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 device 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 device. And, if you configure the device 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 an iPhone 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. 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.

Note that when you first enable Optimized Battery Charging you will not see a benefit immediately; it takes about a week for the charging algorithm to “learn” your usage pattern.

About Optimized Battery Charging on your iPhone - Apple Support

With iPhone 15 series and later Apple added a feature that limits charging to 80%, based on the assumption that leaving a Lithium ion battery at 100% for long periods of time will cause some degradation. While this is meaningful in your EV car, which may remain parked for days at a time, or your laptop that you leave plugged in for days or weeks, it will make little if any difference for an iPhone that is used constantly, and, if Optimized Charging is enabled, will only be at 100% for an hour a day or so. (Newer MacBooks also automatically limit a usually plugged-in device to 80%).

What kind of power sources can I use?

• You can use any Apple power source ever made, as long as it has a USB-A or USB-C output connection.
• Your can use any 3rd party USB-A power source from a reputable manufacturer that is certified as MFI (Made for iOS)
• You can use any USB-C power source from a reputable manufacturer as long as it meets the USB-C/PD (Power Delivery) standard
• The output power of any power source must be at least 5 watts (10 watts for iPads), but there is no upper limit because the phone regulates the charging rate, not the power source: Thus, a 140 watt MacBook charger is OK→140W USB-C Power Adapter - Apple
• You can use any Qi compatible wireless charger with an iPhone 8 or later
• You can use any Qi2/MagSafe compatible charger if your iPhone supports MagSafe
• You should not use ANY power source of unknown provenance, such as gas station, online unbranded or supermarket devices, public charging kiosks, or wall outlets with built-in USB ports.

What about wireless charging?

You can use a Qi wireless charger with iPhone 8 and later model iPhones. You should use at least a 20 watt power source, and the output will be 7.5 watts.

The latest standard is Qi2, or MagSafe charging, available for iPhone 12 and newer; it can charge at a 15 to 25 watt rate depending on iPhone model, and requires a 30 watt or higher power source to get full benefit.

With a true MagSafe disc that is connected to a 20W+ power source and you perfectly align with the sweet spot on the back, you will see a green ring being drawn onscreen showing the level of charge and charging will commence at the higher MagSafe power level (i.e., quicker). If all you see is a green "charging" icon on the screen corner, it will be at the lower-power Qi mode.

You can see that wireless charging, although convenient, is not as energy efficient as charging with a cord. For the low amount of energy required this will not significantly increase the entropy of the universe, but it does mean that the power source and the device will be warmer when charging than with wired charging. It’s also likely that the phone will pause charging at 80% to allow the iPhone to cool off, and display a notification to this effect.

MagSafe info→How to use your MagSafe Charger with iPhone - Apple Support

What if your battery Life isn’t where you think it should be?

Here is Apple’s guidance for managing your battery:

• If the battery in your iPhone drains too quickly - Apple Support
• iPhone Battery and Performance - Apple Support
• About Optimized Battery Charging on your iPhone - Apple Support
• Charge and maintain your iPhone battery - Apple Support

Keep in mind, however, that of all of the billions of smartphones that have been bought over the past 15 years there has not been even one user who was satisfied with their battery performance. Maybe an exaggeration, but not much of one. 

Want still more details?

• 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, may 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 may kill the battery. So if it reaches zero, charge it soon (preferably 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).

Final Notes:

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 14 or earlier (1,000 for an iPhone 15 or later and an iPad). For most people this will be around 2-4 years for the iPhone. iPad batteries will probably outlive the useful life of the device. 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 added 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.

Most of all, don’t obsess over the battery in your device; there is little you can do to either shorten or lengthen the useful life of your battery; it is what it is. You may see videos and articles on how to extend the life of your battery; don’t bother; nothing you do will make any significant difference.