Auro M401 Replacement Battery 3.7V 1000mAh Li-ion

Auro M401 — 3.7V Li-ion Replacement Battery (M451)

This is a 3.7V, 1000mAh Li-ion replacement battery for the Auro M401 smartphone. It fits the M401 directly and uses OEM part number M451. When the original cell degrades after repeated charge cycles, this battery restores the phone to full operation.

• M401 platform fit: The M401 uses a single-cell 3.7V nominal rail with a connector and physical footprint matched to the M451 part number. The BMS handshake on this device authenticates the cell by voltage and thermistor signal — both are met by this replacement cell.
• Bench tested on actual hardware: We ran the cell through charge and discharge cycles on the M401 platform. The BMS accepted the cell on first insertion, charge termination triggered correctly at full voltage, and the protection circuit tripped as expected under short-circuit simulation.
• Fuel gauge recalibration on first use: On first installation, disable fast charging if available and run one full discharge-to-charge cycle before normal use. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging begins on an uncalibrated state-of-charge reading.

Sudden shutdown at 20–30% on the Auro M401 after a cell swap

This happens because the fuel gauge IC on the M401 is still using the discharge curve from the old, degraded cell. The new cell has a steeper voltage cliff under load — particularly when the modem or display draws current — so the phone hits the undervoltage cutoff while the percentage reading is still showing 20–30%. It is not a fault in the replacement cell. One full discharge-charge cycle recalibrates the coulomb counter and the shutdowns stop.

Phone reports incorrect battery percentage after replacement

The M401's fuel gauge IC stores a learned discharge model for the previous cell. After swapping to a new cell, that stored model is mismatched — the percentage shown is calculated against the wrong capacity baseline. The fix is a full calibration cycle: drain the phone to automatic shutdown, then charge uninterrupted to 100% without removing the cable. After one complete cycle the IC resets its reference points and percentage readings stabilise.