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Results of Capacity Testing 18650s for 800+ Cycles
#81
Wolf,
If the current per cell is lowered, is there a point that the interaction that you talk about between the cells no longer occurs?
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#82
(11-29-2019, 01:34 PM)Oz18650 Wrote: Wolf,
If the current per cell is lowered, is there a point that the interaction that you talk about between the cells no longer occurs?
@Oz18650

There will always be a certain amount of interaction when you have cells in parallel. Even when I charge 4 cells in parallel the small differences with IR come into play. I have charged cells on the same charger let them sit for 1 week and then inserted them into my analyser board with no load on them and have had the cells start to exchange electrons. Mind you we are talking ~50mA or so to start at moment of insertion so it's not a lot. This process of the cells balancing without a load can take quite some time as the interaction tapers off to ~5mA or so eventually becoming almost undetectable by the equipment that I have.

To answer your question no I do not think there is ever a point no interaction between cells ever occurs. Especially when charging /discharging.
Different chemistries and IR will see to that.
The best way to think about it is you have a 2 resistors one has a low resistance and the other has a high resistance you put them in parallel.
The current difference between the 2 resistors will always be the same no matter what voltage or load is on the other end.

I can put together a test of 4 batteries 2 with a decent IR and SOH and 2 with a higher IR and a poor SOH and discharge at say 100mA per cell and chart that.
If you want I can also do a second test at 50mA per cell and chart it.
It will be a good experiment to either prove my point or debunk it. Big Grin
 
My bet though is on the interaction being just about as dramatic as a higher current draw just going to take a lot longer.

Wolf
If 18 X 650 = 2200+mAh then we have power! 
May all your Cells have an IR of 75mΩ or less Smile
Last count as of 8/7/2019
Total Number of Cells Recorded and processed                 6149
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2760
For Info Google Drive
Not your average Wolf       
            Cool
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#83
My take on all this is more at the macro level (large solar system battery).   I'm sooooo committed $ wise to 18650...   I need at least 15yrs to have any chance of this paying off $ wise.
Of course its not all about $ but still - it would be nice to get my money back.

I'm getting close to ballooning up my 780ah 18650 batttery bank to 1300ah..  with the goal of keeping daily discharge to 30% DOD in the middle range of the discharge curve.  As the years (hopefully) go by and the battery weakens, I hope to gradually expand my use of the discharge curve to maintain that original 30% power capability.   So even when the battery gets to 50% of its original capacity, perhaps I can expand the range all the way to 4.1 -> 3.3 (or something) and still extract that 30% power (of the original battery strength) to keep the system running.

Its unclear (to me) if it matters wether I'm in top/middle/bottom of the 'middle' of the discharge curve - so for now, its trending more at the bottom - cut-off is just ahead of the discharge curve knee which keeps the hi charge voltage as low as it can be on a daily basis. Not sure if I'll see the top voltage go 'up' as the battery degrades? or maybe I'll have to bring the cut-off higher? Not sure on any of this as the battery is not yet showing degradation that I detect.

It could be that individual pack fail will become predominate as apposed to the 'overall battery' as there will be 70 packs. Not sure on any of this.

This will be a very long experiment (I hope).  But for me, I've come to define my own 'long life' experiment to mean - how long can I get 30% of the original power out of the battery bank.   
1 (of 5) batteries has 486 cycles.
2 and 3 (of 5) have 308 cycles
4 and 5 (of 5) are in progress.

I'm measuring the ah/v of discharge after the PV array goes to 0v in an effort to detect degradation of the 'live system' - but the ah/v seem to vary, maybe because of load or temp.   So far the yearly average is 78ah/v over 213 minutes and 32.1% DOD daily discharge.   Maybe this measurement will pay off in future years.

Its such a long term issue Smile
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#84
So make a entire string with those low ma or low soh cells, is also not a really good idea?
To enlarge your packs in one string with the "low ones", i understand that interaction, but with a complete string.
You must built those packs larger so every string is in the same amp range.
Wouldn't this work?
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#85
Crazy! Kudos for keeping this going! Never expected to have such a quick drop off! Now we know how a battery would perform towards its end of the life. I'm assuming the LG battery would probably follow such a curve at some point in the future, so maybe we can extrapolate some sort of life expectancy when you plot it against it's rated capacity. Too bad you didn't run this on a new LG battery!
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