Technology for low-cost miniature battery power supplies. As with batteries, fuel cells generate electricity through a chemical reaction. However, fuel cells are recharged by refilling or replacing the chemicals (examples include hydrogen, methyl alcohol or aluminum mixed with oxygen from the air) instead of plugging a charger into a wall socket. Fuel cells deliver energy density many times that of lithium and can power sophisticated devices longer. Choosing a stable fuel source that is safe and readily available and decreasing the size for smaller devices while maintaining efficiency remain challenges.