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Originally Posted by OceanSeaSpray
If you never recharge your cells to 100%, you are going to screw them up completely on the long run because the memory effect will make them almost un-rechargeable past a point and this point will keep moving lower and lower over time.
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I’ve read the literature on this effect (in part, thanks to you bringing it to my attention), and I still can’t really marry up what they observe with the practical impact on a real battery pack. Maybe it is so subtle that I can’t understand it, or maybe I am dense. But I will try again.
(I guess I should preface this response with the statement that I’m not personally that concerned about lifetime beyond about 1500 cycles, because for me that is ten years, and that’s way longer than I’ll need. If the effect you are observing only presents after thousands of cycles, I will have concede disinterest and let others worry about whether it’s real and worth mitigating in some way.)
That said, let me take a stab:
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While you don't technically "lose" capacity, the result is the same until remediation can take place.
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If there is a way to remediate the problem, does that still mean the battery is “totally screwed up”? Or do you mean, it will be screwed up only until remediation is undertaken?
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I have been running a very interesting fractional cycling experiment here for 4.5 years and now this bank stays in absorption for over an hour at the end and only reaches 70% SOC when I take it to the termination point of 14.0V and 0.03C.
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I know you said you will give us a detailed write-up when you’ve concluded your remediation experiments, but can you just tell us the basic parameters of the system as a starting point for now? Things like cell type, brand, pack topology, BMS and re-balancing strategy if any, what charge regime, temperatures, rates, cycling frequency?
In addition to being deeply curious about your experiment and this phenomenon (which you and only you continue to bring up over the years, somewhat strangely), part of why I ask about the basic parameters is because some of what you say could be interpreted other ways.
For example, if you are always
charging to 14.0V, that is definitely not a partial charge. (You don’t say you are, but you don’t say you aren’t either. If you aren’t, what is your normal
charging method?)
For another example, it seems to me that an LFP cell that has just experienced normal degradation might exhibit behavior identical to what you describe; namely, charge to a “full voltage” and only measure 70% of the original energy going in. How do you distinguish between normal deterioration and this special effect?
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Recharging the last 30% would take hours and hours at very low current.
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Would take, or does take? I’m guessing you have tried it, and it does
work? And how many hours and hours does it take? Is it, say, 0.015C for 30% = ~20 hours? (And is that holding CV at 3.5Vpc?)
Another question I have is, why did you decide to run your experiment to 70%? (If that is what you decided; it’s not quite clear, but I am presuming.) 70% is an unusual number, because the data suggest that something like 90% is nearly as good... way more
storage, fewer cycles per total energy used, and not much more
heating. I would expect most people would pick 85%, or 90%, or 95%. I chose 92%.
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The next step now is erasing that memory and getting back to normal performance. Basically, it shows very high internal resistance when the SOC approaches this point and it doesn't behave normally at all. Other then that, it performs fine. The internal resistance is generally a bit higher than when it was new, but this is normal with aging.
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It sounds like here you are talking about the same question I had above... how do you distinguish between normal aging and the special effect?
I have a separate question as well: if the experiment is to run the pack for hundreds or thousands of short cycles to 70%, and it works fine at 70% after all those years, what would be the rationale in real life for someone to remediate the pack back to a higher value? If someone has decided to run it at 70% for life, would they care?
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I will publish all the data and results once I have collected more information about releasing the cell memory.
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I’m definitely looking forward to it. As I have written before, I
can see memory “bumps” in my charge curve. But they are so small, and they don’t seem to have any effect on the actual performance of the pack.
I have
not tried taking my cells above the 92% stopping point. I could imagine I might see a large “bump” there in the curve, since they so often reach that point. But... I can’t figure out why I would want to do this either, except for science. Best I can tell, I’m getting the same stored energy today that I did on cycle zero, with the same CV threshold, in about the same amount of time.
What I’m wondering is, at what point would I observe some kind of performance impact that might motivate this testing or some kind of “remediation” process? Maybe you have an opinion from your experiment, already.