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

This is a 1.113 Wh (291.8 mAh at 3.81 V) power pack, labeled A2059.

... 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!