Among the many ways to recharge a battery off-grid, there is the shake charger which gets energy from the mouvement of a magnet in font of a coil. They can be found in some flashlights, but the conventional design (magnet inside the cylindrical coil) leaves a lot of empty volume inside the charger. I decided to invert the design, with a ring-shaped magnet outside of the coil, so that batteries can be placed in the central cylindrical space.