The US Patent and Trademark Office published an article about Apple’s “Charging apparatus for wearable electronic device”. The patent is about manufacturing an inductive charger, implemented in the Apple Watch strap or attached to an existing band.
The illustrations show us a small wire that connects a battery in the strap to the back of the device. The strap itself is really thin, probably in order to be both comfortable to wear and look tolerable.
There’s no knowing if such idea will ever make it to a real product or Apple is just protecting this idea from being used by other companies.
Interestingly enough, the watch on the graphs is actually circular and not a square with round corners. Apple often uses generic illustrations of things in order to broaden the patent applications or actually shows a design that later does not make it to the market, but we can always hope there's more to that.