Xiaomi Mi 11T Replacement Battery BM59 3.87V 4900mAh

Xiaomi 11T / Mi 11T 5G — 3.87V Li-Polymer Replacement Battery (BM59)

The BM59 is a 3.87V, 4900mAh Li-Polymer cell that fits the Xiaomi 11T and Mi 11T 5G, including the Global Dual SIM variant (21081111RG). It powers the phone's display, 5G modem, and processor rail. This replacement targets phones where the original cell no longer holds a full charge or shuts down unexpectedly under load.

• Mi 11T 5G platform fit: The 11T series shares a single battery bay across the standard, Global, and Dual SIM variants. All use the same BM59 connector pinout and BMS handshake, so one cell covers all four model numbers listed above.
• Bench tested on actual hardware: We ran the BM59 through charge and discharge cycles on a 21081111RG unit. The BMS accepted charging without fault flags, and the protection circuit triggered correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: After installation, disable fast charging in Settings and run one full discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before the coulomb counter locks in its reference — skipping this step causes erratic percentage readings for the first several days.

Why the Mi 11T reports wrong battery percentage after a cell swap

The Mi 11T uses a coulomb counter that builds its fuel map against the original cell's charge and discharge curves. When you swap in a new BM59, that stored curve no longer matches the cell in the bay. The fuel gauge IC reads voltage and estimates state of charge against stale data, so the percentage shown can lag or jump until recalibration happens. One full slow charge cycle — standard 18W, not 67W turbo — forces the IC to rebuild its curve against the new cell and resolves the mismatch.

Sudden shutdown at 20–30% on the replacement cell

This happens when the 5G modem or display pulls a current spike the new cell can't sustain at that state of charge, causing voltage to sag below the BMS cutoff threshold — even though the gauge still shows charge remaining. It's a voltage cliff, not a capacity fault. The fix is the same recalibration cycle: drain the phone to auto-shutdown, then charge to 100% without interruption at standard rate. After one complete cycle the fuel gauge anchors the cutoff point accurately and the early shutdowns stop. Check the battery percentage stabilises above 15% before the phone powers off — that's the confirmation the gauge is reading correctly.