About this replacement
Cubot S308 — 3.7V Li-ion Replacement Battery (S308)
This is a 3.7V 1800mAh Li-ion cell for the Cubot S308 smartphone. It replaces the original internal battery when the cell can no longer hold a usable charge between cycles. Capacity is 6.66Wh — matched to the S308's original specification.
- Cubot S308 fit: The S308 uses a fixed internal battery with a specific connector pinout and BMS communication line. This cell matches that pinout, so the phone's charge IC and fuel gauge IC can read the battery state correctly after installation.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench, confirmed the BMS handshake completed without fault codes, and verified the charge IC accepted the cell without triggering a protection cutoff.
- Fuel gauge recalibration after swap: On first use after installation, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before coulomb counting begins — skipping this step causes erratic percentage readings for the first several cycles.
Sudden shutdown at 20–30% on the replacement cell
A new cell has a slightly different voltage-versus-capacity curve than the aged cell it replaces. The S308's fuel gauge IC was calibrated against the old cell's curve, so it misjudges the remaining charge. When the modem or screen draws a short current spike, the voltage drops below the BMS cutoff threshold — typically around 3.0V — before the displayed percentage reaches zero. The fix is one complete discharge cycle with the phone running normally until it shuts off on its own, then a full charge to 100%. That single cycle gives the fuel gauge IC enough data to re-anchor its curve to the new cell.
Phone feels warm near the battery during the first few charges
A fresh cell starts with higher internal impedance than a cell that has been through several cycles. The charge IC pushes current into a higher-resistance load, and that resistance dissipates energy as heat. This is normal for the first two to three charge cycles as the cell's impedance drops toward its rated value. If the warmth persists beyond three full charge cycles or the phone becomes hot to the touch, check that the charge IC has not locked into a reduced-current fault state — clear this by powering the phone fully off, then back on, before connecting the charger.