LFP memory effects thread

[john61ct]

If it's now about longevity, permanent capacity loss in general, fine, of course C-rates and high ambient temps are the major factors.

But none of that is about preventing or recovering from the specific (posited) memory effect the thread started out with.

[evm1024]

THis thread is an off shoot of another thread and even though named memory effects it is about the loss of capacity and the chance of recovery of some or all of that capacity.

It has always been about the recovery of lost capacity. This includes cause and prevention. But of course threads drift. Let me quote the original poster:

This is a starting point for understanding the memory effects that a few of us with LiFePO4 banks have seen.

It would be very nice if we could limit posts to those who have LiFePO4 banks. Of course if someone comes up with something that is directly applicable and not just opinion they should post that too.

This thread is to report observations and actions.

[newhaul]

Originally Posted by john61ct

If it's now about longevity, permanent capacity loss in general, fine, of course C-rates and high ambient temps are the major factors.

But none of that is about preventing or recovering from the specific (posited) memory effect the thread started out with.

all that means is the more information that has been collected the more the study of the subject has evolved and progressed towards an ultimate understanding.

Has anyone recovered any of their ( as reported temporarily lost capacity? No they haven't reported that they have recovered any.

That makes it a permanent loss of capacity .
Now the evolved question is what can we do to mitigate the losses associated with an aging bank .
Without getting ridiculous about it . If doing a monthly 90% for will make it last longer fine but if it requires that to be done every week to get 10,000 cycles instead of 9,000 its not worth it IMO.

we all know batteries regardless of chemistry are a consumable. Eventually they need replacement. Its just a matter of when.

[Dev0n60]

Excellent... I agree ... exercise the pack ... this has solved the issue of 'memory effect' for us... so far...
Measuring capacity by performing a 'load test' is the only way to know what capacity the pack really has.... this involves disconnecting the pack and hooking it up to a load testing device .. we are using the the Array 3721...
Loss of capacity: we lost approx 60AH of capacity on a tested 445ah pack.. we recovered 45ah by 'exercising' the pack after the recommendation from 'Marine How To' (Rod Collins).
The recorded capacity in 2016 using the Array was 445 Ah... this year April 2019... we recorded 383 Ah on a load test (this due to partial recharges over 3 years we assume) .. we are now back up to 428...

[Cpt Pat]

This is just anecdotal, not scientific - but I have charged my pack to 100% about once every 30 cycles for the past 3 years over a use of about 800 cycles; and I haven't seen any sign of memory effect. I determine "100% charge" by manually monitoring charge current and terminating at a charge current of 0.03C. While I do monitor cell voltages and don't let the voltage during charge exceed 3.65 volts on any cell, I use the charge current taper to terminate charging. With my low charge rate (0.2C), the cell voltages top out at about 3.55 volts.

I also calibrate my battery monitor during that 100% SOC charge. The resting voltages and monitor-displayed SOC track exactly down to my stop-discharge voltage (LVCO) of 12.8 volts.

My use is sub-fractional. Max discharge rate: 0.05C, max charge rate: 0.2C. Higher rate usages my not behave as well.

[john61ct]

That is exactly how it's supposed to be done.

As long as your definition of 100% is based on the same endAmps as well as Absorb voltage, then that SoC level will be identical no matter what the earlier current rate.

But keeping that rate lower rather than higher is better for longevity, depending on cell temperatures, cooler temps being more dangerous for higher rates.

[OceanSeaSpray]

Originally Posted by evm1024

Back to the "recovery" attempt on my cells. I've tried a number of charge rates and discharge rate combinations in an effort to recover capacity. Nothing has "worked".

My cells are rock steady at around 500 AH capacity and an ability to discharge for hours at 130 amps.

This leads me to suspect that for whatever reason one of my cells has lost active area and thus "became" a lower capacity cell. This implies that I will get thousands of cycles more out of the bad cell.

Time will tell.

Last year, I also attempted capacity recovery on my then 5.5-year old 4S bank, 77% usable capacity against nominal, first by cycling deeply and recharging to a higher 3.58V/cell and lower 0.01C tail current. Difference less than 1% capacity.

Next, I extended absorption by allowing the current to taper to extremely small values. It is interesting that while I expected it to drop off to zero (as it readily did when the cells were new), I was never able to get there. The additional absorbed capacity was also near negligible.

I then decided to try forcing the issue by raising the absorption voltage to 3.7V/cell and watch what would happen. The voltage step-up caused an initial current inrush followed by the same behaviour. I kept this up for a few full charge/discharge cycles and even discharged down to 2.8V for the lowest cell. I may have gained 2-3% capacity, but some should be attributed to deeper discharge in later cycles, so effectively less than this.

There was no large imbalance showing at the bottom, so all cells are about equal in capacity and behave consistently with reduced capacity.

Conclusions:

1. The capacity loss appears permanent. Aging and degradation of the SEI layer most likely.
2. The bank performs fine in service within its reduced capacity with the following differences: a/ its internal resistance is clearly higher than when new, as illustrated by voltage sag under heavy loads (0.5-1.5C) and b/ the OCV in discharge behaves as if the initial voltage plateau above 13.3V is gone now. The discharge basically occurs around 13.15V or so. This is what had initially prompted me to believe I would be able to recover some capacity at the top.
3. The long partial cycle operation history of the cells didn't result in a memory effects, most likely because the cells were never partly charged and then allowed to sit in this state for a while, which is a condition to perform a successful memory-write cycle, and then they were recharged properly for part of each year with absorption at 3.5V, which erases cell memory.

The cells are the old black plastic-cased prismatic Sinopoly. 3 years ago I was invited to visit a lab in Switzerland where significant research was done in LFP battery applications/management. These people had tested a sample of commonly available prismatic cells (CALB, Winston, Sinopoly and others) to qualify them for a large commercial client operating an extensive fleet of small electric vehicles. The Sinopoly cell had exhibited the fastest rate of capacity loss of all in the lab when measured with high-precision instruments under controlled conditions. Incidentally, the cell eventually selected for the application was Lishen.

I have used these same Sinopoly cells in several other installations, which have all performed flawlessly from a practical point of view, but their present capacity hasn't been quantified. These represent different boats with very different usage profiles. I will try to get this information when I can.

[evm1024]

Originally Posted by OceanSeaSpray

Last year, I also attempted capacity recovery on my then 5.5-year old 4S bank, 77% usable capacity against nominal, first by cycling deeply and recharging to a higher 3.58V/cell and lower 0.01C tail current. Difference less than 1% capacity.

Next, I extended absorption by allowing the current to taper to extremely small values. It is interesting that while I expected it to drop off to zero (as it readily did when the cells were new), I was never able to get there. The additional absorbed capacity was also near negligible.

I then decided to try forcing the issue by raising the absorption voltage to 3.7V/cell and watch what would happen. The voltage step-up caused an initial current inrush followed by the same behaviour. I kept this up for a few full charge/discharge cycles and even discharged down to 2.8V for the lowest cell. I may have gained 2-3% capacity, but some should be attributed to deeper discharge in later cycles, so effectively less than this.

There was no large imbalance showing at the bottom, so all cells are about equal in capacity and behave consistently with reduced capacity.

Conclusions:

1. The capacity loss appears permanent. Aging and degradation of the SEI layer most likely.
2. The bank performs fine in service within its reduced capacity with the following differences: a/ its internal resistance is clearly higher than when new, as illustrated by voltage sag under heavy loads (0.5-1.5C) and b/ the OCV in discharge behaves as if the initial voltage plateau above 13.3V is gone now. The discharge basically occurs around 13.15V or so. This is what had initially prompted me to believe I would be able to recover some capacity at the top.
3. The long partial cycle operation history of the cells didn't result in a memory effects, most likely because the cells were never partly charged and then allowed to sit in this state for a while, which is a condition to perform a successful memory-write cycle, and then they were recharged properly for part of each year with absorption at 3.5V, which erases cell memory.

The cells are the old black plastic-cased prismatic Sinopoly. 3 years ago I was invited to visit a lab in Switzerland where significant research was done in LFP battery applications/management. These people had tested a sample of commonly available prismatic cells (CALB, Winston, Sinopoly and others) to qualify them for a large commercial client operating an extensive fleet of small electric vehicles. The Sinopoly cell had exhibited the fastest rate of capacity loss of all in the lab when measured with high-precision instruments under controlled conditions. Incidentally, the cell eventually selected for the application was Lishen.

I have used these same Sinopoly cells in several other installations, which have all performed flawlessly from a practical point of view, but their present capacity hasn't been quantified. These represent different boats with very different usage profiles. I will try to get this information when I can.

This pretty much mirrors my experience.

One test that I may do after getting the fridge rebuild project done is to charge to 100% SOC (top balanced) then discharge a some rate (30 amps?) and when the "weaker cells" hit the low voltage points recharge those weak cells and keep discharging in an effort to get the capacity of each cell.

More of a curiosity effort than any meaningful test.

And then I'll have to top balance again.