About this replacement
Mobistel Cynus F5 — 3.7V Li-ion Replacement Battery (BTY26184)
This is a 3.7V, 2000mAh Li-ion cell built to replace the original BTY26184 battery in the Mobistel Cynus F5 smartphone and its MT8201w, MT-8201S, and MT-8201B variants. It powers the processor, display, and cellular radio stack. Dimensions are 69.70 × 60.00 × 4.60mm — confirm clearance before fitting.
- Cynus F5 and MT8201 series compatibility: All four models share the same 3.7V battery bay, connector pinout, and BMS handshake protocol, which is why one cell covers the full range. The charge IC expects the same voltage floor and charge termination threshold across all variants.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on the Cynus F5 platform. The BMS accepted charge termination correctly, and protection circuitry tripped as expected at low-voltage cutoff — no false lockouts observed.
- 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 the new cell's discharge curve before the charge controller applies high-current input to an uncalibrated reference point.
Sudden shutdown at 20–30% on the Cynus F5 after a cell swap
This happens because the fuel gauge IC is still running the discharge curve from the old, degraded cell. The new cell has a steeper voltage cliff at the low end — when the modem or display pulls current, terminal voltage drops faster than the IC predicts. The phone reads 25% but the cell is already below the safe floor. One full discharge-charge cycle lets the coulomb counter re-anchor to the new cell's actual curve and eliminates premature cutoff.
Phone feels warm near the battery during the first few charges
A new Li-ion cell has higher internal impedance than a broken-in cell. During the first few charge cycles, the charge IC pushes current into a higher-resistance load, which generates more heat at the cell surface. This is normal and resolves as the cell cycles. If warmth persists beyond the third full charge cycle or the case becomes hot to the touch, stop charging and check that the charge voltage is not exceeding 4.2V per cell.