I've been toying with the idea for something similar, but with performance in mind. The car is a 1991 Honda Accord sedan. I want to keep the original ICE powertrain up front, and add a performance-oriented electric powertrain in back.
My car weighs around 2700-2800 pounds as it sits, and gets around 30mpg combined (35+ highway). My plan was to remove as much weight from the car as possible to compensate for the EV setup, but there is no way to remove enough to even remain close to the factory weight, with both powertrains onboard. That means my fuel mileage will decrease, and battery life will be compromised.
When I heard about the new lithium cells (JR3P) I tried to balance the equation again, completely disregarding cost, but it's still too heavy. I need a pretty substantial pack onboard to provide the short bursts of acceleration needed, and even with lithium, it just gets too heavy.
The biggest problem I have though is that the weight is going to kill the handling. I like the handling characteristics of lightweight cars. My car is lowered, and I plan to improve the handling with occasional upgrades. The dual powertrain concept would completely ruin this for me.
I would think that would be the down side would still be there even for those seeking to improve mileage. Battery life would be severly affected by having to push an extra thousand pounds of ICE powertrain around on electric, and the mileage of the ICE would be even worse dragging an EV powertrain around after the batteries are depleted.
I am still hopeful, but a breakthrough in battery technology that allows an extremely lightweight pack is needed for my purposes. I need to get the entire EV system down to about 500lbs, because I can eliminate that much from the car to compensate. Anyone have a 200lb battery pack in the works, that can put out 300+ volts and...