What is the purpose of PLC CPU battery

Dear Sirs,
I have just joined this PLC news group. I have one querry

this is regarding Omron C200HE PLC, provided with EEPROM memory cassette holding Ladder Logic and provided battery for data ram. If I want to renew the CPU unit, I have to exchange EEPROM memory from old CPU. My doubt is what is the use of the battery provided inside the CPU as the ladder logic is loaded on EEPROM, please explain

thanks & regards
K.Parimalam

The battery is used to mantain the memory in the PLC. Sure you have tape or disk or even Eeprom backup of the program, but you loose what we call "state ram" When the program is set up and tested, the variables of the program are stored in state ram and yes They are stored in the program file. Now say someone or something has changed and the "variable" in your program has been modified by an operator via the operator interface or "HMI"/"OIT". This directly changes the operation of the system. Well those things are not "written" automatically to the "backup" system whether it be a tape or disk or even Eeprom. The battery ensures the "program variables" are held in case of a power outage. Some PLC have the option of "loading from Eprom always" in this case a battery is not required. Just remember if any changes are made to the "variables" in your program they will not be on the Eprom unless you re-burn it. We use Eprom for the burner management systems on our boilers, this is required to "start" from a proven configuration after a catistrophic failure. It is also required by code.
hope this helped clarify your question.

Bitmore

Bitmore is correct, but it's not as scary as he makes it. The C200HE has a separate battery not attached to the board but found on the backside of the CPU. The ladder program is kept as a backup on the EEPROM cassette, but the "variables" (Data Memories) are backed up by the CPU battery. You will need to use software to make a current copy of the "variables" to transfer them to the new PLC.

If you don't have any programming software, you can find a C Series Backup Utility at MrPLC.com to transfer the program, the PLC setup and the "variables".

C Series Backup Tool Version 6.0 @ MrPLC.com

The batteries also keep the clock running (if you've got one).

Wouldn't it be annoying to have to set the system time every time you cycled power to the PLC?

AK