About this replacement
Sendo S300 — 3.7V Li-ion Replacement Battery
This is a 3.7V 650mAh Li-ion cell for the Sendo S300 mobile phone. It fits the S300 directly and restores power to a handset that no longer holds charge or fails to turn on. Capacity is 650mAh (2.41Wh), matching the original cell specification.
- S300 platform fit: The S300 uses a compact single-cell Li-ion pack at 3.7V nominal. This replacement matches that voltage rail and physical form factor, so the phone's charge IC sees the correct cell chemistry and does not throw a fault during the charge cycle.
- Bench tested on actual hardware: We cycled this cell through a full discharge and charge sequence, monitoring BMS cutoff at both ends. The protection circuit tripped correctly at low voltage and accepted a full charge without thermal event or charge IC rejection.
- Fuel gauge recalibration on first use: On first installation, run the battery from full charge down to automatic shutdown — do not interrupt the cycle. This gives the S300's fuel gauge IC one complete reference curve against the new cell before it starts reporting percentage to the OS.
Why the S300 shuts down suddenly at 20–30% remaining
This is a voltage cliff problem, not a capacity problem. When the S300's modem or backlight draws a current spike, a degraded or freshly installed cell can drop below the BMS cutoff threshold for a split second — the phone shuts off even though the fuel gauge still shows charge. The fuel gauge IC was calibrated to the old cell's discharge curve and does not yet know where the new cell's voltage cliff sits. One full discharge-to-shutdown cycle lets the IC remap that threshold accurately.
Phone shows incorrect battery percentage after cell swap
The S300's fuel gauge IC stores a discharge curve from the previous cell. After a replacement, the stored curve no longer matches the new cell's actual voltage-to-capacity profile, so percentage readings jump or stall. The fix is one uninterrupted full cycle — charge to 100%, discharge to automatic shutdown, then recharge fully. After that cycle the IC overwrites the old curve and percentage reporting stabilises. Do not top-up mid-discharge during this first cycle or the recalibration resets.