About this replacement
Samsung Galaxy S20 Ultra — 3.85V Li-Polymer Replacement Battery (EB-BG988ABY)
This is a 3.85V, 4900mAh Li-Polymer replacement battery for the Samsung Galaxy S20 Ultra and Galaxy S20 Ultra 5G. It fits SM-G988Q, SM-G988W, and related SM-G988 variants. It replaces the original EB-BG988ABY cell when the installed battery no longer holds charge through a full day of use.
- SM-G988 series compatibility: All SM-G988 variants share the same battery bay dimensions, connector pinout, and BMS handshake protocol. Swapping within this series does not require firmware changes or hardware modification.
- Bench tested on actual hardware: We ran this cell in an SM-G988W and monitored charge acceptance across the full CC/CV curve. The BMS engaged correctly at both 25W and 45W input, and thermal cutoff held within spec at the top of the charge cycle.
- 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 recalibrate against the new cell's discharge curve before high-current charging pushes load into an uncalibrated coulomb counter.
Sudden shutdown at 20–30% after fitting the replacement cell
This happens because the S20 Ultra's fuel gauge IC still holds the discharge curve from the old, degraded cell. When the new cell hits a voltage point the IC misreads as near-empty, the phone cuts power to protect itself. The phone is not faulty and the replacement cell is not faulty. Run one full discharge to below 5%, then charge uninterrupted to 100% without fast charging enabled — the IC recalibrates its low-voltage cutoff threshold against the new curve. After that cycle, shutdowns at 20–30% stop.
USB-PD fast charge not triggering on the first cycle after replacement
After a cell swap, the S20 Ultra's charge IC sometimes falls back to standard 5V input on the first charge cycle. This occurs because the BMS on the new cell has not yet completed its initial handshake with the device's power management IC. Plug in the original Samsung charger, wait for the charge session to complete fully, then unplug and re-plug — fast charging typically resumes from the second cycle onward. If it does not, check that the USB-C port is clean and that the cable supports 45W negotiation.