Sagem MY-202X WT048000800 Replacement Battery 3.7V 720mAh

Sagem MY-202X / MY-300X Series — 3.7V Li-ion Replacement Battery (WT048000800)

This 3.7V 720mAh Li-ion cell replaces the original battery in the Sagem MY-202X, MY-300X, MY-300Y, MY-301X, and over 18 additional Sagem models. It matches OEM part numbers WT048000800, 188881300, SA7A-SN2, SA7M-SN1, and SAAM-SN2. Physical dimensions are 46.90 × 33.30 × 5.40mm — confirming fit before installation takes 30 seconds and prevents connector stress.

• MY-200 and MY-300 series cell compatibility: These Sagem models share the same battery bay geometry, connector pinout, and 3.7V single-cell architecture. One cell covers the full range without any modification to the battery contacts or housing.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on compatible hardware and confirmed the BMS communicates correctly with the charge IC. Protection circuits for over-voltage, over-current, and short-circuit all triggered at expected thresholds.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging if available and run one full discharge-to-charge cycle before normal use. The fuel gauge IC on these Sagem handsets calibrates its coulomb counter against the cell's discharge curve on the first cycle — skipping this step causes erratic percentage readings for several days.

Why the MY-202X reports the wrong battery percentage after a cell swap

The fuel gauge IC stores a learned discharge curve from the original cell. A new cell has a different internal resistance and a slightly different voltage-to-capacity slope. Until the IC runs at least one complete discharge cycle on the new cell, it maps current state-of-charge against the old curve. The result is percentage readings that run ahead or behind actual charge — commonly showing 100% that drops to 40% within minutes of use. One full discharge below 5% followed by an uninterrupted charge to 100% resets the calibration.

Sudden shutdown at 20–30% remaining on the replacement cell

This happens when the modem radio or display backlight draws a short current spike that pulls cell voltage below the BMS cutoff threshold — even though the reported percentage suggests charge remains. On a new, uncalibrated cell the fuel gauge overestimates remaining capacity, so the voltage cliff arrives earlier than the display indicates. The BMS is cutting off correctly at approximately 3.0V per cell to protect the chemistry. Run two full calibration cycles and the fuel gauge IC will track the voltage curve accurately enough to warn the OS before the cutoff hits.