Shouldn't a couple regen braking events be capable of dumping a boatload of heat?

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.

I think you are missing the most important effect of cold: more air density at terrain level, which creates way more drag on the car.

The density of air at 20ºC (293ºK) is 1.204kg/m^3. At -10ºC (263ºK) its density is 1.341kg/m^3, or 11% denser. For a Model 3 moving at 100kph, about half of its power is used to overcome air drag and half is used to overcome rolling resistance. Air drag is 1/2 * density * velocity^2 * coefficient of drag * area, so an 11% increase in density means an 11% increase in drag, so a ≈5.5% decrease in range. That's significantly less than typical cold weather range loss.

Most of the range loss comes from climate control. In a combustion car, the majority of the energy in the gasoline is turned into heat. In cold weather, you just dump some of this heat into the cabin to keep it warm. An EV's efficiency is a disadvantage in this case, as it needs to use energy from the battery to heat up the cabin.

Everything sum up.

I don't have the exact numbers there but the HVAC system should draw ~2kW at full throttle. Giving an average of, let say, 160Wh/km (speaking of a recent Tesla M3 for example) at highway speed. Over an hour of full throttle heating you would have consumed in heating an extra ~10% of what you spent to move the car. So OK, maybe air drag increase is not the worst offender here but it's not just the HVAC system.

...

While writing this I searched a bit and according to this [1] when there is really cold temperature and the battery still haven't got warm, it can draw 5-6kW at the beginning. Anyway when driving motors heat up, the heat is used to warm the battery, the battery heats up as well and all that heat is used by the heat pump to warm the inside of the vehicle.

[1] https://insideevs.com/news/452464/tesla-model-y-heat-pump-sy...

I have a PHEV and when I turn on the heat as I'm pulling out of my garage, the range is cut by 30%. This has nothing to do with drag since I'm backing out at 2 MPH. It's just about the HVAC system, which uses a lot of power. I typically just use the seat warmer, but when my kids are in the backseat I have to run the heat as well.

I very much doubt this. I've never seen this having a notably measurable effect on ICE vehicles in Massachusetts winters (which can be cold but aren't notably arctic).

Because ICE vehicles don't have "range issues" so you just don't notice it.