Nickel-Hydrogen Battery

A nickel-hydrogen battery is a rechargeable battery that pairs a nickel oxyhydroxide electrode with hydrogen gas held under pressure of up to 1,200 pounds per square inch inside a sealed cell, rather than the solid hydrogen-absorbing alloy used in a nickel-metal hydride battery. During discharge the pressurized hydrogen oxidizes to water while the nickel electrode converts to nickel hydroxide, and because the cell's hydrogen pressure falls as it discharges, that pressure itself serves as a built-in state-of-charge gauge; the design also tolerates overcharging, since any oxygen produced at the nickel electrode simply reacts with hydrogen to reform water. The batteries deliver only about a third the energy density of lithium batteries and self-discharge relatively quickly, but they can survive more than 20,000 charge cycles, which is why they have powered spacecraft including the Hubble Space Telescope and, until 2019, the International Space Station. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/