Compudyne 486DX33 PLC Replacement Battery 3.6V 2700mAh

Compudyne 486DX33 / 4DX266 / 4SX25 — 3.6V Li-MnO2 Replacement Battery

This is a 3.6V 2700mAh lithium manganese dioxide cell that maintains SRAM and real-time clock backup power in Compudyne 486DX33, 4DX266, and 4SX25 programmable logic controllers. When main power is cut or the controller goes through a maintenance cycle, this cell keeps program memory and clock data intact. It is a direct-fit cell matching the original dimensions of 60.55 x 18.50 x 18.35mm.

• 486DX33 / 4DX266 / 4SX25 compatibility: All three Compudyne models share the same backup battery socket and 3.6V SRAM retention rail. One cell fits all three without modification.
• Bench tested on actual hardware: We ran this cell under continuous SRAM retention load and confirmed the BMS holds the output rail steady above the 3.0V memory-retention threshold throughout discharge.
• Hot-swap procedure — mandatory for this controller: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is powered off drains SRAM instantly — program memory is gone and a full reload from the programming device is required.

PLC battery alarm not clearing after a confirmed good installation

On Compudyne controllers, the battery fault flag is set in software and does not clear automatically once a new cell is fitted. The PLC has no way to auto-detect that a fresh cell is in place — it only knows the alarm was triggered. After swapping the cell, you must navigate to the battery status register in the programming software and manually acknowledge or reset the alarm flag. Leaving the alarm active masks any future genuine battery fault, so clearing it immediately is part of the swap procedure.

New cell reading below 3.6V on first installation

Li-MnO2 cells ship in a storage state, typically reading 3.3–3.45V out of the bag. This is normal — the cell is not depleted. Once seated in the PLC, the controller's float circuit brings the terminal voltage up to full rated level within a few hours. Do not reject the cell based on an initial low reading. Confirm the installation is good by checking voltage again after four hours under PLC float charge — it should read at or above 3.5V.