About this replacement
Sony Ericsson Xperia U / ST25i — 3.7V Li-ion Replacement Battery (BA600)
This is a 3.7V, 1000mAh Li-ion replacement battery for the Sony Ericsson Xperia U smartphone. It fits the ST25 and ST25i variants, also sold as the Kumquat. OEM part number BA600 confirms compatibility across this hardware family.
- ST25 / ST25i / Kumquat platform fit: Sony Ericsson used the same board layout, connector pinout, and BMS handshake across all Xperia U variants. The BA600 part number spans the full ST25 family — regional model codes differ, but the battery slot and charge IC are identical.
- Bench tested on actual hardware: We cycled this cell on an ST25i unit and monitored BMS response through charge initiation, trickle phase, and CC/CV transition. The protection circuit tripped at the correct low-voltage cutoff and recovered without reset on reconnection.
- Fuel gauge recalibration on first install: After fitting this cell, run one full discharge down to auto-shutdown, then charge uninterrupted to 100% before normal use. The Xperia U's fuel gauge IC was calibrated to the original cell's discharge curve — skipping this step leaves the coulomb counter reading against the wrong baseline.
Why the Xperia U reports wrong battery percentage after a cell swap
The Xperia U uses a coulomb-counting fuel gauge IC that builds its capacity model from accumulated charge and discharge data over time. When a new cell goes in, that learned model no longer matches the actual cell's voltage-to-capacity curve. The IC will report a percentage based on stale data, causing jumps or early shutdown warnings. One complete discharge-to-shutdown followed by a full uninterrupted charge resets the baseline and brings the percentage reading back in line.
Sudden shutdown at 20–30% on the replacement cell
This happens when the modem or display draws a current spike the cell cannot sustain at low state of charge — voltage collapses below the BMS cutoff threshold even though the gauge still shows charge remaining. On a new Li-ion cell with a slightly higher internal impedance than the aged original, this voltage cliff appears earlier. The fix is completing the full recalibration cycle first. After calibration, the fuel gauge tracks the actual cutoff voltage more accurately and shutdown moves back to the 5–10% range where it belongs.