The inner aluminum plate serves as a door for the battery compartment, which houses a 1,877 mAh battery (7.13 Whr) that more than doubles the iPhone 6s battery capacity of 1,715 mAh. iFixit says the battery is "very strongly adhered in place" with a connector rather than soldering.
Much of what makes Apple's Smart Battery Case "smart" is powered by the iPhone, as the case only has a few other components, including a NXP NX20P3 load switch, also found in Lightning to USB cables, and a NXP 1608A1 charging chip.
iFixit awarded the Smart Battery Case a repairability score of 2 out of 10, with ten being the easiest to repair. While the battery pack can technically be replaced, the case has a strong adhesive lining, and all other repairs would destroy the accessory.