About this replacement
Samsung Galaxy S21 Ultra — 3.87V Li-Polymer Replacement Battery (EB-BG998ABY)
This is a 3.87V, 4900mAh Li-Polymer replacement battery for the Samsung Galaxy S21 Ultra and Galaxy S21 Ultra 5G. It fits SM-G998N, SM-G9980, and related variants where the original EB-BG998ABY has degraded or failed. Capacity is 18.96Wh — identical to the factory specification.
- S21 Ultra and 5G variants: The SM-G998 series shares a single battery footprint, connector pinout, and BMS handshake across all regional builds. One cell fits the full lineup without adapter or modification.
- Bench tested on actual hardware: We ran this cell on an SM-G998B through a full charge cycle, confirmed BMS communication with the charge IC, and verified the fuel gauge IC registered the correct capacity after one complete discharge pass.
- First-cycle fast charge protocol: On first use after installation, disable fast charging and run one full discharge-to-charge cycle. This lets the fuel gauge IC recalibrate against the new cell's discharge curve before high-current fast charging pushes current into an uncalibrated cell.
Sudden shutdown at 20–30% on the replacement cell
This is a voltage cliff issue, not a capacity defect. When the modem, display, and processor draw peak current simultaneously, a freshly installed cell with an uncalibrated fuel gauge IC can drop below the BMS cutoff threshold before the percentage reading reflects it. The phone shuts down because actual cell voltage falls under load, even though the reported percentage looks safe. Run one full discharge cycle without fast charging — this lets the coulomb counter reset its baseline and the shutdowns stop.
Phone will not power on after the battery sat in storage
Li-Polymer cells self-discharge in storage. If a replacement cell has been sitting long enough, cell voltage can fall below 2.5V per cell — the point where the BMS locks out to prevent damage. The phone won't respond to the power button and may show nothing on screen. Connect to a wall charger via USB-C and leave it for 20–30 minutes before pressing power. Once the BMS sees voltage above the recovery threshold, it releases the lockout and charging resumes normally.