About this replacement
Apple Watch Series S6 40mm — 3.85V Li-Polymer Replacement Battery (A2345)
This 3.85V, 260mAh Li-Polymer cell is the direct replacement for the Apple Watch Series S6 40mm. It fits the 40mm case only — not the 44mm variant. Swap it when the original cell no longer holds charge through a full day of normal use.
- S6 40mm cell dimensions: The A2345 cell measures 25.30 × 18.50 × 4.60mm. The 44mm S6 uses a physically larger cell with a different part number. Fitting the wrong size risks pressure damage to the display and incorrect BMS handshake with the S6's power management IC.
- Bench tested on actual hardware: We ran this cell through charge-discharge cycles on S6 hardware. The BMS accepted the cell without error flags, the fuel gauge recalibrated after a full charge from near-zero, and health reporting in watchOS stabilised within two full cycles.
- Post-swap charge protocol: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The S6 fuel gauge IC loses its reference point when the cell is disconnected. Without a full charge cycle from near-zero, the percentage display will read inaccurately for days.
Why the S6 fuel gauge reads wrong after a cell swap
Apple Watch uses a coulomb-counter IC that tracks charge state relative to a known baseline. Removing the cell clears that baseline entirely. When a new cell is fitted, the IC has no reference, so it estimates — often badly. A full charge to 100% from a low state gives the IC a fixed endpoint to anchor its model. Until that anchor exists, watchOS will show erratic percentages that do not reflect actual capacity.
Watch not pairing with iPhone after battery replacement
Removing the battery cuts power to the BLE radio mid-session. The iPhone retains the old session key, but the watch boots with no active session — the two sides are out of sync. This is not a hardware fault. On the iPhone, open the Watch app, go to General → Reset, and select "Erase Apple Watch Content and Settings," then re-pair. The watch will reconnect cleanly and restore from backup.