Why bother with all that messing about with nasty, heavy batteries filled with nasty, expensive, polluting chemicals, over complex electronics just waiting to go wrong or kill you. Why not clockwork regen ?
No - honest, come back, wait...
Basic idea is a big spring. The tension on the spring is added by regen braking until it reaches a maximum level. Once the driver asks for more torque the spring's tension is released and the torque added to the output of the smaller ICE.
I'm no engineer but I suspect the weight of this setup would be no more, and perhaps less than the combined motor and batteries setup of an Insight. And as it is all mechanical fixing it would be simpler.
Plus of course it could be converted to the plug-in instantly :
