About this replacement
Mobistel Cynus F5 — 3.7V Li-ion Replacement Battery (BTY26184)
This is a 3.7V, 1700mAh lithium-ion battery for the Mobistel Cynus F5 smartphone. It also fits the MT-8201S, MT-8201B, and MT-8201w variants. It replaces a degraded original cell that no longer holds sufficient charge through a full day of use.
- Cynus F5 and MT-8201 series compatibility: All four model variants use the same connector pinout, physical footprint (69.88 × 59.87 × 4.60mm), and 3.7V nominal voltage rail, so a single cell covers the entire MT-8201 range without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the Cynus F5 platform. The BMS handshake completed correctly, charge IC accepted the cell without error flags, and voltage held steady under screen-on and modem load.
- Fuel gauge recalibration on first cycle: After fitting this cell, run one full discharge to automatic shutdown, then charge uninterrupted to 100% before normal use. The Cynus F5 fuel gauge IC uses that first cycle to map the new cell's discharge curve — skipping it causes inaccurate percentage readings from day one.
Why the Cynus F5 reports wrong battery percentage after a cell swap
The fuel gauge IC in the MT-8201 platform stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches the actual voltage-to-capacity relationship of the replacement. The chip reads voltage and maps it to the old curve, so the percentage shown can be off by 15–25%. One complete discharge-to-shutdown followed by a full uninterrupted charge resets the coulomb counter and lets the IC build a new curve against the actual cell. After that cycle, percentage reporting stabilises.
Sudden shutdown at 20–30% on the replacement cell
This happens when the cell voltage drops sharply under load — modem transmission bursts and screen-on draw can pull the terminal voltage below the BMS cutoff threshold faster than the fuel gauge IC expects. On a freshly fitted cell, the IC has not yet mapped the low-end of the discharge curve, so it shows 20–30% when the cell is actually near its minimum safe voltage. The BMS cuts power to protect the cell, and the phone shuts off with percentage still showing on screen. Complete the recalibration cycle described above — one full discharge to shutdown, then charge to 100% — and the cutoff will align with a more accurate state-of-charge reading.