I have an iPhone 6S and the battery is starting to reach the end of its useful lifetime. What I am noticing is how I can get the battery down to about 10 or 20 percent, power the phone off (completely, as if to reboot) then come back a few minutes later (not plugging it in) and then the battery reads out higher then when I turned it off. I’ve tested this a few times and the phone seems to be able to do this on command. As far as I know, wireless charging wasn’t a thing until the iPhone X, and I’m using a 6S.

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    Generally you want to avoid letting your phone get down below about 30%, to minimize further wear. Of course, that's 30% of the raw battery capacity, which as the answers discuss might be very different from the percentage displayed. batteryuniversity.com/article/… says that devices like phones usually have 100% as 4.2 volts, so aiming to charge to 70 to 80% is good if you don't need the extra capacity. (Deep discharge hurts more than full charge.) Commented Jan 29, 2023 at 4:19
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    A bit of a electronics background: The state of charge (SOC) of a battery is not something you can directly measure like voltage or current. It is generally modeled on a voltage curve that is fit to the voltages measured after timed outputs of current from a full (specific max voltage) charge. As the battery degrades, this relationship between voltage and useful stored energy starts to degrade & it becomes hard to tell what SOC the battery is at. For ex. if you install Ubuntu on a macbook, it will often show a different SOC than when you boot up on macOS, just due to different software.
    – johnDanger
    Commented Jan 29, 2023 at 5:47
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    @johnDanger: There is also Coulomb counting i.e. recording how much energy goes in and out of the battery and estimating how much is left in there. This is usually done directly in hardware and the charge controller would just give you some number (percent or energy remaining) which the operating system displays.
    – Michael
    Commented Jan 29, 2023 at 15:34
  • Thanks. I did some further research and the faulty relationship between what the phone says and what it actually is can be very different (Samsung note 7 is probably the best example of this) and I plan for taking it in to a battery shop pretty soon. Commented Jan 30, 2023 at 19:15
  • @WendelRareheart Yes, I'd think so as well. Commented Jan 30, 2023 at 19:43

3 Answers 3


The phone's battery percentage is never accurate. Never ever. However, some are better than others. Doesn't matter if you phone is 1 year old or 10, that readout is only a rough guess. And your phone is a demonstration of how some guesses are better than others. As someone who is attempting to create my own tech company, I can tell you that the percent is usually based on how much voltage is being circulated through the device. That's how I build my own phone battery readout. The only problem is just like people, the older a phone gets the worse their memory and therefore guesswork can become, so your phone has to rethink itself every time it reboots because that's how rebooting works.

I'd say that if you really are comfortable with this problem, and it doesn't make the phone unusable, you can probably squeeze a few more years out of it. I am also the owner of a 6S, and I still use it, but I wouldn't count on it to be used as (example) an emergency phone, since mine has the exact same thing going on, but mine shuts down early because it thinks it's lower than it is.

And also, don't be afraid to get that battery replaced if you really are inconvenienced by this. I agree with you, I don't want to pay $400-$1000 dollars for a new phone, just to use for a year or two, then a few years later pull it out of the darn closet and squeeze the last bit of life out of it. I just wanted to say that the guy at the genius bar was lying to you, there is little to no risk in replacing your battery. The only risk would be the guy doing it is on his first day (VERY doubtful) or he screws something up big time (EVEN MORE DOUBTFUL).

The moral of the story is don't be afraid to change that old battery, and don't let your charge get low.

  • Actually I upgraded from a 6 to an X Commented Feb 21, 2023 at 23:27
  • @WendelRareheart cool. Commented Feb 22, 2023 at 0:36

In broad strokes - the phone hasn't a clue how much charge the battery has. Because it's so old & worn out, its guesswork is getting worse. After a reboot it will have another think, but none of the figures will actually be accurate, merely new guesses.

By comparison, by the time I traded up my 6S it had gone through two batteries, lasting about 3 years each. After that point they just got really unreliable & started reporting odd figures & having random crashes, especially in the cold.

I'd say it's time for a new battery, but it's really time for a new phone. [The 6S cannot be upgraded any further, iOS 15 was the last to support it.] A 2nd hand SE 2020 wouldn't be a bad trade up - it looks & feels like the 6s, still has TouchID not the infuriating FaceID;) The batteries are much better too, mine at 18 months old is showing 98% health. You can even use the same case, though you will need to chop a bit out of the camera hole as the SE's camera is larger.


It's well observed that when a battery gets towards the end of life, the % indicator can be unreliable.

Apple's battery replacement charges are quite reasonable, and well worth it, to extend the useful life of a phone.

  • ok. Just a additional question, I look into getting the battery replaced, and the guy at the Genius Bar said that replacing the battery was risky because of the phone’s age. Is this true? Commented Jan 28, 2023 at 20:27
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    I'd call BS on Apple replacing a battery as being "risky." They have the specialized tools and knowledge to do this. Maybe he was trying to steer you to a new phone, but that's just guessing. Replace the battery and Apple is going to be the best bet. And if they do break it (doubtful, but possible) they will replace it. And, as always, when you take any computing device anywhere for service, make a backup! Commented Jan 29, 2023 at 0:21
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    @WendelRareheart I'd guess the main risk is that it's an old phone that might be obsolete in other ways, so you might not get as many years of use out of a new battery because of some other reason for giving up on the phone (like that it becomes too slow to be usable with newer apps or web pages), before the new battery wears out. Commented Jan 29, 2023 at 4:16
  • @WendelRareheart So, did you change your battery? Commented Feb 16, 2023 at 16:30

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