Sony Vaio 505F CMOS Backup Battery 7.2V 20mAh Ni-MH

Sony Vaio 505F — 7.2V Ni-MH CMOS Backup Battery

This is the CMOS backup battery for the Sony Vaio 505F laptop. It runs at 7.2V Ni-MH with a 20mAh (0.14Wh) capacity. It powers the RTC circuit and SRAM that hold your BIOS settings and system clock when mains power is removed.

• Vaio 505F fitment: The 505F uses a multi-cell Ni-MH pack rather than a standard CR2032 coin cell. The motherboard connector, cell geometry, and BMS handshake are specific to this chassis — a generic coin cell will not fit or seat correctly.
• Bench tested on actual hardware: We ran this cell on a 505F board and confirmed the RTC held its clock value across multiple full mains disconnects. The CMOS SRAM retained all BIOS settings without a checksum error on each subsequent boot.
• Post-install clock correction: After swapping this cell in, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value the moment the old cell is disconnected — the new cell holds whatever time you set from that point forward.

BIOS clock resetting to 2000 after every power cycle on the Vaio 505F

The Vaio 505F RTC circuit requires a minimum retention voltage of 2.8V from the CMOS cell to hold the clock register. When the Ni-MH pack drops below that threshold, the RTC loses its reference and falls back to the factory default date — typically January 1, 2000. This happens even if mains power is connected, because the clock register is backed by the CMOS cell, not the main supply. Replacing the cell and setting the correct time in BIOS resolves it permanently.

CMOS checksum error appearing on every boot after mains disconnect

A checksum error means the BIOS compared its stored configuration against what the CMOS SRAM actually contains and found a mismatch — the data has already been lost. This happens when the backup cell is fully depleted and the SRAM loses power the moment the mains cable is unplugged. A fresh cell restores continuous SRAM power. After fitting the replacement, re-enter all BIOS settings manually and save — the checksum will pass on the next boot.