Originally Posted by meb
SPARC by SRAM and the discontinued Sinclair Zeta III are probably the closest systems to what you describe.
Here is a pic of the 172 W Zeta III which was still available about a year ago at this site:
http://www.zapworld.com/ZAPWorld.aspx?id=2242 a pic is still visible although they seem to have cleared them. They pop up on ebay occaisionally. About 9 mph, 10 lbs, you also have a headlight out of the package. It uses a friction belt drive which is vastly superior to the roller friction drives.
SRAM SPARC has 2 100W motors embedded within a S5 internal gear hub, system with batteries adds about 10 lbs. also. Pricey, close to $1000 dollars. Like the Zap DX, runs the current through the motors in series for low speed and in parrallel for high speed.
You could probably get a 100 W scooter and make that conversion for a lighter system.
If you are doing 20 mph, you are probably putting out much more than 100 W though.
With the recharging aspect you desire, the developed Bionx system might be the best way even though it is heavier.
Alternatively, an even heavier Zap DX system still has the regenerative braking (it is no longer marketted but is still present on the newer models as an undocumented feature-it was of limitted benefit so they simply stopped marketting the feature as they did before 2003). Zap might be under 15 lbs with small 2 AH NiMH in place of the SLA battery pack.