I have a UPS system for a PLC that manages a standby 12.5KW, 120/240 
volt generator. The PLC is rated for 12 -24 VDC, but 10 VDC is the real 
minimum before it drops out. Normal power from the 120 through a 12VDC 
output switching power supply. Backup is 2 12 VDC lead acid batteries in 
parallel, everything has a common ground. The 2 batteries are isolated 
by a pair of diodes so the higher voltage one will do the feeding, and 
they are isolated by a DPDT relay so when not needed they are isolated.

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.

To prevent chattering if the house power is intemittent for a time (a 
minute), the PLC monitors the 120 house supply (could be either utility 
or generator), and when first instant the house supply falters, 
activates a 12 VDC dip relay with a set of N.C. contacts in series with 
the 120 volt coil relay, and continues to activate until 1 minute after 
the 120 volt house power has not dropped.

All this works very good when the house is powered by the utility, but 
ONLY when the house is powered by the generator (voltage a nice 120 volt 
+/- 1 volt,  60 hz. +/1 2hz.) (everything else in the house likes the 
generator power with 12/240 changeover only needing to set a few clocks) 
the switchover from house to battery causes the PLC to reset. Note that 
the Dip relay is activated, opening a contact that at this shouldn't 
have much voltage, and the 120 volt coil relay is just dropping out,

Any thoughts would be appreciated. Can I (or do I wnat to) put a big 
capacitor on the PLC supply. I don't have a O..scope, but do have a DMM 
with RS-232, at it's slow (1 /sec.) rate nothing is showing up in the 
PLC supply.

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