Range Loss Over Time, What Can Be Expected, Efficiency, How to Maintain Battery Health

[FlatSix911]

FYI - my July battery report calculates average SOC is 56%.

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[KenC]

FYI - my July battery report calculates average SOC is 56%.

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Protect your EV battery with Owner Insights by Recurrent. This is a community of 20,000 drivers who foster EV adoption by demystifying range and battery health.

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View attachment 835634

Interesting. Why does your charge dot, drift lower over time? Are you only charging once every couple weeks? Mine is always the same-ish.

Apparently, I drove right off their chart in the first image.

In Stats, there's a Smart Battery Prep function, where I set my daily SOC to 60%, then it falls back to 58% after a couple hours. Why do I do this, I've forgotten! Apparently, Recurrent sees the 58% as 57%. Probably rounding down.

 

[AAKEE]

View attachment 835729

I would be very interrested in learning what kind of source they have for the statement that you should keep the SOC between 30 and 80% to minimize degradation.

My guess is that they got cought by the forum rumors….

 

[KenC]

I would be very interrested in learning what kind of source they have for the statement that you should keep the SOC between 30 and 80% to minimize degradation.

My guess is that they got cought by the forum rumors….

Ha, sources! No one needs sources, this is the internet age of anything goes!

 

[tm3lrawd]

I would be very interrested in learning what kind of source they have for the statement that you should keep the SOC between 30 and 80% to minimize degradation.

My guess is that they got cought by the forum rumors….

If 70-80% SOC is OK, why is 20-30% not OK?

 

[jackmott]

If 70-80% SOC is OK, why is 20-30% not OK?

Its just some kind of wrong received wisdom that lithium ion cells don't like to be at low states of charge. Probably because they can brick when they go *below* their 0% state of charge.

 

[3sr+buyer]

Its just some kind of wrong received wisdom that lithium ion cells don't like to be at low states of charge. Probably because they can brick when they go *below* their 0% state of charge.

Probably has to do with the following:

• Comfort against range anxiety for the driver. How many drivers are comfortable charging so that they get home at 1% every day? But that does not have anything to do with what is good or not good for the battery.
• Vampire drain if the car is parked at 1% and not charged for a while, leaving the possibility of bring drained below 0% that may be bad for the battery.

 

[MY-Y]

Its just some kind of wrong received wisdom that lithium ion cells don't like to be at low states of charge. Probably because they can brick when they go *below* their 0% state of charge.

If you have cells in series at a low SoC, a cell can get charged backwards. This is very bad for the cell.

Specifically, at a high current draw, the individual cell's voltage can flip while the current is still flowing in the normal direction. I suspect the car's BMS tries to ensure that this never happens.

Even at low current draw, as a series of cells' SoC dwindles, a cell can hit 0v while the series bank still has a fairly high voltage. At this point, any current draw is charging the cell backwards. Low SoC can cause problems that high SoC doesn't.

 

[jackmott]

If you have cells in series at a low SoC, a cell can get charged backwards. This is very bad for the cell.

Specifically, at a high current draw, the individual cell's voltage can flip while the current is still flowing in the normal direction. I suspect the car's BMS tries to ensure that this never happens.

Even at low current draw, as a series of cells' SoC dwindles, a cell can hit 0v while the series bank still has a fairly high voltage. At this point, any current draw is charging the cell backwards. Low SoC can cause problems that high SoC doesn't.

Is this something that happens when cells are at or above their minim rated voltage? Because 0% on the car is well above 0volts for any cell.

 

[AAKEE]

If 70-80% SOC is OK, why is 20-30% not OK?

20-30% is very good and below is even better, down to 0% on the screen.

In fact, 70-80% SOC is not that good if the car is standing long times with that. Its OK by Tesla, as the battery will hold up u til the warranty is gone, but it probably causes higher degradation, specially in hot climates.

The lower the SOC, the lower the aging from time.
The lower the SOC, the less wear from cycles.
The smaller the cycles the less degradation.

Bold above = What all research about lithium battery aging says from the result of very much research.

70-80% SOC cause quite high degradation so the statement is wrong in both ends.

For the lowest degradation (no range anxiety, no ”only have acess once a week” or practical issues etc), the advice for minimizing the degradation should be to only charge what you need until the next charge, and also to charge as often as possible.

 

[AAKEE]

If you have cells in series at a low SoC, a cell can get charged backwards. This is very bad for the cell.

Specifically, at a high current draw, the individual cell's voltage can flip while the current is still flowing in the normal direction. I suspect the car's BMS tries to ensure that this never happens.

Even at low current draw, as a series of cells' SoC dwindles, a cell can hit 0v while the series bank still has a fairly high voltage. At this point, any current draw is charging the cell backwards. Low SoC can cause problems that high SoC doesn't.

No: polarity switching not an issue with lithium batteries. Forget that part.

100% SOC is defined by the maximum allowed voltage per cell set by the cell manufacturer (4.20v/cell for NCA and NMC)
0% SOC is defined by the lowest allowed discharge voltage, also set by the cell manufacturer.
At 0% there still is charge/energy left in the battery. Not much but enough to still be very safe.
At 0% ve have the least degradation from time for the battery. This refers to 0% True SOC.

This 0% true SOC level is way below Teslas 0% on the screen. Teslas 0% on the screen is circa 4.5% true SOC on the battery.

A NCA battery like Tesla panasonic 2170 will degrade with time according to this picture:
Its quite clear that 0% is the SOC that causes least degradation from time.
Also clear that 70-80% is about as bad as 90 or even 100%.
There is some research reports that actually points out 80% as the worst SOC for calendar aging.

 

[KenC]

I would be very interrested in learning what kind of source they have for the statement that you should keep the SOC between 30 and 80% to minimize degradation.

My guess is that they got cought by the forum rumors….

Okay, I'm sure it has to do with utility. There's no reason why you can't keep your SOC lower than 30%, but how practical is that? At 20%, Tesla turns off your security cameras. In general, keeping one's Tesla at very low SOCs for daily use is not something very many people are considering. It's all about utility, and balancing that against battery longevity.

 

[andy92782]

No: polarity switching not an issue with lithium batteries. Forget that part.

100% SOC is defined by the maximum allowed voltage per cell set by the cell manufacturer (4.20v/cell for NCA and NMC)
0% SOC is defined by the lowest allowed discharge voltage, also set by the cell manufacturer.
At 0% there still is charge/energy left in the battery. Not much but enough to still be very safe.
At 0% ve have the least degradation from time for the battery. This refers to 0% True SOC.

This 0% true SOC level is way below Teslas 0% on the screen. Teslas 0% on the screen is circa 4.5% true SOC on the battery.

A NCA battery like Tesla panasonic 2170 will degrade with time according to this picture:
Its quite clear that 0% is the SOC that causes least degradation from time.
Also clear that 70-80% is about as bad as 90 or even 100%.
There is some research reports that actually points out 80% as the worst SOC for calendar aging.

View attachment 835949

File this under "Stuff I wish I'd known back when my car was new."

My new daily charge setting is 50%. Today that buys me close to 140mi of range. Plenty enough for what I need most of the time. Every so often I need to do a longer drive but it's no big deal; just crank it up to 90% for that one day then revert back to 50% when I no longer need the extra range.

 

[MY-Y]

No: polarity switching not an issue with lithium batteries. Forget that part.
...

I suggest that you don't forget this part. (Otherwise, I wouldn't have posted it.) There are good papers on this subject that are more reliable than forums like this.

"...In this study, focused on the overdischarge phenomenon that is most likely to be encountered in the practical use of electric vehicles and grid storage, the impact of overdischarge on battery performance degradation is analyzed by neutron imaging technology and its safety hazards is systematically explored, combined with multimethods including electrochemical analysis and structural characterization. Results reveal the deterioration of the internal structure of traction batteries" - https://pubs.acs.org/doi/pdf/10.1021/acsami.0c18185

"Lithium-ion batteries connected in series are prone to be overdischarged. Overdischarge results in various side effects, such as capacity degradation and internal short circuit (ISCr)." - Mechanism of the entire overdischarge process and overdischarge-induced internal short circuit in lithium-ion batteries - Scientific Reports

"Another condition, known as overdischarge, is also becoming a common safety issue as greater numbers of cells are being connected in series, as is the case in systems requiring high voltages, such as Electric Vehicles (EVs). This work seeks to explain the mechanisms that cause internal damage to cells during external shorting and overdischarge and to determine the most dangerous conditions that can exist during these types of abuse." -
Overdischarge and External Short Behavior of Lithium-Ion Batteries

 

[DrChaos]

I suggest that you don't forget this part. (Otherwise, I wouldn't have posted it.) There are good papers on this subject that are more reliable than forums like this.

"...In this study, focused on the overdischarge phenomenon that is most likely to be encountered in the practical use of electric vehicles and grid storage, the impact of overdischarge on battery performance degradation is analyzed by neutron imaging technology and its safety hazards is systematically explored, combined with multimethods including electrochemical analysis and structural characterization. Results reveal the deterioration of the internal structure of traction batteries" - https://pubs.acs.org/doi/pdf/10.1021/acsami.0c18185

"Lithium-ion batteries connected in series are prone to be overdischarged. Overdischarge results in various side effects, such as capacity degradation and internal short circuit (ISCr)." - Mechanism of the entire overdischarge process and overdischarge-induced internal short circuit in lithium-ion batteries - Scientific Reports

"Another condition, known as overdischarge, is also becoming a common safety issue as greater numbers of cells are being connected in series, as is the case in systems requiring high voltages, such as Electric Vehicles (EVs). This work seeks to explain the mechanisms that cause internal damage to cells during external shorting and overdischarge and to determine the most dangerous conditions that can exist during these types of abuse." -
Overdischarge and External Short Behavior of Lithium-Ion Batteries

The results are real, but their definition of "overdischarge" is seen in experiments going way way beyond the usual 2.5V low cutoff, all the way to negative voltages (temporarily) and then of course to zero. Important, but only for short circuits and outright failures.

Looks like 0% indicated on Teslas are above 2.5V. I think the calibration of the cars is pretty conservative on the low end (also why 0% indicated is 4.5% actual). Which makes sense as they really want to avoid point cell failures (stranding people and warranty costs), but minimizing slow expected degradation over years isn't something they optimize for as much.

That's something owners need to optimize for.

 

[MY-Y]

I mentioned in my initial post that I suspect the BMS won't allow a cell to go negative. For my 2020 MY, the voltage of each of the 96 packs in series is measured separately. If a pack (listed as a "cell" in scanmytesla) got too low, I think the BMS would limit the current.

My last post was clarifying that having a cell go negative, even for lithium batteries, is a bad thing.

Before I got AB in my 2020 MY, it was very slow at highway speeds if the SoC was below 15% and it was cold. My minivan could easily take it in a rolling race. I suspect this was caused by the BMS aggressively avoiding an over discharge. I'd be interested to know if a non-AB car with newer software is still so slow for cold low SoC highway driving.

 

[bedoig]

...My minivan could easily take it in a rolling race...

I need to get my wife into whatever minivan you're driving

 

[MY-Y]

I need to get my wife into whatever minivan you're driving

It's a hybrid Pacifica. Now that the AB and/or coincidental software updates fixed this, my MY would dust any minivan at 15% SoC, even when cold.

 

[AAKEE]

I suggest that you don't forget this part. (Otherwise, I wouldn't have posted it.) There are good papers on this subject that are more reliable than forums like this.

"...In this study, focused on the overdischarge phenomenon that is most likely to be encountered in the practical use of electric vehicles and grid storage, the impact of overdischarge on battery performance degradation is analyzed by neutron imaging technology and its safety hazards is systematically explored, combined with multimethods including electrochemical analysis and structural characterization. Results reveal the deterioration of the internal structure of traction batteries" - https://pubs.acs.org/doi/pdf/10.1021/acsami.0c18185

"Lithium-ion batteries connected in series are prone to be overdischarged. Overdischarge results in various side effects, such as capacity degradation and internal short circuit (ISCr)." - Mechanism of the entire overdischarge process and overdischarge-induced internal short circuit in lithium-ion batteries - Scientific Reports

"Another condition, known as overdischarge, is also becoming a common safety issue as greater numbers of cells are being connected in series, as is the case in systems requiring high voltages, such as Electric Vehicles (EVs). This work seeks to explain the mechanisms that cause internal damage to cells during external shorting and overdischarge and to determine the most dangerous conditions that can exist during these types of abuse." -
Overdischarge and External Short Behavior of Lithium-Ion Batteries

I think I linked to some research earlier in this thread.
0% true SOC is not overdischarge.

Overdischarge is discharging below the lower voltage limit of the cell. For teslas NCA this limit is 2.5 V/cell ( can be seen in the BMS as pack low volt limit 241V, I think with for example Scan my tesla.
0% on the Tesla screen is > 3Volts, and the BMS shur down the car by opening the high voltage contactors before any cell reach danger voltage.

So in a Tesla there is no risk of getting any cell dangerously low because the BMS protects the battery. It would be possible though, if you actually try to kill the battery: to drive until the car shut down and then leave it for very long time so the voltage drops from self drain to dangerous voltage levels. Self drain is low at low SOC so it might take weeks.

We are protected fron overdischarge in hour Teslas.

For overdischarge, below the voltage limit, small overdischarges most often do not cause any damage at all. You need to overdischarge to a quite low voltage to cause damage.
Range Loss Over Time, What Can Be Expected, Efficiency, How to Maintain Battery Health

 

[AlanSubie4Life]

If a pack (listed as a "cell" in scanmytesla) got too low, I think the BMS would limit the current.

The bricks are balanced by BMS bleed resistors. It’s very important in series packs, as your link above suggests. That’s the point of balancing otherwise cells will eventually (actually quite rapidly) be overdischarged. Potentially imbalance can result in overcharging as well. And actually the BMS can only tolerate relatively small differences in brick capacity, even with the mechanisms it uses to balance.

So anyway the point is that overdischarge is not a problem for the end user. It absolutely is an issue but it’s not important for the owner (with the obvious caveats of course).