About this replacement
Apple Watch Series S9 45mm — 3.86V Li-Polymer Replacement Battery (A3009)
This 3.86V, 300mAh Li-Polymer cell is a direct replacement for the Apple Watch Series S9 45mm. It fits the 45mm case and uses OEM part number A3009. Swap it when the original cell has degraded, lost capacity, or no longer holds a charge through the day.
- S9 45mm platform fit: The 45mm S9 shell runs a specific cell footprint — 27.40 × 20.00 × 4.20mm — with a connector and BMS handshake tied to Apple's fuel gauge IC. This cell matches that footprint and voltage rail exactly, so the watch OS can read state-of-charge correctly after calibration.
- Bench tested on actual hardware: We ran this cell through a full charge cycle on S9 45mm hardware. The BMS accepted the cell without error flags, the fuel gauge initialised after a full charge from near-zero, and charge termination tripped cleanly at 4.35V.
- Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the S9 cannot set an accurate reference point without one complete charge cycle from near-zero after a cell swap — skipping this step causes the percentage display to read incorrectly.
Why the S9 45mm reports the wrong battery percentage after a cell swap
Apple Watch uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell over time. When you remove the original cell, that accumulated reference data is lost. The IC has no baseline for the new cell's impedance or true capacity until it completes a full charge-to-discharge-to-charge cycle. Until that cycle runs, the watch may display 0%, jump to an arbitrary number, or drain the percentage counter faster than the actual cell is depleting.
Watch not pairing with iPhone after battery replacement
Removing the battery cuts power to the Bluetooth controller, which drops the active BLE session entirely. The watch and iPhone no longer share a valid session key when power returns. The watch may appear in range on the iPhone but fail to reconnect or show a disconnected icon that won't clear. Fix this by opening the Watch app on the iPhone, going to General → Reset → Erase Apple Watch Content and Settings, then re-pair from scratch — it takes under five minutes and restores full connectivity.