Capacity fade in EVs

[Posatronic]

I found an interesting study

http://www.myrav4ev.com/forum/viewtopic.php?f=8&t=561&p=13780&hilit=battery+fade#p13780

published on the Rav 4 EV forum.

The author purchased Panasonic cells similar or possibly identical to those used by Tesla, which are presumably also the same or similar to those used in your Mercedes.

He tested the cells in isolation, at room temperature, and subjected them to cycling equivalent to approximately 90K miles. He said this took "many months"

In his tests, fade was about 10% at 90K miles. However, these were at moderate temperatures, and they involved primarily cycling fade, not calendar fade. Typically, a 100K mile car would need 6 to 10 years, depending on usage.

If his tests are indicative, it tends to dispute the often quoted Tesla owner report that suggests about 10% per 100K miles, even with temperature and calendar effects.

Another interesting study

http://jes.ecsdl.org/content/164/1/A6066.full#F6

From the Journal of the Electrochemical Society looks at calendar fade in NCA batteries, similar to those used by Tesla. they basically charged up cells to various states of charge and stored them, at various temperatures, for 9 months. They then measured the battery capacity when fully charged.

Look in particular at Figure 8. It shows, as expected, greater fade at higher storage temperatures and states of charge. But below about 55% SOC, fade is very low and almost independent of SOC. Above 55 %, there is a step increase in fade rate, and the fade increases with higher and higher SOC.

This study gives results which are consistent with an earlier Army study which showed that the cells lasted better when cycled between 0% and 50% compared to cells cycled between 25% and 75% or between 50% and 100%. Many people believe that "mid level" is best for the batteries, but these two studies refute that belief. Lower is better than midway.

Hey first of all, great finds. I like all that data.
Second your interpretation of figure 8 seems a little off.
You should throw out 50c. But would like to see 12.5c, more realistic temptures with battery conditioning.
So looking at just 25c(ideal temp) biggest loss at first is 0-10%. Little more 10-20% then it levels off to 55% for all 4 lines. Then from 55-65 is the step your looking at and goes down and tapers off about level the rest of the way out. At 55% for average of all 4 lines at 25c is dropping .025 capacity. That seems very small. And does not suggest way lower
Than 50% is best. It looks like mid way is just fine, not a big enough difference to leave at 0 SOC. It's such a small difference that then I'd point(55% SOC) is just fine. Then the second big step after 0-10% is 55-65~.
So to me this says having the battery at approximately 55% is just fine and then having the car charged up right before you use the car seems to be the best. So in reality then setting in he Bolt for charging to 40% right away then charge the rest of the way just before you leave seems pretty accurate. ( assuming and based on others peoples "testing" of battery that the battery is bigger then 60kwh like 66-70)
Also if you add that the Prius uses their battery from 60-80% and have one of the best track record for their battery's. ( extremely low replacement of their batteries going bad)
The mid point, to stay charged , seeems very reasonable based on reasearch and then charge right before you leave.
Even the worst 25 c line after 9.6 months is only .05.
If you find more studies I would love to read them. I like to see lots of data.

 

[Nagorak]

From the study it looks like having a non-actively managed battery temperature is also not good. Granted 55 C is pretty hot, but maybe not unreasonable for a car parked outside in the sun in Southern CA. That makes me really reluctant to consider the 2nd gen Nissan Leaf since it is once again going to be air cooled (more accurately not cooled). For those who live in NorCal or PNW maybe it doesn't matter as much.

Edit: Found this humorous...Tesla charging nearly $10K for a "range upgrade" that is nothing more than a software update.

https://electrek.co/2016/12/14/tesla-battery-capacity/

A lot of the Tesla business model rubs me the wrong way. It's one reason I'm sort of reluctant to buy the Model 3 although I have put a reservation down as an option.

They can log into your car whenever they want. A bunch of features are just software locked which strikes me as shady, even though I understand they could have simply not installed those features. On top of that Elon Musk seems like an *** what with canceling people's orders who post complaints.

On the whole I think Tesla has helped EV adoption a lot and hopefully their products are good and they succeed. But I can't be a complete fan of the company.

 

[oilerlord]

A lot of the Tesla business model rubs me the wrong way.

They can log into your car whenever they want. A bunch of features are just software locked which strikes me as shady, even though I understand they could have simply not installed those features.

I can't be a complete fan of the company.

Sounds a lot like the manufacturer of very popular rectangles. People either love or hate the walled garden.

 

[GetOffYourGas]

Sounds a lot like the manufacturer of very popular rectangles. People either love or hate the walled garden.

I love my rectangle. Don't really care about the walled garden.

Same thing with Tesla (for me, anyway). If I love the Model 3, I will buy one. I don't have to be a fan of the company or their practices. To flip it around, there are some very vocal people who claim they will never buy a Bolt simply because of what GM has done or continues to do (e.g. petitioning Trump to lower CAFE standards). And that's independent of how good of a product the Bolt may be.

 

[oilerlord]

I love my rectangle. Don't really care about the walled garden.

Same thing with Tesla (for me, anyway). If I love the Model 3, I will buy one. I don't have to be a fan of the company or their practices. To flip it around, there are some very vocal people who claim they will never buy a Bolt simply because of what GM has done or continues to do (e.g. petitioning Trump to lower CAFE standards). And that's independent of how good of a product the Bolt may be.

Same thing for me. I could care less about dieselgate, the megathreads discussing it, or that VW lied. I love my VW.

 

[WetEV]

I could care less about dieselgate, the megathreads discussing it, or that VW lied. I love my VW.

I'm sure the thousand or so that died from VW's excess pollution would agree with you. As would the thousands that will die if these cars stay in operation.

Of course, much of the death toll is in Europe, where there are more VWs, the population is denser, the miles driven per car is higher, and the atmospheric conditions are less favorable.

http://news.mit.edu/2017/volkswagen-emissions-premature-deaths-europe-0303

 

[oilerlord]

I'm sure the thousand or so that died from VW's excess pollution would agree with you. As would the thousands that will die if these cars stay in operation.

Of course, much of the death toll is in Europe, where there are more VWs, the population is denser, the miles driven per car is higher, and the atmospheric conditions are less favorable.

http://news.mit.edu/2017/volkswagen-emissions-premature-deaths-europe-0303

Fortunately, I offset ALL of my VW's CO2, SO2, and NOx but thanks for the lesson in environmental morality 101. Want to buy some credits? I have plenty to spare:

Clean up your own backyard before you criticize mine.

 

[SparkE]

I could care less about dieselgate, the megathreads discussing it, or that VW lied. I love my VW.

I'm sure the thousand or so that died from VW's excess pollution would agree with you. As would the thousands that will die if these cars stay in operation.

Of course, much of the death toll is in Europe, where there are more VWs, the population is denser, the miles driven per car is higher, and the atmospheric conditions are less favorable.

http://news.mit.edu/2017/volkswagen-emissions-premature-deaths-europe-0303

What, you are surprised the Germans are gassing people to death?

 

[michael]

I found an interesting study

http://www.myrav4ev.com/forum/viewtopic.php?f=8&t=561&p=13780&hilit=battery+fade#p13780

published on the Rav 4 EV forum.

The author purchased Panasonic cells similar or possibly identical to those used by Tesla, which are presumably also the same or similar to those used in your Mercedes.

He tested the cells in isolation, at room temperature, and subjected them to cycling equivalent to approximately 90K miles. He said this took "many months"

In his tests, fade was about 10% at 90K miles. However, these were at moderate temperatures, and they involved primarily cycling fade, not calendar fade. Typically, a 100K mile car would need 6 to 10 years, depending on usage.

If his tests are indicative, it tends to dispute the often quoted Tesla owner report that suggests about 10% per 100K miles, even with temperature and calendar effects.

Another interesting study

http://jes.ecsdl.org/content/164/1/A6066.full#F6

From the Journal of the Electrochemical Society looks at calendar fade in NCA batteries, similar to those used by Tesla. they basically charged up cells to various states of charge and stored them, at various temperatures, for 9 months. They then measured the battery capacity when fully charged.

Look in particular at Figure 8. It shows, as expected, greater fade at higher storage temperatures and states of charge. But below about 55% SOC, fade is very low and almost independent of SOC. Above 55 %, there is a step increase in fade rate, and the fade increases with higher and higher SOC.

This study gives results which are consistent with an earlier Army study which showed that the cells lasted better when cycled between 0% and 50% compared to cells cycled between 25% and 75% or between 50% and 100%. Many people believe that "mid level" is best for the batteries, but these two studies refute that belief. Lower is better than midway.

Hey first of all, great finds. I like all that data.
Second your interpretation of figure 8 seems a little off.
You should throw out 50c. But would like to see 12.5c, more realistic temptures with battery conditioning.
So looking at just 25c(ideal temp) biggest loss at first is 0-10%. Little more 10-20% then it levels off to 55% for all 4 lines. Then from 55-65 is the step your looking at and goes down and tapers off about level the rest of the way out. At 55% for average of all 4 lines at 25c is dropping .025 capacity. That seems very small. And does not suggest way lower
Than 50% is best. It looks like mid way is just fine, not a big enough difference to leave at 0 SOC. It's such a small difference that then I'd point(55% SOC) is just fine. Then the second big step after 0-10% is 55-65~.
So to me this says having the battery at approximately 55% is just fine and then having the car charged up right before you use the car seems to be the best. So in reality then setting in he Bolt for charging to 40% right away then charge the rest of the way just before you leave seems pretty accurate. ( assuming and based on others peoples "testing" of battery that the battery is bigger then 60kwh like 66-70)
Also if you add that the Prius uses their battery from 60-80% and have one of the best track record for their battery's. ( extremely low replacement of their batteries going bad)
The mid point, to stay charged , seeems very reasonable based on reasearch and then charge right before you leave.
Even the worst 25 c line after 9.6 months is only .05.
If you find more studies I would love to read them. I like to see lots of data.

My Focus Electric had liquid battery cooling, and when charging and operating it maintained a temperature right at 98 F (37 C). The system did not cool to anything approaching 25 C and certainly not 12 C.

I don't know how the Bolt TMS will work. But the Ford system, for all practical purposes would heat up to 98F when charging or driving, and would remain very close to that for many hours, due to insulation and large thermal mass.

The point I would like to make is that many people believe the battery does best if charged and discharged above and below the midpoint. This research, together with the Army paper and the writings of Jeff Dahn suggest that the centroid should be lower. The battery has the lowest degradation at zero SOC, not at 50%.

I think the fact that a grossly over-discharged battery will be damaged suggests to many people that a battery at a very low SOC is "unhappy". As best I can see, it's very happy.

 

[elpwr]

It is not corrrct that zero SOC results in minimum degradation.

 

[michael]

I posted an article that says it does. They presented test results and explained the underlying electrochemical mechanism. What is your basis for making that assertion??

This is my very point. A lot of people believe mid levels of charge are best for the battery but scientific evidence shows otherwise

What is your basis?? I have explained mine

 

[elpwr]

I have no intention of arguing the point with you. There is abundant evidence to suggest that the extreme top and bottom SOC's are hard on the battery. Use them only when it is absolutely necessary. And not for storage.

Teslas battery longevity is recognized as being about the best. Tesla strongly recommends keeping the car plugged in at all times. The minimum SOC when plugged in is 50%. I do this with my model S. It is 3.5 years old and has 34,000 miles on it. The battery degradation (rated range reduction) is only 1.8%. I have experienced absolutely minimal battery degradation by avoiding the extreme SOC's where possible, and keeping the car plugged in at all times - as per the recommendation.

 

[elpwr]

I have no intention of arguing the point with you. There is abundant evidence to suggest that the extreme top and bottom SOC's are hard on the battery. Use them only when it is absolutely necessary. And not for storage.

Teslas battery longevity is recognized as being about the best. Tesla strongly recommends keeping the car plugged in at all times. The minimum SOC when plugged in is 50%. I do this with my model S. It is 3.5 years old and has 34,000 miles on it. The battery degradation (rated range reduction) is only 1.8%. I have experienced absolutely minimal battery degradation by avoiding the extreme SOC's where possible, and keeping the car plugged in at all times (around 50% SOC for storage) - as per the recommendation.

 

[elpwr]

Sorry for the double post

 

[oilerlord]

My Focus Electric had liquid battery cooling, and when charging and operating it maintained a temperature right at 98 F (37 C). The system did not cool to anything approaching 25 C and certainly not 12 C.

I don't know how the Bolt TMS will work. But the Ford system, for all practical purposes would heat up to 98F when charging or driving, and would remain very close to that for many hours, due to insulation and large thermal mass.

The point I would like to make is that many people believe the battery does best if charged and discharged above and below the midpoint. This research, together with the Army paper and the writings of Jeff Dahn suggest that the centroid should be lower. The battery has the lowest degradation at zero SOC, not at 50%.

I think the fact that a grossly over-discharged battery will be damaged suggests to many people that a battery at a very low SOC is "unhappy". As best I can see, it's very happy.

I haven't found a paper that's disputed as temperature increases, more degradation occurs. The ambient temperature in my heated garage is maintained at 14C, and doesn't usually climb above that until June, July, and August. The other nine months out of the year, my city's average temperature is typically 11C or less. As much as it sucks to live in a winter city, my battery as you say - is very happy.

When winter gets really cold here, like -20C - I'm not able to play the very low / charge to mid-level SOC game because less than 40 miles of range is too risky for me. As we head into spring, summer, and fall, and as it does appear that low / very low SOC is better than the alternative of high / very high SOC - I'm going to begin the practice of charging to 75% (indicated) which is less than 60% of my total battery capacity, and run it down to 10% indicated SOC . I used to charge to 100% and come home with ~30%. In terms of range & usability, nothing really changes as all I'm doing is shifting the SOC number lower, cycling deeper into the battery. Again, all bets are off in the winter...I'll be charging to 100%.

I also very rarely plug the car in overnight and/or leave the car at 100% SOC for long periods. My work allows me to plug in the car at home during the day (and when the sun is shining on my PV), and more or less manually manage the SOC. I wish the car had the Tesla SOC "slider" but it doesn't, and timers won't work for me because I don't have a fixed 9-5 (or other) schedule.

Cool temperature, low-mid SOC, and efforts to maintain an artificially high battery buffer are what I'm going with until I see several studies that refute this.

Michael, do you think I've got this covered?

 

[oilerlord]

Teslas battery longevity is recognized as being about the best. Tesla strongly recommends keeping the car plugged in at all times. The minimum SOC when plugged in is 50%. I do this with my model S. It is 3.5 years old and has 34,000 miles on it. The battery degradation (rated range reduction) is only 1.8%. I have experienced absolutely minimal battery degradation by avoiding the extreme SOC's where possible, and keeping the car plugged in at all times - as per the recommendation.

Have you changed the default maximum SOC? I don't own a Tesla, but should we assume that rated range reduction is the same as the reduction in battery capacity?

 

[elpwr]

Teslas maximum SOC is infinitely variable between 50% and 100%. I vary maximum SOC depending on what usage I anticipate. As per Teslas recommendation, I only use the top 10% when necessary for a trip and try to avoid the lowest 10% or so of SOC. I have probably only used the highest and lowest SOC maybe a dozen times in the 3 1/2 years of ownership.

The Tesla rated range is tied directly to the condition of the battery. It does not vary with usage. Other Tesla screens give estimated range based on recent driving. So the rated range is used by Tesla owners to determine the condition of the battery.

 

[elpwr]

Beyond the issue of stress on the battery when stored at 0% is this: no immediate availability for use of the vehicle. At least a couple of hours charge time if you suddenly need to run an errand. At 50% SOC I always have at least 130 miles available immediately in the Tesla.

 

[oilerlord]

Beyond the issue of stress on the battery when stored at 0% is this: no immediate availability for use of the vehicle. At least a couple of hours charge time if you suddenly need to run an errand. At 50% SOC I always have at least 130 miles available immediately in the Tesla.

So as I understand it, you typically charge the car to no higher than 50% SOC - is that correct?

I never "store" my car at 0%, or other very low SOC (though other than Internet "advice", I'm not convinced that's bad). I'm only considering cycling deeper into the battery as per the scientific studies that Michael has posted. Instead of charging to an indicated 100%, and running down to ~35%, the change would be limiting the charge to about 75% (again, indicated SOC) and run down below 20%. During the spring, summer, and fall - my SOC would be lower but my range wouldn't be affected that much.

 

[Nagorak]

I posted an article that says it does. They presented test results and explained the underlying electrochemical mechanism. What is your basis for making that assertion??

This is my very point. A lot of people believe mid levels of charge are best for the battery but scientific evidence shows otherwise

What is your basis?? I have explained mine

The issue with that study might be that it appears they studied capacity fade when charged to a specific level and maintained at that level. They didn't look at discharge, so the two aspects aren't mutually exclusive. It could be that storing a battery is best at zero state of charge. However, it could also be that discharging down to 0% is worse than discharging down to 50%.

We need another study that looked at various discharge cycle depths and their impact on battery capacity. Then we could try to combine the two into the most optimal battery management plan.