Mugen Power Batteries Italy, producer of 3rd party batteries for a number of smartphones, has told Windows Phone Italy that they will be releasing battery cases for the Nokia Lumia 920 and HTC 8X.
The case would be similar to the one above, for the HTC One X, and can be added and removed as needed.
The company will also be producing an extended battery for the Nokia Lumia 820, which does have a removable battery and battery cover. It is not known yet if the battery will fit under the same cover, or will come with a new 3rd party cover to accommodate the later battery.
The external battery jacket costs around $100 and an extended battery by itself around $30