Do you need Standard Regen Braking
Yes, do not use "Low" regen braking.
For you:
1) Drive normally, minimizing brake use.
2) Be sure to not let the car cold soak at the top of any mountain. If you do, it will take potentially a long time to get regen back (but it depends on the SoC).
3) Do not charge the vehicle above 80% or so at the top of any mountain if you will not be travelling downhill at freeway speeds (70-80mph) (For example, (not in your neck of the woods, but...) it is usually fine to charge to 90% at Mt. Shasta Supercharger, since you typically travel very fast and need little regen after departure, on I-5 or other major roads.)
You need a warm battery, and an SoC below about 80-85% to get decent regen. A long descent, even with a warm battery, will fill the battery and gradually reduce regen. In addition, you may notice extreme reduction of regen if you start with a cooler battery and try to regen on it - even with modest Soc (70-80%). This can result in a real problem - an unpleasant driving experience (riding the brakes) and potential brake pad/rotor damage for a long, steep hill on a cold, high SoC battery.
So, if you have a high SoC (above ~70%) at the top of the hill, and you stop, and it is cold, it is either best to ensure the car does not cool down (don't stop), or aggressively pre-warm the car (reducing SoC and warming the battery) WELL prior to departure. Just be sure you can still make your charging destination.
All of the regen limitations are communicated via the dots on the left hand side (green side) of the line under the speedometer.
I believe regen would heat the traction battery on a downhill grade.
To some extent you're right, but it's kind of complicated. As you know, a cold battery has limited regen capacity, but there is ALSO a memory effect. The more regen you do, the LESS regen it allows - even with minimal change in SoC. For example, you can start with perhaps 70% regen with a cool battery at the top of a short hill, and by the time you are at the bottom, you might only have 40-50% regen (it has nothing to do with the increase in SoC - the increase in regen dots is way too much for it to be caused by an SoC increase).
I believe the mechanism is that regen is a rate-limited process, limited by the rate at which ions can be incorporated into the anode, and after a certain amount of regen, there is a cloud of lithium ions near the battery anode. If these become too concentrated, they will not be re-incorporated into the lattice properly, and lithium plating can result, destroying the battery.
This rate of reincorporation is strongly dependent on temperature, so a cold battery can be quite limited and the regen available gets WORSE, at least transiently, as you do more regen.
This is what is observed. (And this is speculation on my part about the underlying mechanism, but it's not based on exactly nothing:
Lithium Ion Battery Anode Aging Mechanisms )
Now, of course, EVENTUALLY as you drive the battery will be warmed up (by the car burning energy to do so or otherwise), but the regen process itself is fairly efficient, and in and of itself may not be enough to warm the battery - especially if it starts out limited, you might only have a couple kW of power (in the form of regen heat losses in the battery) to heat the battery (not including the heating generated by the motors running inefficiently to heat the battery).
