About this replacement
MyPhone Q-smart Premium — 3.8V Li-Polymer Replacement Battery (BM-09)
This is a 3.8V, 2300mAh Li-Polymer replacement battery for the MyPhone Q-smart Premium smartphone. It carries OEM part number BM-09 and fits directly into the Q-smart Premium's battery bay. It restores power to calls, messaging, apps, and display functions when the original cell has degraded or failed to hold a charge.
- Q-smart Premium compatibility: The BM-09 cell matches the Q-smart Premium's 3.8V power rail and physical bay dimensions — 76.53 × 59.00 × 3.70mm. The BMS connector pinout aligns with the phone's charge IC, allowing normal charge and discharge handshaking without modifications.
- Bench tested on actual hardware: We cycled this cell through full discharge and charge sequences on the Q-smart Premium. The BMS held the charge cutoff at the correct upper voltage and the fuel gauge IC responded to cell data within two complete cycles.
- First-cycle fuel gauge reset: On first use after installation, disable fast charging and let the phone complete one full discharge-to-charge cycle. This lets the fuel gauge IC recalibrate its discharge curve against the new cell before high-current charging begins on an uncalibrated coulomb counter.
Why the Q-smart Premium reports wrong battery percentage after a cell swap
The phone's fuel gauge IC stores a learned discharge curve from the original cell. When a new BM-09 cell goes in, the IC still references that old curve, so displayed percentage drifts from actual state of charge. This shows up as erratic jumps — the phone might read 45% and drop to 12% without significant use. One full discharge down to automatic shutdown, followed by a complete charge to 100%, forces the IC to rebuild its curve against the new cell's actual chemistry. After that cycle, percentage readings stabilise.
Sudden shutdown at 20–30% on the replacement BM-09 cell
This shutdown happens when the modem or screen draws a current spike that the new cell cannot sustain while the fuel gauge IC is still uncalibrated. The cell voltage sags below the BMS cutoff threshold — typically around 3.2V under load — and the phone shuts off even though the displayed percentage looks safe. It is not a faulty cell. Run the phone through two complete discharge-charge cycles without fast charging, and the IC will map the real voltage cliff of the new cell. Shutdowns at 20–30% should stop after calibration completes.