How to Change an Allen Bradley PLC Battery

Updated July 20, 2017

The lithium battery in the Allen-Bradley Programmable Logic Controllers (PLC) provides backup power to the processor’s memory. Should a power outage of less than 30 minutes long happen, the processor’s memory will use the battery’s backup power to retain information. The lithium battery will last between two to five years, depending on the type of processor and the amount the battery is used. A red "Battery Low" indicator on the front of the processor indicates when the battery voltage is getting low and the battery needs to be replaced.

Investigate the location of the battery. If it is in the front of the processor module, leave the PLC plugged in. If it cannot be located without removing a module from the chassis, unplug the PLC.

Press the retainer clips at both the top and bottom of the processor module, and slide it out. If the battery is located in the front of the processor module, this step is not required.

Inspect for damage to the battery casing or leakage of electrolyte. According to Allen-Bradley’s Guidelines for Handling Lithium Batteries, if the old battery is damaged or leaking, put the damaged battery in two polythene bags; add 28.4gr. of calcium carbonate to the inner bag; and heat seal both bags. Contact a battery disposal company for proper disposal.

Unplug the battery connector, and remove the battery from the retaining clips.

Inspect the replacement battery to ensure that it is identical to the removed battery.

Write the current date on the new battery with a marker. If there is no space to write the date directly on the new battery, use the labels that were provided with the new battery. In addition, place one date label on the front of the battery compartment, and write the current date on this label.

Plug the new battery connector into the socket, and push the battery into the retaining clips.

Slide the processor module into the chassis until the retainer clips click.

Restore power to the PLC.

Ensure that the "Battery Low" indicator located on the front of the processor is off.


Dispose of the old battery properly. Some Allen-Bradley PLCs contain a bit in the status file to indicate that the battery is low. This bit can be used in your application to alert maintenance personnel of a low battery.


Avoid electrostatic damage by touching a grounded object before handling the processors. Do not turn the power off to SLC 5/01 or 5/02 processors. If the power is turned off to these processors and the battery is removed, your application code will be erased. Do not remove the battery and turn the power off to your PLC for more than 30 minutes. You run the risk of erasing the application code from the PLC memory. Do not pick up ruptured or damaged batteries with bare hands or breathe the fumes.

Things You'll Need

  • Marker
  • Replacement battery
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About the Author

Alissa Crowe-Scott has been writing technical documentation, specifically life-cycle documentation, since 1995. She uses her background in project management and engineering experience to write her articles. She graduated from the University of Tennessee with a Bachelor of Science in chemical engineering and received her certification as a project management professional in 2003.