About this replacement
Fujitsu Loox T800 / T810 / T830 — 3.7V Li-Polymer Replacement Battery (S26391-F2061-L400)
This 3.7V, 1530mAh lithium-polymer cell replaces the original battery in the Fujitsu Loox T800, T810, and T830 Windows Mobile smartphones. It slots directly into the battery bay and connects via the original three-pin contact strip. Capacity is 5.66Wh, matching the stock specification for these devices.
- T800, T810, and T830 compatibility: All three Loox models share the same battery bay dimensions, connector pinout, and BMS handshake protocol — the 49.76 × 33.35 × 4.98mm cell fits all three without modification.
- Bench tested on actual hardware: We cycled this cell on a T830 and confirmed the BMS accepted the charge IC handshake on first insertion, held voltage above 3.6V through a full discharge, and tripped the low-voltage cutoff cleanly at the correct threshold.
- Fuel gauge recalibration on first use: After installing this cell, disable any fast-charge mode and run one complete discharge-to-charge cycle before normal use. This lets the fuel gauge IC map its coulomb counter against the new cell's discharge curve — skipping this step causes the OS to report inaccurate percentages for the first several cycles.
Sudden shutdown at 20–30% on the Loox T800
This is a voltage-cliff failure, not a capacity failure. As a lithium-polymer cell ages, its internal resistance rises — under the combined load of the display backlight and GSM radio, terminal voltage drops sharply and hits the BMS cutoff before the fuel gauge reaches zero. The phone shuts down even though the gauge shows charge remaining. A fresh cell with low internal resistance sustains voltage under that combined load, and the shutdowns stop.
OS reporting wrong battery percentage after cell replacement
The Loox fuel gauge IC uses a coulomb counter calibrated to the discharge curve of the original cell. A new cell has a slightly different curve, so the IC reports incorrect percentages until it re-learns the endpoints. To recalibrate, run the device down until it powers off on low battery, then charge uninterrupted to 100% with the screen off. After that single full cycle, percentage readings stabilise to within a few points.