ZTE Nubia Red Magic Li3937T44P6h886639 Replacement Battery 3.85V 3700mAh

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

This is a 3.85V, 3700mAh Li-Polymer replacement battery for the ZTE Nubia Red Magic smartphone, including the Dual SIM and Dual SIM TD-LTE variants (NX609J). It replaces OEM part Li3937T44P6h886639. Fit covers the full NX609J platform — same cell footprint, same connector, same BMS handshake protocol.

• NX609J platform compatibility: The Red Magic, Red Magic Dual SIM, and TD-LTE share the same motherboard power rail at 3.85V nominal with an identical connector pinout and BMS communication line. One cell revision covers all three variants without modification.
• Bench tested on actual hardware: We cycled this cell on an NX609J board and confirmed the BMS completed charge handshake, accepted the full 3700mAh charge current ceiling, and passed voltage regulation checks at both the 4.35V charge cutoff and the 3.0V discharge floor.
• Fuel gauge recalibration on first use: On first install, run one complete discharge-charge cycle with fast charging disabled. The Red Magic's fuel gauge IC holds a learned curve from the old cell — it needs a full slow cycle against the new cell's discharge profile before its coulomb counter reads accurately.

Sudden shutdown at 20–30% on a freshly installed Red Magic cell

The NX609J's Snapdragon 835 pulls hard when the modem hits a busy band or the GPU ramps during gameplay. A new cell with an uncalibrated fuel gauge IC can report 25% remaining while actual resting voltage has already dipped toward 3.5V under that load. The BMS reads a real undervoltage event and cuts output before the OS can log a low-battery warning. One full discharge-charge cycle at standard charge speed resets the coulomb counter and removes the mismatch.

USB fast charge not activating after cell replacement

On the first cycle after a cell swap, the charge IC on the Red Magic may not complete the handshake required to unlock higher current delivery. The new cell presents a higher impedance than a worn cell, which can cause the charge IC to stay in trickle mode as a precaution. Plug in via the original cable and charger, leave it undisturbed through a full charge, then disconnect and reconnect — the IC re-negotiates the charge profile on the second connection. Confirm fast charge is active by checking the charging indicator in the notification bar or Settings → Battery.