August MB415 Smartphone Replacement Battery 3.7V 1200mAh

August MB415 — 3.7V Li-ion Replacement Battery

This is a 3.7V 1200mAh Li-ion cell for the August MB415 smartphone. It replaces the original battery when the existing cell no longer holds a charge or fails to power the phone. Voltage and capacity match the MB415's power rail requirements exactly.

• MB415 fit: The MB415 uses a compact 53.00 × 33.90 × 5.70mm cell on a 3.7V nominal rail. This replacement matches those physical and electrical parameters so the battery seats correctly and the phone's charge IC recognises the cell without error flags.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench, confirmed the BMS communicated correctly with the host device, and verified charge termination fired at the correct cutoff voltage without overshoot.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging starts pushing current into an uncalibrated cell.

Why the MB415 reports wrong battery percentage after a cell swap

The MB415 fuel gauge IC stores a charge curve learned from the original cell. When a new cell goes in, that stored curve no longer matches the actual discharge characteristics of the replacement. The phone reads voltage against the old curve and reports a percentage that drifts from reality — sometimes showing full charge when the cell is near half capacity. One full discharge-charge cycle at standard rate forces the coulomb counter to resynchronise against the new cell and corrects the displayed percentage going forward.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or screen draws a current spike that the cell cannot sustain at low state-of-charge. Voltage drops sharply below the BMS cutoff threshold — typically around 3.0V — and the phone shuts off even though the percentage display suggested charge remained. It is a voltage cliff, not a capacity fault. After the recalibration cycle described above, the fuel gauge tracks the real discharge curve and begins flagging low battery earlier, giving the BMS time to manage load before the cliff is reached.