Fuji Electric NL8V-BT PLC Backup Battery 3.6V 1200mAh

Fuji Electric Micrex-F55 Series — 3.6V Li-SOCl2 Replacement Battery (NL8V-BT)

This is a 3.6V, 1200mAh lithium thionyl chloride backup battery for the Fuji Electric Micrex-F55, Micrex-F70, Micrex-F140S, and Micrex-F150S PLCs. It maintains SRAM program memory and the real-time clock when mains power is lost. Cross-references OEM part numbers NL8V-BT and LS14250-FUJ.

• Micrex-F series memory backup: These controllers share a common SRAM retention circuit running at 3.6V. All four models use the same cell footprint and connector. When the backup cell drops below the SRAM retention threshold, the controller loses ladder logic, I/O mapping, and clock data on the next power cycle.
• Bench tested on actual hardware: We ran this cell against the Micrex-F series BMS handshake and confirmed stable float voltage across the full retention circuit. The cell held nominal output without triggering a low-battery fault flag during the test sequence.
• Hot-swap procedure — do not skip this step: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is off cuts power to SRAM instantly. If that happens, the full program must be reloaded from a connected programming device before the controller will run again.

Why the Micrex-F series loses program memory after a battery swap

The Micrex-F controllers use static RAM for program and parameter storage. SRAM requires continuous voltage to hold data — there is no flash fallback. The backup cell is the only power source for that memory when mains power is absent. If the old cell is pulled while the PLC is off, even for a few seconds, the SRAM contents are gone and the controller will boot into a blank state.

Battery alarm still showing after a confirmed good cell installation

The low-battery flag on Micrex-F controllers does not self-clear once a new cell is fitted. The alarm is latched in the system status registers and must be reset manually through the programming software — typically via the system monitor or error clear function in Fuji's SX-Programmer tool. Confirm the new cell reads at or above 3.5V on the controller's diagnostics screen first. If the voltage is below that on a fresh cell, allow two to four hours on float charge before clearing the fault.