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Discussion Starter · #1 ·
I have built a NiMH conditioner. It's not completely finished - it stills needs a high current discharger that is being built from the carcass of an old car stereo amp (nice heat sinks and enclosure) so I haven't been able to do the all important load testing yet. But it is programmable, I am arriving at a conditioning regimen, and deep discharging is part of the experiment.

I don't have to worry about cell reversal because preventing that was a top design goal! So each cell can be discharged individually through a one ohm resistor. Since the charge current is less than the discharge current, I'm able to charge one cell while discharging its neighbor.

Early tests (and lots of forum posts) suggest that deep discharging heals some NiMH wounds. The problem is that it takes a LOT of time. With the one ohm load on one cell, it takes about 6-10 hours to reach 10 mV and about 20+ hours to reach 5 mV.

The first runs showed interesting behavior when the load was released and the cell voltages recovered. @eq1 has posted on this, but I can't find the chart in this uncurated morass! The deeper I allow the discharge to go, the more readily I see "plateaus" or steps where the slope changes as the voltage increases.

But I have no idea what is going on electrochemically or mechanically inside the cell, and I really would like to know whether what I'm doing is good or bad, where returns begin diminishing, when permanent insult begins, if ever, etc etc, I want to know what the processes are that are happening in the cell, so that my charging profile is informed, not the result of a string of half-finished experiments.

So, how deep should I take these cells? With my one ohm load, is taking them down to, oh, 4 mV over a day good enough? Or am I already into diminishing returns when it reaches 10mV six hours in? Or do I really need to let it discharge for days, weeks, or months? Is there any harm to letting cells sit on a 1 ohm resistor for two months before the first charge? Any benefit?

And finally, most folk aren't crazy enough to build a tester like this (not to mention that it only does two sticks at a time and I'm not terribly inclined to scale it up.) With lithium approaching this is supposed to be a cheap ($) experiment done mostly with available parts.

The common discharging practice for cells in series is the light bulb discharger. But I am concerned about harm that can occur from cell reversal. I will argue that half of the cells in a pack may reverse, and those are all going to be the weaker cells that we want most to recover and do not wish to insult! Is there a maximum safe current that can pass through a cell in reverse indefinitely (ie, for 2-3 months while the rest of the pack is slowly discharging)? The goal, or course, is to get the same benefit as per-cell discharging but doing an entire pack at once, but unlike light-bulb discharging, with automation that ensures that when reversal occurs, it will never be to a degree that damages cells.
 

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Discussion Starter · #2 ·
Also - I want to state that initially I was deeply against deep (below 1.0V/cell) discharging and built the charger to be able to take each cell this far but no further. But a few experiments exposed curative effects on a few cells that were taken to about 100 mV that were very obvious in the data. I am not going to say much more on this until I have a chance to design some experiments around it and finish the high current discharger, but except for the discharger, I'm able to measure enough to be confident running accelerated tests to see whether and how far reversing can go before a cell is damaged. (It seems that several people have done similar tests in the past, but not seeing the data, I want to confirm it before relying on it.)

And to the naysayers, I am very much committed to lithium, but I'm learning so much from this, some of which carries over, that I'm happy to be doing it. Also, I don't much care if I trash a NiMH stick or three in the process. The unobtainable lithium cells? I want to get it right before I play with those.
 

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The common discharging practice for cells in series is the light bulb discharger. But I am concerned about harm that can occur from cell reversal. I will argue that half of the cells in a pack may reverse, and those are all going to be the weaker cells that we want most to recover and do not wish to insult! Is there a maximum safe current that can pass through a cell in reverse indefinitely (ie, for 2-3 months while the rest of the pack is slowly discharging)? The goal, or course, is to get the same benefit as per-cell discharging but doing an entire pack at once, but unlike light-bulb discharging, with automation that ensures that when reversal occurs, it will never be to a degree that damages cells.
The simple fact is nobody here has been able to answer the question of what the maximum safe current passing through a cell in reverse is. We have seen that light bulb discharging sub 300mA doesn't appear to do the cells much harm in reverse. I'm wary to say no harm, because in every case the pack was out of whack and the increase in performance from recovering the lost capacity due to memory effect probably completely masks any issues created from higher internal resistance, overall loss of capacity or high self discharge. My issue with putting cells in reverse is that all the literature tells you not to do it. Just because users on here have no way of measuring what minor harm they have caused doesn't mean it hasn't happened.

It's completely possible to detect cells going into reverse at a pack level. My Simple Arduino Discharger can see it:

Graph time. I just want to illustrate how much more detail is captured by this compared to manually logging a discharge. In the past, I've mused how a shallow jaggedy-looking discharge curve probably points to individual cells dropping out during the discharge. But when does this actually happen? When you manually note the discharge voltages, say every 30 or 60 minutes, you lack the detail to pinpoint it.

This is the kind of shallow, jaggedy discharge curve I'm referring to. Here is a discharge I did when my battery was playing up last winter. Aimed to discharge to 120V, manually noted the voltages every hour. See it's got a very shallow drop-off which is not the expected characteristic curve of a NiMh:

View attachment 88512

After a couple of cycles, the curve looks more like this. Capacity is clearly restored. Much closer curve to what is expected:

View attachment 88513

So in the absence of detail, what actually happened in the top curve? Let's take a look...

Now look at the discharge I logged over the weekend. Carried this out because the pack was starting to get grumpy. Discharged to 140V. Same bulbs used as on those graphs above, but this time logging the detail. Lower rate of discharge kicks in at 145V, you can see the change in slope. There's about 2 hours of discharge missing from the beginning of the plot which I'm not bothered about, but there are two definite steps in the data:

View attachment 88514

Zoom in a bit on the two steps. Both of these relate to a drop of around 2-3 volts. It's not so clear on the second step, but the first one looks like it has two humps. I think both of these steps show two cells dropping out. In the first step they happen very close to one another, and in the second step they both go at about the same time.

View attachment 88515

If the discharge was continued lower, you'd see more and more of these happening. This is interesting stuff.
It wouldn't be too difficult to get the software to detect that and shut off. Recharge and repeat.
 

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The first runs showed interesting behavior when the load was released and the cell voltages recovered. @eq1 has posted on this, but I can't find the chart in this uncurated morass! The deeper I allow the discharge to go, the more readily I see "plateaus" or steps where the slope changes as the voltage increases. But I have no idea what is going on electrochemically or mechanically inside the cell, and I really would like to know whether what I'm doing is good or bad, where returns begin diminishing, when permanent insult begins, if ever, etc etc...
This post and various ones around it, after it, might help...you wrap your brain around some of this:

I don't really have an answer, at least nothing precise. In general, what you see below ~1V is akin to what you see above ~1.2V when you do normal charges and discharges, it's just that different 'active materials' are now in play and they instead control the voltage... For example, instead of Ni/NiO controlling the voltage as it does on the 'normal' plateau, you'll see, say, Co/CoO controlling it, maybe below 1V but above about 0.5V... And other things. At that link there's charts that show something like 2 or 3 different plateaus below about 1V.

It's all pretty complicated stuff, I gather you need excellent facility with at least basic chemistry to at least get somewhere, and probably good facility with electro-chemistry and advanced batteries to really understand what's going on. Over the years I've tried to pick-out some of the pieces and put it all together, but I've only managed a sort of half-assed conceptual understanding, at best...

I don't really have time to get into everything, but I can offer a few...of my own butt-dyno suggestions/observations from working with deep discharge in general.

First thing is that a 1 ohm resistance, if not controlled with a 'duty cycle', I'm pretty sure would just blow by stuff that needs to be discharged. You end up not actually discharging anything. There's probably a lot of stuff that just sits there, such as in large crystalized formations, that can't support the current that a 1 ohm resistance would impart. I think the trick is, not necessarily to go low, but rather, go low enough at as small a current as possible.

The ideal I think would be something like 1V to 0.9V at an infinitely small current. So for example if you can program your device to start pulsing the 1 ohm load at around 1V - and maintain that for who knows how long, maybe until the effective current rate drops to something like 10-20mA - I think that'd be the way to go. I'm pretty convinced that in my deep/super/ultra-deep discharge experiments that it's the low current that's been the essential ingredient, not the super low voltages. The super low voltages instead have been somewhat of a proxy, or maybe red herring, in that, say, if you're doing a stick or even a pack, you have to go so low to ensure that cells simply get dropped to 1V. If you're doing single cells I don't think you need to go low voltage, you just need to go very low current.

Also, I'm not convinced that really low discharges don't do damage, even if cells aren't reversed. Or perhaps, rather, that they can do damage. I'm thinking below 1V. If you want to maximize performance of cells, during the formation stage, when cells are new, supposedly you can do a really low current CV charge, CV at 1V, to maximize how much cobalt gets utilized. The cobalt helps a few things, such as self discharge and capacity retention over time. Along these lines I've thought that it might be possible to damage this 'cobalt conductive matrix' with deep discharges, perhaps particularly if you don't respect some kind of re-formation step in the process... I try to let my cells naturally rebound after any super deep discharging, like for 24 hours. When I haven't, and I've moved more or less straight to charging, even at low current (300-500mA), the voltages end up persistently lower than those cells that had the recuperation period... I can't say I've seen evidence of actual damage though, just lower, different voltages that persist... (which, actually, is a form of damage -- lower voltage in my book=damage).

How deep is too deep?: I think it's hard to grasp this stuff, but when 'we' talk about reversals, I think people are missing the boat. There's no single answer for multiple-cell strings, as it just depends how charged some cells are and how uncharged others are... Basically, it's not about what rate is too high, or how long is too long, exactly, but a matter of how unbalanced/unmatched the cells are - how long will cells 1-3 and 6 continue to discharge through cells 4 and 5, reversing cells 4 and 5 and depleting their negative electrodes (or something like that)? I don't fully understand this stuff, but in general, when you reverse a cell there's chemistry taking place in the reversed cells, and once the active materials for that chemical reaction are depleted, you ruin the cell, it won't come back...

That's a string, that's reversal.

For single cells, the question is about potentially damaging the 'low end structure', I think. My 'work' tells me going lower than 0.9V is unnecessary if you go to 0.9-1V at very low current. I think I'd use 0.5V, for single cells, as an absolute minimum. Self discharging cells tend to peter-out at about 0.5V, and the rejuvenation that happens upon charging self discharged cells is where this whole business started. On my personal pack, I have been upholding performance for quite a while without doing super-deep or ultra-deep discharges (and zero grid charges); I've generally been going down to just neg recal (at about 1 amp discharge) with mostly balanced cells, supplemented with tap discharges down to about...8V total, which would be 0.66V per cell if they were all even. I've upheld the gains, so my conclusion is there's no need to go lower. Oh yeah, but note I'm talking about a string of 12 cells when I'm talking taps, so you kind of have to go lower than you would want to since the cells aren't perfectly matched and balanced.
 

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Discussion Starter · #6 ·
This is all very helpful.

So for my per-cell discharger, after a full charge, I'm going to try a deep discharge to 2 mV per cell. That will take about 48 hours. (Actually, since I can't take the LTC6804 much below 9 volts without it shutting down, I can only do four cells at once, so it will take a week. Two days of rest for the last four. Then two more full charges and two more discharges to 1V, then one more charge and let it sit for a week to monitor self discharge. With high current tests along the way. I've decided to stop the charge at the inflection point when the slope starts decreasing just before the peak, rather than after detecting the peak.

I have observed that the plateaus are more apparent on recovery the deeper you discharge. So we'll see how it goes.

For the pack discharging, I am tempted to just put a 2K-5K resistor across the pack and let it sit for three months. However, I know that if one stick is good and the rest are bad (due to a substitution a while back) once the bad cells reach zero, the good stick could very well cause all the others to reverse for the rest of the discharge. But at a very low current.

I think that yeah, discharging the stick pairs is probably the best approach when time is not concern, short of tearing down the entire pack.

I have some ideas for determining the likelihood of damage, namely, conditioning some cells, then reversing them to and beyond the point of damage (each cell a different amount), then recondition ang compare the before and after. This may reveal a knee where damage increases rapidly and perhaps one can project backwards to find a 'safe zone'.

Thanks all for the tips.
 

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And to the naysayers, I am very much committed to lithium, but I'm learning so much from this, some of which carries over, that I'm happy to be doing it. Also, I don't much care if I trash a NiMH stick or three in the process. The unobtainable lithium cells? I want to get it right before I play with those.
I'm sure you are aware, but just to be sure, Li-Ion is a very different beast than NiMH! If a li-ion cell ever gets discharged below 2.5V or so, it must be discarded. Charging it back up after it's been to a low voltage can cause it to catch fire. And li-ion can't be top-end-balanced the way we do with our packs and grid chargers, hence the need for cell-level pack management. I'm sure you know but just wanted to be certain because it would be a serious safety issue if anybody ever tried to treat a li-ion pack like they do a NiMH one.
 

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...If a li-ion cell ever gets discharged below 2.5V or so, it must be discarded. Charging it back up after it's been to a low voltage can cause it to catch fire...
fyi, I think this is true to the extent that the li-ion cells we're talking about have graphite negative electrodes - which is basically everything out there except lithium-titanate cells. Can't remember exactly what the problems is.
 

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So, how deep should I take these cells? With my one ohm load, is taking them down to, oh, 4 mV over a day good enough? Or am I already into diminishing returns when...
For comparison, it occurred to me that your 1 ohm load is roughly equivalent to something I do all the time: discharge in auto-stop until neg recal. I usually see about a 1 amp load; for your 1 ohm load we're looking at ~1.2V/1 ohm = 1.2A.

Based on my experience with this, I think I can say with almost absolute certainty that this load down to about 1V does no harm. I guess that's not surprising. So the real question is whether going down lower at that load does any good, or whether going down to that level or lower at lower current rate would do any good?

Back in the day I experimented with all sorts of resistances on single cells. I started with 1 ohm, but it was obvious the rate was too high - at least insofar as the goal was to try to replicate manually, with resistors, what happens naturally due to self discharge. I didn't really come to a definitive conclusion on everything, but the resistance I ultimately settled on was something like 37 ohms (on my cell discharge rig). That seemed to produce a small enough load that one or two passes could discharge thoroughly enough that voltage wouldn't rebound above about 1V, and it didn't take too long... Self discharged (OEM) cells usually self discharge down to about 0.48V to 0.78V and hang around there for a long time.

At the time the strategy was either try to replicate the self discharge behavior, or 'burn through' the 'lower plateau' that one researcher postulated was the cause of 'memory effect' (based on thermodynamic data, that plateau was supposed to be at about 0.78V). 1 ohm clearly did not 'burn' through this stuff - or whatever stuff, you can't sustain a discharge at a 1 amp rate, the electro-chemistry simply can't accommodate that rate. It's like trying to gently fill a wind-sock, so it gracefully floats and flutters, yet in gale-force winds. Or trying to stoke a dying camp fire yet using some super-high-powered fan. In either case the sock or the camp fire just blows away or blows out...

37 ohm load on a single cell: 1V/37 ohms = 27mA. 0.7V/37 ohms = 19mA. At the time, the idea was to find a load that would 'burn' above 0.7V without blowing by it. After experimentation it ended up being around this...

So, quickly, back to that question: I say I do 1 amp loads all the time, in the car, down to neg recal, which is voltage starting to tank on at least 1 cell. Does no harm, seems to do good. But, I do seem to need to go either lower, or as low but at lower current, to see full benefits. The problem here is that I'm working with 12-cell strings: I think the palliative impacts I see from doing 'tap discharges' have as much or more to do with the low current and the balancing among cells that happens, rather than the absolute lowness voltage is taken (among other things).

Going down to neg recal at 1 amp - lowest cell probably around 1V at neg recal - that cell sees 'reconditioning'. All the other cells don't. Going down at a lower rate, the tap discharge rate, which is about 38mA, allows more thorough 'burning' at the normal, upper plateau, plus you can take more cells down closer to 1V. I actually do 2 different procedures: I usually just do a little tap balancing once in a while, mostly to gain 'experimental evidence'. And once in a blue moon I'l go down to about 8V (I just did that a few days ago), with the objective being to try to ensure that all cells are completely depleted on the upper plateau, that they are bottom-balanced... You get bottom-balanced cells, and you get the reconditioning effect from dragging all cell voltages down to about 1V at very low current (i.e. thorough 'burning')...

Anyway, all this informed my suggestion earlier that the ideal, at cell-level, would probably be about 0.9-1V CV down to about 20mA. I think were I to build something this is what it'd do. I think I'd choose 1V, I'd program the device to pulse the load and keep increasing the duty cycle until the effective discharge rate reached about 20mA, or maybe 10mA... Couldn't you do that? I don't have much facility with this stuff, but, say, a 50% duty cycle would be, say, a 500ms pulse over a 1000ms period(?). You'd cut the rate in half, from 1 amp to 0.5 amp. A 750ms pulse would be 250mA rate ("pulse" here indicates the time that the resistor is OFF)... 875ms would be 125mA (I think). Something like that...
 

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Discussion Starter · #10 ·
Thank you for the tips and data.

I'm getting impatient because I need to get one or two reconditioned packs ready to go into a car. But I am thinking of letting some sticks sit with one ohm across every cell for several weeks.

I think the conditioning regimen that my Arduino is running now can produce repeatably results and when I'm done building my deep discharger I'll be ready to try to find where things fail.

Thanks for the comment about discharge in autostop, and the amount of current. Remember that with a resistor the current is proportional to voltage. So when my cells are down to 3mV they are pulling 3mA.

I've also decided to give a deep discharge a shot using your BCM HV connector method. I had some BCM connectors I got from Digikey or Mouser and wired up two, one with odd cells jumpered and the other even. They are each on a pack now and I will switch them. I may have widened one contact some time back jamming a voltmeter probe in because the pack would drop out on a bump. So I'm happy to use the proper connectors.

Finally

I'd choose 1V, I'd program the device to pulse the load and keep increasing the duty cycle until the effective discharge rate reached about 20mA, or maybe 10mA... Couldn't you do that?
That was my first plan, which I did, well sorta, and is why I'm now pursuing deep discharging. It didn't exactly work out the way I expected. (I'm not saying much until I can confirm what I saw with more testing, so as not to start unfounded rumors or spread bad info based on a half-baked observation).

And from another thread
As they say: BATT gauge is a finger in the wind at best. At worst it's a pernicious, cynical, evil device deployed by Honda - to manipulate dumb-*** Insight drivers.
That gave me a good laugh!
 

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But I am thinking of letting some sticks sit with one ohm across every cell for several weeks...
Go ahead and try it. But that's what I'm trying to explain, that I seem to be failing at. I guess I don't totally understand it myself, but... As you traverse, say, the 1.2V to 0.7V range, the current that a 1 ohm load induces is so strong that you don't actually discharge anything in that range, and once the potential difference between positive and negative electrodes is reduced to nothing - it's done, there's no more 'discharging' that actually happens. Or at least I'm pretty sure something like this is what happens... It's almost like a one-way trip - you have to get your discharge on along the way, cuz if you don't it ain't gonna happen.

If you let voltage rebound and put the 1 ohm back on, you might make a little more headway. But, a few passes and that's it.

If you stick a larger resistance in the loop, though, you actually end up discharging stuff. There's a lot of things that could explain this. The main thing is probably large crystals - reactions first take place on the surface of NiO crystals, if the crystals are large (=small total surface area) they can't support much current, you need super-low current to break-up the crystals (lots of small crystals=lots of surface area=high current support). Basically, capacity can be locked-up in large crystals, and if you don't use a small enough current you don't get any discharge out of them, and you don't release that capacity...

Hey, here's a good analogy: Think about all the capacity you're not able to access if you do a discharge at say 20 amps. On a typical used stick you might get around, I don't know, 5000mAh discharge at 20 amps. If you lower the current to 6.5 amps you might get 5800. In either case, though, if you let the discharge keep going until zero volts - it's not like you actually end up discharging anything more. It's the same or at least a very similar thing with the 1 ohm load vs. say 37 ohm. I actually think most cells probably don't lose a ton of capacity due to physical/mechanical losses; most would probably discharge about 6500mAh or even more if the load is small enough...

The thing is, though, that this is about more than just quote 'capacity': IF capacity is locked-up, you've not only lost the raw 'amp-hours'; you've lost power density. I don't exactly know how this works, but it seems pretty clear that reduced capacity of this nature means the cell simply doesn't perform as well, doesn't support high current, doesn't put out as much power, across the whole charge state range. The operating voltage is reduced and the active material that's there is sub-par, like in the form of large crystals, or there's water sequestered from the electrolyte in 'gamma' mod NiOOH, or whatever. So, a super low current discharge on the normal plateau can unlock more than just, say, that 6800mAh minus 5800mAh = 1000mAh of 'capacity' - because that 1000mAh of locked-up capacity is actually a reflection of all the capacity (active materials, whatever) - but in a degraded state.
 

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I think try and keep it it all in perspective and the realms of reasonable practicality and repeatability.

I agree that with decent kit, skills and even more time than it normally takes to fix a pack, there is probably a bit extra that can be wringed out of the system by cycling/charging at the cell level etc. Maybe a couple of extra %..

But most of that is completely impracticable for the majority of owners who will have neither the skills or equipment.
Of course it's fine for us mad experimenters ;) We snack on electrolyte and active materials.

My cycling mantra is KIS..

Get a dodgy pack on the bench.

After a cursory check of tap voltages for any wild dead cell outliers or leaky green sticks!
It's a 350ma grid charge for 24/36hrs or so.

Then it's a light bulb 60w 240v on it until the bulb goes out.
I'm not fussy about time here and might leave it several days/week.

The lower it goes the better as far as I am concerned.
I might do two full cycles on the pack and then test it.

Do the taps stay balanced under load?
Are the OCV tap voltages in balance?
What are the taps like after a week or two?

The real test of a pack is back in the car of course...
I usually test packs in my own car as final go/no go check..

If bad sticks show up then I might try and fit a matched replacement stick but only if it is for my own use.
I do not mix and match sticks for customers cars.

If a pack recovers great, if not then it's a new set of matched sticks. Simple.

Owners can also follow the basic cycling procedure with very simple equipment.
It can avoid needless long trips to Hull to visit me and £££ pack replacement.

Most OEM packs I see in the UK recover quite well if caught early enough.

Good luck with the stick/cell mangler/tester.. (y)
 

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^ Right, that's all sensible, regular folk shouldn't bother with 'cell-level' and all that business, etc. But sean's trying to do his thing and he's asking about more arcane stuff...

Oh yeah, also:

I agree that with decent kit, skills and even more time than it normally takes to fix a pack, there is probably a bit extra that can be wringed out of the system by cycling/charging at the cell level etc. Maybe a couple of extra %..
I think there's WAY more that can be wringed out of packs than we would have ever dreamed possible. My current pack is a case in point. I don't think people around here realize just how well my once-defunct pack functions now, most likely due to the way I use it. But, no one cares any longer. I barely do myself.
 

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Discussion Starter · #14 ·
Yeah, what I'm doing is far beyond what the average Insight owner is willing to do. Arguably the t
 

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...The thing is, though, that this is about more than just quote 'capacity': IF capacity is locked-up, you've not only lost the raw 'amp-hours'; you've lost power density. I don't exactly know how this works, but it seems pretty clear that reduced capacity of this nature means the cell simply doesn't perform as well, doesn't support high current, doesn't put out as much power, across the whole charge state range. The operating voltage is reduced and the active material that's there is sub-par, like in the form of large crystals, or there's water sequestered from the electrolyte in 'gamma' mod NiOOH, or whatever. So, a super low current discharge on the normal plateau can unlock more than just, say, that 6800mAh minus 5800mAh = 1000mAh of 'capacity' - because that 1000mAh of locked-up capacity is actually a reflection of all the capacity (active materials, whatever) - but in a degraded state.
Something about this has been...'troubling' me. I realized that this characterization is something that doesn't seem symmetrical/balanced between high charge state usage and low charge state usage. I only want to try to briefly jot down some thoughts on this...

Above I'm saying that lost capacity impacts the performance of the remaining capacity - it's not like you initially have 6500mAh of full performance capacity and lose 1500mAh and the remaining capacity continues to perform as well as it ever did. What I'm saying is that the loss of that 1500mAh is a result of degraded 'material' in general - all the material is degraded and now it doesn't perform as well, that's what the 'lost capacity' is...

That's generally what it seems like - when I use high charge state. But it doesn't happen when I use low charge state.

When I use low charge state it does seem like I lose some capability of cycling high and using high charge state. Not sure if that's a real phenomenon or just a consequence of the BCM programming - that simply won't let you charge higher if the voltage is too high... So, in this situation it seems like I lose high charge state, yet low charge state IS NOT impacted - I can use low at full performance...

So:

-use high charge state, lose the ability to use low charge state AND lose performance at high charge state.

-Use low charge state, lose the ability to use high charge state YET keep performance at low charge state.

That's the basic asymmetry I'm getting at, I think...

I've been trying to sort-out the differences, because they aren't so cut and dried, there's a lot of usage patterns that make a difference, and it's hard to keep them all straight and in mind. At times it has seemed like there's no difference between using high or low, but ultimately there is, just hard to pinpoint exactly why, when, and how. This "asymmetry" is one difference.
 
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