I generally like the angle of this advice, particularly if someone regularly spends time with the pack between 80-30% for long calendar life. I want to add some background and nuance.
As you noticed with the 11% to 2% drop, there are *not always miles below zero*, particularly if your pack has bad balance. It's often the case that there are more miles, but on an out of balance pack zero can really mean zero, or 2% can mean zero. When trying to balance a pack, assuming you can drive after zero is rolling the dice on needing to wait by the road for a flatbed, and it doesn't do you any good in terms of balance to get that low. It's also not really necessary - more on that later.
It's far more damaging to leave the pack at 0% than at 100%. If you follow this advice, don't sweat the time at 100%, it's no big deal even for a few days. Do charge as soon as possible after getting down close to 0%, as if there is any chance the car sits and gets down to the bottom of the buffer it's a big problem. If the car ends up not getting charged for a while at 0% it can do serious long term damage to the battery, particularly if the pack is significantly out of balance. Time at 100% accelerates calendar aging but even a few weeks spent at that level is very little aging in terms of the overall life of the pack. If you will drive within a few days, no need to run the pack down after charging it to 100%, just take care of it with your next drive.
For the underpinning concept that causes the balance errors, look at the graph on this page:
Silicon Lightworks
Note that Tesla uses the battery between about 3% and 98% on that graph, or 2.85v to 4.15v. (more info here
What's the battery voltage from 0% to 100%? )
See how the curve is very flat between about 85% and 25%? That's what confuses the BMS. If you always charge to 80% and never get below 30% your BMS will have a very hard time maintaining long term accuracy. There's just not much voltage difference to measure, so it's stuck counting power in and power out without an absolute reference. Even just daily charging to 90% and occasionally taking the car down to 20% will give the system far more data to work with on current SOC of individual cells.
Personally I would not aim for 0% to rebalance an out of balance pack. Going that low is a harder cycle than a shallower cycle, and cycle life is a significant part of pack aging. Even down to 15% is plenty close enough to the bottom for the BMS to get a proper reading, assuming you've been going to 90% or higher at least every once in a while. I charge to 100% before any trip long enough for more than one Supercharger stop, and get the balance for free that way, because good Supercharger stops are often in the 5-10% range at arrival.
With regular charges to 90% and ordinary use, my pack has remained in excellent balance with below average degradation and very consistent measures of capacity. If you don't want to think about balance ever again, just daily charge to 90% and take the car down below 15% from time to time. That's using these cars on easy mode. It won't get the absolute maximum life out of the pack, but it gets very close and it's simple. The Tesla software takes a lot of care to manage the details of the pack for you so that use in the Daily range and following on-screen warnings means you're taking pretty good care of the battery with little effort.
