T-Mobile Galaxy S II Compatible Battery 3.7V 1300mAh

T-Mobile Galaxy S II — 3.7V Li-ion 1300mAh Replacement Battery

This is a 3.7V Li-ion battery rated at 1300mAh (4.81Wh) for the T-Mobile Samsung Galaxy S II smartphone. It replaces a degraded or failed original cell that can no longer hold a charge through normal daily use. The battery fits the Galaxy S II form factor at 58.50 x 46.00 x 5.20mm and connects to the device's charge IC and fuel gauge circuit.

• Galaxy S II compatibility: The T-Mobile Galaxy S II uses a specific connector pinout that carries both power and fuel gauge data lines. This cell matches that pinout so the phone's charge IC and coulomb counter can communicate with the battery correctly.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the Galaxy S II platform. The BMS held stable voltage throughout load variations and the charge IC accepted the cell without throwing a charge fault.
• Fuel gauge recalibration on first use: On first use after installation, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100% before resuming normal use. This gives the fuel gauge IC a full reference curve against the new cell and prevents erratic percentage readings early on.

Why the Galaxy S II reports wrong battery percentage after a cell swap

The Galaxy S II uses a coulomb counter IC that builds its charge model against the original cell's discharge curve over many cycles. When you swap in a new cell, that learned curve no longer matches the actual cell. The IC will report percentages based on stale calibration data until it relearns the new cell. One full discharge-to-shutdown followed by a full uninterrupted charge resets the reference points the IC uses for percentage calculations.

Sudden shutdown at 20–30% on the replacement cell

This happens when the fuel gauge IC still carries calibration data from the old, degraded cell, which had a compressed voltage curve near the end of discharge. The new cell has a steeper voltage drop at low state-of-charge, so the phone hits the hardware undervoltage cutoff — around 3.4V — before the software gauge reaches 0%. The phone shuts off to protect the cell even though the reported percentage looks healthy. A full discharge cycle followed by a full charge corrects the IC's low-voltage reference point and eliminates premature shutdowns.