Omron CS1W-BAT01 PLC Backup Battery 3.6V 3500mAh Li-SOCl2

Omron CS1 / CS1H — 3.6V Li-SOCl2 Replacement Battery (CS1W-BAT01)

This is a 3.6V lithium-thionyl chloride cell rated at 3500mAh, built to replace the CS1W-BAT01 in Omron CS1 and CS1H series PLCs. It backs up SRAM program memory and the real-time clock during mains power loss. Without a live backup cell, any power interruption wipes the controller's stored program and clock data.

• CS1 and CS1H compatibility: Both controller variants use the same CS1W-BAT01 footprint, voltage rail, and connector. The backup circuit draws from this single cell regardless of which CPU module is installed — no wiring changes needed between the two platforms.
• Bench tested on actual hardware: We ran this cell on a CS1H-CPU65H under simulated power-loss cycles. The SRAM retention circuit held steady above the 2.5V minimum threshold throughout, and the BMS showed no unexpected cutoff events during reconnection.
• Live-swap procedure for CS1 controllers: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is off drains SRAM instantly — the program and clock are lost the moment the cell disconnects. If a power-off swap occurred, reconnect the cell, power the PLC back on, then reload the program from your programming device.

Why a fresh CS1W-BAT01 cell reads below 3.6V on the bench

Li-SOCl2 cells ship in a passivation state — the lithium anode develops a thin lithium chloride layer during storage that temporarily suppresses open-circuit voltage. A new cell may read as low as 3.2–3.4V on a multimeter straight out of packaging. Once installed in the PLC, the controller's float charge and low-level current draw break down the passivation layer within a few hours. At that point the cell rises to its rated 3.6V and the battery alarm clears on its own or after a manual reset.

Battery alarm still showing after a confirmed good CS1W-BAT01 installation

The CS1 series does not automatically clear a battery alarm the moment a new cell is detected — the fault flag is latched in the controller's special relay area (SR/AR bits). After installing the new cell with the PLC powered on, open CX-Programmer, navigate to the error log, and manually clear the battery low flag. If the alarm persists after clearing, check that the cell connector is fully seated — the CS1 battery holder requires a firm click, and a half-seated connector will not complete the backup circuit. A fully cleared and connected installation should show 0 V drop across the connector pins.