To be clear, I was not suggesting that battery heating during charging will make much difference for regen braking. My point is that charging during the evening and then letting the car sit for several hours before preconditioning throws away any battery heating that may occur as it is being charged. BTW I can achieve almost full regen in the morning after preheating in an 8C garage, but at the end of the day with the car parked outside below freezing I have yet to achieve close to the same level of regen even after preconditioning. TezLab shows those drives being 15-20% less efficient than the morning.
If preconditioning is performed only an hour or so after charging the battery pack will still retain some heat from charging. How long do you precondition? I find that at colder temperatures, i.e. below 0C, I need to precondition for at least 30 minutes to have adequate but not nearly full regen. (I try to count the little dots at the top of the Driving screen but they are too small. Up to about 10 dots, the available regen is adequate.) I have driven without preconditioning and the lack of any useful regen caused me to adjust my driving to allow for braking. In a different thread someone posted that during charging the battery was only warmed to 10-12C; when preconditioning the battery would be warmed to 20-22C.
I've only been preconditioning for about 15 mins, just enough to warm the cabin. I haven't been thinking in terms of the extra time that might be required to fully warm the battery because I'm not convinced it's necessary. This morning I left heat on for longer and departed with no regen dots, although 4 or 5 appeared during the drive (20 mins at -9C). This afternoon after being parked the cabin was at -1C and I ran heat for 25 mins. At the end of that period there was no battery symbol on the app climate page but there were still 8 regen dots on the screen. Preconditioning consumed 25 km of nominal range (~ 5% SoC). Regen during the drive was quite adequate though. However, the "limited regen" alert came up for the last 10 mins of the drive even though there was no apparent change in the number of dots from when I set off. In spite of this, the TezLab drive efficiency was still only 55% (44 km used vs 25 km travelled) compared to 78% in the morning, and not much different from my normal afternoon drive performance so I didn't see any advantage from the longer preconditioning period.
I prefer to precondition for longer than ~10 minutes, closer to 30 minutes in cold weather. I appreciate having full or nearly full regen capability when I start driving. Most of my trips are under 5 miles so I don't usually see the limited regen warning unless I make several short trips after preconditioning. About 5 out of 7 mornings I am able to charge for ~90 minutes at a free public charger (6kW) so my efficiency may be low but my cost per mile is practically zero.