Robert/tomo: it seems that all EV batteries have some "headroom" in that the available capacity is always some kWh lower than the absolute total capacity. So road tests will quote stuff like "it has an 85 kWh battery of which 77 kWh is useable". That margin was what Tesla temporarily released, as mentioned by tomo.
My understanding is that this allows the car to maintain its range even with a bit of battery degradation. Also the cells do not have to be charged to the absolute limit to reach the nominal 100% capacity.
Interrogating the battery will show the true available capacity as well as all the other stuff.

tomo,
We were starting to hear some guff about coral bleaching at the end of last week. I think the plan was for a blitz like you are seeing there, but the stabbing at the weekend stole their thunder. I expect they'll regroup and give it another push in a week or two.

The GBR has been "under threat" all the time I've been in Australia, and always sensationalised in the news. Not much fun owning a "crown jewel" if all you do is agonise over the fear of losing it. Half a century passes and we're still agonising. You'd like to hope that the boy who cried wolf effect would kick in, but it seems the collective memory works like Clive Wearing's.

On Tesla being able to tap extra potential in the batteriy in an emergency, I conjecture that Teslas maintain charge state as net charge stored, which involves keeping a tally — like a two-way odometer — of charge in - charge out. The software would have low- and high-water marks for this meter: stop moving when you reach the low; stop charging when you reach the high. Presumably they researched the values for the marks by balancing range against safety and cell longevity. For the emergency they struck a different balance and set a lower low-water mark. One little number buried somewhere in the firmware.

That's probably all fairly obvious to everyone. An interesting question is how they adjust the high-water mark as the cells degrade. Might be empirical, or by mathematical model. Problems with both IMO.

Empirically, you'd have to overcharge occasionally to see when voltage starts to lift rapidly, then set the high mark to some lower value. Obviously there's a cost to longevity in running it up against its limits.

Mathematically, you'd have to trust the quality control of the cell packs, and of the model. Things might get ugly if later cell packs aren't at least as good as the originals.

But the high-water mark would give an excellent indication of battery degradation for that 8-year warranty. It should also be very highly correlated with the car's range.

MikeHig,
I'm sure you're right about EV owners keeping a close eye on range and, really, that matters far more than voltage readings (or whatever) in a data log. That's why I found tomo's suggestion that Tesla might be working a separate set of figures a bit improbable. It *might* come down to a legal wrangle on the wording of Tesla's warranty, but consumer law (in Australia at least) doesn't look kindly on such word games.

I was wondering what sort of figures would be logged. My understanding is that cell voltages tell you *very* little with at least some of the Lithium cell chemistries — that 20% to 80% capacity only makes 0.1V difference or so — so it's very hard to tell from that simple measure. Googling turned up this forum topic which does seem to be logging some useful measurements, though a too short-term to make interesting graphs.

tomo,
Yes: When I said avoid blackouts at all costs, I didn't mean ...

Been listening to more John Anderson podcasts. Enjoyed his chats with Peter Hitchens, Bettina Arndt and Lord Sumption. Sumption completes a square for me. Lots of people believe both climate and COVID alarm, and lots, like me, believe neither. TinyCO2 and Jo Nova were amongst a fair number who accepted alarm over COVID but not climate. Sumption is the first I'm aware of who rejected COVID but accepts climate alarm. Anderson gave him some push-back on climate, but Sumption shrugged it off easily enough. Smart guy, but a bit of a cold fish.