360 N4S Replacement Battery QK-394 3.85V 4900mAh

360 N4S / 1505-A01 / 1505-A02 — 3.85V Li-Polymer Replacement Battery (QK-394)

This is a 3.85V, 4900mAh Li-Polymer replacement cell for the 360 N4S smartphone, covering variants 1505-A01 and 1505-A02. It matches the OEM part number QK-394 and slots into the same battery bay without modification. At 18.87Wh, capacity is on par with what shipped from the factory.

• N4S variant coverage (1505-A01 and 1505-A02): Both variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell fits both without adaptation. The fuel gauge IC on each board reads the same charge curve from this QK-394 format cell.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the N4S board. The BMS accepted the cell without error flags, charge termination triggered correctly at 4.35V, and the protection circuit tripped as expected at the low-voltage cutoff threshold.
• Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter to the new cell's actual discharge curve before high-current charging pushes current into an uncalibrated cell.

Sudden shutdown at 20–30% on the N4S after a cell swap

This is a voltage cliff — not a capacity problem. The N4S modem and display together pull enough current during peak load that a new cell with a slightly different internal resistance profile will sag below the BMS cutoff voltage before the percentage reading hits zero. The fuel gauge IC is still calibrated to the old cell's discharge curve, so it reports 20–30% while the actual terminal voltage under load has already dropped to the cutoff threshold. Fix this by running two full discharge-charge cycles without fast charging — the coulomb counter will re-anchor itself to the new cell's curve and the shutdowns stop. After recalibration, charge termination should land cleanly at 4.35V.

360 N4S not powering on after the replacement cell sat in storage

Li-Polymer cells self-discharge in storage, and if this one dropped below 2.5V per cell the BMS will have locked out to prevent further discharge damage — the phone will show no response to the power button or charger. Connect a USB-C charger and leave it for 15–20 minutes without pressing anything; the charge IC on the N4S board will trickle current into the cell at a reduced rate until voltage climbs above the BMS re-enable threshold. Once the screen shows a charge indicator, the BMS has re-initialised and normal charging resumes. If there is still no response after 30 minutes on charge, check that the battery flex connector is fully seated — an unseated connector gives identical symptoms to a BMS lockout.