If you know your batteries you will already know what the title of this article is saying. But, for all you novices (and for everyone who hates acronyms) let us spell things out a little clearer.
Nickel Metal Hydride (NiMH) or Nickel Cadmium (NiCad) batteries used to be the battery of choice for RC helicopters. But these batteries were heavy and their charge would not last nearly long enough for the expert RC helicopter flyer. This meant that electric helicopters were invariably smaller and more lightweight than their gas or nitro powered cousins and were not so nearly thought of in so high regard. Then came Lithium (Li) batteries.
Lithium batteries are the same type of batteries that can be found in your regular mobile phone. They can be recharged numerous times, and most importantly their charge lasts for a much longer period of time than the previous NiMH and NiCad batteries. Unfortunately Lithium batteries are still heavy and you may need more than one battery attached to your copter to provide sufficient power. However because of technological advancement in the Lithium battery field, electric RC helicopters can now challenge their nitro or gas turbine powered counterparts and be a major player in the world of RC helicopters.