Nubia Red Magic 3s Li-Polymer Replacement Battery 3.85V 4900mAh

Nubia Red Magic 3s / NX629J — 3.85V Li-Polymer Replacement Battery (Li3949T44P6h996644)

This is a 3.85V, 4900mAh Li-Polymer cell for the Nubia Red Magic 3s (NX629J). It replaces the original battery when capacity has dropped due to charge cycles or age. The Red Magic 3s runs a high-refresh display and a thermally demanding processor — both pull hard from the cell under gaming loads.

• Red Magic 3s and NX629J compatibility: Both model identifiers refer to the same hardware revision. They share the same battery bay dimensions (96.84 × 65.10 × 4.16mm), the same flex connector pinout, and the same BMS handshake protocol. No adapter or wiring change is needed.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on a Red Magic 3s unit. The BMS accepted the cell on first connection, thermal cutoff triggered correctly at the expected threshold, and the fuel gauge IC initialised without error flags during the first full cycle.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current fast charging pushes current into an uncalibrated state. Skip this step and the percentage readout will drift.

Sudden shutdown at 20–30% on the Red Magic 3s after a cell swap

This happens because the fuel gauge IC is still calibrated to the old cell's voltage-to-capacity curve. The new cell hits a voltage cliff — a steep drop in terminal voltage under modem and display load — at a point the IC still reads as 20–30%. The phone interprets this as a brownout and shuts down. One full discharge-charge cycle with fast charging disabled resets the coulomb counter and maps the new curve correctly.

USB-PD fast charge not accepting the new cell on first cycle

On first connection, the charge IC may reject fast charging protocol negotiation if the BMS reports a cell impedance outside its learned range. This is normal — the BMS hasn't yet logged a charge history for the new cell. Charge once at standard rate to let the BMS record baseline impedance data. After that first cycle completes, fast charging protocol handshake resumes at full rate, typically confirmed by the phone showing the fast-charge indicator at 5V/2A or above.