About this replacement
Apple Watch Series S8 45mm — 3.86V Li-Polymer Replacement Battery (A2815)
This 3.86V, 300mAh Li-Polymer cell is a direct cell replacement for the Apple Watch Series S8 45mm. It fits the 45mm case specifically — not the 41mm variant. Voltage and capacity match the OEM A2815 spec exactly.
- S8 45mm fitment: The 45mm case uses a larger cell footprint (27.40 × 20.00 × 4.20mm) than the 41mm model. The two cells share the same voltage rail but differ in physical size and capacity. Fitting a 41mm cell into the 45mm chassis leaves the battery unseated and the connector under stress.
- Bench tested on actual hardware: We ran this cell through the S8 BMS handshake and confirmed the fuel gauge IC accepted the new cell reference. The BMS held charge termination at the correct upper voltage threshold with no false fault flags.
- Post-swap charging protocol: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the S8 platform cannot calibrate its state-of-charge reference without a full charge cycle from near-zero following a cell swap. Skipping this step causes the watch to report 0% or shut down unexpectedly.
Why the S8 45mm shuts down before the battery percentage hits zero
The S8 uses a coulomb-counting fuel gauge IC that tracks charge in and out of the cell over time. When a new cell replaces a degraded one, the IC still holds the old cell's impedance profile in memory. The mismatch causes the IC to miscalculate remaining capacity and trigger a low-voltage cutoff well above the actual depletion point. A full charge-to-100% cycle after swap resets the reference and corrects the shutdown threshold.
Magnetic charger not registering after reassembly
If the watch sits on the charger but shows no charging indicator, the charging contacts on the rear of the case have likely shifted during reassembly. The S8 back glass is held by adhesive, and pressure applied off-centre during resealing can tilt the contact pads by fractions of a millimetre. Remove the back, check the contact pads are flush and centred, then reseal. Confirm charging resumes — the charging indicator should appear within 30 seconds of correct contact.