It is well known that Li-Ion batteries should not be deep discharged.
But sometimes they do discharge deeply. Is it OK for the device to remain in such state for a long time (and recharge again only when the device is needed again after a year) or it should be charged back as soon as possible?
In other words, the battery was discharged deeply. Now I need to know the best way to prevent further damage to the battery. Should I recharge it immediately or leave it in a deeply-discharged state until I need it again?
Does deeply discharged battery have higher or lower self-discharge compared to normally charged battery?
Answer
No, it is not OK to have a Li-Ion deeply discharged at all.
Here is why: When discharged below its safe low voltage (exact number different between manufacturers) some of the copper in the anode copper current collector (a part of the battery) can dissolve into the electrolyte. The copper ions (atoms?) then in turn can stick on to the anode during charging by chemical reduction and cause dendrites. The dendrites might cause a short circuit inside the battery. So basically discharging too much is as bad as charging too much. But the dendrites caused by overcharging is formed out of lithium.
Normally the battery pack should have some sort of supervisory circuit that disconnects the cells from the charger or load when the cells are above or below the recommended voltages.
Question 2:
Does a deeply discharged battery have higher or lower self-discharge compared to normally charged battery?
A deeply discharged battery might have a higher self-discharge due to the above mentioned damage.
From what I can see in the data sheet provided by a large manufacturer (under NDA) the best relative (%) capacity retained is at somewhere around 50% charge and at low storage temperature.
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