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LiFePO4 balancing.

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paulca:
Assuming we are talking about the same active capacitve balancers...  aka these:
https://www.aliexpress.com/item/1005005050192601.html

I gather it's a series of logic counters and mosfets.  The tests I seen, A*V in > A*V out.  By a fair margin of double figure percentage.

That is what you would expect with a PWM on a mosfet gate every rise and fall it goes through the variable resistive region.  Every time you turn the mosfet on you have to charge the gate.  Everytime  you turn it off you dump that charge to ground.  It's all wasted.

If you have a LiFePO4 cell at 3.30V and it's neighbour is at 3.20V and you attempt to balance them, you could end up transferring 25% of the capacity of the cell via a 80% efficient balancer.  I think the flat portion on most cells is about 0.5V from 0% to 100% resting OC.  10mV difference can amount to quite a chunk of the capacity.  Its that reason balancing is not recommended specifically for LiFePO4 within that voltage curve region.  Li-Ion is more linear and affords balancing during charge.  I would always balance charge my high current LiPos, unless I'm in a real rush, when "fast charge" canes them at 2C until one cell hits 4.20V and takes half the time.

paulca:
Back on topic.

I have been using the lead-acid boost charge profile at 14.40V to "effectively" top balance the 4S pack through "absorption".

It's been a week or so now and ... it's not exactly working out as planned.

I have one cell stuck in it's flat charge region while the other 3 are climbing up over the 3.65V target.  The highest got to 3.900V while the lowest was still dragging in the flat region at 3.460V

While I was expecting them to appear unbalanced at the top undercharge the self discharge back to the 3.6/3.7 region would effectively equal them out.

That is assuming however that all cells reach their upper voltage curve on every charge.  Once that stops being try the cell imbalance will probably continue to get worse until there is a cell taking far too much voltage on it's own.

While 3.9V is not terminal for a LiFePO4 cell, it's not optimal or recommended.

The only balancer I have had a total "capping" current of 100mA per cell.  Thankfully, the sun is now out of scope on the panel and that 3.9V cell is rapidly falling back under 3.7V.

I suppose it bumps the BMS board up a notch in priorities.  I was hoping to get away with it until the other 4 cells arrive mid april, then I don't need a 4S BMS, but can go to the main range which start at 8S.

I have a 2 Amp balancer/BMS but it's 3S.

When I finish work I suppose I will have to go out and manually top balance them all again... and probably repeat that at least weekly until the other 4 cells arrive.

We don't have much sunny weather planned anyway :)

shapirus:
I'm not sure there is such thing as absorption when it comes to lithium cells. You can't "overcharge" lead-acid cells, because once they reach a certain voltage, they won't be able to get any higher, because this "extra voltage" is "spent" for dissociating the electrolyte, which is known as so-called "boiling". Overcharging them simply means slowly (how slowly? I don't know) destroying their electrolyte, which, in non-sealed (or manually unsealed) batteries, can be restored by bringing the acid-water solution back to the required density.

This is why it is possible to balance lead-acid batteries by charging them at a higher voltage: when some cells begin producing hydrogen and remain at a certain voltage level, the others will eventually catch up and become equalized. This is my understanding of this process, at least.

It apparently doesn't happen like this with lithium batteries, which is why it is important to have their cells balanced, especially as you're nearing 100% SoC, or, rather, as some of the cells are reaching their maximum allowed voltage.

paulca:
There definitely is absorption during charge. 

Take an average small LFP cell.
Resting voltage, lets say 3.4V, close to 90%+ SoC.
Apply 1 Amp of current to it and the cell might rise to 3.8V
Remove that current and it will settle back to 3.4V.
Apply 5 Amp of current to it and the cell might rise to 4.0V
Remove that current and it will settle back to 3.4V.

It's that "boost" differential I was hoping would scale in the same way as the upper voltage curve scales.  When cells have 14.40V across them a lot of those volts are accounted for by the "boost" differential across the cells.  Because there is so little actual capacity up there I was assuming it is caused by the resistence to charge rising as fast as the voltage.  Thus a reasonable amount of time spent unbalanced in that region, assuming of course that no cell exceeds 4.2V seemed likely to balance the pack.

I think ultimately it would/will, but with such high charge currents over 0.1C it does not appear to be working out.

Zucca:
This is why I decided to use my LiFePO4 from 20% to 80% SoC and to avoid any possible chinese or cheap or professional BMS.
BMS does cause more harm than good on the long run.

Yes I had already some fun with thermal events and white smokes events. All because of (poor designed) BMS.
BTW designing a good BMS is a nightmare.

I mean instead to built clever protection in the battery we should use clever chargers and clever loads.

That said, I think it is crucial to have a stupid passive monitoring in the battery pack that reports the voltage of each cell to the outside.
Don't know but I am a fan of stupid battery packs that are not a pain to replace (look at here what a horror movie), and have all the protection outside.

And yes, for portable application where 100% of the SOC needs to be squeeze out, then and only then, I will bent over and install a BMS.

PS: Very interesting discussion, thanks for all the good info here. Especially thanks to Siwastaja for sharing his knowledge!

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