This week we tore down the 44mm Apple Watch Series 5 and discovered only a handful of minor differences from last year’s Series 4. However, we eventually got our hands on inside the 40mm model and noticed a fairly significant change not present on the larger model.

Instead of the usual dark-colored outer foil pouch surrounding the battery (which most lithium-ion batteries sport), we noticed a snazzy new metal casing, most likely aluminum. This is the first time we’ve seen this in an Apple product, and it might tell us something about the future of battery replacements.

The new design of the 40mm Apple Watch Series 5’s battery.

We located what we think is the patent that explains this new design—emphasis on “think.” We’re assuming what’s described in this patent is the same thing we’re seeing with this new Apple Watch battery, and it certainly looks like it, but it’s also possible we’re looking at something unrelated. With that said, we combed through the text and found a couple of interesting things.

First, this new metal casing seems to be a more space-efficient method for sealing up batteries. In the Apple Watch Series 5, the overall size of the battery compartment doesn’t look like it changed much, if at all. But Apple managed to cram in 10% more battery capacity, and this patent seems to explain how. The traditional foil pouches leave a larger seal than this new metal enclosure, meaning Apple can theoretically put previously-wasted space to good use.

New battery design (left) vs. old design (right)

A closer look at the metal casing

Additionally, the metal casing makes the battery physically stronger. This is fantastic for repair purposes, since it’s all too easy to puncture a lithium-ion battery during repairs. The outer metal shell should allow device owners to pry out the battery without risking damage to the internals, reducing the associated fire hazard.

Carefully peeling off the metal enclosure to reveal the battery components inside.

Lastly—and perhaps most importantly—the patent suggests that Apple wants to use this metal casing design to secure the battery to the device with screws, which would allow them to (hopefully) skip the repair-unfriendly glue. What’s more, they’d be able to accomplish this with no extra external brackets that would otherwise take up even more space. This year’s Apple Watch battery was unfortunately still glued in place, despite the new metal-encased battery, but we’re excited by the prospect that things could improve in the near future.

An illustration from the patent showing off the integrated screw holes.

Whether or not they switch to screws, though, this new battery case design is surely a boon for safety and space efficiency—these days, we need every extra hour of battery life we can get.

Again, this new design is only in the smaller 40mm Series 5. We’re not quite sure why that is. Perhaps the smaller model was hurting the most for battery life? Or Apple wants to low-key test this new design? We don’t know, but it looks like something that could easily show up in more products in the future.