About this replacement
Apple Watch Series 9 — 3.85V Li-Polymer Replacement Battery (A2810)
This is a 3.85V, 280mAh Li-Polymer cell (1.08Wh) replacing OEM part A2810 in Apple Watch Series 9. It fits models A2770, A2772, A2773, and A2857. The battery powers the S9 SiP, LTPO display, heart rate sensor array, and Bluetooth/Wi-Fi radios.
- Series 9 cell compatibility: These four model numbers cover the GPS and GPS + Cellular variants of Series 9 in 41mm and 45mm cases. All share the same A2810 cell footprint, voltage rail, and flex connector — one cell fits across the whole lineup.
- Bench tested on actual hardware: We ran A2810 cells through charge and discharge cycles on Series 9 hardware. The fuel gauge IC completed calibration cleanly after a full charge cycle from near-zero, and the BMS held stable cutoff voltage without tripping the over-discharge latch.
- Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before wearing it. The Series 9 fuel gauge IC loses its reference point during cell removal and cannot recalibrate the state-of-charge reading without one full charge cycle from near-zero.
Why the Series 9 fuel gauge reads 0% after a battery swap
The S9 SiP uses a coulomb-counting fuel gauge that tracks charge relative to a stored reference. Removing the old cell wipes that reference entirely. When the new cell lands, the IC has no baseline, so it defaults to 0% or refuses to display a reading. This is not a faulty cell. A single full charge to 100% re-establishes the reference and the percentage will read correctly from that point on.
Watch not pairing to iPhone after battery replacement
Cutting power during a battery swap terminates the active Bluetooth LE session between the watch and iPhone. The watch comes back up without a valid pairing token, so it will not reconnect automatically. On the iPhone, open Settings → Bluetooth, find the watch, tap "Forget This Device," then re-pair through the Watch app. The watch does not need to be reset — only the BLE pairing needs to be re-initiated from the phone side.