The problem is that cold batteries can't charge or discharge quickly. From the battery's point of view, regenerative braking is the same as fast charging (just for a very short period of time). When the battery is very cold (or almost full), Teslas regen as much as they can, then use the friction brakes to keep a consistent feel for one pedal driving. The battery does have fluid pumped through it for heating/cooling, but it can take some time before the pack is warm enough to charge/discharge at its rated capacity. The longest I've had a cold battery notice is 20 minutes, and that was after my car sat at 0ºF overnight.

My thought was to use the regen braking electricity for (more) resistive heating when it has nowhere else to go. It's an extra thing that only gets some use (depending on climate), but not a very expensive extra thing and takes some load off your pad/rotor brakes.

The battery is warmed by the heat pump (or if you have an older model, the resistive heating coils). That component can only take so much power. A more powerful heat pump would cost more, take up more space, and weigh more. My guess is that Tesla designed their heat pump based on these tradeoffs.

At least last time I checked, Teslas had rather anemic battery heaters. IIRC they’re around 6kW.

6kW is not enough to make a big dent in required mechanical braking, and it’s also not enough to quickly heat the battery pack, especially when operated intermittently.

I thought GP was referring to generating heat for HVAC, which is an interesting idea. I assume one challenge would be getting the warmth from the brakes to the cabin.

A gadget to recover heat from a disk brake would certainly be interesting, but it sounds complex, messy and expensive.

A bigger battery/coolant loop heater sounds more straightforward — the mechanism to convert kinetic energy to current is already there.

Yeah, from what I've read, Tesla max regen is 60+kw. Does it just lockout regen braking until the battery pack is "receptive"?

It limits max regen power, and the limit is variable. There’s a cute little indicator on the dash.