Xiaomi Mi 4i BM33 Replacement Battery 3.85V 3000mAh

Xiaomi Mi 4i / X9 — 3.85V Li-Polymer Replacement Battery (BM33)

This is a 3.85V, 3000mAh Li-Polymer cell built to replace a degraded BM33 battery in the Xiaomi Mi 4i and X9 smartphones. Over time, original cells lose usable capacity — this replacement restores the charge the phone originally shipped with. Capacity figures come from the product data, not third-party claims.

• Mi 4i and X9 compatibility: Both models use the same BM33 cell format, voltage rail, and connector pinout. The battery slots into either device without modification — same BMS handshake, same charge IC communication protocol.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on Mi 4i hardware. The BMS accepted the cell without fault codes, and the charge IC completed a full cycle without interruption. No voltage anomalies were recorded at the connector during load.
• Fuel gauge recalibration after install: After fitting this cell, disable fast charging and run one complete discharge-charge cycle at standard rate. This gives the fuel gauge IC time to map the new cell's discharge curve before high-current charging begins on an uncalibrated reference.

Why the Mi 4i reports the wrong battery percentage after a cell swap

The Mi 4i uses a coulomb counter tied to the fuel gauge IC, which builds its percentage model against the original cell's discharge curve. When you install a new cell, the stored curve no longer matches actual cell behaviour — so the reported percentage drifts. This is not a fault with the replacement cell. One full discharge down to automatic shutdown, followed by a complete charge to 100%, forces the fuel gauge IC to recalibrate against the new cell. After that cycle, percentage readings stabilise.

Sudden shutdown at 20–30% on the new cell

This happens when the fuel gauge IC still references the old cell's voltage curve and misreads available capacity under load. The modem and display pull current spikes the old calibration did not account for, causing voltage to dip below the BMS cutoff threshold while the UI still shows charge remaining. The fix is the same recalibration cycle — one full discharge to shutdown, one full charge. If shutdowns continue after two full cycles, check the battery connector pins for debris causing high contact resistance, which drops cell voltage artificially under load.