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Amazfit T-Rex A1919 Replacement Battery 3.8V 390mAh

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From $28.99 USD $35.99
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Fits Amazfit T-Rex and T-Rex PRO smartwatches, replacing OEM battery part numbers A1919, A1918, and A1801.
This 3.8V, 390mAh lithium-polymer cell restores multi-day runtime to the T-Rex after the original degrades from extended use and charge cycles.
Connector is a two-pin pogo contact mounted on a flexible adhesive tab; orientation locks into the watch housing with no physical latch required.
We ran this cell through a full discharge cycle on T-Rex hardware; the fuel gauge IC initialized correctly and BMS reported steady voltage under continuous heart rate monitoring.
After fitting this battery, place the watch on its magnetic charger immediately and charge to 100% before wearing — the fuel gauge cannot calibrate without a complete cycle from near-zero after a cell swap.
Delivery time

This product ships directly from our Manufacturer’s Warehouse and is usually delivered within 5 – 8 business days to your doorstep.

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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

Delivery and Shipping

🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.

Disclaimer

⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.

🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

3.8V

Amp

390mAh

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.

Compatible Models

T-Rex T-Rex PRO

Replaces Part Numbers

A1919 A1918 A1801

Technical Specifications

Voltage3.8V
Amp Hours390mAh
Capacity390mAh
Rate1.48Wh
Net Weight7g /0.25 oz
Gross Weight32g /1.13 oz
Approximate Weight32g /1.13 oz
Dimension 25.00 x 25.00 x 5.00mm

Product Highlights

  • Brand: Amazfit
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Li-Polymer
  • Battery Type: Li-Polymer
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

My T-Rex shows 0% and won't turn on right after fitting the new battery — is it faulty?

The fuel gauge IC on the T-Rex loses its state of charge reference the moment the cell is disconnected. When a new cell goes in, the IC has no baseline and reports zero until it completes a full calibration cycle. Place the watch on its magnetic charger straight away and leave it until it reaches 100% without interruption. After that first full charge, the percentage display will track correctly.

The new battery drains in about a day — the old one used to last several days. What's wrong?

Check whether always-on display is enabled and whether continuous heart rate monitoring is turned on simultaneously — those two features together are the most common cause of single-day drain on the T-Rex. Always-on display alone can cut expected battery life by more than half. Go to Settings → Display on the watch and switch to raise-to-wake, then set heart rate monitoring to 30-minute intervals instead of continuous. If drain still seems excessive after three full charge cycles, the fuel gauge may still be recalibrating — give it a week before drawing conclusions.

The magnetic charger isn't registering — the charging icon never appears after reassembly. What do I check?

The T-Rex charging contacts sit flush on the underside of the case and align to specific poles on the magnetic puck. If the watch back was shifted even slightly during reassembly, the contact pads can land off-centre and carry no current. Remove the watch from the charger, rotate it 180 degrees, and reseat it — the magnet will pull to the correct orientation once aligned. If the icon still doesn't appear, check the contact pads on the watch underside for any adhesive or debris from the repair and clean them with isopropyl alcohol before reseating.

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