About this replacement
Toshiba Dynabook R751 / Satellite Pro R850 — 11.1V Li-ion Replacement Battery (PA3904U-1BRS)
This is an 11.1V, 4400mAh (48.84Wh) Li-ion battery for the Toshiba Dynabook R751, R752, R752/F, and Satellite Pro R850 series laptops. It uses OEM part numbers PA3904U-1BRS, PA3905U-1BRS, PABAS245, and PABAS246. The cell replaces a degraded or failed original pack in these thin-and-light business notebooks.
- Dynabook R751/R752 and Satellite Pro R850 compatibility: These models share the same 11.1V three-cell battery rail, connector pinout, and BMS handshake protocol — one cell fits the full platform without modification.
- Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a Satellite Pro R850. The BMS completed full charge without tripping protection, and the system reported voltage correctly through each cycle.
- Post-install calibration on Dynabook and Satellite Pro R850: After fitting this cell, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle and clears the inaccurate health warning that appears after every cell swap on these models.
BIOS reporting battery health as poor immediately after swapping the R851 or R850 cell
The Satellite Pro R850 BIOS reads health data stored in the old cell's EEPROM. When a new cell installs, that EEPROM data is blank or mismatched, so the BIOS flags the pack as degraded before it has run a single cycle. This is not a fault with the replacement cell. Run one full discharge-to-hibernate followed by an uninterrupted charge to 100% to complete the BIOS learn cycle. After that cycle, the health indicator updates to reflect actual cell capacity.
Laptop shuts down at 20–30% charge shown on Dynabook R751
This happens when the fuel gauge IC on the motherboard is still calibrated to the old, degraded cell. Under full CPU and display load, voltage drops faster than the IC expects, and the system hits its emergency cutoff threshold while the gauge still reads 20–30%. The gauge IC needs two or three full discharge-and-charge cycles against the new cell to recalibrate its voltage-to-capacity curve. After those cycles, the shutdown threshold aligns correctly with the actual remaining charge.