Hyundai MBD125 Replacement Battery 3.7V 1200mAh

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

This is a 3.7V Li-ion cell rated at 1200mAh (4.44Wh), built to fit the Hyundai MBD125 and MBD125 Dual Sim smartphones. It replaces a degraded original cell that can no longer hold voltage under load. Physical dimensions are 53.00 × 33.90 × 5.70mm — confirm fit before installation.

• MBD125 and MBD125 Dual Sim compatibility: Both variants share the same battery bay dimensions, voltage rail, and connector pinout. The Dual Sim model draws slightly more standby current across two active radios, but the charge IC and BMS handshake are identical on both.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the MBD125 platform. The BMS accepted charge correctly, protection cutoff triggered at the expected low-voltage threshold, and the charge IC did not flag a fault.
• First-cycle calibration tip: On first use after installation, disable fast charging if the option is available, then run one full discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before it starts reporting percentage to the OS.

Sudden shutdown at 20–30% on the replacement cell

This happens because the fuel gauge IC is still using the discharge curve it learned from the old, degraded cell. When the new cell hits a voltage point the old curve never reached cleanly, the IC miscalculates remaining capacity and signals a shutdown. The fix is one full discharge cycle — drain the phone until it shuts off automatically, then charge uninterrupted to 100%. After that cycle, the coulomb counter recalibrates against the actual new cell behaviour and the early shutdowns stop.

Phone reports wrong battery percentage after cell swap

The MBD125 fuel gauge IC stores a learned model of the previous cell's internal resistance and capacity fade. A fresh cell with lower impedance produces a different voltage curve, so the IC reads capacity incorrectly — often showing a steep drop or erratic jumping. Run two full discharge-charge cycles without interruption. By the second cycle, the IC's coulomb counter has enough real data to recalibrate, and percentage reporting stabilises. Do not top up frequently during these two cycles — partial charges prevent the counter from completing its recalibration pass.