Why Charging Your Gadgets Over 80% Is Such a Bad Idea
How Tech Works

Why Charging Your Gadgets Over 80% Is Such a Bad Idea

Charging your phone’s battery to 100% is drastically shortening its useful life. But the fix is easy, and while auto companies have known about this and mitigated its consequences for some time, now most major smartphone brands—including Apple, Samsung, and Google—are finally jumping on board. 

When we think about battery longevity, we think about the number of charging cycles that it can stand until it will no longer hold a useful charge. The li-ion batteries used in phones, laptops, and other rechargeable gadgets can be charge-cycled a limited number of times before they no longer hold a useful charge. That’s why we want manufacturers to design devices with replaceable batteries.

But that’s not the whole story. How you charge the battery matters, and keeping the maximum charge below 100% can increase the most relevant stat: the total amount of energy the battery can deliver over its lifetime. 

“That’s my secret. I’m always angry.” — FitBit Charge HR battery, after a lifetime of being charged over 80%

Virtuous Cycle

First, let’s look at what happens inside a lithium-ion battery when you charge it. Here’s iFixit’s resident battery expert, Arthur Shi:

“In general, li-ion batteries don’t like to operate near max capacity. My go-to analogy is a sponge and a cup of water. The sponge represents the graphite anode and the water represents the lithium ions.

“When you charge a li-ion battery, the lithium ions have to embed into the graphite (intercalation). When a sponge is bone dry and you pour water onto it, it readily absorbs water. As it saturates, you have to slow down the water input, or water will start puddling on top of the sponge.”

But this last part is not only slower, it’s also the part that causes the most damage to the battery. 

“Imagine that the water that puddles on top solidifies irreversibly. This would reduce the mating surface area of the sponge and reduce the amount of water that can be recovered from the sponge,” says Shi. “For a li-ion battery, this is known as ‘lithium plating,’ which irreversibly reduces the battery capacity.”

Limiting the maximum charge gives a substantial increase to the overall life of a battery. If we measure the life of a battery not in cycles, but in the number of electrons it can deliver to your phone over its lifetime, then we’re looking at a possible increase of 400%. That is, your battery could deliver 4X the number of electrons. And yes, that means it could last up to four times as long, according to some simple testing:

“[C]harging to 80% instead of 100% multiplies by 4 the amount of energy the battery will have transferred to you over its life—the only tradeoff being to compromise on how much energy you can get out of a full charge (big slices, small cake VS small slices, large cake). This also means you can use your battery for 4 times longer before it gets to the end of its rated life,” “space mechatronics engineer” Mister Mystère wrote on Stack Exchange last year. 

I spoke to iFixit’s independent (and anonymous) consultant, a materials science and engineering PhD and electrical engineer, who confirmed that the general arguments put forth in that Stack Exchange article are good. Although limiting your charge to 80% might not be enough to get the promised 4X increase in lifetime, our consultant said, it might still be one of the best ways to increase longevity. 

Another contributor to that same thread, named Kuba hasn’t forgotten Monica, keeps a device in a remote location, powered by a rechargeable lithium battery and solar panels. The battery packs have more than 20X the required capacity and are kept at only 25% charge. “In those conditions, the batteries last a very long time. Going on 12 years now!” they write. 

Three sets of hands pulling on a Samsung Galaxy S23 Ultra's battery tab.
If all else fails, you can of course change your battery.

The Fix Is In

Now, it’s possible to keep an eye on your charging gadgets and unplug them before they hit 100%, but honestly, that’s never going to work. I have a Shortcut running on my iPhone that notifies me when the battery gets to 80%, but more often than not I still forget to unplug it. 

The good news is that some vendors are starting to build this in. Apple’s Optimized Battery Charging is one take. It learns your daily charging habits, and will charge your iPhone, Apple Watch, or AirPods to 80%, and hold it there, before adding the last 20% just before you need it. If you charge overnight, then it should get topped off just as your alarm wakes you up. “Also, many Samsung phones and tablets have a built-in setting to limit charging to 85% or 80%, respectively,” says Shi.

With laptops, you can install software to do this for you. AlDente Charge Limiter, by AppHouseKitchen, lets you limit your MacBook’s battery to 80% of its maximum (or any other arbitrary percentage), and then top it up to 100% before a trip, for example. And if you leave a modern MacBook plugged in most of the time (hooked up to a monitor and keyboard, for example), then it will notice, and automatically limit charging to 80%.

The problem with third-party apps is that they only run when the computer is on. I use AlDente, but if I apply a software update, or just turn the Mac off while plugged in, it quickly charges back to 100%.

Lenovo’s Vantage Battery Charge Threshold offers more control than Apple’s automatic version, allowing you to specify not only a maximum charge but also to choose a level below which the battery has to drop before it begins charging, so you could have it cycle forever between 40-60% of maximum, for example. With the iPhone 15, Apple has added a setting that lets you limit the charge to 80%. Because it’s built in, it should even work while the iPhone is switched off. 

But why isn’t this standard across all gadgets? After all, a 4X increase in overall battery lifetime is a big deal, lengthening the life of gadgets, and also reducing waste when replacing them. 

One reason might be that the manufacturers don’t care. Charging a battery to 80% means you only get 80% of the maximum run time before the next charge, after all. 

The bottom line is that limiting your batteries’ maximum levels can make a huge difference, saving money and hassle, and avoiding waste. If your device has this option, enable it, and switch it off only when you know you’ll need that extra boost. 

If your device doesn’t have the option, and you can install software to do it for you, then do it. Next time you buy a laptop, phone, or tablet, you should put optimized charging near the top of the list, but for most iFixit readers, the DIY way can be just as effective. You’ll be able to eke out a lot more use from the devices you own right now, and you’ll know exactly when it’s time to replace the battery with a new one.