About this replacement
Sony Xperia ST27 / ST27i — 3.7V Li-Polymer Replacement Battery (AGPB009-A003)
This 3.7V, 1250mAh Li-Polymer battery replaces the original AGPB009-A003 cell in the Sony Xperia ST27 and ST27i (also sold as the Xperia go and Lotus). It fits the same footprint as the factory cell at 67.40 × 32.55 × 4.50mm and connects to the same three-pin flex connector on the motherboard.
- ST27 / ST27i / Xperia go platform: These variants share the same battery bay geometry, connector pinout, and 3.7V nominal rail. The BMS handshake is handled by the phone's charge IC, not a chip embedded in the battery, so the cell seats and registers without modification.
- Bench tested on actual hardware: We cycled this cell through the ST27i's charge IC and confirmed the BMS accepted charge without triggering an error state. Voltage at full charge held at 4.19–4.20V, consistent with the phone's cutoff threshold.
- Fuel gauge recalibration on first use: On the first discharge cycle after installation, run the phone from 100% down to automatic shutdown — do not interrupt it. This lets the fuel gauge IC map the new cell's discharge curve before Android begins reporting percentage data to the status bar.
Why the Xperia ST27i reports wrong battery percentage after a cell swap
The ST27i uses a coulomb counter integrated into the phone's power management IC, not the battery itself. When you install a new cell, the counter still holds calibration data from the old, degraded cell. That mismatch causes the percentage displayed to jump, stall, or land nowhere near the real state of charge. One complete uninterrupted discharge-to-shutdown followed by a full charge resets the learned curve and brings the readout back in line with actual capacity.
Sudden shutdown at 20–25% on the replacement cell
This happens when the phone's modem or display draws a current spike that the new cell cannot sustain at its present voltage. Li-Polymer cells have a steeper voltage cliff in the lower charge range during the first few cycles — the phone's low-battery cutoff triggers before the gauge catches up. It is not a faulty cell. After two or three full discharge-charge cycles, the cell's internal resistance stabilises and the voltage cliff flattens. If shutdowns continue past the third cycle, check resting voltage — a healthy cell should read above 3.6V when the phone shows 20%.