Up-to-today, conversations of ControlLogix CPU battery only exist in sporadic, small chat rooms, and/or email conversation between Allen-Bradley distributors and end-users. I wanted to break out of this “cage” and share some of my professional and technical experience about these batteries, cost, after market solutions to everyone, and hopefully it might allow you to make excellent recommendation to your facility.
ControlLogix CPU battery has to be effectively maintainted on regular basis to ensure data rentention and no impact to the process. Allen-Bradley recommend recycling battery every 2 to 3 years, but they come with a premium price (at about CAD$450 per battery with wires and connector); as quoted to me on Feb 2018. Cost can add up really quickly if you have 48 ControlLogix in your plant or facility.
The table below shows Allen-Bradley catalogue number for batteries that are recommended for 1756-L6*. Note that, 1756-L7* model of the PLC comes with “1756 Energy Storage Modules” which is basically is a capacitor circuitary; see section below. The table was extracted from the ControlLogix System – User Manual, page 75.
When the battery is approximately 95% discharged, low battery warnings are issued via the following means.
- The BAT is solid red
- A minor fault (type 10, code 10) is logged. Table below show all minor fault type 10 which pertains to battery fault (table is from Logix5000 Controllers Major, Minor, and I/O Faults, page 36-37).
Purpose of The Battery
ControlLogix PLC retains important information (such as RTC, PLC configuration, PLC logic, process setpoints) in an EPROM, which is powered via a battery. Naturally, if you remove 120/240VAC power supply to the PLC, continues to power the EPROM. If both the 120/240VAC power supply and the battery were to be removed, the PLC would lose all the information, hence setting back to its “factory” state. For the same reason, PLC battery replace Preventative Maintenance (PM) must be performed on regular interval.
Table below shows average battery life (table is from ControlLogix System, page 77) when power is not applied to ControLogix PLC.
Can ControlLogix battery be changed while PLC is powered with 120/240VAC
Yes you can as the battery is easily accessible from the front face of the CPU, as long as user logic does not have any code that have adverse effect to the process, such as stop process if the battery goes down you can safety change it while the processor is running. Btw, it is uncommon to have such a logic built but nevertheless I figure I should underscore it.
1756 Energy Storage Modules
Instead of a battery, the 1756-L7 and 1756-L7S controllers are shipped with a 1756-ESMCAP energy storage module (ESM) already installed. See images of the 1756-ESMCAP module below. I have a YouTube video on how to remove it at the end of this section.
The table below is from “1756 ControlLogix and GuardLogix Controllers”, (page 39).
After Market Battery
After market 3.6VDC battery can be used for PLC-5 but it has to be Lithium ion type. One take away from this is that soldering leads on a Lithium ion battery is NOT recommended for beginners as there is a serious danger of overheating which can damage the internal barriers and cause excessive internal leakage, which can quickly cascade into a damaged battery, or just about ready to explode.
I don’t work for Allen-Bradley or Rockwell, and I am not saying you cannot use after market batteries but I would be cautious about it. Here is one example of after market battery for PLC-5 with pre-soldered leads.
Difference between Lithium and Lithium-Ion Batteries
Lithium-Ion batteries have a lower self discharge rate than other types of rechargeable batteries. This means that once they are charged to their capacity and disconnected, they will retain their charge for a longer period of time than other types of rechargeable batteries. Depending on the environment condition that they are stored in, Nickel Cadmium (NiCad) and Nickel Metal Hydride (NIMH) batteries loses 1-5% of their charge per day. Lithium-Ion batteries will retain most of their charge even after months of storage.
They have a higher energy density than most other types of rechargeable batteries, which means that for their size and/or weight it can hold much more energy than other rechargeable batteries. In additional to that, Lithium-ion batteries also runs at a higher voltages than other rechargeable batteries (about 3.7VDC against 1.2VDC for Lithium-Ion and NiMH or NiCd respectively).
“Memory Effect” of Nickel Cadmium (NiCad) and Nickel Metal Hydride (NIMH) batteries
Memory effect, also known as battery effect, lazy battery effect, or battery memory, is an effect that is observed in NiCad and NIMH rechargeable batteries that causes them to hold less charge. What it translates to is that, when NiCad and NIMH partially discharged/used before charging, the battery “forgets” that it has the capacity to further discharge all the way down. Th eonly way to remedy this is to fully discharge/drain your NiCad or NIMH battery at least once a month, before fully charging it back again.
For this reason, NiCad and NIMH batteries are not a good solution for critical process environment.
Battery Replacement Compromise
As I was mentioning, it cost about CAD$450 per battery; as quoted to me on Feb 2018. And with 48 ControlLogix in the plant I was working at, the cost was quite astronomical (at CAD$21,600 purely for the batteries without GST).
Given the cost and that the battery can be replaced without powering down the ControlLogix, one of the best way to mitigate this delima would be to approach it with a reactive action. That is, replace the battery only when it has faulted or cannot sustain the require voltage. Locally the PLC will indicate that the battery has fallen below its lower threshold with the “BATT” LED (on the ControlLogix CPU faceplate), but you can further automate by adding the GSV block to your logic to monitor the battery status, and setup to trigger an alarm on your Control Room HMI system. The screenshot is a short logic to monitor the battery status in the ControlLogix.
Make sure that there is no pre-existing logic that would cause major interrupting to the process; such as ESD a compressor or plant when the battery low. Not too sure why one would do that but just wanted to mention it.