About this replacement
Apple Watch Series S9 41mm — 3.87V Li-Polymer Replacement Battery (A2954)
This is a 3.87V, 280mAh lithium-polymer cell for the Apple Watch MR913LL/A (Series S9 41mm). It restores power to the watch's display, heart rate monitor, and notification functions after the original cell has degraded. Fit Model coverage includes MR913LL/A and the Watch Series S9 41mm platform.
- S9 41mm platform fit: The Series S9 41mm case uses a fixed connector orientation and a specific cell footprint — 26.10 x 20.10 x 4.60mm. The A2954 cell matches that footprint exactly, which matters because even a fraction of a millimetre over thickness will prevent the back cover from seating flush and break the magnetic charger contact.
- Bench tested on actual hardware: We ran the A2954 cell through charge and discharge cycles on an S9 41mm unit. The BMS accepted the new cell without flagging an incompatible cell error, and the fuel gauge registered charge input immediately once placed on the magnetic charger.
- Post-swap charge requirement for S9 fuel gauge: After fitting this cell, place the watch on its charger straight away and run it to 100% before wearing it. The S9 fuel gauge IC loses its reference point when the cell is removed — it cannot accurately track state of charge until it completes a full charge cycle from near-zero on the new cell.
Why the S9 magnetic charger shows no connection after a cell swap
The Series S9 back cover doubles as the charging contact surface. If the cover is not pressed fully flush after reassembly, the magnetic charger puck will attract but the charging pins will not make consistent contact. The watch may show the charging animation briefly, then stop. Check that the back glass is seated evenly around all four edges and apply firm even pressure until it clicks. Once correctly seated, place it on the charger and confirm the charging indicator stays on continuously.
Watch not pairing with iPhone after battery replacement
Removing the battery cuts power completely, which terminates the active Bluetooth Low Energy session between the watch and the paired iPhone. When the watch restarts on the new cell, it may not automatically re-establish that session. On the iPhone, open the Watch app, go to General, and tap the watch name — if it shows as unavailable, select "Unpair Apple Watch" and pair it again from scratch. The watch data will restore from the paired iPhone backup once pairing completes.