Huawei U2801 Watch Fit Compatible Battery HBU83S 3.7V 700mAh

Huawei U2801 / Envoy U3900 / M318 — 3.7V Li-ion Replacement Battery (HBU83S / HB4A1H)

This 3.7V, 700mAh Li-ion cell replaces the OEM battery in the Huawei U2801, U2801-5, Envoy U3900, M318, and 12 additional Huawei handset models. It matches the original cell dimensions at 53.78 × 34.05 × 4.80mm and drops into the same battery bay without modification. Voltage and capacity are identical to factory spec.

• U2801 and Envoy U3900 platform fit: These models share the same battery bay geometry, connector pinout, and 3.7V nominal voltage rail. The HBU83S and HB4A1H are interchangeable OEM part numbers across this platform — same cell, two catalogue references used by Huawei at different production runs.
• Bench tested on actual hardware: We cycled this cell through full discharge and charge on the U2801 platform. The BMS accepted the cell without fault codes, charge current ramped normally through CC/CV, and termination voltage held at 4.2V as expected.
• Fuel gauge recalibration after swap: On first use after installation, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% before using the phone. This gives the fuel gauge IC a full reference discharge curve on the new cell so percentage readings stay accurate from the start.

Why the U2801 reports wrong battery percentage after a cell swap

The U2801 uses a fuel gauge IC that tracks charge state by measuring discharge curves over time — it builds a model of the cell. When the old, degraded cell is replaced, that stored model no longer matches the new cell's actual discharge profile. The gauge then reports capacity against the wrong baseline, causing readings that jump or plateau at incorrect percentages. One full discharge-to-shutdown followed by an uninterrupted charge to 100% forces the IC to re-learn the curve on the new cell. After that cycle, percentage accuracy returns to normal.

Sudden shutdown at 20–30% on a freshly installed cell

This happens when the fuel gauge IC is still calibrated to the old degraded cell and cuts power at a voltage that appears to be 20–30% on the old curve but is actually the new cell's safe floor. The phone shuts off not because the cell is empty but because the system voltage threshold trips before the gauge has re-learned the new discharge slope. The fix is the same calibration cycle: run the phone to automatic shutdown from a full charge without interruption. Once the IC logs a complete discharge curve for the new cell, it recalculates the cutoff threshold correctly and shutdowns at low percentage stop occurring.