MegaFon SP-W1 Replacement Battery 3.7V 1100mAh Li-ion

MegaFon SP-W1 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1100mAh Li-ion replacement battery for the MegaFon SP-W1 smartphone. It fits directly into the SP-W1 and restores the phone to working operation when the original cell has degraded or failed. Capacity is 4.07Wh at the rated voltage.

• SP-W1 cell format: The SP-W1 uses a slim 84 × 34 × 4mm cell with a low nominal voltage rail of 3.7V. Any replacement must match these physical dimensions and voltage exactly — the phone's charge IC expects that specific cell geometry and will not seat or make contact with an oversized cell.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench and confirmed the BMS communicates correctly with the SP-W1's charge controller. Cutoff voltages at both ends of the cycle registered within the expected window — no false low-battery shutoffs during our test runs.
• Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one full discharge-charge cycle from 100% down to automatic shutdown, then charge uninterrupted to 100%. This lets the fuel gauge IC map the new cell's discharge curve before resuming normal use.

Why the SP-W1 reports wrong battery percentage after a cell swap

The SP-W1 uses a fuel gauge IC that builds a discharge model from the cell it learns on over time. When you swap the cell, that learned model no longer matches the new cell's actual voltage curve. The IC will apply the old curve to the new cell and report percentages that are early or late by a wide margin. One complete uninterrupted discharge-charge cycle forces the IC to relearn the curve against the actual cell chemistry. After that cycle, percentage reporting stabilises.

Sudden shutdown at 20–30% on the SP-W1 after replacement

This happens when the fuel gauge IC has not yet recalibrated to the new cell and the phone hits an actual voltage floor before the reported percentage reaches zero. Under screen or modem load, the cell voltage drops sharply at the bottom of its curve — faster than the stale gauge predicts. The phone shuts off to protect the cell even though the display showed charge remaining. Run one full cycle as described above and confirm the phone reaches the automatic low-battery shutdown rather than a hard cutoff. If the problem continues after two full cycles, check that cell voltage at shutdown is above 3.2V — below that points to a seating or contact issue.