Programmable logic controllers (PLCs) are rugged, lightweight computers used to automate industrial processes such aswater waste treatment plants, mineral processing plants, and manufacturing plant assembly lines.
All programmable logic controllers are composed of four basic components. These components include the processor section, input/output (I/O) section, programming section, and power supply. The power supply provides the appropriate voltage levels required to power the PLC and its internal components. The power supply consists of a step-down transformer to step down the line voltage and a rectifier to convert it to DC.
The device also features capacitors that filter DC current to ensure a constant supply of output voltage, free of electrical noise and transient voltage spikes. While the main power source for the PLC and its components is the mains power supply, the The PLC also contains a backup PLC battery. In some PLC models, the main power supply unit charges the internal battery. The battery is connected directly to the PLC CPU board. This article provides a detailed description of PLC batteries.
Why are batteries needed in PLCs? (What is the purpose of this)PLC configuration settings, process set points, PLC logic and real-time clock are stored in EPROM (erasable programmable read-only memory), which is battery powered. The battery powers the EPROM in case the PLC power supply is isolated for maintenance or in the event of a power failure. PLC batteries are also known as CMOS batteries, backup batteries, RTC batteries, processor batteries or RAM memory batteries. Therefore, we can say that the PLC battery provides the power required to retain the contents of the processor’s non -volatile memory during a power outage or when the machine is shut down for maintenance or changing locations.
Common types of PLC batteriesTypically, PLC systems use lithium-ion or lithium batteries. Lithium thionyl chloride batteries are the most common battery type used for PLC backup power. They are the most popular as they are ideal for low current applications and provide long term service to PLC batteries. Additionally, the common voltage types for PLC batteries are 3.0 Volts DC and 3.6 Volts DC, although you can get higher voltages by connecting multiple batteries in series. The two most common variations of PLC battery voltage are the 3.6V lithium PLC backup battery and the 3.0V lithium coin cell battery.
How long does a PLC battery last?Depending on whether the PLC uses a capacitor battery assembly or a lithium battery assembly, some of the batteries that power the PLC processor are not rechargeable, while other batteries are rechargeable. Generally, the service life of lithium-type PLC batteries is 2-5 years, depending on the type of processor used, operating environmental conditions and the amount of battery power used. For example, the onboard 3.0 VDC lithium battery can provide backup for the PLC CPU during a power outage lasting several hours or days, after which its charge is completely depleted.
What are the factors that affect the service life of PLC batteries?PLC battery life may be negatively affected by:
frequent power outageshigh operating temperaturePower off the system for an extended period of timeSystem ageIt is recommended to replace the PLC battery every 2 to 3 years to avoid problems such as electrolyte leakage. Additionally, since the above factors have a negative impact on PLC battery life, you should also consider your situationr The PLC system schedules the time to replace the battery.
What happens if the PLC battery dies?As mentioned previously, PLC programs that control manufacturing processes or machinery are typically stored in non-volatile or battery-backed memory. Even if the battery fails or is exhausted, the PLC system can continue to operate normally, and in this case, these programs will not be lost as long as the main power supply remains on and is not turned off.
However, when a power outage occurs or the PLC power is cut off and the
Replace PLC battery is exhausted, the entire programmed software logic and PLC configuration settings will be cleared. This leaves you looking for a program backup, and the PLC system is likely to be out of service after a power outage. Therefore, it is recommended that you regularly check the voltage level of your PLC battery and replace it before it expires or before its 5-year lifespan expires.
How to replace the battery in a PLC?For the reasons discussed in the previous section, the onboard PLC battery in the CPU module should be replaced periodically. The first thing you need to do is determine if the PLC battery needs to be replaced by checking its status. Diagnostics programs are designed to detect types of faults in individual components of a PLC system, such as CPUs and I/O modules.
The CPU marks non-fatal errors as requiring attention. These errors do not prevent the CPU from entering RUN mode, nor do they cause the CPU to change from RUN to STOP mode. Additionally, PLCs have system tags in their applications that detect the occurrence of non-critical errors. “Backup battery low” is an example of a non-fatal error. In this case, the PLC system will issue a warning via the BATT LED on the CPU module.
Typically, the battery (BATT) LED on the CPU flashes or turns yellow or red when the battery voltage is below the low voltage threshold, and turns off when the battery is good (when the battery voltage is above the lower threshold). However, it is difficult to notice that the BATT LED is on because most of the time the LED is locked in a drawer or cabinet in the controller. Therefore, it is important to develop a routine maintenance schedule to check the status of the battery.
A) PLC battery status checkWhen you perform a routine battery status check, follow these steps:
First, pull the system controller cabinet or drawer out of the exciter frame.To expose the CPU module, remove the system controller enclosure cover.Check for red, yellow or flashing BATT LED alarms on the PLC CPU module.If the BATT Alarm LED is “ON” (active), proceed to replace the PLC battery.NOTE: As mentioned previously, the BATT LED alarm will activate when the PLC battery voltage falls below the low threshold; however, the threshold will vary depending on the brand or type of PLC battery used. For example, the sub-threshold voltage of a 3.0 VDC rated lithium battery used in the Allen-Bradley PLC-5 is approximately 2.0 VDC or 2.5 VDC. In this particular PLC-5, the BATT LED alarm first turns “ON” when the remaining battery backup powering the processor (CPU) reaches 10 days, regardless of whether the CPU is powered or not.
B) Replace PLC batteryWhen the PLC battery voltage is below the low threshold and the BATT LED is red, then battery replacement should be considered. Additionally,PLC battery replacement can be done as a common preventive maintenance procedure or when the battery life has expired.