The power packs [Andy] looked at have a TP4221 controlling the charge cycle ... to 5 volts for one or two USB ports while providing automatic shutdown if the LiPo cell voltage drops below 3.2v.

Luckily, [Paul] sent in a great tutorial on building a LiPo charger that works over USB. The board [Paul] came up with is incredibly small – just barely larger than the USB plug itself.