Portable USB Charger

A portable USB charger is a device with an embedded battery or a slot for AA or AAA battery that can charge portable devices on the go via a USB cable that is not necessarily supplied.  Pictured are common portable USB chargers by Duracell and Energizer.

(Yasuhito prefers Duracell chargers in that generally they can be used with the USB cable that was supplied with your mobile device or smartphone.  Energizers tend to be slimmer but wider.)