NEC Aterm MR04LN Replacement Battery 3.8V 2100mAh

NEC Aterm MR04LN — 3.8V Li-ion Replacement Battery (AL1-003988-101)

This 3.8V, 2100mAh Li-ion battery replaces the OEM cell in the NEC Aterm MR04LN mobile hotspot and the PA-MR03LN, PA-MR03LN6B, and AtermMR03LN 3B. All four models share the same battery bay dimensions and connector, so one part number covers the full range. Capacity is 7.98Wh — identical to the original specification.

• MR04LN and MR03LN series compatibility: NEC used the same 59.50 × 55.00 × 5.60mm cell across the MR04LN and MR03LN variants. The voltage rail, connector pinout, and BMS handshake are identical across all four listed models, so no firmware or hardware changes are needed when swapping cells.
• Bench tested on actual hardware: We ran this cell under simulated multi-device hotspot load — simultaneous WiFi client connections plus active cellular data — and monitored the BMS across charge and discharge cycles. The protection circuit responded correctly to overvoltage and undervoltage thresholds with no spurious cutoffs.
• Ventilation during extended sessions: Keep the hotspot on a hard, open surface when multiple devices are connected. The cellular radio pushes to maximum output power when signal is weak, generating heat that accumulates inside the case. A blocked vent or pocket placement compounds thermal stress on the cell.

Why the MR04LN drops connections mid-session under full load

The MR04LN drives two separate radios simultaneously — the cellular modem and the 802.11 WiFi chipset. At maximum connected clients, both radios draw peak current at the same time. If the cell has aged or a replacement cell has high internal resistance, voltage sags below the BMS threshold momentarily, triggering a protection cutoff that drops all connected devices. A fresh cell with low internal resistance holds the voltage rail stable under that combined draw. If dropouts persist after replacement, check that the device firmware is current — older firmware versions have aggressive BMS timeout settings that a firmware update resolves.

MR04LN won't power on after sitting unused for weeks

Li-ion cells in standby devices self-discharge at roughly 2–3% per month, but the MR04LN's idle modem polling accelerates that drain. After extended storage, the cell can fall below the minimum boot voltage the BMS requires — typically around 3.0V — and the device shows no response at the power button. Connect the charger and leave it for at least 20–30 minutes before attempting to power on; the BMS needs enough charge to re-initialise before it will pass current to the motherboard. If the device still won't respond after that, measure charge voltage at the USB port to confirm the charger is delivering the correct 5V output.