Following a mobile phone charging issue I came across this intriguing discussion thread on reviving dead batteries and or devices.

It starts off with a phone but goes not to rechargeable tools (drill, chainsaws etc)

https://www.ukworkshop.co.uk/threads/how-i-charged-an-old-removable-mobile-phone-battery.146855/

I'm sure we could all learn something from this, for example DeWalt device battery terminal

It turns off permanently by design if the residual cell voltage drops below about 2.7v. The normal operating voltage is 3.2v - 4.2v. The 3.2v point is the arbitrary 'zero' point. At some point just below that, the battery is considered to have 'failed' and the circuitry disables it. If the voltage further drops below 2V, the lithium can turn metallic and form web-like structures, which can short-circuit the cell, heat up rapidly and go into thermal runaway. I've seen it twice and nothing can stop it. It's like a jet engine.

E2A: Here's the Battery University site's take on reawakening sleeping batteries:

Battery University

BU-808a: How to Awaken a Sleeping Li-ion

Li-ion batteries contain a protection circuit that shields the battery against abuse. Depending on the manufacturer, the protection circuit of a Li-ion…

Edited Saturday at 04:15 PM3 days by StringJunky

Yeah.
As Stringy says, these 'tendrils' that form internally, short out the battery and make it useless.
I've 'rejuvenated' old Ni-Cd cells and Ni-MH cells in the past, by using a higher charging voltage to burn the 'tendrils' and fix the internal shorting, but Li-Ion have so much protection built in ( some actually microprocessor controlled ) that it's virtually impossible, unless you open the battery case, take out the individual cells, and rejuvenate them individually before reassembling in the battery case.

Well I don't often link to other discussion sites, and I only found that one because I needed to fix a mobile phone, that was apparantly out of battery.

A replacement battery didn't fix it, so I looked for the ideas presented in that discussion concerning the charging circuitry and a loose ribbon cable.

In the event there was no ribbon cable, the micro usb was firmly soldered onto the one piece mainboard.

However disassembly and reassembly brought the old battery and system back to life.

But it still did not like the replacement battery - allegedy the same type number.

Interestingly when I tested the new and old batteries off load they both showefd 4.1 volts - a bit high for the nominal 3.7v.

~As I said there were many lesson for those who read the discussion, not least the need to keep Li-ion batteries topped up every few months.

My usage of portable power tools is diminished these days and rather intermittent and I do find the discipline prolongs active life PAL for those who rember the dogfood adververts.

Another thing to keep in mind ...
As you mentioned, Li-Ion batteries don't like overcharging, and if you keep your battery at 100%, or plugged into the charger for continuous periods, it will 'over-charge', and reduce your battery's longevity.
I know Samsung phones ( such as mine ) have a feature in the settings that only allows a charge to 85% to mitigate this problem.
Not sure about iPhones as I don't have one, but I've ruined many laptop batteries over the years, by keeping the plugged into the charger continuously.

Frequently I do revive lithium battery packs that refuse to charge by opening the case and measuring each cell individual voltage without need to dismantle. If any is noticeably lower or under 2.5V I apply 4V to that one for a minute and when its voltage has raised above 2.5V; it is sensed by the management circuit as 'just discharged' instead of 'defunct' and the recharging can resume.

Only when the application of voltage to a cell does not raise its voltage above the 'acceptable level' then the cell gets replaced to restore a pack.

7 hours ago, MigL said:

Not sure about iPhones

Supposedly i-phones cut off at 100% and don't begin recharging until it goes below a certain threshold (someone told me 75% but I couldn't find any trustworthy confirmation). The battery in mine doesn't seem to have changed significantly in well over 2 years of use.

31 minutes ago, npts2020 said:

Supposedly i-phones cut off at 100% and don't begin recharging until it goes below a certain threshold (someone told me 75% but I couldn't find any trustworthy confirmation). The battery in mine doesn't seem to have changed significantly in well over 2 years of use.

Doubt that they charge to 100%, more like 95%.

8 hours ago, studiot said:

Well I don't often link to other discussion sites, and I only found that one because I needed to fix a mobile phone, that was apparantly out of battery.

A replacement battery didn't fix it, so I looked for the ideas presented in that discussion concerning the charging circuitry and a loose ribbon cable.

In the event there was no ribbon cable, the micro usb was firmly soldered onto the one piece mainboard.

However disassembly and reassembly brought the old battery and system back to life.

But it still did not like the replacement battery - allegedy the same type number.

Interestingly when I tested the new and old batteries off load they both showefd 4.1 volts - a bit high for the nominal 3.7v.

~As I said there were many lesson for those who read the discussion, not least the need to keep Li-ion batteries topped up every few months.

My usage of portable power tools is diminished these days and rather intermittent and I do find the discipline prolongs active life PAL for those who rember the dogfood adververts.

4.1v is 'full'.

37 minutes ago, StringJunky said:

Doubt that they charge to 100%, more like 95%.

IDK but it seems likely to me that 95% is probably what reads as 100% when you turn it on because that is what it always reads whether it's been just long enough to charge or overnight.

1 hour ago, npts2020 said:

IDK but it seems likely to me that 95% is probably what reads as 100% when you turn it on because that is what it always reads whether it's been just long enough to charge or overnight.

Yes, you are probably right.