Eton is one of many companies producing backup battery packs for iPhones and iPads, including its Boost 4200 ($55). As the name suggests, there's a 4200mAh battery inside the slim, black metal housing. Sold with a micro-USB cable, it has two ports: one micro-USB for charging, and one 2.1-Amp full-sized output. To check the remaining power level, you shake the battery and LED lights will show you what's left inside.
