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Level 10

203,508 points

When to charge your iPhone or iPad

There's a lot of myth and folklore surrounding charging iOS devices (or actually any device that uses Lithium technology batteries). A lot of it comes from the advice given for older technologies, such as Nickel-Cadmium or Nickel-Metal-Hydride batteries. None of this applies to Lithium, however, and some of what we "know" from the NiCd and NiMH days is actually harmful to modern battery technology.

Things to understand:

The "charger" for an iOS device is built into the device. It is not the thingy that plugs into the wall, and it is not the cable that connects the thingy that plugs into the wall to the phone. They are just a source of current and a way to get it to the phone, respectively.
Completely draining a Lithium battery, even once, will kill it. (Unlike NiCd and NiMH, which people really would drain completely to prevent "memory effect").
The internal charger is "smart" - It will prevent the device from being overcharged, and it will attempt to prevent the device from totally draining the battery by shutting down the device before the battery is fully depleted.
When the phone shuts off at 0% it really isn't zero; there's still sufficient charge on the device to prevent the battery from going completely flat. Likewise, 100% is not the maximum the battery can store; it stops charging slightly short of maximum to prevent overcharging.
The worst thing you can do is drain the battery to 0%, then not charge it immediately. After it reaches zero and shuts off there's a small amount of energy left, but if you leave it uncharged for long it WILL go flat and kill the battery. So if it reaches zero, charge it soon (within hours). And never leave a phone unused for weeks or months on end without periodically recharging it.
You should only use high quality USB power sources to charge your iOS device. They don't have to be Apple's (although Apple makes good ones), but they should never be cheapo USB sources, both because they may damage the phone and they may even injure you.
The power source needs to supply at least 1 amp to charge an iPhone, and 2 amps to charge an iPad. Note, however that a power source that can supply more than these values is OK to use; the internal battery charger will take only what it needs. So, for example, you can safely charge your iPhone with an iPad USB adapter.
iOS devices fast charge until they reach about 75%; the rate then slows down to prevent overcharging. So it will reach 75% very quickly (under an hour), but it can take a couple of hours more to reach full charge.

So what are the "rules" for charging? The most basic one is charge whenever you want to, for a long as you want to. There's no reason to let the device drain completely before charging (in fact, it's a bad idea to do that on a regular basis), and there's no need to wait until it reaches 100% before removing it from the power source. You can charge when it's at 40% and disconnect when it reaches 80%, or any other values, without hurting the phone.

The Best Practice, however, is to charge the phone overnight, every night. As it stops automatically at 100% you can't overcharge it doing this. You thus start the day with a fully charged phone. And, if you configure the phone for automatic backup using iTunes or iCloud, the phone will back up every night when it has a WiFi connection and is asleep.

iPhone 6, iOS 9.2.1, 128 GB

Posted on Mar 26, 2016 11:09 AM

Best reply

chcn

Level 1

75 points

Posted on Jan 19, 2018 9:33 AM

I too have read a lot about lithium ion batteries, and also have a lot of practical experience with them. I agree with 90% of what you’ve written here (and at When to charge your iPhone or iPad, where I could not comment), but I am curious about your insistence that “the very best strategy” is to leave it plugged in all night every night.

There seems to be plenty of evidence that lithium batteries prefer to live life near the centre of their charge range. Just as leaving them deeply discharged for long is harmful, leaving them fully-charged for a long time is harmful too. See, for example http://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die; search for the text “Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity”. I note that 3.92V appears to be around 65% on an iPhone.

I have experienced this effect with lithium laptop batteris too: keeping it plugged in all the time kills the battery within months. Once I got a replacement battery and programmed my ThinkPad to keep the charge level between something like 40% and 60% most of the time, that battery lasted darned-near forever. (Of course I charged it too 100% when I needed to use it in a mobile application. But at my desk, there was no point.)

I would agree that leaving an iPhone plugged in every night will not hurt it significantly, assuming a normal use case where the user unplugs it and uses it the next day; I’m sure it’s Apple’s standard use case. However, it stands to reason that an even better strategy—purely from the perspective of battery life and ignoring possible lost utility from not having a full charge, nightly backups to iCloud, etc.—would be to only charge it to around 80% except when you needed the full capacity. This is what I have done with my iPhones and have enjoyed excellent battery longevity. My wife and daugher, on the other hand, are both in the charge-it-up-to-100%-every-day camp, and their batteries don’t seem to last as long. Again, nothing wrong with this usage, and it’s what Apple expects. I just question the statement that “the very best strategy” is to leave it plugged in overnight every night. Since there are trade-offs, doesn’t the best strategy depend on what the user values?

Are you saying the lithium battery technology has changed that much in the last 15 years so that there is no longer any harm that results in consistently keeping a battery above 4V?

Or are you saying that the incremental wear that results in charging all the way up every night (resulting in a higher average voltage level over the life of the battery) is too infinitesimally small to be worth the trade-offs? And if that’s what you’re saying, can you quantify it?

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86 replies

y_p_w

Level 6

17,648 points

Jan 21, 2018 5:08 PM in response to Lawrence Finch

Lawrence Finch wrote:

Consider also the Tesla, which is entirely powered by Lithium Ion batteries; there have been no history of battery failures reported. Or my 2012 Plug-in Prius. It's battery capacity has not changed in the almost 6 years I have had it, and I tend to keep it near full charge when idle.

There are reports of catastrophic battery failure in Teslas, but they were attributed to a glitch in the battery management that allowed a deep discharge. However, even Tesla recommends that most owners set up their batteries to charge to 80-90% to improve ultimate battery life. There are some interesting discussions on this on Tesla's own user message board.

Charging recommendations. | Tesla

Of course most iPhone users aren't going to keep their devices for 10 years. I can imagine someone trying to keep a Tesla going for 15 years or longer. I've even seen a second generation Prius still working, and that's how old? I heard that owners have a name for the charge range that avoids extremes to exponentially extend the life - the "sweet spot". I've heard of Prius taxis that easily went 400,000 miles on the original batteries. But of course that's NiMH, but research does seem to indicate that reducing the ultimate charge level will result in Li rechargeable batteries lasting considerably longer.

Feb 7, 2018 6:07 AM in response to Lawrence Finch

Hi Lawrence ,

I discharged my battery completely several times, but I recharged in 15-30 minutes. Is there a problem with the battery ? I mean u say " Completely draining a Lithium battery, even once, will kill it." Is my battery dead or have any other issue ? Sorry for my bad english.

Lawrence Finch Author

Level 10

203,508 points

Feb 7, 2018 7:56 AM in response to hasangundogdu

Draining to zero is not a complete discharge; Apple designed it to display zero and shut down before the battery is fully depleted. So as long as you charge it soon after reaching zero no harm has been done. ("Soon" means within a few days). From what you describe you didn't do any harm to the battery. However, there is no reason to discharge it to zero intentionally; if it happens by accident just charge it, but do not go out of your way to discharge it to zero.

Feb 9, 2018 9:44 AM in response to Lawrence Finch

Heyy sir Can you please help me

I want to know can we charge our iPhone thrice a day? Will it harm my battery in long term?

If I maintain my battery from 50% - 80% then I need to charge it twice to thrice a day

If this is harmful

Then I need to charge my battery to 100% and then use it till 10% this will work in 1 charge whole day

Tell me which one is good for battery performance and health for longer use?

Feb 9, 2018 9:59 AM in response to FarazSidd

I don’t understand why people don’t get it, the phone has a built in charging circuit that will not let it over charge, just charge it when ever you want for as long as you want but try not to let it drop to 0

Lawrence Finch Author

Level 10

203,508 points

Feb 10, 2018 7:00 AM in response to FarazSidd

OK, time to read and understand what it says:

"If you want to store your device long term" That means if you are going to put it in a drawer and not use it for months. "Long term" is not one day. And you are not storing it, you are using it pretty constantly. So the excerpt you quoted has nothing to do with a phone that is powered on and in use.

Yes, if you put it in drawer it is important to power it down, with the battery at around 50%. (When iPhones ship from the factory they are charged to around 50%). And 50% is chosen so the battery is in no risk of discharging to zero while in storage. The reason they don't recommend 100% is for complex technical reasons based on battery technology that aren't important here.

Lawrence Finch Author

Level 10

203,508 points

Feb 10, 2018 8:09 AM in response to FarazSidd

Two things affect the life of a battery: It's actual age, and the number of full charge cycles. Actual age usually isn't an issue, as it's over 5 years. The battery is rated for 500 full charge cycles to 80% capacity, where a full charge cycle means zero to 100%. Charging it from 50% to 100% is one half of a charge cycle, so if you charge it from 50% to 100% twice that counts as one full cycle out of the 500.

But there are other factors that affect battery life, primarily the number and energy consumption of the apps you are using. Get the app Battery Life (it's free) and check the capacity of your battery. If it is over 80% then any reduction in battery life will be due to the apps that you use, how often and how long your calls and texts are, and how strong your cellular signal is. A weak signal will use ten times the amount of energy as a strong signal for voice and data.

Lawrence Finch Author

Level 10

203,508 points

Feb 11, 2018 9:39 AM in response to FarazSidd

What do you man by “properly?”

There are 1095 days in 3 years. The battery will last 3 years if you use about 1/2 of a charge on average each day. one way to help is to leave it connected overnight, so background apps use external power rather than battery power.

Feb 11, 2018 10:36 AM in response to FarazSidd

Hello. If you want to use your battery better and long term than 2 years you remember should do some steps:

- Dont charge until the phone

- Dont take out the cable when battery still not full

- And dont stop the charge battery object before 100%

With these steps im sure you will possible keeping your phone's battery health perfect forever

Goodluck

Feb 12, 2018 8:58 AM in response to Lawrence Finch

Why AppleSupport And Apple Service point provide wrong information that charge your phone once a day

If You connect and Suddenly disconnects the charger you wasted one cycle no Matter how much percent you used ot

Feb 12, 2018 9:47 AM in response to FarazSidd

This is directly from the Apple support site.

Charge your Apple lithium-ion battery whenever you want. There’s no need to let it discharge 100% before recharging. Apple lithium-ion batteries work in charge cycles. You complete one charge cycle when you’ve used (discharged) an amount that equals 100% of your battery’s capacity — but not necessarily all from one charge. For instance, you might use 75% of your battery’s capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle. It could take several days to complete a cycle. The capacity of any type of battery will diminish after a certain amount of recharging. With lithium-ion batteries, the capacity diminishes slightly with each complete charge cycle. Apple lithium-ion batteries are designed to hold at least 80% of their original capacity for a high number of charge cycles, which varies depending on the product.

Lawrence Finch Author

Level 10

203,508 points

Feb 12, 2018 9:53 AM in response to FarazSidd

Sorry, you are wrong. If you connect and disconnect you DO NOT waste any cycles. A "cycle" is zero to 100%. If you charge it from, say, 40% to 50% you have used ONE TENTH of a cycle. If you connect it and then disconnect it immediately, and it charges by 1%, you have used ONE HUNDREDTH of a charge cycle. If you connect and disconnect immediately and the state of charge does not change you have used NO cycles. If it is already at 100% and you plug it in you use NO cycles, because it is already fully charged.

chcn

Level 1

75 points

Feb 12, 2018 2:26 PM in response to Lawrence Finch

Thanks for the thoughtful reply.

Regarding the 3.92V research:

chcn wrote:

There seems to be plenty of evidence that lithium batteries prefer to live life near the centre of their charge range. Just as leaving them deeply discharged for long is harmful, leaving them fully-charged for a long time is harmful too. See, for example http://batteryuniversity.com/learn/article/bu_808b_what_causes_li_ion_to_die; search for the text “Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity”. I note that 3.92V appears to be around 65% on an iPhone.

Yes, that is true for batteries being stored; even Apple says to leave a device to be stored at around 50%. But when a battery is continually being used it isn't the same thing as storage, and an iPhone battery usually cycles over a wide range of SoC.

Note that the reference I gave was for batteries in satellites that were also being cycled, not stored.

Our experiences with keeping lithium ion battries fully charged for lengthy periods appears to have differed, so I remain skeptical that this is the very best way to use them, but I will concede that Apple devices appears to have been designed to be fully charged frequently and that this is not likely to do any harm.

chcn

Level 1

75 points

Feb 12, 2018 2:33 PM in response to Lawrence Finch

That said, if you buy a third-party replacement battery from the iFixit people (something I have not tried and cannot vouch for), their own installation instructions say that, at least the first time, to calibrate the battery gauge you should drain the new battery to below 10%, then charge uninterrupted to 100%. I don’t know if the gauges in genuine OEM Apple batteries benefit from doing this from time to time.

As you point out elsewhere, it is bad to “completely” discharge it and then leave it that way for days on end.

chcn

Level 1

75 points

Feb 12, 2018 2:38 PM in response to Lawrence Finch

Lawrence Finch wrote:

... There are several advantages to charging overnight: the battery will be full at the start of the day; if you turn on iCloud Backups it will back up overnight; it will reduce the total number of charge cycles because background processes will use mains power instead of battery power. ...

Are you sure about this? I’m not sure how the power management in an iPhone works, but it appears to be different from a laptop that can operate with out a battery inserted at all. The mains power is charging the battery, and then the system is using power from the battery. I’m not sure the phone ever runs off of mains power, does it?

When to charge your iPhone or iPad