A teardown of the Apple Watch Series 5 yesterday revealed that the internals are mostly the same as the Series 4. iFixit has now given the 40mm Apple Watch Series 5 the teardown treatment, and it turns out that it houses some major battery changes compared to last year’s model.

According to the teardown, the 40mm Apple Watch Series 5 features a new battery design with a new metal casing, likely made out of aluminum. Citing an Apple patent, iFixit speculates that the metal casing is likely more space-efficient than alternatives.

The battery in the 40mm Series 5 packs to 10% more battery capacity, despite the fact that the battery compartment has changed:

The new metal casing also makes the battery physically stronger, according to iFixit. This makes the battery less of a hazard during repairs because the metal casing can protect against accidental punctures.

The traditional foil pouches leave a larger seal than this new metal enclosure, meaning Apple can theoretically put previously wasted space to good use.

Furthermore, the change suggests that Apple would be able to secure batteries to the device with screws, rather than glue:

It’s unclear why Apple only switched the 40mm Apple Watch Series 5 to this new battery. The 44mm Series 5 features a slightly larger battery, but with the traditional foil pouch exterior. Check out our coverage of yesterday’s Apple Watch Series 5 teardown for more details on the differences and similarities compared to the Series 4.