[ConstantineXVI]

a) Car batteries are massive.

b) The batteries are shaped to the car[1]; standardizing batteries would require sacrificing car design (aka: goodbye back seat).

[1] From http://www.teslamotors.com/models/features#/battery "...integrating with the vehicle in the same way, providing structural, aerodynamic, and handling advantages."

[gnaffle]

It might be possible to have some of the battery packs replaceable and some fixed, so that you can get to 40% charge or so instantly if you need it. I'm a bit more pessimistic on getting the car makers to standardize on a battery size, especially as long as battery technology is a competitive advantage.