Samsung Galaxy S21 FE 5G EB-BG990ABY Replacement Battery 3.85V 4200mAh

Samsung Galaxy S21 FE 5G — 3.85V Li-Polymer Replacement Battery (EB-BG990ABY)

This is a 3.85V, 4200mAh Li-Polymer replacement battery for the Samsung Galaxy S21 FE 5G smartphone. It replaces the original EB-BG990ABY cell and fits SM-G990B, SM-G990B/DS, and the Galaxy S21 Fan Edition 5G variants. Voltage and capacity match the original spec so the charge IC operates within its expected parameters.

• SM-G990B and Fan Edition variants: All models in the S21 FE 5G line share the same 3.85V voltage rail, EB-BG990ABY connector pinout, and BMS handshake protocol — one cell fits the full range without hardware modification.
• Bench tested on actual hardware: We ran this cell through a full charge-discharge cycle on an SM-G990B. The BMS accepted the cell without fault flags, charge IC engaged normally, and the coulomb counter tracked capacity through the full curve.
• Fuel gauge recalibration on first use: On first use after installation, disable fast charging and complete one full discharge-charge cycle at standard speed. This lets the fuel gauge IC map the new cell's discharge curve before high-current fast charging pushes load into an uncalibrated cell.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff failure, not a capacity failure. The S21 FE 5G's modem and AMOLED panel draw bursts of current that cause the cell voltage to sag sharply at lower states of charge. If the fuel gauge IC is still calibrated to the old worn cell's curve, it misreads remaining charge and allows the device to run past the point the new cell can sustain voltage under that load. One full discharge-charge cycle at standard charge speed resets the coulomb counter against the new cell's actual voltage curve. After that cycle, shutdowns at 20–30% typically stop.

USB-PD fast charge not working on first cycle after replacement

Samsung's adaptive fast charge handshake between the charger and the phone's charge IC can fail on the first cycle after a cell swap because the BMS presents different impedance on a new, fully cold cell. The charge IC defaults to standard 5V input rather than negotiating a higher voltage rail. Plug in the supplied or original Samsung charger, let the phone charge to 100% at standard speed without interruption, then unplug and restart the device. On the second charge cycle, fast charge negotiation resumes normally.