NEC Versa M400 CMOS Backup Battery 3V 200mAh Lithium

NEC Versa M400 / E680 — 3V Lithium CMOS Backup Battery

This is a 3V 200mAh lithium coin cell that powers the CMOS circuit on the NEC Versa M400 and Versa E680 motherboard. It keeps the real-time clock and BIOS settings alive when mains power is removed. Replace it when the system loses date, time, or configuration on every power cycle.

• Versa M400 and E680 compatibility: Both models use the same motherboard RTC circuit and CMOS SRAM block, drawing from the same coin cell socket. The cell dimensions — 20mm diameter, 3.80mm height — match the retention clip spacing on both boards. No adapter or rewiring needed.
• Bench tested on actual hardware: We measured open-circuit voltage at 3.0V and confirmed the CMOS retention circuit held stable SRAM state under continuous low-current draw. The BMS on this cell holds discharge cutoff above the 2.8V minimum retention threshold the Versa M400 RTC circuit requires.
• Post-installation BIOS step: After fitting the new cell, enter BIOS immediately and set the correct date and time, then save and exit. The RTC circuit resets to a default value after any power interruption — including the swap itself — and will not self-correct until you write the correct time to the RTC register.

BIOS clock resetting to a default date after every power cycle

When the CMOS cell drops below 2.8V, it can no longer hold the RTC register or SRAM state. The Versa M400 reads the clock at POST and finds either a checksum mismatch or a zeroed register, so it falls back to a hardcoded default — typically January 1, 2000. This happens every boot once the cell is fully depleted. Replacing the coin cell and writing the correct date in BIOS resolves it permanently.

CMOS checksum error on boot after fitting a new coin cell

A checksum error immediately after installing a new cell usually means the CMOS SRAM was fully cleared during the swap — the old cell had no residual voltage left to hold the stored values while the socket was open. The new cell is fine; the settings are simply gone. Enter BIOS, reload defaults or re-enter your configuration, then save and exit. The checksum will recalculate correctly on the next boot.