Sagem MY-V56 Replacement Battery 3.7V 750mAh Li-ion

Sagem MY-V56 / MY-X5-2 Series — 3.7V Li-ion Replacement Battery (188421922)

This 3.7V, 750mAh Li-ion cell replaces the original battery in the Sagem MY-V56, MY-X5-2, MYX5-2, MY-X2, and related candybar-style handsets from the same platform. It matches the OEM dimensions of 53.80 × 33.70 × 5.80mm and drops into the same battery bay without modification. Voltage and capacity match the original spec exactly.

• MY-V56 / MY-X5-2 platform fit: These models share a common battery bay dimensions, connector pinout, and charge IC handshake voltage. The same cell works across the range because the charge management circuit expects 3.7V nominal with a 4.2V charge ceiling — this cell meets both.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a MY-V56 unit. The BMS accepted the charge IC handshake on first connection, charged to 4.2V without interruption, and cutoff terminated correctly at the low-voltage threshold.
• Fuel gauge recalibration on first cycle: After fitting this cell, run one full discharge to automatic shutdown, then charge uninterrupted to 100%. The phone's fuel gauge IC calibrated to the old cell's discharge curve — without one full cycle on the new cell, the percentage readout will drift, typically reporting empty well before the cell is actually depleted.

Why the MY-V56 reports wrong battery percentage after a cell swap

The MY-V56 uses a coulomb-counting fuel gauge IC that builds its charge model from previous cycle history. When you fit a new cell, that stored model no longer matches the actual discharge curve of the replacement. The gauge reads state-of-charge against the wrong reference, so percentages appear inaccurate — often showing 20–30% and then dropping to zero without warning. One full uninterrupted discharge-charge cycle forces the IC to rewrite its reference curve against the new cell. After that cycle, percentage reporting stabilises.

Phone shuts off suddenly at 15–25% with the new cell installed

This is a voltage cliff issue, not a capacity problem. At 15–25% state-of-charge, a fresh cell's internal resistance is still slightly elevated on early cycles — when the screen backlight or RF transmit circuit pulls a short high-current burst, cell voltage sags below the BMS cutoff threshold and the phone drops dead. It typically clears within two to three full charge-discharge cycles as the cell conditions and internal resistance drops. After conditioning, confirm the cell holds above 3.5V under load by charging fully and checking the phone holds a stable percentage reading under normal screen-on use.