My experience is the same as yours. September 2021 SR+ LFP, and I have just over 10,500 miles now, but the ranges posted by Baluchi in each of his updates has matched mine.
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Model 3 SR+ LFP Battery Range, Degradation, etc Discussion
- Thread starter Baluchi
- Start date
yep, 10,500! Thanks I always feel a little bit wimpy after Baluchi posts his latest mileage!
On the bright side, I'm going to have to fork out a bunch of money for a new car way before you do.
I'm just an old guy and really not that technical so would be interested to hear what some of you more techy guys think about my Tesla performance recently.I have a 2022 Model 3 rear drive lfp.11 months old 25k.km no garage, winter tires(15.5mi)Sat.nite -6C/21f.charged 100% scheduled precondition. At 0830 departure time temp-3(26) EPA projected range range427(265). Five adults drove 270km(167mi) Temp between - 3to-5c(23-27f).Arrived home SOC 21% 91km(57).Did have a bit of heat on. what do you think ?
OK, so I reset one of the trip meters to get a more comprehensive picture of the effects of changing from 42 psi to 45 psi — good thing because I guess that one-day drop to 197 watt hours per mile was some kind of anomaly. So far the trip meter that was reset shows a drop per mile to 218 watt hours per mile, down 3 watt hours per mile from when I last reset it about 15 months ago…
I've been driving without the aero covers for the last week, because I broke on of the clips. I've got a new one coming tomorrow, but I've noticed about a 15 watt hour per mile penalty without them.
The last 5 % can take 15-20 minutes longer than any other 5%. I don’t know much about batteries but my speculation it’s slower intentionally but think it also has to do with the BMS doesn’t actually know what 100% is when your are charging. It’s likely part of the recalibration and the reason why you have to charge to 100% at least once a week.
Lithium batteries can only be charge with a specific maxmimum Voltage, to be safe/not degraded etc.
For LFP I don not have the exakt number in a Tesla, so in this example, I use any long range/performance.
Each cell is only allowed to be charged with 4.20V/cell, and a fully charged cell has close to 4.20V resting voltage.
At low SOC we can turn on the voltage tap to reach the desired power of amphere, whatever we use to regulate the charging speed.
This, as the cell might have a resting voltage of 3.3 Volts or so and if we increased the voltage to 4.20V we would more than supercharge the cell.
At high SOC the voltage comes closer to 4.2V, so even if we keep 4.20V the charging power will reduce as the voltage difference reduce as the SOC / cell voltage increase.
Its the Voltage difference that is the driving force to charge the battery. eventually we are very close to the final voltage and the voltage difference will be smaller and smaller, reducing the charging power.
LFP battery voltage will significantly increase at 100% state of charge, so the BMS will know when that is reached. But it may have some difficulty knowing 90% versus 95% versus 99% from cell voltage alone.
90 vs 95% might be hard, but 99% is much easier to see because of how sharp the rise in voltage is above 95%. But from 90-95%, it can be very flat, which is why Tesla recommends charging to 100% regularly instead of just 90%.
OCV-SOC curve for LFP battery at room temperature: (a) 0%-100% SOC; (b) 30%-80% SOC.
When charging to 100% with a regular lithium ion charger, the charger use the decresing current as the measurement for knowing when the charging should be considered finished.
For example when the current has decreased to 1/20 of the set charging current.
As long as the charger is charging, the voltage is kept constant by the charger so its not possible to detect the cells resting voltage.
I would guess that Teslas use the same technique to know when to call the charging finished for a charge to 100%.
For partial charges, the BMS know the SOC before the charge, and has an estimated capacity (”nominal full pack”) so the number of kWh needed can be calculated before the charging starts. That number of kWh ”to charge complete” is present in the BMS and can be seen with Scan My Tesla.
The final SOC for partial charges does not always hit the exact set number, and can be fairly of if the Bms is of and ”would need” a BMS calib.
If the charge overshoots (final SOC > the set charging target) the BMS is probably overestimating the capacity. And, vice versa.
The resulting SOC wont be seen directly after the charge, but after a short time, with preferably a sleep between.
This pic is from a time when my BMS was quite of, underestimating the capacity.
The S point is the start of the sleep with the car stating 55% (55% was the selected target) after the charging finished and 53.86% was the ”real SOC” after a four hour sleep.
For example when the current has decreased to 1/20 of the set charging current.
As long as the charger is charging, the voltage is kept constant by the charger so its not possible to detect the cells resting voltage.
I would guess that Teslas use the same technique to know when to call the charging finished for a charge to 100%.
For partial charges, the BMS know the SOC before the charge, and has an estimated capacity (”nominal full pack”) so the number of kWh needed can be calculated before the charging starts. That number of kWh ”to charge complete” is present in the BMS and can be seen with Scan My Tesla.
The final SOC for partial charges does not always hit the exact set number, and can be fairly of if the Bms is of and ”would need” a BMS calib.
If the charge overshoots (final SOC > the set charging target) the BMS is probably overestimating the capacity. And, vice versa.
The resulting SOC wont be seen directly after the charge, but after a short time, with preferably a sleep between.
This pic is from a time when my BMS was quite of, underestimating the capacity.
The S point is the start of the sleep with the car stating 55% (55% was the selected target) after the charging finished and 53.86% was the ”real SOC” after a four hour sleep.
I've owned the car for four months now, two with covers on and two with covers off, and I've actually gotten better efficiency since taking them off, but that's probably more about me flooring it less and no longer using FSD beta, which I subscribed to for the first couple months. FSD beta accelerates faster than I typically would from a stop and waits later than I typically would before letting off the accelerator (it usually has to use some brakes and loses out on some regen). Also, 90% of my driving is on roads with 45-55 mph speed limits, and I suspect the covers make less of a difference at these lower speeds. When Car and Driver did their controlled test, they measured 8 Wh/mi difference at 50 mph and 19 Wh/mi difference at 90 mph.
This is a study of LFP aging and capacity recovery that might interest you.
TLDR:
- Shallow cycles in the middle of the SOC range (~20-80%) cause higher capacity loss in LFP cells than deep cycles or cycles that cross certain SOC ranges below 20% and above 80%.
- Here's a graph showing capacity loss percentages after 350 Full Equivalent Charges at different cycle ranges. The study says that crossing between voltage plateaus (especially between 1, 2, and 3 or between 4 and 5) is what keeps capacity loss low. As you can see, cycling between 15-35% had the least capacity loss, followed by 0-100% and 10-90%, whereas cycling between 40-60% had the most capacity loss.
- The capacity loss is not due to BMS calibration drift. It's because the voltage curve is so flat in the middle of the SOC range that it causes non-uniform lithium distributions in the electrodes. Higher voltage gradients in the lower and upper SOC ranges help to redistribute lithium more evenly.
- Most (up to 90%) of the capacity loss due to this effect is recoverable with the right procedure.
- The most effective recovery procedure is holding SOC at 0% for 1-3 weeks depending on temperature (1 week at 45 C to 3 weeks at 25 C). Holding SOC at 100% also worked somewhat, but it was much slower and much less effective. Note: When your Tesla gets to 0%, it's really around 4.5% due to the buffer. I'm not sure how that affects the recovery and whether you need to drain it all the way out or not. I also imagine it won't be great for the low voltage battery, but maybe if you keep the car plugged in and not charging at very low SOC, that would work?
Even though most of the capacity loss from this effect is theoretically recoverable, this combined with the fact that deep cycles barely have any affect on LFP batteries (unlike NCA and NCM batteries), led me to avoid the desire to do 40-60% or 30-50% cycles, which would be great for NCA or NCM batteries. Instead, I charge to 100% just before I know I'll be going somewhere, then let the SOC get all the way down to 15-20% before I charge it back to 100% again, kind of like you would do with a gas car. Letting the battery get down low lowers the average SOC, which is good for calendar aging, but this method also lets the SOC cross those areas with higher voltage gradients to keep lithium distributed evenly. I do still plug in at home every time (so I can get preconditioning from wall power, etc), but I stop the charge manually when I'm not ready to charge yet.
If you're set on not charging to 100%, you might at least want to do something like 15-55% cycles instead of like 30-55% or 40-55%, to make sure you're crossing that area with a slightly higher voltage gradient around 18%.
I know the common thinking around here is that Tesla only recommends charging LFP to 100% so that the BMS can properly calibrate, but I wonder if it's also to help prevent the uneven lithium distribution that can be caused by charging only in the middle of the SOC range where the voltage curve is most flat. After all, Tesla does say "To maintain battery health", not "To maintain battery calibration".
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Well first things first: thanks very much for taking the time to post the link to this study and pertinent information from the study.
My battery pretty much sits between 37% and 55% at any given time, and probably (because I don’t drive that much and charge up as soon as I get home) spends most of its time at 55%. I guess according to the chart I am maybe getting somewhere around 7.5% capacity loss per 350 FEC?
Now I am going to try to extrapolate: If an FEC for my car is roughly equivalent to 240 miles of driving, then I would likely average an FEC somewhere around every 10 days that I drive it. Since I drive around 20 days a month, perhaps I tend to complete 2 FEC’s per month. I have had the car for about 20 months, so I have perhaps 40 total FEC’s under my belt so far with this car. This kind of matches up with 43.75, which is the number I get from dividing my total odometer miles (10500) by 240.
If the capacity loss is proportional and I have driven the car for somewhere around 1/8th of the 350 FEC‘s measured in the study, then my capacity loss so far should be 7.5% divided by 8 = a little less than 1%.
Baluchi, on the other hand, who drives about 5 times as much as I do, at this point shows about the same capacity loss as I do. It would be interesting to plug in some his numbers as far as what ranges his battery is usually operating within, and what his expected capacity loss would be for those ranges. Maybe Baluchi and I equal out because though he loses more range than I do because of increased miles, I in turn lose more range because of operating between 37% to 55% most of the time…
Without reading the whole post in detail, I have seen that report and if I remember it correct, the same researchers even got another report out with contradicting result.
Other reports have not seen the same thing for small cycles.
Anyway, a LFP will do several thousand FCE how ever you choose to discharge it, small or full cycles.
So, 4-5000 FCE cycles equals one miljon mile or so. This in turn means that the degradation per year or 10K mile or whatever unit we use that comes from cycles/ driving will be almost neglibe.
Picture shows degradation (full range) on 2021 SR+ with LFP. What we see here is to the absolute largest part calendar aging, and not cyclic aging. If it was cyclic aging, that continues in about the same rate, that battery wouldnt hold for many miles.
The conclusion is, (the same as for NCA and NMC) that if we would like to minimize degradation we need to focus on the calendar aging as it will be the dominating degradation factor for very long on LFP (as the cyclic aging is negligible.
If we would need to prioritize between reducing calendar and cyclic aging the choice is clear.
RichAZ/CapeCod
Member
gtg, I admit I find the discussion as to where to leave my M3 SR LFP (Aug 2022) for six months (May~October) somewhat baffling. In your opinion, should I leave it at 100% SoC or 50% SoC? Car will be plugged in to a 120v outlet, garaged in AZ.
Rich
50%, plugged in.
Will reduce the calendar aging.
RichAZ/CapeCod
Member
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