PLC battery question.

I just have another question..

what is the main purpose we r using battery inside plc..

how many years a usual one will last??

when battery is taken and re inserted will the program will malfunction??

Some PLCs use the battery to maintain the program logic and the data tables during a power off state. others use the battery only to maintain the data tables wiht the program logic being maintained in non-volitile RAM. You must check the specific PLC documentation to determine the specific battery purpose and lifetime (which may vary depending on powered off storage temperature).