Media-Tech MT842 Replacement Battery 3.7V 1200mAh Li-ion

Media-Tech MT842 Series — 3.7V Li-ion Replacement Battery

This is a 3.7V, 1200mAh Li-ion cell for the Media-Tech MT842 smartphone and its variants — MT842R, MT842B, MT842W, and eight additional model suffixes. It fits where the original cell has degraded, swollen, or stopped holding a charge. Physical dimensions are 53.00 × 33.90 × 5.70mm — measure your existing cell before ordering.

• MT842 variant coverage: The MT842 line shares a common board layout and connector pinout across colour and regional variants. One cell spec covers the full range — same voltage rail, same connector, same BMS communication protocol across all listed suffixes.
• Bench tested on actual hardware: We cycled the cell through charge and discharge on a smartphone test rig. The BMS accepted the charge handshake correctly and held voltage through draw spikes without tripping the low-voltage cutoff prematurely.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one full discharge-to-charge cycle at standard rate. The fuel gauge IC in the MT842 calibrates its coulomb counter against the cell's actual discharge curve — skipping this step causes erratic percentage readings until the IC re-anchors its reference points.

Why the MT842 reports wrong battery percentage after a cell swap

The MT842 uses a fuel gauge IC that builds its charge-state model from the discharge curve of the original cell. A new cell has a different internal resistance profile, so the IC's coulomb counter drifts from the real state of charge immediately after installation. The percentage shown on screen can be 15–25% off in either direction until the IC recalibrates. One complete discharge down to automatic shutdown followed by a full charge at standard rate resets the reference. After that cycle the displayed percentage tracks actual cell voltage correctly.

Sudden shutdown at 20–30% on the new cell

This is a voltage cliff, not a capacity fault. When the modem, screen, and CPU pull current simultaneously, the new cell's voltage drops faster than the fuel gauge IC predicts — hitting the BMS low-voltage cutoff while the display still reads 20–30%. It is most common in the first three cycles before the IC recalibrates. Force a full discharge cycle without interruption, then charge to 100% and check whether the shutdowns stop. If the cutoff still triggers above 15%, verify the resting cell voltage with a multimeter — it should read 3.7V or above after a full charge.