Toshiba Satellite 5105-S501 PA3123U-1BRS Replacement Battery 10.8V

Toshiba Satellite 5105-S501 — 10.8V Li-ion Replacement Battery (PA3123U-1BRS)

This is a 10.8V, 4400mAh Li-ion replacement battery for the Toshiba Satellite 5105-S501 and the broader Satellite 5000/5100/5105 series. It cross-references OEM part numbers PA3123U-1BRS, PA3178U-1BRS, PA3211U-1BRS, PABAS018, and several others listed in the compatibility data. The cell fits directly into the battery bay of these late-era Pentium 4-M Satellite notebooks.

• Satellite 5000/5100/5105 series fit: These models share the same 10.8V three-cell-series architecture, battery bay dimensions, and connector pinout. The BMS communication protocol is also common across this generation, so one cell SKU covers the full range without modification.
• Bench tested on actual hardware: We ran this cell through charge, discharge, and a simulated CPU-plus-display load spike. The BMS held the charge rail steady at 10.8V nominal and tripped thermal protection correctly at threshold — no premature cutoff under sustained load.
• Post-install discharge cycle on the Satellite 5100 series: After fitting this battery, run one full discharge to hibernate-cutoff, then charge uninterrupted to 100%. This resets the BIOS battery learn cycle on these Toshiba units and clears the inaccurate health warning that appears after every cell swap.

Why the Satellite 5105 BIOS reports poor battery health immediately after swap

The Satellite 5000/5100/5105 series stores battery health data in EEPROM on the battery's own BMS board. When you fit a new cell, the BIOS reads that EEPROM and compares it against the laptop's stored charge history — the mismatch flags as "poor health" or displays a degraded percentage. This is not a fault with the replacement cell. One full discharge-to-hibernate followed by a full uninterrupted charge gives the BIOS enough cycle data to recalibrate its health register. Most units clear the warning after two full cycles.

Satellite 5105 shutting down at 20–30% battery remaining

This happens when the cell voltage drops below the minimum threshold under combined CPU and display load — even though the fuel gauge still shows charge remaining. The Satellite 5100-series power controller shuts the system down the moment the pack drops below roughly 9.9V under load, which can happen well before the gauge reads zero. A degraded or partially cycled new cell is the most common cause. Confirm the pack voltage under load with a battery diagnostic — if the pack reads below 10.4V at 50% state of charge, the cells need conditioning or the unit is defective.