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never drain the battery to 0% or charge to 100%?

No, the oldest battery types actually recommend that you always discharged to 0 and then recharged to 100 (NiCd batteries suffer from memory effects, so if you partially charge them they lose capacity).

For Li batteries the advice is to avoid discharging fully, and longevity is improved by not fully charging (but obviously at the cost of endurance because you start with less stored energy if it's not fully charged). I'd put more priority on avoiding full discharge, but if 80-85% gives you enough endurance for your needs and it's not too painful to stop charging at that point (which it may be if you charge overnight, unless your phone lets you limit charging) then it's probably worth considering. Of course occasionally running it flat isn't an issue, but you should try to avoid it if you can.

Personally my favourite feature of my current phone is that it lets me turn off that fast charging stuff. I generally charge overnight so don't need the extra thermal stress of a higher charging current, so while it's helpful to have the option if I ever need it I generally am better off without it.
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Personally I think all LI powered devices should automatically turn off charging when full and stay off until unplugged and plugged in again, but no that would be the intelligent thing to do and we'll have none of that! However there are charge alarm apps to the rescue. I use one called simply "charge alarm". At 100 percent the alarm goes off and I unplug my phone right of way. The alarm also sounds when the charge hits 20 percent so I know to plug it in. Both min and max settings are configureable to your preference. I highly recommend this type of app.
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I really don't want an alarm going off an hour after I go to bed ;).

Actually any Li device I've used slows charging as it approaches 100% and then either goes into a very low current "trickle" mode or turns off the charging (which is why some could be left on charge overnight but only be 98% when you unplugged them in the morning, though if they drop too far they will start charging again). No phone I've ever met just keeps charging steadily when already full: that would be dangerous with a Li battery.

My s21 also has the option to limit charging to 85% built in, no alarm app and manual unplugging needed. I'm genuinely quite impressed.

I've read that if you have a regular charging pattern current Sony devices learn that and moderate their charging so that it finishes not long before you normally unplug in the morning.
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Personally I think all LI powered devices should automatically turn off charging when full and stay off until unplugged and plugged in again, but no that would be the intelligent thing to do and we'll have none of that! However there are charge alarm apps to the rescue. I use one called simply "charge alarm". At 100 percent the alarm goes off and I unplug my phone right of way. The alarm also sounds when the charge hits 20 percent so I know to plug it in. Both min and max settings are configureable to your preference. I highly recommend this type of app.

The reason that it is not this way is because these devices run off of the battery, not the charger.

The charger only charges the battery.

That is why you cannot run a phone from a charger alone if there is no battery installed.
(Actually, it is sort of possible, but usually ecceedingly difficult.)

What needs to happen is that devices charge the battery and then switch to running from the charger.
Of course, this would mean that they (or we) would need to provide much better chargers than what we use now.

So it most likely is a combination of a few cost cutting measures (better chargers and phone tech added) and planned obsolecence, where the device must be replaced when the battery is dead.

Like @Hadron says, try not to run it dead. (Your device will shut itself off before you technically hurt the battery, but if your battery is already aged, this can still kill it off.)
The newer devices have very good 'overcharging' protection, both in the phones and the batteries themselves.
So, not long ago, there was a 'scare' about leaving your device on charge for too long, after it is fully charged.
At that period of time, I used my devices as I liked- ran them down more than I wanted and whenever available they were on charge, sometimes for days on end.

I still got 2+ years out of the batteries, and this is within the 'normal' range- although at the lower end of it.

Those devices said something in the manuals about not charging them after 100% was reached.

My new ones both say not to worry about it.
(I wonder if they just want the batteries to wear faster?)

Anyway, as long as you get 2 or more years out of a battery, you are getting the expected life- and with that, I would not go out of my way to change my charging habits.

Life is too short- both for us and these devices- to fret about this thing.
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Perhaps I should have been more precice in my post. I didn't mean to imply that the batt would continue to constantly charge, that would indeed be dangerous. The batt does stop charging when full, and on most current and older phones and other devices, also starts charging again after it starts to be discharged. Not a danger, but this constant topping off keeps the batt at or near 100 percent. While convenient for the user, LI batts don't like remaining in a high charge state for very long as it causes build up on the internal electrodes and thus accelerating degradation over time. That's the reasoning for removing it from the charger when it hits 100 percent or even better about 80 percent or so, like hadron's phone does automatically. (Lucky stiff) I just this summer upgraded from an old Nokia Windows 8 phone and my batt as you can imagine was easily over 5 years old and although it was starting to exhibit some degradation it was actually still working quite well as I retired it. Is any of this actually necessary, honestly no. However if you're looking to maximize your batts longevity the 20/80 percent rule of thumb goes a long way. I personally do 20/100 percent while watching the TV at night before bed. Incidentally I've found an optimized charging option in my settings which I will investigate. Hope that this is info more helpful.
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Basically, for Li-Ion batteries, look at it this way:

A battery is going to survive a certain amount of full charge cycles.
A full charge cycle is from 0% to 100%.

You can charge that battery anywhere in between, any number of times, any amount of percent- but a full cycle is always counted as 100%.

If you went from 0% to 100%, that is one cycle.
If you went from 20% to 80%, then took it off charge and used it back down to 20%, then charged it back up to 80% again, you have used 1.2 cycles. (60% charge added each time, times two.)

So, even though you are correct that Li-Ion batteries are happiest at a charge that is around 75%, the amount of lifespan that this would add to the battery is much surpassed by the fact that the number of full cycles of charge is equivalent to the typical use over two years.

Basically, you can try to add lifespan by fretting over charge percentages- which is annoying at best, and leaving the user with a device never capable of a full charge of useability- or you can think of it as charge cycles.

Look at it this way:
My device is on charge and used constantly (music, videos, movies, news) throughout the night.

At 6:30AM it is off the charger, and I get to work at 8AM.
I get home a little after 6PM, and the phone goes back on the charger at 7:30PM or so.

Normally, It is still at around 60%-80% charge when I plug it back in.

So each day of work, I use 40% to 20% of a charge cycle.
The time spent on charge after 100% is inconsequential because the charge never goes below 100%.
(If it sometimes dropped to 99% then went back up, that would be adding to the cycle tally.)

These are insignificant amounts of charge- literally, tiny pieces of one percent of one cycle.

In the big picture, you will waste more energy, and never enjoy your battery's full capacity, by trying to go about the 20 to 80 thing.

The life you add to the battery will be insignificant as opposed to the hassles involved.

That being said, there are apps available that allow you to control the maximum charge of a device to less than 100%.
But they require a rooted device.

The optimized charging option you mention is a slower charge (medium, or normal) than a quick charge( a fast charger), but faster than a slow charge (like from a computer port, known as USB charging.)

That will most definitely make a difference in the lifespan of the battery.
A slower charge builds up less heat, and is better for the battery.
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Oops, I totally forgot about the whole partial vs full cycle thing. Thanks for the reminder. The count of full cycles does play a huge role in determining the lifespan of a battery. Then there's the issue of the amount of heat that is created during fast charge vs that of a slow charge, that too plays a significant role in a battery's lifespan by, if I remember this correctly, negatively impacting the total count of full cycles that can be achieved before degradation becomes a problem. I have to admit that I slipped on those two important points. Guess I'm getting a little rusty. As far as my other posts go, I'm just going by all of the articles that I've read over the years from the emergence of the LI battery till now. Granted LI battery technology has been progressing steadily, albeit slowly, since its introduction and much of the impact of all of these issues have been attenuated to some degree or another. If I understand you correctly I think that the one thing that we really disagree on is just how much of an impact routinely prolonged high state of charge conditions affect longevity. Since I can't find any documentation that specifically details just exactly how much repeated and prolonged high state of charge conditions effect longevity or even something that compares all of these technological limitations to each other, then unfortunately I have no concrete evidence to support my interpretation of all the documentation that I was able to read. So I guess I can't really argue with you on that point. That's not to say that I'm going change my routine any time soon, after all I have to admit that I can be a bit anal retentive about some things that most people would think isn't a big deal and therefore others may feel that I nitpick. I guess I can't argue against that either. So as much as I believe my interpretation to be the correct one I will however yield to you puppykickr since I can't actually prove said interpretation.

BTW. You also stated that the optimized charging option is actually somewhat slower than fast charging, then I gotta see what happens when I turn it off. As it is now it takes about an hour and forty minutes to go from 20 percent to 100 percent with the optimize option enabled. If you're right about that, then turning it off should be even faster. That would be absolutely awesome in a pinch if I were in a hurry!

Wow, I really need to learn to do paragraphs...
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I have had phones since they had Ni-Cd batteries.

Then came the Ni-Metal Hydride.

I actually like the Ni-Metal Hydrides, they had an outstanding lifespan.

Probably why they quit making them.

Well, that and the Li-Ion pack.a ton of power into a small size.

What you have read over time is probably all correct, in the time frame that it was written.

All of these battery chemistries have gone through changes over the course of the use of each one.

What I have told you is just what I have gathered most recently, and from my most recent devices.
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