MeiZu L681C Replacement Battery 3.9V 4000mAh BT61

MeiZu L681C / M3 Note Series — 3.9V Li-Polymer Replacement Battery (BT61)

This is a 3.9V, 4000mAh Li-Polymer replacement battery carrying OEM part number BT61. It fits the MeiZu L681C, L681Q, L681H, and M3 Note, among other models in this line. The cell matches the original voltage rail and physical footprint, so it seats and connects without modification.

• L681 and M3 Note platform fit: These models share the same battery bay geometry, connector pinout, and BMS handshake protocol. One cell covers the full range — the charge IC on each device reads the same pack ID and accepts the BT61 without flagging an incompatible source.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the M3 Note platform. The BMS accepted the pack on first connection, charge IC entered CC/CV mode normally, and the cell held steady voltage through the full discharge curve without triggering a low-voltage cutoff early.
• Fuel gauge recalibration on first use: After installation, disable fast charging and run one full discharge-to-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes amperage into an uncalibrated counter — preventing the percentage jumps most users see in the first two days.

Sudden shutdown at 20–30% on the BT61 replacement cell

This happens when the fuel gauge IC is still running the old cell's discharge curve. The phone calculates remaining charge against a degraded cell profile, so when the new cell hits a voltage point the old profile maps to 20–30%, the device cuts power rather than reads it correctly. It is not a faulty cell — it is a calibration mismatch. Run one full uninterrupted discharge until automatic shutdown, then charge to 100% without interruption. After that cycle, the coulomb counter resets against the actual BT61 curve and shutdowns stop.

Phone feels warm near the battery bay during first charge after replacement

A new Li-Polymer cell at full factory charge state has higher internal impedance than a cell that has been cycled. The charge IC pushes current into that resistance, and some energy dissipates as heat — this is normal on cycle one. Warmth that fades after the first full charge is the impedance dropping as the cell forms. If the phone stays hot past the second charge cycle and the charge IC has not throttled, check that the replacement is seated flat with no pressure on the cell body, as a bent pouch raises impedance permanently.