Battery pack

Most external battery bpacks use a usb to Apple connector to charge the phone - so if you have an iPhone 5 the usb to lightening connector should work fine -Mophie, Kensignton, Griffin and several others make external battery packs - the voltage has not changed since the 4S so any of them should work fine - they dont need to specifically be compatible with the iPhone 5

Now if you are talking about a battery case which is a case for the Iphone with a built in battery thats a different matter since the case will need a lightning not a 30 pin connector to hold the phone - they are just beginning to show up - most of the case manufacturers who make battery cases still hvent produced them for the iPhone 5 or are saying they will be availble around the end of 2012

one thing i forgot to mention that there are some external battery packs that plug directly into the iPhone - those (if they have the 30 pin connector) will not work with the iPhone 5 - they might work with the lightning to 30 pin connector but perhaps not since they want to fit directly into the connector port and the connector may not allow that to happen - i havent yet seen those type of external batteries for the iPhone 5 but im sure they will be coming

external batteries that plug into the iPhone with a cable should be fine if you have the udb/lightning cable for the iPhone 5