Toshiba Portege 300 CMOS Replacement Battery 7.2V 40mAh

Toshiba Portege 300 / Tecra 500 Series — 7.2V Ni-MH CMOS Backup Battery

This is a 7.2V, 40mAh Ni-MH backup battery for the Toshiba Portege 300, Portege 300CT, Tecra 500, and Tecra 500CDT. It sits on the motherboard and powers the RTC circuit and CMOS SRAM when mains power is removed. When this cell fails, the system loses BIOS settings and the clock resets on every power cycle.

• Portege 300 and Tecra 500 platform fit: These models share the same motherboard backup circuit — same 7.2V rail, same connector pinout, same physical footprint at 70.00 × 12.50 × 6.50 mm. One cell fits all listed variants.
• Bench tested on actual hardware: We cycled this cell through repeated mains disconnects on a Tecra 500CDT. The CMOS SRAM retained all stored settings across each power interruption, and the RTC held accurate time with no resets.
• Post-installation clock correction: After swapping the cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit loses its reference during the swap — any delay before correcting the clock means the system boots with a default date until you do.

BIOS clock resetting to a default date after every power cycle

The RTC circuit on the Portege 300 and Tecra 500 motherboards draws continuous low current from this backup cell to keep the clock running when mains power is absent. Once the cell drops below the minimum SRAM retention voltage — roughly 2.8V for the logic on these boards — the RTC loses its reference and the clock reverts to a factory default on the next boot. A cell that reads above 2.8V on a multimeter but still causes resets is likely at the retention threshold under load, not open circuit. Replacing the cell and immediately setting the correct date in BIOS resolves the cycle.

CMOS checksum error on boot after fitting a replacement cell

A checksum error on boot means the BIOS detected that stored CMOS values no longer match the checksum written when settings were last saved — this happens when the cell drops to zero and SRAM loses power entirely, not just to retention threshold. Fitting a new cell restores power to the SRAM, but the corrupted or cleared data still triggers the error on first boot. Enter BIOS setup immediately after the error prompt, re-enter all settings including date, time, and boot order, then save and exit. The checksum is recalculated on exit and the error will not reappear.