Li-Ion at 3.5v when in storage

[iamlucky13]

Fair points. Personally, I go with 3.75v for storage, which is about 50% for the common high-drain 18650 cells. Though, I doubt 3.9v is a lot worse. It seems the consensus is that most of degradation occurs when cells are kept above 4.1v. And/or stored in warm conditions.

3.9V actually looks like it would be a lot worse. The link to a prior discussion Gauss163 posted above is handy, since Overclocker posted an excerpt of the graphs from the research paper, which I'll copy here.

If you look at the graph for 25 Celsius (room temperature) for the NMC (INR) chemistry, for example, you can see the capacity loss nearly doubles going from 60% SoC to 70% SoC. Outside that range, the trend is much flatter.

The NCA chemistry showed similar trend for capacity loss, although the plateau point looks like it might be closer to 55% SoC.

Extrapolating from that for an NMC cell, the shelf life expectancy to 70% of original capacity is 6 years at 70% SoC, versus almost 10 years at 60% Soc.

So if your strategy is 50% SoC for storage, that looks good. I'm not sure 3.75V is below that 60% level for all cells on the market currently, but HKJ's data should be a good reference source for that question.

 

[Modernflame]

Depends where you live, I guess. A 2-pack of CR123's is $24.99 in the stores, here. Total rip-off. Cheaper on-line, sure, but still a rip-off in Canada. Even on-line, CR123's are almost as expensive as buying a quality 18650.

CR123's do have advantages, though, especially in extreme cold. Eneloops and 18650's don't do well below -20C.

That's a fair point. I'm able to get them very cheap online when purchased in bulk. In a pinch, my local hardware store sells two packs of Surefire branded CR123's for $4.97.

 

[WalkIntoTheLight]

3.9V actually looks like it would be a lot worse. The link to a prior discussion Gauss163 posted above is handy, since Overclocker posted an excerpt of the graphs from the research paper, which I'll copy here.

If you look at the graph for 25 Celsius (room temperature) for the NMC (INR) chemistry, for example, you can see the capacity loss nearly doubles going from 60% SoC to 70% SoC. Outside that range, the trend is much flatter.

A step like that makes me question the experiment. Why would there be a sudden change like that, if it's flat on both ends of that step? I don't know of any reason why the chemistry would not behave as a smooth curve, especially when it's in the middle of the voltage range. The change should be sudden only near the top of the voltage range (and maybe the bottom).

If the experimenters have an answer for why that sudden drop happens at 60% - 70% charge (but not at 50%-60% or 70%-80%), I'd like to see it.

 

[iamlucky13]

A step like that makes me question the experiment. Why would there be a sudden change like that, if it's flat on both ends of that step? I don't know of any reason why the chemistry would not behave as a smooth curve, especially when it's in the middle of the voltage range. The change should be sudden only near the top of the voltage range (and maybe the bottom).

If the experimenters have an answer for why that sudden drop happens at 60% - 70% charge (but not at 50%-60% or 70%-80%), I'd like to see it.

They discuss it, although I don't understand the discussion well enough to try to translate it into lay terms. I get the sense it's that some of the reactions that cause cell aging don't occur as easily below a certain internal voltage threshold.

 

[Gauss163]

A step like that makes me question the experiment. Why would there be a sudden change like that, if it's flat on both ends of that step? I don't know of any reason why the chemistry would not behave as a smooth curve, especially when it's in the middle of the voltage range.

The article explains this matter at length (it is even summarized in the abstract). In brief, those flat plateaus correspond very closely to plateaus in the anode potential (e.g. see the graph below). When the anode drops into a lower plateau potential this aggravates electrolyte reduction, which promotes growth of the SEI (anode passivation layer) and loss of cyclable lithium (the major cause of capacity fade due to calendar (vs. cycle) aging).

 

[markr6]

For a $6 cell, I'm not too worried about charging to 4.1-4.15v and letting it sit, get used, get lost, get up and walk away, etc.

OTOH some people spend $20,000 on a car and rarely wash it, go over on oil changes, and a million other things that are negatively impacting that investment.

 

[Gauss163]

Above someone asked about how storage degradation decays with time, and extrapolation, etc. The study results agree with earlier work that shows that the SEI passivation layer grows roughly linearly as a function of the square root of time. For example, in the graph above the times are 2.1, 4.0, 6.6, 9.6 months, with sqrts = 1.45, 2.0, 2.57, 3.1. So the time sqrts have constant increase about 0.55 and, indeed, the capacity graphs above show constant decrease - the blue curves drop by about the same amount in these intervals.

This means that most of the degradation occurs early on, so to minimize such you should bring them down to 50% SOC or lower as quickly as possible (and delay charging them higher till just before use).

 

[WalkIntoTheLight]

Above someone asked about how storage degradation decays with time, and extrapolation, etc. The study results agree with earlier work that shows that the SEI passivation layer grows roughly linearly as a function of the square root of time. For example, in the graph above the times are 2.1, 4.0, 6.6, 9.6 months, with sqrts = 1.45, 2.0, 2.57, 3.1. So the time sqrts have constant increase about 0.55 and, indeed, the capacity graphs above show constant decrease - the blue curves drop by about the same amount in these intervals.

This means that most of the degradation occurs early on, so to minimize such you should bring them down to 50% SOC or lower as quickly as possible (and delay charging them higher till just before use).

It also seems to imply that degradation slows way down after you've lost about 10% of capacity. So, does this mean a 3000mAh battery that degrades to about 2700mAh, is basically immune from any further rapid degradation?

That might explain why my ~10 year old laptop cells seem to still have descent capacity. They might have lost 20% in the first year or two, but very little after that.

 

[Gauss163]

^^^ The study shows that their NCA/NMC lose about 6-7% capacity when stored starting at 70-100% SOC, 25°C for 1 year. So after 9 years they'd lose about sqrt(9) = 3 times that, about 3*6.5 = 19.5%, so they'd be around 80.5% of initial capacity. Yours were 81.8% (1800/2200) - not far off. That does not include loses due to cycling (maybe you had few cycles?) [Note: these estimates should be coinsidered to be very rough since we don't know the actual parameters for the degradation curve]

High current apps may suffer from much more severe losses due to growth in IR (which does not have the same plateaud step-function curve).

Also keep in mind that the losses increase greatly at higher temperatures, e.g. at 50°C (e.g hot laptop) for 1 year their NMC cells degraded over 30% when stored at 100% SOC and around 20% when stored above 80% SOC (which explains why those Dells with hot-running Pentiums and no battery-saver software often had dead batteries in a years time - even when the battery was little-used).

 

[hiuintahs]

It's nice to be aware of the technicals of lithium ion. Realizing that keeping fully charged unused cells will degrade them, I for the most part haven't worried too much about it.

I have 15 18650 cells. 4 of them have never been charged and a couple of them are several years old. Two are the NCR18650B. Voltage on them is still approximately what they were when new.........3.61v. The other two unused cells are NCR18650GA. They are also about that same voltage. Self-discharge if any is negligible from what I have seen at that voltage level. Temperature is just under 70 degrees year around in my basement.

From comments on the forum I don't use a 18650 lights stored in the car due to the extreme temperature variations. In the cars, I have Fenix LD11's with 14500 cells in them...........so those may have lost some capacity. But I wouldn't know because I don't use them that often. I do keep spare AA's in the car as back up.

The rest of my 18650 batteries, I've probably lost some capacity, but I wouldn't know because I hardly use these lights and the cost of depreciation is negligible from a $ standpoint in my opinion compared to the convenience factor. I do have around 3 dozen CR123A batteries. On camping and road trips I do take spare fully charged 18650's with me that I top off prior to use.

I have a $189 AGM/starter battery in my truck that does not get driven very much. I do pay attention to the voltage on that battery due to the small parasitic drain on most newer vehicles. Unlike lithium ion, lead acid batteries want a full or mostly full charge to extend longevity.

 

[WalkIntoTheLight]

I have 15 18650 cells. 4 of them have never been charged and a couple of them are several years old. Two are the NCR18650B. Voltage on them is still approximately what they were when new.........3.61v. The other two unused cells are NCR18650GA. They are also about that same voltage. Self-discharge if any is negligible from what I have seen at that voltage level. Temperature is just under 70 degrees year around in my basement.

I've noticed that too. I have several cells (different brands) stored in the refrigerator at 3.75v. I checked them after 6 months, and their voltage hasn't budged. I think self-discharge basically stops at those voltage levels, except where heat is maybe a factor. I expect I won't have to charge them back up to 3.75v for many years. I won't bother until they drop to 3.6v.

Even protected cells don't self-discharge much, even though I know they must discharge due to their microprocessor. It must just be a couple of microamps, as it's basically unnoticeable.