What is the purpose of the external battery of the M221?
The help content gives us the information that the battery of the M221 allows:
- to save the content of the whole %MW area ( %MWi with i>50; note: %MW0-%MW49 are also save without battery)
- to save the RTC clock
In reality the battery is always needed/mandatory because without battery the PLC can’t start in RUN after a Power Cycle.
Within %S75 the PLC program can check when the battery has the be replaced before it becomes out of order (average is every 4/5 years):
=> when this bit is Set to 1, it means the external battery power is low and it has to be replaced soon.