About this replacement
Fly FS504 Cirrus 2 — 3.7V Li-ion Replacement Battery (BL9200)
This is a 3.7V, 1800mAh Li-ion replacement cell for the Fly FS504 and Cirrus 2 smartphones. It slots into the original battery bay and connects to the device's charge IC and fuel gauge circuit. Capacity is 1800mAh (6.66Wh), matching the OEM BL9200 specification.
- FS504 and Cirrus 2 compatibility: Both devices share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The BL9200 part number covers both variants — the charge IC expects the same voltage profile from either model.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the FS504 platform. The BMS accepted full charge without triggering overvoltage cutoff, and the fuel gauge IC tracked cell state across the discharge curve without logging errors.
- Fuel gauge recalibration on first use: After fitting this cell, disable fast charging if your device supports it and run one full discharge-charge cycle before normal use. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current charging begins on an uncalibrated cell.
Why the FS504 reports wrong battery percentage after a cell swap
The FS504 uses a fuel gauge IC that stores a learned discharge curve from the previous cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The percentage display reads from the old data, not the new cell. One complete discharge down to automatic shutdown, followed by a full charge to 100%, forces the IC to write a new baseline curve. After that cycle, percentage reporting stabilises.
Sudden shutdown at 20–30% on the replacement cell
This happens when the cell voltage drops below the modem or screen load threshold before the fuel gauge reaches 0%. A fresh cell with an uncalibrated gauge can show 25% on screen while the actual open-circuit voltage has already fallen to around 3.5V under load. The phone's protection circuit cuts power to prevent over-discharge damage. Run that first full calibration cycle and confirm the phone reaches 4.2V at the charge termination point before trusting the percentage display.