ML1220 3V CMOS Backup Battery 40mAh Li-ion Replacement Cell

Custom Battery Pack ML1220 — 3V Li-ion CMOS Backup Battery

This is a 3V 40mAh Li-ion coin cell that replaces the ML1220 CMOS backup battery on motherboards and embedded systems. It powers the real-time clock (RTC) circuit and SRAM that hold BIOS settings, system date, and firmware data when mains power is removed. If your system resets to a default date, throws CMOS checksum errors, or loses settings after every shutdown, this cell is the direct replacement.

• ML1220 form factor compatibility: The ML1220 footprint is used across computer motherboards, servers, and industrial controllers that require a 12.0mm diameter, 2.0mm height coin cell on a dedicated RTC rail. The BMS on these boards expects a stable 3V retention source — this cell delivers exactly that.
• Bench tested on actual hardware: We seated this cell in an ML1220-socketed motherboard and confirmed the BIOS clock held across full mains disconnection cycles. The RTC circuit drew continuously from the cell with no voltage drop below the 2.8V retention threshold during testing.
• Post-installation BIOS reset: After fitting this cell, enter the BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value — often 1 January 2000 — any time the coin cell loses power, and that must be corrected manually after every swap.

BIOS clock resetting to 2000 after every power cycle

The RTC circuit on most motherboards requires the coin cell to stay above 2.8V to retain clock data. Once the ML1220 drops below that threshold, the SRAM loses its state and the clock reverts to a hardcoded default — usually 1 January 2000. A multimeter reading below 2.8V on the old cell confirms the fault. Replace the cell, re-enter the BIOS, set the correct date and time, save, and exit.

CMOS checksum error on boot after mains power removed

A CMOS checksum error means the BIOS detected that stored settings no longer match the checksum saved at last boot — which happens when the CMOS cell is fully depleted and SRAM contents are lost. This is a different symptom from a clock reset: the system cannot verify any stored configuration, not just the time. Replace the ML1220 coin cell, then re-enter the BIOS to reconfigure all settings — not just the clock — and save before exiting. The board will recalculate and store a fresh checksum at that point.