About this replacement
Apple Watch Series 4 44mm — 3.82V Li-Polymer Replacement Battery (A2059)
This is a 3.82V, 290mAh Li-Polymer cell for the Apple Watch Series 4 44mm (A1976). It replaces the original A2059 battery when the watch can no longer hold charge or shuts down unexpectedly. The cell powers the display, processor, heart rate sensor, and all wireless functions.
- Series 4 44mm platform fit: The A2059 footprint is specific to the 44mm Series 4 chassis. The 40mm variant uses a different cell with a different connector orientation. Fitting this cell to any other series risks connector damage and BMS mismatch.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Series 4 unit. The BMS accepted the cell without error flags, and the fuel gauge IC re-calibrated correctly after a full charge from near-zero.
- 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 Series 4 loses its voltage reference during cell removal and cannot read state-of-charge accurately until it completes one full charge cycle from near-zero.
Why the Series 4 reports 0% immediately after a battery swap
The Series 4 uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell. When you remove the old battery, the IC loses its reference point entirely. It has no way to know where the new cell sits in its charge curve until it sees a full cycle. The watch may display 0%, refuse to boot, or shut down within seconds of powering on. Charge the watch to 100% on the magnetic charger before using it — that single full charge resets the IC's baseline and the percentage readout will stabilise.
Watch not pairing with iPhone after battery replacement
Removing the battery kills the BLE session between the watch and the paired iPhone. The phone sees the watch as disconnected and may not automatically re-establish the link once the watch powers back on. The fix is not a software update — it is a manual re-pair. On the iPhone, open the Watch app, tap your watch, select Unpair Apple Watch, then set it up again as a new device. The watch retains its data if a backup completed before the swap.