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An iPhone can go from zero to 50% charge in about 30 minutes using a 20W charger, and then go to being fully charged in under two hours.
Given that an iPhone can charge up so rapidly, it doesn't make sense that we hook it up to a charger for eight hours a day. That works out to a third of its lifetime.
Now, Apple has baked Optimized Battery Charging into iOS, which stops the iPhone from charging once the battery hits 80% and only adds the final 20% in time for you to wake up in the morning. But this still means that the iPhone is attached to power all night.
And it's still charging, albeit at a slower rate. This is how it stays at 80%.
So why is it bad to leave your phone connected to a charger for extended periods?
Heat is a killer of batteries. While there's nothing you can do about the normal wear and tear that a battery experiences from being charged and discharged, heat is something you can do something about. When your iPhone is connected to a charger, it gets warm -- warmer than if it isn't connected, and even though that increase might only be a few degrees above room temperature, doing this overnight, every night, doesn't seem smart to me.
So, I've radically changed how I charge my iPhone.
Rather than charge it at night, I give it a charge or two during the day. This is the charging schedule that works for me:
I put it on for 30 to 45 minutes in the morning after I get up (I also charge up my Apple Watch at the same time).
Then I charge it around mid-afternoon for another 30 minutes or so to bring it to that 80% mark, which usually will be enough to see me through to the next morning.
Note that if I'm going to be spending a lot of time in the car or out and about, I'll also use a car charger or carry a power bank with me.
This system works well, and I believe it puts less stress on the iPhone's battery (time will tell) as well as the charger. It also makes me plan where I need my chargers, so I'm not just relying on a bedside charger.
And when I'm at home, working on the road, or traveling, this charging regimen hasn't let me down.
There are so many good comments here that I feel it's worth taking some time to respond to some of the main themes.
Several people have commented that "the battery does not charge all night," if you leave the device connected to a charger.
Even with the Optimized Battery Charging feature enabled on an iPhone, it's still charging during the night. Why? Because even when it's just sitting on the nightstand the iPhone is using battery, and it's being topped up to stay at that 80% point. You can observe the charging/discharging on iOS chart by going Settings > Battery. Leave the iPhone off charge and it will lose charge overnight. When charging with Optimized Battery Charging enabled it will fast charge to 80% and then stay there until the final charge is added. It's only able to stay at that 80% because the battery is being topped up slowly.
And no, the iPhone doesn't run off the power being supplied to it over the cord, this is why the battery has to charge up a few percent before it will start up when completely flat.
If you have a USB power meter handy, you can watch the iPhone draw charge periodically to keep the battery topped up.
With iOS 13 and later, Optimized Battery Charging is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged. When the feature is enabled, your iPhone will delay charging past 80% in certain situations.
It reduces wear on the battery by limiting the amount of time the battery spends fully charged. It's a good mechanism, but by manually taking control of the charging, I find that I can choose to not push my battery into that fully charged place.
There's also confusion in the comments about how many recharge cycles Apple rates the iPhone's battery as being good for before it's considered worn.
For the iPhone, it's not 1000 or 2000 recharge cycles, but 500.
"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions."
I also want to pick up on a point made several times about chargers, and how smart they are with all their double overcharge protections and all that.
Absolutely, modern chargers are good, with Apple's chargers probably being some of the very best. And even third-party switch mode power supplies are both very efficient and smart in that they can handle a variety of charging protocols.
But I've seen the sketchy chargers that people use. I've also seen chargers "crash" and behave weirdly, which is why it's a good idea to occasionally unplug or switch off chargers so they "reboot" the chip that controls how they operate.
It's also worth bearing in mind that when it comes to chargers and the devices they charge up, the chargers, despite the brains that control charging protocols and watch out for overvoltage and overheating and such, are the dumber part of the equation.
While on the subject of chargers, a few people mentioned using slower 5W chargers to charge their devices. If you're someone who wants to charge overnight then this might be the better way to do it because it completely eliminates the issues associated with overheating and keeping the battery at 100% for extended periods.
Modern lithium-ion batteries are also incredibly smart, and they have safeguards in place against both overcharging and overdischarging (overdischarge is a very big deal because if you allow a lithium-ion battery to discharge below a certain voltage, it won't usually recharge by normal means).
The final point I want to make here is that charging is complex, and manufacturers are all boasting about how fast the battery charges and how long the battery lasts, and this is causing some manufactures to do things that aren't that smart (I'm looking at you, Android handset makers). But what hasn't changed are the habits and beliefs of the end users.
Charging overnight used to be a necessity. Now it's a habit.