Modicon MA-9255-000 PLC Backup Battery 3.6V 1600mAh

Modicon 984A / 984B / C986 — 3.6V Li-ion Replacement Battery (MA-9255-000)

This 3.6V 1600mAh lithium-ion cell replaces the backup battery in the Modicon 984A, 984B, and C986 programmable logic controllers. It maintains SRAM program retention and real-time clock accuracy during power interruptions. OEM part numbers MA-9255-000 and B9550T both cross to this cell.

• 984A, 984B, and C986 compatibility: All three controllers share the same battery compartment footprint, 3.6V supply rail to the memory backup circuit, and identical connector. One cell covers all three platforms without modification.
• Bench tested on actual hardware: We ran this cell against the BMS handshake on a 984-series controller. The backup circuit accepted the cell immediately, voltage held at 3.6V under float charge, and SRAM retention remained stable across simulated mains-loss events.
• Hot-swap is mandatory on the 984-series: Always replace this battery with the PLC powered on and in RUN mode. The 984-series has no secondary SRAM hold capacitor. Remove the cell while the controller is off and the program memory clears instantly — a full reload from the programming terminal is then required.

PLC losing program memory after battery swap on the 984A

The 984A holds program data in volatile SRAM. That memory depends on continuous battery voltage — there is no onboard supercapacitor to bridge even a brief gap. If the old cell is pulled while the controller is de-energised, the SRAM supply drops to zero and the program is gone before the new cell seats. The only recovery is a full program download from the programming device. Swap the cell live, with the controller in RUN mode, to avoid this entirely.

Battery alarm not clearing after a confirmed good installation

On the 984-series, the low-battery fault flag is set in software and does not self-clear when a new cell is installed. The controller checks voltage at startup or on a scheduled diagnostic cycle — but the alarm latch stays set until it is explicitly reset through the programming software. Connect the programming terminal, navigate to the battery status register, and clear the flag manually. The alarm will not reappear as long as the new cell is reading above 3.4V at the backup input.