are a few models out there that are a kind of hybrid. The battery has a USB mini plug at the end where there is usually a LED. This works as a pass
through you can unplug and take with you once charged.