Hyundai MBD125 Replacement Battery 3.7V 1000mAh Li-ion

Hyundai MBD125 / MBD125 Dual Sim — 3.7V Li-ion Replacement Battery

This is a 3.7V 1000mAh Li-ion cell for the Hyundai MBD125 and MBD125 Dual Sim smartphones. It slots into both the single-SIM and dual-SIM variants of that handset. Voltage and physical dimensions match the original cell: 53.00 × 33.90 × 5.70mm.

• MBD125 and MBD125 Dual Sim compatibility: Both variants run the same 3.7V power rail and share an identical battery bay. The connector pinout and BMS handshake are the same across both, so one cell covers the full MBD125 line.
• Bench tested on actual hardware: We cycled this cell through full charge and load discharge on the MBD125 platform. The BMS accepted the cell without error flags, and the protection circuit tripped correctly at low-voltage cutoff.
• First-cycle fuel gauge reset: On first use after installation, disable fast charging and run one complete discharge down to automatic shutdown, then charge to 100% uninterrupted. This lets the fuel gauge IC map the new cell's discharge curve before the OS starts reading percentage from it.

Why the MBD125 reports wrong battery percentage after a cell swap

The fuel gauge IC on the MBD125 stores a learned discharge curve from the original cell. When a new cell goes in, the IC keeps using the old curve, so the percentage readout drifts from the actual state of charge. This mismatch is worst in the 30–10% range, where the new cell's voltage slope differs most from the stored model. One full discharge-charge cycle forces the IC to recalibrate against the new cell. After that cycle, percentage accuracy returns to normal.

Sudden shutdown at 20–25% on the replacement cell

This is a voltage cliff, not a capacity fault. Under modem or screen load, the cell voltage briefly dips below the BMS cutoff threshold even though the fuel gauge still shows charge remaining. It happens most often on a new cell before calibration or on a cell that has been stored discharged for several months. Check resting cell voltage with a multimeter — if it reads below 3.4V after a full charge, the cell has not recovered from deep discharge. A slow trickle charge back to 3.7V open-circuit can recover it; if resting voltage stays below 3.5V, the cell should be replaced.