LFP memory effects thread

[Cpt Pat]

Here's another in vitro protocol using only cell voltage for measuring and terminating charge:

Conditions: 100 AH battery, predetermined resting cell voltage at 100% SOC = 3.35 volts. Current SOC state: 0% SOC (fully discharged). Desired SOC 100%. Acceptable SOC tolerance: - 0%, +5%.

1) Rest battery for 8 hours. (No load, no charge).
2) Measure cell voltage.
3) If voltage is equal to or greater than 3.35 volts, terminate charging.
4) If cell voltage is less than 3.35 volts, charge 5 AH and return to step 1.

This is a "piecewise" approach with 5 AH increments. Starting at 0% SOC, it will take "only" 20 steps, or 20 X 8 hours (160 hours) - plus charging time - to complete the process. That's fine, if you have no other use for the battery and don't mind waiting a week.

That method obviously isn't practical while underway and actually using the battery.

[john61ct]

Originally Posted by Cpt Pat

How do you implement that protocol while underway, with loads connected, alternately charging and discharging the battery at variable rates, while never isolating and resting the battery (i.e., in the real world) where no current taper can be accurately detected?

I believe your case is an extremely low charge rate, below even .1C

with no source available at a higher rate is that right?

I'm not saying I have solutions for you that you would consider acceptable, or that you haven't considered.

But if you did sincerely want to explore the possibilities - where there's a will there's a way IMO, I think better pursued in a different thread.


Maybe revive this one? http://www.cruisersforum.com/forums/...ml#post2790204

or start your own if you like.

[Cpt Pat]

Originally Posted by evm1024

I would like to remind folks that the thread is a working thread not a theory thread. Simply stated (in post #1):

This thread is to report observations and actions. ...

...For those who have LiFePO4 banks and who are experiencing capacity loss and who are trying to see if they can recover that lost capacity.

We are not looking at getting to 100% SOC or even defining what 100% SOC is or how to measure it. There are other better threads for that discussion. We are discussing 30% loss of capacity. Any discussion of the difference between 99.94% SOC and 99.69% SOC are far too subtle for this hands on thread.

And oh by the way the difference (between 99.94 and 99.69) is around 0.25 gram of Li ions transferred. Which is meaningless in the context of this thread.

Let's stay focused.

After three years and 500 cycles, I have avoided any permanent memory effects using a coulomb counting charging process. I believe the alternative process: using voltage detection alone, is responsible in whole or in part for the development of detrimental memory effects by leaving the battery in a cumulatively undercharged state due to premature charge termination based on terminal voltage (while charging) detection alone. I am discussing the details of my method which has prevented the memory effect problem and comparing that method with the voltage-based termination method.

How is that off topic ("LFP memory effects thread") or necessitating a separate thread? I admit I am guilty of conventional wisdom heresy.

If the Internet existed in the days of Galileo, I suspect the Pope would have told him to create a new thread when he asserted the earth revolves around the sun. (Not to compare my intellect with his. I'm stuck with talking about batteries.)

[evm1024]

Originally Posted by Cpt Pat

After three years and 500 cycles, I have avoided any permanent memory effects using a coulomb counting charging process. I believe the alternative process: using voltage detection alone, is responsible in whole or in part for the development of detrimental memory effects by leaving the battery in a progressively undercharged state due to premature charge termination based on terminal voltage (while charging) detection alone. I am discussing the details of my method which has prevented the memory effect problem and comparing that method with the voltage-based termination method.

How is that off topic ("LFP memory effects thread") or necessitating a separate thread. I admit I am guilt of conventional wisdom heresy.

Posting how you go about running your bank is on topic. Telling us that you use coulomb counting as the major determinate of deciding state of charge is on topic.

Explaining what you mean by coulomb counting is on topic.

Debating whether what you mean by coulomb counting is "accurate" or not is off topic. Debating the best way of coulomb counting is off topic.

Heck Debating anything is off topic.

[Cpt Pat]

Well, then, I won't debate with you.


(Where would science be without debate?)


"All things are hidden, obscure and debatable if the cause of the phenomena is unknown, but everything is clear if its cause be known."
--Louis Pasteur


Once we know everything about the memory effect, there will be no further cause for debate. Until then....

[newhaul]

Originally Posted by Cpt Pat

Well, then, I won't debate with you.


(Where would science be without debate?)


"All things are hidden, obscure and debatable if the cause of the phenomena is unknown, but everything is clear if its cause be known."
--Louis Pasteur


Once we know everything about the memory effect, there will be no further cause for debate. Until then....

you are confusing discussion ( which this is )
and . In a debate, opposing arguments are put forward to argue for opposing viewpoints. ( which this isn't )

[Cpt Pat]

Originally Posted by newhaul

you are confusing discussion ( which this is )
and . In a debate, opposing arguments are put forward to argue for opposing viewpoints. ( which this isn't )

OK. But I've forgotten the words to the song Kumbaya and I'm out of practice giving group hugs, so please pardon me if I disagree with someone. I didn't study science in the College of Consensus.

I've found some good reasearch into memory effect here: Comprehensive Study of the Polarization Behavior of LiFePO4 Electrodes Based on a Many-Particle Model. My interpretation in plain English of this paper and others I've read is this:

When charging to a partial SOC (say, 90%), a voltage bump at that SOC occurs in the next charge cycle. This results in a premature termination of the charge in the next charging cycle at the same voltage, when only voltage sensing is used for SOC determination -- slightly before a 90% SOC is reached. At each successive charging cycle, the charge terminates at a lower and lower SOC but at the same voltage because the voltage bump moves to a lower actual SOC during each charge cycle. If this process is allowed to continue, eventually the full battery capacity can't be restored because the battery won't absorb a full charge (you have to dig into the chemistry and morphology discussions to see why).

Counting amp/hours in and out on the other hand causes the voltage bump to be "ignored" during charging, preventing a progressive loss of capacity.

At least that is my highly simplified explanation. Please feel free to disagree with me and express your findings. I don't want to repress anyone's expression of disagreeing information.

[john61ct]

That corresponds with the past discussions I read on other forums, particularly the cause noticed in those days being an automated / algorithmic repetition stop-charging at the same spot.

The solution proposed being variety in the stop-charge point, and especially regularly going higher up the SoC curve than most people think they "should" go in normal usage.

[OceanSeaSpray]

Today I increased the absorption voltage so the highest cells would reach 3.65V - I see some imbalance ranging from 3.51V to 3.65V when charging, but none after the cell voltages settle - and charged to termination again.

The result was disappointing: 72Ah, with a little more uncertainty as the BMS had performed CC from the day before when it was reset at empty. I don't think I made much headway since starting.

The only avenue left now is extended absorption at low current. I noticed that the pack keeps charging even if the voltage is much lower and there is not enough headroom to increase the voltage until a decent charge current is obtained, so it looks like remediation is going to be a lengthy process.

In fact, it might look like absorbing until the current goes down to zero and not require anything more than a modest charging voltage, but this will need to be tested. For now, I am going to return the absorption voltage to 3.5V/cell and lower the residual current limit down near zero. It won't get hit because I will never manage to charge for long enough at the moment, but the idea is to see if I can start gradually pushing back against the cell memory.

I can't draw any conclusions with regard to cell balance. The differences might arise from resistive effects only, so I won't attempt any big corrections at the moment.

[john61ct]

What is preventing you from holding CV / Absorb until current tapers to zero?

Is 3.60V easier for you than 3.65V?

Also, from an earlier post:

Originally Posted by john61ct

I bet if you do your best to restore capacity at these very low charge rates for quite a few cycles and results aren't what you'd like,

getting up to much higher say 1C if feasible may produce a very different result.
…
Not recommending over .3C for normal usage, just for this sort of test/maintenance protocol, occasional use only.

[evm1024]

Originally Posted by OceanSeaSpray

Today I increased the absorption voltage so the highest cells would reach 3.65V - I see some imbalance ranging from 3.51V to 3.65V when charging, but none after the cell voltages settle - and charged to termination again.

The result was disappointing: 72Ah, with a little more uncertainty as the BMS had performed CC from the day before when it was reset at empty. I don't think I made much headway since starting.

The only avenue left now is extended absorption at low current. I noticed that the pack keeps charging even if the voltage is much lower and there is not enough headroom to increase the voltage until a decent charge current is obtained, so it looks like remediation is going to be a lengthy process.

In fact, it might look like absorbing until the current goes down to zero and not require anything more than a modest charging voltage, but this will need to be tested. For now, I am going to return the absorption voltage to 3.5V/cell and lower the residual current limit down near zero. It won't get hit because I will never manage to charge for long enough at the moment, but the idea is to see if I can start gradually pushing back against the cell memory.

I can't draw any conclusions with regard to cell balance. The differences might arise from resistive effects only, so I won't attempt any big corrections at the moment.

Very good info and observations. I think I am at the same point in my recovery process that you are.

On a side note: I should have called this thread "Remediation of significant capacity loss in LiFePO4 cells".

Memory effects has a fairly specific meaning in the science crowd - specifically the "bump" in the charge voltage that happens. A very small bump to be sure.

So for my own data I've finished my discharge cycle at a 10 amp rate. The end results are 502 AH taken out at a 11.6 v cutoff. This will establish a baseline for my remediation efforts.

I agree that balancing does not have a large effect on total capacity. At least not to the tune of 200 AH of loss over the sticker capacity.

Right now my bank is recharging at a 50 amp rate. As time allows (I have a day job) I will break the pack down and charge to 3.65 VPC at a 7.5 amp rate (30 amp bench supply is the limiter) then reassemble and do a discharge to see what if any change in capacity I will get.

A few cycles under that regime then I'm thinking I will change the charge rate to something larger. This assumes that I am not making any capacity gains. If I am making gains then I might just let it go that way for a few cycles to see what happens.

The 30% loss we have seen indicated to me that there are inactive regions in the anode and cathode. These regions may be in area or in depth of the active materials on the electrodes. One of the reasons I am starting with low charge/discharge rates is to minimize nonuniform Li insertion in the electrodes i.e. minimize depth effects.

Actually, as I think about it it may be more productive to use low current charging (to promote uniform Li insertion) and high discharge currents (for full depletion at the "surface" working deeper into the depth of the active materials. Hmm, perhaps I'll try that for a few cycles. Plus it tales a lot less time (5 hours at 100 amps for 500 AH rather than 50 hours @ 10 amps)

Perhaps our folks without a LiFePO4 bank will undertake a literature search on that for us.

[OceanSeaSpray]

Originally Posted by evm1024

On a side note: I should have called this thread "Remediation of significant capacity loss in LiFePO4 cells".

Memory effects has a fairly specific meaning in the science crowd - specifically the "bump" in the charge voltage that happens. A very small bump to be sure.

It is all one and the same here. It initially shows as a small bump, but when left unchecked it leads to what we are experiencing. At least, the title differentiates from capacity loss due to loss of lithium.

Originally Posted by evm1024

Right now my bank is recharging at a 50 amp rate. As time allows (I have a day job) I will break the pack down and charge to 3.65 VPC at a 7.5 amp rate (30 amp bench supply is the limiter) then reassemble and do a discharge to see what if any change in capacity I will get.

My guess is that you will get none if you terminate on the residual current condition, this is essentially what I have tried a few times now.

I will keep charging in series to see what the cell balance looks like, but as it is fairly even at the bottom, I already know that it is not way out.

Originally Posted by evm1024

A few cycles under that regime then I'm thinking I will change the charge rate to something larger. This assumes that I am not making any capacity gains. If I am making gains then I might just let it go that way for a few cycles to see what happens.

The 30% loss we have seen indicated to me that there are inactive regions in the anode and cathode. These regions may be in area or in depth of the active materials on the electrodes. One of the reasons I am starting with low charge/discharge rates is to minimize nonuniform Li insertion in the electrodes i.e. minimize depth effects.

Actually, as I think about it it may be more productive to use low current charging (to promote uniform Li insertion) and high discharge currents (for full depletion at the "surface" working deeper into the depth of the active materials. Hmm, perhaps I'll try that for a few cycles. Plus it tales a lot less time (5 hours at 100 amps for 500 AH rather than 50 hours @ 10 amps)

It is simply impossible to increase the charge rate meaningfully once you hit the wall... the cells are just way too resistive. At this point, the cells are still happy to keep charging and it doesn't actually require a high voltage, but the current is low and it looks like a matter of time... plenty of time. It could take 10-20 hours here by the look of it, unless things suddenly get easier. Before that, I don't think that charge rate matters much, the cells are still easy to charge and the root cause for the memory effect is already in place.

I am not sure whether discharge will have any role to play at all in this. If the memory can't be cleared in one single recharge cycle, then discharge is just the pathway to have another go.

High current operation never provides enough time to fully move the charge carriers anyway.

[Cpt Pat]

OceanSeaSpray: I think you've got a good handle on things. Please post your results when you have them.

[evm1024]

Originally Posted by OceanSeaSpray

SNIP

It is simply impossible to increase the charge rate meaningfully once you hit the wall... the cells are just way too resistive. SNIP

True - but I am not there yet. I have limited the charging current to 50 amps by choice. This works out to C/14 for the sticker capacity of 700 AH or C/10 for the "actual" capacity of 500 AH.

This series of charge/discharge cycles is designed to maximize electrode porosity and to fully (de)lithiate smaller particles of active material before larger particles.

The next series of tests will likely charge (starting at) a 200 amps to 300 amps rate (my 14.4 volt charging capacity is limited to about 300 amps) and discharge at 100 amps.

That series would be designed to move all active particles into a single phase regime.

I'm looking for a log jam break type of change - say a change of capacity from 500 AH to 600 AH - that would be nice. But I will take a number of 20 AH changes. Just gotta find a way to make the change.


I still intend to do 1 or 2 more low current (dis)charge cycles before going over to high current (C/2) cycles.

[Cpt Pat]

For background, here is another write-up on memory effect that describes the chemical mechanism in simpler terms: https://phys.org/news/2013-04-memory...batteries.html

My own conception of the progressive build-up of memory is this (feel free to challenge it):

Expected experimental results: If the goal is to achieve a 90% SOC, and if at a charge current that SOC initially translated to a terminal voltage of 3.45 volts per cell (the cell voltage will be higher with higher currents due to cell series resistance), then terminating charging at 3.45 volts per cell (while charging) would yield a pattern like this:

Charge cycle 1: 3.45 volts = 90% SOC
Charge cycle 100: 3.45 volts = 89% SOC
Charge cycle 1,000: 3.45 volts = 80% SOC

My conclusion: Terminating charging using only a charge termination cut-off voltage will result in a progressive development of under charging.

Using coulomb counting on the other hand, if properly calibrated, will result in the same number of amp/hours discharged being returned during charging, and the terminal voltage can and should be disregarded, provided it will remain within a tolerable maximum limit. I've set that limit at 3.60 volts per cell. In my implementation, that limit has never been exceeded with low charge currents in the range of zero to 0.16C. Additionally, I manually charged to 100% SOC with a constant current (0.16C) and a constant voltage of 3.6 volts per cell (14.4 volts pack voltage) every 25 cycles to calibrate the counter. I defined "100% SOC" as a charge current taper to 0.02C.

Coulomb counting was the only mechanism I could devise to accommodate any degree of charging accuracy with low and varying charge currents (sun-dependent solar and speed-dependent water impeller). That charging regimen appears to have prevented any measurable memory effects. I can't say if charging to 100% SOC every 25 cycles, the coulomb counting method, or the two combined prevented memory effects.