About this replacement
Apple Watch Series 5 44mm — 3.82V Li-Polymer Replacement Battery (A2181)
This is a 3.82V, 290mAh Li-Polymer cell built to the A2181 specification. It fits the Apple Watch Series 5 44mm and Watch SE2 44mm, including MWVF2LL/A and MWW12LLA variants. Swapping this cell restores charge capacity lost to cycle degradation in the original battery.
- Series 5 and SE2 44mm compatibility: Both models share the same 27.10 x 20.50 x 4.60mm cell footprint, flex connector pinout, and BMS handshake protocol — which is why a single A2181 cell covers both platforms.
- Bench tested on actual hardware: We cycled this cell through the S5 PMIC on the bench. The BMS initialised correctly, charge current stepped down as expected at the CV phase, and no over-voltage events were logged.
- Post-swap charging requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC loses its reference point during a cell swap and cannot recalibrate until it completes a full charge cycle from near-zero.
Why the Series 5 fuel gauge reads incorrectly after a cell swap
The Series 5 uses a coulomb-counting fuel gauge IC that tracks charge state relative to a stored battery profile. When the cell is removed, that reference is lost. The IC then estimates state-of-charge from open-circuit voltage alone, which produces inaccurate readings — often 0% or a sudden drop mid-day. A full charge cycle from near-zero lets the IC re-anchor its count to known endpoints and restore accurate percentage reporting.
Watch not pairing with iPhone after battery replacement
Removing the battery clears the active BLE session between the watch and iPhone. The watch reboots without a live pairing context, so the iPhone treats it as an unknown device. This is not a fault with the new cell. Open the Watch app on the iPhone, tap your watch, select "Unpair Apple Watch," then re-pair from scratch — the process takes under five minutes and restores full connectivity.