I have an ancient Lithium Ion power bank here, the USB type you might use to charge a phone. It is optimistically rated at 20,000mAh new.
It had been "misplaced" in my cave, for several years. I found it with minimal remaining charge, but not stone dead. When I then recharged it the first time, it would only accept about 1,000mAh. So I thought great, it's almost dead, it might work to power an Arduino or something like that... so that's what I put it to use for. The use case means it gets drained until it shuts down, every time, before getting recharged.
Except... it has powered the Arduino longer and longer... every time I recharge it, it seems to accept more and more juice. With the last recharge, it accepted 10,397mAh... which is probably getting close to whatever real capacity it had in the first place...
I knew that new Li-Ion batteries have to go through a few cycles to build up to full capacity... but I didn't think this was something the battery could "forget" again?
Is it expected that Li-Ion can "recover" some capability after standing around, with a bit of "massage" like this?