About this replacement
Sagem MYX-8 — 3.7V Li-ion Replacement Battery (XX-8944)
This 3.7V, 1000mAh Li-ion cell replaces the original battery in the Sagem MYX-8 and MYX8 mobile phone. It matches the OEM part number XX-8944 and fits the original battery bay without modification. Capacity figure is taken directly from product data at 3.7Wh.
- MYX-8 and MYX8 compatibility: Both model designations use the same battery bay dimensions — 53.90 x 33.70 x 5.50mm — and the same 3.7V single-cell voltage rail. One cell covers both variants because the connector pinout and BMS handshake requirements are identical across the two SKUs.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the MYX-8 platform. The BMS accepted the charge IC handshake without fault flags, and cell voltage held stable across the full charge curve from 3.0V to 4.2V.
- Fuel gauge recalibration on first install: On first use, disable fast charging if available and run one complete discharge down to automatic cutoff, then charge to 100% uninterrupted. This gives the fuel gauge IC a clean reference curve against the new cell before it starts tracking coulombs under load.
Why the MYX-8 reports the wrong battery percentage after a cell swap
The MYX-8 fuel gauge IC stores a discharge curve calibrated to the original cell. When a new cell goes in, that stored curve no longer matches the actual charge state of the replacement. The gauge reads voltage and compares it against the old map, so it reports figures that are often 10–20% off. One full uninterrupted discharge-charge cycle overwrites the reference and brings percentage reporting back into alignment.
Sudden shutdown at 20–30% remaining on the MYX-8
This happens when the cell voltage drops below the BMS cutoff threshold under load — typically during a screen-on burst or a network search spike — even though the reported percentage still looks healthy. The fuel gauge is reading an uncalibrated curve and misjudging remaining capacity. The cell voltage at cutoff is usually around 3.2–3.3V under that transient load, which the BMS correctly reads as empty. Run one full calibration cycle first; if shutdowns continue after that, check that the battery contacts in the bay are clean and making full contact.