BM4J Redmi Note 8 Replacement Battery 3.85V 4400mAh

Redmi Note 8 Pro — 3.85V Li-Polymer Replacement Battery (BM4J)

This is a 3.85V, 4400mAh Li-Polymer cell that replaces the original BM4J battery in the Redmi Note 8 Pro and related M1908C3J variants. It fits models M1908C3JH, M1908C3JG, and M1908C3JE, among others in the Note 8 Pro line. Physical dimensions are 85.45 × 64.10 × 4.28mm — measure your original before ordering if you are unsure.

• Note 8 Pro series fit: All M1908C3J variants share the same battery bay geometry, connector pinout, and BMS handshake requirements. The BM4J part number covers that full range — the fuel gauge IC on each board reads the same cell parameters.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on a Note 8 Pro board. The BMS accepted the cell without fault flags, and the charge IC reached full termination voltage without thermal shutdown.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging for one complete discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's actual discharge curve before high-current fast charging begins pushing current into an uncalibrated cell — it prevents early percentage-jump errors.

Sudden shutdown at 20–30% on the Redmi Note 8 Pro

This is a voltage-cliff problem, not a capacity problem. When the modem fires at full transmit power or the screen peaks brightness, current draw spikes hard. An aged or poorly matched cell cannot hold voltage under that load — it collapses below the BMS cutoff threshold before the fuel gauge reads zero. A fresh BM4J cell with lower internal resistance sustains voltage through those spikes. After fitting the new cell, let it discharge to around 5% naturally on the first cycle so the coulomb counter anchors its lower calibration point correctly.

USB fast charge not triggering after battery replacement

The Note 8 Pro uses Xiaomi's proprietary fast charge protocol, which the charge IC negotiates over the data lines before ramping current. On the first cycle with a new cell, the BMS sometimes presents parameters the charge IC reads as out-of-range, so it falls back to standard 5V charging instead. This is not a fault — it clears itself after one full standard charge cycle completes and the BMS registers a valid state-of-charge endpoint. If fast charging still does not trigger after two full cycles, check the USB-C port for debris or try a certified Xiaomi charger, as third-party chargers often fail protocol negotiation entirely.