MeiZu Note 5 BA621 Replacement Battery 3.85V 4000mAh

MeiZu Note 5 / M5 Note Series — 3.85V Li-Polymer Replacement Battery (BA621)

The BA621 is a 3.85V, 4000mAh (15.4Wh) Li-Polymer cell that fits the MeiZu Note 5, M5 Note, M5 Note Dual SIM TD-LTE, M621M, and four additional variants in this line. It replaces the original internal battery when capacity fade or charge failure makes the phone unreliable. Dimensions are 82.10 × 62.60 × 4.45mm — match these before installing.

• M5 Note platform fit: These models share the same PCB connector pinout, voltage rail, and BMS handshake protocol. The BA621 cell communicates with the fuel gauge IC on each variant without modification.
• Bench tested on actual hardware: We ran the BA621 through charge and discharge cycles on the M5 Note platform. The BMS accepted charge current correctly, coulomb counter tracking stayed consistent, and no unexpected cutoff events occurred under screen and modem load.
• First-cycle fuel gauge reset: After installation, disable fast charging and complete one full discharge-to-charge cycle at standard current. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes amps into an uncalibrated register.

Sudden shutdown at 20–30% on the BA621 replacement cell

This happens because the fuel gauge IC still holds the discharge curve of the old, degraded cell. When the new cell hits a voltage point the IC mapped to 20–30% on the old cell, it signals low voltage before the actual charge is depleted. The phone shuts down to protect the SoC even though capacity remains. One full discharge cycle — run the phone to auto-shutdown, then charge uninterrupted to 100% — forces the coulomb counter to recalibrate against the new cell. After that cycle, reported percentage and actual voltage should align. Target a resting voltage of 3.85V at full charge to confirm calibration completed correctly.

USB fast charge not working on first cycle after cell swap

MeiZu's proprietary charge protocol handshake runs between the charge IC and the BMS on the new cell. On the first connection after a swap, the charge IC may default to standard 5V input while it confirms BMS compatibility — fast charge appears inactive or the indicator doesn't trigger. This is not a fault with the charger or cable. Complete one standard-current full charge first; the charge IC re-establishes the fast-charge handshake on the second connection. If fast charge still does not trigger after two cycles, check the cable — the protocol requires a cable rated for the charger's output current.