A merchandise guide produced for Apple retailers is showing the clearest evidence yet that Apple is readying a Smart Battery case for the iPhone XS and iPhone XS Max.
A merchandising guide intended for Apple retail employees features a range of iPhone cases including two with the distinctive hump of an extra battery. The guide was produced for Fall 2018 and contains no technical information surrounding wireless charging, and no other indication of the Smart Battery beyond thumbnail images listed as being for the iPhone XS Max and iPhone XS.
The leaked document first reported by Appleosophy is one page from a guide for Stores about both what Apple merchandise is available and how best to display it all. Some 96 different iPhone cases are shown with the instruction that they should be displayed in store "left to right starting with neutrals, moving into cool, and then warm colors."
The two thumbnail images featuring a battery-style hump are both in neutral black. There does not appear to be a white Smart Battery accessory nor any for models but the XS ones.
The leaked image is from page 14 of a document called "Premium Accessory Merchandising Guidelines Fall 2018" specially in a section called "Overview Examples." This suggests that the Smart Battery accessories were expected to be available at least approximately when the iPhone XS and iPhone XS Max launched. It seems unlikely that they will now be added before Christmas and the New Year.
It's previously been believed that Apple was to launch three such accessories, not just two. This was based on part numbers and icons found within watchOS 5.1.2.
In 2015, Apple released a Smart Battery Case for the iPhone 6s which extended the life of the phone up to 25 hours. When you've charged both case and phone together, the Smart Battery Case depletes first so that your iPhone remains fully charged for as long as possible.
This new leak appears to confirm that the new case will retain the 2015 model's hump but positioned lower down the case. The original design's shape was criticized for how it made putting the case on and taking it off an iPhone more difficult versus alternatives.