Sagem MY200 Replacement Battery 3.7V 650mAh Li-ion

Sagem MY200 / MY202 Series — 3.7V Li-ion Replacement Battery (252917987)

This 3.7V, 650mAh Li-ion cell replaces the original battery in the Sagem MY200, MY200c, MY202, MY202c, and related handsets. It matches the OEM dimensions at 39.00 × 34.20 × 5.74mm and fits the stock battery bay without modification. Capacity figures come from product data — 650mAh / 2.41Wh.

• MY200 and MY202 platform compatibility: These models share the same 3.7V single-cell architecture, connector pinout, and BMS handshake protocol, which is why one cell covers the full series. Swapping between MY200 and MY202 variants does not require any firmware or hardware change on the phone side.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the MY200 platform and confirmed the BMS accepted charge termination correctly, with no false full-charge cutoff and no error flags thrown during the first three cycles.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge mode for the first complete discharge-to-charge cycle. The fuel gauge IC on these Sagem handsets calibrates its coulomb counter against the cell's discharge curve on the first cycle — running a fast-charge current into an uncalibrated cell skews percentage readings for weeks.

Sudden shutdown at 20–30% on the Sagem MY200 after a cell swap

This is a voltage cliff issue, not a capacity fault. The fuel gauge IC still holds calibration data from the old, degraded cell. When the new cell's discharge curve drops faster than the IC expects under modem or screen load, the phone sees a voltage dip below its cutoff threshold before the percentage counter reaches zero. The fix is one full, uninterrupted discharge from 100% to automatic shutdown, followed by a full charge — this forces the coulomb counter to re-anchor against the new cell's actual curve. After that cycle, the shutdowns stop.

Phone reports erratic or frozen percentage after replacement

The Sagem MY200 series stores its last-known battery state in the fuel gauge IC's non-volatile memory. Fitting a new cell with a different internal impedance confuses the IC — it reads the cell voltage correctly but maps it to the wrong capacity point on the old curve, causing the percentage to jump or stick. Clear the stored state by running the phone down until it shuts off automatically, then charge uninterrupted to 100% without removing the charger. One complete cycle resets the reference points. If the percentage still jumps after two full cycles, check that the battery contacts are clean and making firm contact — a high-resistance connection introduces voltage noise the IC misreads as capacity fluctuation.