Mobistel Cynus F9 Replacement Battery 3.8V 2000mAh

Mobistel Cynus F9 Series — 3.8V Li-Polymer Replacement Battery (615080005069)

This 3.8V, 2000mAh Li-Polymer battery replaces the original cell in the Mobistel Cynus F9, Cynus F9 4G, and F303-S smartphones. It matches the OEM part number 615080005069 and the original 76.15 × 55.50 × 3.30mm footprint. Rated at 7.6Wh, it restores full power to the device when the original cell has degraded or failed to hold charge.

• Cynus F9, F9 4G, and F303-S fitment: All three models share the same battery bay dimensions, voltage rail, and connector pinout. The same OEM part number 615080005069 covers all three variants, so one cell fits the full lineup without modification.
• Bench tested on actual hardware: We cycled this cell through a full discharge-charge sequence and monitored BMS communication throughout. The protection circuit responded correctly to cutoff thresholds at both ends of the voltage range, and the fuel gauge IC accepted the new cell without triggering fault flags.
• Fuel gauge recalibration on first use: On first installation, disable any fast charging option and run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. This gives the coulomb counter a full sweep of the new cell's discharge curve before normal use begins.

Why the Cynus F9 reports wrong battery percentage after a cell swap

The fuel gauge IC on the Cynus F9 builds its capacity model from previous charge and discharge cycles. When you install a new cell, that stored model no longer matches the actual discharge curve of the replacement. The phone reads voltage and maps it to a percentage using the old curve, so the displayed figure drifts away from reality — often by 10–20% in either direction. One full uninterrupted discharge-to-shutdown cycle followed by a full charge resets the coulomb counter and retrains the gauge against the new cell.

Sudden shutdown at 20–30% charge remaining on the new cell

This happens when the fuel gauge IC has not yet mapped the new cell's voltage-to-capacity curve accurately. Under sudden load — a call, a screen wake, or background sync — the cell voltage drops below the BMS cutoff threshold even though the displayed percentage suggests charge remains. The phone shuts down to protect the cell, not because the cell is faulty. Run one full discharge cycle without interruption; once the gauge recalibrates, shutoffs at 20–30% typically stop. If they continue after two full cycles, check that the resting voltage after a full charge reads at least 4.15V.