About this replacement
Aligator S4000 Series — 3.7V Li-ion Replacement Battery (AS4000BAL)
The AS4000BAL is a 3.7V, 1200mAh Li-ion cell that fits the Aligator S4000, S4000 Dou, and S4020 smartphones. It replaces a degraded or failed original battery without requiring board-level repairs. Voltage and form factor match the original spec exactly.
- S4000, S4000 Dou, and S4020 fit: All three models share the same battery bay dimensions (53.85 × 53.90 × 4.50mm), connector pinout, and 3.7V nominal voltage rail. One cell covers the full production run of this platform.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on the S4000 platform. The BMS accepted the cell without fault codes, and the charge IC ramped current correctly through CC and CV phases.
- Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one complete discharge-charge cycle at standard rate. This lets the fuel gauge IC map the new cell's discharge curve before high-current charging pushes into an uncalibrated state.
Sudden shutdown at 20–30% on the replacement cell
The S4000's fuel gauge IC holds a discharge curve learned from the original cell. A new cell with a different internal impedance profile reaches voltage sag earlier under modem or display load — even when the reported percentage still reads 25%. The OS doesn't see the voltage drop in time and the BMS cuts power before a low-battery shutdown can trigger. One full calibration cycle at standard charge current resets the coulomb counter and resolves premature cutoffs.
Phone won't power on after the battery sat discharged in storage
If a Li-ion cell drops below approximately 2.5V per cell during extended storage, the BMS enters a protection lockout and blocks normal charge current. The phone will show no response to the charger and won't boot. Connect it to a low-current USB source (a 5V/500mA port, not a fast charger) and leave it for 20–30 minutes — the BMS trickle circuit should recover the cell voltage above the lockout threshold. Once the screen shows a charge indicator, switch to the standard charger to complete the cycle.