My casio F91W needs a new CR2016 coin cell. Each cell runs the watch for about a decade and contains only about 36mg of lithium, but I find single-use/disposable lithium offensive on principle.

This watch uses only about 5uW at 3V, so we need to generate very little. Gravity energy storage is laughably impractical compared to even the worst battery chemistries, but for a load this tiny, I'm just making a desk toy.

A CR2016 stores about 800J, which is pretty impressive if you think about the gravitational equivalent: 82kg/180lbs lifted 1 meter/3ft. I don't have a machine shop, so I think I'll shoot for less energy per charge. I'll turn a crank once a week if it saves lithium.

Plan: Suspend a weight (a few kg) about 1-2m from a string wrapped around a spool. The spool applies torque to a rotary generator through a gear train that provides a ratio big enough to accomplish a reasonable runtime. Generator output current provides opposing torque, slowing descent per load.