I saw this at a recent car show: http://www.hymotion.com/products.htm. From my understanding, it is a supplement to the original battery so it does exactly what is being discussed here. I see a couple of issues:

1: If it is to be removable, you need some sort of lift system as it weighs about 300lbs! So you can't just pop it out when you need space like an extra seat.

2: For the Ford Escape at least, I would want to upgrade the motor so I can get better acceleration and get at speeds higher than 40kph. I can stay electric on quiet streets with no traffic behind me, but otherwise I need the ICE.

At least it looks like we're moving in the right direction.

Cheers,

Roch