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If the voltage of a lithium-ion cell drops below a certain level, it's ruined. Lithium-ion batteries age. [Source]

As an owner of quite a lot of expensive mobile devices (lots of Apple stuff for example) I wonder... what if I, say, put my Airpods in a drawer at, say, 3% charge and "forget about them" for a few months?

Li-ion: 5% in 24h, then 1–2% per month (plus 3% for safety circuit) [Source]

Will my device be unusable? As I understand it I won't be able to charge it anymore?

Also: How does Apple (or any other manufacturer) make sure that devices in warehouses etc. don't "die out" during the time they're in there? I know that Apple devices usually come out of the box at least partially charged, but I assume Apple doesn't charge the devices to 100% anyway. But what if the charge drops below "0%" (or: the 'certain level' from above quote)?

Also: what is "ruined"? The above quote implies the cell being unusable, beyond repair, FUBAR. However:

The protection circuit turns off and most chargers will not charge the battery in that state. A “boost” program applying a gentle charge current to wake up the protection circuit often restores the battery to full capacity. [Source]

and

Some battery chargers and analyzers (including Cadex), feature a wake-up feature or “boost” to reactivate and recharge batteries that have fallen asleep [Source]

Would such a 'wake-up' be available in consumer products (laptops, phones, earbuds, whatever) or would this be put only in industrial stuff?

Lithium-ion batteries age. They only last two to three years, even if they are sitting on a shelf unused. So do not "avoid using" the battery with the thought that the battery pack will last five years. It won't. [Source]

Is that information outdated? I have had quite a few mobile devices over the past decade(s) that lasted much longer than that.

I'm trying to make sense of a lot of sources that all seem to contradict each other one way or another.

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  • \$\begingroup\$ Software reported "battery percentage" != state of charge. Still, keep them at 80 % charge (reported or real) for maximum life. \$\endgroup\$ – winny Nov 8 '17 at 14:33
  • \$\begingroup\$ Software reported "battery percentage" != state of charge Hence the quotes on "0%" and 'certain level' reference). I'm aware the reported percentage and actual charge differ; I'm talking about actual percentage/charge, not reported. Thanks for pointing this out though! \$\endgroup\$ – RobIII Nov 8 '17 at 14:36
  • \$\begingroup\$ There are variety of chemistries in Li-Ion family, and variety of technologies to make internals,foils/barriers. Plus electronic "protection" varies on the top of this. So the results vary. Voting to close as "too broad". \$\endgroup\$ – Ale..chenski Nov 8 '17 at 14:54
  • \$\begingroup\$ Check the StackExchange EE for keyword "overdischarge", you will find plenty of information and opinions. \$\endgroup\$ – Ale..chenski Nov 8 '17 at 15:03
  • \$\begingroup\$ Check the StackExchange EE for keyword "overdischarge", you will find **plenty of information and opinions**. That's the problem; I'm having trouble making sense of all the information and opinions and all the information I find only raises more questions. \$\endgroup\$ – RobIII Nov 8 '17 at 15:05
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If the voltage of a lithium-ion cell drops below a certain level, it's ruined.

That statement is a bit short sighted, it depends on the actual type/brand/model/chemistry (there are several Lithium based types) if this is true. According to tests by BigClive (sorry, I cannot recall exactly in which video he mentions this) some batteries can be completely discharged (0 Volts), charged again and then appear to still have their full capacity. Note that the "fully discharged" state did not last long (hours or days perhaps).

Lithium-ion batteries age

That is true but also depends everything I mention above and also how you treat the battery. For example in some military applications Li-Ion cells are charged not up to 4.1 - 4.2 V (like in many consumer products) but to 3.8 V or less. This places less stress in the cells making them last longer.

but I assume Apple doesn't charge the devices to 100% anyway

That will actually be the battery manufacturer producing the batteries in such a way that they are at around 40% to 60% charge when finished. They're not charged or discharged as that takes too long! The charge level can be set by using the proper ratios of chemicals during production. Same as non rechargeable batteries are made to have 100% charge.

If you want to store a device for some time, I suggest charging the battery to a value between 40% to 70 % (my rough estimate). Then if possible remove the battery!

If removing the battery is not possible make sure that the device is switched off and store it in a location where it cannot get very hot or cold.

To protect against fire and overcharging (when the charging circuit in a product breaks) nearly any device will have a battery protection circuit. This will simply disconnect the battery when the battery is in danger. I am quite sure Apple devices have this.

BTW, this does not protect against manufacturing issues with batteries which is often the cause of batteries catching fire.

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  • \$\begingroup\$ BTW, this does not protect against manufacturing issues Manufacturing issues aside, ofcourse 😉 Thanks for answer though! Much appreciated! \$\endgroup\$ – RobIII Nov 8 '17 at 14:48
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I don't think all spectrum of OP questions can be answered within EE site, but I'd like to address the following:

The protection circuit turns off and most chargers will not charge the battery in that state. A “boost” program applying a gentle charge current to wake up the protection circuit often restores the battery to full capacity. [Source] and

Some battery chargers and analyzers (including Cadex), feature a wake-up feature or “boost” to reactivate and recharge batteries that have fallen asleep [Source]

This is just an inflated piece of baloney. The situation is simple: when a battery gets discharged below certain limit set by protection circuitry (say 2.9 or 2.5 V), the circuit disconnects the battery output. The terminal would show "zero voltage", which looks like "dead". However the INPUT path is there. So if you apply a precharge current (usually 100-200 mA), the cell (behind the circuit) will charge up above the cut-off threshold of it, and battery "wakes up". There is no need in any special secret "wake-up" or "re-activation" tools to do so.

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