Hearing Aid Battery Powered Watch

I often find the way to approach a problem, is to look into how people with similar, but much bigger problems are doing things. For example, you need to put up a partition wall. You cut a piece of 2x4, position it, nail into place. A pro wouldn't do this. They'd set up a chop saw cut 6 pieces to identical length and nail-gun it together with galvanized steel plates. Maybe it's worth doing that or maybe it isn't.

Hearing aids have a bigger power problem than watches. But they still need a small power source, so they turn to Zinc-air cells. These work like any other cell, they're a redox reaction where electrical work is extracted from one thing oxidizing and another reducing. But, they use Oxygen in the air as one half of that. Consequently they're ~6x larger in energy density Vs conventional Alkaline watch cells <link>. So, we power our watch with this? Well, sealed inside a waterproof watch, it would quickly use all the O2 inside, and non waterproof watches are a bit silly. So, the cell goes into an externally-vented waterproof compartment, we use the whole watch case as -ve and a watertight electrode contacts the +ve side.

With a standard sized Zinc-air battery, e.g. a 675, your watch would last decades. Even better, the manufacturer could build a cell the size of the whole back, meaning a lifetime of power.