Testing for self-discharging cells

BarryG

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Feb 15, 2019
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Hi,

Sorry if this is a much-asked question, but I can't find a definitive answer:

Like most of you guys on here, I guess, I am processing 18650 cells recovered from laptop batteries (and also some from e-Bike batteries). After checking for obvious visible faults, near-zero volt cells, etc, I start the processing by charging the cells. I label each cell with the "fully-charged" voltage (which, by the way, seems to vary quite a bit - I am using a "home-grown" set-up for charging multiple cells at the same time using TP4056's). I then letthem sit for 3 or 4 weeks and then measure thevoltage of each cellagain.
SO - Here's my question......obviously if the cell voltage is still the same as what it was when "fully charged", then this particular cell 'passes' this test. However, if the cell voltage is far lower than the "fully charged" voltage, then I discard that cell. BUT.....how close do the two measurements have to be, in order for it to be classed as a "good" cell?

I'd be very interested in what others use as a "rule-of-thumb" to determine if a cell discharges too much - and over what time period. For example, I am currently measuring roughly 4 weeks after charging and discard cells if they measuremore than 200 millivolts less than their "fully charged" value. So for example a cell that was originally measured as being 4.10v and is then measured as being only 4.00v after 4 weeks, would be within my limits currently, and would be kept.

Should I be more aggressive with my discarding and only keep cells that have lost less than, say,20 mV over 4 weeks?

Thanks for any and all responses. If I get a lot of answers, I'm happy to compile a bar-chart showing what 'the community' use as their test thresholds.....and report back......
 
Basically if a cell drops below 4.1 after sitting for a month are probably not good for power-pack use. If they drop to 4.0 after 4 months, that's probably fine. But, that's about as low as you'd want to go over that long period of time. Anything lower than 4.0 isn't suitable for power-pack. Might be good for small projects, solar lights, toys, flashlights, etc.

Now, that's *my* rule-of-thumb, others will have different variables. I've had cells in storage for over a year, and if anything is below 4.0V, they get set aside.
 
I would not discard, but reallocate them for something else.
Yes, I keep my 4.1+ as premiums, but others that drop more may have good capacity too.

From my observations, high current ones tend to drop faster over time than standard current ones.
 
Korishan said:
Basically if a cell drops below 4.1 after sitting for a month are probably not good for power-pack use. If they drop to 4.0 after 4 months, that's probably fine. But, that's about as low as you'd want to go over that long period of time. Anything lower than 4.0 isn't suitable for power-pack. Might be good for small projects, solar lights, toys, flashlights, etc.

Now, that's *my* rule-of-thumb, others will have different variables. I've had cells in storage for over a year, and if anything is below 4.0V, they get set aside.

Thanks for the feedback, Korishan.......but from what INITIAL voltage? If you charge them to 4.2v, then that's a drop of 100mV in 4 weeks, but if you charge them only to 4.1v initially, then even a 10mV drop would mean that you discard them.

Also - does anyone else find that when using TP4056's to charge them, they don't all stop charging at the same voltage, but vary quite considerably? Some of mine stop charging at around 4.16v or 4.18v, but some stop below 4.0v


Overmind said:
I would not discard, but reallocate them for something else.
Yes, I keep my 4.1+ as premiums, but others that drop more may have good capacity too.

From my observations, high current ones tend to drop faster over time than standard current ones.

Thanks, Overmind - what would you consider to be too bad to keep, then? I'm trying to get some real figures here, so I need to know what drop you would use as a 'cut-off' for acceptable - and over what period of time.
 
You stated that you charged them up using the TP4056. They charge up to 4.2V +/- 1%. So I was basing it off those numbers.

However, a cell from any charge state above 4.1V shouldn't drop below 4.1V if it's a healthy cell over the course of a few weeks.

And because the TP4056 is a +/- 1%, yes, 4.15V - 4.25V is the range they generally stop at.
If they are stopping lower than 4.15V, then there's something wrong with that TP unit, or, the cell is dropping voltage "that" fast after disconnect. Some cells will drop from from 4.2V down to 4.1 or less within seconds and then stabilize. I've seen it with some of my own cells. I can't explain it, but it can happen. However, those cells I would not use in a power-pack application.
 
Don't measure the voltage right away after taking the cell off the charger. I would wait an hour then record that voltage. Let the internal chemistry settle.

As to what rule of thumb, that might depend on your application. But 2weeks and 200mV is what I am looking at. If they drop more than that I might look at them in 4weeks.
 
Jumping in with a related question

If 3.7 v is the standard for long term storage, why can we not look to verify voltage drop from 3.7

Meaning, is charging up to4.2 the only way to test or could you take the cell to 3.7 then let sit for 4 weeks

Tks
 
Because the drop is in capacity and easier seen att high voltage. Its also clearer sign of the soh of a cell att high voltage.

Think of it like this. IF One cell just drops 50mAh per week that isnt much right? At 4.2v that is visible. At 3.7it isnt....
Imagine you sort your cells with a spreadsheet and then by type of cell.... You then easily can get 80 of those in One pack and hardly none in another. Potentially many Ah per Day need to be balanced.

So you want to easily distinguish cells that could have an issue or in near future Will develop one

The price of eliminating cells with issues are cheap in the beginning of the build phase even though you might rosa cells you think is Good... Imagine the cost later One to swap out cells or packs or whatif that cell catches fire....


Yes it may not be likely that all cells that self discharge end Up in a pack or a fire but lets minimize it :)

Hope it explains the thought behind it
 
I charge all my cells to 4.2V.and do a capacity holding test after minimal one month. Every cell that goes below 4.1V will not go in my power wall. Everything below 4.0V will not even be considered in my clown packs.
 
Make sure you have a decent power supply.
If your power supply can handle, lets say 7 amps, then charge only max 6 tp's at one time.
If you do more, they will divide the remaining power between themselves, but:
If you charge with 10 tp's with a 6A psu 1 will not charge and wait till someone is done.

When i started 6 months ago i was testing them with a 7 amp psu, now i have a psu rated for 30A at 6 volt, he can do 45 tp/tc, but then you can cook on it, literally!

Most important thing i noticed also:
When they turn blue, they are not ready yet, they wait for 5-15 minutes (depends on how much your psu can handle), and get back into charge mode again, to fill the cell completely.
I witness this several times, with my versions of tp's and tc's, with a multi meter attached to it also.
Sinds then i give them some more time, the voltage measured after one hour to a day rest are more precise/closer to 4.2v

My rule of thumb: charge them good, put them aside for 4 weeks then discharge with testing to 2.8v, IF they are holding more than 4.10 volt, some use 4.11 as a border.
Charge them back up with a buck converter to 3.4 volt for long term storage.
Some cells get a "second chance run, i write down the remaining V(example 4.05v) discharge write the mah, charge them to 4.2v set them away for 4 weeks.
Some cells have to "wake up"

At 3.4 volt is inside the cell not much to self discard, at 4 volt there is.
Put them lower then 3.1 v aside the chemistry will get lazy over time.
Perhaps one of the forum members can chime in, that knows the story more precise, i know i am close.....or at least in the neighborhood
 
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