About this replacement
Apple Watch 1st Gen 42mm — 3.8V Li-Polymer Replacement Battery (A1579)
This 3.8V, 240mAh Li-Polymer cell is a direct cell replacement for the first-generation Apple Watch 42mm. It fits the larger case variant of Apple Watch Series 1, sitting behind the display stack in the 28.30 × 20.20 × 5.00mm footprint. Swap it when the original A1579 cell no longer holds charge through a full day of normal use.
- 42mm case compatibility: The 42mm and 38mm Watch 1 cases use different cell footprints and separate OEM part numbers. This A1579 cell fits only the 42mm variant. The connector, ribbon length, and BMS handshake are specific to that chassis — fitting it in the 38mm shell is not possible.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on Watch 1 hardware. The BMS initialised correctly, the fuel gauge registered state-of-charge without error codes, and the cell held voltage within spec across the test window.
- Post-swap charge procedure: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on Watch 1 loses its charge reference during cell removal and cannot recalibrate without a full charge cycle from near-zero after the swap.
Why the Watch 1 fuel gauge reads wildly wrong after a cell swap
The Watch 1 uses a dedicated fuel gauge IC that tracks charge state by counting coulombs in and out of the cell. When the cell is removed, that running count is lost entirely. The IC has no stored reference for the new cell's actual capacity, so it estimates from the first partial charge rather than a calibrated baseline. This produces incorrect percentages — sometimes jumping or dropping 20–30% in one charge step — until you complete at least one full charge cycle to 100%.
Watch not pairing to iPhone after battery replacement
Removing power from the Watch 1 during a battery swap terminates the active BLE session between the watch and the paired iPhone. The watch comes back up as an unpaired device from the phone's perspective, even though the watch itself still shows the pairing stored. To fix this, open the Watch app on the iPhone, go to My Watch → All Watches, tap the info icon next to the watch, and select Unpair Apple Watch — then re-pair from scratch. The watch data restores from iCloud backup once pairing completes.