About this replacement
Omron CJ1M / CJ2M Series — 3V Li-MnO2 Replacement Battery (CJ1W-BAT01)
This is a 3V, 850mAh lithium-manganese dioxide cell that replaces the CJ1W-BAT01 in Omron CJ1M, CJ1G, CJ2M, and CJ2G programmable logic controllers. It maintains SRAM program memory and the real-time clock when the PLC loses main power. Swap it before the controller logs a battery alarm — not after.
- CJ1 and CJ2 platform compatibility: The CJ1M, CJ1G, CJ2M, and CJ2G all share the same battery bay geometry and 3V SRAM retention rail. One cell format covers the full CJ-series lineup because the memory backup circuit is common across these CPU units.
- Bench tested on actual hardware: We confirmed the cell seats correctly in the CJ1W-BAT01 socket, verified the retention voltage held above 2.5V under simulated SRAM load, and confirmed the BMS did not flag a low-battery alarm on fresh installation.
- Hot-swap procedure on the CJ series: Always replace this battery with the PLC powered on and in RUN mode. Removing the old cell while the controller is off will wipe SRAM instantly — the backup circuit has no capacitor to bridge the gap. If the PLC was off during the swap, reload the program from your programming device before restarting production.
PLC battery alarm not clearing after a confirmed good installation
The CJ-series CPU does not auto-clear a battery alarm when a fresh cell is installed. The alarm flag is latched in the system flags area and must be reset manually through CX-Programmer or the Sysmac Studio tool. Navigate to the online monitor, locate the battery alarm flag (typically A402.04), and force-reset it after confirming the new cell reads above 2.7V at the terminals. The alarm will not clear on its own, even with a fully charged cell seated correctly.
New cell reading lower than 3V immediately after installation
Li-MnO2 cells ship in a controlled storage state and often measure 2.75–2.85V out of the packaging. This is normal — it is not a fault cell. Once seated in the PLC, the float charge circuit brings the terminal voltage up over several hours of powered operation. Check the voltage again after the controller has run for four to six hours; it should stabilise at or above 2.9V. If it remains below 2.7V after that period, the cell is suspect and should be replaced.