About this replacement
Motorola WX180 / WX280 Series — 3.7V Li-ion Replacement Battery (SNN1218K)
This is a 3.7V, 650mAh Li-ion cell replacing the original battery in the Motorola WX180, WX280, WX288, and WX390. It fits the same physical bay and connector as OEM part SNN1218K. Capacity figure is taken directly from the product data — 2.41Wh total energy.
- WX-series shared battery platform: The WX180, WX280, WX288, and WX390 all draw from the same 3.7V single-cell architecture with an identical connector pinout and BMS handshake protocol. One cell design spans this range because Motorola kept the same power rail and charge IC across these models.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on WX-series hardware. The BMS accepted the charge IC handshake without fault flags, and cell voltage held stable at 3.7V nominal through multiple cycles with no cutoff events.
- Fuel gauge recalibration on first use: On first use after installation, run one full discharge down to automatic cutoff, then charge uninterrupted to 100% before resuming normal use. This lets the fuel gauge IC map the new cell's discharge curve and stops erratic percentage readings early on.
Why the WX180 reports the wrong battery percentage after a cell swap
The fuel gauge IC on the WX180 stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The IC reads voltage and maps it against the old curve, so the displayed percentage drifts — sometimes showing full charge while voltage is already dropping. One complete discharge-to-cutoff and uninterrupted recharge resets the coulomb counter and forces the IC to build a fresh curve against the new cell.
Sudden shutdown at 20–30% on the replacement cell
This happens when the cell voltage drops sharply under load from the modem or display backlight — a voltage cliff the fuel gauge IC doesn't anticipate from its uncalibrated curve. The phone's protection circuit reads the voltage sag as a critical low and cuts power instantly, even though the displayed percentage looks safe. It is not a faulty cell — it is a calibration gap. Run one full discharge cycle to cutoff, let the IC log the actual cliff point, and the shutdowns stop. After calibration the gauge will begin showing low battery warnings before the cliff, not after it.