LG K52 Replacement Battery BL-T51 3.87V 3900mAh Li-Polymer

LG K52 / K62+ — 3.87V Li-Polymer Replacement Battery (BL-T51)

This is a 3900mAh Li-Polymer cell at 3.87V, replacing part number BL-T51 (also listed as EAC64788701). It fits the LG K52, LMK525BMW, K525BMW, K62+ 2020, and over a dozen related variants. Swap it in when the original cell no longer holds charge or causes unexpected shutdowns.

• K52 and K62+ 2020 compatibility: Both models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — that is why one cell covers both. The 87.50 × 65.90 × 3.84mm footprint matches the original enclosure with no modification needed.
• Bench tested on actual hardware: We ran the BL-T51 cell through full charge and discharge cycles on K52 hardware. The BMS accepted the cell without error, and the charge IC held current within spec through each cycle. No thermal flags were triggered.
• 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 map the new cell's discharge curve before high-current charging pushes into an uncalibrated state. Skip this step and the percentage readout will drift immediately.

Sudden shutdown at 20–30% on a replacement cell

A new cell in an old phone does not mean the fuel gauge IC resets. The coulomb counter inside the K52 is still calibrated to the discharge curve of the worn original cell. When load spikes — modem switching bands, screen at full brightness — the new cell's actual terminal voltage drops below what the IC expects at that state of charge. The phone reads the voltage sag as a critical low and cuts out. One full uninterrupted discharge to auto-off, then a full charge to 100%, forces the IC to relearn the curve. After that cycle, the shutdowns stop.

USB-PD fast charge not activating on the first cycle after a cell swap

The K52 charge IC runs a brief handshake with the BMS before it steps up to fast-charge current. On a new cell fresh out of storage, the BMS may report a state-of-charge value that causes the charge IC to default to standard 5V current as a precaution. This is not a fault — it clears on its own after the first full charge cycle completes. If fast charging still does not activate after two full cycles, check that the charger output voltage is confirmed at 9V under load before assuming the battery is the cause.