I have been working on a UPS that goes the other direction. 120VDC (International Power IHC12-3.4) normal power, with a 12 VDC marine battery as backup)( See my post ot this forum some months ago for more details). To do the switching I use a "ice cube" relay (Square D 8501KP12P14V20) DPDT 120VAC coil). NC contacts to battery, NO contacts to 120VAC supply. The coil is series with 12VDC coil DPDT DIP relay (Clare LX200000) the NC contacts. One of the PLC's outputs controls the DIP relay. When the 120VAC drops, the big relay coil power drops, and the PLC power is switched to the battery. The PLC is monitoring the 120VAC frequency and voltage already since this is a standby generator controller, and recognizing the 120VAC is out, will power the DIP coil, keeping the PLC power to the battery until the 120VAC power is good for a period of time say 15 minutes, to prevent chatter and the fluctuations that might occur as the power company is working on the lines. At this time I have bench tested the setup, and the 120VAC power supply bridges the gap, and even 12 VDC the PLC does not reset. Hopefully in about a week I should have the installation complete and be able to verify the design, will keep posted.