How conservative should I be with self discharge?

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green1

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I'm wondering if I'm not being conservative enough in my resting voltage test. Whenever I search for how much voltage loss is acceptable when sitting I see lots of people say it's important to check, but not a lot of agreement on how much is too much. (Yes I know there are other posts like this, but I'm still not seeing much in the way of good guidelines on this subject)

Currently I test at 6 and 12 days, and I discard anything that's dropped below 4.000V But I see that others use4.1 or4.05, Am I setting myself up for failure using 4.000 after 12 days? Once I account for other factors, and voltage drop,I'm keeping 61% of my harvested cells.Raising to 4.1V would drop that to 47%. I also have to think that if I extended to 30 days (which some advocate for)I'd be ditching more cells as well.

Obviously I want to keep the most possible cells, but I also don't want to end up with a problem pack at the end. Do I need to re-think this and get rid of more of these cells?
 
You just need to be consistant with your testing methods. I find charging in parallel on one charger to be optimal. For example I use a skymax clone with a battery tray that will charge 12 batteries in one shot. This brings all the batteries to the same voltage consistently every time to 4.2V. If I try using the Opus, it varies from charger to charger (i have 3 of them) and sometimes it goes up to 4.18, sometimes it goes up to 4.2V. There's a margin of error in between the Opus by 0.2-0.3V. So I only use the Opus to discharge test only.

Then I know I have a 4.2V across all my charged batteries. I leave them for 1-2 weeks. If they drop below 4.15V I put aside. Now they shouldn't drop any more after that. Ie. if in another week it drops down to 4.10V or 4.00V then I know they're no good. Typically they stabilize and never drop any further. The rate of discharge of lithium batteries is quite slow. I've had batteries stay over 4.15V after 2 months easily.
 
Consistancy. Note that if you dont spread the self dischargers evenly you will get imbalanced packs.. then you need even better balancer.

I do 4.1 after 1 month. U dont want any cells that have degraded to much. As simple as that. I keep around 40% of tested cells at MOST.
 
@not2bme I get the consistency in testing, and I do try to be consistent. Using your 4.15V rule after 12 days would keep only 24% of cells, that's a big hit (and the most conservative number I've seen yet)

I'd love some more opinions, but I'm really starting to get the feeling that my 4.000 rule is a bit too lax, and I really need to up it some, the only real question is by how much.
 
green1 said:
@not2bme I get the consistency in testing, and I do try to be consistent. Using your 4.15V rule after 12 days would keep only 24% of cells, that's a big hit (and the most conservative number I've seen yet)

I'd love some more opinions, but I'm really starting to get the feeling that my 4.000 rule is a bit too lax, and I really need to up it some, the only real question is by how much.
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I charge using OPUSs. After a few weeks, typically they go to 4.1+'ish on my volt meterand sit there. Obviously the OPUS voltage is not 'precise' and my personal voltmeter may be off compared to yours. But in general - the cells will reach a voltage such as 4.12 or 4.15 and then will sit there for weeks... and not significantly move.

I would say is not so much the 'exact' voltagebut the fact that they sit for several weeks at same top voltage measured with same equipment. In general, most of us see 4.1+ stabilization after full charge for many weeks with no change. So full charge -> 4.0v after just a couple of weeks might indicate some self-discharge. Just check again in another 3 weeks and if its still 4.0v then OK but if its 3.95 then not so good.
 
The Opus Charge all from 4.15 to 4.25 so its not valid to say exact 4.15 either. But i would say its quite a difference between 4.0 and 4.1 though. For instance a cell charged to 4.2 v generally stops at 4.16-4.17v after 24h.

All cells have self discharge and the energy discharged is quite consistent on new cells but the voltage dropped isnt.... Why?? Because the capacity curve ;) so a drop to 4.1 v is not same between 2 brands either in terms of wh.

When starting to look at this you Will soon realize that combining 2nd hand cells gets very complex IF you want to raise the bar.... :)
 
Charging is using LiitoKala but I don't use it for voltage measurement I have a dedicated volt meter for that. Charged voltages range from 4.17 to 4.21. All cells are Sanyo, 2 different models (UR18650V and 18650E) which do have different "nominal" voltages (3.7 vs 3.6).

I think I'll bite the bullet and go to 4.1 and toss a bunch of cells, but be more comfortable that my end result will be ok.

Next I have to debate what percentage of new capacity (mAh) I'm willing to tolerate (I have some measuring 100%, most are at about 80-95% but I have some that are only about 50%.) Apart from lower capacity, and all other things being equal, any reason to fear those cells that are only 50% of their new capacity?
 
All below 60% are to be considered dead based on manufacturer datasheets. Or atleast thats how you can intepretate it.

Feel free to look up above to see what they say but i generally dont use below 80% for something that should last several years.
 
daromer said:
All below 60% are to be considered dead based on manufacturer datasheets. Or atleast thats how you can intepretate it.

Feel free to look up above to see what they say but i generally dont use below 80% for something that should last several years.
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What I find odd, is that the newest cells are the ones with the lowest capacity (as a percent). The UR18650V cells seem to be from 2011-2012 and are mostly in the 80-90% range, the UR18650E cells are all from 2015, and so far not one of them has tested over 75%, and most of them are in the 50s. All cells pulled from service within the past few months, and all from the same application and usage patterns. I'm starting to think Sanyo's newer chemistry just isn't as good as it's older one...
 
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