About this replacement
Mobistel Cynus F8 — 3.7V Li-ion Replacement Battery (BTY26189)
This is a 3.7V, 3000mAh (11.1Wh) Li-ion cell for the Mobistel Cynus F8 smartphone. It replaces the original BTY26189 when the factory cell has degraded or failed. Swap it in to restore full power capacity without replacing the entire handset.
- Cynus F8 fit: The Cynus F8 uses a fixed connector pinout and a specific BMS handshake tied to BTY26189. This cell matches that pinout and communicates correctly with the phone's charge IC — no firmware flags, no rejected charge cycles.
- Bench tested on actual hardware: We cycled this cell through the Cynus F8 charge IC and confirmed BMS acceptance, full charge termination at 4.2V, and stable voltage delivery under both screen-on and modem-active loads.
- Fuel gauge recalibration on first install: After fitting this cell, disable fast charging if your device supports it and run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. This gives the fuel gauge IC one full cycle to map the new cell's discharge curve before it starts reporting accurate percentages.
Sudden shutdown at 20–30% on the Cynus F8 after a cell swap
A new cell has a slightly different internal resistance profile than the factory cell. The Cynus F8's fuel gauge IC holds its old calibration data after the swap. Under high-current loads — active LTE, GPS, or screen at full brightness — the new cell's voltage drops faster than the IC expects, and the phone hits its cutoff threshold before the reported percentage reaches zero. The fix is one full uninterrupted discharge-charge cycle. After that cycle, the coulomb counter resets its reference and the shutdowns stop.
Percentage jumping erratically in the first few days after installation
The Cynus F8's fuel gauge IC uses a stored discharge curve from the old cell to estimate remaining capacity. When a new cell goes in, the stored curve no longer matches actual cell behaviour — so percentage readings jump between values that don't reflect real charge state. This is a calibration gap, not a fault with the replacement cell. Run the phone from 100% down to automatic low-battery shutdown twice, charging fully between each cycle, and the IC rebuilds its reference curve against the new cell's actual voltage profile.