About this replacement
Apple Watch Series S7 45mm — 3.85V Li-Polymer Replacement Battery (A2552)
This is a 3.85V, 300mAh Li-Polymer cell for the Apple Watch Series S7 45mm. It replaces part number A2552 and fits the 45mm case only — not the 41mm. Swap this in when the original cell no longer holds charge through a full day of normal use.
- S7 45mm case fit: The 45mm and 41mm S7 share the same voltage rail but use physically different cells. This cell measures 27.40 × 20.00 × 4.30mm, matching the 45mm cavity. Fitting the wrong size stresses the flex connector and risks a BMS fault on first charge.
- Bench tested on actual hardware: We ran this cell through the S7 charge controller and confirmed the BMS accepted the new cell without a protection trip. The fuel gauge IC registered the correct open-circuit voltage on connection.
- Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The S7 fuel gauge IC loses its reference point when the cell is removed. Without a full charge cycle from near-zero, the gauge will report inaccurate percentages and may cut off early.
Why the S7 45mm charge contacts fail to engage after reassembly
The S7 uses a magnetic charging puck that aligns to four contact points on the back crystal. If the rear case shifts even slightly during reassembly, those contacts sit off-centre and the watch draws no current. The charger LED may still glow green because the magnet seats before the contacts do. Re-seat the rear crystal, confirm it clicks flush on all four corners, then place it on the puck again and check for the charging icon within 30 seconds.
Watch not re-pairing to iPhone after battery swap
Removing the cell drops system power completely, which wipes the active BLE session stored in volatile memory. The iPhone retains the old pairing record but the watch boots with no matching session, so the two devices sit waiting for the other to initiate. Go to the Watch app on the iPhone, tap your watch name, select "Unpair Apple Watch," and re-pair from scratch. The process takes under five minutes and restores all synced data from the paired iPhone backup.