NiMH is short for Nickel Metal Hydride Battery. This type of battery technology is similar in uses to the Ni-Cad minus the memory effect. These types of batteries are starting to show up in more and more cordless power tools. They typically cost more then NiCd but less then Li-Ion.  The Lifespan of NiMH batteries are a little better then that of NiCd and a normal charge takes 30 minutes to an hour.