About this replacement
Apple Watch Series 3 GPS 42mm — 3.82V Li-Polymer Replacement Battery (A1875)
This is a 3.82V, 340mAh Li-Polymer cell that directly replaces the A1875 battery in the Apple Watch Series 3 GPS 42mm and Watch Series 3 42mm (A1861). It restores normal smartwatch operation after the original cell has degraded past the point of lasting through a full day. Capacity is sourced from the original cell specification — 340mAh, 1.3Wh.
- Watch Series 3 42mm platform: Both the GPS and GPS + Cellular variants of the 42mm Series 3 use the same A1875 cell footprint — same voltage rail, same flex connector, and the same fuel gauge IC handshake via the S3 SiP. Fitting this cell into either variant works without modification.
- Bench tested on actual hardware: We ran this cell through charge cycles on Series 3 hardware. The fuel gauge IC accepted the new cell without throwing a service warning once a full charge cycle from near-zero was completed. BMS communication stayed stable across multiple cycles.
- Post-swap charge requirement: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before use. The fuel gauge IC on this platform loses its reference point when the cell is swapped — without a full charge cycle from near-zero, the percentage readout will be inaccurate and the watch may shut off unexpectedly at apparent mid-charge levels.
Why the Series 3 shows 0% immediately after a battery swap
The Series 3 uses a coulomb-counting fuel gauge IC inside the S3 SiP. When the cell is disconnected, that IC loses its charge reference entirely. It has no prior data to work from on the new cell, so it defaults to 0% or refuses to display a reading. This is not a fault with the replacement cell. Place the watch on charge immediately after reassembly and let it reach 100% — that single full cycle re-establishes the fuel gauge baseline.
Magnetic charge contacts not making after reassembly
If the watch sits on the charger but shows no charging animation, the rear crystal or case back has shifted slightly during reassembly and the charge contacts are misaligned with the magnetic puck. Press gently on the back of the watch while it rests on the charger — if the animation appears, the back needs reseating and re-adhesion. Clean both the watch contacts and the charger puck with isopropyl alcohol before reseating. The contacts on this model sit at a fixed radial position — even a 1–2mm offset is enough to break the circuit.