Samsung Galaxy S II EB-L1A2GB Replacement Battery 3.7V 1650mAh

AT&T Galaxy S II SGH-I777 — 3.7V Li-ion Replacement Battery (EB-L1A2GB)

This is a 3.7V, 1650mAh Li-ion replacement battery for the AT&T Samsung Galaxy S II (SGH-I777). It replaces OEM part numbers EB-L1A2GB, EB-L1A2GBA, and EB-L1A2GBA/BST. The original cell degrades after repeated charge cycles and this unit slots into the same battery bay using the same connector.

• SGH-I777 fit: The Galaxy S II on AT&T uses a dedicated connector pitch and cell footprint (58.50 x 46.00 x 5.20mm) that differs from international S II variants. This cell matches those dimensions and the BMS handshake the phone expects on boot.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the SGH-I777. The BMS communicated correctly with the phone's fuel gauge IC, and the charge IC accepted current without triggering a fault state.
• Fuel gauge recalibration on first use: After installing this cell, run one complete discharge down to auto-shutdown, then charge uninterrupted to 100% before resuming normal use. This gives the coulomb counter a full reference curve against the new cell and prevents erratic percentage readings from the first day.

Why the SGH-I777 reports wrong battery percentage after a cell swap

The SGH-I777 uses a coulomb counter that builds its charge model against the original cell's discharge curve. When you swap in a new cell, the stored model no longer matches reality. The phone reads voltage and translates it to percentage using old reference data — so it may show 50% when the cell is actually at 30%. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the reference and brings the percentage display back into alignment.

Sudden shutdown at 20–30% on the replacement cell

This happens when the modem or display pulls a current spike the new cell cannot sustain at that state of charge — voltage drops below the BMS cutoff threshold instantly, and the phone shuts off. It is not a defective cell; it is the fuel gauge IC working from a miscalibrated curve and reporting a higher percentage than actual charge remaining. After one full calibration cycle, the percentage display tracks accurately and the early shutdowns stop. If they continue past two full cycles, check that the cell contacts are seated flat — a lifted contact raises internal resistance and causes the same voltage sag under load.