[MannyLopez]

If they could be made with about 1/3 the energy density of regular petrol it'd be a real game-changer. The reason being that a typical petrol engine is about 30% efficient, a good electric motor about 90% or so efficient, so if the batteries were about 1/3 as good a petrol it should be possible to get a car, etc, doing about the same job. (that right?)

there are certain mechanical inefficiencies realised between the flywheel and the wheels