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Cell storage
#1
Hi guy's

What is the proper voltage for a 18650 cell for in storage?
I don't think i will have my wall up and running before the end of this year.

I thought it was 3.5 volt, but someone (i really can't remember who, sorry) told in a post that it was 3.7v.
At 3.7 volt he stated: at 3.7 volt the cell chemistry is at his perfect balance.

The storage temperature will be around 18-20 degrees celsius.
The last stage of my testing board is equipped with a adjustable psu.

I have already a ~1000+ tested, ready to use cells stored at 3.5v and ~250 at 3.7v.
When i tested "ready for use" a 2800 cells i will begin to build my packs, and give them a charge to 4v, discharge to 3.1 and charge up to storage voltige again.

Thanks in advance, best
Still learning English. Learning Li ion and solar technology.

2800 cells in packs Exclamation above 2500mah and 90%soh.
~400 cells between 1800-2500 and above 90%soh(sell?)
600 cells between 2200 - 3000mah, 80-90%soh (sell?)
3000 waiting for testing.
approx: 0 cells not safe for use: dead, heaters, to high ir or sd/vamps

Time is our enemy, must work to, the sun is our friend, must relax to.
With best regards
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#2
3,4-3,5V is the absolute best voltage for storage only.

The lower the storage voltage, the lower the capacity loss, and the lower the self-discharge.

I've had cells sitting in storage at 3,5V since about 2016, and they are still at 3,5V, while the 4,2V cells have gone down significantly in voltage, and have probably lost a bit of capacity.

Also, try to keep the temperature as low as possible. 10-25 degrees celsius is about perfect.
The power of lithium ion is in our hands!
We'll show them what we're made of!
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#3
Balance voltage for Li-ion is 3.7 V, for high power cells is 3.6 V.
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#4
I think i messed up a bit?
There seems to be a difference between balance and storage?
So to balance your cells or packs its 3.7v and for storage it would be 3.45 volts.

Sorry for the inconvenience, i did not want to start a fight.
I was referring to storage, balance will come next year when i will put the packs together.

Thanks
Still learning English. Learning Li ion and solar technology.

2800 cells in packs Exclamation above 2500mah and 90%soh.
~400 cells between 1800-2500 and above 90%soh(sell?)
600 cells between 2200 - 3000mah, 80-90%soh (sell?)
3000 waiting for testing.
approx: 0 cells not safe for use: dead, heaters, to high ir or sd/vamps

Time is our enemy, must work to, the sun is our friend, must relax to.
With best regards
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#5
Why would storage be better at a different than balance voltage ?

I mean the internal chemical balance of a single cell, not the balance of a pack.
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#6
I store my 3.6V nominal cells at 3.7V and my 3.7V nominal cells at 3.8V. It's kind of arbitrary, but that usually puts the cells at about 40-50% charge, which is what i heard from everywhere to do. If I were doing this all over again, I would do 3.5V like BlueSwordM recommends. That puts the cell somewhere between 1 and 10% charge. I dont really understand it, but after opening up several hundred new old stock laptop packs with an average cell voltage of 2.8V, the cells are like new capacity. But anything I find over 3.7V has taken a hit on capacity. The ones I find around 4V have lost 20-30% of their capacity.
Formerly known as Dallski
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#7
(06-07-2019, 01:22 PM)Dallski Wrote: ................... I dont really understand it, but after opening up several hundred new old stock laptop packs with an average cell voltage of 2.8V, the cells are like new capacity. But anything I find over 3.7V has taken a hit on capacity. The ones I find around 4V have lost 20-30% of their capacity.

If you look at this way a fully charged cell has full voltage potential between the anode and cathode.
There is basically a very thin membrane separating the 2. You have a negative side (cathode) brimming with electrons just waiting to be release to supply the positive (anode) sides lack of electrons. The electrons are in a virtual jail cell just waiting to bust out. The membrane separating them is under constant attack and the more electrons there are the greater the force. If you put constant pressure on the membrane it will give up in small areas and allow a small amount of electrons to flow through. Once a microscopic if not molecular sized hole has been discovered there is no repair so it just continues to degrade the cell. If the storage voltage is kept low, the potential attacking the membrane is reduced so that even the most aggressive electron cannot penetrate the membrane hence no degradation to the cell.

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                 6106
Total Cells required for PowIRwall                                   2856
Total Cells ≥2200mAh, ≥80%, ≥35mΩ, ≤75mΩ, ≥4.12V   2694
For Info Google Drive
Not your average Wolf       
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#8
Thanks Wolf! I was just referring to "commonly accepted practices" of storing batteries at 40-60% full. But in practice, I have found that storing cells at like 1-5% seems to work best for the real long term, like 5+ years. The generic manufacturers, at least, seem to build their packs with fully charged 4.2V cells. Within 2-3 years, the crap BMS drains the cells. Some BMSs are less crappy and completely cut off vampire drain once the cells reach 2.8V, which is the voltage most packs I find are at. The worse ones never stop drawing until the cells get to 0V. I'm pretty sure they use fully charged cells because a few packs I have will have 2 sets of cells at like 4V and a third set at like 0V, which indicates the third set was self draining right from the start, and cut off the BMS vampire draw right after the pack was built.
Formerly known as Dallski
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#9
The commonly accepted practices exist mostly for shipping and usual storage.

We want to keep our cells as healthy as possible, so a lower voltage is beneficial.
The power of lithium ion is in our hands!
We'll show them what we're made of!
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#10
the best nos cells i have gotten were from 6 year old hp laptop packs.
i bought 150+ of these packs at an auction.
these were all around 3.4v/cell.
they were Samsung 26f and all were over 2500mah and <60mohm.
had some with Samsung 22b that were 1.8v/cell that were 8 years old.
of 800 cells all were around 2300mah.
i paralleled groups of 10 pr and put the group on a tp4056 and waited.....
payoff was around 800 like new cells.<50mohm.
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