You’re letting your phone die
Lithium ion batteries become volatile when they drop down to low power, so you should avoid getting that pesky “low battery” warning too often. Every full discharge wears down your battery ever so slightly. That being said, Apple and other experts suggest you let it fully run out of juice every one to two months to “calibrate” the battery gauge.
You’re overheating your battery
Okay, this isn’t specifically a charging issue. But it’s still very important. You’ve probably noticed that your phone is annoyingly sluggish after its been sitting out in the sun. That’s because iPhone batteries really, really hate extreme temperatures, and have to work extra hard in high heat or cold. According to Apple, the “comfort zone” for iPhones (as well as iPads, iPods, and Apple Watches) is between 32 and 95 degrees. Bottom line: your phone is basically a needy little computer baby that needs to be bundled up and cooled off on the regular.
You’re leaving your case on while you charge
While we’re on the subject, if your phone feels too hot when it’s plugged in, it’s possible your fancy case might be trapping in the heat that’s produced while charging. So do your phone a favor and take it off.
You’re using the wrong charger
Your Apple-issued charger is designed to cut power when the battery hits 100%, but those cheapo unauthorized third-party USB cords are not, and may do some damage by excessively juicing the battery. Plus, ya know, they could kill you.