Sagem MYC-1 Replacement Battery 3.7V 850mAh Li-ion

Sagem MYC-1 — 3.7V Li-ion Replacement Battery (251358949)

This is a 3.7V, 850mAh lithium-ion replacement battery for the Sagem MYC-1 mobile phone. It fits the MYC-1 and MYC1 variants and slots directly into the original battery compartment. Replaces OEM part numbers 251358949 and BD47-04.

• MYC-1 and MYC1 fit: Both model designations share the same battery bay dimensions, contact layout, and 3.7V nominal rail. One cell covers both.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the MYC-1 platform. The BMS held cutoff correctly at both ends — no false full-charge termination and no over-discharge trip under call load.
• Fuel gauge recalibration after swap: Run one full discharge to low-battery warning, then charge uninterrupted to 100% before normal use. The MYC-1 fuel gauge IC calibrates its reference curve on this first cycle — skipping it causes erratic percentage readings from day one.

Sudden shutdown at 20–30% on the Sagem MYC-1

Early 2000s Li-ion cells have a steep voltage cliff in the lower charge range. When the cell voltage drops below roughly 3.5V under the load of an active call, the phone's protection circuit cuts power before the fuel gauge reaches zero. A new cell with an uncalibrated gauge makes this worse — the reported percentage and actual state of charge don't match. Run one full calibration cycle first. After that, shutdowns below 25% reported charge should stop.

Phone shows wrong battery percentage after fitting the new cell

The MYC-1 fuel gauge IC stores a discharge curve built from the old cell's impedance and capacity. A new 850mAh cell has different internal resistance, so the IC misreads state of charge immediately after swap. The fix is one complete uninterrupted cycle — drain to the low-battery cutoff, then charge to 100% without pulling the phone off charge early. After that cycle the IC resets its reference points and percentage accuracy returns to normal.