Advent 4211 CMOS Backup Battery 3V 200mAh Lithium

Advent 4211 / 4212 — 3V Lithium CMOS Backup Battery

This is the 3V lithium coin cell that keeps the RTC and BIOS settings alive in the Advent 4211 and 4212 when mains power is removed. It runs at 3V with a 200mAh capacity. When this cell drops below the CMOS retention threshold, the motherboard loses its clock and stored settings on every shutdown.

• Advent 4211 and 4212 fit: Both models share the same motherboard CMOS socket and RTC circuit. The coin cell holder accepts a standard 20mm diameter cell. One replacement covers both devices.
• Bench tested on actual hardware: We seated this cell in the socket and cycled the board through cold boot, BIOS save, and full mains disconnection. The RTC held the correct time and BIOS settings on every power-on cycle. The cell measured 3.0V under load.
• After installation — BIOS clock correction: After fitting the new cell, enter BIOS immediately and set the correct date and time, then save and exit. The CMOS circuit powers the RTC directly, and any gap between the old cell dying and the new one seated resets the clock to a default value. The board will not correct this automatically — you must write the right time into NVRAM manually.

BIOS clock resetting to 2000 after every power cycle on the Advent 4211

The RTC on this motherboard draws power exclusively from the coin cell when mains is off. Once cell voltage drops below 2.8V, the SRAM holding the clock value loses retention and reverts to its default state — typically 1 January 2000. This happens even if the machine runs fine while plugged in, because the PSU rail masks the failing cell during operation. Replacing the coin cell is the only fix. After fitting, set the correct date in BIOS and save.

CMOS checksum error on boot after fitting a new coin cell

A checksum error immediately after a new cell swap usually means the BIOS compared its stored configuration against what it read from CMOS and found a mismatch — because the old cell had already let the CMOS contents corrupt before the swap. The new cell is fine; the data it is now backing is invalid. Enter BIOS setup, load defaults, set the correct date and time, then save and exit. The checksum clears once valid data is written back and the cell holds it above 2.8V.