Running a smartphone until it’s dead—a full discharge—every time is not the way to go with modern Lithium-ion batteries. Don’t even let it get that close to 0 percent. That wears out a Lithium-ion battery even faster than normal. Partial discharge is the way to go.
Batteries are on borrowed time from the get-go. The insides are in a state of decay that can’t be helped. Over time, they’re simply going to hold less and less power. If you’ve got an old iPhone 5 or 6 still in use and wonder why it’s only got a charge for a few hours compared to the almost full day you got when it was new, that’s why. The capacity diminishes over time.
The only time you would want to go out of your way to use up a smartphone battery all the way to 0 is to effectively recalibrate the internal sensor that displays your phone’s battery level. It’s hardly guaranteed—in fact, many people don’t think it works at all—but it’s recommended by some, especially if you’ve got a phone that hits 10 percent (or even 20 or 30) and seems to just die.
Even if you do go all the way to shutdown, that may not mean it’s at 0 percent in the battery. Leave the phone be for a few hours, if you think this is worth doing. Then give it a reset (holding down the Home and sleep/wake button simultaneously) for good measure.
Best Thing to Do: Plug the phone in before it asks you to enter a low-power mode(on iPhones, iOS will ask you to turn that on when you hit 20 percent power). Plug it in when the phone is between 30 and 40 percent. Phones will get to 80 percent pretty quick if you’re doing a fast charge. Pull the plug then, as going to full 100 percent when using a high voltage charger can put some strain on the battery. Keep it between 30 and 80 percent charged to increase battery lifespan.
(BTW: Fast charging like we’ve seen in Android phones for a while finally arrived in the iPhone 8 and X. Before, it took an iPhone a couple of hours to go up 50 percent. Now, Apple claims the 8 and the X go from up 50 percent in only 30 minutes with the right chargers. That requires a USB-C power adapter, which in turn means owning a special USB-C-to-Lightning cable, neither of which are included; or using a higher voltage charger like the one from an iPad or even a MacBook.)