HONDA claim to have incorporated in their FCX Concept fuel cell car, a proprietory ultracapacitor. According to their graph, this module stores up to 2 kWh/kg and delivers 1.7 kW/kg. These numbers look too good to be true, a 20-fold improvement in energy density of the Tesla roadster's battery pack. To store 50 kWh would only require 25 kg of mass! http://world.honda.com/FuelCell/FCX/ultracapacitor/charging/ Also, NESSCAP are selling 5 kF ultracapacitors with only 1/3 of a 18650's energy density. http://www.nesscap.com/products_capacitor.htm So how did HONDA do it? Or is there a typo in HONDA's graph?