I’ve always interpreted deep discharge as the battery getting to very low voltage. For example, driving the vehicle until it shuts off and letting it sit for a long time before charging would be an extreme case. Most Lithium-ions are only usable to ~3.0 volts, which is a ~0.5 volt “pad” (aka anti bricking buffer) from 2.5 volts where lithium-ions become unusable (i.e. a brick). We are talking extreme cases here but it’s still generally not advised to allow the battery to get to very low SOC frequently since lower voltages is where Solid Electrolyte Interference (SEI) growth occurs.
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Jeff Dahn's recommendation on long term battery preservation
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I’ve always interpreted deep discharge as the battery getting to very low voltage. For example, driving the vehicle until it shuts off and letting it sit for a long time before charging would be an extreme case. Most Lithium-ions are only usable to ~3.0 volts, which is a ~0.5 volt “pad” (aka anti bricking buffer) from 2.5 volts where lithium-ions become unusable (i.e. a brick). We are talking extreme cases here but it’s still generally not advised to allow the battery to get to very low SOC frequently since lower voltages is where Solid Electrolyte Interference (SEI) growth occurs.
There are a couple of things to consider with respect to maximizing your battery life:
If they charge a 100kWh battery to 90% on a daily basis then they will get 400-700 full cycles out of the battery. A full cycle is around 300 miles (slightly less for X, a bit more for S). This gives them between 120k and 210k miles before their battery is at 90% capacity. That will take them 8 to 14 years to reach that state.
If they charge to 70% on a daily basis then they will get 2400-4000 full cycles of the battery. That means 720k to 1.2m miles before they get to 90% capacity. It will also take them 48 to 80 years to reach 90%. After 8 to 14 years they may have more like 98% capacity on their battery.
If they keep the car that long and then travel back in time to today to try to sell it, it will be the difference between trying to sell a 98kWh car vs a 90kWh car. How much difference in value will they have? Not much, and that is looking at the EV roadmap of today.
Now consider that they are doing this in 8 to 14 years. In 2027-2033, what kind of EVs are available and how much do they cost? How much would their car be worth even if they put it in a museum for those 8-14 years and tried to sell it as "brand new" with a full 100% of battery capacity (assuming ideal battery maintenance)?
This is why it isn't as critical as most people think when they consider how to charge their EV.
Most people have faced maintaining a cell phone battery and that leaves them a little wary about capacity loss. They have seen that their cell phone has a noticeably shorter life after a year or two and they need to get a new phone as a result - so they want to avoid having to do that with their car. This experience over-sensitizes them to the issue.
Consider that if you cared for your Tesla like most people care for their cell phones then you would be driving it til empty every day and then charging it to 100%. In a year you'd have put almost 110k miles on that car and it would be under 90%. Is that something you do with your car? Tesloop did something like that, including abusive Supercharger charging to over 90% more than once a day - and they saw mostly typical expected battery capacity losses over 300k+ miles with a couple of cases of having gotten a car with a bad battery.
Now measure the peace of mind of waking up to 90% of your range every morning vs. waking up to 70% of your range, and how long you think it will take you to want to sell it.
- Cycle ratings are measuring when the battery will reach 90% of usable capacity, not when the battery will die.
- Consider how long it will take to reach the number of cycles.
If they charge a 100kWh battery to 90% on a daily basis then they will get 400-700 full cycles out of the battery. A full cycle is around 300 miles (slightly less for X, a bit more for S). This gives them between 120k and 210k miles before their battery is at 90% capacity. That will take them 8 to 14 years to reach that state.
If they charge to 70% on a daily basis then they will get 2400-4000 full cycles of the battery. That means 720k to 1.2m miles before they get to 90% capacity. It will also take them 48 to 80 years to reach 90%. After 8 to 14 years they may have more like 98% capacity on their battery.
If they keep the car that long and then travel back in time to today to try to sell it, it will be the difference between trying to sell a 98kWh car vs a 90kWh car. How much difference in value will they have? Not much, and that is looking at the EV roadmap of today.
Now consider that they are doing this in 8 to 14 years. In 2027-2033, what kind of EVs are available and how much do they cost? How much would their car be worth even if they put it in a museum for those 8-14 years and tried to sell it as "brand new" with a full 100% of battery capacity (assuming ideal battery maintenance)?
This is why it isn't as critical as most people think when they consider how to charge their EV.
Most people have faced maintaining a cell phone battery and that leaves them a little wary about capacity loss. They have seen that their cell phone has a noticeably shorter life after a year or two and they need to get a new phone as a result - so they want to avoid having to do that with their car. This experience over-sensitizes them to the issue.
Consider that if you cared for your Tesla like most people care for their cell phones then you would be driving it til empty every day and then charging it to 100%. In a year you'd have put almost 110k miles on that car and it would be under 90%. Is that something you do with your car? Tesloop did something like that, including abusive Supercharger charging to over 90% more than once a day - and they saw mostly typical expected battery capacity losses over 300k+ miles with a couple of cases of having gotten a car with a bad battery.
Now measure the peace of mind of waking up to 90% of your range every morning vs. waking up to 70% of your range, and how long you think it will take you to want to sell it.
Where are you getting the data on battery cycle count and battery degradation for Tesla battery packs? I’ve been looking for such data but everything I’ve seen published is on different types of lithium-ion.
Definitely agree you won’t see much difference in degradation until you start getting to 8+ years. I charged to 90% a few times and had no difference in “piece of mind” compared to 70%, because I only use about 10-12% on average per day. So I figure why not just use 70% as my daily charge level since the science/studies show it is slightly better for minimizing battery degradation. I guess 10-15 years from now, the person who buys my car can thank me.
It's not simply just battery cycles and I'm not sure what you're basing this off of. Perhaps in the future when they figure out the nearly perfect additive combination for lithium-ion batteries, we will see 98% capacity retained in 8-14 years but right now, even if you stored a lithium-ion with 40% SOC at 0-deg C, you will ~2% degradation after a year. I realize these aren't Tesla's batteries used in that study but the point is, there would still be some rate of degradation without even ever cycling the battery.
As the Tesloop comparison, they went through many cycles in a short period of time. If you went through those many cycles in a longer period, say 6-8 years, you would see a higher level of degradation. I think Jeff Dahn’s video has some good examples of how it’s not just cycles but also time (among many other things like temperature, charge rate, etc).
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I think his point is simply getting to a low charge state is "fine". The issue would be allowing the battery to remain at a lower SOC without charging it. "SEI is seen as a cause for capacity loss in most graphite-based Li-ion when keeping the charge voltage below 3.92V/cell." (Battery University).
From the articles you linked in your own post just before this. In particular, Table 4 of this article:
How to Prolong Lithium-based Batteries - Battery University
(I just realized that I kept referring to the car as 70%, but I was using the figure from Table 4 for 60% as I thought you had said that you were seeing 3.92v at 70% on your car, but now I realize I was misreading it. And, my deadline for editing that post has expired so it will live on with this discrepancy...
Also, I ballparked how much cycle-related degradation the 70%/60% car might incur in 120k miles or so. It won't be at 100%, but it also won't be at 90% yet (due to cycling degradation). The answer is somewhere between the two. However, since degradation isn't linear it would experience more than a linear amount of degradation. All of the curves shown in any battery degradation graph always start out with steep degradation that levels out over time.
Another thing to keep in mind when you read the graphs is what they mean by a cycle. Typically when talking about the lifetime of a battery they use "cycle" to mean "number of times the full capacity of the battery". But, in graphing some of the study data they can talk about the number of "test cycles" and if the test involves partial discharge and recharge then the number of cycles is misleading. After a number of comments on the article linked above pointing this out, they finally added the footnotes you see now in Figure 6 where they talk about the total energy units available. What looks like severe degradation on the graph (and those of us who have made suggestions about this want them to redraw the graphs) is really not as bad as the graph suggests because you see a ton of cycles on the battery that only used 10% of its capacity per test cycle. You need to divide the cycles on that battery by 10 in order to get an idea of the number of full cycles, i.e. the number of times the full storage of the battery was used, that are represented. In particular, the best line of the graph - for the 75%-65% battery only delivers 90k units over the lifetime of the test whereas the 75%-25% battery delivers 150k units - 2/3 more useful work delivered by the battery that was discharged more and whose graph looks quite a bit more dire.
I guess 10-15 years from now, the person who buys my car can thank me.
Yes, but their thanks may be in single digits of dollars given how much the technology in your car will be worth in 10-15 years.
I didn't mean to imply that cycles were the only factor, but we are talking about how high to charge the battery on a daily basis and there is data that shows how different cycles affect the available charge/range. My figures were the degradation due only to the max charge level of the cycling, but as you point out, even with no cycling the ideal doesn't allow for 100% range over time. However, whatever age related degradation occurs, the additional degradation due to cycling will be on the order of what I listed. In other words, the best difference in range you might see in 8-14 years would be 8% between the car charged to 90% daily vs. the car charged to 70% daily. If there was additional degradation due to age alone, or heat in the environment, or any other factor, then the range difference between the two would be even less than 8%. (Simple math - degrading due to A+B vs A+C are closer than degrading due only to B or C.)
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Ah, got it. I'm not sure exactly what battery cells they used for those tables but it does mention "The readings reflect regular Li-ion charging to 4.20V/cell." and gives a pretty big range, so I'd imagine Tesla batteries fall somewhere in that range (likely the higher end). The battery packs on my 85D reported 3.915/3.918/3.920V (min/avg/max) at 70% displayed SOC (71.6% reported by BMS "SOC UI").
Thanks for clarifying your previous post - makes more sense now. Like you mentioned, it's all a trade off of single digit capacity loss difference in the 8-12 year timeframe versus having the extra range every morning. Personally, I'll take the (best case) 8% of battery capacity after 8 years (again, best case) from just cycling alone because I'm only using ~10% daily (to and from work) and 70% SOC gives me plenty of extra range if I need to do anything else. Now, if the zombie apocalypse breaks out and our power grid goes down, perhaps that extra range would have been nice that day. More reason for me to buy solar for the house I guess.
To your point on value, agreed, it might not be much but 8% of 262 rated range is ~21 miles. How much more would that sell for in 4-6 years from now? Who knows, but I know if I were buying an EV today, I would pay extra for the used Tesla with ~21 of extra range (assuming all else being equal).
This german paper/thesis (In English of course!) measures on Panasonic 18650 NCA cells - that is probably the closest you can get to Tesla 18650 cells
https://mediatum.ub.tum.de/doc/1355829/file.pdf
FlatSix911
Porsche 918 Hybrid
This german paper/thesis (In English of course!) measures on Panasonic 18650 NCA cells - that is probably the closest you can get to Tesla 18650 cells
https://mediatum.ub.tum.de/doc/1355829/file.pdf
Thanks for posting... for those that want a summary of Strategies for Maximizing the Battery Life in Electric Vehicles go to pages 141- 145
Attempting TL;DR:This german paper/thesis (In English of course!) measures on Panasonic 18650 NCA cells - that is probably the closest you can get to Tesla 18650 cells
https://mediatum.ub.tum.de/doc/1355829/file.pdf
1. No apparent lower bound to best storage SoC, store it as low as possible, as cold as possible.
2. High silicon cells suffer from accelerated degradation when stored above 57% SoC (all new Tesla cells)
3. Extremely deep depth of discharge is bad (Tesla doesn't allow you to get that low anyway even at 0%?)
4. Charging while cold is very very bad, capacity degradation from regen charging at 10C worse than at 25C and 40C. Fastest way to kill the battery is to charge to high SoC and have high depth of discharge when cold (figure 86c, 93a)
This german paper/thesis (In English of course!) measures on Panasonic 18650 NCA cells - that is probably the closest you can get to Tesla 18650 cells
https://mediatum.ub.tum.de/doc/1355829/file.pdf
I personally love the recovery shown, at higher SoC, when cycle aging was disrupted for months
Fig 86, 10C, 20% DOD @4.1V.Recovery at High SoC AND as well at Low SoC is confirmed by this: https://www.sciencedirect.com/science/article/pii/S2352152X18306637
but for prismatic NMC cells. However - the low SoC recovery is assumed to be tied to the Carbon Anode, so should apply to NCA as well.
The test documents a flattening of Differential Voltage Analysis (DVA) graphs, due to very reduced 'homogeneity of lithium distribution' (HDL) after extensive cycling (but at very low DOD) So if ever a car producer were to use DVA, to determine cell (rather anode) capacity, they have a problem with users that cycle all the time and are doing so NOT around medium SoC.
If you look at Fig 6, then after extensive shallow cycling at either high or low average SoC, the DVA have no clear Anode Peak to estimate capacity or HLD from (or it is dead wrong as in the P1~80% Soc graph). Only the small DOD cycling around 42% SoC retains the original peak, all other average SoC needs recovery to re-shape the curve, for any estimation to make any sense.
I would love to see the DVA for a Tesla constanly being SUC cycled and never rested at low SoC
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