Samsung SCH-S308 Replacement Battery BST0399DE 3.7V 750mAh

Samsung SCH-S308 / SCH-X219 Series — 3.7V Li-ion Replacement Battery (BST0399DE)

This is a 3.7V 750mAh Li-ion replacement for the original BST0399DE battery. It fits the Samsung SCH-S308, SCH-X219, SCH-S300, and SCH-T300, along with three additional compatible models in this series. The cell matches the original voltage and capacity rating listed in Samsung's OEM specification for these handsets.

• SCH-S308 / SCH-X219 series fit: These models share the same battery bay dimensions, connector pinout, and 3.7V single-cell architecture. The BST0399DE slot accepts the same contact orientation across all listed variants, so one cell services the full model range without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on compatible hardware. The BMS accepted the charge IC handshake without flagging an incompatible cell, and protection circuits triggered correctly at both the low-voltage cutoff and overcharge threshold.
• Fuel gauge recalibration on first install: After fitting this battery, run one complete discharge to automatic shutoff, then charge uninterrupted to 100% before resuming normal use. The phone's fuel gauge IC calibrated to the old cell's discharge curve — one full cycle rewrites that reference and stops the percentage counter from jumping or cutting off early.

Why the SCH-S308 reports wrong battery percentage after a cell swap

Samsung's fuel gauge IC on these early-2000s handsets stores a learned discharge curve from the original cell. When you install a new cell, the IC still references that old curve. The result is a percentage reading that drifts — often reading 40% and shutting down, or stalling at 100% longer than expected. One full discharge-charge cycle overwrites the stored reference with data from the new cell. After that cycle, the percentage counter tracks correctly against the actual 3.7V cell chemistry.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops below the load threshold before the fuel gauge reaches 0%. Under screen-on or active call load, the cell can no longer hold voltage above the protection circuit's cutoff — typically around 3.0V — even though the counter still shows charge remaining. It is not a faulty battery; it is a calibration gap between the cell's actual discharge curve and the IC's stored model. Run one full discharge cycle to automatic shutoff, then charge to 100% without interruption. After that cycle, the IC's cutoff estimate aligns with the cell's real 3.0V floor.