About this replacement
Compaq Presario B2811TX Series — 11.1V Li-ion Replacement Battery (405231-001)
This 11.1V, 4400mAh Li-ion battery replaces the original power cell in the Compaq Presario B2800 series notebooks, including the B2811TX, B2808TX, B2805TX, and B2802TX. It matches the OEM voltage rail and connector used across this platform. Cross-references include part numbers 407672-001 and HSTNN-CB25.
- B2800 series platform fit: All B2800-series models listed share the same 11.1V three-cell configuration, connector pinout, and BMS handshake protocol — which is why one cell covers the full range without modification or adapter.
- Bench tested on actual hardware: We cycled this cell through full charge and discharge on a B2800-series unit. The BMS communicated correctly with the BIOS, reported accurate state-of-charge, and held voltage stable under combined CPU and display load without triggering a low-voltage cutoff prematurely.
- Post-install recalibration cycle: After fitting this cell, run the laptop down to hibernate cutoff on battery power alone, then charge uninterrupted to 100% without unplugging. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears on first boot after any cell swap.
Why the Presario B2811TX shuts down between 20–30% shown charge
The fuel gauge IC in this platform tracks cell voltage and translates it into a percentage for the OS. When a new cell is installed, the IC's internal model is still calibrated to the old cell's degraded voltage curve. Under full CPU plus display load, the new cell's voltage drops faster than the IC predicts — the system sees a sudden voltage cliff and triggers an emergency shutdown. The displayed percentage at cutoff can read anywhere from 18% to 32%. Running two full discharge-to-hibernate and charge-to-100% cycles recalibrates the IC against the new cell's actual curve and eliminates the premature cutoff.
BIOS showing "unknown" or "poor health" immediately after fitting a new cell
The BIOS reads battery health data from the cell's EEPROM — a small memory chip that stores cycle count, rated capacity, and manufacture date. When a replacement cell arrives, its EEPROM data does not match the history the BIOS logged for the original battery, so it flags the status as unknown or degraded. This is a data mismatch, not a fault with the new cell. Run one full discharge to hibernate followed by an uninterrupted charge to 100%, and the BIOS learn cycle will overwrite the stale health flag with accurate data from the new cell.