This is only a discussion concerning cold weather related charging slowdowns. We have gotten a few posts recently about it so I thought I would attempt to address it in detail and hope that new users that have the question can find the information useful.
Ok, so you have recently noticed your supercharging speed slow down and wonder if there might be something wrong. Well chances are, there is nothing wrong with your car and it is due to cold weather effects and it is due to your battery being too cold. When it is too cold, the car limits charging speed to help protect the battery as Li-Ion batteries can be physically damaged if you attempt to charge them at too high a rate when they are too cold.
Charging a Li-Ion battery at 0 degrees C is bad, so as you approach 0C the car restricts the level of charging it can accept whether it be by actual charging, or regenerative braking. At 0C the car will not charge the battery at all until it warms it up first.
Now how can you determine whether this is what is affecting your car? There are a couple of things that you can see on your screen. See the image I posted here of my screen. https://imgur.com/y12MCEi
At the top circled in BLUE, is a snowflake icon. This icon tells you two things, first that your battery is cold. Second, that some of your battery capacity is not available to you until your battery warms up. Circled in RED is the power bar. This is what turns green on the left half when you let off the accelerator and start regenerating. As you can see in the picture the left side of the line is dashed(or is showing dots as we generally say). The more dots you have the less regeneration capability you have due to in this case, cold weather. The other time you can see dots here is when you start going above 90-95% SOC% (state of charge).
So if you see dots on the left side of the power bar, then you are either at a high SOC, and/or your battery is cold and it will have its charging rate restricted. The more dots, the more restricted.
So now you may be asking, how do I warm up the battery. There are multiple ways. You can ensure that your charging finishes close to when you will be leaving in the morning for example by using the scheduled departure function in the car.(there are caveats here concerning your requested SOC vs what you will actually have in the morning but I am not going to get into that here). You can also turn on your climate control from the app some time before you leave(how much time is needed is battery temperature dependent). You could repeatedly mash on the accelerator and let off until the dots go away but you might get motion sick. :) You can also route to a supercharger and the car will "precondition" the battery in route. This does not work very well but will do a bit in an AWD vehicle. Further discussion about preconditioning can be found here https://forums.tesla.com/discussion/170664/battery-heating-explained/p1 It was written last January but I think everything should still be valid even for the Model Y.
Now lets get into a little ScanMyTesla(SMT) detail on those dots for those who just love data!
The number of dots tells you how cold your battery temperature is right...but how cold is it? I used to have a partial chart but I trashed it after the recent UI update because....ahh nevermind... anyway. Using SMT I have the following data, so far.
1.5C - 19 dots
3.5C - 17
4.0C - 17
6.5C - 16
7.0C - 16
9.0C - 14
9.5C - 14
13.0C - 12
13.5C - 11
14.0C - 11
14.5C - 10
15.0C - 9
15.5C - 8 dots
Eventually I will fill this out fully as well as get multiple readings at each temperature/dot level as I did originally. I am sure a lot of people would also like to see a charging level vs temp at multiple x-Temperatures but that would take a long time for me to do and is a lot of work.
Anyway, I hope this helped those new and old.