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 Ion batteries, however, and some of what we "know" from the NiCd and NiMH days is actually harmful to modern battery technology.

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. And you don't have to turn it off to charge it; in fact, you shouldn’t. And you can leave it plugged in while using it if you want to.

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.

With iOS 13 and later iOS devices now have an Optimized Charging option. With this enabled, if you charge overnight the phone will stop charging at 80%, then resume charging in time to reach 100% in time to meet your normal usage pattern. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often; over the long term this will extend the useful life of the battery.*SEE NOTE BELOW The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. The figure below shows how Optimized Charging works; notice that it started charging when put on a wireless charger around 9:30 PM, stopped charging at 80% a short while later, then resumed charging at 6 AM to reach 100% around 7 AM.

You can charge an iPhone, iPad or iPod Touch with any quality USB-A or USB-C power source and matching cable. Essentially any Apple power source and cable that fit into the connectors at either end are compatible and safe to use. There will be performance differences, with USB-C faster than USB-A to charge, but they will all work (except that a USB-A “cube” doesn’t have sufficient power to charge an iPad). For an iPhone 12 or later you can fast charge using a 20 watt or higher power USB-C that meets the PD spec. The Apple 20 watt and higher is PD compatible, but not all 3rd party power sources are yet.

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• The "charger" for an iOS device is built into the device (Specifically, a “U2 Tristar” microcircuit). 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 Ion 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).
• It’s best to charge it when it goes below 20%, primarily to avoid the risk of it going to zero when you don’t have a charger available, and to prevent unexpected shutdowns when you might need the phone. The phone helpfully changes the color of the battery icon to yellow below 20% and enables Low Power mode to preserve the remaining charge for as long as possible.
• Never leave a phone unused for weeks or months on end without periodically recharging it. If it is going to be unused for a long period, Apple recommends leaving it at around 50% charged (not full, and not empty).
• 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.
• You should also use only high quality cables, as cables that do not meet Apple's "Made for iPhone" (MFI) standards can damage your phone (see: Why Counterfeit Lightning Cables Kill iPhones - Motherboard)
• The power source needs to supply at least 1 amp (5 watts) to charge an iPhone, and 2 amps (10 watts) 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 or even a USB-C MacBook power adapter.
• With the iPhone 8 and later you can also benefit from charging with a USB-C power source. This will charge the device much faster, but still safely. See this Apple support link—>Fast charge your iPhone - Apple Support
• For fast charging the iPhone 11 the recommended power source is the 13 watt adapter, and for the iPhone 12 and 13 Apple recommends the 20 watt adapter for the fastest charging. However, you can use a higher power adapter safely; the phone will only take the power it needs. Thus, if you have a 61 watt MacBook adapter it is safe to use that.
• 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.

Finally, keep in mind that batteries are "consumables". Their capacity starts dropping the day they come off the assembly line. Battery life is determined primarily by "full charge cycles" - A full charge cycle is 0% to 100%, or any combination that adds up to 100%, such as 50% to 100% or 30% to 80% twice, 80% to 100% 5 times, etc. The battery will gradually lose capacity, and is rated to remain above 80% capacity (that's total capacity, not charge level) for 500 full charge cycles for an iPhone (1,000 for an iPad). For most people this will be around 2 years for the iPhone. Apple will replace the battery for a reasonable fee (Use this link to find the cost for your model and country→iPhone Battery Replacement - Official Apple Support) if it goes below 80% after the original 1 year warranty or AppleCare+ warranty period. Within the warranty the replacement is free.

While this tip is specifically for iOS devices, the same principles apply to all other Apple devices (watches, AirPods, MacBooks) as well as most other manufacturer’s products that have Lithium chemistry batteries.

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*More about Optimized Charging: I was asked by a user in the iPhone forum (https://discussions.apple.com/thread/251707376?answerId=253281558022#253281558022) why the iPhone does this. There is some evidence that Lithium chemistry batteries should not be left fully charged. So, for example, Tesla recommends not charging the car’s battery beyond 80% unless planning for a long trip, and in fact it can be programmed to do this. That doesn’t mean they should never be fully charged, just that they should not be left at 100% for long periods of time. Most electric bike instructions have the same advice. And Apple recommends storing devices that won’t be used for a while at 50% charge.

When the phone is connected to power and not charging the energy to run the phone comes from the external power source, rather than the battery, thus letting the battery “rest”. And any smartphone is always doing something that uses energy, even when it isn’t actually being used. As the battery’s useful life is measured in full charge cycles, with only a small contribution from chronological age, leaving the battery at 80% but still connected to power achieves 2 aims: Not leaving it at 100% for long periods of time, and reducing the number of full charge cycles while still powering the phone.