Infinix Zero 5 BL-43AX Replacement Battery 3.8V 2700mAh

Infinix Zero 5 / X603 — 3.8V Li-Polymer Replacement Battery (BL-43AX)

This is a 3.8V, 2700mAh Li-Polymer replacement cell for the Infinix Zero 5 (X603). It slots into the same position as the original BL-43AX and connects to the same flex ribbon contacts on the motherboard. Use it when the original cell no longer holds charge through a normal day of calls, browsing, and screen-on time.

• Zero 5 and X603 compatibility: Both model designations reference the same hardware platform. The BL-43AX connector, cell footprint (86.40 × 69.64 × 3.42mm), and BMS handshake spec are identical across both variants — no adapter or modification needed.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Zero 5 platform. The BMS accepted the cell without fault codes, the charge IC ramped current correctly, and the fuel gauge began tracking state-of-charge after one full cycle.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging for the first complete discharge-to-charge cycle. The fuel gauge IC on the Zero 5 is calibrated to the discharge curve of the original cell. One slow cycle lets the coulomb counter re-anchor its zero and full points against the new cell before high-current charging begins.

Why the Zero 5 reports wrong battery percentage after a cell swap

The Zero 5 uses a fuel gauge IC that builds its state-of-charge model from repeated discharge data on the original cell. When you fit a new cell, the IC still references the old discharge curve. This mismatch causes the percentage readout to jump, freeze, or report full charge at a lower actual capacity. One full slow discharge followed by a full slow charge forces the coulomb counter to relearn the new cell's voltage-to-capacity relationship. After that cycle, percentage reporting stabilises.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem radio, display, or both pull current that the cell cannot sustain at its present state of charge — voltage drops below the BMS cutoff threshold even though the UI shows charge remaining. On a new, uncalibrated cell, the fuel gauge IC underestimates how quickly voltage falls under load. The fix is the same recalibration cycle: one full discharge to automatic shutdown, then a full charge without interruption. If shutdowns continue past two cycles, check that the battery flex connector is fully seated — a partial contact raises internal resistance and worsens voltage sag under load.