As shown in the links proposed earlier by H-H, a backup battery will be required iof the load memory of the CPU is based on RAM technology. This wad the case for the older S7-300 CPU (now load memory is based on a MMC, therefore battery is not required) and it is still the case for S7-400 PLCs using a RAM card (there is a possibility of using an EEPROM card instead, and that does niot need a battery support)..
If your present CPU needs a battery to support the load memory and you do not have one in the PLC, do not remove power from the CPU or you will lose the program loaded to the PLC and it will not go to Run mode when powered up, until you download the program once more from the programming device..
Hope this helps,
|=== Edited by dchartier @ 7/27/2014 3:33 PM [GMT ] ===|