Yes, it's likely that the voltage is dropping causing the PLC to reset, 
but doesn't explain why, with the 3 voltages (2 batteries and switching 
supplies, and the 120 volt are stable (though the 1 battery will be a 
volt or so higher due to the generator's engine's alternator (15 amp 
max.) being on line)(more on that below), the relay (have tried 3 
different manufacture's equivalents) is dropping out contacts pulled 
apart by spring, and while on utility power, no problem. The 2 batteries 
are connected to the PLC by diodes as you suggest.

Currently the switching supply is 12.1 volts, the batteries are 
generally in the 13 - 14 volt range. It would be possible to provide a 
higher voltage switching supply, but there is other equipment on line 
including numerous automotive 1" cube relays rated at 12 VDC, 
electronics, and sensors that may have resistor voltage dividers, that 
makes that a poor choice at this time.

After sleeping on it overnight :), the only thing different with the 
genrator running on the 12 VDC side is the generator's engine (27 Hp. 
Kohler) does have an alternator and regulator to service it's battery 
(beefy 12 volt deep cycle marine type lead acid) is on line. Both 
batteries have their own charger/maintainer also. Will do some 
experimenting with taking 1 or the other battery out of the circuit. The 
engine alternator to battery cable is heavy, probably "00", and then 
from the battery to the UPS area is 35' (10m) (one way) of #12 wire. 
Would expect the battery to absorb most spikes, noise, etc.

The PLC does have LED's for pause, RTC Error, and Run Error. To the 
eyeball, the 3 LED's are indicating the same as if powering up the PLC, 
which tends to support the theory of dropped power.

Please keep the ideas coming. :)

Dwayne Reid wrote:
> At 07:06 PM 4/9/2007, Carl Denk wrote:
>
>   
>> The DPDT relay (Square D 8501KP12P14V20) has a 120 volt coil going to
>> the house power with the PLC input going to the common contacts, the 120
>> power supply to the  N.O. contacts so if there is 120 volt power
>> available, the power comes from the 120 volt supply. Each battery goes
>> to one of the N.C. contacts so if the 120 isn't there, the batteries are
>> the supply.
>>     
>
> I take it that the PLC loses power during the time while the relay is 
> switching between its contacts?
>
> If so, I expect that to be the problem.
>
> Can I make a possibly simple suggestion?
>
> Ensure that your DC supply while operating from AC power is higher 
> than the float voltage on the batteries.  Then use diodes to OR all 3 
> sources of power: one diode anode from each of the batteries and from 
> the AC power supply.  The cathodes of all the diodes feed the PLC power input.
>
> By picking the output voltage of the AC-powered DC supply to be 
> higher than what is present on the batteries, you ensure that the 
> batteries are not loaded.  When the AC power disappears, the diodes 
> smoothly transition to whichever battery has the highest voltage.
>
> Easy to test to see if this fixes your problem.  I suspect that it will.
>
> dwayne
>
>   
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