That's neat— I wonder if the gains are simply too small for it to make sense in a mass-market vehicle. Either way, seems like it would be a slam dunk for a delivery truck, city bus, garbage truck, basically any heavy vehicle with a stop-start workflow.

I think it's more that the capacitors are too expensive for mass production - the figures I saw put them at around $10K, for something that contributes a relatively small amount to the performance of the vehicle. Manufacturers would probably prefer to put that towards a bigger battery pack, which would give better overall specs, if a bit less efficiency.

You don't need ultracapacitors, regular ordinary teslas starting with the 2012 model S have had this. It's not quite a decade old.

Do you have a source on that? This article, for example, on the Tesla website is clearly aimed at a lay audience, but nonetheless mentions nothing about an intermediate capacitor bank for absorbing spikes of regen braking energy:

https://www.tesla.com/en_CA/blog/magic-tesla-roadster-regene...

I don't get your question. But that is a roadster, not an S or later. But it presumably doesn't mention intermediate capacitors because they don't use them.

I think you and mikepurvis are both saying the same thing - that Tesla and other electric cars are using batteries only, not capacitors.

I will note that the homebuilt electric cars generally had much smaller battery packs, which could not supply the same peak currents that a 50+kWh battery would have.