SFR S 300+ 3.7V Replacement Battery 35H00077-00M 1250mAh

SFR S 300+ — 3.7V Li-Polymer Replacement Battery (35H00077-00M)

This 3.7V, 1250mAh Li-Polymer battery replaces the original cell in the SFR S 300+ smartphone. It matches the OEM part number 35H00077-00M and fits the physical bay at 66.50 × 44.00 × 4.50mm. Capacity is 4.63Wh — identical to the factory specification.

• S 300+ fit confirmation: The S 300+ uses a compact Li-Polymer pouch cell on a 3.7V nominal rail. The BMS connector pinout on this replacement matches that rail exactly, so the charge IC recognises the cell and applies the correct charge termination voltage of 4.2V.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the S 300+ platform. The BMS accepted the cell without a handshake error, and the charge IC progressed normally through pre-charge, CC, and CV phases to full termination.
• Fuel gauge recalibration on first use: After installing this replacement, disable fast charging for one full discharge-charge cycle. The fuel gauge IC is still calibrated against the old cell's discharge curve. One full cycle at standard current lets the coulomb counter map the new cell before high-current charging begins.

Why the S 300+ reports wrong battery percentage after a cell swap

The S 300+ uses a fuel gauge IC that builds its state-of-charge model from previous cycle data stored on the old cell. When a new cell goes in, that stored model no longer matches the actual discharge curve. The IC reports percentage based on stale data until it observes a full discharge-to-charge cycle on the replacement cell. This causes readings that are too high early in the discharge and too low near the bottom. One complete cycle from 100% down to automatic shutdown and back to full charge resets the coulomb counter reference point.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or display pulls a burst of current and the cell voltage drops below the BMS cutoff threshold — even though the fuel gauge still shows charge remaining. A new Li-Polymer cell has slightly higher internal impedance in its first few cycles, which amplifies the voltage sag under load. The fuel gauge percentage does not track that instantaneous sag, so the BMS trips the cutoff before the OS can log a low-battery warning. Run two to three full discharge-charge cycles and the cell's impedance drops, reducing sag. If shutdown persists below 25%, check that the connector is fully seated — a partial connection raises contact resistance and worsens sag under the same load.