Mustang M31 Replacement Battery 3.7V 900mAh Li-ion

Mustang M31 — 3.7V Li-ion Replacement Battery

This is a 3.7V, 900mAh (3.33Wh) lithium-ion cell for the Mustang M31 smartphone. It replaces the original battery when the cell no longer holds a usable charge or fails to power the device. Dimensions are 46.20 × 34.14 × 5.68mm — confirm against your existing cell before fitting.

• M31 cell compatibility: The M31 runs a compact battery bay sized for this specific footprint. A mis-sized cell will stress the connector tab and can lift the back cover. Match all three dimensions before installation.
• Bench tested on actual hardware: We cycled the cell through charge and discharge on the bench. The BMS accepted charge current correctly and voltage held steady through a full discharge without mid-cycle cutoff.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging if available and run one full discharge-to-charge cycle. The fuel gauge IC on the M31 is calibrated to the original cell curve — a full cycle lets it map the new cell before resuming normal operation.

Sudden shutdown at 20–30% on the Mustang M31 after a cell swap

This happens because the fuel gauge IC still references the old cell's discharge curve. When the new cell hits a voltage point the IC hasn't mapped yet, it can't accurately predict remaining capacity. Under load — screen-on, active data, or a call — voltage sags below what the IC expects, and the phone shuts down to protect the system. One full discharge and recharge cycle from 100% down to automatic shutdown, without interruption, lets the coulomb counter recalibrate against the new cell.

M31 not powering on after the replacement cell sat in storage

Li-ion cells in storage self-discharge slowly. If the cell drops below approximately 2.5V, the BMS enters lockout mode and will not pass current to the device. The phone appears completely dead — no charge indicator, no boot. Connect the phone to a wall charger rather than a PC port and leave it for 15–20 minutes before attempting to power on; a wall adapter delivers enough current to push the BMS out of lockout where a low-amperage USB port may not.