kind of batteries

Some automakers are beginning to use Lithium Ion batteries in cars that will be released shortly which do have higher density levels than say, a nickel metal battery. However, nickel metal batteries are more widespread (i.e. Prius, and all those hybrid cars that are out on the market right now) and a little cheaper at the moment. LIthium ion batteries are expected to get cheaper as time passes and economies of scale get gearing up and what not. As for putting a battery on a bike, I am not a grease monkey, so I am not in a position to make a recommendation, but I know some people on this site are, so keep asking around. : )