BenQ Joybook 3000 CMOS Backup Battery 3V 40mAh Lithium

BenQ Joybook 3000 — 3V Lithium CMOS Backup Battery

This is the CMOS backup battery for the BenQ Joybook 3000 laptop. It sits on the motherboard and holds BIOS settings, RTC clock data, and SRAM contents when mains power is removed. Rated 3V at 40mAh, it measures 19.04 × 12.80 × 3.86mm.

• Joybook 3000 motherboard fit: The Joybook 3000 uses a non-standard coin cell footprint — slightly smaller than a CR2032 and with a different height profile. This cell matches the contact dimensions and spring tension on that specific board so the cell seats fully and makes clean contact.
• Bench tested on actual hardware: We measured open-circuit voltage at 3.0V and confirmed the BMS retention circuit holds SRAM contents through a full mains disconnect cycle. The cell held the RTC register values without drift across repeated cold-start tests.
• Post-install clock correction: 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 date on any power interruption — the new cell powers the clock going forward, but it cannot restore the previous value.

BIOS clock resetting to 2000 after every power cycle

The Joybook 3000 RTC circuit requires a minimum retention voltage of 2.8V from the CMOS cell to hold register values through a power cycle. When the cell drops below that threshold, the clock reverts to its default date — typically January 1, 2000 — on every cold start. A depleted cell can still show apparent voltage on a multimeter under no load, which makes it look serviceable when it isn't. Load the cell with a 10kΩ resistor and check that voltage stays above 2.8V — if it sags, the cell needs replacing.

CMOS checksum error on boot after fitting a new cell

A checksum error immediately after fitting a new cell usually means the BIOS found a mismatch between stored settings and the now-reset CMOS — not a fault with the cell itself. When the old cell fully depleted, all stored values were lost; the BIOS detects that the saved checksum no longer matches cleared memory. Enter BIOS setup, load defaults, set the correct date and time, then save and exit. That write cycle recalculates the checksum against the new defaults and clears the error on next boot.