Motorola FB55 Moto X Force Replacement Battery 3.8V 3450mAh

Motorola Moto X Force — 3.8V Li-Polymer Replacement Battery (FB55 / SNN5958A)

This 3.8V, 3450mAh Li-Polymer cell replaces the original FB55 / SNN5958A battery in the Motorola Moto X Force and Moto X Force Dual SIM. It fits the XT1581 and related 64GB Dual SIM TD-LTE variants. The Moto X Force is a 2015 Android smartphone, and this cell matches the physical footprint at 103.50 × 45.65 × 5.18mm.

• XT1581 and Dual SIM variant compatibility: The single-SIM and Dual SIM Moto X Force variants share the same battery bay, connector pin-out, and BMS handshake. The FB55 cell works across all of them without modification — the voltage rail and connector are identical across the entire XT1581 family.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on a Moto X Force unit and monitored BMS communication. The charge IC accepted the new cell without fault codes, and the BMS transitioned cleanly through trickle, CC, and CV charge phases.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated coulomb counter.

Sudden shutdown at 20–30% on the Moto X Force after a cell swap

A new Li-Polymer cell has a slightly different voltage-to-capacity curve than the aged cell it replaces. The Moto X Force fuel gauge IC retains the old curve in memory, so it misjudges when the new cell hits its voltage cliff. Under modem or display load, the cell voltage drops below 3.2V faster than the OS expects, triggering an emergency shutdown. One complete discharge cycle below 5% and a full charge to 100% resets the coulomb counter and aligns the gauge to the new cell curve.

USB charging not detected after deep discharge in storage

If the FB55 cell drops below approximately 2.5V during storage, the BMS enters a lockout state and the phone will not respond to a standard USB charge cable. The charge IC requires a minimum cell voltage to initialise the charging circuit. Connect the phone to a wall charger — not a PC USB port — and leave it undisturbed for 20–30 minutes to allow trickle current to bring the cell back above the BMS re-entry threshold of around 2.9V, at which point normal charging will resume.