I've got a bit of a theory (theory in the commonly used sense of a wild guess, not a scientific theory) as to what condition Z might be related to. I'm not sure if this has been covered here to date...
What if condition Z is not a hither-to-unknown battery defect, but instead a failure of the control system meant to manage the battery temperature? This might be analogous to the MCAS system that has been the cause for the grounding of the Boeing 737 Max fleet. In that instance, Boeing relied on the design of this control system to account for differences in how the planes would fly so that they wouldn't have to do significant retraining of pilots. Instead, faulty information was fed to the control system, causing the pilots to lose control of the plane, and disaster occurred.
Here's the story I'm telling myself about how batterygate and chargegate may have come about:
Tesla battery experts are smart and certainly know of the potential for dendrite formation, lithium plating, etc. Since the effect of these occurring could be detected by a spike in temperature, they carefully monitor the temperature of the modules and account for this by kicking in cooling and shutting off or rapidly tapering charging current. In the original 85 "A" battery configuration, they have revision 1 of the cooling and temperature monitoring system. This allows in their testing for safe charging at the original supercharging rates, starting at 90kW, tapering to about 60kW at 50% SOC. The battery engineers then develop the "B" packs (and later) with revision 2 of the cooling and temperature monitoring system. They implement a new temperature control system in this "B" pack, with a different, more robust temperature monitoring system and a more effective coolant loop. In new testing, they are able to charge faster without seeing any signs of dendrite formation (120kW to start, tapering to ~75kW at 50% SOC). This system has some minor revisions ("C" and "D" packs) but largely continues up to when we get a change in cell chemistry (90s). March 2019 rolls around, everything is looking good, but suddenly we have some battery fires. Tesla looks into it and the common thread seems to be some erroneous information being fed to the BMS by the temperature sensors. The self-immolating cars seem to have a defect that should have been detected and mitigated by the BMS but wasn't. So, they start looking for sensors that may be reporting erroneous information. Those cars are capped as this mitigates the potential for forming a short in the cell when charged to higher voltages. The original "A" pack design doesn't have this failure mode. 90s and later also have a different design that doesn't show this failure mode. As an additional mitigating factor, they revert to the original "A" pack taper profiles for later packs (though uncapped ones can still do 120+ kW to start...maybe some other portion of the design limited the "A"s to 90kW). Newer chemistry packs also get a slight reduction in supercharging speeds at higher SOC to mitigate any future problems with those.
Now, this whole story and idea may be completely and totally wrong. It does explain why there seems to be absolutely no correlation between past charging behavior and capping - the failure is not necessarily in the chemistry of the cell (except maybe in some very rare circumstances), but instead is in the sensing system meant to mitigate the potential for thermal runaway. Failure of a temperature sensor, or any other electronic sensor, would be expected to be random and not correlated with charging behavior. Like in the Boeing example, the engineers had more confidence in their control system than maybe they should have had. This also may explain why there was a seeming correlation with the battery pre-heating update. Thoughts?
