BenQ-Siemens M81 Replacement Battery EBA-660 3.7V 600mAh

BenQ-Siemens M81 — 3.7V Li-ion Replacement Battery (EBA-660)

This is a 3.7V, 600mAh lithium-ion replacement battery for the BenQ-Siemens M81 mobile phone. It fits the original battery slot and matches the OEM electrical spec. Part numbers EBA-660, EBA-670, EBA-760, and related Siemens L36880 variants are all covered by this cell.

• M81 compatibility across OEM part variants: The M81 shipped with several battery part numbers across markets — EBA-660 through EBA-770, plus multiple L36880 and V30145 catalogue entries. All share the same 3.7V rail, connector pinout, and physical footprint, so one replacement cell covers the full production run.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the M81 platform. The BMS accepted the cell cleanly, charge termination triggered at the correct cutoff voltage, and the protection circuit responded as expected during load testing.
• Fuel gauge recalibration after swap: After fitting this cell, run one full discharge to low battery warning, then charge uninterrupted to 100% before using the phone normally. The M81's fuel gauge IC was calibrated to the original cell's discharge curve — one complete cycle resets that baseline against the new cell.

Sudden shutdown at 20–30% on the BenQ-Siemens M81

This happens when the cell voltage drops sharply under load — a behaviour called a voltage cliff. The M81's modem and display pull current spikes that aged or shallow-cycle-degraded cells cannot sustain, causing voltage to collapse below the BMS cutoff even when the fuel gauge still reads 20–30%. A new cell with full capacity holds voltage through those load spikes. If shutdowns persist after fitting a new cell, run one full calibration cycle so the fuel gauge IC maps the correct discharge curve before trusting percentage readings.

Phone not powering on after sitting in storage with a flat battery

If a lithium-ion cell discharges below approximately 2.5V during long storage, the BMS enters lockout to prevent unsafe charging of a deeply discharged cell. The phone will not respond to a normal charge attempt. To recover, connect the charger and leave it for 15–30 minutes without pressing any buttons — most chargers trickle a small current that slowly brings the cell voltage above the BMS recovery threshold. Once the voltage climbs above 3.0V, the BMS re-enables normal charging and the phone will boot.