It does move a lot, but generally only 1%.
How does it move then in general? The same direction (up / down) or does it vary?
I had not driven before that big jump right after a short charge. Battery should have been 'cold' in that it was not in active use, although my actual temps here are 70 degrees.
As it was not driven the SOC before the charge should be very correct (OCV read).
I've been running it below 10% in the last week because I wanted to see if the BMS might rebalance the cells.
There is no ”rebalancing” of the cells at low SOC.
”Calibration” of the BMS (i.e showing the battery capacity to the BMS) can be done by letting the car getting low SOC, and then sleep (no sentry, no cabin overheat, no summon and no peaking in the tesla app).
It would need to sleep a few hours to get the steady OCV = true SOC number.
Then it need to see the high SOC side fairly close in time, the same is valid for that sleep as the low SOC sleep.
There is no balancing of the cells at low SOC. Balancing is only discharging the high cells to put them at about the same voltage as the lower ones. Tesla balances by burning of energy from the high ones.
There are no reasons to balance at low SOC. It would only create a need for more balancing after recharging, as the imbalance comes from that the cells have slightly different capacity.
A full charge need balancing to actually make all cells reach 100% or close to that.
The prior month I was running the car at max charge often, which is 90% in my case because this is an SR, and I'm capped at 90%.
You should not be capped at 90%.
There is a recommendation for daily chsrge that until recently was 90% (might have been changed to 80% for your car) but when needed you should be able to set 100% and also reach 100%.
If it doesnt, you might wanna contact Tesla for a check.
Measuring SOC should be a fast operation.
Yes, it only takes like 1/100 second.
It will be correct if the car was sleeping.
Pretty sure my TeslaFi data is accurate because on several occasions, I've gone to check what the car reports on the main screen as a sanity check of SOC, and they matched.
I do not mean that the SOC number is recorded wrong but I have seen glitches where the car ”was sleeping” according to teslafi (no data received from the car) but in reality it was not sleeping (charhing or other).
A loss of data due to a connection failure or something like that during a charge would make the SOC jump.
My best guess here, and it's strictly a guess with no backing data, is that the battery SOC can vary this much based on thermal characteristics that are hidden in the battery system.
The true measured SOC doesnt change with temperature. When reading the SOC from the BMS it is not drooping with temperature. It is very constant. This is because of that the lithium battery voltage is more or less constant with varying temperature. The change for a bery big range of temperature might be a mV, which is about 0.1%.
So the SOC number in the BMS do not change due to cold battery.
The car displays a lower SOC though, that is calculated by a formula to reflect the lower output due to higher internal resistance and maybe need for battery heating (active heating during a drive only happens if the battery is sub freezing).
The battery might very well be kept warm until it decides it can no longer afford the expense, and lets it cool to ambient, which could in theory push the SOC off this much
The car never keeps the battery warm when parked.
I did log all BMS data via teslalogger and I also had a tablet in front of the steering wheel on my M3P showing everything.
The screen I setup fot driving included the battery temp.
Data was logged for two years, but when I changed car for the logging I might have caused an erase of the old data unfortunately. Need to check my raspberry pie, I hope it still is there as it shows just how things actually works.
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In this scenario, there is no outside thermal change, my temperature has varied between 68F and 72F in the garage with no change during day. Multiple days of sitting without driving, until I did the small charge shown in the graph. In theory that might have warmed up the battery as part of the process, and thus drift up the 4% as a reaction. All speculation of course.
At around 70C there are no changes to the displayed SOC due to the temperature.
Teslafi get the real SOC and also the displayed. You can look at the status on your car at teslafi, SOC is shown both as the normal SOC with warm battery (BMS-SOC) and the displayed SOC.
Heres my car at 9C ambient (varied between 4-9C that day). After been parked for 10 hrs, the cell temp was 10C (50F)
Displays 24%
SOC according to the BMS 25.2%
I just wish it was more predictable, or they'd report on battery temperature if it matters this much. If I was doing the software, I'd either give you the info you need to decide, or more likely hide it behind a 4% range check when I know it might be off from days of sitting. I'd just like to have the certainty you get with an ICE car, where you can drive down to 0% for sure, with no ambiguity. This setup is more like an old VW reserve tank where I can't remember if I flipped the lever back or not.
The SOC number will not be of from days of sitting, it will be very accurate becauae of the sleeping gives the real OCV reading.