> It's a small trailer with a single wheel (I think) and motor and
> batteries. It goes behind your car and gives it a boost from the
> electric motor, which is run by the batteries which of course plug in
> to charge. The idea is not to fully power a car, but to increase its
> range and mileage.

I think this idea is backwards!  build a electric vanagon with modest range and build a trailer holding a couple of batteries and a generator so you could go on long trips and leave it off  around town,
with the other method you have a trailer all the time.
mike