>> The best supercaps out there have roughly 10% of the energy density
>> per volume of lead acid batteries. Energy density per mass is probably
>> more like 20% of lead acid due to the lower density of the capacitor.

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

"In terms of energy density, existing commercial electric double-layer
capacitors range around 0.5 to 30 W=B7h/kg, with the standardized cells
available from Maxwell Technologies rated at 6 W=B7h/kg and ACT in
production of 30 Wh/kg. Note however that ACT's capacitor is actually
a Lithium ion capacitor, known also as a "hybrid capacitor". Experimental
electric double-layer capacitors from the MIT LEES project have
demonstrated densities of 30 W=B7h/kg and appear to be scalable to 60 W=B7h=
/kg
in the short term, while EEStor claims their examples will offer capacities
about 400 W=B7h/kg. For comparison, a conventional lead-acid battery is
typically 30 to 40 W=B7h/kg and modern lithium-ion batteries are about
160 W=B7h/kg. Also, gasoline has a net calorific value (NCV) of around
12,000 W=B7h/kg, which in automobile applications operates at 20%
tank-to-wheel efficiency giving an effective energy density of 2,400 W=B7h/=
kg"


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