Sony SmartWatch 2 AHB412033PS Replacement Battery 3.7V 180mAh

Sony SmartWatch 2 SW2 — 3.7V Li-Polymer Replacement Battery (AHB412033PS)

This is a 3.7V 180mAh Li-Polymer cell replacing part number AHB412033PS in the Sony SmartWatch 2 (SW2). It powers the display, processor, Bluetooth radio, and notification handling. Fit it when the original cell no longer holds charge through a normal day of use.

• SmartWatch 2 and SW2 compatibility: Both model names refer to the same hardware generation. They share the same battery bay dimensions (31.90 × 20.10 × 4.20mm), the same flex connector, and the same BMS communication protocol — so one cell covers both.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the SW2 platform. The BMS accepted the cell without fault codes, charge termination triggered correctly at full capacity, and low-voltage cutoff engaged at the expected threshold without damage to the cell.
• Fuel gauge re-initialisation after swap: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The SW2 fuel gauge IC loses its calibration reference when the original cell is removed. Without a full charge cycle from near-zero, the percentage readout will be inaccurate and the watch may shut down before 0% is displayed.

Why the SW2 reports 0% and shuts down right after a battery swap

The SW2 uses a coulomb-counting fuel gauge IC that tracks charge state continuously. When you remove the old cell, the IC loses its accumulated reference data entirely. The new cell arrives partially charged from storage, but the IC has no baseline to measure from, so it defaults to or misreports a near-empty state. Charge the watch to 100% immediately after reassembly — this re-anchors the IC's upper reference point and clears the false empty reading.

SW2 not pairing with the phone after battery replacement

Removing the battery terminates the active Bluetooth Low Energy session between the watch and the paired phone. The phone retains the old pairing record, but the watch starts without session context, causing a handshake mismatch. Go into your phone's Bluetooth settings, forget the SmartWatch 2 device entry, then re-initiate pairing from the Sony SmartConnect app. The watch will appear as a new device and pair cleanly.