About this replacement
Yusun Q1 / M1 / M2 Series — 3.7V Li-ion Replacement Battery (LA20 / LA-20A)
This is a 3.7V, 2000mAh Li-ion cell for the Yusun Q1, M1, M2, and M1Y smartphones. It replaces OEM part numbers LA20 and LA-20A directly. When the original cell degrades and can no longer hold voltage under screen or modem load, this swap restores normal operation.
- Q1 / M1 / M2 platform compatibility: These models share the same battery bay dimensions (60.45 × 55.61 × 4.80mm), connector pinout, and BMS handshake protocol — one cell fits all without modification.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on a Q1 unit. The BMS accepted the cell without fault codes, and the charge IC held a steady 4.2V cutoff at full charge.
- Fuel gauge recalibration on first use: On first 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 fast charging pushes high current into an uncalibrated state.
Sudden shutdown at 20–30% on a freshly installed cell
A new Li-ion cell has a slightly different voltage-to-capacity curve than the worn cell it replaces. The phone's fuel gauge IC is still calibrated to the old curve, so it misreads remaining capacity. When modem transmit bursts or screen brightness spikes draw current, the cell voltage dips below the BMS cutoff threshold — the phone shuts off even though the gauge shows charge remaining. One full discharge cycle down to automatic power-off, followed by an uninterrupted charge to 100%, forces the coulomb counter to relearn the new curve. After that cycle, shutdowns at 20–30% typically stop.
Phone won't power on after sitting in storage — BMS lockout
If a replacement cell has been stored discharged for an extended period, the cell voltage can drop below 2.5V per cell — the threshold where most BMS circuits lock out to prevent damage. Plugging straight into a wall charger often shows nothing on screen. Connect via USB to a powered computer first; the lower current draw (500mA) is enough to nudge the cell above the BMS re-enable threshold without tripping overcurrent protection. Once the screen shows a charge indicator, switch to a wall adapter to complete the charge normally.