Nubia Red Magic 5G Replacement Battery LI3945T44P8H526391 3.85V 4000mAh

Nubia Red Magic 5G NX659J — 3.85V Li-Polymer Replacement Battery (LI3945T44P8H526391)

This 3.85V, 4000mAh Li-Polymer cell replaces the original battery in the Nubia Red Magic 5G (NX659J). It matches OEM part number LI3945T44P8H526391 and fits the same connector and physical footprint. If your Red Magic 5G is shutting down unexpectedly or no longer holding a charge, this cell addresses the degraded original.

• Red Magic 5G and NX659J compatibility: Both model designations share the same motherboard, battery connector, and BMS handshake protocol. One cell fits both. No hardware modification is needed — the flex connector and retention bracket transfer directly from the original cell.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the NX659J board. The BMS accepted the cell without fault flags. Charge current ramped normally through CC and CV phases, and the fuel gauge IC registered expected coulomb counts across the cycle.
• First-cycle fast charge caution on the Red Magic 5G: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard current. The fuel gauge IC needs to calibrate its discharge curve against the new cell before the Red Magic's high-current fast charge pushes elevated current into an uncalibrated cell — skipping this step can cause erratic percentage readings for weeks.

Sudden shutdown at 20–30% on the Red Magic 5G after a cell swap

This is a voltage cliff issue, not a capacity problem. The Red Magic 5G drives a 144Hz display and a 5G modem simultaneously — both pull sharp current spikes. If the fuel gauge IC is still calibrated to the old cell's discharge curve, it reports a higher state of charge than the new cell's actual voltage supports under that load. When the cell voltage drops below roughly 3.5V under peak draw, the BMS cuts output before the OS registers 0%. Run two full discharge-charge cycles at standard charge rate to let the coulomb counter re-map against the new cell.

USB-PD or Red Magic fast charge not activating after replacement

Fast charge protocols on the NX659J — including the proprietary 65W path — require the charge IC to handshake with both the cable and the battery BMS before enabling high-current mode. On a new cell, the BMS sometimes holds the charge IC in a low-current trickle state for the first partial cycle, particularly if the cell shipped below 3.6V. Plug in at standard USB current first, let the cell reach 50% charge, then reconnect with the original fast-charge adapter. The fast-charge indicator should activate at that point.