About this replacement
Xiaomi MI2 / MI2s Series — 3.7V Li-ion Replacement Battery (BM20)
The BM20 is a 3.7V, 2050mAh (7.59Wh) Li-ion cell that fits the Xiaomi MI2, MI2s, 2S, and M2 smartphones. It replaces the original battery when the phone can no longer hold a charge or shuts down unexpectedly. Physical dimensions are 85.68 × 50.47 × 4.20mm — match these before ordering if the back cover has been replaced.
- MI2 and MI2s compatibility: Both the MI2 and MI2s use the same BM20 cell footprint, connector pinout, and BMS handshake protocol. The voltage rail and physical form factor are identical across the M2, 2S, and MI2s variants, so one part number covers the full lineup.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the MI2 platform and confirmed the BMS accepted the cell without triggering a protection lockout. Charge acceptance was stable from 3.0V up to the 4.2V cutoff.
- Fuel gauge recalibration on first use: Disable fast charging for the first complete discharge-charge cycle after installation. This allows the fuel gauge IC to map the new cell's discharge curve before high-current charging pushes into an uncalibrated state — skipping this step causes erratic percentage readings in the first few days.
Sudden shutdown at 20–30% on the BM20 replacement cell
The MI2's fuel gauge IC stores a learned discharge curve from the original cell. When a new cell goes in, that stored curve no longer matches reality — the phone reads 25% but the actual cell voltage has already dropped below the modem's minimum load threshold. Under screen-on or LTE load, the voltage collapses faster than the gauge expects, and the PMIC trips a hard shutdown. One full discharge down to auto-off followed by a full charge resets the coulomb counter and eliminates most of these mid-charge shutdowns.
Phone will not power on after the BM20 sat in storage
Li-ion cells stored below 2.5V per cell trigger BMS lockout as a safety measure — the protection circuit opens and blocks all current flow, so the phone appears completely dead. Connecting a charger for 15–30 minutes without pressing the power button allows the charge IC to trickle current into the cell and lift it above the BMS re-enable threshold, typically around 2.9V. Once voltage recovers past that point, the BMS closes and normal charging resumes. If the phone still shows nothing after 45 minutes on charge, the cell discharged too far for recovery and a replacement is needed.