MaxCom MM131 Replacement Battery 3.7V 900mAh Li-ion

MaxCom MM131 — 3.7V Li-ion Replacement Battery

This is a 3.7V 900mAh Li-ion cell for the MaxCom MM131 mobile phone. It fits the MM131 directly and restores power to calling, messaging, and basic phone functions. Capacity is 900mAh (3.33Wh) — matched to the original cell specification.

• MM131 cell fit: The MM131 uses a compact 46.20 × 34.14 × 5.68mm cell footprint with a low 3.7V nominal rail. This replacement matches those dimensions and the voltage rail the phone's charge IC expects. A mismatched voltage rail causes the BMS to reject charging entirely.
• Bench tested on actual hardware: We cycled this cell through full charge and discharge on the bench. The BMS held cutoff at the correct thresholds at both ends — no false full-charge trips, no undervoltage lockout during normal draw from the modem and display.
• Fuel gauge recalibration on first cycle: On first use after installation, run one complete discharge-charge cycle with fast charging disabled. The MM131's fuel gauge IC is calibrated to the old cell's discharge curve. Skipping this step causes erratic percentage readings until the IC maps the new cell's actual capacity curve.

Sudden shutdown at 20–30% on the MM131 after a cell swap

This happens when the fuel gauge IC still holds the old cell's discharge curve in memory. The phone thinks it has 25% remaining, but the new cell's voltage drops below the modem's minimum sustain voltage under load — triggering an emergency shutdown. The fix is a full discharge to auto-off followed by a full charge without interruption. After one complete cycle, the coulomb counter resets its baseline and the percentage readout tracks the actual cell state accurately.

MM131 not powering on after the battery sat in storage

Li-ion cells self-discharge during storage. If the cell drops below approximately 2.5V, the BMS enters lockout mode to prevent damage — and the phone will not respond to the power button at all. Connect the phone to a charger and leave it for 15–20 minutes before attempting to power on. Most charge ICs on budget handsets like the MM131 apply a low trickle current first to bring the cell back above the BMS re-enable threshold, which sits around 2.9V before normal charge current resumes.