UPS for PLC controlling standby generator

[cdenk]

I Have a PLC (Tri-PLC T100MD888) that will be managing a Tri-Fuel standby generator. Normal power for the PLC will be a 120 VAC to 12VDC linear power supply. The 120 VAC will either be power company or from the generator. There is a delay between switch overs of up to a minute or more. The generator power supply is a 12VDC lead acid battery that will be charged and maintained by the generator's charging system and a 120 VAC charger. The issue is, the brief period while the generator comes up to speed, or switching back to power company power, the PLC needs to remain up and running, otherwise when it would reset, the generator would stop, and oscillation between power sources would result. Have thought about parallel diodes, but the lead acid battery and charges could frequently have voltage above the 12VDC linear power supply. How can I keep the PLC from resetting?  

[support]

Is the 12V lead acid battery used to start up the diesel generator? When you crank the engine the voltage would drop to may be 4V because the engine takes several tens or hundreds of amps of current! That's would cause the PLC to reset if the PLC is powered from the same 12V source.  You may want to use a separate 12V battery to power the PLC so that it is not affected by the startup voltage drop.

[cdenk]

Yes, the lead-acid battery is used to start the generator, but it's a gasoline 27 hp., 2 cyl. Kohler engine. No load battery voltage is 12.7 volts, cranking 12.0 volts. The battery is severely oversized with respect to the engine for normal applications. Was thinking the battery is there, and kept charged, why not use it as the UPS, and eliminate one more piece of equipment to maintain (a separate UPS), but maybe that's a better way to go, a UPS meant for a PC could easily bridge the power outages and aren't that expensive. I was thinking an electronic relay circuit controlled by the 120 volt supply would switch quick enough to keep the PLC from resetting .

[support]

For system operating at 24V and if you put a large enough capacitor at the PLC power terminal it may be able to buffer the voltage drop long enough for the relay to switch over. But at 12V there is not much margin for the voltage to drop before the PLC is reset. So for your application I would certainly recommend a separate backup power for the PLC.

[cdenk]

The 12 volt selected since everything else is 12 volt, sounds like an UPS is the way to go. Thanks for the quick replies, good support!