How about this for a ultra-light jump-start backup battery?
Apparently this crazy 8Ah battery has a 40C (40 x capacity) burst current rate, meaning it could deliver 320A to the starter for a few seconds.
And it only weighs 0.64kg!
http://www.hobbyking.co.uk/hobbyking...arehouse_.html
Beats the price of these,
http://www.cnet.com/news/pocket-sized-jump-starters/
Although I'd like to see a nice compact capacitor based hand-crank jump start pack?
(not that I ever needed one for the D2, even with a almost dead 8 year old lead acid battery it still started eventually)