About this replacement
Fujitsu Lifebook T4220 — 7.2V Ni-MH CMOS Backup Battery (6/V40H)
This is the CMOS backup battery for the Fujitsu Lifebook T4210, T4215, and T4220. It runs at 7.2V with a 40mAh capacity and uses Ni-MH chemistry. Its job is to keep the RTC circuit and BIOS SRAM powered when the main battery is disconnected or the laptop is off.
- T4210, T4215, T4220 platform fit: These three models share the same CMOS circuit architecture, connector type, and OEM part reference (6/V40H / 313-016). The retention voltage threshold and SRAM backup requirements are identical across the series, so one cell covers all three.
- Bench tested on actual hardware: We confirmed connector engagement, polarity, and that the BIOS retained date, time, and boot order settings through a full main battery disconnect cycle. The cell held SRAM state without mains power present.
- Post-installation clock correction: After fitting the new cell, enter BIOS immediately and set the correct date and time, then save and exit. The CMOS cell powers the RTC circuit, and any interruption during the swap resets the clock to a factory default — typically January 2000 — which must be corrected manually before the OS boots.
BIOS clock resetting to 2000 after every power cycle
When the CMOS cell drops below the minimum retention voltage of 2.8V, the RTC circuit loses power and resets to its factory default date — on the T4220, that's typically 1 January 2000. A new main battery or AC adapter will not fix this. The CMOS cell is a separate circuit that only the backup cell feeds. Replace the 6/V40H cell, enter BIOS, set the correct date and time, and save before exiting.
CMOS checksum error on boot after main battery swap
A checksum error at POST means the BIOS detected that stored settings no longer match what the CMOS SRAM contains — this happens when the backup cell is fully depleted and SRAM lost power during the swap. The error is not caused by the new main battery. Fitting a fresh 6/V40H cell restores SRAM voltage, but you must re-enter all BIOS settings manually once the cell is seated. Confirm the cell is reading above 2.8V retention voltage before closing the chassis.