Doctor Drivel wrote:
Have you ever weighed a full fuel tank -£50s worth?. Per weight modern
state-of-the-art batteries pack a hell of a lot of energy. These batteries
are not available in volume yet.

Lithium Polymer (one of the best candidates for this - as you yourself
have stated) has an energy density of 166Wh/kg[1]

Petrol is of the order of 12000-13000Wh/kg[2][3] depending on the
source.

I make this about 70-80 times more efficient to carry your energy in
petrol than a battery if you ignore the fact that the fuel gets used
up. Assume that the tank averages a half-empty state you are about 150
times better off.

Out of interest diesel is about 13% better as far as gravimetric energy
density is concerned[3]


Refs:
[1] http://www.sanyo.com/batteries/lithpol.cfm
[2] http://hypertextbook.com/facts/2003/ArthurGolnik.shtml
[3] http://xtronics.com/reference/energy_density.htm