About this replacement
Fly FS518 Cirrus 13 — 3.7V Li-Polymer Replacement Battery (BL9601)
This is a 3.7V, 2200mAh Li-Polymer replacement battery for the Fly FS518 and Cirrus 13 smartphones. It carries OEM part number BL9601 and slots into the same battery bay as the original cell. Use it when the factory battery no longer holds a charge or the phone shuts down unexpectedly under load.
- FS518 and Cirrus 13 compatibility: Both models run on the same 3.7V Li-Polymer cell with identical connector pinout and BMS handshake protocol, so one part number covers both. No adapter or modification needed.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the FS518 chassis. The BMS accepted the charge IC handshake correctly on the first cycle, and protection cutoffs triggered at expected voltage thresholds.
- Fuel gauge recalibration on first use: On first use after installation, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes current into an uncalibrated state.
Sudden shutdown at 20–30% on the FS518 after a cell swap
This happens because the fuel gauge IC is still calibrated to the old cell's discharge curve. The new cell hits a voltage cliff under modem or screen load before the gauge reads zero — the phone interprets it as a crash, not a low-battery shutdown. The fix is one full uninterrupted discharge-charge cycle with the screen on and mobile data active. After that cycle, the coulomb counter resets against the new cell and the percentage readout stabilises.
Phone warm near the battery compartment on the first few charges
A new Li-Polymer cell has higher internal impedance than a worn cell. The charge IC on the FS518 pushes the same current profile it used on the degraded original, which generates more heat until the cell's impedance drops after a few cycles. This is normal and typically resolves within three to five charge cycles. If the phone stays warm beyond that point, check that the battery contacts are seated flush — a partial contact forces the IC to compensate with higher voltage.