About this replacement
Apple iWatch Series 4 40mm — 3.85V Li-Polymer Replacement Battery (A2058)
This is a 3.85V, 220mAh Li-Polymer cell replacing part number A2058 in the Apple iWatch Series 4 40mm. It fits models A1975, A1977, and A2007 — all 40mm Series 4 variants sharing the same battery bay and connector. Swapping this cell restores the watch's ability to run fitness tracking, heart rate monitoring, and notifications at full capacity.
- Series 4 40mm compatibility: The A1975, A1977, and A2007 model numbers cover GPS, GPS + Cellular, and regional variants of the 40mm Series 4. All share the same 23.80 × 18.92 × 4.44mm battery footprint, the same flex connector, and the same BMS communication protocol — one cell covers the entire 40mm Series 4 lineup.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a Series 4 unit, confirmed BMS handshake on reconnect, and verified the fuel gauge IC accepted the new cell without throwing a service warning on the watch face.
- Fuel gauge calibration after swap: Place the watch on its magnetic charger immediately after fitting this cell and charge to 100% before powering the device on. The Series 4 fuel gauge IC loses its reference point when the original cell is removed — skipping this step causes the percentage readout to jump or read incorrectly from the first boot.
Why the Series 4 shows a service alert after a battery swap
watchOS includes a battery health check that flags replacement cells until the fuel gauge IC completes a full calibration cycle. The alert appears in Settings → Battery → Battery Health and typically reads as "Unable to determine battery health." This is a calibration state, not a fault. Running one full charge from near-zero to 100% and then using the watch until it prompts a charge clears the flag in most cases.
Watch not pairing with iPhone after battery replacement
Removing the battery terminates the active BLE session between the watch and the paired iPhone. When the watch reboots with a new cell, the phone-side Bluetooth stack does not automatically re-establish the link. Open the Watch app on the iPhone, go to General → Reset, and select "Reset Network Settings" on the watch — this clears the stale session. After that, keep the watch and phone within one metre until the pairing indicator confirms connection.