Sagem MY401C Replacement Battery 3.7V 650mAh Li-ion

Sagem MY401C / MY401L — 3.7V Li-ion Replacement Battery (252917966)

This is a 3.7V 650mAh Li-ion cell for the Sagem MY401C and MY401L mobile phones. It replaces OEM part 252917966 and fits both the MY-401C and MY-401L variants. Capacity is 2.41Wh — matching the original cell specification.

• MY401C and MY401L compatibility: Both models share the same battery bay dimensions, connector pinout, and 3.7V nominal voltage rail. The 47.56 × 34.11 × 4.44mm cell seats flush without modification, and the BMS handshake runs on the same single-wire data line across both variants.
• Bench tested on actual hardware: We cycled this cell through charge and load on the MY401C platform. The BMS accepted charge at the expected 4.2V cutoff and tripped the low-voltage cutout cleanly before cell damage — no spurious trips at mid-state-of-charge.
• Fuel gauge recalibration on first install: After fitting this cell, disable any fast-charge mode and run one full discharge-charge cycle before normal use. The fuel gauge IC on these Sagem phones calibrates its coulomb counter against the discharge curve of the first full cycle — skipping this step leaves the IC reading against the old cell's curve.

Sudden shutdown at 20–30% on the MY401C after a cell swap

This is a voltage cliff problem, not a capacity problem. The fuel gauge IC still holds calibration data from the degraded original cell, so its state-of-charge curve does not match the new cell's actual discharge profile. When the modem fires a transmit burst or the backlight draws peak current, the phone's supply voltage dips below the shutdown threshold even though the reported percentage looks safe. Run one full discharge down to automatic power-off, then charge uninterrupted to 100% — this re-anchors the coulomb counter to the new cell. After that single cycle, the percentage readout tracks the real remaining charge accurately.

MY401C will not power on after sitting in storage with a flat battery

If a Li-ion cell self-discharges below approximately 2.5V per cell in storage, the BMS locks out to prevent charging into a potentially damaged cell — the phone shows nothing when you press the power button or connect a charger. Connect the phone to a low-current USB source (a standard 500mA USB-A port, not a fast charger) and leave it for 20–30 minutes without pressing anything. The charge IC delivers a trickle pre-charge that nudges the cell above the BMS re-enable threshold, typically around 2.9V, at which point the BMS releases the lockout and normal charging resumes.