Lithium - Ion Batteries
(An article published in the March 2003 edition of Sloping Off)
This technology has been around for a few years now, most prominently in mobile phones.
The battery illustrated here (with a 7 cell 250mah NiCd above for comparison) is distributed by Perkins and they are, I believe, the first to be distributed widely by the modelling trade in the UK, available in 700mah and 1400mah forms. I bought mine from Channel 4 models. The batteries cost £15 each and the charger will cost you £15 - and yes, you must use a Li-Ion charger for these batteries!
The only other thing to watch is that they must not be discharged below about 2.6v per cell, i.e. 5.2v for these two cell batteries. A quick test using a GWS 2amp speed controller (with a NiCd battery!) showed that the auto cut-off operated substantially below this voltage so, either buy the speed controller that is sold especially for use with these cells, or take care! I have so far flown for 20 - 25mins without coming close to flattening a 700mah pack.
The figures in the table were measured using my Walrus as an example, and say it all really: More power, less weight and, most important of all, with two models equipped with these, you can fly all evening without having to take along charger or a 12v battery. The burning question now is, how long before this technology reaches the power tool market so we can use it in the big electric models!?
Postscript: Since writing this article, the shape of the 700mah battery has changed. Basically it has been folded in half so that the two cells lay on top of each other rather than side by side. This makes the battery similar in size and shape to a PP3 battery rather than an overgrown after-dinner mint.