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I am going to store used li-ion batteries for some long time >1 year, and I have read that optimal storage conditions are:

  • cool ambient temperatures, like 2-5 degrees celsium
  • 3.82V voltage per cell.

What I cannot find is what is correct operation sequence to prepare a used battery for cold storage (which is initially in random state)?

  • Should I charge it to 4.2V then discharge to 3.82V?
  • Should I discharge it to 3.6V then charge to 3.82V?
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  • $\begingroup$ From what I remember, just bring it to 3.82V from wherever it is, direct path. If it's used, the only difference the full charge or discharge will make is taking one charge/discharge cycle off its lifetime. $\endgroup$ – SF. Jul 13 '18 at 10:22
  • $\begingroup$ @SF. I think there are more (maybe minor or practically insignificant) differences, which I dont know about yet. But I am asking out of curiosity, if someone knows real theory behind the scenes. $\endgroup$ – xakepp35 Jul 15 '18 at 20:25
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You should first "balance charge" the battery to full capacity using a suitable charger. Batteries do vary between manufacturers, and between battery types so check the manufacturer's rated voltage. Most modern chargers are smart enough to recognize the change of resistance as the battery charges to prevent over charging.

Once the battery cells maintain the same voltage, the battery is considered to be balanced and fully charged.

Now discharge the battery gently so it does not warm up significantly, until the rated cell "storage voltage" is reached. Older batteries may take several cycles to achieve a stable and even cell voltage for each cell or may not balance at all.

If an individual cell voltage/internal resistance is significantly different than the other cells, the battery can still be stored, but of course not at the manufacturer's optimum voltages. The cell to cell difference should be less than 0.1v.

You can keep the battery in a cool space at the storage voltage for more than 12 months.

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