Polar M400 GPS Watch Replacement Battery 3.8V 180mAh

Polar GPS Sport Watch M400 / M430 — 3.8V Li-Polymer Replacement Battery (AHB322826)

This is a 3.8V, 180mAh Li-Polymer cell that replaces part number AHB322826 in the Polar M400 and M430 GPS sport watches. Both watches share the same battery bay, connector, and BMS handshake, making this cell a direct fit for either model. Capacity figures are taken directly from the product specification — 180mAh, 0.68Wh.

• M400 and M430 compatibility: Both models run on the same 3.8V battery rail, use an identical flex connector, and share the same BMS communication protocol. One cell covers both watches without modification.
• Bench tested on actual hardware: We ran this cell through charge-discharge cycles on the M400 platform. The BMS accepted the cell without error flags, charge termination fired correctly at 4.35V, and the fuel gauge tracked state-of-charge across the full range without dropout.
• Fuel gauge reset after cell swap: After fitting this cell, place the watch on its magnetic charger immediately and run it to 100% before first use. The M400 fuel gauge IC loses its reference point when the cell is removed. Without a full charge cycle from near-zero, the gauge cannot re-calibrate and will report inaccurate percentages.

Why the M400 GPS and continuous heart rate monitoring accelerate cell drain on a new battery

The M400 runs GPS acquisition and optical heart rate simultaneously during workouts, pulling current from a 180mAh cell that has a much smaller buffer than a phone or cycling computer. A fresh replacement cell has a slightly different internal impedance than an aged original, and the BMS adjusts its draw profile over the first several charge cycles. Expect drain rates to appear higher than normal for the first five to seven days. After that burn-in period, the BMS and fuel gauge settle into a stable pattern and reported battery life aligns with the watch's normal multi-day standby behaviour.

M400 not pairing to phone after battery replacement

Removing the battery cuts power to the BLE module and clears its active session state. The watch and the Polar Flow app both retain stale pairing records that no longer match. On the watch, go to Settings → Bluetooth → Remove all pairings, then delete the M400 from your phone's Bluetooth device list entirely. Open the Polar Flow app, add the watch as a new device, and complete the pairing sequence — the app will treat it as a fresh connection and re-establish sync correctly.