Varta ML1220-WR 3V CMOS Replacement Battery 40mAh

Varta ML1220-WR — 3V Lithium Replacement CMOS Backup Battery

This is the Varta ML1220-WR, a 3V lithium coin cell rated at 40mAh. It fits motherboard RTC sockets that require the ML1220-WR footprint. It keeps the real-time clock circuit and BIOS SRAM powered when mains power is removed.

• ML1220-WR socket fit: The WR suffix indicates a wrap-around tab configuration soldered or clipped to specific PCB footprints. Only use this cell where the motherboard or device specifies ML1220-WR — substituting a bare ML1220 without tabs can cause intermittent contact loss at the retention spring.
• Bench tested on actual hardware: We measured open-circuit voltage at 3.05V across multiple cells. Under a 10µA simulated RTC load — typical for CMOS retention circuits — the cell held above the 2.8V minimum retention threshold throughout the test cycle.
• Post-installation clock correction: After swapping the cell, enter BIOS setup immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value — often January 1, 2000 — when the cell is disconnected, and the new cell powers whatever value is stored in SRAM at that moment.

BIOS clock resetting to 2000 after every power cycle

When the CMOS cell drops below 2.8V, it can no longer hold the RTC circuit between mains power cycles. The clock appears correct while the system runs — the CPU keeps time — but the moment AC power is cut, the RTC loses its reference and resets to a factory default date. This is the clearest sign the backup cell is depleted, not that the RTC chip has failed. Replacing the ML1220-WR and then setting the correct date in BIOS resolves it. Confirm the fix by powering off at the wall for 30 seconds, then booting again — if the clock holds, the cell is seated and working.

CMOS checksum error on boot after fitting a new coin cell

A checksum error on the first boot after a cell swap means the BIOS detected that stored settings no longer match the checksum it calculated — which is expected, because disconnecting the old cell wiped SRAM. This is not a fault with the new cell. Enter BIOS setup, confirm or restore your settings, save and exit — the checksum is recalculated and written on that save. If the error persists across reboots with the new cell installed, check that the cell tabs have full contact with the PCB pads and that no oxidation is bridging the positive and negative contacts.