this post was submitted on 16 Feb 2024
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About 4, I'd start long term storage from 80% because self-discharge rate is 30% per year in room temperature, or 15% per year in the fridge, which is the best storage temperature. Also, Battery University said in some article that 65% charge is optimal for storage, which is ~3.95V/cell at rest for most chemistries.
The reason I said 40-60% is because over that entire range both self-discharge and permanent capacity loss happen at their slowest rates because that is the flat range of the voltage curve where the cell is close to its nominal 3.7V voltage. The self-discharge with starting at 80% will maybe buy you an extra couple months before the cell becomes unusable, but you would experience more irreversible capacity loss.