The battery in an Apple Watch 2 can fail, and even swell up. This is a rare problem, but Apple will fix the watch if it occurs.

Swelling is a problem that has long plagued lithium-ion batteries from laptops, phones, and other electronics. The only fix is a replacement of the battery.

Apple alerted its stores on Friday about this Apple Watch battery issue. Its warning says, “Under certain conditions, some Apple Watch Series 2 devices may not power on or they may experience an expanded battery. Apple will service eligible devices free of charge.”

This offer applies only to the 42mm Apple Watch 2. The offer is good for up to three years after the date of purchase. The details of this repair policy can be found in document SN4534 in Apple’s internal GSX portal, according to MacRumors.

Apple Watch battery getting bulgy

Lithium batteries swell because gasses are building up inside. The gas comes from oxidation of battery’s electrolyte (the chemical that allows electricity to flow between the cathode and anode). The root cause is the battery being overcharged. Apple designs its devices to prevent this from happening, so the flaw is in the design, not the way the wearable is charged. Still, it happens from time-to-time.

This issue may cause the battery to fail before it starts to swell. That’s the best-case scenario because a bulging battery often damages the Apple Watch’s casing and/or display.