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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

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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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Lawrence Finch Author

Level 10

204,218 points

Feb 12, 2018 4:02 PM in response to chcn

Thank you for raising this question; it's something I've thought about for a while. Let's say I'm 99% sure, based on the fact that a battery does not have separate input and output terminals; the same terminals are used for charging and discharging. And I'm not sure it's different from a laptop, because there's no way to disconnect the battery to see if it will still work if plugged in. That would be an interesting experiment, but I don't see any way to do it, as disconnecting the battery also disconnects the battery monitoring circuit, which would throw other artifacts into the experiment. So the only way to try it is if you have a throw-away phone you could disassemble and measure current flow into and out of the battery.

From personal experience I can leave my phone plugged in while using it, and it stays at 100%. Most commonly when I'm using navigation apps in a car. So if it stays at 100% any energy used by the phone is not coming from the battery; the only place it can come from is the power source.

BTW, I really appreciate your input into this discussion. It makes me think.

Feb 12, 2018 8:53 PM in response to chcn

What would need to be calibrated? The charger is charging the battery based on voltage, current draw and temperature, when the voltage gets to 80% full the charging circuit slows to trickle charge then stops at 100% so it basically doesn’t matter where the charging started.

Feb 12, 2018 11:06 PM in response to Lawrence Finch

Hello Lawrence

Thank you for the wonderful explanation.

I drive Uber part time and of course I drive with charge cable plugged.

Between the trips I switch off the engine and it stops charging then it starts charging when the engine is back on.

Sometimes the battery is full sometimes not.

I do that quiet often in daily basis.

My understanding after reading your article is that it wouldn’t really affect the phone/battery much in short term.

Am I right?

Kind regards,

Chris

Lawrence Finch Author

Level 10

204,218 points

Feb 13, 2018 6:25 AM in response to MikePlaga

MikePlaga wrote:

What would need to be calibrated? The charger is charging the battery based on voltage, current draw and temperature, when the voltage gets to 80% full the charging circuit slows to trickle charge then stops at 100% so it basically doesn’t matter where the charging started.

You need to read up on Lithium technology batteries. One of their characteristics is the output voltage is almost constant except at the ends of the charge cycle. So you cannot determine whether a battery is at 30% or 60%, because the voltage difference is too small to measure. The only places you can be sure are full charge and almost discharged, when there is a steep rise or drop in voltage, respectively. So the battery gauge needs to know these 2 values. It then monitors current demand out and current in from a charger and calculates the state of charge. About the only battery that has a relatively linear relationship to state of charge and voltage is lead-acid (car) batteries. Other storage technologies (NiCd, NiMH, etc) do not.

chcn

Level 1

75 points

Feb 13, 2018 10:13 AM in response to Lawrence Finch

You are likely right, but I have a few thoughts to complicate matters, inserted into your text below:

Lawrence Finch wrote:

Thank you for raising this question; it's something I've thought about for a while. Let's say I'm 99% sure [that an iPhone can run entirely off of mains power and therefore not engage the battery], based on the fact that a battery does not have separate input and output terminals; the same terminals are used for charging and discharging.

CHCN: But those are the terminals from the phone's power management/charging circuitry (which I'll just call the power supply) to the battery pack, so I'm not sure any conclusion can be drawn from it. I see two possibilities:

(1) The battery is in parallel with the power supply and the rest of the phone's electronics, and the voltage out from the power supply is simply raised/lowered (or turned off entirely? see below*) as needed to achieve the desired charging rate (like the arrangement with the alternator, voltage regulator, and battery in a car).

(2) The battery is isolated from the phone's electronics by the power supply, and the power supply can send current from the mains to the battery and/or the rest of the phone, or direct power from the battery to the rest of the phone, depending on what is needed or whether it is plugged in.

We always tell people that they can't overcharge an iPhone battery by leaving it plugged it, so I had always assumed that something like #2 was true. *And indeed you see that if you leave the phone plugged in and use it while full (occasionally monitoring the state with an app like Battery Life), it will stop charging and the voltage will start to drop off for a while, before it resumes charging back up to its max of around 4.34V. But I have realized that some version of #1 can be true and I would still see that behaviour.

Anyway, it probably is #1, as I imagine #2 would be needlessly complicated and would loose the smoothing advantage of the battery acting like a giant capacitor; #2 could be especially problematic when plugging and unplugging the phone.

Assuming it's #1 (and the battery is therefore always physically connected to the rest of the phone), I was thinking that your assertion that the phone can run entirely off mains power wasn't true. But maybe it adds up to the same thing. A fully-charged battery under scenario #1 will not be discharging or charging much, and therefore the cycle count won't be increasing (much), which was all you were trying to say.

Lawrence: And I'm not sure it's different from a laptop, because there's no way to disconnect the battery to see if it will still work if plugged in.

CHCN: If you mean there's no way to do that with a laptop, I used to do it with my ThinkPad all the time. Before I discovered the battery management software that allowed me to specify charge limits, I used to charge the battery to around 50% and then remove it and put it in my desk. But you may be saying there's no way to do it with an iPhone. I actually did read online today where someone attempted just that (they applied power directly to the phone, isolating the battery), and the phone refused to start. Apparently it needs to talk to the on-board battery charge IC to be happy.

Lawrence: That would be an interesting experiment, but I don't see any way to do it, as disconnecting the battery also disconnects the battery monitoring circuit, which would throw other artifacts into the experiment. So the only way to try it is if you have a throw-away phone you could disassemble and measure current flow into and out of the battery.

From personal experience I can leave my phone plugged in while using it, and it stays at 100%. Most commonly when I'm using navigation apps in a car. So if it stays at 100% any energy used by the phone is not coming from the battery; the only place it can come from is the power source.

CHCN: Are you sure about this? As noted above, I have monitored closely with Battery Life to see the voltage level and charging/not charging state change, and the charging cycles on an off when using the phone when fully charged. I'm not sure if this is a matter of the battery voltage naturally drifting down when a charge is no longer applied, so the phone is just trying to keep the battery full, or if it is actually deliberately using the battery instead of using mains power. To minimize the use/cycle count of the battery, I would expect either complete isolation once full (#2 above), or an output from the power supply that matches the phone's requirements so that the phone doesn't draw down the battery (some version of #1 with a regulated voltage, rather than cycling on and off). But instead I observe the charging kicking in and out, cycling the battery over and over (in admittedly small cycles). Maybe that's better than the larger cycles that would result from leaving it unplugged all night as I do, or maybe it's a wash. If it's a wash, I prefer the lower average voltage level that results from only charging when needed instead of holding it at the highest charge.

Lawrence: BTW, I really appreciate your input into this discussion. It makes me think.

CHCN: I'm very glad to hear it's not annoying.

One last thought: When I fly with my iPad (which I assume is similar to an iPhone in these respects), I find that most portable chargers cannot keep up with the discharge rate of the iPad running the ForeFlight navigation software. (It draws more than 1A, and many of those portable chargers only supplied 1A, even if they advertised more.) So, even plugged in, it would gradually discharge (until I found a powerful enough mobile charger that would actually supply 2.4A). This suggests that it's #1 (it's all in parallel), and not completely isolated and running off mains, which wouldn't have even been possible, unlike my ThinkPad running completely off it's mains power supply.

chcn

Level 1

75 points

Feb 13, 2018 10:33 AM in response to MikePlaga

MikePlaga wrote:

What would need to be calibrated? The charger is charging the battery based on voltage, current draw and temperature, when the voltage gets to 80% full the charging circuit slows to trickle charge then stops at 100% so it basically doesn’t matter where the charging started.

As Lawrence hints at above, the point of the calibration isn’t so much to know when to stop charging; it’s to be able to calculate state of charge so that the user interface can present a bar graph and/or percentage figure to the user that decreases roughly linearly with usage, so the user can judge how much time they have left. (The voltage of a lithium battery does descend with use, but it’s not linear and it’s not sufficient on its own to gauge the state of charge.)

The lithium ion battery packs in smartphones and laptops have intelligent “gas gauge” chips attached to them that collect a bunch of data to aid in this estimation process, and I think Apple layers on even more smarts in this regard.

For one person’s interesting look at the gas gauge IC on one of these batteries, and the data it contains, see here: https://ripitapart.com/tag/iphone-battery-pinout/

chcn

Level 1

75 points

Feb 13, 2018 10:59 AM in response to chcn

chcn wrote:

Lawrence: ... From personal experience I can leave my phone plugged in while using it, and it stays at 100%. Most commonly when I'm using navigation apps in a car. So if it stays at 100% any energy used by the phone is not coming from the battery; the only place it can come from is the power source.

CHCN: Are you sure about this? As noted above, I have monitored closely with Battery Life to see the voltage level and charging/not charging state change, and the charging cycles on an off when using the phone when [plugged in and] fully charged. ...

P.S., While I have definitely experienced this cycling behaviour in the past, I’m re-running this experiment on my iPhone X today. My experiences using my iPad while charging, and still seeing a net discharge, make me wonder if it depends on the current drawn and the current available from the charger. With the screen off in the overnight configuration you suggest, there would be more than enough power from even the small 1A charger to run the phone off of mains power. Whereas, maybe when I observed this cycling behaviour I was doing something more power-hungry (at the very least the screen would have been on). I’ll report back what I observe today.

Lawrence Finch Author

Level 10

204,218 points

Feb 16, 2018 8:19 AM in response to chcn

I just did an experiment. I put a USB power meter in the circuit to the phone and charged to 100%. As expected, the charging current dropped to zero with the phone locked. I then unlocked it and used the phone. As soon as I unlocked it the input current jumped to 1 amp. As I used it the SOC stayed at 100%, but the current varied from .5 amps to 1.2 amps depending on the apps I was running. So apparently the phone was running off line power and not the battery.

Lawrence Finch Author

Level 10

204,218 points

Feb 17, 2018 6:43 AM in response to takith

It won’t hurt to fast charge it. It doesn’t heat enough to hurt the battery, and it only fast charges up to 50%, then slows to a more normal charge rate. And at 80% it starts to taper to an even slower charge, stopping completely at 100%.

Lawrence Finch Author

Level 10

204,218 points

Feb 17, 2018 6:45 AM in response to FarazSidd

There were some apps that would report charge cycles, but Apple took away the ability for apps to measure cycles. Coconut Battery or Mac is the only one that I know of currently. Although iOS 11.3 may have that feature built in.

Lawrence Finch Author

Level 10

204,218 points

Feb 17, 2018 9:56 AM in response to FarazSidd

When you charge the charging stops at 100%. If you use the phone (even background) the battery will discharge to supply power. When it drops by 4% the charger comes back on. In earlier versions this confused people, so the battery percentage in the top bar stays at 100% as long as the cable is plugged in. Battery Life shows the actual state of charge during this process.

Feb 17, 2018 5:05 PM in response to Lawrence Finch

I changed my battery because it was bloated. After one month (now) my iphone can't start without plugging the charger. What should i do? Shall i need to change my battery again? :( please notice me sir

Lawrence Finch Author

Level 10

204,218 points

Feb 18, 2018 9:05 AM in response to FarazSidd

There is no such thing as the efficiency of a battery. (Well, there is, but it has nothing to do with the Capacity or the Charge).

Capacity is how much energy the battery can hold, compared to the battery's original capacity in Milliampere-hours (MaH). So if the battery is specified as 1,000 MaH, but when fully charged it only can supply 900 MaH then its capacity is 90%.

Charge is the current amount of energy that is remaining in the battery compared to the current capacity. So in the same example, if the state of charge is 900 MaH then that will display as 100%.

Both values will vary slightly as the measurement process is not perfectly precise.

Footnote:

Efficiency of a battery is a measure of the energy lost to heat when charging or discharging. For example, if the battery's capacity is 1,000 MaH (5 Watt-Hours) the charger will have to actually supply more than 1,000 MaH because no process is 100% efficient [Second Law of Thermodynamics]. Let's say to reach 1,000 MaH (5 Watt-Hours) the charger provides 1200 MaH (6 Watt-Hours), with 200 MaH (1 Watt-Hour) wasted. That means the efficiency of the charge process is 80%. But that number is not measured or displayed anywhere. And that 1 Watt-Hour will make the phone warmer.

y_p_w

Level 6

17,648 points

Feb 19, 2018 11:09 AM in response to Lawrence Finch

Lawrence Finch wrote:

Capacity is how much energy the battery can hold, compared to the battery's original capacity in Milliampere-hours (MaH). So if the battery is specified as 1,000 MaH, but when fully charged it only can supply 900 MaH then its capacity is 90%.

Charge is the current amount of energy that is remaining in the battery compared to the current capacity. So in the same example, if the state of charge is 900 MaH then that will display as 100%.

Of course any value reported is only an estimate with anything from small to fairly substantial swings on estimated capacity. It's not quite like the old days where NiMH computer batteries were monitored for voltage and internal resistances to get a percentage. That doesn't tell much with lithium rechargeable batteries.

A lot of people complain because older batteries often have these quick jumps in reported remaining battery capacity. There's nothing all that unusual, but people expect their devices to be perfect.

Feb 9, 2018 9:51 PM in response to Lawrence Finch

Read yhis

When to charge your iPhone or iPad