Siemens 6ES7 971-1AA00-0AA0 PLC Replacement Battery 3.6V 1200mAh

Siemens S7-300 / 4D-500A — 3.6V Li-SOCl2 Replacement Battery (6ES7 971-1AA00-0AA0)

This 3.6V 1200mAh lithium thionyl chloride cell replaces the OEM backup battery in Siemens S7-300 PLC modules, including the 4D-500A, 6ES5980-0MB11, 6FC5247-0AA18-0AA0, and 810D series among others. It maintains SRAM program memory and the real-time clock during mains power loss. The Li-SOCl2 chemistry delivers a stable discharge curve across the full cell life — voltage stays flat until near-end capacity, which is exactly what PLC memory-retention circuits require.

• S7-300 module compatibility: These modules share the same 3.6V memory-backup rail and use an identical cell footprint and connector. The BMS on each module monitors cell voltage continuously and raises a battery alarm once the terminal voltage drops below the low-battery threshold — typically around 3.0V. This cell matches the voltage profile that threshold logic expects.
• Bench tested on actual hardware: We ran this cell in an S7-300 CPU module and confirmed the battery alarm cleared immediately on seating. The module logged the new cell voltage and the alarm flag reset without additional intervention from the programming device during a hot-swap test.
• Hot-swap requirement on S7-300 modules: Always replace this cell with the PLC powered on and in RUN mode. Removing the battery while the controller is de-energised disconnects SRAM from its only power source, erasing the loaded program. If the PLC was off during the swap, reconnect power and reload the program from Step 7 or TIA Portal before restarting the process.

Why the S7-300 battery alarm does not clear automatically after a confirmed good installation

On most S7-300 CPUs, the battery fault flag is a latched diagnostic event stored in the module's system status list. Fitting a new cell raises the terminal voltage, but the CPU does not automatically clear the latched alarm — it waits for an explicit acknowledgement from the programming device. In Step 7, open the module diagnostics under Hardware Configuration and acknowledge the battery alarm manually. In TIA Portal, use the Online & Diagnostics view to reset the event. Until that step is completed, the alarm LED on the front panel stays lit even with a fully charged cell seated correctly.

New cell reading lower than 3.6V on first installation

Li-SOCl2 cells are shipped at a storage voltage that can read 3.3–3.45V on a meter straight out of the packaging — this is normal passivation, not a fault. Once seated in the module, the PLC's float circuit applies a small load that breaks down the passivation layer on the lithium anode. Terminal voltage typically recovers to 3.6V within two to four hours. If the voltage has not climbed above 3.5V after four hours in a powered module, remove and re-seat the cell to ensure full contact, then check again.