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Confusion on keeping the charge limit at 100% and charge fully once per week

Hi all,

I need some advice on the wording in the Tesla app (not on how to charge as this has been answered in other threads).

I've got a 2021 Model 3, Standard Range Plus, which has an LFP battery based on Control > Software > Additional Info. My charging limit bar is a 0 - 100 where you can change it to whatever limit you need. I set it to 80%. If it helps, I travel ~200km/day, consuming half my battery capacity, it dropps from 80% to 20% daily.

I was told to never charge it to 100% unless I'm doing long distance travels where I'd need the whole battery. Instead, to charge it to 80%, which I've done, and I understand the reasoning. All good there.

The confusion..

Few months back, the Tesla app says to maintain battery health, keep the charge limit at 100% and charge fully once per week.

The wording is confusing to me..

"To maintain battery health, keep the charge limit at 100% and charge fully once per week".

Here's another example too,

This doesn't make sense, as it contradicts itself.
  • If I keep the limit at 100% it'll charge to 100% every time.
  • Otherwise, to do what the app wants me to, I'd have to manually set it to 80% 6 days a week, and manually to 100% the 7th day, which again contradicts what the app is asking me to do (i.e., keep the charge limit at 100%).
In other words, it's impossible to keep the charge limit at 100% and charge fully once per week. It's either/or, not both.

Is it just me? Or is what they're trying to say worded incorrectly? Or did I misunderstand it?
 
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I got my M3 in march 22. Since i have had the car i charge it to 100% daily without a fail as i do 100+ miles a day on it and already done 22k miles on it.

When i got the car my battery use to charge to 270 miles then 269 then 267 and now at 266. Lately on a full charge it shows between 263-265 but this could be because of winter/cold here in the UK..

degration of the battery hasnt been that bad to be honest
 
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AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
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Looking at the 2000 mark, the difference between "good" and "bad" charging is 92% versus 86%. This 6% difference is about 15 miles, which is pretty negligible, even for a small battery.
Also two variables here, depth of discharge and also average SOC. Average SOC may not matter much in this case since time period is small, but hard to say.

And major contributor in reality is calendar aging, which is better at lower average SOC.
 
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Olle

Active Member
Jul 17, 2013
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2000 cycles on your graph represents about 260,000 miles at 50% of a full LFP pack, which covers most use cases. The data for 4000, 6000 or even extrapolation to 14000 cycles isn't valid for most users, since almost no one puts more than 500,000 miles on a car.

Looking at the 2000 mark, the difference between "good" and "bad" charging is 92% versus 86%. This 6% difference is about 15 miles, which is pretty negligible, even for a small battery.
Since the graph doesn't specify LFP as the cathode, this probably refers to Li-Ion in general. Reading further in Battery University we see;
1667442844634.png

However, since LFP is not more tolerant to low SOC and has more self discharge, keeping a low daily charge risks instead damaging the cells by dipping into low SOC more often. But like I wrote above, if they are resistor balanced only at Vmax, not charging to 100% every now and then will cause major damage.
 
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Instead, you can use bleed resistors [...] Side note: This is how many NMC and NCA packs worked in the old days and the reason for the unfortunate advice/rumor to charge other Teslas to 100% once a month helps with balancing.
That's still how they work. There isn't circuitry to actively balance individual cells. Balancing at 100% isn't a rumor - it's absolutely true but probably not necessary for most folks.

If a Vmax balanced LFP pack would not get a chance to reach 100% often enough, it would drift out of balance and not only lose capacity for the session but also lose physical cell capacity at an accelerating pace as the weaker cells increasingly hit their max/min, they degrade more than the others, which in turn puts the pack out of balance quicker for each session, which in turn degrades weak cells quicker yet.
If there were imbalanced strings of LFP due to individual degraded cells, "accelerated degradation" would be lesser for LFP since the voltage curve is so flat across discharge. Normally the concept here is that degraded individual cells will have a lower voltage than their peers and would need to pull a higher amperage to match the power output of higher voltage peers, leading to higher cycle count over time. However in a string of batteries in an EV the current is the same across all cells in the string regardless of degradation status, so it's not that simple. Additionally, "weaker cells" would be much worse for ternary cells since voltage varies much more significantly over discharge.

However, since LFP is not more tolerant to low SOC and has more self discharge, keeping a low daily charge risks instead damaging the cells by dipping into low SOC more often.
Neither LFP nor ternary Li-Ion are damaged by low SOC. This is an extremely common myth. 0% SOC is actually best for minimizing battery calendar aging but would be a terrible user experience 😉. Overdischarging can permanently damage lithium batteries but you would need to reach approx -10% SOC to cause irreversible damage. Internal hardware and software cuts off around 2.5% SOC so you would need to cut into the battery pack and attach some bare wires and short out the battery to cause overdischarge damage.
 
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Some people may not be able to fully charge the vehicle every time they plug it in, thus, on a LFP battery equipped Model 3, Tesla is advising you that you should both keep the charge limit at 100% and also fully charge it to 100% at least once per week.

Where it appears you are getting confused is the "I was told to never charge it to 100%" statement, as that is not applicable to the LFP batteries. The other thing it appears that you are forgetting is what I said above, that not everyone can charge to 100% for many reasons, even if they have it set that way (dont have home charging, charging on a slow connection, etc etc.
I hope you have got your answer, but there have been some real experts presenting their views here, so a great topic.
Short answer - if you have an LFP battery (which most new cars would have), then leave the charging limit at 100%.
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
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But then if we should leave the limit at 100%, why give us a limit slider in the first place? 🤔

Because people would lose their minds about:


"why is tesla removing choice of charge level from my vehicle and not everyone elses ?!?!?!?!??!?!?!!?!?!?!?!??!?!?!?!??!?!?!!?!!?!??!?!?!??!? COME ON ELON ?!?!?!?!?!?!?!?!?!?!?!?!?!??!?!?!??!?!?!?!?

(Some variation of which would be all over the internet if that were to happen)
 
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But then if we should leave the limit at 100%, why give us a limit slider in the first place? 🤔
Someone who lives on top of a hill where driving down regenerates x% may want to be able to set the charge level to (100-x)% so as not to waste the potential energy from being on top of a hill that could go into the battery if the battery is not completely full.
 
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Olle

Active Member
Jul 17, 2013
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That's still how they work. There isn't circuitry to actively balance individual cells. Balancing at 100% isn't a rumor - it's absolutely true but probably not necessary for most folks.
I highly doubt that present day ternary (thanks for that word, I learnt something new) Tesla packs balance only around 100%. My cars would be out of balance bec I never charge to 100%. I do however agree with you that Tesla doesn't use active balancing if you by that mean shifting energy between cells. It still seems to be resistor balancing, but employed over the whole SOC range as needed. Since top balancing means that all bricks reach Vmax simultaneously, you maximize energy since energy=V*SOC*Q. Perfect for LFP since it is on the lower side for acceptable range. There are many other contradicting considerations, for example balancing for power, which tries to find an individual state for each brick that has the lowest resistance. This is btw what might be going on in Ludicrous and Plaid packs when you initiate the max power / launch mode.

Here is what wk057 writes on the topic of top vs whole SOC range balance:
Balancing used to only happen at a higher SoC, but some years ago Tesla revamped the BMS and it balances as needed across almost the whole SoC range based on accurate data calculated for each brick. (As in, if it calculates that a brick will go out of balance, it proactively balances it when there is a load/charge and such to keep it in line, as it has enough data to calculate out how much use of the balance bleeders is needed).

(Edit: I did post this info in various places over the past few years, but seems the old info is what gets found more often.)
If there were imbalanced strings of LFP due to individual degraded cells, "accelerated degradation" would be lesser for LFP since the voltage curve is so flat across discharge. Normally the concept here is that degraded individual cells will have a lower voltage than their peers and would need to pull a higher amperage to match the power output of higher voltage peers, leading to higher cycle count over time. However in a string of batteries in an EV the current is the same across all cells in the string regardless of degradation status, so it's not that simple. Additionally, "weaker cells" would be much worse for ternary cells since voltage varies much more significantly over discharge.
True that LFP has a flatter voltage(soc) curve. Unfortunately that makes balancing and in turn degradation worse. Take a look at this curve of a 2S 18650 LFP string, and imagine a cell voltage of 3.2V (*2=6.4V in the chart):
1667591773509.png

source for fig 2
As you can see, the SOC could be anything between 5% and 90% at that voltage and a voltage triggered balancing function is unlikely to know what the cell SOC is. Balancing would happen at the wrong times unless it is at Vmax where the hysteresis curves converge. This is one of the reasons why LFP are typically top balanced and necessitate periodic charging to 100%.
Mid range balanced packs, which I suspect that ternary Tesla packs are nowadays, can still have phenomenal SOC estimation on the P group(brick) level due to self correcting prediction and estimation filters, that take an enormous amount of factors into account and even preemptively balance based on predicted drift. Here is where I think Tesla is really coming into its own with software and AI as their forte.
Neither LFP nor ternary Li-Ion are damaged by low SOC. This is an extremely common myth. 0% SOC is actually best for minimizing battery calendar aging but would be a terrible user experience 😉. Overdischarging can permanently damage lithium batteries but you would need to reach approx -10% SOC to cause irreversible damage. Internal hardware and software cuts off around 2.5% SOC so you would need to cut into the battery pack and attach some bare wires and short out the battery to cause overdischarge damage.
Never knew of this. Good to know, thanks!
 
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First, let's look at some relevant quotes from the owners manual:


Model 3 has one of the most sophisticated battery systems in the world. The most important way to preserve the high voltage Battery is to LEAVE YOUR VEHICLE PLUGGED IN when you are not using it.
When left idle and unplugged, your vehicle periodically uses energy from the Battery for system tests and recharging the low voltage battery when necessary.
There is no advantage to waiting until the Battery’s level is low before charging. In fact, the Battery performs best when charged regularly.

This portion essentially states that whether or not if you are charging, the vehicle should be plugged in. For most people with Level 2 charging at their house/apartment, it should pretty much be plugged in every night, if it is charging or not.


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.

This section is a bit confusing to me. They state you should charge to 100% AT LEAST once a week, but should you be charging to 100% every day?? It's worded a bit weird. What's the best method to preserve battery? How do you guys charge your LFP batteries?

I currently charge to 100% once a week, then charge to 80% the rest of the 6 days. However, I drive my Tesla a bit infrequently so on the days I charge to 100%, the car would sometimes just sit in the garage for a couple days before I drive it down to 80% or lower.

What charging practice would you recommend for my situation?
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
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Nov 28, 2018
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First, let's look at some relevant quotes from the owners manual:




This portion essentially states that whether or not if you are charging, the vehicle should be plugged in. For most people with Level 2 charging at their house/apartment, it should pretty much be plugged in every night, if it is charging or not.




This section is a bit confusing to me. They state you should charge to 100% AT LEAST once a week, but should you be charging to 100% every day?? It's worded a bit weird. What's the best method to preserve battery? How do you guys charge your LFP batteries?

I currently charge to 100% once a week, then charge to 80% the rest of the 6 days. However, I drive my Tesla a bit infrequently so on the days I charge to 100%, the car would sometimes just sit in the garage for a couple days before I drive it down to 80% or lower.

What charging practice would you recommend for my situation?

This thread I moved your new thread into has an existing discussion on this topic.

For more discussion than this on the topic of "how to charge LFP batteries", you could consider looking through / joining the discussion in this existing 40+ page thread on the topic:

 
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AAKEE

Active Member
Jan 8, 2021
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Thanks!

So setting it to 100% and charging fully is for those that may not be able to fully charge all the time. But for us that have a charger at home, then we should just keep doing what we do, which is to set the limit to 80% except once a week, to 100%.

Does that sound about right ?

Ta.

There is a lot of myths around lithium batteries. In fact, it actually seems to be more myths around than facts.

For starters, 100% isnt that bad on lithium batteies as people think (its a myth).

For LFP’s the research show that 100% is better than 80-90%.
You need to go below 70% in SOC to see a clearly lower degradation from time.

Someone posted a “Battery university” picture about cycles. Cycles is Not the concern of LFP’s. They can handle a lot of cycles and even if they are slightly affected by the size of the cycles, it of no noticable impact for your car.

Calendar aging is what your battery will degrade noticable from. Not very much but much much more than from cycles.

This is a picture from a lithium battery research report. Actual chemistry is LFP.
The uppermost blue line is how the LFP is affected from SOC x TIME x TEMP for the first 1.9 months.
While its clear that low SOC is good, we also can see that 100% is not worse than 80-90%, in fact it cause lower degradation.
You need to be below ~70% SOC to have less degradation.
990302C9-4BA4-4967-89A1-2CD92EAA07C4.jpeg


If I had a LFP Tesla, I wouldnt worry.

If I would try to reduce degradation while still following Teslas advice (to not have the risk of the BMS loosing control and running out of juice), I would charge to 100% once a week and I would try to stay below 70% otherwise, if I didnt need more than 70% SOC. I wouldnt let the car stand with more than 70% except for the full charge needed.
There is no need to leave the car at 100% ( it is not dangerous either) but a full charge set the marker for the BMS instantly (100% is 100%).

The “avoid region” for LFP is 70/75 to 95%.
Its not dangerous but it is the region that cause highercalendar aging.
At really high ambient temps like 40C or above I would try to stay below 70%.
 
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How do you guys charge your LFP batteries?

I currently charge to 100% once a week, then charge to 80% the rest of the 6 days. However, I drive my Tesla a bit infrequently so on the days I charge to 100%, the car would sometimes just sit in the garage for a couple days before I drive it down to 80% or lower.

What charging practice would you recommend for my situation?
100% approximately once per week, ideally timed so that at least 30+% will be used shortly after finishing charging, to minimize extended parking at >70%.

65-70% otherwise.

Generally use scheduled departure charging.
 
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dmurphy

Active Member
Supporting Member
That's still how they work. There isn't circuitry to actively balance individual cells. Balancing at 100% isn't a rumor - it's absolutely true but probably not necessary for most folks.

I wish I had a more recent Theory of Operations guide for 3/Y. The one I’ve seen says that balancing happens > 85% SoC and of course takes a very long time - days. If it’s changed since publication, I’d sure love to see the new data.

I’m from the “set it and forget it” camp. Is it possible I could squeeze more out of the battery? Maybe. But I’d rather not think about it and just drive the car. :)
 
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If I had a LFP Tesla, I wouldnt worry.

If I would try to reduce degradation while still following Teslas advice (to not have the risk of the BMS loosing control and running out of juice), I would charge to 100% once a week and I would try to stay below 70% otherwise, if I didnt need more than 70% SOC. I wouldnt let the car stand with more than 70% except for the full charge needed.
Tack, AAKEE, I have been using your graph (from [IOP - 2016] Calendar Aging of Lithium-Ion Batteries) to help me make my decisions.

What would you do if you had an LFP M3 that was driven, on average, 25 miles per week?
 
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AAKEE

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Jan 8, 2021
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Tack, AAKEE, I have been using your graph (from [IOP - 2016] Calendar Aging of Lithium-Ion Batteries) to help me make my decisions.

What would you do if you had an LFP M3 that was driven, on average, 25 miles per week?

If depends.

1) I would not worry.
2) Charge to 100% once a week or maybe every other week.
—————————————-
(Nerd level below.)
3) If energy costs was not an issue, use sentry to bleed off SOC to 70% to reduce the SOC above 70% if the climate was very warm (warmer than 25C or so in average). This during the hot period, not during cold season.
 
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