MWG 401 Replacement Battery UBI-4-1300 3.7V 1700mAh Li-ion

MWG 401 — 3.7V Li-ion Replacement Battery (UBI-4-1300)

This is a 3.7V, 1700mAh Li-ion replacement battery for the MWG 401 smartphone. It replaces OEM part number UBI-4-1300 and slots into the same battery bay as the original cell. Use this when the factory battery no longer holds charge or triggers unexpected shutdowns.

• MWG 401 fitment: The 401 uses a single-cell 3.7V nominal Li-ion architecture with a BMS that checks cell voltage at power-on. This replacement matches the connector pinout and voltage profile that BMS handshake expects on boot.
• Bench tested on actual hardware: We ran this cell through full charge-discharge cycles on a controlled rig. The BMS accepted the cell without protection-circuit lockout, and charge termination triggered correctly at 4.2V.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one full discharge-to-charge cycle before normal use. The fuel gauge IC on the MWG 401 calibrates its coulomb counter against the cell's discharge curve — skipping this step causes erratic percentage readings in the first few days.

Sudden shutdown at 20–30% on the MWG 401 after a cell swap

This is a voltage cliff issue, not a capacity defect. Under modem transmission or screen-at-full-brightness load, the new cell's internal resistance causes a sharper voltage drop than the fuel gauge IC predicts from its old lookup table. The phone interprets this drop as a critically low cell and cuts power to protect the hardware. One full discharge-to-shutdown cycle followed by a complete charge resets the coulomb counter and shifts the IC's cutoff mapping to match the new cell's actual discharge curve.

MWG 401 not powering on after sitting in storage with a flat battery

If the cell discharged below approximately 2.5V in storage, the BMS enters lockout mode and blocks current flow to protect the cell from damage. The phone will not respond to a short press of the power button and may show no charging animation. Connect to a low-current USB charger — not a fast charger — and leave it for 20–30 minutes without attempting to power on. Once the cell climbs back above 3.0V, the BMS releases its lockout and the phone will boot normally.