40mm Apple Watch Series 5 Features New Battery Design, 10% More Capacity

A teardown of the 44mm Apple Watch Series 5 conducted by repair site iFixit found few design changes compared to the 44mm Apple Watch Series 4, but a subsequent teardown of the 40mm model has found some more significant design deviations.

The battery inside of the ‌Apple Watch‌ Series 5 has a new metal casing, likely constructed from aluminum, rather than the foil pouch that surrounds most lithium-ion batteries.

Based on an Apple patent, iFixit suggests that the new metal casing is designed to be a more space-efficient method for sealing batteries. The size of the battery compartment in the Series 5 doesn't appear to have changed significantly over the Series 4 model, but the battery in the Series 5 offers 10 percent larger capacity than the Series 4 (0.944Wh vs. 0.858Wh).

Foil pouches feature a larger seal than the metal enclosure, so with the design change, Apple may have been able to use the space saved for a slightly larger battery.

New battery design on left, old design on right
iFixit says that the metal casing also makes the battery physically stronger, which is ideal for repair purposes. An aluminum enclosure makes it more difficult to puncture during repairs.

The new design is limited to the 40mm ‌Apple Watch‌ Series 5 and was not seen in the 44mm Series 5 model. iFixit says that it's not clear why the updated design was used in one watch and not the other, but it could be that the smaller watch needed additional battery or Apple wanted a "low-key" way to test the design.