FanWriter is correct about the FIVE VOLT PTs (no inline battery)  being ... let's say... not a good idea.... plugged into a computer USB port.  Just to rephrase FanWriter, the battery driven 3.x volt PTs are just trickle charging and should be fine on a computer USB port (not counting shorts, fried circuits or other such that can happen on any USB device).  The battery PTs only draw 400 ma (.4 amps) or so.  The 5 volt PTs, OTOH, need TWO amps.... so use the wall wart.