Sanyo ML1220-WR CMOS Backup Battery 3V 40mAh Lithium

Sanyo ML1220-WR — 3V Lithium CMOS Replacement Battery

The Sanyo ML1220-WR is a 3V lithium coin cell rated at 40mAh (0.12Wh). It backs the RTC circuit and SRAM on motherboards and embedded electronics, keeping BIOS settings and clock data alive when mains power is removed. It replaces the original Sanyo ML1220-WR cell directly.

• CMOS and RTC retention: This cell powers the real-time clock and SRAM independently of the main power supply. When the cell drops below the board's minimum retention voltage — typically 2.8V — the RTC loses its reference and BIOS settings revert to factory defaults on the next boot.
• Bench tested on actual hardware: We measured open-circuit voltage across a batch at incoming inspection. Every cell read between 3.0V and 3.05V. We also confirmed the cell footprint and contact polarity against the ML1220-WR datasheet before shipping.
• Post-installation 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, and any interruption during the swap resets the clock to a default epoch value — usually January 1, 2000. Saving corrected values writes them to SRAM, which the new cell then holds.

BIOS clock resetting to 2000 after every power cycle

When the coin cell falls below approximately 2.8V, the RTC circuit loses its reference voltage and cannot hold the time register between power cycles. The board still boots — main power handles that — but the clock defaults to a hardcoded epoch, typically January 1, 2000. Replacing the cell restores continuous RTC power. After fitting, enter BIOS, correct the date and time, and save before exiting.

CMOS checksum error appearing on boot screen

A checksum error means the firmware compared stored CMOS values against a checksum and found a mismatch — usually because the cell fully depleted and SRAM lost its contents entirely. The board flags this to warn that settings are no longer trustworthy. Fitting a fresh ML1220-WR cell and then loading BIOS defaults clears the error. Confirm the cell reads at least 3.0V before installing to rule out a storage-depleted unit.