The key point is that all li-ion batteries, even when brand new, are:
- At risk of self-combustion if charged beyond certain voltage / current limits (which vary greatly based on conditions and the how the various aging processes have affected the cells)
- At risk of unacceptable levels of degradation if charged beyond certain voltage / current limits (same as above)
Which is why you always have limits coded into the BMS. Again, that's the BMS's job.
A key limiting factor in charging is anode intercalation rates. Intercalation of lithium ions into the anode is energetically preferable to plating lithium; they "want" to intercalate rather than plate out. But the rate at which intercalation can proceed varies depending on the conditions - and each of these can be seen in how the BMS manages charging.
- Ion mobility, and thus intercalation rates, are strongly dependent on temperature. In practice, you see this as very slow charging until the battery warms up.
- Intercalation slows down the more lithium there is already intercalated in the anode. The BMS responds with what you see as taper.
- Above a certain point, intercalation has slowed to such an extent that you can A) no longer reliably ensure no lithium plating, and or B) the high reactivity of the anode makes for an unacceptable level of reaction with the electrolyte, and thus degradation. You see this as the cutoff.
It's long been clear that the packs in question were not living up to the desired longevity standards.
- First off, we got the nerfed Supercharging after a fixed number of sessions in 2017. This clearly spells out Tesla's concern: after a given number of sessions, they can no longer trust that intercalation is proceeding fast enough for the full ~120kW charging. Ion mobility is suffering as the packs age and in particular as they are supercharged - hence the limits which quite clearly spell out the problem.
- Now we get this - a voltage limit, and possibly a further charging limit. A voltage limit at the upper end would more commonly be interpreted as protecting the battery from excessive anode-electrolyte reactions - and that might well be. But combined with the charge rate (and thus intercalation) limits, it looks more like they cannot trust the reliability of intercalation at the upper end of the packs. The problem could also be both.
Why didn't Tesla's accelerated aging tests detect this problem? It's hard to say; accelerated aging is a very difficult thing to do reliably (it's even possible that there were QC issues on the anode powder that were out of Tesla's control). I'd love to know what went wrong, but I doubt we ever will. But acceptable intercalation rates have clearly fallen far faster than Tesla expected them to vs. how these packs were testing when new. As they age, the anodes just aren't soaking up the lithium like they're supposed to - at least not reliably.
Can you force them to charge faster? Of course you can. Just like you can try to force
new cells with
new chemistries to charge much faster and to maximum theoretical voltages than they already do. But it's the BMS's job to stop you from doing what's
possible, and instead limit you to what's
reasonable. Sure, it "works" - but you're taking away 9s in that reliability game. Whether the issue is safety or longevity, it's the BMS's job to stop you from hurting yourself. The more a manufacturer learns, the more they change the BMS's algorithm. If the news you learn is good, charge rates, tapers, max voltages etc improve. If the news is bad.... well, this happens. And it sucks for those affected.
But this is the BMS's job. And it is
absolutely right to update the BMS based on the information that you collect over time as to how packs with that chemistry are aging - for good or for bad.
To sum up:
- Packs are always limited. From the very day you pick up the car. That's the BMS's job.
- The limits in the BMS should be updated based on the latest information - good or bad.
- The ability of lithium to be reliably intercalated into the anodes clearly has degraded faster than the company anticipated - hence the chain of downgrades over time.
- Nothing in the warranty guarantees a minimum range
- People absolutely have a right to be upset about how much the range is dropping
- Tesla should try to do something for them, without setting a trend of "non-warranty-covered fixes will occur at Tesla's cost any time people are disappointed with something". What would be appropriate is reasonably up for debate.
- Rolling back the changes and pretending that anode degradation isn't happening, however, is not a reasonable choice.
- Tesla failing to communicate when there's bad news they have to deal with is understandable (who wants to broadcast negative PR when you can just handle the issue silently?), but incredibly frustrating for affected individuals.
That's just my two krónur.