BA600 Sony Ericsson Xperia U Replacement Battery 3.7V 1300mAh

Sony Ericsson Xperia U / ST25i — 3.7V Li-ion Replacement Battery (BA600)

This is a 3.7V, 1300mAh Li-ion replacement cell for the Sony Ericsson Xperia U (ST25, ST25i, Kumquat). It replaces OEM part number BA600 directly. When the original cell degrades and the phone starts shutting down unexpectedly or refusing to hold charge, this is the swap.

• ST25 and ST25i compatibility: Both ST25 variants use the same battery bay dimensions, connector pinout, and BMS handshake as the Xperia U. The BA600 footprint — 64.10 × 43.90 × 4.00mm — is shared across that entire compact platform, so the cell seats and communicates correctly on all listed models.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on an ST25i unit. The BMS accepted the charge IC handshake without error, held voltage above 3.6V through the mid-discharge range, and tripped the low-voltage cutoff at the correct threshold rather than dropping the phone unexpectedly.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge mode for the first full discharge-charge cycle. The fuel gauge IC on the ST25 is calibrated to the old cell's discharge curve. Running one slow cycle first lets the coulomb counter re-anchor to the new cell before high-current charging begins.

Sudden shutdown at 20–30% on the Xperia U after a cell swap

This is a voltage cliff issue, not a capacity problem. Under peak load — mobile data, screen-on, or audio playback simultaneously — the new cell draws enough current to pull its terminal voltage below the shutdown threshold even when the reported state-of-charge still looks healthy. The fuel gauge IC is working from a stale discharge curve mapped to the old, degraded cell. One full discharge down to automatic shutdown, followed by an uninterrupted charge to 100%, resets the coulomb counter reference and typically resolves the premature cutoff.

Phone shows wrong battery percentage after installing the BA600

The ST25i stores the previous cell's discharge profile in the fuel gauge IC. A new cell with a tighter, steeper discharge curve will read as either artificially high or drop percentage in large jumps. This is not a fault with the replacement cell — it is the gauge reporting against the wrong reference map. Force a full discharge until the phone powers off on its own, then charge uninterrupted to 100% without removing the charger. After that single cycle, percentage reporting should track within a few points of actual remaining capacity.