Samsung SGH-D848 Replacement Battery 3.7V 750mAh Li-ion

Samsung SGH-D848 / SGH-D840 — 3.7V Li-ion Replacement Battery (AB394635AEC/STD)

This is a 3.7V, 750mAh Li-ion replacement battery for the Samsung SGH-D848, SGH-D840, and M359 mobile phones. It replaces OEM part numbers AB394635AEC/STD, AB394635CC, and BST31388E. Fits the original battery bay without modification.

• SGH-D848, SGH-D840, and M359 compatibility: All three models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — one cell fits all three without adapter or wiring change.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the SGH-D840 platform. The BMS negotiated charge termination correctly at 4.2V and protection cutoff triggered cleanly at the low-voltage threshold — no false trips under normal call and screen load.
• Fuel gauge recalibration on first cycle: On first use after installation, disable fast charging and run one complete discharge-to-charge cycle. The phone's fuel gauge IC is calibrated to the old cell's discharge curve. A full cycle against the new cell lets the coulomb counter reset its reference before high-current charging pushes into an uncalibrated state.

Why the SGH-D848 shuts down suddenly at 20–30% after a cell swap

The phone's fuel gauge IC builds a discharge model based on the old cell's internal resistance and voltage curve. A new cell has a different impedance profile, so the gauge misreads remaining capacity. When the modem fires or the screen peaks load, the actual cell voltage drops below the protection threshold faster than the gauge predicted — and the phone cuts out. One full discharge-to-shutdown followed by a full charge resets the coulomb counter against the new cell's real curve and eliminates the premature cutoff.

Phone won't power on after the replacement battery sat in storage

Li-ion cells self-discharge during storage. If the cell voltage dropped below 2.5V, the BMS enters lockout mode to prevent charging a deeply discharged cell at full current. The phone will show no response to the power button and may not display a charging indicator. Connect the phone to a low-current USB charger — 500mA wall adapter or a PC USB port — and leave it for 20–30 minutes. Once the cell recovers above 2.9V, the BMS releases the lockout and the phone boots normally.