That's pretty cool, but twice the cost for LiIon batteries?
The extra 100lbs from the Pb batteries would negate your gain in fuel efficiency, wouldn't it? I mean, if a car gets 300 miles a tank already, then this system only gives you a 10% improvement. However it's been said that for every 100lbs you add to a vehicle, fuel effiency is reduced by 3%. So take that 10% and subtract 9% for the 300lbs from the Pb battery system and 6% for the 200lbs from the LiIon system. The idea is great, but it really doesn't help much if you ask me.