[Merad]

But, isn't the lifetime of the battery pack significantly lower than the (presumed) lifetime of the actual car? If your Model S stays on the road 20 years but requires 3-4 battery replacements, that's several big hits against its environmental friendliness.

[digikata]

The lifetime of the battery will likely be much longer than it's life in the car - if the plans for 'second life' re-use as static energy storage devices come to pass.

[CydeWeys]

The lifetimes of battery packs have been greatly underestimated by detractors for quite some time. Hybrids have been on the US market since 1999, and battery life has never been a serious issue. The new batteries are far higher quality than the old ones anyway.

3-4 battery replacements over 20 years isn't anything close to what a Model S will require. Do you have a source for those figures?