In the nicest possible way, why should someone have to justify themselves to control a machine they’ve paid for?
If you can’t control something so basic about your computer such as removing things you don’t want how can you be sure it’s acting in your interests in general?
The discourse around Apple products is bizarre. I was recently looking up why the setting to stop charging my Macbook at 80% didn't seem to do anything and the answer is that the computer can choose to ignore the setting at its discretion, effectively making the setting worthless. The common sentiment about this I found online was that the computer knows better than you and if you want to always limit charging to 80% you're obviously wrong.
What if I told you that unless the battery is occasionally charged to 100% to calibrate the readings, the firmware actually can't tell if it's truly charged to 80%?
What if I told you that the goal of limiting charging to 80% is not to very precisely hit that particular percentage, but to avoid the damage resulting from going all the way to 100%? If total battery capacity has degraded to the point where the voltage level that used to correspond to 80% now is 85%, then so be it. At least it's not a full charge.
An alternative is to reckon the percentage relative to the design capacity, so that a full charge can be less than 100%. That's what the i3 window manager's status bar shows by default, for example: https://i3wm.org/docs/i3status.html#_battery
Lithium batteries are tricky that way, though. The whole middle of their voltage curve (typically from ~20% to ~80%) is very flat. It falls off pretty rapidly below ~20%, and it rises rapidly above ~80%.
So the extremes at top and the bottom are easy to detect, but the middle is hard.
Coulomb counters are often used to keep track of state-of-charge around the middle, but they don't keep track of battery aging, self-discharge, or some other stuff. So an occasional trip up to 100% is a useful calibration function.
If the top (100%) SoC of a cell was, say, 4.2v at the beginning of its life, then it's still 4.2v near the end. That part doesn't change as time moves on and things age.
And yeah: Cell longevity is maximized by avoiding extremes in charge state, but that's also a function of the duration it is held at that extreme. Briefly charging to 100% is a very different thing than bringing it to 100% and leaving it that way for -- say -- days, weeks, or months.
It's alright. Part of keeping the battery at 80% (or 60%, or any other figure) means knowing where 80% is. Taking a brief trip up to 100% and down towards 0% every now and then is a requirement of knowing the present capacity, and therefore where 80% lives, and isn't particularly traumatic -- even for well-aged cells. Balancing is also important for longevity and requires a trip to the top.
The rest of the time, when things are neither being calibrated nor balanced, then it can stay at no more than 80% -- yeah? Yeah.
(And sure, I hear you. You want it to be some other way, and that's fine. You're allowed to want things. I'd offer to apologize on behalf of physics but physics doesn't care about apologies.)
Going all the way to 100% doesn't automatically damage your battery. It's charging it to 100% all the time, and also operating it at 100% at long periods of time that reduces the lifespan.
I'm sure there's some arithmetic where more accurately charging to 80% instead of 85% wins out over never going to 100% in terms of battery life. Not saying Apple engineers did this, but I wouldn't assume they hadn't.
Even with the battery charge limit the device will charge to 100% every so often. To claim that this “didn't seem to do anything” is just Apple Derangement Syndrome. The algorithms were engineered to know more than you about battery chemistry.
A few replies explained that the machine has a good reason to ignore you. I'm sold, they seem like good arguments!
But can empathize with your frustration. It looks like a bug, and I'd like the machine to explain that it's acting intentionally rather than having to dig around to figure out what's going on. It's like having an employee who simply ignores the spec rather than pointing out where it's not going to work.
On the other hand, maybe we're not the target demographic: a lot of people are happier with clicking a box and assuming everything will go to plan. The risky part of the design is that it requires enormous trust in the brand and minimal hacking from the user (which apple mostly has these days): the moment people have reason to believe their system is buggy, they need a bit more feedback.
User wants to see nothing above 80%? Fine. The charging system does whatever it needs to do, and the number on the screen never exceeds 80%. Done.
(The coolant temperature gauge on the E36 BMW is nerfed this way. A whole wide range of coolant temperatures don't move the needle at all from its nominal operating position of a bit less than 12 o'clock. It's deliberate because they don't want questions about why the engine temperature visibly varies, and it's annoying because it makes things harder to troubleshoot than they could be on an old car. The needle on the gauge is driven by a stepper motor and can display any value its controller is programmed to make it display and it's programmed to present a nerfed value.
A non-zero number of folks have un-nerfed it by using different driver electronics.)
It's an interesting parallel with cars: the target consumer pays a bunch of money for a new model, takes it in for service when something is wrong, and sells it before any problems come up.
The 2nd, 3rd, 4th hand buyer, on the other hand, wants all the data, and is happy to know that the internals are a bit more complicated than "full" vs "empty". I guess one explanation for this phenomena is that the 2nd hand buyers aren't the main profit stream.
I programmed industrial LiFePo4 batteries before and we solved it by monitoring the amount of electricity that went to the cells through the wire and the voltage of the cell blocks. You don't need to charge to 100% to balance. You can charge also to a lower voltage with a special balancer (resistors that reduced incoming electricity). If you have good hardware and a good software, it is possible. So it is just apple's crap charging hardware.
Resistors used like that are notorious for being both large, and getting hot when used. Both things that are tricky to cram into a small handheld device.
You also have to wonder what the point of doing all this faffing is. Mobile phone batteries are optimised for energy density above almost anything else, which comes with tradeoffs, include much more rapid natural degradation overtime that can’t be reduced or limited by gentle charging cycles. A phones battery is basically guaranteed to be unhealthy after 4-5 years regardless of what you do or don’t do.
Just because two batteries have the same basic chemistry (Lithium-ion of example), doesn’t mean their behaviour is the same. There’s a lot of possible customisation and tweaking within that single chemistry envelope.
You can kill a battery much faster than 5 years if you don't treat it right.
My personal experience is that modern devices are far better about battery management, my phones used to wear out much sooner than 5 years back when smartphones were a new concept and Li-Ion was new and fancy tech. So all that "faffing" seems to be doing something useful, because even my quite old and forgotten tablet that I forget to charge seems to be doing quite well.
If you can’t control something so basic about your computer such as removing things you don’t want how can you be sure it’s acting in your interests in general?