LFP batteries and Sentry mode

[trintrax]

One question, forgive me if it has been discussed....
The manual states for LFP charge to 100% always, minimum once a week AND let the car sleep by NOT using sentry mode when not needed.

Do they mean not to use sentry mode when not needed in general OR when currently charging/charged and plugged in??


From the manual:
If your vehicle is equipped with an LFP Battery, Tesla recommends that you keep your charge limit set to 100%, even for daily use, and that you also fully charge to 100% at least once per week. If Model 3 has been parked for longer than a week, Tesla recommends driving as you normally would and charge to 100% at your earliest convenience.

In addition, a best practice is to allow Model 3 to "sleep" regularly by parking it with Sentry Mode disabled, when possible. Consider using the Exclude Home, Exclude Work, and Exclude Favorites settings to prevent Sentry Mode from automatically activating at locations it is not needed (see Using Sentry Mode).


The reason I am asking is that I have my car at home without sentry but then charge at work every day with sentry on. Should I rather keep it off then?

 

[AlanSubie4Life]

One question, forgive me if it has been discussed....
The manual states for LFP charge to 100% always, minimum once a week AND let the car sleep by NOT using sentry mode when not needed.

Do they mean not to use sentry mode when not needed in general OR when currently charging/charged and plugged in??


From the manual:
If your vehicle is equipped with an LFP Battery, Tesla recommends that you keep your charge limit set to 100%, even for daily use, and that you also fully charge to 100% at least once per week. If Model 3 has been parked for longer than a week, Tesla recommends driving as you normally would and charge to 100% at your earliest convenience.

In addition, a best practice is to allow Model 3 to "sleep" regularly by parking it with Sentry Mode disabled, when possible. Consider using the Exclude Home, Exclude Work, and Exclude Favorites settings to prevent Sentry Mode from automatically activating at locations it is not needed (see Using Sentry Mode).


The reason I am asking is that I have my car at home without sentry but then charge at work every day with sentry on. Should I rather keep it off then?

Good question.

It’s not clear whether not ever having the contactors open at 100% (while they do open at home at less than 100%) is going to have a big impact on capacity estimates being accurate.

It probably won’t matter much, is my guess; those 100% charges probably don’t have to coincide with contactors being open even though it won’t get a good OCV (open-circuit voltage) read at 100%.

But I don’t know for sure; it depends how fragile the estimation for LFP is.

For sure Sentry does not matter when the car is charging - the issue is what happens after the charge completes (plugged in does not matter either) and how important the OCV (contactors open) at that endpoint is.

 

[ATPMSD]

As long as Sentry mode is on the car does not sleep. By allowing the car to sleep the BMS can assess the low-voltage battery. Also, by allowing the car to go to sleep the HV contactor opens, and later re-closes. Think of it as never opening a car door as opposed to opening and closing it to allow the mechanical parts to move.

Not really sure which of the two is important, probably allowing the BMS to access the system. Just a guess.

 

[AlanSubie4Life]

low-voltage

High-voltage

access

Assess

Typos. Just for clarity!

 

[trintrax]

So it seems that charging for 10h at work to 100% with sentry on and never sleeping, but then not charging at home with 90% and no sentry on and sleeping is the right thing to do….

If I understand it right the BMS will assess the HV system at any time while sleeping, and not just when plugged in at 100% and sleeping.

 

[stopcrazypp]

So it seems that charging for 10h at work to 100% with sentry on and never sleeping, but then not charging at home with 90% and no sentry on and sleeping is the right thing to do….

If I understand it right the BMS will assess the HV system at any time while sleeping, and not just when plugged in at 100% and sleeping.

Yes, BMS assess the system all the time when sleeping, but I think for LFP Tesla specifically wants assessment to happen at 100% (although it's not clear if sleeping is really necessary for this). You can compromise by occasionally having sentry disabled when you charge to 100% (does not necessarily have to be at work if you have other charging options).

Note the whole charge to 100% isn't really that necessary anyways if you don't intend to run your car to very low SOCs. Although LFP is more resistant to it, it's still not good for the battery cells to charge to 100% or close all the time. I think Tesla started requesting it for LFP because the default previously practically never charges to 100% and this led to bad capacity estimates for LFP by the BMS where when people run car to low SOCs (off top of head I believe 5% or lower) and car sudden shuts down because it ran out of charge unexpectedly (and supposedly this had damaged some 12V batteries too).

From below article, supposedly the car will automatically set to charge to 100% when it feels the estimate has drifted.
Tesla using one of the big LFP battery advantages to remedy dead 12V units on the new 2022 Model 3

 

[ATPMSD]

High-voltage

Low-voltage. If the car never goes to sleep the BMS cannot evaluate the low-voltage battery. And thank you for flagging the “assess” typo!

 

[AlanSubie4Life]

Low-voltage. If the car never goes to sleep the BMS cannot evaluate the low-voltage battery. And thank you for flagging the “assess” typo!

Ah, I see what you mean, though also it is high voltage which is what confused me.

Though technically you’d hope they didn’t have to sleep to assess the low-voltage battery otherwise they have a potential bootstrapping problem and the car could end up disabled. Off topic of course.

 

[Abdul2021]

So can you delay charging 100% every week to once a month.
My daily commute is 15-20% and through out I keep my SOC from 30-50%.
nowdays temp stays at or above 100F (40C), what is the best practice to charge in this sweltering heat in Texas?

 

[Big Earl]

Because BMS calibration happens when the car is asleep, I think it’s important for the car to be able to sleep at 100% state of charge. This would require you to turn off sentry mode at work so the car can sleep when it’s finished charging.

 

[Ther0ck]

So can you delay charging 100% every week to once a month.
My daily commute is 15-20% and through out I keep my SOC from 30-50%.
nowdays temp stays at or above 100F (40C), what is the best practice to charge in this sweltering heat in Texas?

I'm sure your question was answered here :

Model 3 SR+ LFP Battery Range, Degradation, etc Discussion

Now that LFP batteries have been operating for a couple months in the US, I thought it would be useful to start a thread for people to post their experiences with degradation of LFP batteries. A major benefit of these batteries is supposed to be resistant to degradation, so let's test that claim...

teslamotorsclub.com

 

[trintrax]

So if I understand it right then the car has to charge to 100% to calibrate the battery(just for the percentage display)
AND sleep to balance the battery cells(all cells at the same voltage).
Björn I think once said Tesla’s battery balancing happens as long it is over 90%.
But I am really grasping straws here. Am I right?

So back to my original question:
Charging at work to 100% with sentry on(good calibration) and then at home sleeping with sentry off and SOC over 90%(good balancing) sounds like a good solution??

 

[AlanSubie4Life]

Björn I think once said Tesla’s battery balancing happens as long it is over 90%.

You don't need to worry about the balancing. That's not the issue here, and the BMS will make sure the batteries are balanced. I've heard they only balance at high SOC but I'm not sure exactly how true that is (it sounds like it has to be above 85%, but it doesn't matter whether the car is sleeping), and also that's probably not the reason for the sleeping recommendation from Tesla (because for the NCAs that sleeping recommendation doesn't exist and they need to be balanced too).

This is mostly guessing:
As I understand it: The issue with LFP is that the voltage curve is very flat (flatter than NCA) for a wide range of SOC and then finally climbs more steeply near 100%. So the problem is that if you never get to that high SOC point, the BMS may lose track of how much charge there is (because a huge range of SOCs look nearly the same).

This is made worse if you also don't get a good voltage read on that flat part. Letting the car sleep (contactors open) allows the car to measure the open-circuit voltage accurately (it can also measure it when NOT sleeping and just makes an estimate of what the actual open circuit voltage is), which takes some time (it takes time for the voltage to settle, etc.). I actually think the sleeping would be most critical in the FLATTEST portion of the curve, since that's where it is most important to get as close to the correct voltage measurement as possible, and if it sleeps it doesn't have to make the guesstimate from a loaded voltage measurement.

I would think it would be less critical on the steep section, since once it gets there, it's going to be pretty evident to the BMS that the battery is near 100%, open circuit or not. It'll measure the voltage regardless and reassess where it thinks the battery is at and somehow also figure out how much energy it contains. Open circuit voltage or not, the adjustments it makes to align those measurements (open circuit vs. loaded) are going to be close enough (large errors in voltage mean relatively small errors in capacity on a steep section of the curve - the opposite of the situation on a flat portion).

So I actually think you probably don't have much to worry about if you just charge to 100% occasionally at work, don't worry about it sleeping, and then just sleep it at home at 90% (even though that's already on the flat part of the curve).

And in the end this probably doesn't matter unless you drive your car below about 10-15%. And even then Tesla has made some changes which grab extra buffer, it can go as high as 11% instead of the normal 4.5% (presumably in advance - this would manifest (I think) as a significant reduction in rated range in the display for a given SOC!) if the battery is likely uncalibrated, to try to avoid problems. So it's probably less of an issue anyway than it was.

 

[stopcrazypp]

So if I understand it right then the car has to charge to 100% to calibrate the battery(just for the percentage display)
AND sleep to balance the battery cells(all cells at the same voltage).
Björn I think once said Tesla’s battery balancing happens as long it is over 90%.
But I am really grasping straws here. Am I right?

So back to my original question:
Charging at work to 100% with sentry on(good calibration) and then at home sleeping with sentry off and SOC over 90%(good balancing) sounds like a good solution??

This seems to be the most reliable info based on the service manual for the Model 3:
Charge to 90% or 80% daily?

Someone linked a reddit thread that has the direct quote of the Model 3 service manual:

https://www.reddit.com/r/teslamotors/comments/ehshn7

Quote of the relevant trigger for capacity calculation:

Notice in the figure above that the SOC was flat for a long time before and after the drive, this results in the most accurate voltage/SOC measurement which makes the CAC calculation accurate. Therefore in a worst case example, if a driver constantly charged and drove without ever letting the vehicle rest at a fixed SOC, then CAC algorithm will not update during that time.

So it basically needs to sit at a given SOC for a while to calculate capacity, but doesn't say HV contactor has to be open.

For balancing, it does require HV contactor be open (so you must let it sleep, plus need SOC high enough to be more than 4V per brick, which apparently is ~85%):

The HVBMS will order Batman to put that bleed resistor across the brick with the highest voltage when Delta V is > 5mv MinBrickV > 4.0v (~85% SOC) && HVBMS State == STAND BY.

Note however LFP BMS may not necessarily operate in the same way. Again it bears repeating, if you don't intend to ever drive below 5%, you really don't have to stress out that much about about this as you won't be bumping into scenarios of possible sudden shut down even if your capacity calculation is not that accurate. Balancing also doesn't need to be done that often. Again, even though LFP is more resistant, it's still not good for it to always be at high SOC or to use large DOD.

 

[AlanSubie4Life]

So it basically needs to sit at a given SOC for a while to calculate capacity, but doesn't say HV contactor has to be open.

Yes, it can measure the voltage whether or not the contactor is open. I think though, given that people are recommended even with NCA to let their vehicle sleep for the best estimates (by the service centers usually), it's probably a good idea to get that open-circuit voltage measurement, in general, from time to time.

So for NCA it is a "nice to have" sort of thing....while for LFP, the accuracy of the read is apparently so important for correct estimation, it may be slightly more than just "nice to have," though it may depend on your other practices.

That being said, I suspect that if the OP just charges to 100%, this second piece of best practice (letting the car sleep) in the LFP manual doesn't really matter very much. It doesn't really make sense that it would, and the manual doesn't really say you need to do both - it just lists the additional best practice.

The way I read it, that second "best practice" is a good idea in the intervening time between 100% charges, to ensure that the very best SOC estimate is arrived at in the absence of a charge to the steep part of the curve (where it's going to easily get a good estimate of where things are at even if it's not sleeping).