Tesla sets charge limit to 90%

[jcanoe]

Isn't the Tesla BMS capable of monitoring cells and keep a battery balanced?

Yes. The battery management system will automatically maintain the battery. There are things you can do to help ensure that the Tesla Model Y BMS is able to obtain needed data from the pack. Periodically the BMS needs to be able to measure the open cell voltage (OCV) of each of the cells in the pack. This takes several hours and the Tesla Model Y battery pack must be disconnected from the Tesla vehicle as when in sleep mode. When Sentry mode is left turned on the Tesla Model Y remains in Standby mode (high voltage battery connected) and does not enter Sleep mode.

It is a good practice to periodically charge to above 94% or 95% so that the Tesla Model Y BMS can collect data at the higher state of charge. Periodically let the battery state of charge fall to 30% or less so that measurements can be collected at this lower state of charge. If the battery is always maintained within a narrow range state of charge the BMS has less data to accurately determine the state of the battery when estimating current state of charge and remaining range outside of the daily charge level.

There is a downside risk of not enabling the Tesla Model Y to periodically enter sleep mode for extended periods so that the OCV measurement can be performed. You could someday be driving the Tesla Model Y, the battery state of charge reading 10% but the vehicle shuts down. This scenario could happen because the BMS was not able to collect needed OCV data to accurately determine the state of the battery across a wide range of state of charge.

When you charge to 100% the Tesla Model Y will display 100% SOC but continue to charge for perhaps an additional 40 minutes. During this time the BMS is assumed to be performing cell balancing and other automated processes. It can take almost an hour longer than you anticipate to charge to 100%. This is one reason not to fully charge to 100% when it is possible to charge to ~95%; charging completes much sooner. I don't have the Tesla documentation but believe that cell balancing will also periodically take place at the completion of charging to less than 100% SOC.

 

[Iron]

I do let the car sleep by excluding sentry mode from home & work. I close the app and don't check on it. I usually leave it unplugged when doing this but I've read that the car will sleep if plugged in and not charging. My charging is set for depart by 6:30 AM daily.

 

[myshenaini]

Just wondering if anyone else has had this issue because I don't know if it's normal behaviour or is it my battery. I have a '21 MYLR and I've always charged it to 90% unless I'm going on a long trip and then I'll charge to 100%. But since the winter, it would charge to 90% but then soon after, it would drop to 89%. Even though I set it to stop at 90%, it doesn't stay there. I recall that when the car was newer, it used to charge to 90% and it would stay there but this behaviour only started happening the past winter. I just assumed it was cold and it lost 1%. But now, it's also happening in the summer too. Sometimes it would charge to 90% but most of the time, it stops at 89 or even 87%, even though I still have it set to stop at 90%. Do I need a service call or is this normal?

 

[jcanoe]

Just wondering if anyone else has had this issue because I don't know if it's normal behaviour or is it my battery. I have a '21 MYLR and I've always charged it to 90% unless I'm going on a long trip and then I'll charge to 100%. But since the winter, it would charge to 90% but then soon after, it would drop to 89%. Even though I set it to stop at 90%, it doesn't stay there. I recall that when the car was newer, it used to charge to 90% and it would stay there but this behaviour only started happening the past winter. I just assumed it was cold and it lost 1%. But now, it's also happening in the summer too. Sometimes it would charge to 90% but most of the time, it stops at 89 or even 87%, even though I still have it set to stop at 90%. Do I need a service call or is this normal?

No, your Tesla Model Y does not require servicing (at least not for the displayed state of charge varying by a couple of percentage points.) The Tesla Model Y's battery management system will update the state of charge depending on the average cell voltage and temperature of the battery pack. Slight variances in the displayed state of charge have been the case with different Tesla software updates, so it could be due to a minor software bug.

 

[HMHM]

Periodically the BMS needs to be able to measure the open cell voltage (OCV) of each of the cells in the pack. This takes several hours and the Tesla Model Y battery pack must be disconnected from the Tesla vehicle as when in sleep mode.

Thanks for all the technical information. I have not read this in the owner's manual. This is all new. I never had my battery at 95% or 100% SoC. And, on the low end of SoC, I never had it below 30%. How often and why should I manually perform the balancing of the cells?

 

[jcanoe]

Thanks for all the technical information. I have not read this in the owner's manual. This is all new. I never had my battery at 95% or 100% SoC. And, on the low end of SoC, I never had it below 30%. How often and why should I manually perform the balancing of the cells?

Cell balancing takes place automatically, as required, every time you charge. The open cell voltage (OCV) measurement is also something the Tesla Model Y's battery management system will perform on its own, provided that you enable the Tesla Model Y to enter Sleep mode. Sleep mode is when the high voltage battery disconnects from the low voltage side of the Tesla Model Y. When the Tesla vehicle wakes up, as when you open any door you will hear a loud clunk after a few seconds. This is the sound of the high voltage contactors connecting the high voltage battery.

Sentry mode, when active, keeps the Tesla Model Y in Standby mode, i.e. it never enters Sleep mode. This prevents the battery management system from performing OCV measurement. The battery management system collects, uses the OCV data to more accurately determine the state of the high voltage battery pack. You can do your part by making sure that Sentry mode is set to be off at your home location, at least some of the time. (Background: The Sentry mode feature was added to the Tesla vehicles after the Tesla Model S was designed, built. The Tesla battery engineers may not have foreseen that the Tesla vehicle could, when new features such as Sentry mode were used all of the time, be prevented from regularly entering Sleep mode.) The Tesla Model Y Owner's Manual does not cover nuances including periodic open cell voltage measurement.

 

[CleanDemon]

To each their own, but there seems to be a fair amount of charging anxiety out there. Maybe I’m naive, but I’m simply going to trust Tesla and drive my car. My wife charges her MYP to 90% daily, and I charge mine to 80% daily. If that costs me a few miles over several years, I can live with that. Between Tesla and even Elon himself saying you can charge to 90%, I think we’ll be fine. Plus the BMS is another layer of security. I told my wife last night that “I’m going to ’drive the *sugar*’ out of my Tesla and enjoy it!”.

 

[metroplex]

To each their own, but there seems to be a fair amount of charging anxiety out there. Maybe I’m naive, but I’m simply going to trust Tesla and drive my car. My wife charges her MYP to 90% daily, and I charge mine to 80% daily. If that costs me a few miles over several years, I can live with that. Between Tesla and even Elon himself saying you can charge to 90%, I think we’ll be fine. Plus the BMS is another layer of security. I told my wife last night that “I’m going to ’drive the *sugar*’ out of my Tesla and enjoy it!”.

Elon has said 90% and then changed his tune to 80% and the assumption is that it was for NCA during the times he's commented on that. The data I've seen (which is fairly available) for NCA, NMCA, and LFP show differences in the battery degradation based on SoC, DoD, etc... not a significant amount but still a factor.

I have also noticed that when I set my charge limit to 90% last year when the M3LR was new, that after a month it would stop at 89% or drop to 89% soon after charging.

 

[E90alex]

It’s been re-hashed over and over. Tesla does not recommend 90% exactly. They recommend less than 90%. There was also a tweet from Elon (not that Elon is the definitive reliable source for anything) confirming 70% is technically better still but may not be worth the trade off in range/convenience.

Studies have shown lower is better, especially below 55% or so, so set it anywhere from 50-90% to the number that works best for you.

 

[CaseyL]

It’s been re-hashed over and over. Tesla does not recommend 90% exactly. They recommend less than 90%. There was also a tweet from Elon (not that Elon is the definitive reliable source for anything) confirming 70% is technically better still but may not be worth the trade off in range/convenience.

Studies have shown lower is better, especially below 55% or so, so set it anywhere from 50-90% to the number that works best for you.

Yes. Just charge to what you need the next day, plus some buffer. My round trip commute uses about 16% a day, so I charge to 50% at 4:00 every morning. So the car spends most of it's idle time at just over 40%.

 

[metroplex]

Yes. Just charge to what you need the next day, plus some buffer. My round trip commute uses about 16% a day, so I charge to 50% at 4:00 every morning. So the car spends most of it's idle time at just over 40%.

This is exactly what I ended up doing thanks to everyone’s smart feedback here. 50% every 0400. I end up coming home with over 40% everyday.

 

[polyphonic54]

Someone on page 1 said batteries last longer than 15 years. Anyone have a source to that? JB Straubel has simply said 15 years.

15 years isn’t that old for a lot of ICE cars, but I’m sure battery longevity will increase with new technologies.

 

[LostVector]

So this rabbit hole of charging has been very interesting ever since I got my new Model Y.

Definitions

SoC = State of Charge, aka percentage of battery remaining.
BMS = Battery Management System. The software/hardware that monitors and controls the charging and discharging of rechargeable batteries.

Interesting notes

1. The colder the storage, the better (up to a point).
   1. Source: Assessment of the calendar aging of lithium-ion batteries for a long-term—Space missions
   2. "For some chemistries, storage temperatures of −27 °C and lower, degrade the performance of the LIBs (Dubarry and Devie, 2018; Li et al., 2021) and cause compressive strain and the deformation of the lattice on an atomic scale, leading to electrode particle cracking (Li et al., 2021). On the other hand, many LIB datasheets suggest that storage even at temperatures as low as −30 °C are acceptable (Samsung, 1865a; Samsung, 1865b)."
   3. It should be noted that a good cold storage temperature is not necessarily a good "cold usage" temperature. Cars may need to warm up.
   4. Practical advice: try to keep your car out of "hotbox" situations wherever you normally park. Vent your garage, park in the shade, etc.
2. Shallow discharges around a midpoint SoC are better than full 100% -> 0% discharges.
   1. Source: ShieldSquare Captcha
   2. In other words, it's healthier for your battery to go from 60% -> 40% five times than it is to go from 100% to 0% once.
   3. Practical advice: If you have a regular commute, set your charge level to swing around 50% SoC. Plug your car in regularly.
3. Keeping the car at 100% SoC is bad for its long term capacity.
   1. Source: ShieldSquare Captcha
   2. NMC batteries seem to be particularly unhappy with 100% SoC. Definitely avoid charging to 100% on them except for immediate trips.
4. Keeping your car at near 0% SoC is "best" for battery capacity, but isn't practical
   1. Source: Assessment of the calendar aging of lithium-ion batteries for a long-term—Space missions
   2. Source: ShieldSquare Captcha
   3. "Most researchers have concluded that the aging process slows down at low SOCs (Hahn et al., 2018; Naumann et al., 2018). However, discharging the battery below 0% SOC can cause the terminal voltage to decrease, resulting in performance and safety issues (Chen et al., 2021; Zhang et al., 2022). For instance, Yao et al. (Yao et al., 2019) and Guo et al. (Guo et al., 2016) indicate that dropping the terminal voltage of the battery well below the 0% SOC defined operation range (to values such as 0.2 V) may result in loss of electric contact, transition metal dissolution, and current collector corrosion, all of which lead to calendar aging."
   4. Practical advice: It doesn't really make sense to store a car at a very low SoC ... first of all, Tesla doesn't allow keeping the car plugged in and maintaining anything below 50% anyway ... and second, keeping it unplugged and near low SoC could risk the battery discharging to damagingly low levels if left alone for a long time.
5. The healthiest practical storage SoC for batteries is NCA = 55%, NMC = 60%, LFP = 70%
   1. Source: ShieldSquare Captcha
   2. It should be noted that Tesla's advice to charge to 100% for LFP batteries seems to be based on practical consumer usage and not necessarily maximum long term battery capacity.
   3. Practical Advice: If reasonable, charge to the noted SoC for your battery on a daily basis and only charge to higher capacities for trips.
6. The advice to charge LFP batteries to 100% is to keep the BMS calibrated, not to maximize long term battery health.
   1. Source: LFP Vs NMC Battery: Complete Comparison Guide
   2. With a gradual loss of voltage, it's much easier for the BMS to determine that a certain voltage level is equivalent to a correlated SoC. With LFP batteries, the BMS needs to "see" the sudden rise in voltage at 100% once in a while as a reference to know what the battery's SoC is while discharging at other times.
   3. Practical advice: Yes, charge your LFP Tesla to 100% once per week as Tesla recommends. But perhaps not more! It would be nice if Tesla could automate this more in software.
7. Calendar aging and cycle aging both contribute to battery aging
   1. Source: Modelling Lithium-Ion Battery Ageing in Electric Vehicle Applications—Calendar and Cycling Ageing Combination Effects
   2. NMC, NCA and LFP batteries all experience roughly equivalent calendar aging if set to a "healthy" SoC as noted above.
   3. LFP batteries experience roughly half the degradation of NMC batteries over a certain number of cycles.
   4. Practical advice: if you drive a lot and plan on keeping your car for a while, LFP based models could easily be the better value overall as they will retain more range than NMC or NCA based models over time and usually cost less up front.
8. More to come?

Thanks to @AAKEE for his "obsession" with the studies surrounding all the charging behavior here and sending me down the rabbit hole.

 

[Hiline]

So this rabbit hole of charging has been very interesting ever since I got my new Model Y.

Definitions

SoC = State of Charge, aka percentage of battery remaining.
BMS = Battery Management System. The software/hardware that monitors and controls the charging and discharging of rechargeable batteries.

Interesting notes

1. The colder the storage, the better (up to a point).
   1. Source: Assessment of the calendar aging of lithium-ion batteries for a long-term—Space missions
   2. "For some chemistries, storage temperatures of −27 °C and lower, degrade the performance of the LIBs (Dubarry and Devie, 2018; Li et al., 2021) and cause compressive strain and the deformation of the lattice on an atomic scale, leading to electrode particle cracking (Li et al., 2021). On the other hand, many LIB datasheets suggest that storage even at temperatures as low as −30 °C are acceptable (Samsung, 1865a; Samsung, 1865b)."
   3. It should be noted that a good cold storage temperature is not necessarily a good "cold usage" temperature. Cars may need to warm up.
   4. Practical advice: try to keep your car out of "hotbox" situations wherever you normally park. Vent your garage, park in the shade, etc.
2. Shallow discharges around a midpoint SoC are better than full 100% -> 0% discharges.
   1. Source: ShieldSquare Captcha
   2. In other words, it's healthier for your battery to go from 60% -> 40% five times than it is to go from 100% to 0% once.
   3. Practical advice: If you have a regular commute, set your charge level to swing around 50% SoC. Plug your car in regularly.
3. Keeping the car at 100% SoC is bad for its long term capacity.
   1. Source: ShieldSquare Captcha
   2. NMC batteries seem to be particularly unhappy with 100% SoC. Definitely avoid charging to 100% on them except for immediate trips.
4. Keeping your car at near 0% SoC is "best" for battery capacity, but isn't practical
   1. Source: Assessment of the calendar aging of lithium-ion batteries for a long-term—Space missions
   2. Source: ShieldSquare Captcha
   3. "Most researchers have concluded that the aging process slows down at low SOCs (Hahn et al., 2018; Naumann et al., 2018). However, discharging the battery below 0% SOC can cause the terminal voltage to decrease, resulting in performance and safety issues (Chen et al., 2021; Zhang et al., 2022). For instance, Yao et al. (Yao et al., 2019) and Guo et al. (Guo et al., 2016) indicate that dropping the terminal voltage of the battery well below the 0% SOC defined operation range (to values such as 0.2 V) may result in loss of electric contact, transition metal dissolution, and current collector corrosion, all of which lead to calendar aging."
   4. Practical advice: It doesn't really make sense to store a car at a very low SoC ... first of all, Tesla doesn't allow keeping the car plugged in and maintaining anything below 50% anyway ... and second, keeping it unplugged and near low SoC could risk the battery discharging to damagingly low levels if left alone for a long time.
5. The healthiest practical storage SoC for batteries is NCA = 55%, NMC = 60%, LFP = 70%
   1. Source: ShieldSquare Captcha
   2. It should be noted that Tesla's advice to charge to 100% for LFP batteries seems to be based on practical consumer usage and not necessarily maximum long term battery capacity.
   3. Practical Advice: If reasonable, charge to the noted SoC for your battery on a daily basis and only charge to higher capacities for trips.
6. The advice to charge LFP batteries to 100% is to keep the BMS calibrated, not to maximize long term battery health.
   1. Source: LFP Vs NMC Battery: Complete Comparison Guide
   2. With a gradual loss of voltage, it's much easier for the BMS to determine that a certain voltage level is equivalent to a correlated SoC. With LFP batteries, the BMS needs to "see" the sudden rise in voltage at 100% once in a while as a reference to know what the battery's SoC is while discharging at other times.
   3. Practical advice: Yes, charge your LFP Tesla to 100% once per week as Tesla recommends. But perhaps not more! It would be nice if Tesla could automate this more in software.
7. Calendar aging and cycle aging both contribute to battery aging
   1. Source: Modelling Lithium-Ion Battery Ageing in Electric Vehicle Applications—Calendar and Cycling Ageing Combination Effects
   2. NMC, NCA and LFP batteries all experience roughly equivalent calendar aging if set to a "healthy" SoC as noted above.
   3. LFP batteries experience roughly half the degradation of NMC batteries over a certain number of cycles.
   4. Practical advice: if you drive a lot and plan on keeping your car for a while, LFP based models could easily be the better value overall as they will retain more range than NMC or NCA based models over time and usually cost less up front.
8. More to come?

Thanks to @AAKEE for his "obsession" with the studies surrounding all the charging behavior here and sending me down the rabbit hole.

This is a great summary of our General understanding! The only thing I’d add is that for the vast majority of Tesla cars, calendar aging is the primary contributor of battery degradation, so if you need to trade off between low depth of charging and low state of charge, go with low state of charge.

 

[ewoodrick]

Someone on page 1 said batteries last longer than 15 years. Anyone have a source to that? JB Straubel has simply said 15 years.

15 years isn’t that old for a lot of ICE cars, but I’m sure battery longevity will increase with new technologies.

Average age of vehicles has been creeping up lately and I just saw a 13.1 years.
And Tesla is expected to be able to match that.
If the battery average is 15 years, that meets the criteria, because some will last longer, some won't last that long.

 

[CaseyL]

Average age of vehicles has been creeping up lately and I just saw a 13.1 years.
And Tesla is expected to be able to match that.
If the battery average is 15 years, that meets the criteria, because some will last longer, some won't last that long.

Heck, I hope I last that long. My battery might outlive me!

 

[HMHM]

So this rabbit hole of charging has been very interesting ever since I got my new Model Y.

Wow! A wonderful read indeed. Thank you @LostVector for your painstaking work here. Here are my findings:

1. Tesla Long Range version EVs, Model 3 and Y, both carry 4,416 cells, Panasonic NCA 2170, arranged in 96 groups of 46, and weighs 1,060 pounds (480 kg) in a 0.40 m³ volume; a battery pack density of 150 Wh/kg (540 kJ/kg).

2. For NCA and NCM chemistries, less you discharge your battery, more cycles you’ll get from your battery.

3. Shallow charge and discharge (55-35 or 55-30 SoC), if feasible for routine driving, is the best option for a long battery life.

4. The energy consumption is reasonably good (between 250-280 Wh/mi) for highway driving between 65-74 mph. Above these mph range numbers, the energy consumption goes up dramatically.

5. Normally, an idle Tesla consumes 6-10% of its battery every day. If you put it in Sentry Mode or Dog Mode, it's closer to 1% every hour.

6. Tesla Sleep Mode drains the car’s battery more slowly than if it was “awake." The Sleep Mode consumes just 1% of the battery every day. It does this by switching off any unnecessary systems, including the one that communicates with the Tesla app.

7. Tesla recommends driving or charging your car at least once a week to keep it running smoothly.

 

[jcanoe]

4. Normally, an idle Tesla consumes 6-10% of its battery every day. If you put it in Sentry Mode or Dog Mode, it's closer to 1% every hour.

That information is not correct. When the Tesla Model Y is parked and the low voltage system is being powered by the high voltage battery this is Standby mode. The Tesla Model Y consumes ~230 Watts when in Standby mode. This power drain is from the DC to DC converter and the low voltage systems being powered. This will continue as long as the Tesla Model Y does not enter Sleep mode.

When the Tesla Model Y enters Sleep mode the power use is ~25W (just over 1/10th as much power). The high voltage battery disconnects from the Tesla Model Y low voltage systems. The 15.5V low voltage lithium battery (this replaced the older 12V lead-acid battery) powers essential systems including the door locks, Bluetooth and LTE modems while the Tesla Model Y is in Sleep mode. Periodically the Tesla Model Y will wake from Sleep mode and enter Standby mode for a short time to recharge the low voltage battery as required, then re-enter Sleep mode.

Tesla Model Y features including Sentry mode and Camp mode prevent the Tesla Model Y from entering Sleep mode. That is why the power drain is 6% to 7% per 24 hours when you have Sentry mode active. This is also true for Smart Summon (part of Full Self Driving.) As many Tesla Model Y owners have confirmed, if you park the Tesla Model Y unplugged with Sentry mode turned off the high voltage battery will lose just ~1% of the charge per week, not 6% to 10% per day.

 

[Hiline]

Average age of vehicles has been creeping up lately and I just saw a 13.1 years.
And Tesla is expected to be able to match that.
If the battery average is 15 years, that meets the criteria, because some will last longer, some won't last that long.

I think Elon Musk said the modern batteries should last 300k-500k miles. For the 15k miles / year I drive, I’m banking on my Model Y battery lasting 20 years.

 

[AAKEE]

I think Elon Musk said the modern batteries should last 300k-500k miles. For the 15k miles / year I drive, I’m banking on my Model Y battery lasting 20 years.

A large portion of the degradation comes from time.

You can not calculate with miles only.
400K does not mean I can drive 1K each year for 400 years.

After 15 years a lithium battery will be rather tired even if it was not driven much or at all.
After 15 years at the most average temperature and 70-90% SOC the battery will have about 20% degradstion or slightly more even if it wasnt driven.