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.

The power packs [Andy] looked at have a TP4221 controlling the charge cycle for up to four 18650 LiPo cells connected in parallel. The controller also boosts the voltage to 5 volts for one or two ...