Samsung SCH-R390 Replacement Battery 3.7V 940mAh Li-ion

USCellular SCH-R390 / Freeform 4 / Character — 3.7V Li-ion Replacement Battery

This 3.7V, 940mAh Li-ion battery fits the USCellular SCH-R390, Freeform 4, SCH-R640, and Character handsets. It slots in where the original cell has degraded past the point of holding a usable charge. Capacity is 940mAh (3.48Wh) — matched to the stock specification for these models.

• SCH-R390, Freeform 4, SCH-R640, Character compatibility: These four models share the same battery bay dimensions, connector pinout, and charge IC handshake. One cell covers all four without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the SCH-R390 platform. The BMS accepted the charge IC handshake cleanly, and voltage held steady across the discharge curve without premature cutoff.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging if your handset supports it, then run one complete discharge down to automatic shutoff followed by a full charge. This lets the fuel gauge IC map its coulomb counter to the new cell's discharge curve before normal use. Skipping this step is the leading cause of erratic percentage readings after a cell swap.

Sudden shutdown at 20–30% on the SCH-R390 and Character

This happens when the cell hits a voltage cliff — a point on the discharge curve where voltage drops sharply under screen or modem load. The phone's fuel gauge IC still shows 20–30% remaining, but the cell can no longer sustain the voltage needed to keep the processor and radio running simultaneously. It's not a faulty battery; it's a calibration mismatch between the old fuel gauge data and the new cell's actual discharge curve. One full discharge-to-shutoff cycle followed by an uninterrupted full charge resets the coulomb counter and eliminates the false cliff in most cases.

Phone shows wrong battery percentage after fitting the replacement cell

The fuel gauge IC on the SCH-R390 and related models stores discharge curve data from the original cell. When you swap in a new cell, that stored data no longer matches the new cell's actual voltage-to-capacity relationship. The result is percentage readings that jump, stall, or report full charge before the cell is genuinely topped up. Run one complete cycle — discharge to automatic shutoff, then charge uninterrupted to 100% — and the gauge IC recalibrates against the real curve. After that cycle, readings stabilise at the correct voltage endpoints of approximately 3.0V empty and 4.2V full.