BMS Drift - does it really matter?

[Tam]

...I don't know why you would say that...

Because you can read from Jerome Guillen I (former VP, WW sales and service):

"Thank you for your message and bringing this concern to my attention.

Recently, some Model S owners have been concerned regarding displayed decreases in their vehicle's predicted range. We understand the importance of accurate range prediction, and have taken these concerns seriously. After investigating many of the cases, we have found no indications of actual battery problems in the vast majority of these cases. However, we have uncovered a software problem that, under certain charging conditions, yields incorrectly low range estimates over time.

The range estimate displayed in the vehicle is based on several different factors. One important factor is the battery's actual capacity at a given point in its life. This is the amount of energy the battery can hold when fully charged. Since battery capacity cannot be measured without fully charging and discharging the battery, this value is calculated in software. We have found that in cases where the battery is consistently charged to a lower state of charge- between 60-80% -capacity estimation becomes less accurate and tends to underestimate the true capacity of the battery. The result is an incorrect reduction in the displayed range estimate. This does not affect the true range of the vehicle, as the end-of-drive conditions are based on real-time battery measurements of reducing battery power rather than software estimates. In any case we recognize the inconvenience and negative user experience associated with this incorrect and reduced range estimate and we are developing a solution.

The inaccuracy of the capacity algorithm will show up in any Model S that is regularly charged under the conditions mentioned above (it seems it is the case for your Model S). We will develop and implement a firmware updates in the coming weeks (timing TBD) to address the concern you outlined. That being said, the amount of actual energy stored in the battery has not changed. The physical distance you can drive the vehicle from full to empty remains the same, only the displayed estimate has changed. You are correct that avoiding charging to high states of charge optimizes battery capacity retention. You should continue this behavior as best meets your daily range needs. We also recommend opportunistically charging, i.e, charge frequently, charge often. Avoiding deep discharges is another best practice for optimum capacity retention. The advice by the Service Center to "let the range drop down to about 20 miles and then charge it to max Daily Range" is misleading. They are correct that it will mitigate the inaccuracy of the capacity algorithm, resulting in a higher displayed range, but it is misleading as the actual amount of stored energy does not change.

I hope that this addresses your concern. Please let me know if I can be of any additional assistance.

Thanks for your continued support. Best regards,

Jerome Guillen I VP, WW sales and service"

 

[AlanSubie4Life]

Because you can read from Jerome Guillen I (former VP, WW sales and service):

This is from 6 years ago. If it still represents the state of affairs today, that would be troubling. And in fact they say they're fixing this problem, but they've gone far beyond that now I am sure.

Like I said, if you don't believe me, it's super easy to verify. Just do long continuous drives without stopping, and log your kWh used precisely on the trip meter. And then do that over time. It's EXTREMELY consistent. You'll find that the results imply that if you have fewer rated miles, you have less energy available.

I am SURE there are occasional exceptions (cold weather will certainly lead to shifts in the results because the battery "magically" will gain energy as it warms, and sometimes there are even imbalance or BMS issues), but that's not what I'm talking about here. I'm talking about the normal state of affairs, for most drivers, under typical conditions, over time.

 

[Tam]

...This is from 6 years ago...

Human behaviors still persist: Many people still want to set their charge at 80% which is specifically mentioned 6 years ago as the cause for more frequent BMS drift.

The firmware has improved the calculation so that owners don't have to harm their battery by churning it through repeated deep discharges and full charges: Just set at 90% and the BMS will eventually recalculate itself.

Some people report that they could drive beyond 0% or 0 miles. I would say that's an example of BMS drift that reports lower capacity when the car still has more than what it calculates as 0 miles 0%.

 

[AlanSubie4Life]

Some people report that they could drive beyond 0% or 0 miles. I would say that's an example of BMS drift that reports lower capacity when the car still has more than what it calculates as 0 miles 0%.

No, it's not. I would never recommend anyone attempt to use it (because voltage sag under load could cause the car to shut down at any time), but ALL Model 3 vehicles reserve 4.5% of their 100% charge capacity below 0%. That's the reason for the 0.955 factor above.

And indeed, there are videos online of people driving (very slowly!) 20+ km after the meter shows 0%.

That specific observation is unrelated to BMS drift.

There is limited evidence that the majority of people see any recovery in their rated miles. When they're gone, they're gone, because that's how entropy works.

Again, there are ALWAYS exceptions and "strange" behavior due to temperature shifts, etc. But I have monitored my car very carefully on numerous trips - all the way from 100% to 5% - and it's always ~230Wh/rmi. And now I have 286-290 of them, rather than the 310 I started with.

No big deal, but it's also very real (that’s 4.6kWh!).