Sony Ericsson Xperia X5 Replacement Battery 3.7V 480mAh

Sony Ericsson Xperia X5 — 3.7V Li-Polymer Replacement Battery (1221-5975)

This is a 3.7V Li-Polymer cell rated at 480mAh (1.78Wh), replacing OEM part 1221-5975 in the Sony Ericsson Xperia X5 smartphone. The Xperia X5 uses a compact pouch cell format measuring 37.30 × 31.82 × 5.32mm — physical fitment is precise, so those dimensions matter. If the original cell has lost capacity or triggers sudden shutdowns, this is the direct replacement.

• Xperia X5 fitment: The 1221-5975 cell uses a specific connector pinout and pouch dimensions tied to the X5 chassis. A BMS mismatch on voltage reporting will cause the phone to misread state-of-charge from day one — this cell matches the OEM communication protocol so the fuel gauge IC reads correctly after one calibration cycle.
• Bench tested on actual hardware: We ran charge and discharge cycles to confirm BMS handshake, checked cutoff voltage at 3.0V under load, and verified no thermal spike during the first charge on a cold cell. The cell accepted a full charge without triggering protection cutoff.
• First-cycle fuel gauge reset on the X5: After fitting this cell, disable fast charging and run one complete discharge down to automatic shutdown, then charge uninterrupted to 100%. This gives the X5's coulomb counter a clean reference point before fast-charge current is applied to an uncalibrated cell.

Why the Xperia X5 reports wrong battery percentage after a cell swap

The X5 uses a coulomb counter that builds its discharge curve from the original cell's impedance profile. When a new cell goes in, that stored curve no longer matches actual cell behaviour — the phone trusts its old data, not the new cell. Until the counter recalibrates, percentage readings can be off by 15–25%. One full discharge-to-shutdown followed by an uninterrupted charge to 100% resets the reference and brings the gauge back into alignment.

Sudden shutdown at 20–30% on the Xperia X5 after replacement

A new Li-Polymer cell at rest holds voltage well, but under the combined load of the modem radio and display the cell voltage can sag sharply — hitting the BMS cutoff threshold even when the fuel gauge still shows charge remaining. This is a voltage cliff, not a capacity problem. The BMS trips at 3.0V per cell to protect the pouch from damage. Run two full calibration cycles and the fuel gauge IC will learn where the real voltage cliff sits on this cell, pushing the reported percentage closer to the actual cutoff point.