Allen-Bradley PLC backup battery, very short life

[drussell]

This is on an Allen-Bradley MicroLogix 1400.

Has anyone ever seen a case where one of these PLCs, powered up 24/7, drains the backup battery within a matter of a few months at most? 

I was surprised originally to find the battery dead several months ago, since the whole unit is only a few years old and typically the batteries last for 10+ years on a system that is constantly powered because the lifetime is essentially the shelf life of the battery and 20 years is not uncommon on these cells.  I just chalked it up to being a defective battery that had faulted internally and self-discharged earlier than expected, got another good quality 14250 lithium primary cell and stuck it in, expecting it to work for many, many, many years.

Well, someone threw a breaker the other day trying to reset something and the PLC lost all its programming.  I tested the cell.  *0* volts across it.  Something is obviously killing these backup batteries. 

I'm guessing it is a leaky diode in the PLC or something, has anyone ever seen this before on these units?  It seems to be a quite strange and rare problem...

[tecman]

My guess is a leaky capacitor in the battery circuit.  There is usually a diode separating the battery from the internal supply voltage.  If it were leaking you might damage the battery with overvoltage.  A leaky capacitor is much more likely.

paul

[Kalin]

I'm not sure is micrologix are the same as compact logic in this regard but I believe you can read out the battery voltage from rslogix and keep an eye on it that way. I would also check for corrosion or detritus on the battery contacts, it could be bridging and causing a slow drain.

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[drussell]

My guess is a leaky capacitor in the battery circuit.

How do you figure?

There is usually a diode separating the battery from the internal supply voltage.

Indeed....

If it were leaking you might damage the battery with overvoltage.

I assume you mean the diode.  Yes, perhaps it would, and that is my line of thought, however, that depends on what the nominal rail voltage to the SRAM is.  Probably 3.3 volts, so a 3.6 volt lithium primary probably can't be back-fed from that.  I find it unlikely that the cell died from trying to be "recharged" from power feeding backwards into it, though it might be being slowly discharged somehow, it really depends on the voltages in that circuit.

A leaky capacitor is much more likely.

I don't follow...  Why would they put any kind of capacitor before the diode on the battery side?  It would serve no purpose except to cause potential leakage and shortened reduced life expectancy. 

Obviously, though, I have no schematic of the PLC itself, so I have no idea what they've done.

[drussell]

I'm not sure is micrologix are the same as compact logic in this regard but I believe you can read out the battery voltage from rslogix and keep an eye on it that way.

I am not the builder of this system, though I suppose I could try to at least load up whatever software talks to these units, find a serial port and hope that they don't have everything protected... see if I can talk to it at all, at least...

I would also check for corrosion or detritus on the battery contacts, it could be bridging and causing a slow drain.

The battery "contacts" are a .100" header with a connector, wire leads to the battery.  It is not a problem with the connection to the battery.

The multimeter that I used to check the current into the battery terminals from a pair of AAs temporarily wired in series to provide "life support" shows 0.00 mA on the 40mA (10 uA resolution) range...  virtually nothing flowing into/out of the battery when I check it.  Just enough to turn off the LOW BATT indicator on the LCD. 

I'm assuming it is some kind of intermittent fault and somehow temperature/vibration/something-or-other sensitive since it seems to be absolutely fine at the moment. 

It seems to be quite the heisenbug.   

[Zero999]

Have you contacted Allen-Bradley or Rockwell Automation, as I believe they are today?

I'm surprised modern PLCs need a backup battery for anything other than a real time clock. I would have thought they would use non-volatile memory these days.

[Kalin]

I read the documentation on the Micrologix line and it seems what you are describing the battery for the RTC Dying. There is a separate memory module (1766-MM1) for storing recipes etc. but that is not for your ladder logic. those should be saved in a nonvolatile eeprom. I do recall having a similar issue with a compactlogix PLC and I can't remember specifics of how to fix it off the top of my head but i do remember that there was some configuration on how to handle loss of power and what to do in case of an unexpected reset. We had added a compact flash card for program memory and therefore the default location that it loaded the program from in case of loss of power had nothing in it and it made it seem that the program was lost. It will be difficult for you to do anything without access to the programming software for the controller. its not like you can just connect with a serial cable and program it. There are serious protections in place against things like that. 
Hope this helps.
Kalin