Sony Ericsson Z800 BST-33 Replacement Battery 3.7V 900mAh

Sony Ericsson Z800 Series — 3.7V Li-ion Replacement Battery (BST-33)

This is a 3.7V, 900mAh Li-ion replacement cell for the Sony Ericsson Z800, V800, V800i, and V802, plus over 49 additional compatible handsets sharing the BST-33 form factor. It fits the original battery bay without modification. Use the part number BST-33 to confirm fit before ordering.

• Z800 and V800 series compatibility: These models share the BST-33 footprint, connector pinout, and 3.7V nominal rail. The BMS on each handset communicates with the battery over the same three-pin interface, so one cell covers the full lineup without any adapter or firmware workaround.
• Bench tested on actual hardware: We cycled the BST-33 replacement through charge and discharge on a Z800 unit. The BMS accepted the cell without error flags, held voltage flat through mid-discharge, and triggered the low-battery cutoff at the correct threshold rather than dropping unexpectedly under load.
• Fuel gauge recalibration on first use: After installing a new cell, run one full discharge to automatic shutdown before recharging. The Z800's fuel gauge IC was calibrated against the original cell's discharge curve — skipping this step leaves it reading against stale data, which causes erratic percentage jumps for the first several days of use.

Why the Z800 reports wrong battery percentage after a BST-33 cell swap

The Z800 uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell over time. When you swap the physical cell, the IC retains its old baseline — it has no way to know the new cell's actual capacity. The result is percentage readings that lag reality, often showing 100% well before the cell is full or dropping to 15% faster than expected. One complete discharge-to-shutdown followed by a full uninterrupted charge resets the baseline and brings the gauge back into alignment with the new cell.

Sudden shutdown at 20–30% on the replacement cell

This is a voltage cliff issue, not a capacity problem. Under peak load — screen on, active call, or data transfer — the cell voltage sags momentarily below the BMS cutoff threshold even though the fuel gauge still shows charge remaining. It happens most often on cells that haven't been through a full calibration cycle yet, or on handsets where the charge IC is pushing current into a new high-impedance cell. Run one full cycle first and check that the cell reaches 4.2V at end-of-charge before assuming a fault.