iFixit:

In one of the least Apple-y moves in history, Apple released the Smart Battery Case, and it’s ugly. Rumors about this unexpected accessory are flying thick and fast, let’s open this pack up to see how it works.

Great shots as usual, a lot of interesting information, and one interesting tidbit:

With 7.13 Whr (1877 mAh at 3.8 V) capacity, this battery more than doubles your iPhone 6s (6.55 Whr) capacity.