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Should LFP batteries be charged to 100%?

Adopado

Active Member
Aug 19, 2019
4,851
3,672
Scotland
I work in the battery world but other opinions will exist! I'll dump thoughts in simple terms to try and make it readily understandable. Hope it helps.

That was a great read. Good to have all this stuff in one place that will make a useful reference.

Interesting on the charging speed potential of LFP. Clearly, from reports of LFP SR+ owners, this has not been achieved yet but reinforces the views of other commentators that it is likely that software changes will release the full potential in the near future. Do you have any comment regarding the temperature related performance issues that some have raised? Is this just a heat pump management thing or are LFP packs more likely to be affected more than the other Tesla batteries?
 

m3p_uk

Member
Aug 1, 2019
667
168
UK
I work in the battery world but other opinions will exist! I'll dump thoughts in simple terms to try and make it readily understandable. Hope it helps.

LFP, lithium ferro-phosphate is a different cell chemistry, common in China and often used with high power low range applications (buses/trucks). LFP is different to li-ion but neither worse nor better. It charges/discharges very easily, has an exceptional cycle life (you can charge/discharge many times with very little degradation) but at a cost of being less energy dense, so you need more volume to fit the same capacity of battery. You can very roughly equate cycle life to total lifetime mileage of the car - more cycles is more miles before pack needs replacing.

This makes sense for SR+ model, it has space for a very large li-ion battery so using less energy dense LFP Tesla can still achieve the required energy capacity that the SR+ model spec requires. i.e. SR+ with LFP has a bigger battery volume than SR+ with Li-ion.

Interestingly because LFP can discharge so rapidly, Tesla could probably make this model faster without undue harm to the battery. I expect they don't do this purely to keep model performance differences as a marketing ploy, not that the SR+ is slow anyway.

The car probably has an internal buffer, i.e. unused capacity at both ends of the voltage range of the cell. So when you charge to 100% it is likely actually less than this in reality. Same on discharge, when it states 0% they'll be a little left - evidenced by Carwow driving for miles after hitting 0% in their tests. High buffer helps protect the battery in early days and then can be used as the battery does degrade to maintain the stated range. So as capacity diminishes, move towards filling the battery to the real 100% capacity. On the 0% end of the capacity, by leaving some spare energy in reserve, you avoid the car damaging the cells as it continues to discharge to varying degrees when not in use. I expect at some point the car will completely turn off to protect the battery which will need some additional measures to open the charge flap and reenable charging (guessing). LFP models will likely 'supercharge' more easily than the Li-ion cars, certainly at a cell level they absorb energy far more readily but the car software will control it. There is a relationship here though with capacity, a larger battery (LR/Perf) can charge faster than SR+ simply because it has a bigger bucket to fill, so even though the SR+LFP has faster charging capability, because it is a smaller bucket it may actually charge at a similar or slower rate to the bigger siblings. It should however charge faster than the SR+Li-ion.

80% is often cited as an optimal charge level. This is a compromise but a good one - the less energy you store in the battery the more relaxed it is and the less damage caused. Think of the cell like a balloon filled with water, you can fill it to absolutely full but it'll be very stretched and tight and will become weaker - best analogy I can think of! If you can get away with only 50% in the battery and comfortably do all your journeys, then do that and recharge to 50% each day, it will benefit the pack. 80% means you get decent range and helps not stress the battery. 100% is for the days you are doing a long journey and want decent buffer to reach the next charging stop - I've charged my M3LR to 100% twice times in 18 months and even then it wasn't really necessary.

Another aspect of 80% is this is about the point that the pack reaches full charge voltage and switches from a constant current to constant voltage phase. Details are easy to find on the net but in simple terms, charging slows down from about 80% capacity, you'll see this if you watch it at a supercharger. This is why if you can charge to 80% or less and have enough range to get to the next supercharger, that will probably be quicker than charging to 100% and putting less charge in at the next charger or destination when you don't care anyway. As chargers get busier you could hypothesise that Tesla may start nudging up the kWh price after 80% charged to encourage drivers to move off to free up pumps. Note at home on a regularly 7kW charger you won't see this slowing of charge as the rate is already very slow compared to what the battery is capable of accepting.

Charging to 100% for LFP makes some sense, as said they have impressive cycle life even with full charge and discharge, so it will degrade the cell but by an appreciably smaller margin than for the Li-ion pack. By way of example, li-ion based cells get somewhere around 500 cycles before they are judged end of life (which in battery world is actually only 80% of original capacity). LFP will often achieve 2000 - 5000 cycles. Hence why Tesla isn't bothered about you charging to 100% to maximise range as you'll still get more cycle life. You can do the sums but even with 500 cycles of ~200 miles range, that's a 100,000 mile battery that's only lost 20% of capacity -ish. With the SR+LFP model, you're probably going past 500,000 miles before that happens.

The remaining range of the car is a very challenging prediction that the car computer makes and will constantly update. For most lithium based rechargeable systems an occasional 100% charge helps calibrate the algorithm that determines capacity remaining (and thereby range). Otherwise the car is trying to track capacity without a good known starting point. That means it is constantly trying to monitor exact energy consumption at all times (including when not in use) and estimate what is left. Add in that it has to predict temperature as this has an effect of the battery and car efficiency, as well as the parasitic drains on the battery e.g. if it is very cold tomorrow, it won't go as far as it is currently predicting today. ICE cars are no different, they use more fuel for the same journey if it's colder, or if the driver simply chooses to travel more quickly. I suspect that people never gave much thought to the predicted range remaining on their old diesels as it made no difference if you just needed to refill a little earlier given how fuel is so readily available. Remember in the end the range is only a guess and it has no real reflection on the actual true capacity of the battery. I would argue that charging to full and draining to nearly empty to 'calibrate' the range is futile and only helps the human feel better, it doesn't make any difference to the battery itself.

So called phantom drain is a thing, the car uses energy when it's sat doing nothing. Especially if it *is* doing something like cabin pre-heating or Sentry Mode or downloading an update and so on. This is a bit different to your old ICE dinosaur which did tend to do truly nothing when turned off, but then it didn't get regular updates, watch for intruders, defrost the screen while you were eating your cornflakes and so on. Also remember that whilst you have 'paid for' the lost miles, by comparison you've paid a lot less for the actual miles you have journeyed compared to an ICE car so let it slide!

As many others have mentioned, once you're past range anxiety and switch to percentage remaining rather than miles, you'll quickly get used to charging when you need to and relying on superchargers for longer journeys. My own way of using my car is to charge to about 85% once a week timed to finish by the time I'm about to do a commute. This journey gets the car below 80% and I carry on through the weekly commutes. I can usually get a full week of work and back plus a bit of pottering at the weekend from a single weekly charge, particularly as it gets warmer. I tend to not let the car dip below 20% purely because of the reduction on the GO pedal but there isn't any reason not to go lower. You can charge more frequently without any detrimental affect, I just avoid it because it's not necessary and is one less thing to do. With what I now know, I could have leased an SR+ and it would have been absolutely perfect and not affected me very much at all - a few more charges on the long journeys but nothing that I couldn't have managed. Trouble is, now I've had the LR and got used to the performance of it, tough to go back to anything slower next time.


Does charging from 20% to 80% do the same damage to the battery as charging from 20% to 50% twice?
Both versions would be adding 60% battery but thinking that the cycles/wear would be better with the lower charging level?
 

mpandrew

Member
Sep 23, 2019
240
267
Somerset, UK
Do you have any comment regarding the temperature related performance issues that some have raised?
I've not read these, can you point me at them?
Does charging from 20% to 80% do the same damage to the battery as charging from 20% to 50% twice?
I'd expect the decrease in performance to be pretty similar for the two examples you mention. If you made the second option 70%-100% twice, I would expect that would be more detrimental.
 

nufan

Member
Nov 6, 2019
156
137
UK
@Rajanm1: If you want to understand battery healthcare - you might want to throw in the concept of how fast you’re doing your +60% charging too....

If I understand prior forum posts well...fast Supercharging is more painful for the cells when compared with slower charging at home. Is that right mpandrew?

(If you sit in your car at a SC and hear the loud metallic bangs going on below your feet - you’ll sympathise with whatever cell torture is going on down there! Expansion I think).
 
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mpandrew

Member
Sep 23, 2019
240
267
Somerset, UK
If you want to understand battery healthcare - you might want to throw in the concept of how fast you’re doing your +60% charging too....

If I understand prior forum posts well...fast Supercharging is more painful for the cells when compared with slower charging at home. Is that right mpandrew?

(If you sit in your car at a SC and hear the loud metallic bangs going on below your feet - you’ll sympathise with whatever cell torture is going on down there! Expansion I think).
Definitely, if you only trickle charge at home from 20% to 80% somewhere down the line the 3rd or 4th owner is going to love you for it! That is the challenge of course, if you're only leasing for a few years many people may choose to just charge to 100% and not worry about the consequences.

The loud bangs are alarming, but my hunch is you're hearing the metal casing that holds the cells flexing as things warm up in there rather than the cells making the noise! Have a look on the net, the engineering in their packs is very clever, lots of liquid glycol sloshing around warming/cooling cells.
 
  • Informative
Reactions: nufan

Adopado

Active Member
Aug 19, 2019
4,851
3,672
Scotland
I work in the battery world but other opinions will exist! I'll dump thoughts in simple terms to try and make it readily understandable. Hope it helps.

LFP, lithium ferro-phosphate is a different cell chemistry, common in China and often used with high power low range applications (buses/trucks). LFP is different to li-ion but neither worse nor better. It charges/discharges very easily, has an exceptional cycle life (you can charge/discharge many times with very little degradation) but at a cost of being less energy dense, so you need more volume to fit the same capacity of battery. You can very roughly equate cycle life to total lifetime mileage of the car - more cycles is more miles before pack needs replacing.

This makes sense for SR+ model, it has space for a very large li-ion battery so using less energy dense LFP Tesla can still achieve the required energy capacity that the SR+ model spec requires. i.e. SR+ with LFP has a bigger battery volume than SR+ with Li-ion.

Interestingly because LFP can discharge so rapidly, Tesla could probably make this model faster without undue harm to the battery. I expect they don't do this purely to keep model performance differences as a marketing ploy, not that the SR+ is slow anyway.

The car probably has an internal buffer, i.e. unused capacity at both ends of the voltage range of the cell. So when you charge to 100% it is likely actually less than this in reality. Same on discharge, when it states 0% they'll be a little left - evidenced by Carwow driving for miles after hitting 0% in their tests. High buffer helps protect the battery in early days and then can be used as the battery does degrade to maintain the stated range. So as capacity diminishes, move towards filling the battery to the real 100% capacity. On the 0% end of the capacity, by leaving some spare energy in reserve, you avoid the car damaging the cells as it continues to discharge to varying degrees when not in use. I expect at some point the car will completely turn off to protect the battery which will need some additional measures to open the charge flap and reenable charging (guessing). LFP models will likely 'supercharge' more easily than the Li-ion cars, certainly at a cell level they absorb energy far more readily but the car software will control it. There is a relationship here though with capacity, a larger battery (LR/Perf) can charge faster than SR+ simply because it has a bigger bucket to fill, so even though the SR+LFP has faster charging capability, because it is a smaller bucket it may actually charge at a similar or slower rate to the bigger siblings. It should however charge faster than the SR+Li-ion.

80% is often cited as an optimal charge level. This is a compromise but a good one - the less energy you store in the battery the more relaxed it is and the less damage caused. Think of the cell like a balloon filled with water, you can fill it to absolutely full but it'll be very stretched and tight and will become weaker - best analogy I can think of! If you can get away with only 50% in the battery and comfortably do all your journeys, then do that and recharge to 50% each day, it will benefit the pack. 80% means you get decent range and helps not stress the battery. 100% is for the days you are doing a long journey and want decent buffer to reach the next charging stop - I've charged my M3LR to 100% twice times in 18 months and even then it wasn't really necessary.

Another aspect of 80% is this is about the point that the pack reaches full charge voltage and switches from a constant current to constant voltage phase. Details are easy to find on the net but in simple terms, charging slows down from about 80% capacity, you'll see this if you watch it at a supercharger. This is why if you can charge to 80% or less and have enough range to get to the next supercharger, that will probably be quicker than charging to 100% and putting less charge in at the next charger or destination when you don't care anyway. As chargers get busier you could hypothesise that Tesla may start nudging up the kWh price after 80% charged to encourage drivers to move off to free up pumps. Note at home on a regularly 7kW charger you won't see this slowing of charge as the rate is already very slow compared to what the battery is capable of accepting.

Charging to 100% for LFP makes some sense, as said they have impressive cycle life even with full charge and discharge, so it will degrade the cell but by an appreciably smaller margin than for the Li-ion pack. By way of example, li-ion based cells get somewhere around 500 cycles before they are judged end of life (which in battery world is actually only 80% of original capacity). LFP will often achieve 2000 - 5000 cycles. Hence why Tesla isn't bothered about you charging to 100% to maximise range as you'll still get more cycle life. You can do the sums but even with 500 cycles of ~200 miles range, that's a 100,000 mile battery that's only lost 20% of capacity -ish. With the SR+LFP model, you're probably going past 500,000 miles before that happens.

The remaining range of the car is a very challenging prediction that the car computer makes and will constantly update. For most lithium based rechargeable systems an occasional 100% charge helps calibrate the algorithm that determines capacity remaining (and thereby range). Otherwise the car is trying to track capacity without a good known starting point. That means it is constantly trying to monitor exact energy consumption at all times (including when not in use) and estimate what is left. Add in that it has to predict temperature as this has an effect of the battery and car efficiency, as well as the parasitic drains on the battery e.g. if it is very cold tomorrow, it won't go as far as it is currently predicting today. ICE cars are no different, they use more fuel for the same journey if it's colder, or if the driver simply chooses to travel more quickly. I suspect that people never gave much thought to the predicted range remaining on their old diesels as it made no difference if you just needed to refill a little earlier given how fuel is so readily available. Remember in the end the range is only a guess and it has no real reflection on the actual true capacity of the battery. I would argue that charging to full and draining to nearly empty to 'calibrate' the range is futile and only helps the human feel better, it doesn't make any difference to the battery itself.

So called phantom drain is a thing, the car uses energy when it's sat doing nothing. Especially if it *is* doing something like cabin pre-heating or Sentry Mode or downloading an update and so on. This is a bit different to your old ICE dinosaur which did tend to do truly nothing when turned off, but then it didn't get regular updates, watch for intruders, defrost the screen while you were eating your cornflakes and so on. Also remember that whilst you have 'paid for' the lost miles, by comparison you've paid a lot less for the actual miles you have journeyed compared to an ICE car so let it slide!

As many others have mentioned, once you're past range anxiety and switch to percentage remaining rather than miles, you'll quickly get used to charging when you need to and relying on superchargers for longer journeys. My own way of using my car is to charge to about 85% once a week timed to finish by the time I'm about to do a commute. This journey gets the car below 80% and I carry on through the weekly commutes. I can usually get a full week of work and back plus a bit of pottering at the weekend from a single weekly charge, particularly as it gets warmer. I tend to not let the car dip below 20% purely because of the reduction on the GO pedal but there isn't any reason not to go lower. You can charge more frequently without any detrimental affect, I just avoid it because it's not necessary and is one less thing to do. With what I now know, I could have leased an SR+ and it would have been absolutely perfect and not affected me very much at all - a few more charges on the long journeys but nothing that I couldn't have managed. Trouble is, now I've had the LR and got used to the performance of it, tough to go back to anything slower next time.
I have read that the new LFP batteries have the old "memory effect" that the old Nickel batteries had. If this is the case, it seems like a step backwards. I assume that's another reason Tesla is promoting charging to 100%.
 
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Mrklaw

Member
Mar 5, 2020
610
343
Berkshire
That was a great read. Good to have all this stuff in one place that will make a useful reference.

Interesting on the charging speed potential of LFP. Clearly, from reports of LFP SR+ owners, this has not been achieved yet but reinforces the views of other commentators that it is likely that software changes will release the full potential in the near future. Do you have any comment regarding the temperature related performance issues that some have raised? Is this just a heat pump management thing or are LFP packs more likely to be affected more than the other Tesla batteries?

If it was heatpump related, then you'd expect it to affect Fremont heatpump cars too, not just MiC or LFP battery models. is there any grouped data published by eg teslafi that would let you see non heatpump models vs heatpump?
 

Adopado

Active Member
Aug 19, 2019
4,851
3,672
Scotland
If it was heatpump related, then you'd expect it to affect Fremont heatpump cars too, not just MiC or LFP battery models. is there any grouped data published by eg teslafi that would let you see non heatpump models vs heatpump?

My speculation was that if the LFP battery packs were more sensitive to temperature then the tweaking of heat pump function in software may benefit them more than other types. I have no data!
 

mpandrew

Member
Sep 23, 2019
240
267
Somerset, UK
I have read that the new LFP batteries have the old "memory effect" that the old Nickel batteries had. If this is the case, it seems like a step backwards. I assume that's another reason Tesla is promoting charging to 100%.
Not in my experience, none of the lithium chemistries have memory effect. I think Tesla are saying that 100% does little to impact battery life so it promotes having full range available each day making the SR+ a more attractive proposition.
 

pdk42

Active Member
Jul 17, 2019
1,172
1,204
Leamington
I work in the battery world but other opinions will exist! I'll dump thoughts in simple terms to try and make it readily understandable. Hope it helps.
Great read - thanks. On a haïr-splitting point, LFP cells are still Li ion batteries since they work by lithium ion transfer across the electrolyte.
 

browellm

Member
Oct 4, 2019
765
756
Notts
Don't be anxious

This is sage advice. Remember that the internet is full of mad-keen owners looking to min/max every aspect of how their car operates. Whilst this is a lot of fun, it also creates a lot of anxiety for new owners - what if I do this, or don't do this etc.

Regardless of what battery you have, who made it, what the capacity is, what the temperature is and whether you preheated it or not, the truth is, the cars are designed to be run by people who barely give the owner's manual or internet a first or second glance - witness thousands of cars supercharging to 100% on journeys and other suboptimal behaviours.

If you do go outside the parameters the BMS thinks will affect the battery long-term, you'll get a gentle nudge on the car's display. So yeah, don't sweat it and have fun.
 
Last edited:

boogle

Member
Aug 22, 2020
26
47
UK
To add to what's been said, I was watching a Bjorn live stream with the Model S a while back and someone in the chat had access to the Model 3 service manual. He specified when cell balancing, and capacity re-calculation happens, at least as far as the manual goes:

The capacity calculations are performed in the following conditions, and generally only if the BMS sees the need to:
  • Sleep at over 90% SOC, this 'updates' the top voltage
  • Sleep at under 10% SOC, this 'updates' the lower voltage
Cell balancing happens when the cells are over 4V (85%), it uses very small bleed resistors: "24 hours above 90% SOC can reduce pack imbalance by 1mV"

The following isn't from the manual or chat, but just what I know as a layman in terms of batteries and messing with electronics here and there. The battery state of charge is calculated based on the voltage. The manufacturer will set a 'maximum' and a 'minimum' voltage, these will be somewhat far from the absolute limits since if you go to them the chances of permanent damage are high. The problem is the voltage doesn't fall linearly, so for the sake of argument (not real values!) the voltages might be:
  • 100%: 4.2V
  • 90%: 4.18V
  • 80%: 4.15V
  • 70%: 4.12V
  • 60%: 4.10V
  • 50%: 4V
  • 40%: 3.9V
  • 30%: 3.75V
  • 20%: 3.6V
  • 10%: 3.4V
  • 0%: 3.2V
  • Bricked: 2.8V
This is why so many cars and electronics struggle to provide a nice smooth % progression since the voltage isn't linear, and when the difference is small, really small changes can swing the % quite a lot.

The problem with LFP is that the voltage curve is even flatter, especially in the upper ranges making it super hard to guess the real % and as the BMS slowly drifts it looks like a memory, or range loss but isn't at all. If you charge to 100% periodically then the BMS can reset where it is and the range stays. It's important to note you never lose capacity, it's merely the monitoring that's deviating. You can't restore real degredation.

I've also heard second hand, from a Tesla tech, that the normal Lithium batteries (the ones with lots of Nickle, aka not Iron [LFP]) have a flat spot around 70% (65-75?). So if you keep your SOC around 70% a lot, it can cause the range display to get inaccurate and you'll need to do the whole charge to >90%, discharge to <10% dance a few times to get the accuracy back. You aren't undoing degredation, just bringing back the accuracy of the SOC % calculation.
 

Billbrown1982

TM3 LR 2021 | Red | FSD
Dec 21, 2020
534
295
Basildon
After spending 57k on the car plus another 3k in PPF - I would love to be keeping this car for a good 10 years+

Its going to take 6 before I start really "saving" money as then my running costs are basically 0 after the finance is paid off.

BUT, if in 6-8 years time, my car was still worth a decent chunk and I could upgrade for a relatively small extension to the finance...sure i'd be tempted.

I just worry about keeping the car to a point where its lost so much value (mainly due to battery degradation) that I end up stuck with a worthless dud and have to start this process all over again when I eventually upgrade.
 

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