When should you replace the back-up battery on your S7 300/400 controller?
We recommend the following
- Change the backup battery of the S7-300 controller once
CPUs with MMC do not need a backup battery or storage battery.
- Calculate the buffer times of the S7-400 controller and
then change the backup battery accordingly - at the latest after one year
(see Entry ID: 1117740, "Automation System S7-400 Module Specifications", section 3.3).
If you are using a storage battery, then this must not be replaced. You can only use a storage battery in an S7-300 CPU. If you are using a storage battery instead of a backup battery, then if there is a power failure, only the time of day and date of the hardware clock will be buffered and the "BATF" LED lights.
Always change the backup battery in "Power ON" mode, otherwise you will lose data in the user memory and the internal CPU clock will stop.
For further details, see manuals:
"SIMATIC PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP", section 1.1.3 "Backup battery/accumulator" (Entry ID: 8860591)
"Automation System S7-400 Hardware and Installation"; section 7.1 "Replacing the Backup Battery" (Entry ID: 1117849)
Battery error message, Lithium battery, life cycle