About this replacement
Apple Watch Series S6 44mm — 3.85V Li-Polymer Replacement Battery (A2327)
This 3.85V, 300mAh Li-Polymer cell is a direct swap for the Apple Watch Series 6 44mm. It replaces part number A2327 — the original cell Apple fits at the factory. Voltage, dimensions (27.70 × 20.10 × 4.40mm), and connector all match the OEM specification.
- Series 6 44mm platform fit: The S6 44mm and 40mm share the same chassis generation but carry different cell footprints and capacities. This cell is sized and rated for the 44mm case only — the 40mm uses a different part.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Series 6 44mm unit. The BMS accepted the cell, reported state-of-charge correctly after a full charge cycle, and held voltage under continuous heart rate and GPS polling without dropping below the BMS cutoff threshold.
- Post-swap charging requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before use. The fuel gauge IC loses its reference point when the original cell is removed. Without a full charge cycle from near-zero, watchOS will report inaccurate battery percentages — sometimes 0% on a charged cell.
Why the Series 6 fuel gauge IC loses calibration after a cell swap
The Apple Watch Series 6 uses a coulomb-counting fuel gauge IC to track battery state. This IC holds a learned capacity model built from the original cell's charge and discharge history. When the cell is physically removed, that reference is lost — the IC starts with no baseline. A single full charge cycle from near-zero allows the IC to re-anchor its model to the new cell's actual capacity, after which percentage readings stabilise.
Watch not pairing with iPhone after battery replacement
Removing power during a swap terminates the active Bluetooth Low Energy session between the watch and the paired iPhone. The watch does not automatically re-establish this session on reboot — it presents as an unpaired device. To fix this, open the Watch app on the iPhone, go to My Watch → All Watches, tap the watch name, select Unpair Apple Watch, then re-pair from scratch. This resets the BLE handshake and restores full connectivity.