Plusfon U120e Replacement Battery 3.7V 700mAh Li-ion

Plusfon U120e / 401 / 401i — 3.7V Li-ion Replacement Battery

This is a 3.7V, 700mAh Li-ion cell for the Plusfon U120e, 401, and 401i mobile phones. It fits directly into the original battery bay and restores power when the factory cell has degraded or failed completely. Capacity figure is 700mAh (2.59Wh) as specified in the product data.

• U120e, 401, and 401i compatibility: These three models share the same battery bay dimensions (53.78 × 34.05 × 4.80mm), voltage rail, and connector pinout — the same cell services all three without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge under load. The BMS held cutoff correctly at both the low-voltage floor and the 4.2V charge ceiling with no thermal event.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate discharge curve for the new cell before any high-current charging pushes current into an uncalibrated state.

Why the U120e reports wrong battery percentage after a cell swap

The phone's fuel gauge IC stores a discharge curve calibrated to the original cell. When a new cell goes in, the IC keeps using that old curve, so percentage readings drift — often reading 100% when the cell is partially charged or dropping sharply without warning. The fix is one full uninterrupted discharge to auto-off, followed by a full charge to 100% without interruption. After that single cycle, the coulomb counter recalibrates and percentage tracking stabilises.

Sudden shutdown at 20–30% remaining on the replacement cell

This is a voltage cliff — not a capacity fault. Under peak modem or screen load, the cell voltage sags below the BMS cutoff threshold before the displayed percentage reaches zero. It happens most often when the fuel gauge IC is still running on the old cell's curve. Run the recalibration cycle first. If shutdowns continue after one full cycle, check that cell voltage at power-off is at or above 3.4V — if it's dropping below that under load, the cell itself has an internal resistance issue.