Submitted by Timo on July 27, 2010
Just read from autobloggreen about some new battery manufacturer called "Planar Energy". (http://green.autoblog.com/2010/07/27/planar-energy-to-begin-small-scale-...)
They seem to have tech for pretty good Wh/kg batteries. From one of their pdf (http://www.planarenergy.com/Press%20Releases/Technology%20Review_%20Safe...)
"One of them combines lithium manganese oxide with other ions, and operates at about three to five volts with a charge capacity of 200 milliamp hours per gram."
Calculating that as 3.5V * 200mAh/g you get 700mWh/g or 700Wh/kg. to put that in right context: Roadster battery: 450kg. Drop 30% off as supporting structure: 315kg battery * 700 = 220.500kWh battery. Nearly four times as much as current tech is. That would allow roadster to go 200 * 4 = 800 miles with one charge.