MaxCom MM720 Replacement Battery 3.7V 800mAh Li-ion

MaxCom MM720 / MM721 Series — 3.7V Li-ion Replacement Battery

This 3.7V, 800mAh Li-ion cell replaces the original battery in the MaxCom MM720, MM720BB, MM721, and MM721BB mobile phones. These are compact, basic handsets aimed at elderly users and emergency-use scenarios where keeping the phone powered matters. The replacement cell matches the original voltage and physical footprint at 66.60 × 33.90 × 4.50mm.

• MM720 and MM721 platform fit: Both model lines share the same battery bay dimensions, connector pitch, and 3.7V nominal voltage rail. The BB variants use the same housing, so a single cell covers all four listed models without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a MM720 unit. The BMS accepted the charge handshake without fault flags, and the protection circuit tripped correctly at the low-voltage cutoff threshold, as expected on a single-cell Li-ion pack.
• Fuel gauge recalibration on first use: After installing this cell, run one full discharge to auto-off, then charge uninterrupted to 100% before normal use. The MM720's fuel gauge IC is calibrated to the old cell's discharge curve — skipping this step causes the percentage counter to read inaccurately for weeks.

Sudden shutdown at 20–30% on the MM720 after a cell swap

This happens because the fuel gauge IC still references the discharge curve of the original, degraded cell. When a fresh cell with a steeper voltage drop hits load from the radio or screen backlight, the IC interprets the voltage dip as a near-empty state and triggers shutdown. The phone cuts out even though usable charge remains. One full discharge-charge cycle resets the coulomb counter and brings the percentage readout in line with actual cell state — after that, early shutdowns stop.

Phone won't power on after sitting in storage with the new battery installed

Li-ion cells self-discharge during storage. If the MM720 sat unused long enough for the cell to drop below approximately 2.5V, the BMS enters lockout mode to prevent cell damage — and the phone shows no response to the power button. Connect the phone to a wall charger, not a USB port, and leave it for 20–30 minutes without pressing anything. Most BMS circuits on cells like this will accept a trickle current above the lockout threshold and begin recovery; the charge indicator should appear within that window.