What Does NIMH Mean On Batteries?

NiMH means nickel-metal hydride. A little history might be interesting. One of the most successful exotic batteries is the nickel-hydrogen battery. It uses the same nickel-hydroxide positive electrode and KOH electrolyte as the Nickel-Cadmium Battery, but it uses hydrogen gas to replace the cadmium in the negative electrode. It needs a pressure vessel to hold the hydrogen gas. The Ni-MH Battery is mostly used in low-orbit satellites, which charge and discharge the batteries on every pass around the earth, and so need a long cycle-life battery. Nickel-hydrogen cells have a cycle life of tens of thousands of cycles. 

A different way to store hydrogen will be in intermetallic compounds known as metal-hydrides. Some metals have space in their nuclear lattices into accomodate hydrogen atoms. The hydrogen could be forced to enter and depart the metallic matrix by electrochemical means. Thus the Nickel Metal Hydride Battery is a version in the Nickel Hydrogen Battery, using a brand new, low-pressure, method to store hydrogen. Regrettably, because the metal-hydrides corrode when exposed to KOH, they're much less long lived as nickel-hydrogen batteries.