About this replacement
Apple Watch Series 3 GPS 38mm — 3.81V Li-Polymer Replacement Battery (A1847)
The A1847 is the OEM-matching cell for Apple Watch Series 3 GPS 38mm (A1860). It runs at 3.81V with a 260mAh (0.99Wh) capacity. This Li-Polymer cell fits the 38mm case form factor — not the 42mm variant.
- Series 3 38mm platform fit: The Watch Series 3 GPS 38mm and Watch Series 3 38mm (A1860) share the same 25.06 × 16.50 × 5.10mm cell bay, voltage rail, and flex connector. A1847 is the correct part for both listings — the same BMS handshake applies across the GPS-only configuration.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Series 3 38mm unit. The BMS accepted the cell without fault flags, the fuel gauge initialised correctly after a full charge cycle, and heart rate plus GPS sensors drew current within expected parameters.
- Post-swap charging requirement: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on this platform cannot set an accurate reference point without a full charge cycle from near-zero following a cell swap — skipping this step causes the percentage display to read incorrectly for days.
Why the Series 3 fuel gauge loses calibration after a cell swap
The Series 3 uses a coulomb-counting fuel gauge IC that tracks capacity relative to a stored charge reference. Removing the old cell wipes that reference from volatile memory. The new cell arrives partially charged, so the IC has no fixed zero-point to count from. Until a full charge cycle resets the reference, the gauge extrapolates — producing readings that swing between 0% and artificially high values within the same hour.
Watch not pairing to iPhone after battery replacement
Removing power during a battery swap terminates the active BLE session between the watch and iPhone. The watch comes back up as an unrecognised peripheral because the encrypted pairing keys are stored on the watch side but the phone-side session state is stale. Open the Watch app on iPhone, tap your watch, select "Unpair Apple Watch," then re-pair from scratch. The watch does not need to be erased — unpairing and re-pairing restores the encrypted channel without data loss if iCloud backup is active.