Also, HybridsPlus in Boulder Colorado is doing something similar.
Hymotion leaves the stock battery in place, and adds 7x more capacity.
HybridsPlus removes the stock battery, and adds one with 15x the capacity.
http://hybrids-plus.com/index.html
I am at the top of the list at HybridsPlus to test drive their first plug-in Escape ( a 2007) when it is ready, any day now. -John