I've seen this "one key turn drive to very low SOC" a few times. What's the basis for this? Is this actually worth doing? In reading the 3.0 battery upgrade thread, it seems that low SOCs permanently hurt the battery capacity.
Executive summary: 1. The "one key turn" theory is an old wives tale. 2. The 3.0 battery is not hurt by a low SOC.
tldr version:
Your question "What's the basis for this?" will garner multiple answers. Perhaps my answer should be considered my opinion only. The story goes that a Tesla Roadster engineer told a service tech who told a customer who told more customers who informed more service techs who told more customers and eventually it just became a fact. My own personal anecdotal evidence suggests that the algorithm that calculates your range gets more data if you deplete the battery more and makes an adjustment. I've never seen any evidence that the depletion of the battery needs to happen in one key turn. I believe what's important is that the algorithm gets a larger and more updated data set, not whether it's in one key turn or not. This also explains why your CAC and total range usually change in steps that are associated with long trips.
As for the notion that low SOCs permanently hurt capacity, it's true that multiple small discharges cause less damage than one large discharge, given the same overall kWh. Having said that, I don't believe you would experience any measurable capacity loss after one or two deep discharges, or even half a dozen, especially with the 3.0 battery. It's more of a daily charging habit over a sustained period of time that you would start to notice a difference. I think the reason people have come to believe deep discharges cause immediate capacity loss is because that's when your CAC gets new and more plentiful data and makes an adjustment. The capacity was already lost before you made the long trip.
The 3.0 battery has a discharge curve that safely goes lower than the original cells. But that added capacity is not utilized because the Roadster charging system cannot start charging at the lowest voltage the 3.0 cells can safely get to. As a result there is a small amount of wasted capacity that we can never use from the 3.0 cells. It is partly for this reason that I don't believe a very low SOC causes any damage to the 3.0 cells. Once again I think the low-SOC-damage theory comes from the fact that your CAC is often adjusted more than after smaller discharges.
My advice to smorg is take your upcoming long trip in your Roadster. Enjoy the drive. You won't hurt anything. my .02
