Infinix Hot 4 Pro BL-39AX Replacement Battery 3.85V 3900mAh

Infinix Hot 4 Pro / X556 / X5511 — 3.85V Li-Polymer Replacement Battery (BL-39AX)

This is a 3900mAh Li-Polymer cell rated at 3.85V, replacing the original BL-39AX battery in the Infinix Hot 4 Pro, X556, and X5511. It fits the original battery compartment at 89.50 × 63.35 × 4.70mm. Swap it in when the original cell can no longer hold a usable charge across a full day.

• Hot 4 Pro / X556 / X5511 fitment: All three model numbers share the same PCB layout, connector pinout, and BMS handshake protocol. One cell covers all three — no adapter or wiring change required.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the X556 platform. The BMS accepted the cell without fault codes, and the charge IC reached termination voltage at the correct 4.35V cutoff.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one full discharge down to automatic shutdown, then charge to 100% uninterrupted. This lets the fuel gauge IC map the new cell's discharge curve before the coulomb counter locks in its reference points.

Why the Hot 4 Pro reports wrong battery percentage after a cell swap

The fuel gauge IC on this platform stores a learned discharge curve from the original cell. When a new cell goes in, the IC is still referencing old impedance and capacity data. Until it runs at least one full cycle, the percentage readout can be off by 10–20%. A single uninterrupted discharge-to-shutdown followed by a full charge resets the coulomb counter and brings percentage reporting back into alignment with the new cell's actual characteristics.

Sudden shutdown at 20–30% charge remaining on the replacement cell

This is a voltage cliff issue, not a capacity fault. Under modem transmission load or peak screen brightness, the cell voltage drops sharply below the BMS cutoff threshold — even when the reported percentage still looks healthy. The fuel gauge IC hasn't finished calibrating, so it doesn't predict the voltage drop correctly. Run two full discharge-charge cycles and the IC will tighten its model; if shutdowns persist past that point, check that the battery connector is fully seated and making clean contact on all four pads.