Samsung SGH-D307 Replacement Battery BST4839SAB 3.7V 850mAh

Samsung SGH-D307 — 3.7V Li-ion Replacement Battery (BST4839SAB)

This is a 3.7V, 850mAh Li-ion cell built to the BST4839SAB specification for the Samsung SGH-D307 mobile phone. It replaces original cells that have degraded, lost capacity, or failed to hold a charge. Voltage and capacity match the SGH-D307's original power requirements exactly.

• SGH-D307 fit: The BST4839SAB form factor and contact pin layout are specific to the SGH-D307 chassis. The cell voltage matches the phone's charge IC input range, so the BMS handshake completes without triggering a fault state.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the SGH-D307 platform. The BMS accepted the cell on first connection, charge IC engaged correctly, and the fuel gauge IC began tracking the new discharge curve without manual intervention.
• Fuel gauge recalibration on first cycle: After fitting this replacement cell, run one full discharge to auto-shutoff followed by a full charge before normal use. The SGH-D307's fuel gauge IC was calibrated to your old cell's curve — skipping this step causes percentage readings to drift or jump within the first few days.

Sudden shutdown at 20–30% on the SGH-D307 after a cell swap

The SGH-D307 shuts down abruptly at 20–30% when the fuel gauge IC is still mapped to the old cell's discharge curve. The new cell hits its actual voltage floor faster than the IC expects, so the phone cuts power before the display reaches 0%. This is not a fault in the replacement cell — it is a calibration mismatch. Run one full discharge-to-shutoff cycle followed by a full uninterrupted charge, and the fuel gauge IC will remap its coulomb counter to the new cell's curve.

Phone shows correct percentage but drops dead under screen or call load

An 850mAh cell at the end of its first few cycles can show stable resting voltage but fail to sustain that voltage when the display backlight or GSM radio draws peak current. The internal resistance of a brand-new cell is slightly higher than a broken-in one, and the voltage sag under load briefly dips below the BMS cutoff threshold. This typically resolves after three to five full charge-discharge cycles as internal resistance normalises. If the problem persists beyond five cycles, check that the battery contacts on the phone chassis are clean and making full contact.