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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?

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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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    \$\begingroup\$ So this copper keeps on dissolving more and more even when the (discharged) cell lays on a shelf? \$\endgroup\$
    – Vi0
    Apr 10, 2015 at 21:28
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    \$\begingroup\$ So the dissolution keeps going because of state of being undervolted, or because of the act of reaching low voltage? \$\endgroup\$
    – Vi0
    Apr 10, 2015 at 21:35
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    \$\begingroup\$ So would it be correctly assessed to say that it is not dangerous to deeply discharge a Li-Ion battery, but only to attempt to charge it again? \$\endgroup\$
    – Dolda2000
    Dec 17, 2016 at 21:22
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    \$\begingroup\$ Yes, it is dangerous to attempt to charge a deeply discharged Lithium battery. Most Lithium charger ICs measure each cell's voltage when charging begins and if the voltage is below a minimum of 2.5V to 3.0V it attempts a charge at a very low current . If the voltage does not rise then the charger IC stops charging and alerts an alarm. \$\endgroup\$
    – Audioguru
    Aug 15, 2021 at 13:51
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    \$\begingroup\$ @Dejvid_no1, sounds like you are presenting guess work instead of scientifical facts, or you are negligent in how you express yourself. First you say "no, [not] at all" — it's never safe to fully discharge. Then you go on to state that problems happen "during charging" — which is a different activity. Finally you claim that a "deeply discharged battery have higher self-discharge", which at this point even my uneducated brain has to rule out as just plain illogical. An empty battery should have nothing left to discharge, i.e., zero self-discharge. \$\endgroup\$
    – Martin Andersson
    Aug 12 at 16:11
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Bigclive says there won't be any internal short circuit before the cells have been reverse charged to -12% which equals to about negative 2 volts. It will grow nasty copper spikes if the cell gets pushed beyond -12% (-2 V), then it's dead-dead.

The research he found

The main concern is with series connected cells: they're not perfectly balanced, so when one goes flat the others will reverse charge it and potentially kill it or at least weaken it.

Of course, they're not going to be happy about being drained to 0. This just says that it's a lot less worse than what was previously believed.

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    \$\begingroup\$ Why the downvote? \$\endgroup\$
    – Vi0
    Mar 12, 2018 at 16:55
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    \$\begingroup\$ Honestly, this answer isn't very on topic for this question. And perhaps someone thinks the less bad stuff that happens before this really bad stuff, is bad enough to be your primary concern? \$\endgroup\$
    – Oskar Skog
    Mar 13, 2018 at 13:24
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Deeply discharged Li-Ion won't last a year, especially in storage where large ambient temperature changes are possible. It is recommended to store Li-Ion half-charged, to prevent "overcharged state" (i.e., when fully charged cell cools down to below 0C).

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The batteries have protections for over and undercharging, check you battery model if it has these protections. If yes, it is safe. Li-ion batteries are very slow in discharging when not in any device, which may drain it. But it won't drain below the protection. If you have a voltage meter, and feel unsure, you can check that there is a small charge for safety. There are issues which may happen, if really emptied, like change of currents way due to "short circuiting".

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