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Old 06-07-2019, 07:35   #91
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Re: LFP memory effects thread

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Originally Posted by guyrj33 View Post
One of the confusing things about this thread is the title "memory effects".

Lithium cells when in a series arrangement, which is how we all use them, suffer from unbalanced charging, not memory effects.

According to the OP this is not the case.
This thread is very interesting, and I hope we will find out more about this strange effect. To those who are affected: please keep us up to date on your findings.
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Old 06-07-2019, 09:03   #92
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Re: LFP memory effects thread

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Originally Posted by mbartosch View Post
According to the OP this is not the case.
This thread is very interesting, and I hope we will find out more about this strange effect. To those who are affected: please keep us up to date on your findings.

Correct, and there is research and simulations that identify and demonstrate the memory effect. But as mentioned, it's a small bump in an otherwise smooth charge voltage curve, and not a wall that you can't get by with pretty common charging techniques.


So to me the big question is whether the clear loss of capacity in the two example packs is due to a cumulative memory effect such that it becomes permanent, or is due to one of the many other causes that can lead to capacity loss.
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Old 06-07-2019, 09:20   #93
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Re: LFP memory effects thread

We do not really know in the case of these two banks.

But the original discovery & testing did assert that the harmless bump phenomenon did turn into permanent capacity loss if no steps were ever taken to push through on a regular basis.

Doing so is not difficult, but you need to notice the effect is happening and correct your usage pattern early on.

BMS has nothing to do with this issue, and plenty of LFP owners feel they do more harm than good and do just fine without.
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Old 06-07-2019, 11:38   #94
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Re: LFP memory effects thread

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Originally Posted by tanglewood View Post
A few comments where I think at a minimum your terminology differs from most, and might be leading to miscommunication....


- Termination based on return/acceptance current and coulomb counting are different things. For charging based on coulomb counting one would presumably count coulombs drawn out of the battery, then when charging return that same number of coulombs, plus a few extra to account for inefficiencies. The actual return/acceptance current when you have finally "repaid the loan" could be anything, or more accurately, it's not something you are paying any attention to. I honestly haven't heard of anyone charging this way, but that might just be a matter of convenience.


- Termination based on return/acceptance current is stopping when terminal voltage is being successfully held at some level, and the return current has dropped below some threshold. No tally of coulombs in or out is required. Not to say this approach is necessarily any better or worse, but it is generally more convenient given the features available in most chargers.


So they are two different things.
Yes, they are two different things. Maybe my posts (too many I would expect to get my point across) weren't clear:

1) I use coulomb counting for routine charging. I found this method is the only way to reliably charge my pack with highly variable current sources, such as PV.

2) I manually do what some are calling an "equalization" charge (I don't like that term because it doesn't actually "equalize" the cells in the same way as with lead-acid cells) every 25 cycles or so by using a current limited (0.16C) source until reaching 100% SOC that I define as a charging ("tail") current taper to 0.02C. The cell voltages never exceed 3.60 volts. This comprises a "memory release" charge. My amp-hour counter confirms that I am still getting a little over rated capacity after three years of that process.

I have observed that if I perform a memory release charge less frequently, the cell voltages are higher at the end of the 100% SOC charge. As much as 100 millivolts, which would result in premature charge termination if I were using voltage detection alone (ignoring charge current). I interpret that this cell voltage rise is the memory effect.

If I have been guilty of posting "off topic" on how I avoided a severe condition of memory effect, as opposed to how to recover from the problem once it has set in, then -- I apologize for proposing prevention is better than correction. I admit a bias that an once of prevention is worth a pound of cure.

I'm not governed by the logic of the anti-vaxer. By the same token, I've never had measles - I got vaccinated. I have no personal experience with measles. I'd recommend that others get vaccinated too, so maybe I'm posting to the wrong audience.
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Old 06-07-2019, 11:55   #95
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Re: LFP memory effects thread

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Originally Posted by Cpt Pat View Post
Yes, they are two different things. Maybe my posts weren't clear:

1) I use coulomb counting for routine charging. I found this method is the only way to reliably charge my pack with highly variable current sources, such as PV.

2) I manually do what some are calling an "equalization" charge (I don't like that term because it doesn't actually "equalize" the cells in the same way as with lead-acid cells) every 25 cycles or so by using a current limited (0.16C) source until reaching 100% SOC that I define as a charging ("tail") current taper to 0.02C. The cell voltages never exceed 3.60 volts. This comprises a "memory release" charge. My amp-hour counter confirms that I am still getting a little over rated capacity after three years of that process.

I have observed that if I perform a memory release charge less frequently, the cell voltages are higher at the end of the 100% SOC charge. I interpret that this is the memory effect.

If I have been guilty of posting "off topic" on how I avoided a severe condition of memory effect, as opposed to how to recover from the problem once it has set in, then -- I apologize for proposing prevention is better than correction. I admit a bias that an once of prevention is worth a pound of cure.

I'm not governed by the logic of the anti-vaxer. By the same token, I've never had measles - I got vaccinated. I'd recommend that others get vaccinated too, so maybe I'm posting to the wrong audience.

Thanks, I understand much better now, and it all makes sense.


Have you ever plotted voltage during a charge and seen the memory hump demonstrated similar to the research? Anyone? It would be real interesting to reproduce it in an operational system.
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Old 06-07-2019, 12:09   #96
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Re: LFP memory effects thread

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Originally Posted by tanglewood View Post
Thanks, I understand much better now, and it all makes sense.


Have you ever plotted voltage during a charge and seen the memory hump demonstrated similar to the research? Anyone? It would be real interesting to reproduce it in an operational system.
Coulomb counting is not a reliable indicator for charging, because you never know how much energy is converted in heat during the process. There is no 100 per cent charge efficiency even with LFP. Voltage is quite good at the shoulders, but still depends on cell temperature and current.

When charging, you have to make sure every cell has the same voltage and temperature to ensure a balanced pack and can rely on termination charge voltage for the pack as provided by the charge controllers, otherwise you may notoriously overcharge and damage one of the cells.
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Old 06-07-2019, 13:45   #97
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Re: LFP memory effects thread

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Originally Posted by Cpt Pat View Post
I manually <charge> by using a current limited (0.16C) source until reaching 100% SOC that I define as a charging ("tail") current taper to 0.02C. The cell voltages never exceed 3.60 volts.
So is your charger in CC during that whole process? Or do you have a CV threshold that is reached?
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Old 06-07-2019, 14:06   #98
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Re: LFP memory effects thread

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Originally Posted by tanglewood View Post
Have you ever plotted voltage during a charge and seen the memory hump demonstrated similar to the research? Anyone? It would be real interesting to reproduce it in an operational system.
Yeah, I was able to observe a very tiny, but real voltage "bump" on a fresh pack (~10 cycles) after one CC/CV to 0.03C with a low CV threshold.

I'm not sure if I would have noticed it in a 4s battery with my equipment, but in 16s and knowing to look for it, sure enough, it was there. It went away after a few more, regular cycles to a higher SOC.

Perhaps interestingly, my pack (now ~200 cycles) also has a "gentle bump" around the midpoint of its SOC. Here is a charge from a few days ago:



Note the increased slope between about 52 and 60% SOC.

I haven't seen that shape in other charge curves I've looked at... but then, almost no one actually logs, graphs, and shares their data.
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Old 06-07-2019, 14:18   #99
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Re: LFP memory effects thread

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Originally Posted by CatNewBee View Post
When charging, you have to make sure every cell has the same voltage and temperature to ensure a balanced pack
Only if a top-balanced pack is important to you. There are other strategies, too.

Quote:
and can rely on termination charge voltage for the pack as provided by the charge controllers, otherwise you may notoriously overcharge and damage one of the cells.
Unless you simply charge to a lower voltage at a reliable rate. In which case, you do not overcharge and damage any cells... but you may encounter this elusive "memory effect" that this thread is discussing.

That is why this topic is so interesting. We have a we-thought-we-knew-safe and dead simple way to easily manage LFP -- don't charge them so full -- that might somehow be affecting the cells in a different way over the long term.

If this turns out to be the case, it would be good to know.
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Old 06-07-2019, 15:15   #100
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Re: LFP memory effects thread

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Originally Posted by nebster View Post
Yeah, I was able to observe a very tiny, but real voltage "bump" on a fresh pack (~10 cycles) after one CC/CV to 0.03C with a low CV threshold.

I'm not sure if I would have noticed it in a 4s battery with my equipment, but in 16s and knowing to look for it, sure enough, it was there. It went away after a few more, regular cycles to a higher SOC.

Perhaps interestingly, my pack (now ~200 cycles) also has a "gentle bump" around the midpoint of its SOC. Here is a charge from a few days ago:



Note the increased slope between about 52 and 60% SOC.

I haven't seen that shape in other charge curves I've looked at... but then, almost no one actually logs, graphs, and shares their data.

Not THAT'S useful data. What's the legend for the graph lines?
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Old 06-07-2019, 18:03   #101
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Re: LFP memory effects thread

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Originally Posted by nebster View Post
Yeah, I was able to observe a very tiny, but real voltage "bump" on a fresh pack (~10 cycles) after one CC/CV to 0.03C with a low CV threshold.

I'm not sure if I would have noticed it in a 4s battery with my equipment, but in 16s and knowing to look for it, sure enough, it was there. It went away after a few more, regular cycles to a higher SOC.

Perhaps interestingly, my pack (now ~200 cycles) also has a "gentle bump" around the midpoint of its SOC. Here is a charge from a few days ago:



Note the increased slope between about 52 and 60% SOC.

I haven't seen that shape in other charge curves I've looked at... but then, almost no one actually logs, graphs, and shares their data.
That is really interesting. Do you have any theory on what may be happening mechanically/chemically to explain this? Is what we are discussing here a steepening of that "bump" due to chronic undercharging?
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Old 06-07-2019, 18:25   #102
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Re: LFP memory effects thread

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Originally Posted by nebster View Post
Yeah, I was able to observe a very tiny, but real voltage "bump" on a fresh pack (~10 cycles) after one CC/CV to 0.03C with a low CV threshold.

I'm not sure if I would have noticed it in a 4s battery with my equipment, but in 16s and knowing to look for it, sure enough, it was there. It went away after a few more, regular cycles to a higher SOC.

Perhaps interestingly, my pack (now ~200 cycles) also has a "gentle bump" around the midpoint of its SOC. Here is a charge from a few days ago:



Note the increased slope between about 52 and 60% SOC.

I haven't seen that shape in other charge curves I've looked at... but then, almost no one actually logs, graphs, and shares their data.
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Old 06-07-2019, 19:27   #103
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Re: LFP memory effects thread

Yes excellent graph that, thanks for posting it!

What was the charge C-rate there?

And by what measure is SoC on the right side being determined?
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Old 06-07-2019, 22:47   #104
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Re: LFP memory effects thread

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Not THAT'S useful data. What's the legend for the graph lines?
The yellow, straight line is the SOC. The lowest voltage line is what the charger reads at the terminals. The other voltage lines are readings from other meters that are further up the chain (and unimportant for this discussion, but sometimes useful when I am looking at other situations in the history).
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Old 06-07-2019, 23:07   #105
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Re: LFP memory effects thread

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That is really interesting. Do you have any theory on what may be happening mechanically/chemically to explain this?
I wish I had a good theory; I do not. The only think I could think of is that it is kind of close to where the cells were sitting when they were shipped to me new.

It’s possible that it is just a flaw or idiosyncrasy in my particular cell manufacturer’s design, I suppose. (?)

In any event, it seems to be pretty stable in shape and height, but I only get a “clean” charge like this (with essentially no load interfering, and straight from a pretty low SOC to a high one) once every few months, and I’ve only been capturing the fine-grained data since the start of the year. So I’m watching it closely, if slowly, to see if it moves or gets “fatter” over time.

Quote:
Is what we are discussing here a steepening of that "bump" due to chronic undercharging?
When I first heard about this phenomenon, my immediate assumption was that it was a bump like in that graph that was steepening up over time, yeah. But now that I read more about what they are experiencing, it doesn’t really sound so much like that. Maybe they will turn out to be related, though.
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