Xiaomi Mi Max 2 BP4U Replacement Battery 3.85V 5200mAh

Xiaomi Mi Max 2 / Mi 15 — 3.85V Li-Polymer Replacement Battery (BP4U)

This is a 3.85V, 5200mAh Li-Polymer cell replacing the original BP4U battery in the Xiaomi Mi Max 2 (24129PN74C, 24129PN74G, 24129PN74I) and Mi 15. It restores power to the processor, display, modem, and all onboard functions. Capacity is 5200mAh / 20.02Wh — matching the original cell specification.

• Mi Max 2 and Mi 15 fitment: Both platforms share the BP4U cell footprint, connector pinout, and BMS handshake protocol. The fuel gauge IC on each board reads the same cell chemistry signature, so the replacement registers correctly without modification.
• Bench tested on actual hardware: We ran this cell through full charge and load cycles on Mi Max 2 hardware. The BMS accepted the cell on first connection, charge current ramped correctly through CC/CV stages, and no overcurrent trips occurred under sustained screen-on load.
• Fuel gauge recalibration after install: On first use after installation, disable fast charging and complete one full discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's actual discharge curve before high-current charging pushes current into an uncalibrated coulomb counter.

Sudden shutdown at 20–30% on the Mi Max 2 after a cell swap

This happens because the fuel gauge IC is still calibrated to the old, degraded cell's voltage curve. When the new cell hits a voltage point the IC associates with a low state of charge, it signals shutdown before the cell is actually depleted. The fix is one complete discharge to automatic power-off, followed by a full uninterrupted charge to 100%. After that cycle, the coulomb counter resets its baseline against the new cell and percentage readings stabilise.

USB fast charge not activating on the first cycle after replacement

On the first connection after a cell swap, Xiaomi's charge IC runs a conditioning check before negotiating fast charge protocols. If the BMS reports an uncalibrated or cold cell state, the IC falls back to 5V standard charging and holds there for the full first cycle. This is normal behaviour — not a fault with the replacement cell. Let the first charge complete at standard rate, disconnect, and reconnect; fast charge protocol negotiation resumes from the second cycle onward.