Here's the built-in battery powering the Watch—no self-winding action here!

... Or it might be 1.12 Wh, depending on which side of this battery you believe.

Regardless, that's down about 17% in capacity from the 1.34 Wh of the 42 mm Series 3 (although still more than the 38 mm model's 1.07 Wh battery from our most recent teardown).

Now who wants to do the math on volume? 0.7 mm thinner but 2 mm taller comes out to …

… just enough to give the battery its daily 18 hours of runtime, apparently!