Hyundai BP-121 Smartphone Replacement Battery 3.7V 900mAh

Hyundai BP-121 — 3.7V Li-ion Replacement Battery

The BP-121 is a 3.7V, 900mAh Li-ion cell for Hyundai mobile phones and smartphones. It replaces an original battery that has degraded through repeated charge cycles or lost the ability to hold a usable charge. Dimensions are 53.00 × 34.00 × 4.80mm — confirm physical fit against the original before installing.

• OEM part match: This cell shares the BP-121 part number used across Hyundai's compact smartphone lineup. The voltage rail, connector pinout, and BMS communication lines match the original spec so the phone's charge IC recognises the cell correctly.
• Bench tested on actual hardware: We cycled the BP-121 through charge and discharge on the bench. The BMS engaged cutoff cleanly at low-voltage threshold and accepted charge current without fault flags from the charge controller.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC build an accurate discharge curve for the new cell before high-current charging pushes current into an uncalibrated coulomb counter.

Sudden shutdown at 20–30% on the BP-121 replacement cell

This happens when the fuel gauge IC is still calibrated to the old cell's discharge curve. The new cell hits a voltage cliff under modem or display load before the reported percentage reaches zero. The phone interprets the voltage drop as a critical low-battery event and shuts down. Run one full discharge to 0% and a full charge to 100% to let the coulomb counter reset its reference points against the new cell.

Phone warm near the battery during the first few charging cycles

A new Li-ion cell has higher internal impedance than a broken-in cell. During early cycles, the charge IC pushes current into a higher-resistance load, which generates more heat than you'd see from a well-cycled battery. This is normal and settles after three to five full cycles as the cell's impedance drops. If the phone remains warm after five cycles, check that the charge port isn't partially blocked and that the phone's thermal management app shows charge current dropping to trickle before 100%.