Why does an iPhone ignore a user-set charge limit?
Apple added optimized charging for iOS, iPadOS, macOS, and watchOS about five years ago, with iPhone and iPad support coming a little earlier than the others. Optimized charging should extend the battery life on your various devices by preventing certain kinds of wear.
However, Six Colors reader Dan says he’s not seeing this with a new iPhone. After upgrading from an iPhone XR to an iPhone 16 Plus, Dan set the maximum charging to 80%. He had previously avoided using “smart” charging in Settings:
I never go through [80%] in a day. That being said, periodically, after having my phone wirelessly charging next to my bed all night, it shows 100% charge. There’s been no change in my settings, so I’m a little confused if this is an Apple Intelligence snafu. Or?
Why would his iPhone ignore his attempts to limit charging? We need to dig into how Apple balances battery safety and reducing wear against our stated preferences.
(As always, you can skip ahead if you’d like to bypass the exposition.)
Mmm…donuts
Charging a lithium-ion battery resembles a classic segment in “The Simpsons” in which Homer sells his soul to the devil. While waiting for a trial over his fate, the devil temporarily sends Homer to hell, where a demon feeds him endless donuts. Homer, delighted, swells in size as he says, “More!” (The demon: “I don’t understand it. James Coco went mad in fifteen minutes!”)
While Homer’s insatiable donut need led him to grow in size without consequence, if he were a Li-ion battery, the calories (electrons) in the donuts would have led to an out-of-control thermal reaction, and he would burst into flames. (He was already in hell—would it have mattered?)
When you connect a power system drawing juice from line voltage or a battery pack to a device with a Li-ion battery, the device’s power management circuitry essentially stuffs donuts—er, electrons into a body (the internal battery) that has both limited capacity and parameters that require careful stuffing.
Charging is a process of moving electrons at some density and speed from one place to another, where they can be stored for later “retrieval.” In a lithium-ion battery, charging forces lithium ions to move out of one end of the battery (the “cathode”) and migrate physically across it to be packed neatly into graphite layers at the other end (the “anode”). (The actual electrochemistry is far more complicated than that—and far more than I can explain!) Think of it like airplane baggage handlers needing to efficiently pack luggage into an airplane’s hold.
When a battery is discharging to power a laptop or handheld device, the process happens in reverse: the lithium ions unpack themselves from the graphite layers and migrate back across the battery, while electrons flow through the external circuit to power the device.
This sounds very orderly and neat, but there are significant safety risks. With a battery depleted of electrons, the initial charging must be slow to prevent the battery from being destroyed or overheating. This is like trying to rehydrate a dried sponge: pouring water on it quickly just causes the liquid to spatter everywhere. Now imagine that the sponge could burst into flame, too!
There’s a long, moderate pace of charging between empty and full when charging can happen very quickly as there is enough free chemistry in the battery to absorb energy safely. This is the sweet spot where Apple offers its fast charging option on newer devices, adding a 50% charge in about 30 minutes.1
As a Li-ion battery reaches full, charging slows dramatically. Packing the battery full of energy can break down layers within the battery and cause a thermal runaway. While electronic devices have safeguards against this, something like a phone being crushed in an airplane seat or a faulty component in a battery is why you’ve seen videos of phones on planes and electric cars on car carriers suddenly emitting gouts of fire.
This all sounds dangerous. Why does the worst rarely happen?
It’s a numbers game
I know you have suspected that the “empty” line on your gas gauge means there’s still something left in your internal-combustion engine’s reserves. True! Most cars can drive dozens of miles after that. The same is sort of true of lithium-ion batteries.
A truly depleted battery may become unusable, as the chemistry and structure inside can no longer accept a charge. So a battery stops releasing power, and a connected electronic device shows 0% remaining, when it still has a healthy 10% reserve that you can’t tap but which prevents its demise.
Apple’s advice when storing your equipment for a long period without it being plugged in for weeks to months is to charge it 50%. The company notes, “If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge.” Even powered down, a lithium-ion battery slowly gives up a little energy over time, so Apple recommends recharging to 50% every six months.
If the battery is depleted somewhat but not too much, “…it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it.” You may have seen this when there’s a red outline of a battery and your device won’t power on at all. It has to bump the battery to its minimum level before it boots up.
At the other end of the scale, “100%” is probably 80–90% of the stuffed-to-the-gills capacity of a battery. Having that extra overhead provides a safety margin. (Modern batteries and charging circuitry have many other safety precautions.) That “100%” decreases as a battery ages, and some of its internal chemicals have “aged” in Apple’s terminology.
Knowing these extremes helps us understand what Apple offers with battery optimization controls. Before optimization, circuitry would charge an iPad, iPhone, Mac laptop, or Apple Watch to 100% if it was plugged in long enough to an energy source. It would also continue to top up to 100% as the battery consumed power through normal use.
Starting about a decade ago with Macs and a few years later with other hardware, Apple engineered passthrough AC power. If the battery is charged to the optimized level or 100%, depending on the operating system version and battery settings, the device draws power from a connected AC adapter, which, again, reduces wear on the battery.
To adjust charging behavior on an iPhone, go to Settings > Battery > Charging.2 On the last several models of iPhone, you can enable Optimized Battery Charging or use a slider from 80% to 100% to set a charge limit.3 Using the slider disables Optimize Battery Charging. If you slide back to 100%, a dialog appears letting you choose among Allow Until Tomorrow, Set Limit to 100%, or Cancel. If you choose Set Limit to 100% and try to re-enable Optimized Battery Charging, you can then choose Turn Off Until Tomorrow, Turn Off, or Cancel.
I still haven’t answered Dan’s question! But now you have all the backstory.
You have the power!
When battery optimization is enabled, Apple tracks your pattern of battery usage over time. It uses this—and potentially other clues—to charge your device’s battery past 80% to 100% only when you’re plugged into power and the algorithm’s analysis expects you will be unplugging it within the next few hours. The operating system uses that time to charge you to 100% so that you have as much power as possible at the point in time when you’re most likely to want to have as much battery life as possible.
Otherwise, your device stops charging its battery at 80% and will use AC power preferentially. If the battery drops below 80% by some margin, it will try to charge you back to 80%.
What Dan appears to be experiencing is what Apple describes as an “occasional” override: your iPhone charges to 100% when the charge limit is set for it to recalibrate how it calculcates the percentage of your battery used. This detail is in a footnote at the “About Charge Limit” support page. “Occasionally” might be more like “regular” for someone who only plugs in once per day with an 80% charge limit set. I understand why Dan wondered if Apple Intelligence could be involved, since Apple has intertwined it throughout its operating systems, but the analysis for optimization uses plain old machine learning.4
Now back to determining the maximum number of donuts I can fit into a cell phone battery.
Update: Two readers pointed to the footnote cited a couple of paragraphs above as the likely culprit. I thought Apple might mean a much longer period by “occasionally,” but this is likely the answer unless a bug in the charging algorithm remains the culprit.
[Got a question for the column? You can email glenn@sixcolors.com or use /glenn in our subscriber-only Discord community.]
- Using fast charging puts more wear on the battery, but battery optimization reduces it, a neat balancing act on overall lifetime. ↩
- This option isn’t available on iPad—either in iPadOS 18 or 26’s current beta. A Mac’s options are currently limited to disabling optimization altogether or disabling it “until tomorrow” via System Settings > Battery and clicking the info button on Battery Health. ↩
- The current iOS 26 beta displays different options and uses a different visual presentation. It’s not clear how optimization settings will appear in the final release. ↩
- Machine learning is absolutely a form of what’s called “artificial intelligence.” It involves using training sets of data that are marked for an algorithm to analyze and then produce weighted results. While this might be a billion images to help a deep-learning algorithm to tell you whether or not there’s a cat in a photo, here it’s comparing some large set of usage patterns—perhaps even artificially created—and then comparing them against yours. It uses scoring to determine when you’re most likely to be a few hours away from needing 100%. ↩
[Glenn Fleishman is a printing and comics historian, Jeopardy champion, and serial Kickstarterer. His latest books are Six Centuries of Type & Printing (Aperiodical LLC) and How Comics Are Made (Andrews McMeel Publishing).]
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