I haven't done a serious study of this but did check things out to satisfy my own (limited) curiosity. The brake lights come on, it seems, at varying regen values depending on the speed (amount?) you are decelerating. So at highway speed and slowing down, I noticed it was around -30. At city road speed (50-70kph or 30-40mph), it was somewhere around -15. Then after that it's a real challenge to get them to light up at all without the brake pedal. Since I often modulate the fun pedal coasting up to a red light, I think that means my brake lights are often NOT displayed even though I'm slowing down somewhat. However I also watch the behaviour behind me and change my deceleration rate depending on what the other driver is doing (i.e., delay any regen braking until closer to the stopped traffic, etc).
I'm still getting used to estimating exactly when to fully release the fun pedal so that a) I come to the near-stop on regen only at the perfect distance behind the stopped traffic or at the red light, and b) I get maximum regen on the car. Not concerned about refilling the battery charge; it's just a fun challenge for me. Yes, I'm easily amused - comes from all those schoolroom times of being told to go sit in the corner. :tongue: