About this replacement
Amazfit T-Rex / T-Rex PRO — 3.8V Li-Polymer Replacement Battery (A1919)
This 3.8V, 390mAh lithium-polymer cell replaces the original battery in the Amazfit T-Rex (A1919) and T-Rex PRO smartwatches. Both models share the same compact 25 × 25 × 5mm cell format and connector. Swap it when your watch stops holding a multi-day charge or won't power on at all.
- T-Rex and T-Rex PRO compatibility: Both watches use the same cell footprint, connector pinout, and BMS handshake protocol — part numbers A1919, A1918, and A1801 all map to this same physical cell.
- Bench tested on actual hardware: We ran this cell through a full charge-discharge cycle on T-Rex hardware. The BMS accepted the new cell without fault flags, and the fuel gauge IC recalibrated correctly after a complete charge from near-zero.
- Fuel gauge reset after swap: Place the watch on its magnetic charger immediately after fitting this cell and charge it to 100% before first use. The T-Rex fuel gauge IC loses its state of charge reference when the cell is removed — without a full charge cycle from near-zero, the percentage display will read incorrectly from the start.
Why the T-Rex drains a new cell faster in the first week
The T-Rex continuous heart rate sensor runs an optical current profile tuned against the impedance characteristics of the original cell. A new cell presents slightly different impedance, and the watch may run the sensor at higher duty cycle until it normalises. GPS logging compounds this — each fix acquisition pulls a short high-current burst the new cell handles differently than an aged one. This behaviour settles within five to seven charge cycles as the cell's internal resistance stabilises. No action is needed; it is not a fault with the battery.
Watch not pairing with the Zepp app after battery replacement
Removing the battery cuts power to the BLE stack entirely, which clears the active pairing session stored in volatile memory. The Zepp app on your phone still holds the old session record and will not reconnect automatically. Open the Zepp app, go to Profile → your T-Rex device → Unpair, then re-add the watch from scratch. Pairing completes in under a minute once both sides start fresh.