BLP751 Oppo K5 Replacement Battery 3.85V 3900mAh

Oppo K5 / Realme X2 — 3.85V Li-Polymer Replacement Battery (BLP751)

The BLP751 is a 3.85V, 3900mAh Li-Polymer cell for the Oppo K5, Realme X2 Premium Edition, and RMX1991/PCNM00 variants. It replaces an original cell that no longer holds charge or causes unexpected shutdowns. Capacity figure is 15.02Wh as rated.

• K5 and Realme X2 cross-compatibility: Both handsets share the same battery bay dimensions, connector pinout, and BMS handshake protocol — the 73.50 × 62.16 × 5.00mm cell fits either chassis without modification.
• Bench tested on actual hardware: We ran the BLP751 through charge and discharge cycles on a K5 unit. The BMS accepted charge from both standard and VOOC fast-charge inputs, and cell voltage held above 3.6V through the mid-cycle discharge range.
• Fuel gauge recalibration on first install: 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 current into an uncalibrated coulomb counter.

Sudden shutdown at 20–30% on the BLP751 replacement cell

This happens because the fuel gauge IC is still running the discharge curve of the old, degraded cell. The new cell's voltage drops faster under modem and screen load than the IC expects, so the OS reads 25% while actual cell voltage has already crossed the hardware cutoff threshold. The phone cuts out to protect the cell, not because the battery is faulty. One full manual discharge to auto-off followed by an uninterrupted charge to 100% resets the coulomb counter and eliminates the premature cutoff.

VOOC fast charge not activating after cell replacement

On the first cycle after swapping the BLP751 in, the charge IC may fall back to standard 5V/1A input instead of negotiating the VOOC handshake. This is a BMS initialisation behaviour — the controller defaults to safe mode until it has completed one full charge cycle and confirmed cell impedance is within spec. Charge fully at standard rate first, then reboot the handset. VOOC negotiation typically resumes from the second charge cycle once cell voltage is confirmed stable at 4.35V.