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Results of Capacity Testing 18650s for 1,600+ 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
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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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#86
Hi Generic,
I would be happy to send you some LG cells for your cycle testing I you would like.
I have some packs that I am pretty sure should have had zero cycles. The packs also have a date for when the packs were made. They may be good for testing "as new" cells.
Let me know if you are interested.
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#87
(01-28-2019, 05:54 AM)Generic Wrote: **Go to the bottom of this post for my most recent results** 805 cycles as of 11/24/2019

Can you post the raw data that you used to make the graphs?
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#88
I've finally hit over 1,000 cycles. Took 53 weeks, but here we are! And sorry for not updating at 900 cycles, I had everything ready, but it was right as the New Year hit, and I never got around to posting it. Right around 900 cycles is when the THLD cell ran out of steam, so I ran that one up to Cycle 922. Cycle 923 was at 500mA and it tested at 190mAh; Cycle 924 was at 300mA and it tested at 300mAh; and Cycle 925 was at 200mAh and it tested at 402mA. Anyway, here are my usual charts:







Also, I know someone asked if the cells were self-discharging. I have been taking measurements of the voltage of the CJ and THLD cells every 25 cycles and there's not much of a story to report. As of February 4, the CJ cell is still sitting at 4.15 Volts, and the THLD cell is also at 4.15 Volts. They are not self discharging at all, despite being end-of-life cells.

I'm coming to the conclusion that the ASO cells are most likely OK to mix into a powerwall with LG, Samsung, Sony, Sanyo, and BAK cells. They seem to have decent Cobalt content, at least on par with LG's value cells. I'm also pretty sure they are not manufactured by Samsung, as is commonly believed. They are just too different with the positive cap and with the top cell crimp. Lastly, I have verified that they have both PTC and CID protections, so they are on par with genuine cells with regard to safety, capacity, and lifetime.
bogptrsn, Redpacket, rev0 And 4 others like this post
Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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#89
very nice and good work
Thanks, big thumbs up
Cobalt experiments it is, for my own new project ZcBr2 cells!
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#90
100kwh-hunter - I saw that post, good luck with that! I've subscribed to that thread, I think a lot of us might copy you if you are successful with it!
Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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