Sony Ericsson Xperia S BA800 Replacement Battery 3.7V 1700mAh

Sony Ericsson Xperia S / LT26i — 3.7V Li-ion Replacement Battery (BA800)

This is a 3.7V, 1700mAh Li-ion replacement battery for the Sony Ericsson Xperia S smartphone. It fits the LT26 and LT26i variants, along with 14 additional compatible models sharing the same BA800 battery footprint. The OEM part number is BA800.

• LT26 / LT26i platform compatibility: The Xperia S, LT26, and LT26i all share the same 3.7V battery bay, connector pinout, and BMS handshake protocol. Any device that accepted the original BA800 will accept this cell without modification.
• Bench tested on actual hardware: We ran this cell in an LT26i and cycled it through full discharge and charge. The BMS accepted the cell on first insertion, charge termination triggered correctly at 4.2V, and no protection trip occurred during the discharge curve.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging and run one complete discharge-to-charge cycle before relying on the percentage indicator. The Xperia S fuel gauge IC was calibrated against the original cell's discharge curve — one full cycle resets that reference against the new cell.

Why the Xperia S reports wrong battery percentage after a cell swap

The Xperia S uses a coulomb-counting fuel gauge IC that tracks charge by integrating current flow over time. When you swap the cell, the IC still holds the old cell's internal resistance model and discharge curve in memory. This mismatch causes the reported percentage to lag, jump, or read incorrectly — particularly between 20% and 80%. One full discharge and charge cycle forces the IC to re-anchor its model to the new cell's actual behaviour.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under load from the modem or display — a voltage cliff the fuel gauge did not predict because its reference curve was built on the old, degraded cell. The new cell has lower internal resistance but the IC still triggers a low-voltage cutoff early, based on stale data. It is not a faulty cell. Run two or three full discharge-charge cycles to let the coulomb counter rebuild an accurate state-of-charge map. After calibration, the shutdowns stop and the gauge tracks correctly down toward 3.2V per cell before the BMS trips.