About this replacement
Xiaomi Mi 9T / Mi 9T Pro — 3.85V Li-Polymer Replacement Battery (BM4G)
This is a 3.85V, 4000mAh (15.4Wh) lithium-polymer cell for the Xiaomi Mi 9T and Mi 9T Pro. OEM part number BM4G. It fits the pop-up camera variants of the Mi 9T lineup — both share the same battery bay, connector, and BMS handshake protocol.
- Mi 9T and Mi 9T Pro compatibility: Both models run the same battery bay geometry and use the same four-pin connector with identical BMS communication. One cell fits both — no hardware modification needed.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on a Mi 9T unit. The BMS accepted the cell on first connection, charge termination triggered correctly at 4.4V, and no thermal flags appeared during the fast-charge phase.
- First-cycle fast charge protocol: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC recalibrate its coulomb counter against the new cell's discharge curve before high-current charging pushes current into an uncalibrated state.
Sudden shutdown at 20–30% on the BM4G replacement cell
This happens when the fuel gauge IC is still calibrated to the old cell's discharge curve. A fresh cell has a steeper voltage cliff under modem or display load — the phone reads 25% but the actual cell voltage drops below the BMS cutoff threshold under that load spike. The BMS trips and the phone shuts down hard. One full discharge-charge cycle without fast charging corrects the coulomb counter and the shutdowns stop.
Phone not powering on after the BM4G sat in storage
Lithium-polymer cells stored below approximately 2.5V per cell trigger BMS lockout — the protection circuit opens and blocks current flow to prevent damage. A standard charger cannot restart the phone in this state. Connect the phone to a 5V/1A USB-A charger — not a fast charger — and leave it for 20–30 minutes without attempting to power on. The charge IC delivers a low-current trickle that brings the cell above the BMS re-enable threshold, typically around 2.9V, after which normal charging resumes.