Hello AkiraJn;

The attached screenshots are taken from the S7-400 Module data, and show the power supply modules LEDs and controls.

https://support.industry.siemens.com/cs/fr/en/view/1117740

It seems that in your case you can switch off the BATT switch, thus terminating the battery monitoring functions, without harm to the system. The active LEDs indicating a battery failure should turn off.

And do not turn off the power supply switch as you will lose the program in RAM without battery support. Just remember to change the BATT switch back on when you install the new battery. 

The appearance of the EXTF LED on the CPU could be caused by many types of fault; the best way of locating the cause of failure is the CPU's diagnostic buffer, which collects status messages on all events that affect the CPU. Either use CTRL+D in Simatic Manager, or open the Menu PLC--> System Information to see the messages available in the diagnostic buffer.

You can save the messages from the diagnostic buffer (there is a Save As button on that page) and post the contents of the saved text file here as an attachment so we may help you decipher the copntents, if they are difficult to understand. It takes a bit of getting used to.

Hope this helps,

Daniel Chartier