About this replacement
Mobistel Cynus F4 — 3.7V Li-ion Replacement Battery (BTY26183)
This is a 3.7V, 1500mAh Li-ion cell for the Mobistel Cynus F4 smartphone (also listed as MT-7521w and MT-7521B). It replaces the original BTY26183 when the factory cell can no longer hold a usable charge. Dimensions are 55.90 x 53.95 x 4.95mm — confirm these against your existing cell before installing.
- Cynus F4 / MT-7521 series fit: The MT-7521w and MT-7521B are regional variants of the same hardware platform. They share the same battery bay dimensions, connector pinout, and charge IC handshake, so a single cell covers all three model codes.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Cynus F4 platform. The BMS accepted the cell without fault flags, and the charge IC completed a full CC/CV cycle to 4.2V without thermal interruption.
- Fuel gauge recalibration after swap: On first use after installation, disable fast charging and let the phone run through one complete discharge-charge cycle at standard current. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging begins on an uncalibrated cell.
Why the Cynus F4 reports wrong battery percentage after a cell swap
The Cynus F4 uses a coulomb counter that tracks charge in and out of the original cell. When the cell is replaced, the counter's internal model no longer matches the new cell's actual discharge curve. The phone reads a voltage that looks normal but maps it to the wrong state-of-charge value. One full discharge to automatic shutdown followed by an uninterrupted charge to 100% resets the calibration reference point.
Sudden shutdown at 20–30% remaining on the replacement cell
This happens when the cell voltage drops sharply under load — typically during a call, video playback, or a burst of mobile data — before the fuel gauge reaches 0%. The modem and display together pull enough current to drag cell voltage below the BMS cutoff threshold while the reported percentage still shows charge remaining. It is a voltage cliff, not a capacity problem. Charge the phone to 100%, then run one full uninterrupted discharge cycle so the fuel gauge IC can anchor its low-voltage cutoff point to the new cell's actual knee voltage, which on a fresh BTY26183 cell is around 3.4–3.5V under load.