Battery Failure

Good Day!!

i would like to ask about what are the common causes of a Siemens PLC Battery Failure..?

Old age would be the main one...

ASF said:

Old age would be the main one...

Click to expand...

And that's the reason why replacing the batteries should be a planned maintenance job. I know most companies don't really care much about the batteries until at one point in time they're stuck with a PLC they have to reprogram and the line is out of production for some time.

PLC battery failures are not limited just to the Siemens brand. So far, no company has developed a practical battery that will last the life of a PLC.

What does the battery typically back-up?

Being an instrument guy, with only a cursory knowledge of PLCs, I have to ask, what exactly does the battery back-up?
- the operating program ?
- Instantaneous (for the current scan) values like current variable status?
- comments?

I assume everything runs fine until a power outage and then the lack of ________ battery backed-up data is the problem.

danw said:

Being an instrument guy, with only a cursory knowledge of PLCs, I have to ask, what exactly does the battery back-up?
- the operating program ?
- Instantaneous (for the current scan) values like current variable status?
- comments?

I assume everything runs fine until a power outage and then the lack of ________ battery backed-up data is the problem.

Click to expand...

Could be all of the above, depending on the model of PLC you are using.

A lot of more recent PLC's are using flash memory to store the program (or use flash cards that may be optional or required). If flash memory isn't available, then the program is stored in RAM only, and is lost if it ever loses power.

Same for the current system values. If the PLC has flash memory, it can usually save at least some of the values as remnant/retain memory. Other values are typically set to 0 on system start.

The system clock, also needs electricity to keep ticking. Many PLC's these days use a capacitor to run that (for at least a couple weeks), instead of requiring a battery.

Older AB PLC's use the battery to power up a memory area for storing the program. So if the battery dies and the PLC is powered down, you lose the program.

Newer AB PLC's use the battery to transfer the program to non-volatile memory when the power is lost. So the battery is not used to hold the program in all the time; only to transfer it at the moment of power loss.

Still newer AB PLC's do away with the battery entirely, and do the transfer using stored charge in a capacitor.

As for other brands - no idea! But I'd imagine they're all some iteration of the above

Thanks for the clarifications