About this replacement
Toshiba Satellite A10 Series — 2.4V Ni-MH CMOS Backup Battery (P000257640)
This is the Ni-MH CMOS backup battery for the Toshiba Satellite A10 series laptops, including the A10-131, A10-501, A10-511, and A10-521. It runs at 2.4V with a 20mAh capacity and sits on the motherboard to power the RTC circuit and retain BIOS settings when mains power is removed. OEM part number P000257640 confirms fitment across the full A10 range.
- A10 series motherboard fitment: The A10 platform uses a dedicated Ni-MH backup cell rather than a standard coin cell — the voltage rail, connector, and physical dimensions (27.83 × 12.78 × 4.50mm) are specific to this board. Swapping to an incompatible cell type will not seat correctly and may not satisfy the BMS retention threshold.
- Bench tested on actual hardware: We measured open-circuit voltage on fresh cells before shipping and confirmed the RTC circuit holds BIOS settings and clock data with the cell installed and mains power fully disconnected.
- Post-install BIOS step: After fitting this cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value on any power interruption — the cell will hold whatever time you write to it, but it cannot recover a timestamp it never received.
BIOS clock resetting to year 2000 after every power cycle on the Satellite A10
The Satellite A10 RTC circuit requires the CMOS cell to stay above the minimum retention voltage — typically 2.8V — to hold the clock register between power cycles. When the cell drops below that threshold, the RTC loses its reference and falls back to a hard-coded default date, usually January 1, 2000. A depleted original cell will still show some voltage on a multimeter but cannot sustain the micro-draw of the SRAM and RTC circuit overnight. Replacing the cell and writing the correct timestamp into BIOS is the only fix — the circuit itself is almost never the cause.
CMOS checksum error on boot after the A10 has been unplugged overnight
A checksum error on boot means the BIOS compared the stored configuration data to its checksum and found a mismatch — which happens when SRAM loses power mid-session and the values corrupt or clear. This is a later-stage symptom than simple clock drift: it means the cell can no longer sustain even the minimum retention voltage for the SRAM block, not just the RTC. Fitting a fresh cell stops the SRAM from losing power, and the checksum error will not return once you save a clean BIOS configuration. After installing, enter BIOS, confirm all settings, and save — the checksum is recalculated on that save action.