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I read somewhere that Tesla is actually doing that. But the battery heater only got around 4kW of power, that's nothing compared to 60kW of potential regen. You'd need a truly enormous heater to dissipate this much energy :)
Has anyone managed to receive the notifications displayed in the App, e.g. when starting or stopping the charger? This could help to tune the API poll timing and hence let the car sleep better.