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?

the question is, how much self-discharge would that ultracapacitor have? Self-discharge has always been a noticeable problem with electrolytic capacitors, though i don't know about these ultra-capacitors. If it discharges itself by even 10% per day, problems of overall energy efficiency arise

I'm not an electrical engineer so I may have this totally messed up. I believe that there's some kind of problem with scaling up ultracapacitors. The numbers on the Honda were 2 grams/watt I think. Like I said, I don't know what I'm talking about on this, but the factor that doesn't scale is the K factor, which I think is the dialectric coefficient. I'm not sure why you couldn't just have 1,000 2 gram capacitors or something along those lines and still get the finished product of 2kg instead of 2 g. If there's someone that knows more I'd love to see an explanation from an expert.

The Honda Ultracapacitor cannot be 2 wHr/g but must be 2wHr/kg as it shows the ultracapacitors as only being marginaly better than standard capacitors. Also I couldn't find any mention of a 5 kF capacitor at Nesscap.