NEC MAY-BD0015 N331i Replacement Battery 3.7V 550mAh

NEC N331i / N700 Series — 3.7V Li-ion Replacement Battery (MAY-BD0015)

This is a 3.7V 550mAh Li-ion cell that replaces the original MAY-BD0015 battery. It fits the NEC N331i, N700, N708, and N710, along with three additional models in the same series. The connector and BMS handshake match the original so the phone recognises the cell on first boot.

• N331i / N700 series platform compatibility: These models share the same voltage rail, physical footprint, and connector pinout. The BMS on each device reads the same cell identification lines, so one cell covers the full group without adaptation.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the N700 platform. The BMS accepted the cell without a lockout event, and charge termination triggered correctly at the expected cutoff voltage.
• Fuel gauge recalibration on first use: On first installation, run one complete discharge down to auto-shutdown, then charge fully without interruption. This gives the fuel gauge IC a clean reference curve against the new cell — skip this step and the percentage display may drift or jump for several days.

Why the N331i reports wrong battery percentage after a cell swap

The N331i uses a fuel gauge IC that stores a discharge curve calibrated to the original cell's impedance profile. When a new cell goes in, the IC applies the old curve to a physically different cell, so percentage readings diverge from actual charge state. This is most visible between 40% and 15%, where the old curve drops steeply but the new cell still has usable capacity. One full uninterrupted discharge-to-shutdown followed by a complete charge resets the coulomb counter and brings the displayed percentage back in line with real capacity.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or display draws a current spike that the cell cannot sustain while holding its voltage above the BMS cutoff threshold. The BMS reads a voltage collapse — even briefly — and trips the protection circuit, cutting power before the displayed percentage reaches zero. It is not a capacity defect; it is a voltage-cliff event on an uncalibrated cell under load. Run the recalibration cycle first: one full discharge to auto-shutdown, then charge to 100% without interruption, and confirm the cell reads at least 4.1V at full charge before returning to normal use.