[NickM]

Charging the battery fast isn't necessarily so bad in and of itself, but fast charging tends to generate a lot of heat, and high heat is bad for lithium ion longevity. The Model S has a battery cooling system built into the car which kicks on when needed, so this is not such a big issue.

The Leaf doesn't have any battery chillers though, so fast charging may indeed be bad for it. Also, even though most DC fast chargers only put out 50kW (as opposed to 120kW for Superchargers), if you consider how much smaller the Leaf pack is, you're actually charging the battery faster than a Supercharger can charge a Tesla.

That said, this study seems to indicate that fast charging isn't all that bad for Leafs anyway: http://insideevs.com/idaho-national-laboratory-dc-quick-char...