Så har WK057 taget et godt kig på tingene. Han kan se der er ændret i BMS, men har endnu ikke et klarrt overblik over hvad disse ændringer indebærer.
WK057 - indlæg #1117 på TMC tråden:
'I have not been able to 100% confirm anything as of yet. This is definitely a pretty complex change to the SoC calculation system overall.
First, for quite some time (~5 years or so) Tesla has used their battery management system to calculate out a lot of variables about the entire system in a very granular way. A lot of these variables are deduced from various readings and are either not measured directly with sensors or otherwise impossible to measure anyway. Nevertheless, the BMS calculates this data.
One of the data bits that Tesla calculates these days, with a pretty high degree of accuracy, is the SoC and capacity of individual cell groups within the pack. This is an indirectly calculated value. They also calculate a bunch of other variables for each cell group, including estimated internal resistance, power dissipation, a couple of lifetime stress factors, and more recently a few new variables I haven't been able to figure out what they are with certainty.
However, in the handful of data logs I've gathered from affected 85 vehicles, the issue that is outward facing (loss of range) is due to one or more cell groups showing a lower max capacity than other groups. This also seems to correlate a bit with a couple of the new variables, but, again, not really sure what they are (not sure of scaling, either, so really difficult to determine what they're measuring/computing here). Only one of these vehicles did I have both before and after logs, and in that case the groups in question were on par with the other groups up until the update that caused this issue.
Long story short, this is definitely caused by the a recent software change, and is due to some new variable they're calculating. I'm not 100% sure what those variables are, however, but my gut is inclined to give them the benefit of the doubt on this one for the time being. My notes on the ongoing BMS software changes over the years have almost always been to improve some aspect of the system, unlike things like the horrible decline in the UI, for example. Either from a usability standpoint (such tweaking variables to eke out a higher charge rate, or make thermal management a little more efficient) or a safety/reliability standpoint (such as modeling and estimating the stress on cell level fusing to prevent physical damage and subsequent capacity loss).
Tesla's BMS is pretty incredible. They know their batteries, and they can infer a lot about their condition as a result. The BMS models so many different variables, most of which are not exposed to the user or even on CAN in any way without some prodding and/or modification. To my knowledge, there isn't a better BMS setup out there.
This particular situation doesn't seem to be a screw up... if it's reducing the max charge, it's doing it for a reason. I do think Tesla should make that reason known, however, to affected owners. My suspicion is that doing so would open them up to large warranty replacement costs, though... and I'm not really sure where that would sit legally overall. If the reduction is due to some kind of variable that's a result of regular lithium ion degradation from normal use, then, while crappy for the owner, it may be for the best. If it's due to some now-detectable defect, however, I'd say Tesla should be liable to correct the issue with a replacement pack.
This is some speculation, but in either case, I'm inclined to believe that there is a potential safety issue involved here... and Tesla's silence on the situation with lack of a clear response to affected owners leads me to believe that the issue lies in the now-detectable defect category. Again, speculation... but it fits.
All of that said, I went and reset the NVRAM of my one customer's BMS who was affected by this, as an experiment of sorts (at their request after discussing the situation in detail). Basically, the BMS will recalibrate and recalculate all of its internal variables over the next couple of weeks. I modified the software to expose pretty much everything possible for logging, and I'm hoping I'll be able to better monitor it and get a better idea what's going on over the next few weeks, assuming the issue resurfaces.'
WK057 tilføjer i et senere indlæg (#1135)
Overall, as I said, I'm inclined to believe it's not a mistake and is related to some sort of actual safety issue that they're mitigating. I'm just not sure if it's a defect or wear based issue at hand here. Given my experiences with Tesla over the past six years or so, while I'm definitely willing to give the benefit of the doubt that it isn't a screw up of some kind and is actually a justified change... my willingness to cut them slack on it ends there. Regardless of the reasoning, that reasoning should be provided to affected owners in black and white plain language without any issues. My worry is that this is a larger issue than it appears to be, and maybe it is something recall-worthy... and Tesla is being shady about it trying to sweep the issue under the rug with software limitations that they won't explain. No proof of that, but it wouldn't surprise me in the slightest.
Just imagine how much it'd cost Tesla to have to recall and replace even a small percentage of 85 packs. There's something like ~90,000 cars out there with 85 packs, and another ~40,000 or so with 85-type cells. If say 5% of those needed to be replaced, that's like ~$125,000,000 in parts, not counting labor or anything. Would be a bad hit for sure.
Anyway... will update when/if I have more info.