You’ve gotta love the teardowns.
With the new iPhone 6s Smart Battery Case in mind, the cool cats at iFixit have performed a full teardown of the external battery. What they found after removing massive amounts of adhesive, a lining that had to be heated before it could be stripped away, three proprietary screws and a metal reinforcement plate was a 7.13 Whr (1877 mAH at 3.8 V) capacity battery.