T-Mobile MF64 Li-ion Replacement Battery 3.7V 1800mAh

T-Mobile MF64 / Z64 Series — 3.7V Li-ion Replacement Battery (Li3823T43P3h735350)

This is a 3.7V, 1800mAh Li-ion battery for the T-Mobile MF64, Z64, and Z64 4G HotSpot. It replaces OEM part Li3823T43P3h735350 when the original cell no longer holds a usable charge. Capacity is rated at 6.66Wh — matching the original cell specification.

• MF64, Z64, and Z64 4G HotSpot fit: All three models share the same PCB footprint, connector pinout, and BMS handshake protocol. The Li3823T43P3h735350 part number spans the full range because ZTE used the same charge IC and voltage rail across each variant.
• Bench tested on actual hardware: We ran this cell through full charge-discharge cycles on the MF64 platform. The BMS accepted the cell without fault codes, held voltage through screen-on modem load, and hit rated capacity within two cycles.
• Fuel gauge recalibration after swap: On first use after installing this cell, disable fast charging and run one complete discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter to the new cell's discharge curve before high-current charging begins on an uncalibrated cell.

Why the MF64 reports wrong battery percentage after a cell swap

The fuel gauge IC stores a learned discharge curve from the old cell in non-volatile memory. When a new cell goes in, that stored curve no longer matches actual cell behaviour, so the percentage readout drifts — often reading high early and dropping fast below 30%. One full slow-rate discharge cycle from 100% to automatic shutoff, followed by a full charge, forces the coulomb counter to reset against the new cell. After that cycle, percentage reporting stabilises. If the gauge still reads erratically after two cycles, check that the connector is fully seated — a loose pin creates resistance that skews voltage measurement at the IC.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff failure — not a capacity problem. Under combined modem transmission and screen load, the cell's internal resistance causes a voltage sag that briefly pulls the cell below the BMS cutoff threshold, even though the fuel gauge still shows charge remaining. It most commonly appears in the first few cycles before the cell's impedance settles. Run two full discharge-charge cycles to let the cell condition, then recheck. If shutdowns persist, measure resting voltage with a multimeter — a healthy cell at 20% reported charge should read no lower than 3.5V at rest.