An illustration of a smartphone with its battery charged to 80 percent.

Apple’s upcoming iOS 13, due out in mid-September, will have a simple technique to help your iPhone battery last longer: It won’t charge the battery all the way.

On its page listing iOS 13 features, Apple says “battery optimization” will “slow the rate of battery aging by reducing the time your iPhone spends fully charged. iPhone learns from your daily charging routine so it can wait to finish charging past 80 percent until you need to use it.”

Your iPhone already knows quite a bit about your daily routine, including when you head home from the office and when you go to sleep. With this information, it can anticipate when you’ll be plugging in to charge your phone for long period without needing a full 100 percent charge.

In practice, this means that if, like many smartphone users, you plug your phone in to charge overnight, the device will charge just partway, then stay there while you sleep. It will then charge to 100 percent at your regular waking time.

Why would keeping your battery charged to just 80 percent help it last longer? It all has to do with how lithium-ion, or Li-ion, batteries wear out over time. These batteries, which power the iPhone and most other electronic gadgets on the market, degrade slightly with every charging cycle. They can only charge up and then discharge to run your device so many times. 

More on iPhone Battery Life

When you keep your phone plugged in for extended periods, it doesn’t charge to 100 percent and stay there. Instead, the phone draws power from the battery to maintain contact with nearby cell towers and for other tasks. This will discharge your battery ever so slightly, and your battery will then recharge, and then discharge again—essentially, wasting battery cycles. Keeping your phone charged at 100 percent also adds heat to your battery, and Li-ion batteries don’t like that.

“I don’t sleep with my phone plugged in overnight,” says K.M. Abraham, Ph.D., one of the pioneers of the Li-ion battery and a professor at Northeastern University in Boston. “I charge it to 80 or 90 percent, and then I remove it from the charger.”

“It’s like if you take a glass and fill it up to its rim all the time,” says James Dickerson, chief scientific officer at Consumer Reports. “At some point, the way batteries operate, the rim starts to degrade. You won’t be able to retain that amount of water, or that amount of charge, so the battery starts to get worse.”

Li-ion batteries have a Goldilocks zone of charge of between 40 and 80 percent, according to experts. In a perfect world, we would all charge our phones to 80 percent, unplug them, and then plug them back in when they got close to 40 percent. Apple’s battery optimization feature will keep you from overcharging on the high end—though it won’t find a plug for you once the battery starts to run down.

This isn’t Apple’s first attempt to address declining iPhone battery life. In late 2017, the company cut its battery replacement charge to $29, a price drop of $50, after fielding complaints about its decision to quietly slow some iPhones to compensate for worn-down batteries and prevent phones from spontaneously shutting down.

While iOS 13 won’t be available until mid-September, a number of iOS mobile apps, including Battery HD+ and Charge Alarm Pro, are already meant to do the same thing. Accu​Battery is a similar app for Android devices. Consumer Reports has not tested these apps and can’t vouch for their efficacy.

Consumer Reports maintains a list of the top smartphones for battery life, along with tips for extending battery life. For instance, don’t leave your phone out in the sun, or in your car on a hot day. Temperatures higher than 95° F can do permanent damage.

If charging to 80 percent means that batteries last longer, why don’t phone makers have all their products charge to 80 percent, then stop? The reason is simple: Battery would run down more quickly during the day, and daily battery life is a high priority for lots of smartphone users.

“Phone manufacturers don’t care if the battery doesn’t degrade as fast if they can get 2 hours more of daily run time,” Abraham says. In the past, this was a good trade-off for many smarphone owners because cellular contracts meant so many people upgraded to a new phone every 22 months or so. Battery life wouldn’t degrade much by then.

However, consumer habits are changing. Research firm Kantar Group has found that in the past couple of years, people have begun waiting an average of two months longer to buy a new phone, with consumers now waiting to 24.1 months to replace their phone.

There are good reasons for that, in addition to the decline of the two-year contract. Big, beautiful displays; high-quality cameras; and water resistance have been standard on high-end phones for a few years now. And some newer phone developments, such as the demise of the 3.5-mm audio jack, are turn-offs for many potential shoppers.

The pace of innovation has slowed: The difference between a smartphone you bought in 2010 and one you bought in 2012 was immense; the difference between a smartphone you bought in 2017 and one available in 2019 is much more incremental.

As people hold on to their phone for longer periods, more manufacturers may start to follow Apple’s lead.  

“If a smartphone manufacturer is smart, they will have to take this into consideration,” Dickerson says. “Manufacturers will lose customers if their batteries start dying. They will attribute it to being a characteristic of the phone.”