About this replacement
Sony VAIO VGN-FZ15 Series — 11.1V Li-ion Replacement Battery (VGP-BPS8)
This is an 11.1V, 4400mAh (48.84Wh) Li-ion replacement battery for the Sony VAIO VGN-FZ15 series notebook. It fits the VGN-FZ15, VGN-FZ15G, VGN-FZ15L, VGN-FZ15M, and over 235 additional VAIO models that use the VGP-BPS8, VGP-BPS8A, VGP-BPS8B, VGP-BPL8, or VGP-BPL8A part numbers. Dimensions are 137.00 × 78.80 × 20.60mm — physically identical to the original pack.
- VGN-FZ series compatibility: Every model in this range runs an 11.1V three-cell rail with the same dock connector and BMS handshake protocol. The VAIO SMBus talks to the battery's protection circuit on pin contacts at a fixed address — so any cell in this group either communicates correctly or it doesn't charge at all. This pack uses the same address and protocol.
- Bench tested on actual hardware: We ran this cell on a VGN-FZ series unit and monitored the BMS handshake through the full charge cycle. The protection circuit responded to Sony's charge controller without negotiation errors, and the pack held steady voltage under combined CPU and display load without triggering undervoltage cutoff.
- First-cycle recalibration on the VGN-FZ15: After installing, run one complete discharge down to hibernate-cutoff, then charge uninterrupted to 100%. The VAIO's battery learn cycle uses that data to reset its internal fuel gauge IC. Skipping this step causes the BIOS to carry over EEPROM state from the old pack and report inaccurate charge percentages for weeks.
Why the VGN-FZ15 BIOS flags a new battery as poor health immediately after swap
The VAIO's battery health indicator reads EEPROM data written by the original cell over its lifespan — cycle count, degradation flags, and rated Wh. When a new pack installs, that EEPROM presents a fresh state that doesn't match the historical data the BIOS cached from the old battery. Sony's firmware interprets the mismatch as a suspect cell rather than a new one. Running one full discharge-to-hibernate then a complete uninterrupted charge resolves this — the BIOS overwrites its cached data with the new cell's actual readings and the health warning clears.
VGN-FZ15 shutting down abruptly at 20–30% charge remaining
This happens when the cell voltage drops below a sustainable threshold under combined CPU, display, and I/O load — even though the fuel gauge still shows 20–30% remaining. Aged or shallow-cycled cells lose the ability to sustain voltage under real draw, hitting the BMS undervoltage cutoff before the displayed percentage reaches zero. The fuel gauge IC has not yet calibrated its discharge curve to the new cell's chemistry. After two full discharge-to-hibernate and charge-to-100% cycles, the gauge recalibrates and the shutdown point shifts back toward 0–5% as expected.