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 cellsstart 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 pointno 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.
My bet though is on the interaction being just about as dramatic as a higher current draw just going to take a lot longer.
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 dischargeafter 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.
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?
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!
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.
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 tosafety, capacity, and lifetime.
The graphs are very interesting.
It's great that the good cells are still at 80% capacity after 1000 full cycles.
Shows good promise for those building powerwalls which will be less stressful on the cells.
I have cells I have stored at various states of charge, just by chance (if I noticed the discharge part was done I pulled it off the charger), so I'll be retesting those in some time to see how that affects capacity.
If you have the results for tests 101-105, it would be interesting to me to be able to compare the rate of degradation over time. Ie. Is it a straight line from test 1-5 to tests 201-205, going through tests 101-105? OR does the rate of capacity drop increase (or decrease) over time? Tests at 300, 400 and 500 cycles would really show how this progresses, but it is a lot to ask. I think this would help people to decide how important SOH (based on current capacity compared to original capacit) is when deciding on using cells. Some people only use cells with at least 80% of original capacity, while others use a different % of original capacity, while others use "must have at least 2000mah capacity", while others use "must have at least 1200mah capacity". All of these approaches seem to work, however I don't think there is much info on " How will the resulting packs last over time".
I have done storage test and i would say that normal storage is less than 6% degradation per year. As specs from manufactures and other tests.
I had like 2% degradation per year in average on the cells i tested.
Good job!! It'svery interesting data. I am doing similar tests with Liitokala NCR18650B cells (from aliexpress). In my case, I have built a tester that performs automatic charge and discharge cycles with Arduino. For the charge I use a TP4056 (1A), and for the discharge a resistance that I have made (2A approx.). Every 30 cycles I perform a test with the OPUS to measure the capacity of the cell. For now I have 180 cycles. I leave you a graph with the result:
Update time! Sorry I never got around to posting at 1100 cycles, I had everything ready, but didn't get a chance to log in here and post it. Here is the degradation chart, at least:
So, I have made it past 1200 cycles! Right at Cycle 1175, the Opus fan finally gave up (it was moving, just not enough to remove much heat), and you can clearly see a spike in the results. I have several replacement fans on hand, so it didn't cost me any time.Otherwise, it's been pretty uneventful. The THLD cell is sitting at 4.13V and the CJ cell is sitting at 4.14V after almost 4 months, so they are not self-dischargers, despite being end of life. I'm not going to be checking those voltages anymore. The ASO and LG cell are continuing their very slow, almost linear, decline. On the one hand, I'm wishing they would just die already so I can end this test and not feel guilty about ending it early, but on the other hand, I'm rooting for these cells to keep going because the results of this test are so much bigger than the fate of 2 random cells. Anyway, here's where we are at:
And the degradation:
@Diegobg - Nice work! The 2 Amp discharge is putting more stress on the cells, so it looks like they are degrading faster, about 9% loss in capacity, right? Are the cells heating up at all?