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d0ubleth1nk

CompactLogix CPU Battery

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I’m investigating some unusual events with a CompactLogix controlling a small sewage plant. The RPU was installed in April (2010). In August, there was an extended power outage, where the genset failed and the UPS for the panel eventually died. When power came back, there was no program in the RPU as the CPU battery was dead. Then there were issues with a communication loss from the CPU to an IO module, that faulted the processor almost daily. The CPU and PS were replaced, and a new battery came with the new CPU. Everything ran fine until last week. Again the CPU faulted from loss of comms to some IO card, and the battery was dead again. After a series of tests I figured out there was a problem with a dosing pump connected to an AO card where the pump was not properly grounded and there was 48VAC potential at the pump AI. The AO card is rated for 30 VDC max. I replaced the AO card, added some signal isolators and everything seems to working fine. The client has major reservations about the dead PLC battery, and all I can tell them is that the AO card problem must have killed the batteries, but not the CPU’s. Any idea’s how the CPU batteries have been dieing?

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I've heard of bad backplanes causing problems like that. They carry the voltage from the power supply to the cards, and a short in them can leach the battery down to nothing. The CompactLogix doesn't have a separate backplane, so the problem would be in the portion of the backplane in one or more of the cards. In this case, it seems like your dosing pump may have had a power spike that damaged the backplane in the AO card, but not enough to completely kill everything. Just a theory.

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I had a similar issue with a Control Logix system running Control Net. Redundant PLCs in two racks, each with two power supplies, redundant output power supples, but only one control net card in the IO Rack. The system lost is memory and after a call to RA tech support, it turns out there is a batch of bad ICs from Amtel made in an 18 month period. Out of 5 control logix cards, only the control net card had the bad IC, no spare for it, and it was replaced on a recall at no cost. What happened was the control net card would work for a while, then fail. Failure time kept getting shorter and shorter. The bad control net card caused the processor to loose its memory and batteries went dead. Going to see if the program can be saved in non volitale (sp) memory. What we found out there was no alarm sent on the failure. The system just quit working. I am ok with a bad battery, but not Ok with no alarm.

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HI, Is it possible the genset is providing poor quality power to the CPU? I prefer to use a UPS with built in constantly active power inverter if the line is noisy. Might want to load a memory storage card with the program file & set the CPU to load up the program when needed. BD

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