Amazfit GTR 42mm PL412120H Replacement Battery 3.85V 190mAh

Amazfit GTR 42mm — 3.85V Li-Polymer Replacement Battery (PL412120H)

This is a 3.85V, 190mAh Li-Polymer cell for the Amazfit GTR 42mm smartwatch. It replaces the original PL412120H battery when the watch no longer holds a charge or fails to power on. It fits the GTR 42mm chassis and connects to the existing flex cable and fuel gauge IC without modification.

• GTR 42mm platform fit: The 42mm GTR uses a compact Li-Polymer cell on a 3.85V rail with a fuel gauge IC that tracks state-of-charge. The PL412120H footprint — 21.40 x 19.70 x 3.60mm — matches the battery bay directly. The BMS handshake is handled by the watch's main board, not the cell itself.
• Bench tested on actual hardware: We ran this cell through charge cycles on a GTR 42mm unit. The fuel gauge IC accepted the new cell, reported charge state correctly after a full cycle, and the BMS did not trigger an overcurrent event during sensor polling loads.
• Post-swap charge ritual for the GTR 42mm: After fitting the new cell, place the watch on its charger immediately and charge to 100% before first use. The GTR 42mm fuel gauge IC 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 display 0% or shut off despite having charge.

Why the GTR 42mm fuel gauge reads wrong after a battery swap

The GTR 42mm uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell over time. When the cell is removed, that accumulated reference is lost. The IC restarts without a known baseline, so it guesses — usually wrong. A full charge cycle from near-zero lets the IC lock onto the new cell's actual capacity and recalibrate its reference point. Until that cycle completes, percentage readings and low-battery warnings will be unreliable.

GTR 42mm not pairing to phone after battery replacement

Removing the battery cuts power to the BLE stack entirely, which drops the active pairing session stored in the watch's memory. The phone still shows the watch as a paired device, but the watch no longer recognises the bond. Open the Zepp app, go to your device settings, and remove the GTR 42mm from the paired devices list. Then re-pair from scratch — the watch will generate a new bond key and the connection will re-establish at the standard BLE 2.4GHz handshake.