Step-by-Step PLC Battery Replacement and Maintenance Procedure

Programmable Logic Controllers commonly referred to as PLC are important components of the current industrial automation systems. Their design is robust, small form factor, and are used for tasks such as controlling water purification, mining operations, assembly lines, etc. There is robustness especially in physical environments, and the performance of PLCs is not affected by factors like high or low temperatures, dust or humidity. In general, PLCs are crucial for every occasion, which requires the governing and command of a single machine or even an entire production line.

PLCs consist of four primary components:

Processor Section: The processor of the system that performs the logic.

Input/Output (I/O) Section: Assignment of tasks for data exchange between the system and other devices.

Programming Interface: Enables the user to feed as well as alter instructions which are used in the computer program.

Power Supply: Transforms the electrical high voltage (120V or 240V AC) into a low DC voltage of 24V that is required in the operation of the system.

The main power always supplies the system; however, there is backup batteries to power it and at the same time the PLC store other important information.

Click here for Understanding PLC Racks and Chassis: Types, Differences, and Purposes

What is a PLC battery?

Programmable Logic Controller Battery: PLC battery is an important component of the system as it allows for retention of the programmed logic in the system memory when there is a power breakdown. It is however, important to ensure that the PLC battery is maintained, Charged and replaced on time to avoid loss of data and many bad outcomes.

A PLC battery is an acid battery which supplies power in case of mains failure to the memory of the PLC where all the software logic and settings are stored. This must be checked and maintained frequently so that voltage’s values of the battery goes with the recommended values for avoiding data loss, system downtime.

Why does a PLC need a Battery?

A PLC operates with its memory in erasable programmable read-only memory (EPROM), where it also preserves important information such as configuration details, process data, and logical applications. The battery makes certain this data is retained in case the main power supply is compromised as could be the case during maintenance or when there are power surges.

Because of this, the batteries are referred to as CMOS batteries, backup batteries or RAM memory batteries. They serve as protection mechanisms of the processor’s non-volatile power to the processor.

Capacitor Assembly Batteries: Use for short-term purposes, which means that it is ideal to provide backup for up to 3 days.

Lithium Battery Assemblies: Able to support up to five years, it provides better support for the need of long-term data storage applications.

With a faulty battery, a power surge might mean losing basic settings and computer programming, therefore battery checks are crucial.

Types of PLC Batteries

• Generally the PLC system employs lithium ion or lithium batteries. Among all kinds of batteries, Lithium-Thionyl Chloride batteries are the most widely used battery for PLC power backup. 
• They are most preferred since they are highly suitable for low current applications and they provide longer service for the PLC battery. 
• Besides, the voltage variants of the PLC batteries which are frequently implemented are 3.0 Volt DC, 3.6 VDC, whereas you can get even higher voltage by joining the batteries consecutively. 
• These two are the typical voltage ranges of the different types of PLC backup batteries agreed on as follows: 3V6 Li – lithium backup battery for PLCs, 3V0 Li – lithium coin cell backup battery for PLCs.

How Long Does a PLC Battery Last?

The duration of battery life in any given PLC depends with the type of battery and the frequency of use of the battery. The span of lithium batteries is 2-5 years on average. Some of the battery-operated systems make use of rechargeable batteries, while other incorporation non rechargeable batteries.

Factors that affect PLC battery life 

Frequent power outages.

High operating temperatures

Powering off the PLC system for long time

The age of the PLC system.

It is advisable to replace the battery after 2-3 years to avoid getting surprised by frequent system breakdowns.

What Consequences Arise If PLC Battery is Discharged?

If the battery of the PLC is dead but the main power supply is on then the system will function properly. On the other hand, the utilization of non-volatile memory is that if the power supply is interrupted, all the data that is stored in the region can be lost. Other items include the code logic, the configuration settings as well as the information stored on the real time clock.

To avoid this, it’s crucial to:

It is recommended to keep track of the state of the battery on a more or less frequent basis.

Change the battery before it goes bad or reaches the time that it will be of no use anymore.

How to Change the Battery in a PLC

The battery in the CPU module of the PLC is another important element because it provides standby power to nonvolatile memory. This guide also tells you when to replace the battery or battery pack in a PLC, how to do it, and how to measure voltage.

Checking PLC Battery Status

Routine Battery Status Check:

• Take out the system controller cabinet or drawer located on the exciter rack.
• Pull the upper tray on the front side of the cabinet of the system controller to access the CPU module.
• An LED alarm visible on the PLC CPU module will be red, yellow, or flickering when the BATT input is impaired.
• If BATT LED is on, then battery voltage is low and battery needs replacement as voltage goes below the lower limit.

Important note:

Note that the voltage threshold is different for each model of PLC. For example, for 3.0 VDC lithium battery the threshold in an Allen-Bradley PLC-5 is about 2.0-2.5 VDC. The BATT LED illuminates when the remaining backup power can last for approximately ten days for the CPU.

It is advisable to establish routine check-ups on the device as one might not easily spot the BATT LED, most specifically when well contained within the cabinet of the controller. The routine schedule for inspections is another critical factor to consider as it ensures timely detection.

How do I change the battery in my PLC?

Replacing the PLC Battery

If the BATT LED is red, the battery voltage has reached a critically low level and must be replaced as soon as possible. Battery replacement can also be part of the preventative maintenance.

General Battery Replacement Steps:

Locate the Battery:

Find out an location of a battery on the CPU module. If the controller is easily accessible, the replacement can be done with the power supply remaining on to retain the program and settings.

If the battery is sealed and cannot be disconnected unless the CPU module is removed, take the power supply off (assuming the memory is not backed up by something like a supercapacitor).

How to remove PLC battery?

Remove the Battery:

Pull clips and then take out the battery carefully ly.

If the battery is to be located at the front of the CPU module, the battery connectors should be disconnected without actually having to remove the CPU module.

Inspect the Replacement Battery:

When buying a new battery ensure that it is of the same voltage capacity, current rating, size and compatibility as the first one. Cross check on the specifications in order not to harm the PLC.

Install the New Battery:

Plug in the new battery to the specific socket and fix it in position using the clips provided.

Replace the CPU module and turn on the power if it was off. In case the battery was installed when the power was on, nothing more needs to be done.

Dispose of the Old Battery:

Always discharge lithium batteries properly especially when they are damaged or possess leakage. They should be kept in two heat-sealed polyethylene bags with calcium carbonate to neutralize leakage. It is important when dealing with hazardous wastes you have to follow local laws as to the disposal of wastes.

Manufacturer-Specific Notes:

Under the general field of setup and programming, refer to the instructions from the specific PLC manufacturer for your model to prevent memory or configuration loss. In certain complex systems like the FANUC CNC machines, numerous batteries are in use that call for further measures.

How to check PLC battery?

Testing PLC Battery Voltage

Electrical testing checks voltage regularly to ensure that batteries are in good conditions. A digital multimeter is advised because of its reliability and its capacity to measure a wide variety of voltages and currents.

Steps to Test Voltage:

Prepare for Testing:

Take off the battery that is connected in the CPU module. If removal involves the disconnecting of the power supply then one should back up the module’s memory.

On the battery, a small mark is placed, which indicates the voltage, for example 3.0V or 3.6V.

Set Up the Multimeter:

Set the multimeter to the DC voltage or DCV mode. Select a range above the battery nominal voltage (5V, 10V, 20V or the desired voltage).

Measure the Voltage:

Using the multimeter touch the red probe on the “+” terminal of the battery and black probe on the ‘-’ terminal of the battery.

Do not move the probes around and note the voltage on the screen.

Interpret Results:

Fully Charged: 3.0VDC or above for the 3.0VDC battery and 3.6VDC—3.7VDC for the 3.6VDC battery.

Functional but Low: 2.0–2.5 VDC for 3.0 VDC batteries, 2.4–2.9 VDC for 3.6 VDC batteries.

Replace Immediately: Lesser than 2.0 volts for 3.0 volts batteries and below 2.4 volts for 3.6 volt batteries.

Additional Testing Tips:

If the BATT LED comes on after replacing the battery, return the new battery to the voltage test to ensure that you put a good one into the unit.

It is also important to double check the multimeter probes when making connections to avoid giving wrong readings.

PLC Battery Maintenance Checklist – Download

This Below checklist will help ensure the PLC battery is well-maintained, reducing risks of data loss and system downtime.

Refer the below link to download the excel form of PLC Battery Maintenance Checklist 

Click here for more Essential Instrumentation activities Checklists

FAQ on PLC Battery

How often should I replace my PLC battery?

The length of time for replacing the PLC batteries depends on the type and usage of the PLCs. For example, Allen-Bradley SLC500 and PLC5 models are best replaced annually, often this is recommended particularly when the PLC is switched frequently. Batteries above can last longer, however, it is not recommended to go beyond 3 years of service interval for continuous running systems.

What is the voltage of a PLC battery?

PLC batteries are usually of 3.0V or 3.6V, the voltage depending on the model of the battery and the manufacturer who produced it. For instance, the Siemens S7-300 uses a 3.0V lithium battery having nominal capacity of 950 mAh.

One has to use a replacement battery, which possess the same features as the original battery in order to enhance its function.

What does a PLC battery warning mean?

A PLC battery warning, normally viewed by a ‘BAT’ or ‘BATT’ LED on the CPU module means that the battery voltage is low, that is below a specific value. For example, in the settings of the Allen-Bradley processors, an ‘’Active’’ battery light automatically turns on when backup power will take only 10 days of charging; this notifies the administrator that the battery requires replacement before total system failure.

Test your knowledge on PLC hardware and Refer the below link for the Quiz on Understanding PLC Hardware Components

Where is the battery in a PLC?

The position of the battery in a particular model of the PLC can be different. Normally it is located in the CPU module or cabinet of the system controller. In some designs, the battery is located on the front side of the CPU module and can be replaced without having to remove the entire module. In others, it may be necessary to get to the battery through the cabinet or drawer of the system controller. For specific steps, concerning the place to find the PLC battery and how to replace it, always consult the manufacturer’s manual.

Its therefore important to periodically check or replace the battery of a PLC so as to protect the system and avoid possible breakdown.

Click here for Understanding Remote I/O in PLC Control Systems