With a slight modification of my Tiny personal flashlight/generator idea, you can create a wearable battery recharger for batteries.
Start with an open cell foam pad 6 inches wide, ½ inch thick and long enough to wrap around the front of a persons stomach. Seal the foam in an airtight bag with
two hose connectors and then wrap an adjustable nylon belt around the bag so that a standard breathing motion will squeeze the foam filled bag against the belt to cause air to be expelled from the bag. Place another bag of identical volume on the outside of the belt on the back of the person, and connected at both ends, so that the air expelled from the first bag can fill the second bag and when the person exhales, air can be sucked from the second bag back into the first by the expansion of the foam. Now install a one way reed valve in one connection and a small Tesla turbine generator across the other. Since we know the frequency for breathing stays pretty constant, the generator could be tuned to produce just the right voltage for charging batteries. The batteries could then be used to run all your other electronics.
This could be of great use in third world countries anywhere else where minimal power generation is required in remote areas.
This idea is very similar to Breathing Generator except in method. Piezo generators are only 10% efficient and this design should be much better. Maybe 50%, the only inefficiencies are from air drag and that mostly from the foam which could probably be fixed by using foam in a eggshell or similar arrangement.
The parts for the Tesla turbine are: a turbine housing, the turbine made of stacked plastic disks and spacers, turbine shaft made from a radially polarized magnet that comes to a point at each end, two jewel pin bearings, 6 coils, a three phase rectifier and a battery holder.