The Clever Engineering Behind the New iPhone XS Battery

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The overwhelming theme of iPhone XS reviews? That Apple’s latest flagship smartphone doesn’t differ much from last year’s iPhone X. It looks and works about the same, and still costs a bundle. But those similarities belie one very important difference, an Apple innovation that hides beneath the surface: a battery with a most unlikely shape.

The iPhone X, as well as this year’s iPhone XS Max, both rely on a dual-cell battery for power. That is to say, they essentially use two rectangular lithium-ion units, placed perpendicular to each other, to keep those smartphones humming. Together, they form the shape of an L, snuggled up inside your phone.

There’s nothing particularly exciting about that. You’ll see some variation on it inside any smartphone that you crack open today. (Note: Please don’t crack open your smartphone.) That’s just how batteries for mobile devices, generally outsourced to third-party manufacturers, get made. But not so in the iPhone XS, which as a recent iFixit teardown shows, trades that dual-cell design for a single, continuous L-shaped battery.

Taking the L

The obvious first question: why? The answer’s not as complicated as it might seem, although it comes with an asterisk. By having one big battery instead of two conjoined, you can ditch a little bit of packaging and eliminate the small gap between them, maximizing your capacity. Picture two small train cars in a row. Next to those, put another car that’s as long as both small cars combined. You can fit more into the single, because you’re eliminating two walls and some in-between space. The same principle applies.

“They’re trying to miniaturize everything. You have to pack in as much energy density as you can into this batteries,” says Venkat Srinivasan, deputy director of the Joint Center for Energy Storage Research. “You’re constantly looking to ask yourself, can I decrease something that is not active in the battery? Can I make my current collectors thinner, can I make my separator thinner, and ultimately can I also make my packaging thinner?”

Every mobile company pursues some version of this, sometimes too vigorously. (See Samsung’s exploding Galaxy Note 7 debacle.) Apple is the first, though, to attempt to turn the corner. And that may be the hardest trick to pull off yet.

X-ray images of (from left) the iPhone X, iPhone XS, and iPhone XS Max.
iFixit

You can likely credit Apple’s ability to make an L-shaped battery to a patent it filed back in 2012, detailing a new way fit a lithium-ion battery’s guts into whatever shape it wants. It first applied those wiles in 2015, introducing a new “terraced” battery design in that year’s new MacBook. In that instance, it curved and stacked battery layers to fit every spare corner of the chassis.

The iPhone XS battery appears to take that same breakthrough, and put it in a phone for the first time. Theoretically, that maxes out the available battery real estate. “The form factor on the outside is fixed by a design team, and the components take up what they take up. Everything else that is left, if you can pack it with the battery, that’s fantastic,” says Srinivasan. “With the L-shaped design, if you can minimize the packaging as much as you can, you get a little bit of a bump.”

But you also run into some potentially scary chemistry. Which is where that asterisk comes in.

Another Notch

The iPhone X famously popularized the “notch,” the front-facing black swoop that houses an array of camera components. The iPhone XS battery has a notch as well, a sort of buffer area around that inside corner, designed to maintain the battery’s integrity as it angles around. In fact, the inside of that L-shaped casing, the actual battery looks more like a J.

The reason: Batteries hate edges. And the more of them you introduce, the more potential problems you create.

“There are two problems to having an inward corner on a battery like this. It’s a very intricate, complex process to fold the layers of the battery in the first place,” says iFixit writer Taylor Dixon. “But once the layers are folded, if you have an inward corner, that corner is harder to maintain a flat seal on, and that sealant is also more susceptible to breaking or corrosion during thermal expansion, when the battery’s hot.”

Enter the notch, which creates a sand trap-like buffer around the corner. Rather than a sharp inside angle, the iPhone XS battery actually has a curve, which should help its long-term integrity, and keep it out of Note 7 territory. Dixon says notch also leaves enough room for Apple to put a proper seal on the battery, without protruding beyond the confines of the L-shaped packaging.

The upshot of the notch, though, appears to be that Apple has actually given up, not gained, capacity. The iPhone XS battery weighs in at 2,659mAh; last year’s iPhone X edges it out at 2,716mAh. It’s possible that the overall footprint of the XS battery is smaller than that of the X, to make way for other components; the newer model has more RAM and a bigger camera sensor. But Dixon believes they’re comparable, and that the notch offsets the gains made by switching from dual-cell to single. That may also explain why the XS Max skipped out on the transition this year. Apple did not respond to requests for comment.

Those trade-offs don’t make the breakthrough any less exciting, though, especially given what it portends. “I definitely know down the road this could enable bigger batteries in smaller devices, because they’re living out the dream of filling out the whole chassis with the battery,” says Dixon.

Which is about all that Apple or any company can do, at least until the next breakthrough battery material comes along. Which could be a while.

“I’m a battery scientist, I’m always looking to discover new materials. And that’s hard, it’s really hard to do that,” says Srinivasan. “In the meantime, companies are also trying to say, while we wait for these battery materials to get better, can you find a way to squeeze more into whatever space we have? You can see how these designs come about. Here’s a little bit more you can get.”


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