About this replacement
MeiZu M3s / Y685 Series — 3.85V Li-Polymer Replacement Battery (BT15)
This is a 3.85V, 2800mAh lithium-polymer cell replacing the OEM BT15 battery in the MeiZu M3s and Y685 series smartphones (Y685C, Y685M, Y685Q). It fits the original battery bay without modification and connects to the same flex ribbon connector as the factory unit. Capacity is rated at 10.78Wh.
- M3s and Y685 platform fit: The M3s and Y685 variants share the same physical footprint (70.92 × 59.60 × 3.58mm), the same 3.85V nominal rail, and the same BT15 BMS connector pinout — so one cell covers all listed models without adapter or modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the M3s mainboard, confirmed the BMS handshake completed correctly, and verified the charge IC accepted the full 2800mAh without triggering a protection cutoff.
- First-cycle fuel gauge reset: After installation, disable fast charging and run one complete discharge-to-charge cycle at standard current. The M3s fuel gauge IC calibrates its coulomb counter against the new cell's discharge curve during this first pass — skipping it causes the percentage display to drift for days.
Why the M3s reports wrong battery percentage after a cell swap
The M3s uses a fuel gauge IC that builds its battery model from accumulated charge and discharge data. When you swap the cell, that stored model still reflects the old cell's impedance and capacity curve. The IC feeds the OS percentage figures based on a curve that no longer matches the new cell. One full slow-rate discharge-to-charge cycle gives the coulomb counter enough data to re-anchor its calculations to the replacement cell.
Sudden shutdown at 20–30% remaining on the replacement cell
This happens when the modem or display spikes current demand and the cell voltage drops sharply — faster than the fuel gauge IC predicted. The phone's protection circuit reads that voltage cliff as a dead cell and shuts down, even though the percentage display said 25%. It is more noticeable on a fresh replacement because the fuel gauge curve is still uncalibrated. Run two full discharge-charge cycles at standard rate and confirm the cell voltage at shutdown reads no lower than 3.4V using a USB voltage logger or the phone's engineering mode.