Realme 9 BLP909 Replacement Battery 3.87V 4500mAh

Realme 9 Series — 3.87V Li-Polymer Replacement Battery (BLP909)

The BLP909 is the OEM-matching lithium-polymer cell for the Realme 9, Realme 9 5G 2022, and RMX3474. It runs at 3.87V with a 4500mAh (17.42Wh) capacity. This battery replaces a degraded or failed original cell in those three models only.

• Realme 9, 9 5G 2022, and RMX3474 fit: All three models share the same connector pinout, BMS handshake protocol, and physical envelope — 92.10 × 64.50 × 4.50mm — so one cell covers the full group without modification.
• Bench tested on actual hardware: We cycled the BLP909 through charge and discharge on the Realme 9 platform. The BMS accepted the cell, protection thresholds triggered correctly at both ends of the voltage window, and the charge IC completed a full cycle without fault flags.
• Fuel gauge recalibration on first use: After fitting the new cell, disable fast charging and run one complete discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve before high-current charging pushes load into an uncalibrated state.

Sudden shutdown at 20–30% on the Realme 9 after a cell swap

A new cell does not automatically inherit the fuel gauge IC's learned discharge curve from the old battery. When the phone reaches the steeper part of the voltage curve under modem or display load, the cell voltage drops faster than the fuel gauge predicts — triggering a hardware undervoltage cutoff before the reported percentage hits zero. The phone reads 25% but the cell is already below the safe floor under load. One full slow-charge cycle resets the coulomb counter and the shutdowns stop.

USB-PD fast charge not accepted on first cycle after replacement

On the first charge after installation, the charge IC may not negotiate fast-charge protocol with the new cell's BMS until it completes an initial handshake cycle. The phone charges slowly or shows standard charge speed despite being on a fast-charge adapter. Disconnect, reconnect the cable, and confirm the charger brick is the original Realme unit — third-party adapters with incorrect voltage steps will fail the BMS negotiation. After the first full cycle completes, fast charging resumes normally.