uNu Exera Battery Case & Duo Charging Dock for iPhone 4 & 4S - Black The exera power system includes one exera battery case, three 1700mah battery modules, and one multi-functional duo charging dock (hub). the Exera Battery Case features a swappable 1700mah battery module, a removable battery housing, an interchangable slim backcover, and a bumper with a great selection of different colors By using the two integrated Exera battery ports users can connect and charge Exera batteries directly to the hub, eliminating the need for a phone-charging cable. Plus, swapping depleted batteries with fully-charged batteries from the hub allows for a constantly replenished source of battery power. This comprehensive hub is equipped with two Exera battery ports and an additional USB outlet allowing for up to three devices to simultaneously connect. By charging multiple batteries at once uNu has eliminated constantly swapping charged and un-charged batteries. Charge up to 3 batteries at once! Via the USB port and two Exera battery ports users can charge an iPhone 4/4S and battery case, plus two additional Exera batteries. In a single charging session users have access to a possible 4 charged batteries (integrated iPhone battery, and three Exera batteries)