Xiaomi Mi 4A Replacement Battery BN30 3.85V 3100mAh

Xiaomi Mi 4A / Redmi 4A — 3.85V Li-Polymer Replacement Battery (BN30)

This is a 3.85V, 3100mAh (11.94Wh) lithium-polymer cell replacing the OEM BN30 battery in the Xiaomi Mi 4A and Redmi 4A. Both devices share the same battery bay dimensions and connector pinout, so one cell covers the full compatibility list. Dimensions are 71.80 × 59.20 × 4.40 mm — measure your bay before ordering if your unit has been repaired before.

• Mi 4A and Redmi 4A compatibility: Both models run the same 3.85V power rail, use the same flex connector, and negotiate charge current through the same BMS handshake — one cell spec covers both.
• Bench tested on actual hardware: We ran this cell through a full charge cycle on a Mi 4A unit. The BMS accepted charge handshake without fault codes, and the protection circuit tripped correctly at the expected over-voltage threshold.
• Fuel gauge recalibration on first install: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard current. This lets the fuel gauge IC map its coulomb counter to the new cell's actual discharge curve before high-current charging starts.

Sudden shutdown at 20–30% on the BN30 replacement cell

This is a voltage cliff issue, not a capacity defect. Under modem TX bursts or peak screen backlight load, a new cell that hasn't been calibrated yet cannot sustain the voltage the SoC expects — so the phone shuts off even though the fuel gauge still shows charge remaining. The fix is a full discharge-charge cycle with no fast charging on the first pass. After calibration, the fuel gauge IC tracks the cell's actual minimum sustainable voltage and stops triggering low-voltage shutoff prematurely.

Phone won't power on after the BN30 sat in storage

Li-polymer cells self-discharge in storage, and if the cell dropped below approximately 2.5V per cell, the BMS enters lockout mode and blocks charge input to prevent thermal events on a deeply discharged cell. Plugging in normally often shows no response. Connect the phone to a low-current charger — a USB port rather than a wall adapter — and leave it for 20 to 30 minutes without pressing the power button. This trickle current is usually enough to bring the cell above the BMS re-enable threshold, after which normal charging resumes.