This is so cool!!!

[Cheesewiz]

I hope that @Will Prowse doesn't already have intellectual property rights on this subject, but, this is so cool. My new handmade 48v 72ah cylindrical cell battery bank is getting close to 500 cycle ah, and balance between my 12p cells was drifting. at first I thought it was because my balance current was too low for the size of the battery Bank, then I realized that if you are balancing to within 2/1000v, balancing stops when the low cell gets within that tolerance. A low cell never catches up with the higher ones.

Yesterday i got up early before solar charging and shut my power system off... all but the bms. I let the battery sit until voltage stabilized, then i manually balanced the cells by hand with 10ohm electronic resistors... This took a while....???... Then, i figured i should just manually balance the pack through the entire charge and I gave the 2 lowest voltage cells a slight advantage over the other 14 that were all nearly perfect for each other.
My #1 cell had the lowest voltage with a cell diff of up to .015 of top cell after nightly discarge at 50% DOD. Now, it is amazing to watch discharge as the #1 cell is highest right up to a moment before it drops 1/1000v. The other 15 cells fall like dominos after that with the last one being cell #16, then #1 is highest voltage cell again... And in case you didn't guess, cell #16 was the other unbalaced cell.

 

[BarkingSpider]

I’ve found you can’t always apply basic logic to LifePO4 cells and their voltages in relation to their ability to provide power, especially older / used cells.

The cells that are lowest voltage for me when my bank is fully charged are often the highest when I’m at 90% discharged. Also the lowest voltage cells when the other are fully charged often take longer to catch up, and if you let them, then your results come out different again. So cells often charge / discharge at different speeds. The voltage is not always an indicator of capacity.

Even with my huge BYD (640a/h at 24v >= 15kwh) the tiny Battgo cell balancer does wonders with balancing cells, I leave it running all the time. My capacity has increased by using it. It reduces even a 500mv variance to less than 20mv in less than a day.

Im closely watching this by recording all metrics of my system 24hrs a day and graphing the results. See my other posts if your interested.

Im beginning to wonder if the best approach is just build a big LifePO4 bank and just run it between 90% and 10% and not worry about the cell voltages outside of a weekly check. It seems to take care of itself. Just my opinion. ;-)

 

[MisterSandals]

The cells that are lowest voltage for me when my bank is fully charged are often the highest when I’m at 90% discharged.

There is actually a good reason for this: those cells have more capacity.
They store more energy so it takes longer to charge (will show lowest voltage when others are fully charged). Similarly, they have more capacity so they discharge the slowest (will show highest voltage when <entire battery> is 90% discharged.

 

[BarkingSpider]

Good point. That makes perfect sense now you say it. Your battery bank is like a chain, its only as strong as the weakest link. Taking care of that weakest link pays dividends!