Nubia Red Magic Li3937T44P6h886639 Compatible Battery 3.85V 3700mAh

Nubia Red Magic NX609J — 3.85V Li-Polymer Replacement Battery (Li3937T44P6h886639)

This 3.85V, 3700mAh (14.25Wh) Li-Polymer cell replaces the original battery in the Nubia Red Magic, Red Magic Dual SIM, Red Magic Dual SIM TD-LTE, and NX609J. It matches the OEM part number Li3937T44P6h886639 and fits the original battery bay without modification. Voltage and capacity specs are sourced directly from the OEM reference — not approximated.

• NX609J platform fit: The Red Magic line shares a single battery footprint across its Dual SIM and TD-LTE variants. All use the same connector position, cell dimensions (87.10 × 65.12 × 3.88mm), and BMS handshake — so one cell covers the full NX609J family.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the NX609J platform. The BMS accepted the cell without fault codes, and the charge IC completed a full constant-current to constant-voltage taper normally.
• Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one full discharge-charge cycle at standard rate. This lets the fuel gauge IC build an accurate discharge curve against the new cell before high-current charging begins on an uncalibrated reference.

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

The Red Magic's fuel gauge IC stores a discharge model calibrated to the old cell. A new cell with a slightly different internal resistance profile reaches its voltage floor faster under GPU or modem load than the IC expects. The phone reads 25% but the cell voltage has already dropped below the shutdown threshold — typically around 3.2V under load on this platform. One complete slow discharge-charge cycle rewrites the coulomb counter reference and eliminates the premature cutoff.

USB fast charge not activating on first cycle after replacement

After installation, the charge IC may default to a low-current trickle profile until it completes an initial cell validation pass. This is not a fault — it is the IC being conservative with an unrecognised impedance signature from a fresh cell. Let the phone complete one full charge at standard current without interruption. On the next charge cycle, the fast charge handshake should negotiate normally and current ramp up to the expected rate.