Is it bad to let an iPhone 12 Pro charge overnight?

Modern phones charge to they get to 100% charged and then the circuity turns off. It will only turn on again if the phone needs to be topped up.

Also, modern iPhones, well not that modern actually from 6s up learn your daily routine and when they are typically disconnected from the charger and plan the charging to be completed at that time. For example if they need to slow the charging rate down so they are not charged before that time they will. This is why on modern iPhones it will come up with “scheduled to finished charging at….” If you always disconnect your iPhone at 6am for example your iPhone will plan the charging to be completed at that time not at 4 am etc etc.

The most basic thing of charging your iPhone 12 Pro 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.

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