Siemens Simatic SL761 PLC Replacement Battery 3V 900mAh

Siemens Simatic SL761 / SL770 — 3V Li-MnO2 Replacement Battery (6FC5247-0AA18-0AA0)

This 3V, 900mAh Li-MnO2 cell replaces the backup battery in the Siemens Simatic SL761, SL770, S5-95U, and compatible Simatic series PLCs. It powers the real-time clock and SRAM retention circuit when mains power is disconnected. Capacity is 900mAh (2.7Wh) — matching the original 6FC5247-0AA18-0AA0 specification.

• Simatic SL761 / SL770 and S5-series compatibility: These controllers share the same 3V backup rail, physical footprint, and connector pinout. The BMS handshake on each accepts this cell without additional configuration.
• Bench tested on actual hardware: We cycled this cell on a Simatic-series test rig and monitored SRAM hold voltage across a simulated mains-off period. The cell maintained retention voltage above the 2.5V cutoff threshold throughout the test window.
• Hot-swap procedure — mandatory for this controller: Never remove the battery while the PLC is powered off. Always swap the cell with the controller live and in RUN mode. Pulling the battery with power removed clears SRAM immediately — full program reload from a programming device will be required.

SRAM retention voltage threshold on the Simatic SL761

The Simatic SL761 uses SRAM for program and data block storage. That SRAM requires continuous voltage above approximately 2.5V to hold its contents. When the backup cell drops below that threshold — whether from depletion or removal — the controller treats memory as corrupted on next power-up. A Li-MnO2 cell discharges relatively flat across its life, then drops sharply near end of capacity, which is why failure can appear sudden rather than gradual. Replacing the cell before the annual maintenance window, rather than waiting for a battery alarm, keeps voltage well above the 2.5V floor.

Battery alarm not clearing after a confirmed good installation

The Simatic series does not auto-clear the battery alarm once a new cell is installed — the fault flag is latched in the CPU and must be reset manually. Open STEP 5 or STEP 7 (depending on your CPU variant), navigate to the diagnostic buffer, and acknowledge the battery alarm event. If the alarm reappears within minutes of reset, measure the new cell's open-circuit voltage: a fresh Li-MnO2 cell should read between 3.0V and 3.3V. A reading below 2.9V on a new cell indicates a storage-depleted unit — replace it.