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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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    \$\begingroup\$ Which part of "should not be deep discharged" is confusing you? \$\endgroup\$ – Dave Tweed Apr 10 '15 at 13:35
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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 '15 at 21:28
  • \$\begingroup\$ As I understand it, the copper dissolves when the voltages is low enough but the dendrites only forms during charging. \$\endgroup\$ – Dejvid_no1 Apr 10 '15 at 21:31
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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 '15 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 '16 at 21:22
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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 '18 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 '18 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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