Fujitsu Loox T800 Replacement Battery 3.7V 3060mAh

Fujitsu Loox T800 / T810 / T830 — 3.7V Li-Polymer Replacement Battery (S26391-F2061-L400)

This 3.7V, 3060mAh Li-Polymer cell replaces the original battery in the Fujitsu Loox T800, T810, and T830 handheld devices. The Loox series are mid-2000s Windows Mobile PDAs — compact, touchscreen mobile computers used for field data entry and personal organisation. Capacity figures come from the product data, not third-party sources.

• Loox T800, T810, and T830 compatibility: All three models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. The voltage rail is identical across the series, so one cell covers all three variants without modification.
• Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the Loox platform. The BMS accepted the cell without fault codes, and the protection circuit responded correctly to both low-voltage cutoff and overcharge conditions.
• Fuel gauge recalibration on first use: After fitting this cell, disable any fast-charge accessory for the first complete discharge-to-charge cycle. The Loox fuel gauge IC is calibrated to the original cell's discharge curve. One uninterrupted slow cycle lets it recalibrate against the new cell before higher current is applied to an uncalibrated state.

Why the Loox T800 reports wrong battery percentage after a cell swap

The Loox T800 uses a fuel gauge IC that tracks charge state by measuring voltage curves and coulomb counting against a stored cell profile. When a new cell goes in, the IC still references the old degraded cell's discharge curve. This mismatch causes the percentage display to read high early in discharge and then drop suddenly. One full slow discharge from 100% to auto-shutdown, followed by a full charge, rewrites the reference and aligns the gauge to the new cell.

Sudden shutdown at 20–25% on the Loox after fitting a new cell

This happens when the fuel gauge IC cuts power based on a voltage threshold calibrated to the old, high-impedance cell. A fresh Li-Polymer cell has lower internal resistance, so it holds voltage higher for longer — then drops steeply near depletion. The device interprets the voltage cliff as an immediate low-battery event and shuts down before the percentage counter catches up. Complete one full discharge cycle without interruption, letting the device reach the auto-shutdown point near 3.0V per cell, then charge fully to reset the threshold mapping.