Poco X3 GT Replacement Battery BM57 3.87V 4900mAh

Poco X3 GT — 3.87V Li-Polymer Replacement Battery (BM57)

This is a 4900mAh Li-Polymer cell built to the BM57 spec for the Poco X3 GT (model 21061110AG). It runs at 3.87V nominal, matching the phone's power management IC thresholds exactly. The original BM57 degrades over charge cycles — this cell restores full capacity to the mainboard's power rails.

• X3 GT and 21061110AG compatibility: Both model identifiers point to the same hardware revision — same battery bay dimensions (90.60 × 63.70 × 5.00mm), same 3.87V power management rail, and same BM57 connector pinout. One cell fits both references.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the X3 GT mainboard. The BMS handshake completed correctly, charge IC accepted current without fault flags, and the fuel gauge IC tracked state-of-charge without erratic jumps.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one complete discharge-charge cycle at standard rate. The X3 GT's fuel gauge IC was calibrated to the old cell's discharge curve — skipping this step causes percentage jumps and premature low-battery warnings on the new cell.

Why the X3 GT reports wrong battery percentage after a cell swap

The X3 GT uses a coulomb-counter fuel gauge IC that builds its discharge curve model against the old cell over hundreds of cycles. When you install a fresh BM57, that model no longer matches the new cell's actual voltage-versus-capacity curve. The IC keeps reading state-of-charge from its stored profile, so it reports incorrect percentages. One full discharge below 10% followed by an uninterrupted charge to 100% forces the IC to rewrite its reference curve against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC hasn't recalibrated and the new cell hits a voltage cliff — real cell voltage drops below the modem's or display's minimum rail voltage before the reported percentage reaches zero. The phone's PMIC triggers an emergency shutdown to protect the mainboard, not because the battery is truly flat. Run the phone down to automatic power-off twice, charging fully between each cycle. After two full cycles the coulomb counter recalibrates and the shutdown should move to below 5%.