Samsung Galaxy S21 5G Replacement Battery EB-BG991ABY 3.87V 3900mAh

Samsung Galaxy S21 5G — 3.87V Li-Polymer Replacement Battery (EB-BG991ABY)

This is a 3900mAh Li-Polymer cell at 3.87V, replacing part number EB-BG991ABY in the Samsung Galaxy S21 5G. It fits SM-G991U1, SM-G991B/DS, SM-G991D, and the wider S21 5G variant range. Capacity figure is taken from the product specification — 15.09Wh total energy.

• S21 5G variant coverage: The SM-G991 series shares a unified battery bay geometry, connector pinout, and BMS handshake protocol across its regional SKUs — single-SIM, dual-SIM, and carrier-locked models all draw from the same cell specification, which is why one part number covers the full lineup.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on an SM-G991B test unit. The BMS accepted the charge handshake without error flags, held voltage stable under display and modem load, and the protection circuit tripped correctly at the low-voltage cutoff threshold.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging for one complete discharge-charge cycle. This lets the fuel gauge IC map the new cell's discharge curve before Samsung's adaptive charging protocol pushes high current into an uncalibrated cell — skipping this step causes percentage drift in early cycles.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff problem, not a capacity problem. The S21 5G's modem and display together draw sharp current spikes — if the new cell's internal impedance is slightly higher than the original, terminal voltage collapses under that load even when the coulomb counter still shows charge remaining. The phone's protection circuit reads that voltage drop as a hard cutoff condition and shuts down before the percentage reaches zero. Let the phone complete two or three full discharge-charge cycles unconstrained; internal resistance typically drops as the cell's electrolyte settles, and the fuel gauge IC recalibrates its cutoff threshold to match the actual cell curve.

USB-PD fast charge not activating on first cycle after swap

After a cell replacement, the charge IC on the S21 5G sometimes withholds fast-charge negotiation on the first cycle because the BMS presents an unrecognised state-of-charge signature to the USB-PD controller. The phone charges at standard rate — not because the adapter is wrong, but because the controller defaults to conservative current limits until it trusts the cell state. Plug into the original Samsung fast charger, let the battery charge to 100% and drain below 15% once without interruption, then reconnect. Fast charge should negotiate correctly from the second cycle onward.