Fly FS454 Replacement Battery BL6425 3.7V 1500mAh Li-ion

Fly FS454 Nimbus 8 — 3.7V Li-ion Replacement Battery (BL6425)

This is a 3.7V, 1500mAh Li-ion replacement battery for the Fly FS454 and Nimbus 8 smartphones. It replaces OEM part number BL6425. Fit this when the original cell no longer holds a charge or causes unexpected shutdowns.

• FS454 and Nimbus 8 compatibility: Both the FS454 and Nimbus 8 use the same BL6425 cell format, connector pinout, and 3.7V nominal voltage rail. The BMS handshake matches the charge IC on both models, so the phone accepts the cell without throwing a battery warning.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the FS454 platform. The BMS reached full charge cutoff cleanly at 4.2V and engaged low-voltage protection at the correct floor, with no spurious cutoff events during mid-cycle draws.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. This lets the fuel gauge IC map its coulomb counter against the new cell's actual discharge curve before any high-current charging begins on an uncalibrated cell.

Why the FS454 reports wrong battery percentage after a cell swap

The fuel gauge IC on the FS454 stores a learned discharge curve from the old cell. When a new cell goes in, that stored curve no longer matches the actual voltage-versus-capacity relationship of the replacement. The phone reads voltage and maps it to the wrong percentage using the old data. One full discharge and recharge cycle forces the coulomb counter to reset and relearn the new cell's curve. After that cycle, percentage readings stabilise.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under a modem or screen load spike — a voltage cliff the fuel gauge did not predict because it hadn't yet calibrated to the new cell. The phone's protection circuit reads the voltage dip as a hard cutoff condition and shuts down even though the percentage display still shows charge remaining. It is not a faulty cell. Let the phone power back on, allow the battery to drain fully to auto-shutdown, then charge uninterrupted to 100%. That single cycle recalibrates the gauge and eliminates the cliff shutdown.