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A Programmable Logic Controller (PLC) is a ruggedized digital computer used for industrial automation. It’s designed to control the automation of electromechanical industrial processes, such as automated packaging equipment, assembly line machinery, material handling systems, and lighting systems. It accomplishes this by continuously monitoring the state of field input devices and making decisions based on a user-defined program to control connected field output devices. The PLC programs used to control automated industrial machinery or manufacturing processes are usually stored in a non-volatile or battery-backed PLC memory.
A typical PLC system consists of four main components: namely, a Central Processing Unit (CPU), a programming device, input and output (I/O) modules, and a power supply unit. The power supply unit provides the appropriate voltage levels required for the internal operations of the PLC and/or powering the I/O modules. It does this by converting the incoming line voltage (normally 240 or 120 Volts AC) into the required DC voltage (normally 5 to 24 Volts DC). PLC power supplies are designed to provide a constant output voltage that is free of transient voltage spikes and other types of electrical noise. Note, the PLC power supply can be a separate unit or built into the PLC’s processor section.
While the power supply unit is the primary source of power for the other PLC components, PLCs also includes an internal battery that prevents memory loss in case the external power supply is removed. This battery is normally connected directly to the CPU module, and in some PLC models, it is charged by the main power supply unit.
PLC control systems use either Lithium or Lithium-ion batteries, with Lithium-Thionyl Chloride being the most common type of PLC battery. Lithium-Thionyl Chloride PLC batteries are well suited for low-current applications where long service life for the battery is desired. The two most common voltage variants for Lithium-type PLC batteries are 3.0 VDC and 3.6 VDC, but higher voltages can be achieved by connecting multiple batteries in series.
The purpose of the PLC battery is to provide the necessary power required to retain the contents of the volatile RAM memory (located within the PLC processor) during power outages or when the PLC system is disconnected from the power source for maintenance or a change of location.
Most PLCs use two types of memory, Electrically Erasable Programmable Read-Only Memory (EEPROM) and Random-Access Memory (RAM). EEPROM does not require a constant power source to maintain its memory contents, as it retains them indefinitely. Also, there is a limit to the number of times the EEPROM can be written but its contents can be read infinitely. In contrast, the contents of RAM memory can be read from and written infinitely, but RAM requires a constant power source to maintain its memory contents. So, the contents of RAM memory will be lost in case the external power to the PLC system is disconnected without battery backup.
Normally, the contents of RAM are maintained by a 5 VDC internal power supply only while the PLC system is powered by an external source (usually 240 or 120 VAC). When the external power to the PLC system is switched off or isolated for maintenance, then the RAM contents can be maintained by the installed PLC battery. This prevents loss of the PLC’s programmed software logic, configuration settings, real-time clock, and process set points that are stored in the volatile RAM memory. That is why the PLC battery is sometimes referred to as a backup battery, RAM memory battery, processor battery, CMOS battery, or RTC battery.
Capacitor-type PLC batteries usually back up the contents of the processor’s volatile memory for up to 72 hours during a power outage. So if the power supply to the PLC system is restored after 72 hours, the PLC will return to operation mode with the configuration settings and program values that were present prior to the power failure.
On the other hand, Lithium-type PLC batteries provide backup power to the processor’s volatile memory should a power outage last for less than 30 minutes. But such batteries have a service life of two to five years, depending on the amount of power drawn from the battery and the type of processor being used. Thus, if a PLC battery is not drained it can back up the RAM memory data for at most five years. It is, therefore, necessary to regularly check and maintain Lithium-type PLC batteries, to ensure that their voltage levels are within the recommended values.
Capacitor-type PLC batteries are rechargeable while Lithium-type PLC batteries are not rechargeable; but, Lithium-ion batteries are rechargeable. And as previously stated, the average service life of a Lithium-type PLC battery is 2-5 years, depending on the amount of voltage being drawn out, the operating environmental conditions, and the type of PLC processor used. Therefore, it’s recommended that you replace a Lithium-type PLC battery after every 2 or 3 years, to avoid battery failure problems such as electrolyte leakage or memory loss. However, there are several factors that can negatively affect the service life of Lithium-type PLC batteries, including:
For example, if a PLC system experiences frequent power outages for a prolonged period of time, an on-board 3.0 VDC Lithium-type PLC battery can provide backup power to the processor’s memory for several hours or days, after which its charge will get completely depleted. At that point the PLC battery is said to be dead–the voltage level of the PLC battery is below a functional amount–and it will need to be replaced.
As mentioned earlier, user-defined PLC programs that control automated industrial machinery or manufacturing processes in a PLC control system are usually stored in a non-volatile or battery-backed memory located within the PLC processor. Hence, the PLC system can still operate normally even with a dead or depleted battery as long its input power supply is not interrupted; in such a case, the battery-backed programs are not lost.
However, if a power outage occurs or the external power to the PLC system is removed while the PLC battery is dead, the process set points, configuration settings, and the entire programmed software logic stored in the battery-backed processor memory will be lost. This leaves the user looking for external program backups and the PLC control system will probably be out of service beyond the power failure. For this reason, it’s advisable that you regularly check the voltage levels of your PLC battery and replace it if its voltage is below the recommended value. Later on, we’ll discuss how to use a Multimeter to check the voltage levels of a PLC battery.
The first thing you’ll need to do to establish if a PLC battery is dead and due for replacement is to check its status via the “BATT (battery) LED” located on the CPU module. PLCs have built-in system diagnostics that detect various types of faults in their components including the processor, memory, internal battery, and I/O modules. The faults that do not prevent the PLC from activating into “RUN Mode”, or those which don’t interrupt its normal functioning (i.e. not switching it from “RUN Mode” to “STOP Mode”) are flagged by the CPU as non-critical errors that require attention. An example of a non-critical PLC error is a “battery low voltage” indicator.
If the voltage of the backup battery is getting low, the CPU will provide a warning signal via the BATT LED on the CPU module. Generally, this BATT LED flickers or turns yellow/red when the voltage of the PLC battery is below the specified low-threshold voltage. Therefore, a flickering BATT LED or a red/yellow “Battery Low” indicator shows that the charge of the PLC battery is depleted (the battery is dead) and needs to be replaced. But when the PLC battery is in good working condition (i.e. its voltage value is as recommended), the BATT LED is normally off.
However, it’s hardly possible to notice a flickering or a yellow/red BATT LED other than when a PLC malfunctions, because in most cases the BATT LED is locked up in the PLC’s enclosure or cabinet. Hence, it’s necessary to set up a routine maintenance schedule for the PLC system that involves checking the BATT LED status to determine the condition of the installed PLC battery.
Note: The lower-than-threshold voltage values of PLC batteries enough to activate a BATT LED vary depending on the model and type of PLC battery in use. For example, the Allen-Bradley PLC-5/40 Lithium-type PLC battery rated at 3.0 VDC, has a low-threshold voltage value of approximately 2.5 to 2.0 VDC. While other PLC battery models like the Mitsubishi A6BAT Li-ion PLC battery, with a voltage rating of 3.6 VDC, have a different low threshold value.
In addition to checking the status of the BATT LED, taking routine voltage measurements of a PLC battery can help determine if the battery is dead or in good working condition. Also, sometimes you may have replaced an old/faulty PLC battery with a new one, but the BATT LED continues to flicker or it’s red/yellow even though the programmed logic and configuration settings are still retained in RAM memory. This would necessitate the need to test the voltage of the new PLC battery, as part of diagnostics and troubleshooting.
Digital Multimeters are often used to test PLC batteries because they’re multifunctional devices that can directly measure voltage and other electrical parameters such as resistance, current, frequency, and capacitance. Also, digital multimeters have a high electrical resistance ranging from 1 Megaohm (MΩ) to 10 MΩ; so, they can measure voltage much more accurately. In addition, they include an onboard LED or TFT LCD screen with a backlight, which offers exceptional visibility to users.
Testing a PLC battery using a digital multimeter will involve a number of steps, including disconnecting the PLC battery, visually inspecting the battery, setting up the digital multimeter, and finally performing the voltage/amperage test. Let’s discuss each one of these steps.
The first step when testing a PLC battery with a digital multimeter is to locate the battery and remove it from the CPU module or from the power circuit it is connected to. If a rechargeable battery is in use, meaning that it’s part of an electrical circuit that uses the external power source supplying power to the PLC, then you should always isolate the PLC power supply unit. But this step is not necessary if you’re planning to test a new battery that’s yet to be installed in the PLC system.
Note: Always be gentle when disconnecting a PLC battery to ensure that its terminals do not get damaged.
After disconnecting the PLC battery from the power circuit of the PLC system, the next step will be to physically inspect it for any obvious signs of damage. Check for the following:
Visual inspection of the physical condition of a PLC battery helps to rule out any external defects that would not be picked up when testing its working condition using a digital multimeter. If you identify any of the defects listed above be sure to replace the battery. If the overall physical condition of the battery checks okay, then proceed to test it with the multimeter.
When testing a PLC battery using a digital multimeter, it’s recommended that you test both its voltage level and the amount of current/amperage it’s supplying. The location of the multimeter dial determines the type of test you can perform on the battery, either voltage or current test.
The first step in setting up the multimeter is to put the testing leads/probes in the correct location. Generally, the testing probes of digital multimeters are colored red and black, and they both have specific locations. The black testing probe should be plugged into the COM port on the multimeter while the red testing probe should be plugged into the Voltage port (often indicated as VΩmA or mAVΩ port). Some multimeters have multiple locations for the red testing leads if they’re fused.
Once the two testing leads are connected as required, you can then turn on the multimeter. Next, move the multimeter dial to the DC Voltage setting, which is usually indicated as either DCV or just V with a superscripted straight line having three dots below it. When testing PLC batteries, ensure that you don’t use the AC voltage setting which is indicated by either ACV or just V accompanied by a superscripted curved line, on most digital multimeters. This is because PLC batteries run on DC voltage; so, testing for AC voltage won’t yield correct results.
After you have selected the DC voltage mode, the next thing to do will be to set the voltage range of your multimeter. Some digital multimeters have an auto-range feature where they’ll change the testing range depending on the connection level of voltage. But if your multimeter does not include this function, then you should set it up to test higher than the actual voltage rating of your PLC battery. For example, if you’re testing a 3.6V PLC battery you should set up your multimeter to test between 0V to 5 V DC. To determine the voltage range to set up on the multimeter, check the specified voltage level of the PLC battery to be tested. You can find the voltage rating of a battery on its label or in the provided user manual.
This is exactly the same process you should follow when setting up a multimeter to test the amperage of a PLC battery. The only difference will be the dial located on the multimeter. Because when testing for the level of current being supplied by a PLC battery, the multimeter dial should be turned to the DC Current setting.
After setting up the multimeter correctly, you can now test the voltage/ current level of your PLC battery. Let’s start with voltage testing.
When testing the voltage level of a PLC battery, place the battery such that its terminals face you or its connectors are close to you. Then, connect the red testing probe of the multimeter to the positive (“+”) terminal or the red connector of the PLC battery. Next, connect the black testing probe of the multimeter to the negative (“-“) terminal or the black/blue connector of the PLC battery. While holding the two testing probes in position (as described above), take note of the voltage reading on the display of your multimeter.
If you’re testing a 3.0V PLC battery, a battery in good working condition will show a voltage reading of between 2.5 V to 3.0 V. Anything less than 2.0 V is an indication that the battery is dead and should be replaced, because the low-threshold voltage of PLC batteries rated at 3.0 V is usually between 2.0 V to 2.5 V. On the other hand, if you’re testing a 3.6V PLC battery, a good battery will show a voltage reading of 2.9 V to 3.6 V. Anything less than 2.4 V can show that the battery is dead and should be replaced, as the low-threshold voltage of 3.6V PLC batteries is normally between 2.4 V to 2.9 V. A general rule of thumb is, if the voltage level of any battery reads half or less than the recommended operating voltage then the battery is considered faulty/dead and not fit for use–it will need replacing.
When testing the amperage or the level of current a PLC battery is supplying you should follow the same steps, but ensure that the multimeter dial is turned to the correct location–DC Current Mode with the correct testing range set up. In such a case, the multimeter reading should show a current level close to the specified amperage rating of the PLC battery being tested, anything considerably less than this indicates that the PLC battery is faulty and not working properly.
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