LFP memory effects thread

[john61ct]

Originally Posted by ghrt

I have an unused 400ah Sinopoly bank, sit for 2 years.
In the first cycle, discharging @10amps, I saw VPCs dropping 0.2mV and then getting back 0.1-0.15mV.

Are you thinking just sitting might produce a memory effect?

And what makes you think your bank has suffered a capacity loss? Only directly testing capacity is a reliable indicator of that parameter.

Voltage sag under load and the subsequent bounceback is not a precise indicator of anything, and that level looks completely healthy to me.

[ghrt]

Maybe it had. It's for sure not voltage sag under load followed by a bounce back, because the bounce was also under the same continous load.

I don't know yet if it has lost any capacity, but capacity available in the first cycle was exactly 400ah though I've charged and discharged with 10 amps. At these rates I was expecting more than 400ah. I suspect a cell is not that good as the others but it's too early to draw conclusions and too late to complain

From experience shared here, I must prepare for an "equalization" at the top about 50 cycles, or maybe switch to top balancing.
Bottom bal. would be better in my case, as solar panels provide less then needed and battery is discharging until I get shore power (or turn off the fridge).

[john61ct]

Not quite clear.

Are you saying you've already done a precisely timed CC load test from 100% charged to 3Vpc?

Or did you get that 400Ah actual capacity estimate from a totalizer counting Wh/Ah?

Please do **not** regularly cycle to such a low DoD, unless you don't care about longevity, the bank lifespan will be very short doing that often.

Find a way to get more energy input, or as you say, stop consuming at 12.4-12.6V ~3.1Vpc, depending on discharge C-rate.

Originally Posted by ghrt

Bottom bal. would be better in my case, as solar panels provide less then needed and battery is discharging until I get shore power (or turn off the fridge).

Bottom balancing is fine, but then you need to be vigilant about manually monitoring for imbalances at the per-cell level whenever you approach the bottom of your load cycles.

Automating active rebalancing there requires a lot more than your average cheapie (or even expensive) BMS.

There is little point balancing in the middle ranges, unless you keep it going all the time.

[evm1024]

I've been busy elsewhere (new job) so have not been able to get to testing.

But, I did find some time to do about 5 cycles charging to 14.2 v @ a tail current of around 12 amps with a bulk charge rate of 100 a. After pulling the (oh god, let's not argue about the term) surface (residual) charge off the cells which causes them to stabilize around 3.345 vpc (13.38v).

Then a discharge phase at 10 amps to a cutoff voltage of 11.5v.

This yields a capacity of 515 ah on average which is about 73% of the sticker capacity.

In the 5 full cycles the capacity has not really varied. No capacity recovery noted.

We can call this the charge high, discharge low regime is we like.

The bank is top balanced and at "full" charge the cells are within 6 mV of each other. At near full discharge the cells voltages are widely spaced. This is consistent with cells of differing capacity. In the last discharge cyclt I let the lowest cell voltage drop to the spec sheet abs min voltage before stopping discharge. The cell voltages were 3.134, 2.777, 2.505, 3.085 volts. I did have to disable the BMS to prevent it opening the circuit.

The obvious conclusion is that the 2 middle cells have a much lower capacity then the other 2.

It will be interesting to see if the capacity remains the same with the completion of the next discharge phase some time tomorrow.

I was thinking that over the weekend I might try a charge to 3.8 vpc and then a 10 a discharge phase to see if that has any effect on the low capacity cells.

With that done I may try a few cycles of 100 amp charge and 100 amp discharge to see if the total capacity changes. And then follow that with a few cycles of low charge / high discharge....

As a side not this last charge cycle had the cells at 3.352, 3.350, 3.350 and 3.352. Just a 2 mV difference at 100% SOC.

One further "crazy" thought I've had would be to discharge until the weak cells were at 3.00 vpc then charge the 2 weak cells to 80% SOC and then continue discharging until the "strong" cells reached 3.00 vpc. This would give me a good idea of the capacity of the strong cells.

[Cpt Pat]

Originally Posted by evm1024

At near full discharge the cells voltages are widely spaced. This is consistent with cells of differing capacity. In the last discharge cyclt I let the lowest cell voltage drop to the spec sheet abs min voltage before stopping discharge. The cell voltages were 3.134, 2.777, 2.505, 3.085 volts.

Could you have lost capacity in the two middle cells because they cannot dissipate heat as readily as the two cells on the ends: thermal aging?

[evm1024]

Originally Posted by Cpt Pat

Could you have lost capacity in the two middle cells because they cannot dissipate heat as readily as the two cells on the ends: thermal aging?

Not likely but worth a thought. The bank has never broken a sweat. Also, I am always feeling them for heat during charging and I can say that I've never felt them at more than ambient temp.

Most likely was that they were B grade form the start.

[CatNewBee]

Interesting, my inner cells are always a little higher when charging than the cells on the edges. Noticed that from the first tests. I assumed then, it was just a coincidence.

When installed on the boat permanently I swapped the outer and inner cells. Now the same effect is noticeable, the inner cells tend to be fuller then the outer cells, cell 2 and 3 are often used to balance cell 1 and 4.

I have no reasonable explanation for this yet,

[john61ct]

At low sub-C rates and avoiding overcharging, I doubt internally generated heat is much of a cycling-lifespan longevity factor.

However ambient temperature definitely has a huge impact on aging in general, even taking cycling out of the picture, aka calendar aging.

Also, ESR / internal resistance goes down as temperature goes up, to the point that used in competition, racers heat their batteries beforehand to get better performance, as in higher / faster power discharge rates per gram of weight.

So I'm thinking that phenomenon could account for those variations, rather than necessarily quality variation issues.

[Cpt Pat]

Here's a very comprehensive paper on LiFePO4 thermal aging effects: https://www.nrel.gov/docs/fy18osti/70616.pdf

[newhaul]

Here is a new study on ev bank charging and cooling and capacity loss effects
https://www.cnet.com/google-amp/news...y-degradation/

Seems that best is to charge slow and keep battery cool.

[evm1024]

Originally Posted by newhaul

Here is a new study on ev bank charging and cooling and capacity loss effects
https://www.cnet.com/google-amp/news...y-degradation/

Seems that best is to charge slow and keep battery cool.

That is why I laugh at all the talk of charging at high fraction of "C".

I've set my alternator to limit out at about 100 amps ( 0.14C or 0.2C)

The inverter/charger is limited to 50 amps ( 0.07C or 0.1C)

The 2 different numbers are because my cells are 700 AH nameplate and 500 AH actual. At least one is bad as noted in this thread as well as elsewhere on CF.

No fuss, no muss and the cells get charged at fractional C.


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.

[Montanan]

I was going to post a reply to this thread, but . . .


I forgot what I was going to post.

[tanglewood]

The negative effects of high C charging, and high temps, typically a direct result of high C charging and discharging, has figured prominently in the literature for a very long time. So nothing really earth shattering. But these are all multi-C rates.



To me the important thing is that a house battery system typically operates at the other end of the spectrum from all the literature. And all the batteries we typically use (LFP) call for "normal" charge in the .25 to .5C range, and discharge in the .5 to maybe 1C range. But those are still huge numbers. For a 500Ah battery, .5C is 250A charge rate, and will fully recharge in 2 hrs. And a .5C discharge rate will drain the battery in 2 hrs. Who's boat works like that? Not mine. Even a 20hr discharge would be quick, which is .05C if my math is right. That's tiny. And all this is reflected in temp rise of packs. I see only a couple degrees rise when charging at .3C. It's immaterial.

[john61ct]

This thread hasn't been about supposed memory effects for a long time now, wouldn't it be better to keep it useful for that?

Created a new thread here http://www.cruisersforum.com/forums/...es-228043.html

[newhaul]

Originally Posted by john61ct

This thread hasn't been about supposed memory effects for a long time now, wouldn't it be better to keep it useful for that?

Created a new thread here http://www.cruisersforum.com/forums/...es-228043.html

actually incorrect again the thread is all about memory effects . What causes it and what may mitigate it . It seems the biggest issues are high C rates of charging. As well as high temperatures during said charging.

For those of us actually using Lfp the charge rate is not an issue . So that leaves us with temperatures and just good old aging. ( I have not noticed any temperature changes when charging mine at .3C or lower so temperature is out. So it comes down to ageing . Is it like humans ? ( exercising regularly staves off effects of aging) should we all start using the bank down to 90% dod on a monthly basis followed by a full charge and top balance? ( to fully exercise the bank and stretch the muscles and tendons so to speak to keep it limber)
I don't know but these are the questions we are trying to answer. Creating more spread out threads just dilute the already small pool of knowledge and users that we can draw on to answer these questions.