When not to rescue a pre-used 18650 cell? What low voltage

[brumbrum]

At what low voltage is it bad to revive a laptop 18650 cell?

What can happen to a cell where the voltage has dropped below a certain point? And what can happen to it when trickled charged back to 4.2v?

Anyone understand this kind of thing? I don't, hence the questions.

Thanks.

 

[liveforphysics]

At ~2.1v, the battery becomes a copper ion battery and starts eating the current collecting foil of the anode while its continuing to have voltage. Upon recharge, these copper ions in the electrolyte will be driven from the solution as the voltage goes above 2.2-2.5v, and may form metallic copper dendrites that can piece the separator and cause internal shorts that range from being minor self-discharge annoyances to fireballs.

 

[brumbrum]

Woo, that's a serious answer. Thanks for the pointers. 2.2 it is then.

Thanks

 

[999zip999]

2.5V or better above.

 

[arkmundi]

Voltage on full charge is no indicator. Its amperage on discharge that must be measured. So hooked up to good multimeter during a discharge test. The internal drain will be obvious.

 

[999zip999]

If 2.5v or. below it's best for the recycler. To start. Meaning much work ahead in testing.

 

[Spicerack]

I used 3V as my cutoff. If I were to revive a 2.5-3V cell, I'd want to cycle it a few times and check for heating upon charging, self discharge rate, max voltage and IR.

My 360 cell (used laptop cells) battery pack (15S24P) has been going well for more than two years now and very rarely gets balanced (once so far). I bulk charge with a meanwell set at 62V.

 

[brumbrum]

I have built a number of packs with 18650 cells, some are fine and keep their charge and dont get warm on use, some get hot spots on discharge, so i do not tend to use them. I have an impedance meter but all it seems to tell me is the cells i have tested are all around 30 Mohms.

I have a couple of hundred but charged a lot of them to 4.2v without taking a voltage reading before hand

 

[liveforphysics]

When you have selected cells, charge to 4.15vdc and keep there for 24hrs with a contant voltage supply. Ensure the cell is at a stable temperature for this process. Remove from the power supply, wait 2 hours and record the voltage with a precision tool voltmeter (at minimum 4 digits). Store at a consistent temperature for a week and record voltage again. Plot the self-discharge differences and cull anything that isn't extremely low. Self-discharge is a strong indicator you do not want that cell in your pack.

 

[brumbrum]

Thanks very much for your time and advice.
Cheers.