BUS-11 Sony Ericsson T28 Replacement Battery 3.7V 950mAh

Sony Ericsson T28 / T39 Series — 3.7V Li-ion Replacement Battery (BUS-11)

This is a 3.7V, 950mAh Li-ion replacement cell for the Sony Ericsson BUS-11 battery. It fits the T28, T29, T36, and T39 series mobile phones. Slot it in when the original cell no longer holds a charge or the phone refuses to power on.

• T28 / T29 / T36 / T39 platform fit: These four models share the same battery bay dimensions, 3.7V rail, and connector pinout. The BUS-11 spec covers all of them — no adapter or modification needed.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the T28 platform. The BMS accepted charge current without interruption, and the protection circuit tripped correctly at the low-voltage threshold during deep discharge testing.
• Fuel gauge recalibration on first use: After fitting this cell, run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. The phone's fuel gauge IC was calibrated to the old cell's discharge curve — this single full cycle resets it against the new cell so percentage readings stay accurate.

Why the T28 reports wrong battery percentage after a cell swap

The T28 uses a coulomb-counting fuel gauge IC that tracks capacity based on the previous cell's charge curve stored in memory. A new cell has a different internal resistance and discharge profile, so the IC reads those values against the wrong baseline. The result is a percentage that jumps around or reads higher or lower than actual state of charge. One full discharge-to-shutdown followed by an uninterrupted charge to 100% forces the IC to rewrite its reference curve against the new cell.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load — a voltage cliff — before the fuel gauge has recalibrated. The phone's protection circuit sees terminal voltage fall below roughly 3.0V during a burst of RF transmission or backlight draw and cuts power to protect the cell. The percentage shown at cutoff looks high because the gauge is still reading from the old curve. Run that first full calibration cycle and the shutdowns stop — the gauge then tracks actual voltage accurately enough to warn before the cliff.