how often do i recharge my 18650 batterys in storage?

i have many stored in plastic 18650 cases. how often do i need to recharge them?

I only have 4 for 2 hosts. About every two months or so, I use up one of the cell's capacity, rest it, charge it, and place it ready at the end of the my rotating 18650 line. So IOW, I'd like to know, too.

if you Must keep them charged and they don`t have a protection circuit on them, then check on them every couple of years. I had one cell fully charged that got completely ignored in some camping gear for over 4 years, took 20 mins on the charger to top it back up!

I read they are best stored at 3.65-3.75V if not going to be used for awhile.

Often enough so that they are **certain** to stay above say 3.2V

Do not store at higher than 3.7V or so

30-40% SoC is a good starting point

Start off checking monthly, scale back over time as you get familiar with the actual discharge rate

Cold temperatures helps a lot, also for longevity

What is "awhile?" A month? 6 months? 2 years?

Something just occurred to me. I don't doubt the advice, and I follow it. But why would long term self-discharge of a full Li-ion battery adversely affect it in any way? Isn't self-discharge the same as an extremely low amp draw? Are such extremely low amp draw applications not suitable for Li-ion?

Generally speaking charging up batteries to full charge just for them to sit is going to do more harm than good. It is best to leave cells charged to about 50% capacity or ~3.6V if they are going to be sitting for along period of time.

Charging cells, especially charging them quickly, is ultimately what wears most on a battery's life span. "Topping" cells off is probably the worst practical thing you can do to a battery.

If I know I'm going to leave an 18650 in a light for along time, I will probably only charge to 4.0 or 4.1V since you drop from 4.2V massively quick even in 21700 cells. Any typical high quality cell from brands like LG, Samsung, Sony, or Panasonic will drop at most 10% of charge over the course of a year from fully charged.

Please note as I say batteries here I'm thinking specifically about Lithium ion.

chillinn said:

.... Something just occurred to me. I don't doubt the advice, and I follow it. But why would long term self-discharge of a full Li-ion battery adversely affect it in any way? Isn't self-discharge the same as an extremely low amp draw? Are such extremely low amp draw applications not suitable for Li-ion?

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EDIT (see discussion with @HKJ below) ...

Li-Ion cells will eventually self-discharge to an unsafe voltage. Even "protected" batteries are not immune from (albeit slow) self-discharge.

archimedes said:

They will eventually self-discharge to an unsafe voltage. Even if "protected" those circuits may not trip the over-discharge detection, because the drain is so low and slow.

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actually With protection, even in storage you have to check them more often because the circuit itself uses power, that`s why I mentioned them in my first post. Unprotected and out of circuit (in a plastic box for instance) they`ll be good for years.

archimedes said:

They will eventually self-discharge to an unsafe voltage. Even if "protected" those circuits may not trip the over-discharge detection, because the drain is so low and slow.

Click to expand...

Right, but then why not leave them at full charge, and instead store at 3.7V? Wouldn't a full charge give them the ability to store longer? I always took the advice to leave them at 3.7V for storage at face value... should have asked "why" earlier.

chillinn said:

Right, but then why not leave them at full charge, and instead store at 3.7V? Wouldn't a full charge give them the ability to store longer? I always took the advice to leave them at 3.7V for storage at face value... should have asked "why" earlier.

Click to expand...

From what I understand it stresses the cell and lowers the amount of charge discharge cycles so does heat, for instance a cell charged to 4.2v will give you roughly 300-500 charge cycles, But... if you charge them to 4.1v you get about 600-1000 cycles!
So you can do it, but it will reduce the active life of the cell, charge it to 3.9v and you can get about 3000 cycles out of it!!!! and when you look at the dischage curve of Li-ION they spend most of their time delivering power at 3.7v anyway, so it`s no great loss only charging to 4.1 or 3.9 even.

edited to add: I found this, it may be helpful; https://batteryuniversity.com/index.php/learn/article/bu_808b_what_causes_li_ion_to_die

I charge them to 3.75v, then put them in long-term storage. I check them every 2 years. They lose very little voltage over that time, so I suspect I could leave them for over 5 years no problem.

The total amount of time spent at high SoC should be very low for all LI chemistries, as a lifetime percentage.

I would not let my cells sit near Full more than a few hours before discharging.

If a big race required doing so overnight once a month, that's not so bad.

_____
The **process** of self discharging is NP, nor are extremely low amp draw applications.

You just must avoid allowing that to continue until the cell goes dead flat, even just once can destroy the cells, or t leas take away 90% of their useful lifespan.

Yes, yes, and yes.

High SoC (most especially at elevated ambient temperature) is bad for capacity.

Low SoC will eventually drain, with or without protection circuits, into unsafe voltage.

If they have been too low, for too long, when recharged they may go

archimedes said:

Even if "protected" those circuits may not trip the over-discharge detection, because the drain is so low and slow.

Click to expand...

Protection circuits do not have an special circuit to disable them at low current, they work at any current!
But they have nothing to do with self discharge, except they increase it a bit due to their current draw, that is very small.

Generally checking a battery once a year is more than enough.

I know about the ideal storage voltage of around 3,7V. But I just have too many batteries to keep track on so my batteries usually are more or less fully charged. And especially with Li-ions(because of the very low self discharge) I usually never need to check out that they don't become depleted. Yes, it has happened in a light because of parasitic drain.

Saw a germane article linked from Phys.org this morning, Predicting the Slow Death of Lithium Ion Batteries


I'll leave it there, but unfortunately it is thin on details, just talks about an algorithm and a model, doesn't explain what it is. Sorry, I posted too fast.

HKJ said:

Protection circuits do not have an special circuit to disable them at low current, they work at any current!
But they have nothing to do with self discharge, except they increase it a bit due to their current draw, that is very small.

Generally checking a battery once a year is more than enough.

Click to expand...

I did not mean to imply that protection circuits are intentionally disabled at any current draw, instead that they do not prevent self-discharge.

archimedes said:

I did not mean to imply that protection circuits are intentionally disabled at any current draw, instead that they do not prevent self-discharge.

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Then please write it to not start that old idea again.
I once did a test to show that protection also works at low current: https://lygte-info.dk/info/DischargeProtectionTest UK.html

HKJ said:

Then please write it to not start that old idea again.
I once did a test to show that protection also works at low current: https://lygte-info.dk/info/DischargeProtectionTest UK.html

Click to expand...

Excellent article, hadn't seen that one. Thank you for linking it.

archimedes said:

.... Li-Ion cells will eventually self-discharge to an unsafe voltage. Even "protected" batteries are not immune from (albeit slow) self-discharge.

Click to expand...

Hopefully this is clearer.