Cell storage

100kwh-hunter

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Mar 2, 2019
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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 aadjustable 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 storagevoltige again.

Thanks in advance, best
 
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.
 
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
 
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.
 
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.
 
Dallski said:
................... 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
 
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.
 
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 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.
 
BlueSwordM said:
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.

Do you know the scientific reason why? Everything I have seen restricts depth of discharge to like 30% or so, but what we have seen is that low voltage is actually good for the cells when talking about storage.

Oh and lucky find, kc8adu!
 
yes that was a dumb luck find.
came to the auction for the spare cnc controls i knew they had and got their cache of new laptop packs they never used and left sit in the supply room.
i have found that <50% charged and low as possible temp short of freezing is best for shelf life.
 
@Dallski, there are 2 factors that hurt lithium ion's cell capacity over time.

1. Temperature.
2. Storage voltage.

Since lithium ion cells are chemical batteries, the main 2 factors affecting storage capacity are the ones noted above.

Higher temperature means parasitic reactions occur faster.
Higher voltages means more of these parasitic reactions can occur(ions passing through the separator, a semi-permeable membrane), and self-discharge, and capacity loss, is higher over time.

The main reason for keeping around 30% charge is so that you can actually use them. At 10% charge when in storage(3,3-3,5V), it's almost nothing.

The lower the initial storing voltage, the lower the actual self-discharge, and the lower the damage to the cell.

That's why storing cells at full voltage isn't a good idea. The voltage may go down over time, but you may permanently lose capacity, and it'll lose voltage more rapidly compared to the other one, and if left for long enough, can go down in voltage further than a 3,4-3,5V stored cell.
 
Thanks for the explanation. So apparently, according to Paul Voelker's research as cited by Wikipedia, storing at >3.6V initiates electrolyte oxidation and induces SEI layer formation, while storing at less than 2V degrades the cathode and releases oxygen.

So there you have it. Store cells between 2V and 3.6V for best results.
 
Great read!
I had to read the papers twice to fully understand.
It explains a lot why and how to storage at a certain voltage.
So i will stick with my 3.45 volts.

Thanks all
 
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